Optical Stark effect in the four-wave mixing and stimulated Raman spectra ofN2

The influence of the optical Stark effect on spectral line shapes in four-wave-mixing Raman spectroscopy (FWMRS) and stimulated Raman spectroscopy (SRS) is investigated experimentally and theoretically. While both high-resolution spectroscopic techniques commonly use focused Gaussian beams with rather high laser intensities, the contribution of the optical Stark effect to the line shapes has not been well explored. Using an experimental setup, capable of rapid alternation between the simultaneous measurement of coherent Stokes Raman spectroscopy and inverse Raman spectroscopy spectra at low (mild-focusing) and high (tight-focusing) intensities, together with a sophisticated frequency reference scheme, we are able to perform a rather direct comparison between Stark-broadened and non-Stark-broadened spectra of both classes of Raman spectroscopies. Experimental spectra of lines in the vibrational Q branch of molecular nitrogen (${\mathrm{N}}_2$) are obtained for low and high pump-laser peak intensities in copropagating focused Gaussian beams. At high pump intensities, the optical Stark effect leads to strong asymmetric broadening for both types of coherent Raman spectroscopy. Our experimental results demonstrate that SRS spectra show more Stark shift and broadening than their FWMRS counterparts. Using the theory of signal generation in focused Gaussian beams, we have generated theoretical Stark-broadened spectra for both spectroscopies. While the experimental SRS spectra show excellent agreement with theoretical calculations, simultaneously measured FWMRS spectra do not perfectly agree with our theory. We demonstrate that this discrepancy can be partly reconciled by the inclusion of a resonant fifth-order process in the commonly employed third-order theory of FWMRS.