Frequency and focal volume dependence of laser-induced breakdown in wide band gap insulators

The frequency and focal volume dependence of the laser induced breakdown fields of selected wide band gap insulators were investigated. Measurements were conducted with pulsed (31 to 140 nsec), simple spatial mode lasers at wavelengths of 10.6, 3.8, 2.7, and 1.06 microns. The results indicate that multiphoton assisted electron avalanche breakdown is a likely mechanism for laser induced failure in these materials over the wavelength range studied. For small focal volumes, volumes smaller than the inverse of the free electron density, the frequency dependence of breakdown is determined by multiphoton ionization of impurity levels. This initiates an avalanche and leads to the observation that breakdown fields decrease with increased laser frequency. For large focal volumes, the free electrons initially present in the crystal initiate the breakdown, and the frequency dependence is determined by the electron phonon relaxation time. Thus, for large focal volumes, the laser induced breakdown fields increase with increasing laser frequency. From these results it is clear that crystal defects and impurities play an important role in what has been called "intrinsic" damage.