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Experimental evidence for a metastable state in FeTe1-xSex following coherent-phonon excitation

L. X. Yang, G. Rohde, Y. J. Chen, W. J. Shi, Z. K. Liu, F. Chen, Y. L. Chen, K. Rossnagel, and M. Bauer, J. Electr. Spectr. Rel. Phen. 250, 147085 (2021) [Click for Abstract]

Changes of the electronic structure of FeTe1-xSex upon ultrafast excitation with near-infrared laser pulses are investigated using time- and angle-resolved extreme ultraviolet photoemission spectroscopy. At different temperatures and for different doping levels we observe a global oscillation of energy bands near the Fermi energy driven by the coherent excitation of the A1g(Te) phonon mode in FeTe1-xSex (x = 0, 0.3). Prominently, photo-induced band shifts and a spectral weight reduction persist for at least 300 ps, long after the decay of the photoexcited carriers and the damping of the coherent phonons. We argue that the system escapes from the original equilibrium state and is transiently trapped in a possibly metastable state envisioning a purely optical means of manipulating the electronic structure in an iron-based superconductor.

Micro-spectroscopy of buried short-range surface plasmon polaritons supported by thin polycrystalline gold films

M. Großmann, M. Black, J. Jaruschewski, A. Klick, T. Leißner, J. Fiutowski, H.-G. Rubahn, and M. Bauer, Plasmonics 16, 737 (2021) [Click for Abstract]

The dispersive properties of short-range surface plasmon polaritons are investigated at the buried interfaces in vacuum/Au/fused silica and vacuum/Au/SiO2/Si multilayer systems for different gold film thicknesses of up to 50 nm using two-photon photoemission electron microscopy. The experimental data agrees excellently with results of transfer matrix method simulations, emphasizing the sensitivity of the plasmonic wave vector to the thickness of the gold film and an ultrathin native substrate oxide layer. The results furthermore illustrate the exceptional qualification of low-energy electron photoemission techniques in studying electronic excitations at buried interfaces.


Bypassing the structural bottleneck in the ultrafast melting of electronic order

L. X. Yang, G. Rohde, K. Hanff, A. Stange, R. Xiong, J. Shi, M. Bauer, and K. Rossnagel, Phys. Rev. Lett. 125, 266402 (2020) [Click for Abstract]

The emergent properties of quantum materials, such as symmetry-broken phases and associated spectral gaps, can be effectively manipulated by ultrashort photon pulses. Photoexcitation generally results in a complex non-equilibrium electron and lattice dynamics that involves multiple processes on distinct timescales16, and a common conception is that for times shorter than about 100 fs the gap in the electronic spectrum is not seriously affected by lattice vibrations. Here, we directly monitor the photo-induced collapse of the spectral gap in a canonical charge-density-wave (CDW) material, Rb0.3MoO3. We find that ultrafast (60 fs) vibrational disordering due to efficient electron energy dissipation quenches the gap significantly faster than the typical structural bottleneck time corresponding to one half-cycle oscillation (300 fs) of the coherent CDW amplitude mode. This result not only demonstrates the hitherto neglected importance of incoherent lattice motion in the photo-induced quenching of electronic order, but also resolves the perennial debate about the nature of the spectral gap in a strongly coupled electron-lattice system.


A combined laser-based ARPES and 2PPES study of Td-WTe2

P. Hein, S. Jauernik, H. Erk, L. Yang, Y. Qi, Y. Sun, C. Felser, and  M. Bauer, J. Phys. Condens. Matter, J . Phys. Condens. Matter 32, 345503 (2020), [Click for Abstract]

Laser-based angle-resolved photoemission spectroscopy (ARPES) and two-photon photoemission spectroscopy (2PPES) are employed to study the valence electronic structure of the Weyl semimetal candidate Td-WTe2 along two high symmetry directions and for binding energies between approx. -1 eV and 5 eV. The experimental data show a good agreement with band structure calculations. Polarization dependent measurements provide furthermore information on initial and intermediate state symmetry properties with respect to the mirror plane of the Td structure of WTe2.

Mode-resolved reciprocal space mapping of electron-phonon interaction in the Weyl semimetal candidate Td-WTe2

P. Hein, S. Jauernik, H. Erk, L. Yang, Y. Qi, Y. Sun, C. Felser, M. Bauer, Nat. Commun. 11, 2613 (2020),  [Click for Abstract]

The excitation of coherent phonons provides unique capabilities to control fundamental properties of quantum materials on ultrafast time scales.  Recently, it was predicted that a topologically protected Weyl semimetal phase in the transition metal dichalcogenide Td-WTe2 can be controlled and, ultimately, be destroyed upon the coherent excitation of an interlayer shear mode. By monitoring electronic structure changes with femtosecond resolution, we provide here direct experimental evidence that the shear mode acts on the electronic states near the  phase-defining Weyl points. Furthermore, we observe a periodic reduction in the spin splitting of bands, a distinct electronic signature of the Weyl phase-stabilizing non-centrosymmetric Td ground state of WTe2. The comparison with higher-frequency coherent phonon modes finally proves the shear mode-selectivity of the observed changes in the electronic structure. Our real-time observations reveal direct experimental insights into electronic processes that are of vital importance for a coherent phonon-induced topological phase transition in Td-WTe2.

Coherent modulation of the electron temperature and electron-phonon couplings in a 2D material

Y. Zhang, X. Shi, W. You, Z. Tao, Y. Zhong, F. Cheenicode Kabeer, P. Maldonado, P.M. Oppeneer, M. Bauer, K. Rossnagel, H. Kapteyn, M. Murnane, Proc. Natl. Acad. Sci. 117, 8788 (2020), [Click for Abstract]

Ultrashort light pulses can selectively excite charges, spins and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge density wave (CDW) material 1T-TaSe2. After exciting the material with a short light pulse, spatial smearing of the electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This indicates a bi-directional energy exchange between the electrons and the strongly-coupled phonons. By tuning the laser excitation fluence, we can control the magnitude of the electron temperature modulation, from ~200K in the case of weak excitation, to ~1000K for strong laser excitation. This is accompanied by a switching of the dominant mechanism from anharmonic phonon-phonon coupling to coherent electron-phonon coupling, as manifested by a phase change of  pi in the electron temperature modulation. Our approach thus opens up possibilities for coherently manipulating the interactions and properties of quasi-2D and other quantum materials using light.



80% Valley Polarization of Free Carriers in Singly Oriented Single-Layer WS2 on Au(111)

H. Beyer, G. Rohde, A. Grubisic Cabo, A. Stange, T. Jacobsen, L. Bignardi, D. Lizzit, P. Lacovig, C. E. Sanders, S. Lizzit, K. Rossnagel, P. Hofmann, and M. Bauer, Phys. Rev. Lett. 123, 236802 (2019),  DOI: /10.1103/PhysRevLett.123.23680 [Click for Abstract]

We employ time- and angle-resolved photoemission spectroscopy to study the spin- and valley-selective photoexcitation and dynamics of free carriers at the K and K' points in singly-oriented single layer WS2/Au(111). Our results reveal that in the valence band maximum an ultimate valley polarization of free holes of 84% can be achieved upon excitation with circularly polarized light at room temperature. Notably, we observe a significantly smaller valley polarization for the photoexcited free electrons in the conduction band minimum. Clear differences in the carrier dynamics between electrons and holes imply intervalley scattering processes into dark states being responsible for the efficient depolarization of the excited electron population.

Femtosecond time-resolved photoemission electron microscopy operated at sample illumination from the rear side

A. Klick, M. Großmann, M. Beewen, P. Bittorf, J. Fiutowski, T. Leißner, H.-G. Rubahn, C. Reinhardt, H.-J. Elmers, and M. Bauer, Rev. Sci. Instr. 90, 053704 (2019),   DOI: 10.1063/1.5088031 [Click for Abstract]

We present an advanced experimental setup for time-resolved photoemission electron microscopy (PEEM) with sub-20 fs resolution, which allows for normal incidence and highly local sample excitation with ultrashort laser pulses. The scheme makes use of a sample rear side illumination geometry that enables us to confine the sample illumination spot to a diameter as small as 6 μm. We demonstrate an operation mode in which the spatio-temporal dynamics following a highly local excitation of the sample is globally probed with a laser pulse illuminating the sample from the front side. Furthermore, we show that the scheme can also be operated in a time-resolved normal incidence two-photon PEEM mode with interferometric resolution, a technique providing a direct and intuitive real-time view onto the propagation of surface plasmon polaritons.

Ultrafast electron calometry uncovers a new long-lived metastable stat in 1T-TaSe2 mediated by mode-selective electron-phonon coupling

X. Shi, W. You, Y. Zhang, Z. Tao, P.M. Oppeneer, X. Wu, R. Thomale, K. Rossnagel, M. Bauer, H. Kapteyn, and M. Murnane, Science Advances 5, eaav4449 (2019),   DOI: 10.1126/sciadv.aav4449 [Click for Abstract]

Quantum materials represent one of the most promising frontiers in the quest for faster, lightweight, energy efficient technologies. However, their inherent complexity and rich phase landscape make them challenging to understand or manipulate in useful ways. Here we present a new ultrafast electron calorimetry technique that can systematically uncover new phases of quantum matter. Using time- and angle-resolved photoemission spectroscopy, we measure the dynamic electron temperature, band structure and heat capacity. We then show that this is a very sensitive probe of phase changes in materials, because electrons react very quickly, and moreover generally are the smallest component of the total heat capacity. This allows us to uncover a new long-lived metastable state in the charge density wave material 1T-TaSe2, that is distinct from all of the known equilibrium phases: it is characterized by a significantly reduced effective heat capacity that is only 30% of the normal value, due to selective electron-phonon coupling to a subset of phonon modes. As a result, significantly less energy is required to melt the charge order and transform the state of the material than under thermal equilibrium conditions.


Ultrafast formation of a Fermi-Dirac distributed electron gas
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G. Rohde, A. Stange, A. Müller, M. Behrendt, L.-P. Oloff, K. Hanff, T. Albert, P. Hein, K. Rossnagel, and M. Bauer, Phys. Rev. Lett.,  Phys. Rev. Lett. 121, 256401 (2018), [Click for Abstract]

Time- and angle-resolved photoelectron spectroscopy with 13 fs temporal resolution is used to follow the different stages in the formation of a Fermi-Dirac distributed electron gas in graphite after absorption of an intense 7 fs laser pulse. Within the first 50 fs after excitation a sequence of time frames is resolved which are characterized by different energy and momentum exchange processes among the involved photonic, electronic, and phononic degrees of freedom. The results reveal experimentally the complexity of the transition from a nascent non-thermal towards a thermal electron distribution due to the different timescales associated with the involved interaction processes.

Measuring the plasma-wall charge by infrared spectroscopy

K. Rasek, F.X. Bronold, M. Bauer, and H. Fehske, Europhysics Letters, 124, 25001 (2018), [Click for Abstract]

We show that the charge accumulated by a dielectric plasma-facing solid can be measured by infrared spectroscopy. The approach utilizes a stack of materials supporting a surface plasmon resonance in the infrared. For frequencies near the Berreman resonance of the layer facing the plasma the reflectivity dip - measured from the back of the stack, not in contact with the plasma - depends strongly on the angle of incidence making it an ideal sensor for the changes of the layer's dielectric function due to the polarizability of the trapped surplus charges. The charge-induced shifts of the dip, both as a function of the angle and the frequency of the incident infrared light, are large enough to be measurable by attenuated total reflection setups.

Detection and characterization of attenuated multimode waveguiding in SiO2 slabs using photoemission electron microscopy

A. Klick, R. Wagner, M. Großmann, L.F. Kadem, T. Leißner, H.-G. Rubahn, C. Selhuber-Unkel, and M. Bauer, Phys. Rev. B 98, 085128 (2018), [Click for Abstract]

Multimode waveguiding in the visible and near ultraviolet spectral regime is observed and characterized in thermally grown SiO2 layers on silicon using photoemission electron microscopy (PEEM). Comparison with Finite-Element-Method simulations allow identifying order and character of the attenuated modes. Real-time investigations on mode propagation support these findings and give additionally evidence for the existence of radiative modes. Finally, the presented experimental results illustrate how a defined deposition of gold nanoparticles can substantially enhance the sensitivity of the PEEM technique to electromagnetic field modes supported by thin dielectric and insulating layers.

Probing long-range structural order in SnPc/Ag(111) by umklapp process assisted low-energy ARPES

S. Jauernik, P. Hein, M. Gurgel, J. Falke, and M. Bauer, Phys. Rev. B 97, 125413 (2018), [Click for Abstract]

Laser-based angle resolved photoelectron spectroscopy is used to study the valence electronic structure of tin-phthalocyanine (SnPc) adsorbed on silver Ag(111). Upon adsorption of SnPc, strongly dispersing bands are observed which are identified as secondary Mahan cones formed by surface umklapp processes acting onto photoelectrons from the silver substrate as they transit through the ordered adsorbate layer. We show that the photoemission data carry quantitative structural information on the adsorbate layer similar to what can be obtained from a conventional LEED study. More specifically, we compare photoemission data and LEED data p robing an incommensurate to commensurate structural phase transition of the adsorbate layer. Based on our results we propose that Mahan-cone spectroscopy operated in a pump-probe configuration can be used in the future to probe structural dynamics at surfaces with a temporal resolution in the sub-100 fs regime.tem.


Single-mode to multi-mode crossover in thin-load polymethyl-methacrylate plasmonic waveguides

M. Großmann, M. Thomaschewski, A. Klick, A. J. Goszczak, E. K. Sobolewska, T. Leißner, J. Adam, J. Fiutowski, H.-G. Rubahn, and M. Bauer, Plasmonics, 13, 1441 (2018), doi:10.1007/s11468-017-0649-3 [Click for Abstract]

Mode character and mode dispersion of sub-60 nm thick polymethyl-methacrylate dielectric-loaded surface plasmon-polariton waveguides (DLSPPW) are investigated using photoemission electron microscopy and finite-element-method simulations. Experiment and simulation show excellent agreement and allow identifying a crossover from single-mode to multi-mode waveguiding as a function of excitation wavelength λ and DLSSPW cross-section. Experiment and simulations yield, furthermore, indications for the formation of a surface plasmon-polariton cavity mode in the close vicinity of the waveguides.

