Characterization of signs change of nonlinear refraction in ZnSe based on a modified double 4f imaging system with a phase object

Signs change of nonlinear refractive index in ZnSe is observed by employing a modified double 4f imaging system at the wavelength of 800 nm using picosecond pulses with different pulse energies. This process results from the competition of the bound electronic nonlinear refraction and the free carrier refraction. At low intensity, positive nonlinear refraction is obtained, which is attributed to bound electrons. As the increase of laser beam intensity, the nonlinear refractive index become small, and changes to negative. This is ascribed to free carriers generated by two-photon absorption. Additionally, the nonlinear refractive index of bound electron and the refractive index change of free carrier are determined unambiguously by a simple method.