Defect generation in fused silica under high intensity laser interaction

Interaction of high intensity laser radiation with fused silica mainly introduces strong temperature increase in the region where this radiation is absorbed. The local temperature rises up to 2500 K or higher. At these temperatures high concentrations of thermal intrinsic point defects are generated. Estimated concentrations of equilibrium oxygen excess defects at 2500 K reaches 7·10¹⁸ cm^-3 and the main part of these defects are non-bridging oxygen atoms. Before equilibrium large part of oxygen excess defects are oxygen molecules. Presence of oxygen molecules in concentration 3·10¹⁸ cm^-3 creates the local internal pressure 1,0 atm. at 2500 K. As the result the boiling and evaporation of the material from the region where high intensity laser radiation is absorbed occurs. Remaining material contains increased concentrations of oxygen deficit point defects, such as three-and two-fold coordinated silicon atoms, as well as oxygen excess point defects which are tightly bonded to the glass network, such as non-bridging oxygen atoms. Presence of these defects causes the strong increase of UV-wavelength laser radiation absorption. This stimulates additional laser interaction with fused silica, New features in defect generation appears when very short (femtosecond) laser pulses are used. In this case the laser pulss length is comparable with the period of main atomic vibrations in fused silica. Such conditions opens new subtreshold intrinsic defect generation mechanisms.