Electronic structure and ultrafast dynamics of FeAs-based superconductors by angle- and time-resolved photoemission spectroscopy

I. Avigo, S. Thirupathaiah, E. D. L. Rienks, L. Rettig, A. Charnukha, M. Ligges, R. Cortes, J. Nayak, H. S. Jeevan, T. Wolf, Y. Huang, S. Wurmehl, M. I. Sturza, P. Gegenwart, M. S. Golden, L. X. Yang, K. Rossnagel, M. Bauer, B. Büchner, M. Vojta, M. Wolf, C. Felser, J. Fink, U. Bovensiepen, Phys. Status Solidi 254, 1600382 (2017) doi:10.1002/pssb.201600382 [Click for Abstract]

In this article we review our angle- and time-resolved photoemission studies (ARPES and trARPES) on various ferropnictides. In the ARPES studies we focus first on the band structure as a function of control parameters. We find near optimally ”doped” compounds a Lifshitz transition of hole/electron pocket vanishing type. Second we investigated the inelastic scattering rates as a function of the control parameter. In contrast to the heavily discussed quantum critical scenario we find no enhancement of the scattering rate near optimally ”doping”. Correlation effects which show up by the non-Fermi-liquid behavior of the scattering rates, together with the Lifshitz transition offer a new explanation for the strange normal state properties and suggests an interpolating superconducting state between BCS and BE condensation. Adding femtosecond time resolution to ARPES provides complementary information on electron and lattice dynamics. We report on the response of the chemical potential by a collective periodic variation coupled coherent optical phonons in combination with incoherent electron and phonon dynamics described by a three temperature heat bath model. We quantify electron phonon coupling in terms of l<w2> and show that the analysis of the electron excess energy relaxation is a robust approach. The spin density wave ordering leads to a pronounced momentum dependent relaxation dynamics. In the vicinity of kF hot electrons dissipate their energy by electron-phonon coupling with a characteristic time constant of 200 fs. Electrons at the center of the hole pocket exhibit a four time slower relaxation which is explained by spin-dependent dynamics with its smaller relaxation phase space. This finding has implications beyond the material class of Fepnictides because it establishes experimental access to spin-dependent dynamics in materials with spin density waves.


Momentum-resolved hot electron dynamics at the 2H-MoS2 surface

P. Hein, A. Stange, K. Hanff, L.X. Yang, G. Rohde, K. Rossnagel, and M. Bauer, Phys. Rev. B 94, 205406 (2016) Editors Suggestion; doi:10.1103/PhysRevB.94.205406 [Click for Abstract]

Time- and angle-resolved photoelectron spectroscopy (trARPES) is employed to study hot electron dynamics in the conduction band of photo-excited 2H-MoS2. Momentum-dependent rise times of up to 150 fs after near-ultraviolet photo-excitation and decay times in the order of several 100 fs allow us to locate areas of light absorption of the conduction band energy landscape as well as to track the relaxation of hot electrons into the lowest energy states. The conduction band minima are finally depopulated within 1 ps, although a residual population remains up to the maximum investigated pump-probe delay of 15 ps. The presence of the fast depopulation channel differs from the results of experiments of bulk MoS2 performed with all-optical methods. It conforms, however, with recent findings for monolayer MoS2. We attribute this similarity to defect and surface states being of considerable relevance for the near-surface electron dynamics of bulk MoS2 as probed in a trARPES experiment.

Time-resolved ARPES with sub-15 fs temporal and near Fourier-limited spectral resolution

G. Rohde, A. Hendel, A. Stange, K. Hanff, L.-P. Olof, L. X. Yang, K. Rossnagel, and M. Bauer, Rev. Sci. Instr. 87, 103102 (2016)doi:10.1063/1.4963668 [Click for Abstract]

An experimental setup for time- and angle-resolved photoelectron spectroscopy with sub-15 fs temporal resolution is presented. A hollow-fiber compressor is used for the generation of 6.5 fs white light pump pulses, and a high-harmonic generation source delivers 11 fs probe pulses at a photon energy of 22.1 eV. Photoemission transients probing a near-infrared interband transition in 1T-TiSe2 yield a characteristic signal rise time of 8 fs corresponding to 13 fs FWHM of the pump-probe cross-correlation signal. Notably, the energy resolution of the setup conforms to typical values reported in conventional time-resolved photoemission studies using high harmonics, and an ultimate resolution of 170meV is feasible.

Self-amplified photo-induced gap quenching in a correlated electron material

S. Mathias, S. Eich, J. Urbancic, S. Michael, A.V. Carr, A. Stange, T. Popmintchev, T. Rohwer, M. Wiesenmayer, A. Ruffing, S. Jakobs, S. Hellmann, P. Matyba, C. Chen, L. Kipp, M. Bauer, H. C. Kapteyn, H.C. Schneider, K. Rossnagel, M. M. Murnane, and M. Aeschlimann, Nature Communications, 7, 12902 (2016); doi:10.1038/ncomms12902 [Click for Abstract]

Capturing the dynamic electronic band structure of a correlated material presents a powerful capability for uncovering the complex couplings between the electronic and structural degrees of freedom. When combined with ultrafast laser excitation, new phases of matter can result, since far-from-equilibrium excited states are instantaneously populated. Here, we elucidate a general relation between ultrafast non-equilibrium electron dynamics and the size of the characteristic energy gap in a correlated electron material. We show that carrier multiplication via impact ionization can be one of the most important energy relaxation pathways, and that the speed of carrier multiplication critically depends on the size of the energy gap. In the case of the charge-density wave material 1T-TiSe2, we show that carrier multiplication and gap dynamics mutually amplify each other, which explains – for the first time on a microscopic level - the extremely fast response of this material to ultrafast optical excitation.

Enhancement of charge ordering by dynamic electron-phonon coupling

A. Singer, S.K.K Patel, R. Krukej, V. Uhlir, J. Wingert, S. Festersen, D. Zhu, J.M. Glownia, H. Lemke, S. Nelson, M. Kozina, K. Rossnagel, M. Bauer, B.M. Murphy, O.M. Magnussen, E.E. Fullerton, and O.G. Shpyrko; Phys. Rev. Lett. 117, 056401 (2016); doi:10.1103/PhysRevLett.117.056401 [Click for Abstract]

Symmetry breaking and the emergence of order is one of the most fascinating phenomena in condensed matter physics and leads to a plethora of intriguing ground states such as in antiferromagnets, Mott insulators, superconductors, and density-wave systems. Exploiting non-equilibrium dynamics of matter following ultrafast external excitation can provide even more striking routes to symmetry-lowered, ordered states, for instance, by accessing hidden equilibrium states in the free-energy landscape or dynamic stabilization of non-equilibrium states. This is remarkable because ultrafast excitation typically creates disorder, reduces the order parameter, and raises the symmetry. Here, we demonstrate for the case of antiferromagnetic chromium that moderate ultra-fast photo-excitation can transiently enhance the charge-density-wave (CDW) order by up to 30% above its equilibrium value, while strong excitations lead to an oscillating, large-amplitude CDW state that persists above the equilibrium transition temperature. Both effects result from dynamic electron-phonon interactions, providing an efficient mechanism to selectively transform a broad excitation of the electronic order into a well defined, long-lived coherent lattice vibration. This mechanism may be exploited to transiently enhance order parameters in other systems with coupled electronic and lattice orders.

Pump laser-induced space-charge effects in HHG-driven time- and angle-resolved photoelectron spectroscopy

L.-P. Oloff, K. Hanff, A. Stange, G. Rohde, F. Diekmann, M. Bauer, and K. Rossnagel; J. Appl. Phys. 119, 225106 (2016); doi:10.1063/1.4953643 [Click for Abstract]

With the advent of ultrashort-pulsed extreme ultraviolet sources, such as free-electron lasers or high-harmonic-generation (HHG) sources, a new research field for photoelectron spectroscopy has opened up in terms of femtosecond time-resolved pump-probe experiments. The impact of the high peak brilliance of these novel sources on photoemission spectra, so called vacuum space-charge eects arising from the Coulomb interaction of the photoemitted electrons, has been studied extensively. However, possible distortions of the energy and momentum distribution of the photoelectrons arising from the low photon energy pump pulse due to the emission of electrons via multiphoton excitation channels have not been studied in detail yet. Here we systematically investigate these pump laser-induced space-charge effects in a HHG-based experiment for the test case of graphite (HOPG). Specically, we determine how the key parameters of the pump pulses - the excitation density, wavelength, spot size and emitted electron energy distribution - affect the measured time-dependent energy and momentum distributions of the probe photoelectrons. The results are well reproduced by a simple mean-field model, which could open a path for the correction of pump laser-induced space-charge effects and thus towards probing ultrafast electron dynamics in strongly excited materials.

Amplitude and phase of surface plasmon polaritons excited at a step edge

A. Klick, S. de la Cruz, C. Lemke, M. Großmann, H. Beyer, J. Fiutowski, H.-G. Rubahn, E.R. Médes, and M. Bauer; Appl. Phys. B (2016) 122:79, doi:10.1007/s00340-016-6350-y [Click for Abstract]

A combined experimental and theoretical study on the laser-induced excitation of surface plasmon polaritons (SPP) at well-defined step edges of a gold-vacuum interface is presented. As a relevant parameter determining the coupling efficiency between laser field and SPP we identify the ratio between step height h and excitation wavelength λ. For specific values of h / λ an almost complete suppression of SPP excitation is observed arising from a destructive superposition of incident and reflected laser field in the presence of the step edge. Experiment and theory show, furthermore, that next to the amplitude also the phase of the SPP is affected by the superposition.


Hot Electron Cooling in Graphite: Supercollision versus Hot Phonon Decay

A. Stange, C. Sohrt, L.X. Yang, G. Rohde, K. Janssen, P. Hein, L.-P. Oloff, K. Hanff, K. Rossnagel, and M. Bauer; Phys. Rev. B 92, 184303 (2015), doi:10.1103/PhysRevB.92.184303 [Click for Abstract]

Disorder-assisted electron-phonon scattering processes (supercollision processes) have been reported to dominate the cooling of hot carriers in graphene. Here we determine to what extent this type of relaxation mechanism governs the hot carrier dynamics in the parent compound graphite. Electron temperature transients derived from time- and angle-resolved extreme ultraviolet photoemission spectra are analyzed based on a three-temperature model which considers electron gas, optical phonons, and acoustic phonons as coupled subsystems. In the probed uence regime of 0.035 - 1.4 mJ/cm2 we find no indications for supercollision processes being involved in the cooling of the hot carriers. The data are, by contrast, compatible with a hot-phonon assisted mechanism involving anharmonic coupling between optical phonons and acoustic phonons, a process which has previously been suggested for graphite. We attribute the striking difference to the reported findings for graphene to the low defect density of highly ordered pyrolitic graphite.

Light-triggered Control of Plasmonic Refraction and Group Delay by Photochromic Molecular Switches

M. Großmann, A. Klick, C. Lemke, J. Falke, M. Black, J. Fiutowski, A. Goszczak, E. Sobolewska, A. Usman Zillohu, M. Keshavarz Hedayati, H.-G. Rubahn, F. Faupel, M. Elbahri, and M.Bauer; ACS Photonics 2 (2015) 1327 - 1332, doi:10.1021/acsphotonics.5b00315 [Click for Abstract]

An interface supporting plasmonic switching is prepared from a gold substrate coated with a polymer film doped with photochromic molecular switches. A reversible light-induced change in the surface plasmon polariton dispersion curve of the interface is experimentally demonstrated evidencing reversible switching of surface plasmon polariton group and phase velocity. The switching capabilities of the interface are furthermore successfully applied to achieve focus control of a plasmonic lens. The results imply the realization of non-volatile and reversible plasmonic switching units providing complex functionalities based on surface plasmon refraction and group delay.

Hot electron lifetimes in metals probed by time-resolved two-photon photoemission

M. Bauer, A. Marienfeld, and M. Aeschlimann; Prog. Surf. Sci. 90 (2015) 319 - 376, doi:10.1016/j.progsurf.2015.05.001 [Click for Abstract]

This review reports on experimental and theoretical results on the inelastic decay of optically excited volume electrons in different types of metals, including simple metals (Al), noble metals (Au, Ag, Cu), transition metals (Ta, Mo, Rh, Co, Fe, Ni) and rare earth metals (Gd, Tb, Yb, La). The comparison of the different materials and material classes provides particular insight into the relevance of the localization and delocalization of electronic states for inelastic carrier scattering processes. The discussion of the data illustrates furthermore the capabilities and limitations of the time-resolved two-photon photoemission technique as well as current theoretical approaches in analyzing and determining inelastic lifetimes of excited electrons.


Plasmonic black metal polarizers for ultra-short laser pulses

T. Søndergaard, E. Skovsen, C. Lemke, T. Holmgaard, T. Leissner, R. L. Eriksen, J. Beermann, M. Bauer, K. Pedersen, and S. I. Bozhevolnyi; Proc. SPIE 9163, Plasmonics: Metallic Nanostructures and Their Optical Properties XII, 916308 (September 10, 2014)  doi:10.1117/12.2061943 [Click for Abstract]

This paper considers a range of plasmonic-black-metal polarizers suitable for ultra-short pulses. The polarizers consist of a metal surface being nanostructured with a periodic array of ultra-sharp grooves with periods of 250-350 nanometers, and groove depths around 500 nanometers. The surfaces can be designed such that practically all incident light with electric field perpendicular to the groove direction is absorbed. The efficient absorption is due to incident light being coupled into gap-plasmon polaritons that propagate downwards in the gaps between groove walls towards the groove bottom, where it is then subsequently absorbed during propagation. Reflection is largely avoided due to an adiabatic groove taper design. The other polarization, however, is very efficiently reflected, and the main point of this paper is that the reflection is with negligible dispersive stretching even for ultra-short pulses of 5-10 femtoseconds temporal width in the visible and near-infrared. Temporal pulse shapes after reflection are calculated by decomposing the incident laser pulse into its Fourier components, multiplying with the reflection coefficient in the frequency domain, and then Fouriertransforming the product back to the time-domain. Reflection of pulses is compared for polarizers based on different metals (gold, nickel, chromium). Polarizers are studied for two groove-array designs and two directions of light incidence, center wavelengths 650 nm and 800 nm, and pulse widths 5 fs and 10 fs for the incident pulse. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Does the excitation wavelength affect the ultrafast quenching dynamics of the charge-density wave in 1T-TiSe2?

G. Rohde, T. Rohwer, A. Stange, C. Sohrt, K. Hanff, L. X. Yang, L. Kipp, K. Rossnagel, M. Bauer; J. Electr. Spectr. Rel. Phen. 195 (2014) 244-248 doi:10.1016/j.elspec.2014.05.009 [Click for Abstract]

The ultrafast spectral responses of a charge-ordered state in 1T-TiSe2 to the photo-excitation with laser-pulses of 790 nm and 395 nm center wavelength are compared. Time- and angle-resolved photoemission reveals pronounced differences in the energy-momentum distribution of the nascent electron gas; a distinct effect of the distribution character on the destruction dynamics of the charge-ordered state is, however, not observed.

