| Abstract [eng] |
Supercontinuum generation is an effect produced by filamentation of ultrashort laser pulses in nonlinear media and resulting in a broadband coherent radiation. At present, there is an increasing demand of coherent broadband light sources in the ultraviolet spectral range. However, supercontinuum generation in ultraviolet faces many challenges such as lack of suitable non-linear materials and optimization of experimental settings. This work aims at a comparative study of the specific features of ultraviolet supercontinuum generation in sapphire, BaF2 and CaF2 crystals pumped by second harmonic pulses from Yb:KGW laser (having a repetition rate of 10 kHz) in tight and loose focusing conditions. Firstly, 5 mm--long sapphire, 6 mm--long BaF2 and 5 mm--long CaF2 plates were pumped using tightly focused (NA~=~0.012) pulses with an energy of 0.17uJ, 0.2uJ and 0.4uJ, respectively. In the second experiment 15 mm--long sapphire, 25 mm--long BaF2 and CaF2 crystals were pumped with loosely focused (NA = 0.004) pulses with an energy of 0.34 uJ, 0.3 uJ and 0.48 uJ, respectively. We observed spectral broadening down to ~380 nm, ~310 nm and ~300 nm in sapphire, BaF2 and CaF2, respectively. Also supercontinuum spectrum broadens more symmetrically with loosely focused pulses. Experiments show that supercontinuum spectrum shrinks rapidly under tight focusing conditions in short sample and shrinks much slower (~30 times in CaF2 crystal) using long sample and loose focusing conditions. This is likely due to 1.7 times lower maximum plasma density and ultimately lower number of long-lived color centers that can rapidly evolve into optical damage of the material due to laser pulses at high repetition rates. |
