Photon absorption of conduction-band electrons and their effects on laser-induced damage to optical materials

The one- and two-photon absorption rates of conduction-band electrons (CBE) in optically transparent materials under the radiation of high-intensity lasers at visible and ultraviolet wavelengths are calculated by the quantum field approach. When both hole-assisted linear absorption and nonlinear absorption of CBE are considered, the rates of total energy gain from the laser field will be enhanced by a factor of 2. The bounding intensity for the avalanche dominating damage process thus decreases by about one-third. The avalanche rate includes not only the term proportional to the laser intensity, but also the terms proportional to the square of the laser intensity and to the product of the laser intensity and hole density. The present result gives a lower damage threshold than previous ones.