Generation of a femtosecond vacuum ultraviolet optical pulse by four-wave Raman mixing

The pressure dependence of the emission intensities measured for Raman sidebands arising through three-color four-wave difference-frequency mixing (FWRM) was examined using a 200 nm probe beam. Using a three-color FWRM scheme, it was possible to efficiently generate Raman sidebands in the vicinity of the probe beam. By introducing a two-color pump pulse prior to the probe pulse, the coherent vibrational motion of hydrogen was induced immediately after the pump pulse and continued thereafter on a time scale of at least several picoseconds. The intensity of the anti-Stokes sideband increased with increasing hydrogen pressure at low pressures. The ratio of the intensities measured for cascade Raman emissions approached 27% at high pressures. When the fourth harmonic emission was employed as a probe beam, the anti-Stokes emission was observed at 185 nm. The conversion efficiency and the pulse energy were estimated to be 16% and 1.6 μJ, respectively. The conversion efficiency could be increased if a probe laser with a better beam quality were used and even a higher value could be obtained if a pump laser with a higher output power were to be used, in conjunction with a capillary waveguide to increase the interaction length with a Raman-active medium of hydrogen.