Influence of iron and neodymium doping on ZnSe crystals' optical properties studied by femtosecond pump-probe spectroscopy

Bulk zinc selenide crystals doped with different elements have been widely used for the laser emission in the middle IR frequency range. Here, we investigate the influence of iron and iron-neodymium volume and surface doping on the optical transmission properties of ZnSe crystals in the visible and NIR range (400-1000 nm) by the linear absorption spectroscopy and nonlinear femtosecond pump supercontinuum probe spectroscopy. We measure and calculate the linear optical transmission and absorption coefficient for different dopant concentrations. These results then used to analyze and model the induced nonlinear absorption pumped by 800 nm femtosecond pulses with the power up to 300 mW. To extract two-photon absorption coefficients, the common theoretical approach is modified for the case of high linear absorption. Our findings reveal that dopant concentration and type of surface-volume doping affect the linear and nonlinear absorption properties of Fe and Nd doped ZnSe crystals, but no direct dependence is found. At certain concentrations, the homogeneity and lattice of the crystal can be damaged causing defects arise and interaction between impurities, which strongly influence on the structure and properties of doped crystals. These results could enable the integration of such crystals into the field of NIR femtosecond optics and terahertz phonics as perspective components.