Ultrafast modulation of the chemical potential in BaFe2As2 by coherent phonons

L. X. Yang, G. Rohde, T. Rohwer, A. Stange, K. Hanff, C. Sohrt, L. Rettig, R. Cortes, F. Chen, D. L. Feng, T. Wolf, B. Kamble, I. Eremin, T. Popmintchev, M. M. Murnane, H. C. Kapteyn, L. Kipp, J. Fink, M. Bauer, U. Bovensiepen, K. Rossnagel; Phys. Rev. Lett. 112 (2014) 207001 doi:10.1103/PhysRevLett.112.207001 [Click for Abstract]

Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in BaFe2As2 around the high-symmetry points Γ and M. A global oscillation of the Fermi level at the frequency of the A1g(As) phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the A1g phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling.

Time- and angle-resolved photoemission spectroscopy with optimized high-harmonic pulses using frequency-doubled Ti:Sapphire lasers

S. Eich, A. Stange, A. V. Carr, J. Urbancic, T. Popmintechev, M. Wiesenmayer, K. Jansen, A. Ruffing, S. Jakobs, T. Rohwer, S. Hellmann, C. Chen, P. Matyba, L. Kipp, K. Rossnagel, M. Bauer, M. M. Murnane, H. C. Kapteyn, S. Mathias, M. Aeschlimann, J. Electr. Spectr. Rel. Phen 195 (2014) 231–236 doi:10.1016/j.elspec.2014.04.013 [Click for Abstract]

Time- and angle-resolved photoemission spectroscopy (trARPES) using femtosecond extreme ultraviolet high harmonics has recently emerged as a powerful tool for investigating ultrafast quasiparticle dynamics in correlated-electron materials. However, the full potential of this approach has not yet been achieved because, to date, high harmonics generated by 800 nm wavelength Ti:Sapphire lasers required a trade-off between photon flux, energy and time resolution. Photoemission spectroscopy requires a quasi-monochromatic output, but dispersive optical elements that select a single harmonic can significantly reduce the photon flux and time resolution. Here we show that 400 nm driven high harmonic extreme-ultraviolet trARPES is superior to using 800 nm laser drivers since it eliminates the need for any spectral selection, thereby increasing photon flux and energy resolution to <150 meV while preserving excellent time resolution of about 30 fs.

How fast can a Peierls-Mott insulator be melted?

C. Sohrt, A. Stange, M. Bauer, and K. Rossnagel; Faraday discussions, 2014, Advance Article doi:10.1039/c4fd00042k [Click for Abstract]

Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to directly determine the momentum-dependent electronic structure dynamics in the layered Peierls-Mott insulators 1T-TaS2 and 1T-TaSe2 on the sub-300-fs time scale. Extracted spectroscopic order parameters display a global two-time-scale dynamics indicating a quasi-instantaneous loss of the electronic orders and a subsequent coherent suppression of the lattice distortion on a time scale related to the frequency of the charge-density-wave amplitude mode. After one half-cycle of coherent amplitude-mode vibration, a crossover state between insulator and metal with partially _lled-in and partially closed Mott and Peierls gaps is reached. The results are discussed within the wider context of electronic order quenching in complex materials.

The Interplay between Localized and Propagating Plasmonic Excitations tracked in Space and Time

C. lemke, T. Leißner, A. Evlyukhin, J. W. Radke, A. Klick, J. Fiutowski, J. Kjelstrup-Hansen, H. -G. Rubahn, B. Chichkov, C. Reinhardt, M. Bauer; Nano Letters 14 (2014) 2431 doi:10.1021/nl500106z [Click for Abstract]

The mutual coupling and coherent interplay of propagating and localized surface plasmons is demonstrated and imaged with high spatial resolution and on a sub-femtosecond time scale. The analysis of the data provide direct information on aspects such as signal transport, plasmonic coupling, the relevance of loss channels, and the coherent control of optical near fields via interference of plasmonic excitations.

The complex dispersion relation of surface plasmon polaritons at gold/para-Hexaphenylene interfaces

C. Lemke, T. Leißner, A. Klick, J. Fiutowski, J. W. Radke, J. Kjelstrup-Hansen, H. -G. Rubahn, M. Bauer; Appl. Phys. B 116 (2014) 585 doi:10.1007/s00340-013-5737-2 [Click for Abstract]

Two-Photon Photoemission Electron Microscopy is used to measure the real and imaginary parts of the dispersion relation of surface plasmon polaritons at different interface systems. The comparison between calculations and experiments at a gold/vacuum interface demonstrates the ability of the presented approach. A systematic study of the dispersion relation of dielectric-loaded gold surfaces shows how effective the propagation of surface plasmon polaritons can be tuned by adjustment of the dielectric film thickness. Deviations of the experimental results from effective index calculations indicate the relevance of thin film peculiarities arising from the details of the growth process.


Spatio-temporal characterization of SPP pulse propagation in two-dimensional plasmonic focusing devices

C. Lemke, C. Schneider, T. Leißner, D. Bayer, J. W. Radke, A. Fischer, P. Melchior, A. B. Evlyukhin, B. N. Chichkov, C. Reinhardt, M. Bauer, M. Aeschlimann; Nano Letters 13 (2013) 1053 doi:10.1021/nl3042849 [Click for Abstract]

The spatio-temporal evolution of a SPP wave packet with femtosecond duration is experimentally investigated in two different plasmonic focusing structures. A two-dimensional reconstruction of the plasmonic field in space and time is possible by the numerical analysis of interferometric time-resolved photoemission electron microscopy data. We show that the time-integrated and time-resolved view onto the wave packet dynamics allow one characterizing and comparing the capabilities of two-dimensional components for use in plasmonic devices operation with ultrafast pulses.

SPP propagation in organic nanofiber based plasmonic waveguides

T. Leißner, C. Lemke, S. Jauernik, M. Müller, F. Fiutowski, L. Tavares, J. Kjelstrup-Hansen, O. Magnussen, H. -G. Rubahn, M. Bauer; Optics Express 21 (2013) 8251  doi:10.1364/OE.21.008251 [Click for Abstract]

Plasmonic wave packet propagation is monitored in dielectric-loaded surface plasmon polariton waveguides (DLSPPW) realized from para-hexaphenylene (p-6P) nanofibers deposited onto a 60 nm thick gold film. Using interferometric time resolved two-photon photoemission electron microscopy we are able to determine phase and group velocity of the SPP wave guiding mode as well as the effective propagation length along the fiber-gold interface. We furthermore observe that the propagation properties of the SPP waveguiding mode are governed by the cross section of the waveguide.

Morphological tuning of the plasmon dispersion in dielectric-loaded nanofiber waveguides

T. Leißner, C. Lemke, J. Fiutowski, J. W. Radke, A. Klick, L. Tavares, J. Kjelstrup-Hansen, H. -G. Rubahn, M. Bauer; Phys. Rev. Lett. 111 (2013) 46802 doi:10.1103/PhysRevLett.111.046802 [Click for Abstract]

Understanding the impact of lateral mode confinement in plasmonic waveguides is of fundamental interest regarding potential applications in plasmonic devices. The knowledge of the frequency-wave vector dispersion relation provides the full information on electro-magnetic field propagation in a waveguide. In this paper two-photon photoemission electron microscopy (2P-PEEM) is used to measure the real-part of the surface plasmon polariton (SPP) dispersion relation in the near infrared spectral regime for individual nanoscale plasmonic waveguides, which were formed by deposition of para-Hexaphenylene (p-6P) based nanofibers on top of a gold film. Characterization by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM) provides accurate information on the dimensions of the investigated waveguides and enables us to quantify the effect of mode confinement by comparison with experimental results from continuous p-6P films on a gold substrate and calculations based on the effective index method (EIM). Our results show that the structural control of the cross-sectional dimension of a p-6p nanofiber provides a promising means for the customized design of plasmonic waveguides.

Tracking the relaxation pathway of photo-excited electrons in 1T-TiSe2

G. Rohde, T. Rohwer, C. Sohrt, A. Stange, S. Hellmann, L. Yang, K. Hanff, A. Carr, M. M. Murnane, H. Kapteyn, L. Kipp, K. Rossnagel, M. Bauer; European Physical Journal Special Topics 222 (2013) 997 doi:10.1140/epjst/e2013-01901-5 [Click for Abstract]

The ultrafast dynamics of excited electrons in 1T-TiSe2 after absorption of a 390 nm light pulse is probed by time- and angle-resolved photoemission spectroscopy using femtosecond XUV pulses. It is demonstrated that the experimental approach can provide a comprehensive view of hot carrier motion in momentum space during relaxation back to equilibrium. This capability opens a new avenue in the investigation of energy dissipation processes in solids after intense optical excitation.

Measurement of surface plasmon autocorrelation functions

C. Lemke, T. Leißner, A. Klick, J. W. Radke, J. Fiutowski, J. Kjelstrup-Hansen, H. -G. Rubahn, M. Bauer; Optics Express 21 27392 (2013) doi:10.1364/OE.21.027392 [Click for Abstract]

In this paper we demonstrate the realization of an autocorrelator for the characterization of ultrashort surface plasmon polariton (SPP) pulses. A wedge shaped structure is used to continuously increase the time delay between two interfering SPPs. The autocorrelation signal is monitored by non-linear two-photon photoemission electron microscopy. The presented approach is applicable to other SPP sensitive detection schemes that provide only moderate spatial resolution and may therefore be of general interest in the field of ultrafast plasmonics.

Plasmonic black gold based broadband polarizers for ultra-short laser pulses

E. Skovsen, T. Sondergaard, C. Lemke, T. Holmgaard, T. Leissner, R. Eriksen, J. Beermann, M. Bauer, K. Pedersen, S. Bozhevolnyi; Appl. Phys. Lett. 103 (2013) 211102 doi:10.1063/1.4830040

Time-domain evidence for an excitonic insulator

S. Hellmann, T. Rohwer, M. Kalläne, K. Hanff, A. Carr, M. M. Murnane, H. C. Kapteyn, L. Kipp, M. Bauer, K. Rossnagel; EPJ Web of Conferences 41, 03022 (2013) doi:10.1051/epjconf/20134103022 [Click for Abstract]

Time- and angle-resolved photoemission spectroscopy using a high-harmonic-generation source is employed to classify the potntial excitonic insulator 1T-TiSe2 and te reference Peierls-Mott insulator 1T-TaS2 on the basis of their melting times.

A direct view onto the carrier dynamics in graphite at the H point

A. Stange, C. Sohrt, T. Rohwer, S. Hellmann, G. Rohde, L. Kipp, K. Rossnagel, M. Bauer; EPJ Web of Conferences 41, 04022 (2013) doi:10.1051/epjconf/20134104022 [Click for Abstract]

Time-resolved XUV photoemission spectroscopy is employed to monitor the dynamics of photo-excited carriers in graphite at the boundary of the Brillouin zone. The experiment provides direct access to the momentum region relevant for optical excitation and relaxation.


Time-domain classification of charge-density-wave insulators

S. Hellmann, T. Rohwer, M. Kalläne, K. Hanff, C. Sohrt, A. Stange, A. Carr, M. M. Murnane, H. C. Kapteyn, L. Kipp, M. Bauer, K. Rossnagel; Nat. Commun. 3 (2012) 1069 doi:10.1038/ncomms2078 [Click for Abstract]

Distinguishing insulators by the dominant type of interaction is a central problem in condensed matter physics. Basic models include the Bloch-Wilson and the Peierls insulator due to electron – lattice interactions, the Mott and the excitonic insulator caused by electron – electron interactions, and the Anderson insulator arising from electron – impurity interactions. In real materials, however, all the interactions are simultaneously present so that classifi cation is often not straightforward. Here, we show that time- and angle-resolved photoemission spectroscopy can directly measure the melting times of electronic order parameters and thus identify — via systematic temporal discrimination of elementary electronic and structural processes — the dominant interaction. Specifi cally, we resolve the debates about the nature of two peculiar charge-density-wave states in the family of transition-metal dichalcogenides, and show that Rb intercalated 1 T -TaS 2 is a Peierls insulator and that the ultrafast response of 1 T -TiSe 2 is highly suggestive of an excitonic insulator.

Optimal open-loop near-field control of plasmonic nanostructures

M. Aeschlimann, M. Bauer, D. Bayer, T. Brixner, S. Cunovic, A. Fischer, P. Melchior, W. Pfeiffer, M. Rohmer, C. Schneider, C. Strüber, P. Tuchscherer, D. V. Voronine; New J. Phys. 14, (2012) 033030 doi:10.1088/1367-2630/14/3/033030 [Click for Abstract]

Optimal open-loop control, i.e., the application of an analytically derived control rule, is demonstrated for nanooptical excitations using polarization-shaped laser pulses. Optimal spatial near-field localization in gold nanoprisms and excitation switching is realized by applying a π shift to the relative phase of the two polarization components. The achieved near-field switching confirms theoretical predictions, proves the applicability of predefined control rules in nanooptical light-matter interaction, and reveals local mode interference as an important control mechanism.

Application of a grating coupler for surface plasmon polariton excitation in a photoemission electron microscopy experiment

T. Leissner, S. Jauernik, C. Lemke, J. Fiutowski, J. Fiutowski, J. Kjelstrup-Hansen, H. -G. Rubahn, M. Bauer; Proc. SPIE 8424, 84241D doi:10.1117/12.921761 [Click for Abstract]

Surface plasmon polariton (SPP) excitation at a gold-vacuum interface via 800 nm light pulses mediated by a periodic array of gold ridges is probed at high lateral resolution by means of photoemission electron microscopy (PEEM). We directly monitor and quantify the coupling properties as a function of the number of grating ridges and compare the PEEM results with analytic calculations. An increase in the coupling efficiency of ≈ 3 is observed when increasing the number of ridges from 1 to 6. We observe, however, that a further addition of ridges is rather ineffective. This saturation behavior is assigned to the grazing incidence excitation geometry intrinsic to a conventional PEEM scheme and the limited propagation distance of the SPP modes at the gold-vacuum interface at the used wavelength.

Mapping surface plasmon polariton propagation via counter-propagating light pulses

Ch. Lemke, T. Leißner, S. Jauernik, A. Klick, J. Fiutowski, J. Kjelstrup-Hansen, H. -G. Rubahn, M. Bauer; Optics Express 20 (2012) 12877 doi:10.1364/OE.20.012877 [Click for Abstract]

In an interferometric time-resolved photoemission electron microscopy (ITR-PEEM) experiment, the near-field associated with surface plasmon polaritons (SPP) can be locally sensed via interference with ultrashort laser pulses. Here, we present ITR-PEEM data of SPP propagation at a gold vacuum interface recorded in a counter-propagating pump-probe geometry. In comparison to former work this approach provides a very intuitive real-time access to the SPP wave packet. The quantitative analysis of the PEEM data enables us to determine in a rather direct manner the propagation characteristics of the SPP.

