Investigations in the nonlinear behavior of dielectrics by using ultrashort pulses (Best Oral Presentation)

Ultra-short pulse systems are considered as innovative laser sources for a variety of applications in micro material structuring, medicine and diagnostics. Current commercial systems are still lacking in output power limiting the throughput and the economic efficiency within a production line. In the optimization of ultra short pulse sources of the next generation, special effects in optical components during interaction with ultra-short pulses play a major role. Especially, low damage thresholds and non-linear absorptance have already been observed within the activities of the EUREKA-project CHOCLAB II, which are concentrated on the evaluation of multiple-pulse damage and the absorptance of fs-optical components according to the International Standards ISO 11254-2 and ISO 11551. In this paper, a theoretical model on the basis of photo- and avalanche ionization is presented describing the incidence of damage as a consequence of a sufficient high density of conduction band electrons. Furthermore, the influence of the Kerr-effect and conduction band electrons on the optical properties of dielectrics is investigated theoretically. From our calculations, a significant increase in reflectance due to the dominant Kerr-effect can be deduced as well as a noticeable increase in absorptance induced by free electron heating already at energy density values clearly below the damage threshold. Finally, results of an experimental investigation in the influence of the internal field strength in a dielectric layer stack on the damage threshold are described. The experiments clearly support the assumption already stated in other publications, that the field intensity formed by the optical design plays a key role for damage resistance of optical coatings for ultrashort pulses.