Nonlinear laser-induced damage and absorptance effects in dielectric coatings by using ultra-short pulses

Abstract

The power handling capability of optical components is still one of the most important limitations for the further improvement of ultra-short pulse lasers in respect of average power and pulse energy. Laser-induced damage of functional dielectric coatings on laser crystals, pockels cells, out-coupling polarizers and compressor gratings is severely inhibiting the wide dispersion of ultra-short pulse laser systems especially in industrial production environments. Since the underlying physical causes for laser-induced damage with ultra-short pulses are distinctly differing from those in the nanosecond time scale, novel approaches must be found for an unambiguous improvement in damage resistance of optical coatings.In previous investigations, the band-gap of the coating material and the maximum field strength in the layer stack were identified as most important influences on the laser-induced damage with ultra-short pulses. Furthermore, a significant nonlinear increase of absorptance in dielectric coatings was found to be strongly related to the band-gap of the material. These effects were traced back to the multi-photon and avalanche-ionization as driving mechanisms for producing a critical conduction band population. In the current investigations, numerous model layer systems were investigated concerning laser-induced damage and non-linear absorptance. Adapting the ion beam sputtering coating process for achieving co-deposition of high and low index materials, coatings with continuously tunable refractive indices were produced. The results of the experiments exhibit a strong correlation of the damage threshold to the controllable shifting band-gaps of the coating materials.