Laser-induced damage of transparent solids by femtosecond laser pulses

Abstract

Our paper is devoted to theoretical analysis of mechanisms of laser-induced dmaage of transparent solids by femtosecond laser pulses in single-shot regime. The duration of the pulses is so small that the phonon sub-system practically does not take part in the processes occurring during the direct action of laser pulse. It means that the process of direct damage starts with a certain delay after the laser pulse. We have come to conclusion that it is reasonable to separate out three main stages of the process of macroscopic damage: 1) the direct laser-solid interaction during pulse action including mulitphoton absorption, excitation of the electron subsystem near the material surface and fast leaving of the irradiated area by electrons (e.g., through photoelectron emission); 2) fast after-action including breaking of electrical neutrality in thin near-surface layer and acceleration of ions; 3) slow or delayed after-action including moving of fast ions into bulk accompanied by heating up of the material through collisions resulting in macroscopic thermal damage. In this presentation we focus on the first two stages, i.e., excitation of the electron sub-system, electron emission and development of electrostatic instability often referred to as Coulomb explosion. Estimations performed on the basis of the Keldysh formula show possibility to reach extremely high density of electrons in conduction band (up to 50% of total number of valence electrons) at laser intensity slightly above 10 TW per sq. cm. The electrons can leave the irradiated area before the laser pulse ends. We utilize Keldysh formula to estimate the total number of electrons lost through emission and show the number to be high enough for significant breaking of electrical neutrality and fomration of relatively large positive charge localized in the irradiated area. Assuming the multiphoton ionization to give the dominant contribution to absorption, we estimate the total number of electrons lost through emission and show the number to be high enough for significant breaking of electrical neutrality and formation of relatively large positive charge localized in the irradiated area. Assuming the mulitphoton ionization to give the dominant contribution to absorption, we estimate the characteristic thickness of the ionized layer and show the positive charge to be localized in the layer which is approximately 1 micrometer thick. Then we estimate velocities and energies of ions accelerated by the laser-induced charge and show possibility of appearing ions with MeV energies. The penetration depth characteristic of those ions is an order of 10 micrometers what implies possibility of heating and thermal damage of the material with formation of deep craters.