Near resonant nanosecond laser driven nonlinear optical response in As50S50 thin films

Nanosecond near resonant excitation in As50S50 thin films leads to strong nonlinear optical response, i.e. nonlinear absorption coefficient up to 4 x 106 cm/GW and nonlinear refractive index of 8.5 cm2/GW, both of which is the strongest ever reported in amorphous semiconductors. We propose a five-level energy model to explain such effect which indicates that nonlinear process is reverse saturable absorption in nature, mediated by excited state absorption from triplet-triplet transition. On the other hand, observation of negative nonlinear refractive index reveals the occurrence of self-defocusing effect. Finally, benefitting from the strong nonlinear response, we demonstrate a promising application of As50S50 thin films as an optical limiter for optoelectronic sensors.