Time-resolved mueller polarimeter

The temperature and electric field induced in a target by the plasma jet is investigated using Mueller polarimetry. In this novel optical technique polarized light is sent through an electro-optic BSO crystal for which the refractive index depends on both the electric field and the temperature gradients¹⁷. Due to the examination of the material the plasma jet is positioned at a 45 degree impact angle, in order not to block the polarized light that is send through the target, see Fig. 4. The experimental setup is shown with more detail in^17,27.

The Mueller matrix of the targeted electro-optic BSO crystal is measured by generating and analyzing different states of polarized light, using the Polarizer State Generator (PSG) and the Polarizer State Analyzer (PSA). Both the PSG and PSA consist of a polarizer and two ferro-electric liquid crystals, for which the orientation can be switched between two states. Hence, 16 different states can be sequentially examined which allows for the acquisition of the 4 × 4 Mueller matrix of the examined sample, i.e. the electro-optic BSO under exposure of the plasma jet.

By measuring the full Mueller matrix of the sample the birefringence is examined, following the logarithmic decomposition^49,50. Diattenuation and depolarization are examined automatically as well, which opens interesting possibilities for future in-situ examinations of temperature sensitive targets directly. The acquisition of the polarized light can be timed within the applied AC period for time resolved Mueller polarimetry. The optical characterization of this highly dynamical system is not possible with a conventional static Mueller polarimetry setup. Since time resolved measurements are necessary for this work, an automatic control of the PSG and PSA states synchornized with the iCCD camera has been implemented.

The Mueller matrix is obtained twice while the plasma jet is active. Once before a new ionization wave has impacted and once right after, with an exposure time of 1 μs. This is achieved by sychronizing the switching of the polarization states and the acquisitions at the iCCD camera with an external trigger, related to the rise of the positive half period of the applied high voltage AC sine wave. A more comprehensive description of the acquisition and theory behind the time resolved imaging Mueller polarimeter can be found in^17,27 but we remind some of the key points here.