Nonlinear absorptance of optical parametric amplified ultrashort pulses in dielectric coating materials

Ultra-short pulse laser systems with high peak power densities are increasingly applied in fundamental and industrial research. Furthermore, these radiation sources are also considered as promising tools for innovative applications in the fields of precise micro-machining and medicine applications. For an improvement of production throughput and economic efficiency, the development of femtosecond laser systems with output powers beyond the actual level of about a few Watts is highly demanded. Further progresses in performance are mainly inhibited by the damage handling capability of laser optical components. A promising strategy for an improvement of present fs-optics is the utilization of high band-gap coating materials. Several investigations in modeling of the damage mechanisms in dielectrics were performed recently. Typically, damage occurs if a critical conduction band population was generated by multi-photon and avalanche-ionization during the initial stage of the ultrashort pulse. Nevertheless, the influence of multi-photon excitation and electron donators (color centers) in the band-gap as sources of initial electrons is still unclear. For studying non-linear absorption effects of dielectric coating materials near the transition wavelengths between two orders of multi-photon absorption, a femtosecond laser system equipped with an optical parametric amplifier was utilized providing ultrashort pulses over a wide wavelength range. The laser-calorimetric measurements indicate a drastic change in the non-linear absorptance behavior for the investigated dielectrics. The results underline the dominant role of multi-photon excitation compared to intra-band electron donators for the generation of conduction band electrons in the case of high performance coatings manufactured by ion beam sputtering.