Laser-induced damage studies in optical elements using X-ray laser interferometric microscopy

Results of a novel X-ray laser application, aimed at understanding the microscopic effects involved in formation of laser-induced damage in optical materials exposed to sub-ns laser pulses, will be presented. Specifically, we studied thin plane beamsplitters that are presently the weakest element of the next generation of high-energy lasers (LMJ, NIF), with permanent damage threshold below 20 J/cm(2). Standard fused silica substrates and a model system, containing well-defined micron grooves as seeding sites to trigger damage when irradiated by 438 nm laser pulses, were in situ probed by a neon-like zinc X-ray laser delivering up to 10 mJ at 21.2 nm. The probing beamline employed a double Lloyd's mirror interferometer, used in conjunction with an imaging mirror to provide magnification of similar to 8. In conjunction with an array of in-situ optical diagnostics, one of the questions addressed was whether the damage (transient or permanent) on the rear surface of the beamsplitter occurs during or after the laser pulse, i.e. whether it is due to local electrical fields or to other processes. Another issue, examined by both the X-ray interferometric microscopy and the optical diagnostics, is whether a local rear-surface modification is associated with non-linear effects (self-focusing, filamentation) of the laser beam in the bulk.