Optical nonlinearities in direct and indirect band gap crystals on the Z-scan technique with nanosecond laser

The free carrier optical nonlinearities in direct and indirect band gap crystals have been investigated by the use of a single beam Z-scan technique. The values of two-photon absorption coefficient (β), nonlinear index n2(w), the total free carrier absorption cross-section (σab) and the change in the refractive index per unit carrier density (σnr) are determined in both direct and indirect band gap crystals. Smaller values of (β) and (σab) in the case of indirect band gap crystals have been attributed to phonon assisted transitions, while the smaller value of n2(w) and (σnr) is due to the rise in saturation taking place in the absorption.