Z-scan investigation of third-order optical nonlinearities in second-harmonic-generation crystals

Abstract

KTiOP04 (KTP), KTiOAs04 (KTA), /2-BaB204 (BBO), LiB305 (LBO) and LiNb03 are important nonlinear optical materials for second harmonic generation (SHG) and optical parametric oscillation (OPO). We present a comprehensive investigation on the two-photon absorption (TPA) and bound electronic Kerr nonlinearity in these crystals using a picosecond, 532-nm-wavelength laser beam. By using Z-scan technique the nonlinear refractive indexes and two-photon absorption coefficients in the z-cut crystals are measured, as well as in KTP, BBO and LBO along the phase-matching angles for the SHG of 1064-nm radiation. The obtained TPA-coefficient and nonlinear-refractive-index data for the SHG crystals have extended the database of third-order optical nonlinearities in nonlinear optical crystals. The microscopic origin of the observed refractive nonlinearity can be understood in terms of bound electronic effects, and the theoretical predictions are in agreement with our measurements. Finally, we introduce an experimentally simple method to study the laser-induced damage in these crystals by extending the Z-scan technique. The measured damage threshold is inversely proportional to the nonlinear refractive index and the thickness of a crystal. The effect of self-focusing on laser-induced damage in these crystals is also discussed.