Picosecond VUV pulse generation by dual-wavelength-pumped Raman-resonant FWM

In recent papers we proposed dual-wavelength-pumped Raman-resonant four-wave mixing (FWM) where the primary pump (P1) energy can be concentrically converted into its anti-Stokes field AS1(P1), or vice versa, under the phase-matched wave-vector configuration shown in Fig. 1. In this paper we show theoretically that this process is applicable to the generation of intense picosecond pulses in the VUV region, where most of the pulse-length-control devices, such as Pockels cells and saturable absorbers, cannot be used. A straightforward way to generate picosecond radiation by this process is to induce the Raman conversion of P1 during a short temporal period by use of a picosecond P2 pulse. However, it is better for the postamplification by a gain medium to generate a short pulse at the same wavelength as P1, rather than at its Raman-shifted wavelengths. For this purpose we propose to arrange the wave-vector configuration of the incident fields as shown in Fig. 2. If the P1 pulse, assumed to be in the VUV region, is intense enough for the Raman conversion by itself, the S1(P1) can grow without P2. When a picosecond P2 pulse with a longer wavelength is applied in the direction shown in Fig. 2 after the buildup of the S1(P1) wave, the P1 field will rapidly rise again because of the onset of FWM, but its direction is different from that of the initial P1. After the passage of the P2 pulse, the split P1 emission will rapidly deplete again because of damping by the remaining phonon that drives P1-to-S1(P1) conversion. As a result, one obtains a picosecond P1 pulse spatially separated from the incident P1 pulse. A typical numerical result for the temporal pulse shapes for P1-chain components after 20 cm of propagation in a 20-atm molecular-hydrogen medium is shown in Fig. 3. We assume an F₂ laser for P1, with a wavelength of 157.6 nm, and a Kr laser for P2, with a wavelength of 248 nm. The initial peak intensities of the P1, P2, and S1(P1) seed pulses are 40, 300, and 4.5 MW/cm², respectively. All the initial pulse shapes are hyper-Gaussian, and the pulse widths of initial P1, P2, and S1(P2) pulses are 645, 17, and 645 ps (FWHM), respectively.