Time-resolved X-ray photoelectron spectroscopy at FLASH

S. Hellmann, C. Sohrt, M. Beye, T. Rohwer, F. Sorgenfrei, M. Marczynski-Bühlow, M. Kalläne, H. Redlin, F. Hennies, M. Bauer, A. Föhlisch, L. Kipp W. Wurth, K. Rossnagel; New Journal of Physics, New J. Phys. 14 (2012) 013062 doi:10.1088/1367-2630/14/1/013062 [Click for Abstract]

The technique of time-resolved pump-probe X-ray photoelectron spectroscopy using the free-electron laser in Hamburg, FLASH, is described in detail. Particular foci lie on the macrobunch resolving detection scheme, the role of vacuum space-charge effects, and the synchronization of pump and probe lasers. In an exemplary case study, the complete Ta 4f core-level dynamics in the layered charge-density-wave compound 1T–TaS2 in response to impulsive optical excitation is measured on the sub-picosecond to nanosecond timescale. The observed multicomponent dynamics is related to the intrinsic melting and reformation of the chargedensity wave as well as to extrinsic pump-laser induced vacuum space-charge effects.

Reversible Switching in Self-assembled Monolayers of Azobenzene Thiolates on Au(111) Probed by Threshold Photoemission

N. Heinemann, J. Grunau, T. Leißner, O. Andreyev, S. Kuhn, U. Jung, D. Zargarani, R. Herges, O. Magnussen, M. Bauer; Chemical Physics 402 (2012) 22 doi:10.1016/j.chemphys.2012.03.025 [Click for Abstract]

The reversible photo- and thermally activated isomerization of the molecular switch 3-(4-(4-Hexyl-phenylazo)-phenoxy)-propane-1-thiol (ABT)1 deposited by liquid phase self-assembly on Au (111) (ABT/Au) was studied using laser-based photoelectron spectroscopy. Differences in the molecular dipole moment characteristic for the trans and cis isomer of ABT are monitored via changes in the sample work function accessible by detection of the threshold energy for photoemission. A quantitative analysis of our data shows that the fraction of molecules within the densely packed monolayer that undergoes a switching process is of the order of 1 %. This result indicates the relevance of substrate and/or film defects required to overcome the steric and/or electronic hindrance of the isomerisation reaction in a densely packed monolayer.

Probing the switching state of a surface-mounted azobenzene derivative using femtosecond XUV photoemission

J. Grunau, N. Heinemann, T. Rohwer, D. Zargarani, S. Kuhn, U. Jung, L. Kipp, O. Magnussen, R. Herges, M. Bauer; Proc. SPIE 8260, 82600Y (2012) doi:10.1117/12.906886 [Click for Abstract]

Photoemission spectroscopy using femtosecond XUV light pulses is applied to probe the isomerization state of the molecular switch 3-(4-(4-hexyl-phenylazo)-phenoxy)-propane-1-thiol deposited by liquid phase self-assembly on Au(111). Spectral shifts of valence-electronic signatures that we associate with the carbon C2s orbital enable us to distinguish the trans and the cis isomerization state of the adsorbed molecules. These preliminary results envision the potential to probe reversible switching processes of surface-mounted molecules in real time by tracking the temporal evolution of the electronic and nuclear degrees of freedom in a femtosecond XUV photoemission experiment.


Collapse of long-range charge order tracked by time-resolved photoemission at high momenta

T. Rohwer, S. Hellmann, M. Wiesenmayer, C. Sohrt, A. Stange, B. Slomski, A. Carr, Y. Liu, L. Miaja Avila, M. Kalläne, S. Mathias, L. Kipp, K. Rossnagel, M. Bauer; Nature 471, (2011) 490 doi:10.1038/nature09829 [Click for Abstract]

In condensed matter systems, intense femtosecond light pulses can transform electronic, magnetic and structural order on the fundamental timescales of electronic and atomic motion. This phenomenon is particularly attractive in the study and in the control of materials whose physical properties are governed by the interactions between multiple degrees of freedom. Time- and angle-resolved photoemission spectroscopy is in this context the potentially most direct and comprehensive, energy- and momentum-selective probe of the ultrafast processes that couple to the electronic degrees of freedom. Previously, the capability of such studies to access electron momentum space further away from zero momentum was, however, restricted due to limitations of the available probing photon energy. Here, employing femtosecond extreme ultraviolet pulses delivered by a high-harmonic-generation source, we use time- and angle-resolved photoemission spectroscopy to measure the photo-induced vaporisation of a charge-ordered state in the potential excitonic insulator 1T-TiSe2. Via stroboscopic imaging of electronic band dispersions at large momentum, in the vicinity of the edge of the first Brillouin zone, we reveal that the collapse of atomic-scale periodic long-range order happens on a timescale as short as 20 femtoseconds. The surprisingly fast response of the system is assigned to screening by the transient generation of free charge carriers. Similar screening scenarios will likely be relevant in other photo-induced solid-state transitions and may generally determine the response times. Moreover, since electron states with large momenta govern fundamental electronic properties in condensed matter systems, we anticipate that the experimental advance represented by the present study will allow novel insights into the ultrafast dynamics and microscopic mechanisms of electronic phenomena in a wide range of materials.

CDW-superlattice suppression probed in time-resolved XUV-photoemission at the border of the Brillouin zone

T. Rohwer, S. Hellmann, M. Wiesenmayer, C. Sohrt, A. Stange, B. Slomski, M. Kalläne, S. Mathias, L. Kipp, K. Rossnagel, M. Bauer; In "Ultrafast Phenomena XVII", Oxford University Press, Inc. (2011) 161 doi:10.1364/UP.2010.MG2

Deterministic Control in Subwavelength Field Localization in Plasmonic Nanoantennas

M. Aeschlimann, M. Bauer, D. Bayer, T. Brixner, S. Cunovic, A. Fischer, P. Melchior, W. Pfeiffer, M. Rohmer, C. Schneider, C. Strüber, P. Tuchscherer, D. V. Voronine; In "Ultrafast Phenomena XVII", Oxford University Press, Inc. (2011) 667 doi:10.1364/UP.2010.WA2

Two-Photon Photoemission from ex-situ prepared Butanethiol SAMs on Au(111)

N. Heinemann, T. Leissner, J. Grunau, T. Rohwer, O. Andreev, M. Bauer; Chemical Physics 382 (2011) 1 doi:10.1016/j.chemphys.2011.03.002 [Click for Abstract]

Self-assembled monolayers (SAMs) of butanethiol on a Au(111) single crystalline surface in the p× √3 lying-down phase prepared by deposition from solution were studied with two-photon photoemission (2PPE) spectroscopy. The spectra reveal clear signatures of two unoccupied resonance states at energies E-EF = 3.7 eV and 3.9 eV. The low-energy state is assigned to the characteristic σ*-resonance associated with the Au-S bond of the thiolate. The energy of the other resonance state agrees well with an interface state reported before for different alkanethiol SAMs on Au(111) in a standing-up phase. The 2PPE data provide furthermore indications that the high quality of the ex-situ prepared SAMs support the formation of image potential states.

Surface plasmon polariton emission prompted by organic nanofibers on thin gold films

T. Leissner, K. Thilsing-Hansen, C. Lemke, S. Jauernik, J. Kjelstrup-Hansen, M. Bauer, H. -G. Rubahn; Plasmonics 7 (2012) 253 doi:10.1007/s11468-011-9301-9 [Click for Abstract]

The excitation of surface plasmon polaritons (SPP) at a gold-vacuum interface by femtosecond light-pulses mediated by organic nanofiber-induced dielectric perturbations is observed using interferometric time-resolved photoemission electron microscopy (ITR-PEEM). The experimental data are quantitatively reproduced by analytic simulations, where the nanofibers are considered as superior source of the SPP emission. The flexibility and tuneability of phenylene-based nanofibers in their morphology and intrinsic optical properties open up future applications to fabricate custom-designed nanoscale sources of SPP’s.


Spatiotemporal control of nanooptical excitations

M. Aeschlimann, M. Bauer, D. Bayer, T. Brixner, S. Cunovic, F. Dimler, A. Fischer, W. Pfeiffer, M. Rohmer, C. Schneider, F. Steeb, C. Strüber, D. V. Voronine; Proc. Natl. Acad. Sci., (2010) doi:10.1073/pnas.0913556107 [Click for Abstract]

The most general investigation and exploitation of light-induced processes require simultaneous control over spatial and temporal properties of the electromagnetic field on a femtosecond time and nanometer length scale. Based on the combination of polarization pulse shaping and time-resolved two-photon photoemission electron microscopy, we demonstrate such control over nanoscale spatial and ultrafast temporal degrees of freedom of an electromagnetic excitation in the vicinity of a nanostructure. The time-resolved cross-correlation measurement of the local photoemission yield reveals the switching of the nanolocalized optical near-field distribution with a lateral resolution well below the diffraction limit and a temporal resolution on the femtosecond time scale. In addition, successful adaptive spatiotemporal control demonstrates the flexibility of the method. This flexible simultaneous control of temporal and spatial properties of nanophotonic excitations opens new possibilities to tailor and optimize the light– matter interaction in spectroscopic methods as well as in nanophotonic applications.

Quantum-Well-Induced Giant Spin-Orbit Splitting

S. Mathias, A. Ruffing, F. Deicke, M. Wiesenmayer, I. Sakar, G. Bihlmayer, E. . Chulkov, Yu. M. Koroteev, P. M. Echenique, M. Bauer, M. Aeschlimann; Phys. Rev. Lett. 104 (2010) 066802 doi:10.1103/PhysRevLett.104.066802 [Click for Abstract]

We report on the observation of a giant spin-orbit splitting of quantum-well states in the unoccupied electronic structure of a Bi monolayer on Cu(111). Up to now, Rashba-type splittings of this size have been reported exclusively for surface states in a partial band gap. With these quantum-well states we have experimentally identified a second class of states that show a huge spin-orbit splitting. First-principles electronic structure calculations show that the origin of the spin-orbit splitting is due to the perpendicular potential at the surface and interface of the ultrathin Bi film. This finding allows for the direct possibility to tailor spin-orbit splitting by means of thin-film nanofabrication.

Ultrafast melting of a charge-density wave probed on the atomic scale

S. Hellmann, M. Beye, C. Sohrt, T. Rohwer, F. Sorgenfrei, H. Redlin, M. Kalläne, M. Marczynski-Bühlow, M. Bauer, A. Föhlisch, L. Kipp, W. Wurth , K. Rossnagel; Phys. Rev. Lett. 105 (2010) 187401 doi:10.1103/PhysRevLett.105.187401 [Click for Abstract]

Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics in the charge-density-wave/Mott insulator 1T-TaS2. After strong photoexcitation, a prompt loss of charge order and subsequent fast equilibration dynamics of the electron-lattice system are observed. On the time scale of electron-phonon thermalization, about 1 ps, the system is driven across a phase transition from a long-range charge ordered state to a quasi-equilibrium state with domain-like short-range charge order. The experiment opens the way to study the nonequilibrium dynamics of condensed matter systems with full elemental, chemical, and atomic site selectivity.

Spectroscopy and population decay of a Ti excess state in layered TiSe2

M. Wiesenmayer, S. Hilgenfeldt, S. Mathias, F. Steeb, T. Rohwer, M. Bauer; Phys. Rev. B 82 (2010) 035422 doi:10.1103/PhysRevB.82.035422 [Click for Abstract]

Two-photon photoelemission (2PPE) spectroscopy is used to map the momentum dependent energy distribution of electronic excitations in the layered 1T-TiSe2 transition metal dichalcogenide compound. A comparison of the experimental results with previous calculations based on the local density functional approach enables us to identify the second Ti 3d conduction band and a localized excitation state arising from the presence of excess titanium atoms in the van der Waals gap of the crystal. Time-resolved 2PPE measurements show clear differences in the lifetime between the two states, indicative for the decoupling of the Ti excess atoms from the bulk electronic structure.

Probing adsorbate dynamics with chirped laser pulses in a single pulse scheme

F. Steeb, S. Mathias, M. Wiesenmayer, A. Fischer, M. Aeschlimann, M. Bauer, J. P. Gauyacq; Phys. Rev. B 82 (2010) 165430 doi:10.1103/PhysRevB.82.165430 [Click for Abstract]

Femtosecond dynamics of the model-like adsorption system Cs/Cu(111) is probed by two-photon photoelectron spectroscopy (2PPE) using phase-modulated (chirped) laser pulses. The experimental data are quantitatively modeled within a wavepacket propagation approach under explicit consideration of the adsorbate motion. The results enable us to assign characteristic chirped-pulse 2PPE features to the ultrafast adsorbate dynamics associated with the excited state lifetime and the adsorbate motion and to improve on the qualitative interpretation of experimental data as published in reference [D D]. Our results show that non-linear photoemission with a chirped pulse in a single-pulse scheme can complement real-time studies based on pump-probe schemes to gain quantitative insights into the femtosecond dynamics of ultrafast surface processes.

Band structure dependence of hot-electron lifetimes in a Pb/Cu(111) quantum-well system

S. Mathias, A. Ruffing, F. Deicke, M. Wiesenmayer, M. Aeschlimann, M. Bauer; Phys. Rev. B 81, 15542 (2010) doi:10.1103/PhysRevB.81.155429 [Click for Abstract]

The band-structure dependence of the inelastic lifetime of electrons is investigated in a Pb quantum-well system on a Cu(111) substrate with femtosecond time- and angle-resolved two-photon photoemission. For a single monolayer of Pb on Cu(111), we find an unoccupied quantum-well state with a free-electronlike parabolic dispersion around the Γ-point, and with negative dispersion for finite momentum. Our investigation of this state is the first momentum-resolved study of lifetimes in a quantum-well system. We demonstrate the importance of intrasubband-scattering processes for the decay of hot electrons at finite momentum. Furthermore, we find that the competition between intersubband- and intrasubband-scattering processes directly induces a momentum anisotropy in the hot-electron lifetimes. This momentum anisotropy is strongly dependent on the specific electronic band dispersion. We compare our findings with previous investigations of ultrafast electron dynamics in model-like surface-state systems and with ultrafast electron dynamics in Pb full-bulk material. Our findings suggest that the peculiar electronic structure of quantum-well systems can be used to tune ultrafast dynamical properties in metals.

Time-resolved photoelectron nano-spectroscopy of individual silver particles: perspectives and limitations

M. Rohmer, M. Bauer, T. Leissner, Ch. Schneider, A. Fischer, G. Niedner-Schatteburg, B. v. Issendorff, M. Aeschlimann; Phys. Stat. Sol. (b) 247 (2010) 1133 doi:10.1002/pssb.200945479 [Click for Abstract]

Simultaneous time- and energy-resolved two-photon photoemission with nanometer resolution is demonstrated for the first time. We monitor the energy dependence of the decay dynamics of electron excitations in individual silver particles, which were deposited from a gas aggregation cluster source onto a silicon substrate. We show furthermore that the near-field enhancement due to plasmon- resonant excitation is an efficient means to address individual nanometer sized particles using Photoemission Electron Microscopy.

