Ge/YbF3 and ZnS/YbF3 Thin-Film Combinations Open New Perspectives in Production of Broadband Optics for Mid-Infrared Laser Applications

Recently, the mid-infrared (MIR) spectral region has become more and more interesting for bio-medical laser applications since large organic molecules exhibit their fundamental vibrational and rotational modes in this range[1]. In this rapidly developing research area, high-quality optical interference coatings are highly demanded. In many laser systems (see, for instance [2]), it is required to separate initial high-power near-infrared beams originated from a driving laser from lower power MIR components emerged in a nonlinear crystal through a nonlinear process (for example, frequency difference generation). The corresponding multilayer dichroic filters reflecting the radiation in the near-infrared range and transmitting the MIR radiation are required. The coatings exhibit desired reflectance and phase in a near-infrared range and high transmittance in a required MIR region. The acceptable levels of reflectance and transmittance, width of the high-reflectance and high-transmittance ranges, polarization states as well as the angle of incidence are dependent on the laser setup. Along with the specified spectral performance, non-optical properties such as good adhesion, low mechanical stress, thermal stability, are of outmost importance. Low stress is crucial in order not to deform the laser beam wave front of the entire laser system and to avoid thermal lensing. Therefore, the choice of the appropriate evaporation materials and optimization of the high-vacuum deposition processes on the one hand and selection of optimal design solutions on the other hand are key elements in development of advanced multilayer optics for laser systems.