Prebreakdown energy absorption from intense laser pulses at 532 nm in NaCl

Details of the photoacoustic measurement and model calculations of prebreakdown energy deposition in NaCl at 532 nm [Jones, Shen, Braunlich, Kelly, and Epifanov, Phys. Rev. B 35, 894 (1987)] are provided. 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 K are attained for nondamaging pulses at 532 nm. Interpretation of the experimental data considering secondary energy absorption by free carriers and transient crystal defects yields the result that the generalized four-photon cross section lies in the range 1×${10}^{\mathrm{-}114}$ ${\mathrm{cm}}^8$ ${\sec }^3$≤${\mathrm{\sigma }}^{\mathrm{}\left(4\right)\mathrm{}}$≤20×${10}^{\mathrm{-}114}$ ${\mathrm{cm}}^8$ ${\sec }^3$. It is further shown that avalanche generation of free carriers is not evident even for these large temperature increases. The calibrated results were obtained from reactive-atmosphere-processed ultrapure NaCl crystals; the importance of sample purity is discussed.