Two realizations of the time-resolved pump-probe method have been applied to study comparatively the nonlinear optical response of chalcogenide glasses of the system As-S-Se illuminated by femtosecond laser pulses near their fundamental absorption band edge and near the two-photon absorption band edge. In such conditions, charge carrier’s photo-excitation is going on with participation of gap states. By comparison of the results of measurements and numerical modeling, we have demonstrated that character of the nonlinear optical response depends on how a glass samples was illuminated. If the pump pulse frequency corresponded to the Urbach tail of a glass sample, the carriers trapping and excitons formation was followed by the excitons absorption and excitation of conduction electrons. If a sample was illuminated at the red-edge of its Urbach tail, the photo-excitation was going through the gap states, but excitation of conduction electrons was followed by excitons formation. Due to these peculiarities, calculated spacial distribution of the photo-induced refractive index variation in the sample was different depending on the illumination conditions.
Elena A. Romanova, Andrei V. Afanasiev, Alexander P. Velmuzhov, Maxim V. Sukhanov, and Vladimir S. Shiryaev, "Role of gap states in chalcogenide glass response to photo-excitation by the high-intensity femtosecond laser pulses," Proc. SPIE 10683, Fiber Lasers and Glass Photonics: Materials through Applications, 106832F (Presented at SPIE Photonics Europe: April 26, 2018; Published: 17 May 2018); https://doi.org/10.1117/12.2306582.
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