Knowledge Bank

With the use of background-rejection $techniques(1)$, the minor species detectivity of coherent anti-Stokes Raman scattering (CARS) is no longer limited by the nonresonant back-ground susceptibility. The CARS detectivity increases with increasing laser powers, limited now only by the perturbing effect of very high optical power $densities.(2)$ In this paper we report investigations of background free CARS near the limits imposed by stimulated Raman scattering and dielectric breakdown. We report a new effect on CARS linewidths associated with very high optical power densities: We observed a marked asymmetric broadening of spectral linewidths which depends on the laser power. This effect is consistent with preliminary calculations of the optical Stark effect on vibrational transitions. CARS spectra from a gas cell and a methane diffusion flame are presented, demonstrating the perturbations caused by high power densities, as well as the high detectivity and spatial resolution made possible by the crossed-beam, background rejection configuration.