Absorption spectra and nonlinear transmission (at $\lambda=2940$ nm) of a diffusion-doped Fe$^{2+}$:ZnSe single crystal

Transmission spectra of a ZnSe sample diffusion-doped with Fe$^{2+}$ ions have been measured in the wavelength range 500–7000 nm. A broad absorption band in the range 500–1500 nm has been observed in both doped and undoped regions of the sample. This band is possibly due to deviations from stoichiometry in the course of diffusion doping. The transmission of the Fe$^{2+}$:ZnSe sample at a wavelength of 2940 nm has been measured at various dopant concentrations and high peak pulse intensities (up to 8 MW  cm$^{-2}$). The samples have been shown to be incompletely bleached: during a laser pulse, the transmission first increases, reaches a maximum, and then falls off. Our results suggest that the incomplete bleaching cannot be accounted for by excited-state absorption. The incomplete bleaching (as well as the transmission maximum) is due to the heating of the sample, which leads to a reduction in upper level lifetime and, accordingly, to an increase in absorption saturation intensity.