We present the results of experimental and numerical studies of filamentation of high power, mid-IR laser pulses in different transparent solids. In the experiments, 12-mJ, 80-fs pulses at the central wavelength of 3.9 μm were generated by the sub-terawatt OPCPA laser system operating at 20 Hz repetition rate [1]. The measurements were performed in both multiple- and single-filament regimes. In multiple filamentation measurements, the «12 mm-diameter beam with up to 6 mJ energy (∼103 Pcr) was loosely focused by a f=1.5 m focusing mirror. The samples were placed in the converging beam 60 cm before the geometrical focus. In the single-filament case, the 5 mm-diameter beam was focused by a f=75 mm lens on the entrance surface of a sample while the energy of the pulses did not exceed several μJ. Filaments where generated in a 1cm or 5cm long CaF2 and YAG crystals, as well as in a 1cm long Sapphire and a 1 cm long LiSGaF crystals. The spectra measured after the filamentation in 5 cm long CaF2 crystal are presented in Fig.1. Very surprisingly, for all materials in both single- and multiple-filaments regimes we did observe neither significant spectral broadening around the fundamental wavelength nor continuum generation in the near-IR-visible spectral range. Instead, strong UV emission at different wavelengths for different materials was detected. Conversion efficiency ∼10-3 from the fundamental mid-IR pulse to the 390 nm emission in the 5 cm long CaF2 rod was measured with the UV spectral intensity at least an order of magnitude higher than the intensity of the third harmonic.
展开▼
