Photoacoustic method for orientation and optical characterization of nonlinear crystals

Second-harmonic conversion of the 532-nm pulsed output of a doubled Nd:YAG laser in KDP was characterized by analyzing the changes in the acoustic signals generated in the crystal at different orientation conditions and for several incident fluences. Using a piezoelectric transducer, phase-matching condition was determined by maximizing the amplitude of the acoustic signals detected. The angular orientation for optimum harmonic efficiency was obtained with the same precision compared with the conventional optical procedure. The origin of the photoacoustic signals is the relaxed energy following the absorption of 266-nm photons. To determine the mechanisms of the 266-nm absorption processes, we also performed experiments under direct illumination with the 266-nm emission of the quadrupled Nd:YAG laser. A combination of a linear and nonlinear process occurs. Direct absorption by KDP as well as the participation of transient defects produced in the material were analyzed. Received: 27 July 1998 / Revised version: 19 March 1999 / Published online: 19 August 1999