Switching from negative to positive nonlinear absorption in p type 0.5 at% Sn doped GaSe semiconductor crystal

The nonlinear absorption properties of p type 0.5 at% Sn doped GaSe crystal have been studied by using open-aperture Z-scan technique under 1064 nm wavelength and 4 ns or 65 ps pulse duration. A switching from negative nonlinear absorption (saturated absorption) to positive nonlinear absorption (two photon absorption) has been observed as the input laser irradiance increases from 0.049 GW/cm2 to 0.106 GW/cm2. The nonlinear absorption coefficient increases monotonically with the increase of pulse duration from 65 ps to 4 ns.