High repetition rate green-pumped supercontinuum generation in calcium fluoride

Supercontinuum (SC) generation in the ultraviolet (UV) spectral range is a formidable task due to lack of suitable nonlinear solid-state materials, since spectral broadening toward shortest wavelengths is constrained by low-order nonlinear absorption and large chromatic dispersion, see e.g. [1] . In that regard, the largest spectral blue-shifts are attained in alkali metal fluorides, which exhibit the largest energy bandgaps among solid-state dielectric materials, and the most reliable performance was achieved in calcium fluoride (CaF 2 ) crystal [2] – [4] . However, although CaF 2 formally exhibits high optical damage threshold as exposed to a single femtosecond laser pulse [5] , it suffers from heat accumulation and color center formation that lead to rapid optical degradation and eventually optical damage under irradiation with repetitive pulses. Therefore to produce stable performance in a standard SC generation setup (relatively tight focusing into a few mm long sample), the material has to be continuously translated or rotated with respect to the incident laser beam even at fairly low (e.g. 1 kHz) repetition rates.