A new regime of femtosecond mid-infrared filamentation in transparent solids

Summary form only given. 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 P_cr) 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. In the numerical simulations we solve 1D Maxwell´s equations using a Pseudo-Spectral-Space-Domain (PSSD) technique that includes full material dispersion and nonlinearity [2]. The numerical results for the CaF2 crystal, shown in Fig.1b, agree very well with the experimental measuremen- s. Numerical modeling reveals that the observed UV emission is qualitatively analogous to dispersive wave emission from 1D solitons but here induced by the filaments [3]. The input pulse undergoes hardly any spectral broadening due to SPM or other nonlinear effects. Around z=0.8 mm pulse self-steepening forms a shock front and resonantly transfers energy into the peak at ω=4.8 rad/fs, i.e. λ=390 nm, as it is observed in the experiments. We underline complete absence of features of super-continuum generation and the negligible pump reshaping aside from the relatively efficient formation of the UV band. This scenario is confirmed by the 3D numerical simulations (Fig.1c) describing full beam dynamics of ultrashort mid-IR pulses in solids through 3D supercomputations in parallel codes [4]. Similar agreement between numerical simulations and experimental measurements were found for all materials tested.