Two-Photon Photoemission of Plasmonic Nanostructures with High Temporal and Lateral Resolution

M. Bauer, D. Bayer, C. Wiemann, M. Aeschlimann; in 'Nonlinear Dynamics of Nanosystems', Eds.: Radons, Rumpf, Schuster, Whiley-VCH, Weinheim (2010) doi:10.1002/9783527629374.ch14 [Click for Abstract]

In this contribution we illustrate the potential of the time-resolved two-photon photoemission technique (TR-2PPE) to study static and dynamic properties of localized surface plasmons (LSP) in silver nanoparticles. In a first example, we show that conventional (laterally integrating) TR-2PPE is a highly sensitive tool for the detection of LSP resonances in metallic nanoparticles. This technique enables us to address the LSP induced enhancement of the local electric field as well as the decay dynamics of the LSP excitation. Studies at a lateral resolution well below the optical diffraction limit were performed with a detection scheme combining the time-resolved two-photon photoemission and the photoemission electron microscopy (PEEM). Operated at a typical resolution in the sub-30 nm regime, this technique enables systematic studies of the local dynamics associated with the excitation of single nanoparticles. Experimental examples include the real-time observation of the retardation of the LSP mode across a single nanoparticles and the coupling between neighbouring nanoparticles induced by the dipolar field associated with the LSP excitation.

Time-resolved photoelectron spectroscopy at surfaces using femtosecond XUV-pulses

S. Mathias, M. Bauer, M. Aeschlimann, L. Miaja-Avila, H. C. Kapteyn, M. M. Murnane; in "Dynamics at Solid State Surfaces and Interfaces", eds: U. Bovensiepen, H. Petek, M. Wolf, Whiley-VCH, Berlin (2010) doi:10.1002/9783527633418.ch21 [Click for Abstract]

Ultrashort XUV pulses from laser-based high-harmonic sources and free-electron lasers are an exceptional and promising tool for monitoring ultrafast dynamical processes at surfaces. In this contribution, we review recent progress in time-resolved photoelectron spectroscopy studies of surfaces that employ these light sources, with particular emphasis on technical aspects, pioneering experiments, and future perspectives.


Quantum oscillations in coupled two-dimensional electron systems

S. Mathias, S. V. Eremeev, E. V. Chulkov, M. Aeschlimann, M. Bauer; Phys. Rev. Lett. 103 (2009) 026802 doi:10.1103/PhysRevLett.103.026802 [Click for Abstract]

Quantum oscillations of the electron phonon coupling of a Shockley surface state induced by the coupling to a metallic quantum well are observed experimentally as the thickness of the quantum well is increased. Microscopic calculations allow us to assign these oscillations to changes in the Eliashberg function, which are caused by a modulation in the wavefunction overlap between the 2D surface state band and the 2D electron system of the quantum well. The findings are important in the context of the control of surface processes such as film growth and surface chemical reactions.

The nature of a nonlinear excitation pathway from the Shockley surface state as probed by chirped pulse two photon photoemission

F. Steeb, S. Mathias, A. Fischer, M. Wiesenmayer, M. Aeschlimann, M. Bauer; New Journal of Physics 11 (2009) 013016 doi:10.1088/1367-2630/11/1/013016 [Click for Abstract]

Phase-modulated femtosecond laser pulses are used to study the spectral response of a non-resonant two photon excitation from the Cu(111) Shockley surface state. Controlled variations in the spectral phase of the laser pulse were introduced using a tuneable Fork prism phase modulator and resulted in a shift in the peak-position (of up to 110 meV), variations in the spectral width (up to 88 meV) and changes in the asymmetry of the surface state peak as detected by two-photon photoemission. A satisfactory quantitative model of the experimental results can only be achieved if the complete spectral phase up to the third order dispersion terms is taken into account. Of particular note, we find that a consistent description of this two photon absorption process does not require coupling of the excitation to an intermediate copper bulk state, which contradicts the previous results of Petek and coworkers [1]. [1] Petek H, Heberle A P, Nessler W, Nagano H, Kubota S, Matsunami S, Moriya N and Ogawa S 1997 Phys. Rev. Lett. 79 4649

Determination of spin injection and transport in a ferromagnet/organic semiconductor heterojunction by two-photon photoemission

M. Cinchetti, K. Heimer, J. -P. Wüstenberg, O. Andreyev, M. Bauer, S. Lach, C. Ziegler, Y. Gao, M. Aeschlimann; Nature Materials, 8 (2009) 115 doi:10.1038/nmat2334 [Click for Abstract]

A fundamental prerequisite for the implementation of organic semiconductors (OSCs) in spintronics devices is the still missing basic knowledge about spin injection and transport in OSCs. Here, we consider a model system consisting of a high-quality interface between the ferromagnet cobalt and the OSC copper phthalocyanine (CuPc). We focus on interfacial effects on spin injection and on the spin transport properties of CuPc. Using spin-resolved two-photon photoemission, we have measured directly and in situ the efficiency of spin injection at the cobalt–CuPc interface. We report a spin injection efficiency of 85–90% for injection into unoccupied molecular orbitals of CuPc. Moreover, we estimate an electron inelastic mean free path in CuPc in the range of 1 nm and a 10–30 times higher quasi-elastic spin-flip length. We demonstrate that quasi-elastic spin-flip processes with energy loss less than or equal to200 meV are the dominant microscopic mechanism limiting the spin diffusion length in CuPc.

Time and angle resolved photoemission spectroscopy using femtosecond visible and high-harmonic light

S. Mathias, M. Wiesenmayer, F. Deicke, A. Ruffing, L. Miaja-Avila, M. M. Murnane, H. C. Kapteyn, M. Bauer, M. Aeschlimann; J. Phys.: Conference series 148 (2009) 012042 doi:10.1088/1742-6596/148/1/012042 [Click for Abstract]

The angle resolved photoelectron spectroscopy (ARPES) has emerged as a leading technique in identifying static key properties of complex systems such as the electronic band structure of adsorbed molecules, ultrathin quantum-well films or high temperature superconductors. We efficiently combined ARPES by using a two-dimensional analyzer for parallel energy (E) and momentum (k||) detection with femtosecond time-resolved spectroscopies. Using time and angle resolved two photon photoemission (2PPE) with visible light pulses, the hot electron dynamics in complex electronic structures are directly accessible by means of angle resolved hot electron lifetime mapping. Furthermore, femtosecond ARPES spectra recorded with high harmonic generation (HHG) light pulses are presented, showing the potential of this technique for future investigations of surface dynamics and photo-induced phase transition processes.

Time and space resolved studies on metallic nanoparticles

D. Bayer, J. Lange, C. Wiemann, M. Rohmer, M. Bauer, M. Aeschlimann; In "Physics and Engineering of New Materials", p. 61, Eds. D.T. Cat, A. Pucci and K. Wandelt, Springer Proceedings in Physics 127 (2009) doi:10.1007/978-3-540-88201-5_7 [Click for Abstract]

The dynamics of laser-excited electronic excitations (localized surface plasmons) in spherical Ag nanoparticles is studied by phase and time resolved two photon photoemission (TR-2PPE) and photoelectron emission microscopy (TR-PEEM). A two-dimensional array of nearly identical, parallelly oriented particles is deposited lithographically on a transparent ITO covered glass substrate. We are able to show that the parallel acquisition mode of the PEEM enables us to resolve local variations in the ultrafast electron dynamics in the nanoparticles with an accuracy of 1fs and a lateral resolution in the nanometer regime. A qualitative interpretation of the mapped inhomogeneities in the local electron dynamics is provided.

Simultaneous Spatial and Temporal Control of Nanooptical Fields

M. Aeschlimann, M. Bauer, D. Bayer, T. Brixner, S. Cunovic, F. Dimler, A. Fischer, W. Pfeiffer, M. Rohmer, C. Schneider, F. Steeb, C. Strüber, D. V. Voronine; In “Ultrafast Phenomena XVI“, Springer Series in Chemical Physics, 705 (2009) doi:10.1007/978-3-540-95946-5[Click for Abstract]

Using time-resolved two-photon photoemission electron microscopy we demonstrate simultaneous spatial and temporal control of nanooptical fields. Cross correlation measurements reveal the ultrafast spatial switching of the local excitation on a subdiffraction length scale.


Direct measurement of core-level relaxation dynamics on a surface-adsorbate system

L. Miaja-Avila, G. Saathoff, S. Mathias, J. Yin, C. La-o-vorakiat, M. Bauer, M. Aeschlimann, M. M. Murnane, H. C. Kapteyn; Phys. Rev. Lett. 101 (2008) 046101 doi:10.1103/PhysRevLett.101.046101 [Click for Abstract]

The coupling between electronic states of an adsorbate and the surface on which it resides is fundamental to the understanding of many surface interactions. In this work, we present the first direct time-resolved observations of the lifetime of core-excited states of an atom adsorbed onto a surface. By comparing laser-assisted photoemission from a substrate with a delayed Auger decay process from an adsorbate, we measure the lifetime of the 4d-1 core level of Xenon on Pt(111) to be 7.1 ± 1.1 fs. This result opens up time domain measurements of highly-excited state dynamics at surfaces where, because of complex interactions, energy-resolved measurements may provide incomplete information.

The lifetime of an adsorbate excitation modified by a tuneable two-dimensional substrate

M. Wiesenmayer, M. Bauer, S. Mathias, M. Wessendorf, E. V. Chulkov, V. M. Silkin, A. G. Borisov, J. -P. Gauyacq, P. M. Echenique, M. Aeschlimann; Phys. Rev. B 78, 245410 (2008) doi:10.1103/PhysRevB.78.245410 [Click for Abstract]

The coupling efficiency between an adsorbate and a two-dimensional substrate is probed by real-time monitoring of the ultrafast charge transfer between a Cs atom and an ultrathin silver film of varying thickness adsorbed on a Cu(111) surface. For the first two monolayers of the silver film, a reduction in the resonance lifetime of the cesium 6s-6p hybrid state of approximately 35% is observed. When the silver coverage further increases, the resonance lifetime stays constant at a value close to the value for Cs adsorption on a bulk Ag(111) surface. Both the one-electron resonant and the multielectron inelastic contributions to the adsorbate-substrate charge transfer are theoretically evaluated based on wave-packet propagation and GW approximation. The results support the experimental findings and allow us to assign the observed dependence of the lifetime change in the multielectron inelastic contribution to the electron transfer rate between the Cs resonance and the very top atomic layers of the substrate.

Electron emission from films of Ag and Au nanoparticles excited by a femtosecond pump-probe laser

A. Gloskovskii, D. A. Valdaitsev, M. Cinchetti, S. A. Nepijko, J. Lange, M. Aeschlimann, M. Bauer, M. Klimenkov, L. V. Viduta, P. M. Tomchuk , G. Schönhense; Phys. Rev B 77 (2008) 195427 doi:10.1103/PhysRevB.77.195427 [Click for Abstract]

Electron emission from Ag and Au nanoparticle films was studied under excitation with femtosecond-laser pulses with photon energies of 1.55 and 3.1 eV. Films were grown on a glass substrate with particle sizes from the nanometer range to a continuous layer. The transition from a continuous film to a nanoparticle film is accompanied by an increase in photoemission current by more than an order of magnitude. Pump-and-probe experiments with variable delay gave information on the lifetime of the intermediate states. At a fixed pulse power, the emission yield increases as the temporal width of the laser pulses is decreased. Experimental results are interpreted in terms of two different electron emission mechanisms, i.e., multiphoton photoemission and thermionic emission or thermally assisted multiphoton photoemission. The first mechanism prevails for continuous films and larger particles with sizes above several tens of nanometers; the second one prevails for smaller nanoparticles with sizes of a few nanometers.

Spin injection and spin dynamics at the CuPc/GaAs interface studied with ultraviolet photoemission spectroscopy and two-photon photoemission spectroscopy

H. J. Ding, Y. L. Gao, M- Cinchetti, J. P. Wustenberg JP, M. Sanchez-Albaneda, O. Andreyev, M. Bauer, M. Aeschlimann; Phys. Rev. B 78 (2008) 075311 doi:10.1103/PhysRevB.78.075311 [Click for Abstract]

Interface formation between p-type GaAs(100) and the organic semiconductor copper phthalocyanine (CuPc) has been studied with ultraviolet photoemission spectroscopy and two-photon photoemission spectroscopy (2PPE). Spin-resolved 2PPE measurements show a highly efficient spin injection of hot electrons from GaAs into CuPc, demonstrating that spin-polarized electrons originating from the GaAs substrate can be injected into CuPc without any substantial spin-flip scattering at the interface. Furthermore, spin- and time-resolved 2PPE measurements are employed to study the temporal evolution of the spin polarization injected into the organic layer in the range up to 1 ps after injection from the GaAs substrate. The results show that the degree of spin polarization of electrons injected through the GaAs/CuPc interface into molecular orbitals just above the lowest unoccupied molecular orbital onset of CuPc is preserved longer than the spin polarization of electrons injected in energetically higher lying states.

Dynamics of the coercivity in ultrafast pump–probe experiments

T. Roth, D. Steil, D. Hoffmann, M. Bauer, M. Cinchetti, M. Aeschlimann; J. Phys. D: Appl. Phys. 41 (2008) 164001 doi:10.1088/0022-3727/41/16/164001 [Click for Abstract]

In this paper we devote ourselves to the coercivity as a measured variable in femtosecond magnetism. We stress the fact that an all optical pump–probe technique is in general not suitable to gain access to the time-dependent behaviour of the coercivity, since the switching in a fixed external field is an irreversible process. We comment on the possible mechanisms leading to the observed reduction in the coercivity with increasing pump power and propose a potential solution to clarify the origin of such behaviour.

Time-resolved 2PPE and time-resolved PEEM as a probe of LSP's in silver nanoparticles

D. Bayer, C. Wiemann, O. Gaier, M. Bauer, M. Aeschlimann ; Journal of Nanomaterials 2008 (2008) 249514 doi:10.1155/2008/249514 [Click for Abstract]

The time-resolved two-photon photoemission technique (TR-2PPE) has been applied to study static and dynamic properties of localized surface plasmons (LSP) in silver nanoparticles. Laterally integrated measurements show the difference between LSP excitation and non-resonant single electron-hole pair creation. Studies below the optical diffraction limit were performed with the detection method of time-resolved photoemission electron microscopy (TR-PEEM). This microscopy technique with a resolution down to 40nm enables a systematic study of retardation effects across single nanoparticles. In addition, as will be shown in this paper, it is a highly sensitive sensor for coupling effects between nanoparticles.

