The values of the damage thresholds of some alkali halide crystals, sapphire and fused quartz at λ = 0.266 μm are presented.
The role of electron avalanche in the damage of real wide-gap dielectrics in the UV region is discussed.
Some peculiarities of the interaction of intense UV radiation with dielectrics (nonlinear absorption, photoionization of impurities, self-focusing and others) are considered.
Experiments on laser damage induced by two synchronous, crossed laser beams with different frequencies (λ1 = 1.06 μm and λ2 = 0.266 μm) are described. This method makes it possible to establish the influence of “seeding” electrons on the development of laser-produced damage, as well as to determine if the damage mechanisms are similar at different frequencies.
In order to record directly and estimate concentrations of the free carriers excited by UV laser radiation, d.c. photoconductivity experiments have been carried out.
The results of a theoretical analysis of the electron avalanche process at high frequencies of the electromagnetic field are presented. The dependence of the avalanche rate on radiation intensity and of the critical field on frequency are obtained for nanosecond and picosecond pulse durations. The relative role of electron avalanche and multiphoton ionization in laser damage of transparent dielectrics is discussed and it is shown that it is strongly influenced by the pulse widths.
Peculiarities of laser-induced damage in transparent dielectrics caused by electron avalanche in the case of deterrent lack of seed electrons are discussed. Statistical models of the avalanche process initiated by multiphoton ionization of host atoms or impurities are described. Based on the breakdown probability expressions derived, the damage threshold dependence upon the temperature and the focal spot size are discussed. The theoretical results are found to be in good qualitative agreement with the experimental data for the laser-induced damage in alkali-halide crystals.
We present direct experimental evidence that significant amounts of energy can be deposited in NaCl by short laser pulses of 532 nm wavelength without electron avalanche formation. The mechanism is four-photon free carrier generation and free carrier absorption with contributions by self-trapped holes. We also report the first reliable measurements of four-photon absorption cross sections in any wide-gap ionic solid.
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