Contributions of multiphoton absorption to laser−induced intrinsic damage in NaCl

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Abstract
Intrinsic damage of NaCl upon exposure to intense laser pulses is discussed in terms of electron kinetic processes. Avalanche ionization is confirmed to be the dominant mechanism at the ruby frequency. However, compared to pure avalanche breakdown, both five−photon absorption and, to a lesser extent, the presence of F centers reduce the damage threshold by providing a large concentration of free carriers at the onset of avalanche ionization. Three−photon absorption contributes extensively to damage at the doubled ruby frequency. The dependence of the damage threshold on the laser pulse length is calculated and it is shown that its measurement can provide information on the role of multiphoton absorption in the damage process. At a given wavelength multiphoton contributions become increasingly more important as the laser pulse length decreases.
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