Hot-electron dynamics in thin films of sodium-doped perylene-3,4,9,10-tetracarboxylic dianhydride

J. Wüsten, S. Berger, M. Salomo, A. Mönnich, M. Bauer, M. Aeschlimann, Ch. Ziegler; ; Phys. Rev. B 78, 195326 (2008) doi:10.1103/PhysRevB.78.195326 [Click for Abstract]

Time resolved 2 photon photoemission (2PPE) measurements of the doping process of perylene-3,4,9,10-tetracarboxylicdianhydride (PTCDA) thin films with sodium give insight into the dynamics of hot electrons in this system. Interaction with sodium results in a charge transfer from sodium to PTCDA (”n-doping”) and is accompanied by an increase of the lifetime of electrons excited to unoccupied states 1.2 - 1.8 eV above the Fermi level. This result can be considered from two physical viewpoints, the one-electron picture, treated in ”classical” photoemission spectroscopy theory, and the many-electron excitations as commonly considered in optical spectroscopy. As thin PTCDA layers on a Ag(111) surface show up similar features in ultraviolet spectroscopy as sodium doped PTCDA, this system also was investigated by 2PPE. An increase of lifetime can be observed for thin PTCDA coverages.

Direct Measurement of Core-Level Relaxation Dynamics on a Surface-Adsorbate System using Ultrafast X-Rays

L. Miaja-Avila, G. Saathoff, S. Mathias, J. Yin, C. La-o-vorakiat, M. Bauer, M. Aeschlimann, M. M. Murnane, H. C. Kapteyn; 2008 Conference on Lasers and Electro-Optics & Quantum Electronics And Laser Science Conference, Vols. 1-9 Book Series: IEEE Lasers and Electro-Optics Society (LEOS) Annual Meeting, 3096-3097 (2008) 


Adaptive sub-wavelength control of nano-optical fields

M. Aeschlimann, M. Bauer, D. Bayer, T. Brixner, F. Javier Carcia de Abajo, W. Pfeiffer, M. Rohmer, Ch. Spindler, F. Steeb; Nature 446 (2007) 301 doi:10.1038/nature05595 [Click for Abstract]

Adaptive shaping of the phase and amplitude of femtosecond laser pulses has been developed into an efficient tool for the directed manipulation of interference phenomena, thus providing coherent control over various quantum-mechanical systems. Temporal resolution in the femtosecond or even attosecond range has been demonstrated, but spatial resolution is limited by diffraction to approximately half the wavelength of the light field (that is, several hundred nanometres). Theory has indicated that the spatial limitation to coherent control can be overcome with the illumination of nanostructures: the spatial near-field distribution was shown to depend on the linear chirp of an irradiating laser pulse. An extension of this idea to adaptive control, combining multiparameter pulse shaping with a learning algorithm, demonstrated the generation of user-specified optical near-field distributions in an optimal and flexible fashion. Shaping of the polarization of the laser pulse provides a particularly efficient and versatile nano-optical manipulation method. Here we demonstrate the feasibility of this concept experimentally, by tailoring the optical near field in the vicinity of silver nanostructures through adaptive polarization shaping of femtosecond laser pulses and then probing the lateral field distribution by twophoton photoemission electron microscopy. In this combination of adaptive control and nano-optics, we achieve subwavelength dynamic localization of electromagnetic intensity on the nanometre scale and thus overcome the spatial restrictions of conventional optics. This experimental realization of theoretical suggestions opens a number of perspectives in coherent control, nano optics, nonlinear spectroscopy, and other research fields in which optical investigations are carried out with spatial or temporal resolution.

Adaptive Control of Nanoscopic Photoelectron Emission

M. Aeschlimann, M. Bauer, D. Bayer, T. Brixner, F. J. García de Abajo, W. Pfeiffer, M. Rohmer, C. Spindler, F. Steeb; in Ultrafast Phenomena XV: Proceedings of the 15th International Conference, (2007) 633 doi:10.1007/978-3-540-68781-8_203 [Click for Abstract]

We show experimentally that optimally polarization-shaped femtosecond laser pulses provide spatial control over electron photoemission from nanostructures. Emission patterns are manipulated with subdiffraction resolution, illustrating the potential of electric near-field control in nanophotonics.

Phase propagation of localized surface plasmons probed by time-resolved photoemission electron microscopy

M. Bauer, C. Wiemann, J. Lange, D. Bayer, M. Rohmer, M. Aeschlimann; Appl. Phys. A 88 (2007) 473 doi:10.1007/s00339-007-4056-z [Click for Abstract]

In combining time-resolved two-photon photoemission (TR-2PPE) and photoemission electron microscopy (PEEM) the ultra-fast dynamics of collective electron excitations in silver nanoparticles (localized surface plasmons – LSPs) is probed at fs and nm resolution. Here we demonstrate that the sampling of the LSP dynamics by means of time-resolved PEEM enables detailed insight into the propagation processes associated with these excitations. In phase-integrated as well as phase-resolved measurements we observe spatio-temporal modulations in the photoemission yield from a single nanoparticle. These modulations are assigned to local variations in the electric near field as a result of the phase propagation of a plasmonic excitation through the particle. Furthermore, the control of the phase between the fs pump and probe laser pulses used for these experiments can be utilized for an external manipulation of the nanoscale electric near-field distribution at these particles.

Excited states in the alkali/noble metal surface systems: A model system for the study of charge transfer dynamics at surfaces

J. P. Gauyacq, A. G. Borisov, M. Bauer; Progress in Surface Science 82 (2007) 244–292 doi:10.1016/j.progsurf.2007.03.006 [Click for Abstract]

The low coverage adsorption of alkalis on metal surfaces induces excited states localised on the adsorbate. In the case of noble metal substrates, these excited states can exhibit a very long lifetime, up to tens of fs in the Cs/Cu(111) system. We review recent experimental and theoretical investigations of alkalis adsorbed on noble metal surfaces, with emphasis on the characteristics of the alkaliinduced excited states, the origin of their long lifetimes, and the consequences for the adsorbate dynamics. The possibility of long-lived resonances in other adsorbate/substrate systems is also discussed.

Spin- and time-resolved photoemission studies of thin Co2FeSi Heusler alloy films

J. P. Wüstenberg, M. Cinchetti, M. Sánchez Albaneda, M. Bauer, M. Aeschlimann; J. Magn. Magn. Mat. 316 (2007) e411-e414 doi:10.1016/j.jmmm.2007.02.164 [Click for Abstract]

We have studied the possibly half metallic Co2FeSi full Heusler alloy by means of spin- and time-resolved photoemission spectroscopy. For excitation, the second and fourth harmonic of femtosecond Ti:sapphire lasers were used, with photon energies of 3.1 eV and 5.9 eV , respectively. We compare the dependence of the measured surface spin polarization on the particular photoemission mechanism, i.e. 1-photon-photoemission (1PPE) or 2-photon photoemission (2PPE). The observed differences in the spin polarization can be explained by a spin-dependent lifetime effect occurring in the 2-photon absorption process. The difference in escape depth of the two methods in this case suggests that the observed reduction of spin polarization (compared to the bulk) cannot be attributed just to the outermost surface layer but takes place at least 4 − 6 nm away from the surface.

Angle Resolved Photoemission Spectroscopy with a Femtosecond High Harmonic Light Source using a 2D Imaging Electron Analyzer

S. Mathias, L. Miaja-Avila, M. M. Murnane, H. Kapteyn, M. Aeschlimann, M. Bauer; Rev. Sci. Instr. 78 (2007) 083105 doi:10.1063/1.2773783 [Click for Abstract]

An experimental setup for time- and angle-resolved photoemission spectroscopy using a femtosecond 1 kHz high harmonic light source and a 2D electron analyzer for parallel energy and momentum detection is presented. A selection of the 27th harmonic (41.85eV) from the harmonic spectrum of the light source is achieved with a multilayer Mo/Si double mirror monochromator. The extinction efficiency of the monochromator in selecting this harmonic is shown to be better than 7:1, while the transmitted bandwidth of the selected harmonic is capable of supporting temporal pulse widths as short as 5 fs. The recorded E(k) photoelectron spectrum from a Cu(111) surface demonstrates an angular resolution of better than 0.6° (=0,03 Å-1 at Ekin,e- =36 eV). Used in a pump-probe configuration, the described experimental setup represents a powerful experimental tool for studying the femtosecond dynamics of ultrafast surface processes in real-time.

Local 2PPE-yield enhancement in a defined periodic silver nanodisc array

C. Wiemann, D. Bayer, M. Rohmer, M. Aeschlimann, M. Bauer; Surf. Sci. 601 (2007) 4714 doi:10.1016/j.susc.2007.05.040 [Click for Abstract]

Well-prepared periodic arrays of silver nanoparticles are investigated by means of linear and non-linear photoemission electron microscopy. The structures show homogeneous photoemission for UV excitation in the linear photoemission regime whereas striking inhomogeneities are mapped in the case of the nonlinear (2 Photon) excitation using ultrashort 400 nm laser pulses. A detailed analysis enables to assign these inhomogeneities to defect induced electron momentum transfer processes only effective for the 2 photon excitation process. We propose this mechanism to be of relevance for the appearance of so-called hotspots in nonlinear photoemission as identified in other 2PPE studies in the past. Furthermore, the complementarity between all-optical studies and nonlinear photoemission studies of localized surface plasmons in nanoparticles is discussed.

Experimental time-resolved photoemission and ab initio GW+T study of lifetimes of excited electrons in ytterbium

A. Marienfeld, M. Cinchetti, M. Bauer, M. Aeschlimann, V. P. Zhukov, E. V. Chulkov, P. M. Echenique; J. Phys.: Condens. Matter 19 (2007) 496213 doi:10.1088/0953-8984/19/49/496213 [Click for Abstract]

In this paper we give a detailed analysis of the difference between the lifetimes of Yb quantum-well states measured by scanning tunnelling spectroscopy (STS) in [Phys. Rev. Lett. 94, 126804 (2005)] and lifetimes of bulk Yb obtained by means of time-resolved two-photon photoemission spectroscopy (TR-2PPE). In particular, we show that in spite of a seeming disagreement with the TR-2PPE measurements, the inelastic lifetimes yielded in STS experiments are also close to the inelastic lifetimes of bulk states which emphasizes the complementarity of both methods. Our approach is supported by ab initio electron self-energy calculations performed within the GW and GW+T approximations. Moreover we analyze the impact of the 4f-states on the lifetimes. We show that the GW-term of the inverse lifetime (linewidth) is markedly less than the experimental linewidth. The agreement with experimental data is recovered when both electron-hole and electron-electron terms of the T-matrix are included in the linewidth calculations.

Mapping the femtosecond dynamics of supported clusters with nanometer resolution

M. Rohmer, F. Ghaleh, M. Aeschlimann, M. Bauer, H. Hövel; Eur. Phys. J. D 45, (2007) 491 doi:10.1140/epjd/e2007-00248-8 [Click for Abstract]

In this paper we present a combined STM, SEM and time-resolved PEEM study of silver clusters on a nano-patterned HOPG-substrate, exhibiting areas of different defect type and defect densities. The areas show small but distinct differences in the femtosecond dynamics associated with electronic excitations in the clusters. We assign these differences to variations in the cluster size distribution and variations in the cluster-substrate interaction as governed by the bonding to the different defect types.

Excited electron dynamics in bulk ytterbium: Time-resolved two-photon photoemission and GW+T ab initio calculations

V. P. Zhukov, E. V. Chulkov, P. M. Echenique, A. Marienfeld, M. Bauer, M. Aeschlimann; Phys. Rev. B 76, 193107 (2007) doi:10.1103/PhysRevB.76.193107 [Click for Abstract]

The excited electron dynamics in ytterbium is investigated by means of the time-resolved two-photon photoemission spectroscopy and ab initio GW+T-matrix approach. We show that the standard GW approach with core 4f-orbitals fails to describe the experimental energy trend of the lifetimes. However, a fine agreement with the experimental data is achieved within GW+T-matrix approach when 4f-orbitals are included in a valence basis set. In contrast to previous STS data, we show that at small excitation energy the energy dependence of lifetimes strongly deviates from that for the free electron gas. We also argue that the e-ph coupling in Yb should be weak.


Quantum-Well Wavefunction Localization and the Electron-Phonon Interaction in Thin Ag Nanofilms

S. Mathias, M. Wiesenmayer, M. Aeschlimann, M. Bauer; Phys. Rev. Lett. 97, (2006) 236809 doi:10.1103/PhysRevLett.97.236809 [Click for Abstract]

The electron-phonon interaction in thin Ag-nanofilms epitaxially grown on Cu(111) is investigated by temperature-dependent and angle-resolved photoemission from silver quantum-well states. Clear oscillations in the electron-phonon coupling parameter as a function of the silver film thickness are observed. We relate these oscillations to a sudden wavefunction localization of quantum-well states within the silver film at distinct film thicknesses. This mechanism is different from other thin film systems where quantum oscillations are related to the Fermi-level crossing of quantum well states.

Energy-Resolved Electron-Spin Dynamics at Surfaces of p-Doped GaAs

H. C. Schneider, J. P. Wuestenberg, O. Andreyev, K. Hiebbner, L. Guo, J. Lange, L. Schreiber, B. Beschoten, M. Bauer, M. Aeschlimann; Phys. Rev. B 73, 081302(R) (2006) doi:10.1103/PhysRevB.73.081302 [Click for Abstract]

Electron-spin relaxation at different surfaces of p-doped GaAs is investigated by means of spin, time, and energy-resolved two-photon photoemission. These results are contrasted with bulk results obtained by time-resolved Faraday rotation measurements as well as calculations of the Bir-Aronov-Pikus spin-flip mechanism. Due to the reduced hole density in the band bending region at the (100) surface the spin-relaxation time increases over two orders of magnitude towards lower energies. At the flat-band (011) surface a constant spin relaxation time in agreement with our measurements and calculations for bulk GaAs is obtained.

Space Charge Effects in Photoemission with a low repetition, high intensity femtosecond laser source

S. Passlack, S. Mathias, O. Andreyev, D. Mittnacht, M. Aeschlimann, M. Bauer; J. Appl. Phys. 100, (2006) 024912 doi:10.1063/1.2217985 [Click for Abstract]

In this paper, we present experimental results on the effect of space-charging in photoelectron spectroscopy from a surface using a pulsed and intense femtosecond light source. We particularly focus on a quantitative evaluation of the induced spectral broadening. Our results are compared with analytic calculations based on energy conservation considerations as well as with experimental results from measurements using picosecond pulses for the excitation process. As a measure of space charge effects, we monitored the angular and energy distribution of the photoemission from the occupied Shockley surface state of Cu(111) as a function of the total number N of the photoemitted electrons per laser pulse. Our results show that spectral distortions exist for the entire laser fluence regime probed. The energetic broadening of the surface state peak can be fitted with remarkable accuracy by a dependence, in agreement with the theoretical predictions and different from the experimental picosecond results, where a dominating linear dependence has been reported. In addition to a pure energetic broadening of the photoemission spectra, we also identify modifications in the angular distribution of the photoemitted electrons due to space charge effects.

