The multiphoton photoconductivity properties of ZnSe and CdSe direct band gap crystals have been studied using frequency doubled Nd:YAG laser excitation. Two-photon absorption in both the crystals has been observed. The photoconductivity measurements and nonlinearity in Z-scan traces confirms the existence of lower valence band, from where one-photon transitions are forbidden but two-photon transitions are allowed. The value of two-photon absorption cross-section 2 in both crystals has been calculated by using second order time dependent perturbation theory. gap is lower than photon energy of a laser. In this chapter, studies on the charge (Q) induced inside the crystals by photoconductivity mechanism and optical nonlinearities using the Z-scan technique for ZnSe and CdSe direct band gap crystals in open and closed aperture have been carried out.
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