New regime of fs-pulse stimulated Raman scattering

We present the results of experimental and theoretical studies of fs-pulse Raman-frequency conversion in gases in the regime of medium pumping with pulses shorter than the molecular vibrational period T_v. The spectral transformation of both intense excitation pulses and variably delayed injection pulses is analyzed as a function of laser parameters and gas pressure. For intense 40 fs pump pulses propagating in a SF₆-filled hollow waveguide, we observed a continuous (pressure-dependent) redshift of the carrier frequency, amounting to several thousand cm^у. By transmitting a weak injection pulse with a narrow spectrum through impulsively excited SF₆, we observed multiple Stokes and anti-Stokes lines of the injection radiation corresponding to scattering on the symmetric vibrational mode of SF₆. Since the pumping and scattering processes are decoupled in this delayed-pulse scheme, the effect of Kerr self-action on the injection field scattering was completely eliminated. Our physical model of the fs-pulse stimulated Raman scattering is in good quantitative agreement with the observed effects.