Morphological Modifications of Ag/Cu(111) probed by Photoemission Spectroscopy of Quantum Well States and the Shockely Surface State

S. Mathias, M. Wessendorf, S. Passlack, M. Aeschlimann, M. Bauer; Appl. Phys. A 82 (2006) 439 doi:10.1007/s00339-005-3369-z [Click for Abstract]

Epitaxial ultra-thin Ag films grown on Cu(111) have been investigated by angle-resolved photoemission spectroscopy. The thickness dependence of the binding energy for the Shockley surface state at 300 K could be determined accurately in films up to 5 ML thick. Furthermore, we observe drastic changes in the film morphology after annealing to 450 K. Spectral modifications in the shape of the quantum-well states (QWS), characteristic for these ultra-thin silver films, prove that the surface morphology is homogeneous. The photoemission spectra also indicate that the silver film bifurcates to form a film exhibiting two distinct film thicknesses. For all levels of silver coverage, we identify surface regions that are 2 ML thick, while the thickness of the remaining surface depends on the amount of deposited silver. The almost purely Lorentzian line-shape of the spectral features corresponding to the two different surface regions show that both surface areas are atomically flat.

Local correlation of Photoemission Electron Microscopy and STM at a defined cluster substrate system

M. Rohmer, C. Wiemann, M. Munzinger, L. Guo, M. Aeschlimann, M. Bauer; Appl. Phys. A 82 (2006) 87 doi:10.1007/s00339-005-3343-9 [Click for Abstract]

We describe a technique that enables photoelectron spectroscopy and STM imaging of supported clusters from identical surface areas of a size of a few µm² at a lateral resolution in the low nanometer regime. In this way we are able to locally correlate properties regarding the electronic structure of the clusters and their topography. The use of a photoemission electron microscope (PEEM) allows one to probe the local distribution of the photoemission yield from the sample at a resolution down to 20 nm. An STM-tip is used to remove clusters from their position and set local, well-defined markers at the surface that are clearly visible in the PEEM images. These markers act as reference points to identify surface areas in the PEEM image that have formerly been imaged by an STM. The present accuracy of this local correlation technique is at least 300 nm. We propose a scheme to further improve this correlation so that in future experiments even selected single clusters, which have been characterized by STM, can be addressed by local photoelectron spectroscopy as well as local time-resolved photoelectron spectroscopy.

Probing femtosecond plasmon dynamics with nanometer resolution

J. Lange, D. Bayer, M. Rohmer, C. Wiemann, O. Gaier, M. Aeschlimann, M. Bauer; Proc. SPIE Vol. 6195, (2006) 61950Z doi:10.1117/12.663097 [Click for Abstract]

In combining time-resolved two-photon photoemission (TR-2PPE) and photoemission electron microscopy (PEEM) the ultrafast dynamics of collective electron excitations in silver nanoparticles (localized surface plasmons – LSP) is probed at femtosecond and nanometer resolution. In two examples we illustrate that a phase-resolved (interferometric) sampling of the LSP-dynamics enables detailed insight into dephasing and propagation processes associated with these excitations. For two close-lying silver nano-dots (diameter 200 nm) we are able to distinguish small particle to particle variations in the plasmon eigenfrequency, which typically give rise to inhomogeneous line-broadening of the plasmon resonance in lateral integrating frequency domain measurements. The observed spatio-temporal modulations in the photoemission yield from a single nanoparticle can be interpreted as local variation in the electric near-field and result from the phase propagation of the plasmon through the particle. Furthermore, we show that the control of the phase between the used femtosecond pump and probe laser pulses used for these experiments can be utilized for an external manipulation of the nanoscale electric near-field distribution at these particles.

Spin-resolved two-photon photoemission study of the surface resonance state on Co/Cu(001)

O. Andreyev, Yu. M. Koroteev, M. Sánchez Albaneda, M. Cinchetti, G. Bihlmayer, E. V. Chulkov, J. Lange, F. Steeb, M. Bauer, P. M. Echenique, S. Blügel, M. Aeschlimann; Phys. Rev. B 74 (2006) 195416 doi:10.1103/PhysRevB.74.195416 [Click for Abstract]

Bulk and surface states of a clean and Cs-doped surface of a Co film grown on Cu(001)have been studied by spin-resolved photoemission (SR-PE) and compared with band structure calculation results. One-photon (1PPE) and two-photon (2PPE) photoemission spectra from clean Co films are found to be dominated by a peak located at a binding energy of about 0.4 eV with respect to EF, which is assigned to the spin up 3d bulk state. Slight Cs-doping of a Co(001) surface shifts an image potential state in resonance with the sp-states of the conduction band. SR-2PPE study of the optically-induced electron population in such an image resonance reveals a strong dependence on the set polarization of the laser light. We are able to directly detect the spin polarization of electrons photoemitted from the image resonance state, which can be varied from highlypolarized (about bulk values) to almost unpolarized when tuning light polarization of the pump laser pulse from s to p.

Spin-Flip Processes and Ultrafast Magnetization Dynamics in Co: Unifying the Microscopic and Macroscopic View of Femtosecond Magnetism

M. Cinchetti M. Sánchez Albaneda, D. Hoffmann, T. Roth, J. -P. Wüstenberg, M. Krauß, O. Andreyev, H. C. Schneider, M. Bauer, M. Aeschlimann; Phys. Rev. Lett. 97 (2006) 177201 doi:10.1103/PhysRevLett.97.177201 [Click for Abstract]

The femtosecond magnetization dynamics of a thin cobalt film excited with ultrashort laser pulses has been studied using two complementary pump-probe techniques, namely, spin-, energy-, and time-resolved photoemission and the time-resolved magneto-optical Kerr effect. Combining the two methods, it is possible to identify the microscopic electron spin-flip mechanisms responsible for the ultrafast macroscopic magnetization dynamics of the cobalt film. In particular, we show that electron-magnon excitation does not affect the overall magnetization even though it is an efficient spin-flip channel on the sub-200 fs time scale. Instead, we find experimental evidence for the relevance of Elliott-Yafet-type spin-flip processes for the ultrafast demagnetization taking place on a time scale of 300 fs.

Experimental time-resolved photoemission and ab initio study lifetimes of excited electrons in Mo and Rh

A. Mönnich, J. Lange, M. Bauer, M. Aeschlimann, I. A. Nechaev, V. P. Zhukov, P. M. Echenique, E. V. Chulkov; Phys. Rev. B 74 (2006) 035102 doi:10.1103/PhysRevB.74.035102 [Click for Abstract]

We have studied the relaxation dynamics of optically excited electrons in molybdenum and rhodium by means of time resolved two-photon photoemission spectroscopy (TR-2PPE) and ab initio electron self-energy calculations performed within the GW and GW + T approximations. Both theoretical approaches reproduce qualitatively the experimentally observed trends and differences in the lifetimes of excited electrons in molybdenum and rhodium. For excitation energies exceeding the Fermi energy by more than 1 eV, the GW +T theory yields lifetimes in quantitative agreement with the experimental results. As one of the relevant mechanisms causing different excited state lifetime in Mo and Rh we identify the occupation of the 4d bands. An increasing occupation of the 4d bands results in an efficient decrease of the lifetime even for rather small excitation energies of a few 100 meV.


The lateral Photoemission Distribution from a defined Cluster/Substrate System as probed by Photoemission Electron Microscopy

M. Munzinger, C. Wiemann, M. Rohmer, L. Guo, M. Aeschlimann, M. Bauer; New J. Phys. 7, (2005) 68 doi:10.1088/1367-2630/7/1/068 [Click for Abstract]

We used photoemission electron microscopy (PEEM) to investigate the lateral distribution of the photoemission yield from a defined system of silver clusters supported by a highly oriented pyrolytic graphite (HOPG) substrate. For threshold photoemission using conventional photoemission (PE) and twophoton photoemission (2PPE) we find that distinct, well-separated emitters are responsible for the measured integral photoemission yield. Complementary characterization of the surface usingSTMshows that the emitter density as probed by PEEM is reduced by about three orders of magnitude in comparison to the actual cluster density. Wavelength and light polarization scans in combination with two-photon-PEEM clearly show that the origin of the 2PPE signal is related to small silver particles. Furthermore, the PEEM differentiates between inhomogeneous and homogeneous broadening effects in the 2PPE signal. This observation allows one to assign the origin of the local photoemission signal to either a distinct single silver particle or a number of coherently coupled silver particles. We conclude that the 2PPE-yield is highly selective with respect to specific properties of the supported silver particles. Our results show that in future experiments, PEEM as a highly local field probe, may be a key tool in the identification of these properties.

Spin Dynamics of GaAs(100) by Two Photon Photoemission

L. Guo, J. P. Wüstenberg, O. Andreyev, M. Bauer, M. Aeschlimann; Acta Physica Sinica Vol. 54 No. 7 (2005)

Femtosecond ultraviolet photoemission for the study of ultrafast surface processes (topical review)

M. Bauer; J. Phys. D: Appl. Phys. 38 (2005) R253-R267 doi:10.1088/0022-3727/38/16/R01 [Click for Abstract]

Conventional ultraviolet photoelectron spectroscopy (UPS) has been successfully used for decades to study static electronic properties of surfaces and their interaction with adsorbates. The recent progress in the creation of femtosecond and attosecond VUV pulses by means of high-order harmonic generation opens the potential to operate UPS in a time-resolved mode, which is capable of monitoring ultra-fast dynamical changes of surface properties. Particularly, it is now possible to evaluate the temporal evolution of chemical surface reactions on their fundamental time scales. This paper reviews technical aspects of time-resolved UPS and its application to the study of ultra-fast surface processes. Special emphasis is placed on the creation of ultra-short VUVpulses by means of high-order harmonic generation and also on the description of the stroboscopic experimental approach that enables temporal resolutions on the order of femtoseconds. This review will show that time-resolved UPS is indeed capable of probing with femtosecond resolution dynamic surface processes related, for example, to electronic excitations at surfaces or to the chemical state of adsorbates on a femtosecond time scale.

Time-resolved 2PPE: Probing adsorbate motion on femtosecond time-scales - what is the role of the laser bandwidth?

M. Bauer, M. Wessendorf, D. Hoffmann, C. Wiemann, A. Mönnich, M. Aeschlimann; Appl. Phys. A 80, (2005) 987 doi:10.1007/s00339-004-3126-8 [Click for Abstract]

Recently it has been shown that time-resolved two photon photoemission spectroscopy (TR 2PPE) is capable of probing the atomic motion of an adsorbate after excitation by a femtosecond laser-pulse. In this paper we address the question in how far the bandwidth of the used laser pulse is of importance with respect to the sensitivity of TR 2PPE to the actual nuclear dynamics. In particular, we find that the use of laser pulses of different spectral bandwidth may simplify the interpretation of the measured TR 2PPE autocorrelation traces and help to distinguish between purely electron related contributions and the nuclear dynamics. Our experimental results indicate that for pulses of small bandwidth only the lifetime of the electronic adsorbate excitation at the ground state equilibrium distance is probed. This information can be used as input for the interpretation of autocorrelation traces obtained with large bandwidth pulses, which in addition contain information about the nuclear dynamics of the adsorbate.

Irradiation of supported silver and gold nanoparticles with continuous-wave, nanosecond and femtosecond laser light: a comparative study

F. Hubenthal, M. Aschinger, M. Bauer, D. Blazquez Sanchez, N. Borg, M. Brezeanu, R. Frese, C. Hendrich, B. Krohn, M. Aeschlimann, F. Träger; Proc. of the SPIE 5838 (2005) 224 doi:10.1117/12.608560 [Click for Abstract]

Modification of metal nanoparticles with laser light has been a well-known technique for several years. Still, selective tailoring of certain sizes or shapes of nanoparticles has remained a challenge. In this paper, we present recent studies on tailoring the size and shape of supported nanoparticles with continuous-wave and femtosecond pulsed laser light and compare them to our results obtained with ns pulsed laser light. The underlying method is based on the size and shape dependent plasmon resonance frequencies of the nanoparticles. In principle, irradiation with a given laser photon energy excites and heats nanoparticles of certain sizes or/and shapes and leads to difusion and evaporation of surface atoms. Thus, tailoring the dimensions of the nanoparticles can be accomplished. In our experiments, gold and silver nanoparticles were prepared under ultrahigh vacuum conditions by deposition of atoms and subsequent diffusion and nucleation, i.e. Volmer-Weber growth. This gives particle ensembles with size and shape distributions of approximately 30 % - 40 %. The nanoparticle ensembles were irradiated with laser light either during or after growth. It turns out, that irradiation with cw or ns laser light makes possible selective modifcation of the nanoparticles. In contrast, application of fs laser pulses results in non-selective modifcation. For example, post-grown irradiation of supported gold nanoparticles with ns laser pulses (hv = 1.9 eV) causes a clear reduction of the width of the surface plasmon resonance from 0.52 eV to 0.20 eV (HWHM). Similar experiments were carried out with fs pulsed laser light (hv = 1.55 eV), which result in a slightly reduced line width but also, to an overall decrease of the extinction. A comparison of all experiments revealed, that for size or shape tailoring of supported metal nanoparticles best results have been achieved with ns pulsed laser light.


Lifetimes of excited electrons in Ta: experimental time-resolved photoemission data and first-principle GW+T theory

V. P. Zhukov, O. Andreyev, D. Hoffmann, M. Bauer, M. Aeschlimann, E. V. Chulkov, P. M. Echenique; Physical Review B 70, 233106 (2004) doi:10.1103/PhysRevB.70.233106 [Click for Abstract]

Time-resolved two-photon photoemission spectroscopy and first-principles GW and GW+T theories have been used to study excited electron lifetimes in tantalum. The GW+T approach includes evaluation of the lowest self-energy term of the many-body perturbation theory in the GW approximation and higher terms in the T-matrix approximation. The GW+T calculated lifetimes are in good agreement with the measured lifetimes at excitation energies above 1.6 eV. At lower energies, a slightly worse agreement between theoretical and experimental data is obtained which we refer to as the influence of cascade processes.

