X. A. Shen

Recent Progress On Laser-Induced Modifications And Intrinsic Bulk Damage Of Wide-Gap Optical Materials

In this paper we provide a comprehensive review of our recent work on the nonlinear interaction between high intensity pulsed laser beams and transparent solids. New experimental techniques used to measure multiphoton absorption and energy deposition in wide-gap alkali halides in the prebreakdown regime have led to hard evidence refuting the avalanche model of laser-induced damage at visible laser wavelengths. These measurements, performed in specially purified materials, have allowed the discovery of the roles of laser-induced excitations in energy absorption, leading to the conclusion that virtually all lattice heating occurs via a nonlinear absorption of laser photons by multi-photon-excited free electrons. These results yield an experimentally confirmed theoretical definition of intrinsic, single pulse laser damage thresholds at 532 nm wavelength in three- and four-photon bandgap alkali halides. Extending this work to multipulse effects in the subthreshold intensity regime, we have formulated a new model of bulk damage based on thermomechanical stress induced by accumulation of multiphoton-generated lattice defects.

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

Prebreakdown temperature increases exceeding 300 K in NaCl exposed to 80 psec pulses at 532 nm are reported along with four‐photon absorption cross sections in NaCl and KBr.

Non-Avalanche Dielectric Breakdown in Wide-Band-Gap Insulators at DC and Optical Frequencies

It is the purpose of this paper to put recent novel experimental efforts to understand dielectric breakdown in wide-band-gap materials in various fields into a common framework and outline significant changes in the understanding of dielectric breakdown at optical frequencies and free electron heating under DC conditions. New experimental techniques used to measure multiphoton absorption and energy deposition in wide-band-gap alkali halides in the prebreakdown regime have led to hard evidence refuting the avalanche model of laser-induced damage at visible laser wavelength. The experiments show that virtually all lattice heating occurs via nonlinear absorption of laser photons by multiphoton excited free electrons. Direct measurements of free electron heating by DC fields in thin SiO 2 -films and direct measurements of electron-phonon scattering rates of energetic free electrons impose a new understanding of carrier heating. The scattering of free electrons with non-polar acoustic phonons is found to be the dominant interaction in preventing the free carriers from reaching energies high enough to cause impact ionization and initiate avalanche breakdown. These results unambiguously show that the role of avalanche breakdown under DC conditions has been overestimated in the past.