Diagnostics

We have worked on a number of diagnostic tools for complex plasma research:

  • Angular and polarization resolved Mie scattering
    Based on a setup developed by Asnaz et al. we studied the achievable resolution for size measurements using angular and polarization resolved Mie scattering. The basic idea is that the angular resolved scattered ligth of a particle already gives  reasonable information on its size. If this information is combined with polarization the size resolution can be increased significantly. A scetch of the angular and polarization resolved scattered light intensity is depicted below. Especially the merging and splitting of intensity maxima as a function of polarization angle provides rich information. We demonstrated that the achievable resolution of this setup is below a nanometer (see Kohlmann et al.).

    Lichtstreuung an einem Partikel (Skizze)









     
  • An optical tweezer for complex plasmas
    To manipulate position and dynamics of single particles in a dusty plasma is a sphisticated task. The concept of optical twezers, which have been invented by Ashkin et al., is not easily transfered to complex plasmas. Conventional tweezers require optices with a very high aperture. However, the experimental setup with a vacuum system, electrodes and a volume for plasma generation inhibit a straight forward application of Ashkins concept. That tweezers can nevertheless be realized to tweeze single dust particles was shown by us in 2015 (see Schablinski et al.). Our tweezer uses a moderate aperture and in addition makes use of confinement forces in the plasma itself (see sketch below).

    Forces on particles in an optical tweezer

    See F. Wieben, J. Schablinski, and D. Block,  Modification of microparticles due to intense laser manipulation.  Physics of Plasmas 26, 033701 (2019) https://aip.scitation.org/doi/10.1063/1.5090452 for some further reading and application of the tweezer setup.
     
  • Laser heating
    To manipulate the particle dynamics in 2D and 3D dust clouds is a well established technique. Our laser heating setup uses four laser beams to
    achieve a perfect Maxwellian velocity distribution. The intensities of the beams are matched to generate zero momentum transfer on average and achieve artefact free velocity spectra. Thus, this setup can be regarded as a perfect thermostat for 2D plasma clouds. A detailed description of its function is found in our paper in Physic of Plasmas. The image below shows the current setup in our experiment.

    Laser heatting setuo
     
  • Digital holography
    To measure the positions in 3D of particles in dust clouds is still a challenge. Basically to techniques have shown to be successfull: stereoscopic imaging (developed and operated by AG Melzer at University Greifswald) and digital holography. Our experiments in 2008 (see Kroll et al.) were the first to show that holographic imaging of dust clouds is possible and yields precise information on 3D positions.