Electronic Surface Structure of n-ML Ag/Cu(111) and Cs/n-ML Ag/Cu(111) as investigated by 2PPE and STS

M. Wessendorf, C. Wiemann, M. Bauer, M. Aeschlimann, M. A. Schneider, H. Brune, K. Kern; Appl. Phys. A 78, (2004) 183 doi:10.1007/s00339-003-2313-3 [Click for Abstract]

We investigated the electronic structure of epitaxially grown silver films on Cu(111) with and without adsorption of cesium by means of scanning tunneling spectroscopy and two-photon photoemission. This system has been chosen as a model system to engineer and measure the dynamics of charge-transfer processes between an adsorbate and a heterogeneous substrate. Special emphasis has been laid on the investigation of the energy shift of the Shockley-type surface state and an excited cesium resonance as a function of Ag film thickness. For the cesium resonance we observe an increase in line width with increasing layer thickness.


Hot-Electron-Driven Charge Transfer Processes on Surfaces

C. -F. Lei, M. Bauer, K. Read, R. Tobey, M. Murnane, H. Kapteyn; in Ultrafast Phenomena XIII: Proceedings of the 13th International Conference, (2003/2004) 313

Time-resolved UPS: a new experimental technique for the study of surface chemical reactions on femtosecond timescales

M. Bauer, C. Lei, R. Tobey, M. M. Murnane, H. C. Kapteyn; Surf. Sci. 532 535, (2003), 1159 doi:10.1016/S0039-6028(03)00414-X [Click for Abstract]

Recent progress in the generation of ultrashort XUV-pulses by means of high harmonic generation provides a means to monitor the dynamics of surface chemical reactions using photoemission spectroscopy. In this paper we describe details of an experimental setup for time-resolved photoemission spectroscopy using high harmonic generation. We als present results where the different steps involved in the laser-induced change of an adsorbate–surface bond were monitored with sub-100 femtosecond time-resolution.


Imaging Sub-Nanosecond Processes Using Photoemission Electron Microscopy

G. Schönhense, A. Oelsner, A. Krasyuk, C. M. Schneider, M. Bauer, M. Aeschlimann; Recent Trends in Charged Particle Optics and Surface Physics Instrumentation, 8th Seminar, Brno (2002)

Dynamics of excited electrons in metals, thin films and nanostructures

M. Bauer, M. Aeschlimann; J. Electr. Spectr. 124, (2002) 225 doi:10.1016/S0368-2048(02)00056-7 [Click for Abstract]

The immense progress in the field of ultrashort pulsed lasers made it possible to study ultrafast dynamics of photoexcited hot electrons in metals by means of a variety of pump-probe techniques. Time-resolved two-photon photoemission has the capability of directly monitoring the dynamics of electrons with specific energy and momentum during the course of the transformation of a nascent (as photoexcited) nonthermal electron distribution to an excited Fermi–Dirac distribution. The main purpose of this investigation was to gain a basic understanding of the dynamics of single excited electrons at a metal surface, particularly in an energy region which is important for surface photochemistry and catalytic model reactions (E ,E,E ). In these studies, the roles of secondary electrons and transport effects in equal pulse cross-correlation F Vac experiments were considered. The results demonstrate the feasibility of studying electron relaxation in noble and transition metals as a function of band structure, spin-polarization, surface morphology and dimensionality. We also present an extension of the common time-resolved two photon photoemission method to higher energies (hn.20 eV, UPS mode) and high lateral resolution (PEEM mode).

Time-resolved two photon photoemission electron microscopy

O. Schmidt, M. Bauer, C. Wiemann, R. Porath, M. Scharte, O. Andreyev, G. Schönhense, M. Aeschlimann; Appl. Phys. B 74, (2002) 223 doi:10.1007/s003400200803 [Click for Abstract]

Femtosecond, time-resolved two photon photoemission has been used to map the dynamics of photo-excited electrons at a structured metal/semiconductor surface. A photoemission microscope was employed as a spatially resolving electron detector. This novel setup has the potential to visualize variations of hot electron lifetimes in the femtosecond regime on heterogeneous sample surfaces and nanostructures.

Hot-electron-driven charge transfer processes on O2/Pt(111) surface probed by ultrafast extreme-ultraviolet pulses

C. Lei, M. Bauer, K. Read, R. Tobey, Y. Liu, T. Popmintchev, M. M. Murnane, H. C. Kapteyn; PRB 66 (2002) 245420 doi:10.1103/PhysRevB.66.245420 [Click for Abstract]

We use ultrafast extreme-ultraviolet photoelectron spectroscopy to directly monitor the electron dynamics and the characteristic valence-band photoelectron spectra associated with a hot-electron mediated surface chemical reaction. By adsorbing molecular oxygen onto a Pt(111) surface and exciting it with an ultrafast laser pulse, charge transfer induced changes in the platinum-oxygen bond were observed on femtosecond time scales. By simultaneously monitoring both the hot-electron distribution at the Fermi edge and the valence-band photoemission spectra, it was determined that the thermalization of the hot-electron gas precedes significant changes in the O2/Pt bond.


Direct Observation of Surface Chemistry Using Ultrafast Soft-X-Ray Pulses
see also: "Shooting an X-ray movie" Phys. Rev. Focus

M. Bauer, C. Lei, K. Read, R. Tobey, J. Gland, M. M. Murnane, H. Kapteyn; Phys. Rev. Lett. 87, (2001) 025501; doi:10.1103/PhysRevLett.87.025501 [Click for Abstract]

We present the first demonstration of the use of ultrafast extreme-ultraviolet pulses to directly monitor a surface chemical reaction on femtosecond time scales. By adsorbing molecular oxygen onto a platinum surface and exciting it with an ultrafast laser pulse, changes in the oxygen-platinum chemical bond on a subpicosecond time scale were observed through changes in the photoelectron spectra. This work demonstrates a powerful new technique for studying reactions of interest in catalysis and for probing changes of local order on surfaces on their fundamental time scale.


Transport and dynamics of optically excited electrons in metals

M. Aeschlimann, M. Bauer, S. Pawlik, R. Knorren, G. Bouzerar, K. H. Bennemann; Appl. Phys. A 71 (2000) 1 doi:10.1007/s003390000704 [Click for Abstract]

Time-resolved two-photon photoemission, based on the equal-pulse correlation technique, is used to measure the energy relaxation and the transport of the photoexcited carriers in thin Ag and Au films. The energy-dependent relaxation time shows a significant thickness dependence in the Ag film, whereas for Au a much smaller effect is observed. These experimental observations are compared with a theoretical model based on the Boltzmann equation, which includes secondary (Auger) electrons and transport. A good agreement between experimental and theoretical results is found for Au. However, in our calculations, we did not find any significant change in the thickness dependence in the case of Ag. In order to explain the strong effect in Ag, we discuss the possibility of surface excitations.


Snapshot of electronic surface excitation: Observing adsorbate dynamics with femtosecond time-resolution

M. Bauer, S. Pawlik, M. Aeschlimann; Proc. of the 10th Annual Symposium of the Center for Photoinduced Charge Transfer, Rochester (1999) doi:10.1142/9789812793294_0016

Decay dynamics of photoexcited alkali chemisorbates: Real-time investigations in the femtosecond regime

M. Bauer, S. Pawlik, M. Aeschlimann; Phys. Rev. B 60 (1999) 5016 doi:10.1103/PhysRevB.60.5016 [Click for Abstract]

The inelastic decay time of photoexcited cesium and sodium adsorbed on different single-crystal surfaces has been investigated by means of time-resolved two-photon photoemission. Especially in the case of cesium, we observe a surprisingly high lifetime. For Cs/Cu(111) we obtain a value of 15 fs. Intra-atomic hybridization, the specific band structure of the substrate, and adsorption site effects may be responsible for this behavior. These different mechanisms are discussed in detail.


Electron dynamics of aluminum investigated by means of time-resolved photoemission

M. Bauer, S. Pawlik, M. Aeschlimann; Proc. of the SPIE, 3272 (1998) 201 doi:10.1117/12.307123 [Click for Abstract]

Femtosecond time-resolved two photon photoemission has been used to investigate the dynamics of photoexcited electrons at a polycrystalline Al surface. The measured relaxation time data are very different from the behavior predicted for a Fermi Liquid. We observed a distinct increase in the decay rate of the excited states. The origin of this strong deviation from the theoretical prediction may be transport effects or band structure effects induced by the periodic crystal lattice.

Direct transition in the system Ag(111) studied by one and two photon photoemission

S. Pawlik, R. Burgermeister, M. Bauer, M. Aeschlimann; Surf. Sci., 402-404 (1998) 556 doi:10.1016/S0039-6028(97)01021-2 [Click for Abstract]

We report a Two-photon Photoemission study of an excited state of cesium adsorbed on a Cu(111) single crystal surface at submonolayer cesium coverage. The dependence of cesium excitation on the laser polarization enables us to make statements about the symmetry of the intermediate as well as the initial state of the 2PPE process.

Symmetry properties of an alkali excitation at a noble metal surface as investigated by two-photon photoemission

M. Bauer, S. Pawlik, R. Burgermeister, M. Aeschlimann; Surf. Sci., 402-404 (1998) 62 doi:10.1016/S0039-6028(97)01042-X [Click for Abstract]

We report a Two-photon Photoemission study of an excited state of cesium adsorbed on a Cu(111) single crystal surface at submonolayer cesium coverage. The dependence of cesium excitation on the laser polarization enables us to make statements about the symmetry of the intermediate as well as the initial state of the 2PPE process.

Spin-dependent electron dynamics investigated by means of time- and spin-resolved photoemission

M. Aeschlimann, R. Burgermeister, S. Pawlik, M. Bauer, D. Oberli, W. Weber; J. Electr. Spectr., 88-91 (1998) 179 doi:10.1016/S0368-2048(97)00110-2 [Click for Abstract]

The power of time- and spin-resolved two photon photoemission is demonstrated. This method allows the determination of the spin-dependent decay of an excited electron population in ferromagnetic materials by means of a real time experiment. The lifetime of majority-spin electrons is found to be longer compared with that of minority-spin electrons for both cobalt and iron. This study shows that the relaxation dynamics of excited electrons is strongly influenced by the imbalance between majority- and minority-spin electrons in a ferromagnet.


The resonance lifetime and energy of an excited Cs-state on Cu(111)

M. Bauer, S. Pawlik, M. Aeschlimann; Phys. Rev. B. 55 (1997) 10040 doi:10.1103/PhysRevB.55.10040 [Click for Abstract]

Using time-resolved two-photon photoemission spectroscopy, we investigated the resonance lifetime and energy of an excited Cs-state on Cu(111) in the low-coverage case. We found a pronounced lifetime increase of up to 11 3fs on a copper surface covered with cesium as compared to a clean copper surface in the energy range of an excited electronic Cs-state in the spectrum. This result is in agreement with recent theoretical predictions of hybridization among excited atomic levels of alkali atoms near metal surfaces.

Ultrafast spin-dependent electron dynamics in fcc Co

M. Aeschlimann, M. Bauer, S. Pawlik, W. Weber, R. Burgermeister, D. Oberli, H. C. Siegmann; Phys. Rev. Lett. 79 (1997) 5158 doi:10.1103/PhysRevLett.79.5158 [Click for Abstract]

The unambigious observation of a spin dependent lifetime in a 200 A-thick fcc Co film on Cu(100) substrate by means of a femtosecond realtime experiment is reported. Using time- and spin resolved two photon photoemission technique, a ration of majority- to minority-spin lifetimes of 1.4 is obtained at low energies. The results demonstrate the feasibility of studying spin-dependent electron relaxation in ferromagnetic solids directly in the time domain and provide a framework for understanding the dynamics of electron transport in ferromagnetic solids and thin films.

Femtosecond lifetime investigations of hot electrons trapped by adsorbate states: Atomic Oxygen on Cu(111)

M. Bauer, S. Pawlik, M. Aeschlimann; Surf. Sci. 377-379 (1997) 350 doi:10.1016/S0039-6028(96)01415-X [Click for Abstract]

We investigated an excited electronic state induced by atomic oxygen adsorbed on Cu( 111) by means of time-resolved two-photon photoemission spectroscopy. In contrast to a system investigated previously, cesium adsorbed on Cu( ill), no increase in the relaxation time of the photoexcited electrons due to the resonance time of the excited oxygen state was detected within our time resolution. The difference between the two systems will be discussed, taking the specific adsorption sites of the adsorbates on the Cu( 111) surface into consideration.

Lifetime difference of photoexcited electrons between intraband and interband transitions

S. Pawlik, M. Bauer, M. Aeschlimann; Surf. Sci. 377-379 (1997) 206 doi:10.1016/S0039-6028(96)01353-2 [Click for Abstract]

Using time-resolved two-photon photoemission, we show that the inelastic lifetime r,, of optically excited electrons of copper depends crucially on whether the electron is excited by an intraband or an interband transition. These results indicate clearly that z,, of photoexcited electrons is determined not only by the available phase space for the inelastic scattering process, but also depends on the initial state of the excited electron.


Dynamics of photoinduced surface reactions

S. Pawlik, M. Bauer, M. Aeschlimann; in "Femtochemistry: Ultrafast Chemical and Physical Processes in Molecular Systems", Edited by: M. Chergui, World Scientific Publishing (1996)

Competing nonradiative channels for hot electron induced surface photochemistry

M. Aeschlimann, M. Bauer, S. Pawlik; Chem. Phys., 205 (1996) 127 doi:10.1016/0301-0104(95)00372-X [Click for Abstract]

We report experiments in which we investigated the ultrafast dynamics of competing nonradiative channels for hot electron mechanisms in various polycrystalline metal samples. Time resolved two-photon photoemission, based on the equal pulse correlation technique, is used to measure the energy relaxation and the transport of the photoexcited carriers. In these studies the role of coherent effects in auto- and crosscorrelation experiments has been considered. While the inelastic lifetime of Ag is in qualitative and quantitative agreement with the Fermi liquid theory, the result obtained for Au is very different. The measured inelastic relaxation time for transition metals with unoccupied d orbitals is shorter as compared to the noble metals. The results demonstrate the feasibility of studying electron relaxation in noble and transition metals directly in the time domain and provide a framework for understanding the dynamics of hot electron transfer from a metal surface to the adsorbate.


Femtosecond Time-Resolved Measurement of Electron Relaxation at Metal Surfaces

M. Aeschlimann, M. Bauer, S. Pawlik; Ber. Bunsenges. Phys. Chem. 99 (1995) 1504 doi:10.1002/bbpc.199500115 [Click for Abstract]

Time resolved two-photon photoemission, based on the equal pulse correlation technique, is used to measure the energy relaxation of the photoexcited electrons in various polycrystalline metal samples. While for Ag the inelastic lifetime of excited electrons is, within our time resolution, in agreement with the Fermi-liquid theory, the result obtained for Au is very different. Scanning tunneling microscopy studies reveal that the enhanced inelastic lifetime of gold is not caused by surface effects. The measured inelastic relaxation time for transition metals with unoccupied d-orbitals is shorter as compared to the noble metals, indicating that the inelastic decay depends on the number of unoccupied d-orbitals below the probed energy state.