Significant intensity dependence of anisotropic two-photon absorption in ZnSe and GaP

Abstract

Using polarized femtosecond laser transmission measurements toward (100), (110), (111) ZnSe, and (100) GaP, we show that in a wide laser irradiation intensity range over two orders of magnitude before the laser damage threshold, a periodic amplitude modulation always appears clearly in the angle-dependent two-photon absorption (TPA) curves for (100), (110) ZnSe, and (100) GaP. It also exhibits a close connection between the orientation-dependent TPA and the ionic bonding symmetrical characteristics of specific crystal plane, thus revealing the origin of the orientation dependence of nonlinear absorption in the strong field laser-solid interaction. Interestingly, the modulation amplitudes of orientation-dependent TPA exhibit significant intensity dependence, with the maxima occurring around 1011 W cm−2. For the orientation-dependent nonlinear absorption in solids, such a nonmonotonic relationship between the irradiation intensity and the modulation amplitude has not been revealed previously in the near-damage-threshold regime by direct transmission measurements. In particular, with the intensity approaching the laser damage threshold, the significant descending trend of modulation amplitude confirms the consistent characteristics of saturation or even attenuation for the orientation dependence of TPA in the two crystals, which might be considered as a product of absorption saturation phenomenon of TPA at high irradiation intensity and would provide new understanding about the orientation-dependent TPA in the near-damage-threshold regime.