LASER-INDUCED MODIFICATIONS AND THE MECHANISM OF INTRINSIC DAMAGE IN WIDE GAP OPTICAL-MATERIALS

A brief outline of our previous investigations into the mechanism of intrinsic bulk laser induced damage and our main conclusions serve as the introduction to exciting new results obtained by Shen et al. [Phys. Rev. Lett. (June 5. 1989)]: who measured the free electron absorption of 1064 nm photons photoacoustically in NaCl and SiO2. The electrons were generated with a 266 nm pump pulse by two- or three-photon transitions from the valence band. For a given pump energy which is chosen sufficiently low to avoid heating of the interaction volume by itself, the dependence of the photoacoustic signal and temperature rise were observed as a function of the energy of the 1064 nm pulse and found to be in agreement with the theory of free carrier heating by Epifanov et al. Temperatures approaching the melting point were produced in the materials under prebreakdown conditions. This work provides direct experimental proof that free electron absorption is the primary mechanism for heating wide-gap optical materials by intense laser pulses in the visible and near infrared, confirming the indirect measurements reported earlier by us. No evidence of electron impact ionization and ensuing avalanche formation is found up to the intrinsic damage threshold, throwing doubt onto the commonly held belief that it is responsible for free electron generation and eventual breakdown at 1064 nm. © 1990.