Saturable absorption of Bi_2-XSb_XTe_3-YSe_Y quaternary solid solutions

Nonlinear light absorption in variously thick films of Bi_2-xSb_xTe_3-ySe_y quaternary solid solution crystalline grown on sapphire substrates by means of MOCVD [1] was investigated. We measured the transmission coefficient of laser pulses through samples as a function of the incident laser radiation intensity. A pulsed Nd³⁺: YAG laser was used as a source of laser radiation (wavelength λ=1064 nm, duration of pulses τ≈35 ps, pulse repetition period 7 ns). The nonlinear absorption in Bi_1.25Sb_0.75Te_1.65Se_1.35 films was observed starting from the saturation intensity I_sat≈50 MW/cm². The data can be explained by a process of filling states in a quantum well [2]. We have not observed the nonlinear absorption in two of the investigated films and, moreover, these films were destroyed at a high radiation intensity. An earlier work [3] demonstrated a change in the type of conductivity of a semiconductor film to metallic at a high radiation intensity. Since metal films are known to be destroyed at high radiation intensities, this phenomenon can explain the destruction of the investigated films. It was also found that a nonlinear change of the transmission of quaternary solid solutions can occur only for compositions with a Sb content of about 2 times less Sb than Bi, and the film thickness should not exceed 12 nanometers.