As₂Se₃ films were deposited on silicon substrate by thermal evaporation due to the robust stoichiometry of bonding in this glass (Pressure < 2.10^-5 Torr, deposition rate ~ 100 Å/s). They were annealed for 2 hours at 180 ^oC under vacuum before measurements.

The samples were illuminated in nitrogen atmosphere with a HeNe laser (633 nm) or an Argon laser (514 nm). The area of illumination was large enough to enable ellipsometric measurement on a homogeneous area while the intensity of the illumination was kept similar to the cutback measurements. The illumination time was ~ 2h, much longer than the characteristic time determined by the cutback measurements for films of similar thickness (~ 500 nm). The sample refractive index was then measured for three different incidence angles (72, 73 and 74 degrees) in the range 800 nm – 1800 nm. A filter was used to cut all wavelengths below 800 nm that could induce photobleaching or photodarkening of the film during the measurement.


Such cycle was repeated three times for each wavelength of illumination to ensure reproducibility. To extract the thickness of the deposited film, the ellipsometric parameters were fitted using the Levenberg-Marquardt regression method with a Cauchy-like dispersion relation:

where λ is the wavelength and A,B,C,D,E,F are fitting parameters. The deduced dispersion relation was used to generate starting values in the Levenberg-Marquardt regression method to calculate the n and k values at every wavelength (independently of any model).

The following graph shows the increase in the real part of the refractive index (n) of As₂Se₃ when illuminated at 633 nm and 514 nm due to photodarkening. The imaginary part of the refractive index (k) was too low to accurately measure any change but over all did seem to increase with illumination. The darker curves below represent the experimental data for three cycles in the annealed (blue) and photodarkened (red and green) states while the lighter curves are just the average of these three curves for each state. This graph clearly shows that the annealed state can be recovered upon thermal annealing at the glass transition temperature and that the index change induced by photodarkening is very reproducible. At 1.55 microns, the index change was equal to 0.04 (1.44 %) and 0.085 (3 %) for illumination at 633 nm and 514 nm, respectively.