Density matrix treatment of photodarkening kinetics in amorphous chalcogenide As_2S_3 thin films

Photoinduced changes in the optical properties of amorphous chalcogenide As(2)S(3) thin films are explained by using the density matrix formalism. The treatment based on the double well potential model for a four-level system gives an improved analytic expression for nonlinear optical absorption coefficients and explicitly shows the break of reciprocity law between illumination intensity and exposure. Photodarkening and photobleaching kinetics obtained theoretically are compared with experimental results obtained in the present work.