Observation of three-photon absorption and saturation of two-photon absorption in amorphous nanolayered Se/As₂S₃ thin film structures

We have studied the nonlinear optical properties of nanolayered Se/ As2 S3 film with a modulation period of 10 nm and a total thickness of 1.15 μm at two [1064 nm (8 ns) and 800 nm (20 ps)] wavelengths using the standard Z -scan technique. Three-photon absorption was observed at off-resonant excitation and saturation of two-photon absorption at quasiresonant excitation. The observation of the saturation of two-photon absorption is because the pulse duration is shorter than the thermalization time of the photocreated carriers in their bands and three-photon absorption is due to high excitation irradiance.