Photoinduced dichroism in amorphous As2Se3 thin films

Abstract

The dichroism induced by a polarized beam of near bandgap light was measured in thin films of amorphous As2Se3. The polarized pump-beam causes a decrease in the absorption coefficient for light polarized in the same direction, and the resulting anisotropy in absorption coefficients is referred to as dichroism. An unpolarized beam of light passing through the sample will be preferentially absorbed in the same direction as the pump-beam polarization direction. That preferential absorption can be measured as a function of time to represent the time dependence of the induced dichroism.

The time dependence of the induced dichroism follows a stretched exponential of the form exp((-t/ô)^â). Three parameters from the fitting function are of interest; the saturation level Asat, the time constant ô, and the stretching exponent â. The kinetics of dichroism were extracted from measurements on samples of various thicknesses, samples subjected to various pump-beam intensities, samples doped with small quantities of iodine (0.1%), and for samples in the presence of a DC electric field. It was found that the induced dichroism saturates at a value that depends linearly on sample thickness, but is independent of inducing beam intensity. The time constant (ô) was found to have an inverse relationship with pump-beam intensity and decreases with increasing sample thickness. The stretching exponent â has little or no dependence on either intensity or thickness. The addition of iodine (0.1%) increased the time constant (ô) in some but not all cases; meanwhile the DC electric field had no effect on any of the fitting parameters.

Measurement of the kinetics of the induced dichroism in amorphous As2Se3 can be used to constrain models of the phenomenon in Chalcogenide glasses. The results of the intensity dependence study fit well within the framework of the current theoretical models; however, the iodine doping and electric field measurements do not. The model predicts a decrease in the rate for samples doped with iodine, and those subjected to a transverse electric field; neither effects were observed. Providing a framework for photoinduced dichroism is important if device level applications are to be realized using the effect. In order for any material to be used in commercially viable applications it needs to be cheap and easy to produce; and it needs to be completely described from an optical and electrical standpoint. A better understanding of photoinduced dichroism in amorphous As2Se3 may lead to the realization of optical switches, optical memory, polarizers, waveguides, and diffraction gratings.