Direct measurement of nonlinear energy deposition from an intense 532-nm photon field into alkali halides

We describe a photoacoustic method for the direct measurement of the energy absorbed by wide-gap optical materials during non-linear interaction with intense laser beams and present results obtained with ultrapure NaCl crystals exposed to 80-ps pulses from a frequency-doubled Nd:YAG laser. The measured temperature rise of the interaction volume depends approximately on the fourth power of the laser peak flux F and exceeds 300 K at F = 1.45 × 1030 photons/cm2s without laser-induced breakdown. We find no evidence of free-electron avalanche formation up to these temperatures and show that the mechanism of energy deposition is four-photon electron-hole pair generation and subsequent single-photon absorption by free electrons and self-trapped holes with small contributions from self-trapped excitons. We report the first photoacoustically measured four-photon absorption cross section: σ(4) = 2 × 10-113cm8s3in NaCl for linearly polarized 532-nm photons. This value is corroborated by σ(4) = 0.94 × 10-113 cm8 s3 obtained in physically similar KBr at 40 K by measuring the recombination luminescence of four-photon-generated self-trapped excitons.