Picosecond z-scan investigation of two-photon absorption and bound electronic self-focusing in second-harmonic generation crystals

Abstract

We present our investigation of two-photon absorption and self-focusing in second-harmonic-generation crystals on a picosecond 532-nm-wavelength beam using the z-scan technique. By measuring the crystal transmittance as a function of the crystal position with respect to the focus, the two-photon absorption coefficients were determined to be 0.041 plus or minus 0.008, 0.10 plus or minus 0.02, and 0.24 plus or minus 0.06 cm/GW for KTPA, KTA, and LiNbO 3, respectively. With inserting an aperture in front of the detector that recorded the transmitted laser pulse energy, self-focusing effects manifested themselves in the z scans on the three crystals at input irradiances of higher than a few GW/cm 2. The quantitative data confirm that the optical Kerr nonlinearity should be responsible for the observed self-focusing and n 2 equals (4.6 plus or minus 0.9), 3.6 plus or minus 0.7), and 5.3 plus or minus 1.0) multiplied by 10 -6 cm 2GW in KTP, KTA and LiNbO 3, respectively. The microscopic origin of the measured n 2 can be understood in terms of bound electronic effects. The theoretical predictions are in agreement with our measurements. Finally, effects of the observed refractive nonlinearity on laser-induced damage are discussed.