Multi-mJ Raman frequency shifter for high-field THz generation in organic crystals

Summary form only given. Present THz source technology, with peak fields > 1 GV/m, are realized by optical rectification (OR) of femtosecond pulses in organic crystals [1-3]. The main challenge for such source is the need of high-energy pump laser at non-standard wavelengths (1.2-1.5 μm) for the optimal phase matching conditions. Ti:sapphire driven optical parametric amplifiers are the most common technique to shift the spectrum towards longer wavelengths. These converters, however, suffer for limited pulse energy and poor output beam quality which are detrimental for further upscaling of the THz sources. Here we report high-efficiency and high-energy frequency down-conversion by stimulated Raman scattering (SRS) in high-pressure H2[4]. In the experimental setup a femtosecond Ti:sapphire laser amplifier delivering 30 mJ pulses at 100 Hz stimulates the Raman process [5]. The laser spectrum is centered at 820 nm with bandwidth of 30 nm. A double-pulse pumping scheme of the active medium is used in the SRS. Two orthogonally polarized pulses with equal energy are delayed by 250 ps prior to the Raman cell. The first pulse generates a coherent phonon wave in H2, on which SRS of the delayed replica occurs with high efficiency. In order to limit parasitic effects we employ chirped laser pulses of hundred ps to seed and pump the SRS. After amplification the Stokes pulse at 1.28 μm is re-compressed by means of a gratings compressor.