Photoacoustic characterization of transient defects in potassium dihydrogen phosphate crystals

Abstract

Transient defects in potassium dihydrogen phosphate (KDP) were characterized by using the acoustic signals generated in the crystal when it is impinged with pulsed laser radiation. These defects are produced by simultaneous absorption of two λ=266nm photons and they show linear absorption in the visible and UV spectral region. The decay kinetics of the defects has been studied by a new method based on the analysis of the acoustic signal generated by visible pulses. The acoustic measurement of the decay time shows a nonexponential decay and it is free from thermal lensing or beam deformation by other causes, effects that can alter the pure optical measurements. We propose that the origin of the photoacoustic signal is the heat released by the deexcitation of the energy levels of the defects when they are excited by visible pulses. This mechanism, optical absorption and nonradiative relaxation of defects, could be the reason for some depletion in the yield of several devices based on KDP. This phenomena must be carefully taken in account, when KDP crystals are used in combination with Nd:YAG (YAG, yttrium aluminum garnet) lasers for second-harmonic generation from λ=532nm to λ=266nm.