Photoacoustic Measurement of Pre-Breakdown Nonlinear Energy Absorption from Intense Laser Pulses at 532 NM in Sodium-Chloride

The pulsed photoacoustic technique is employed in the measurement of pre-breakdown absorption of 532 nm laser photons in NaCl. The primary absorption process is shown to be four-photon excitation of valence electrons to the conduction band. Calibration of the acoustic system by two-photon absorption of 266 nm laser light indicates focal point temperature increases over 300(DEGREES)C are attained for nondamaging pulses at 532 nm. Interpretation of the experimental data considering secondary energy absorption by free carriers (or polarons) and transient crystal defects yields the result that the generalized four-photon cross section lies in the range 1 x 10('-114) cm('8) sec('3) (LESSTHEQ) (sigma)('(4)) (LESSTHEQ) 20 x 10('-114) cm('8) sec('3). It is further shown that avalanche generation of free carriers is not evident in these results. The calibrated results were obtained from reactive-atmospheric-processed ultrapure NaCl crystals; the importance of sample purity is discussed. A theory of acoustic pulse generation by multiphoton absorption for sharply focused pulses of Gaussian profile is also presented.