Optical nonlinearities in direct and indirect band gap crystals on the -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 n 2 ( w ) , the total free carrier absorption cross-section ( σ ab ) and the change in the refractive index per unit carrier density ( σ nr ) are determined in both direct and indirect band gap crystals. Smaller values of ( β ) and ( σ ab ) in the case of indirect band gap crystals have been attributed to phonon assisted transitions, while the smaller value of n 2 ( w ) and ( σ nr ) is due to the rise in saturation taking place in the absorption.