Abstract

Coherent anti-Stokes Raman scattering (CARS) is an important tool in the study of dynamical processes in combustion, aerodynamics, fluid flow, and plasma physics. Frequently, molecular densities are low and available pump and Stokes intensities are limited. To compensate, resonant CARS is often used. To enhance separation of the anti-Stokes signal from the pump and Stokes signals, a folded geometry called BOXCARS is often employed (Fig. 1). In resonant BOXCARS, the pump and Stokes frequencies are chosen so that one-photon and/or three-photon resonances enhance the anti-Stokes signal. These multiple resonances can also be used to simplify spectra in molecules with a complex energy-level structure, such as NO₂. However, large field strengths near resonances can cause populations to change and/or levels to shift and mix (the optical Stark effect). This limits the resonance enhancement and causes deviations from linearity to occur. We report here a systematic study of the effect of these processes and gi^ve the conditions for obtaining optimum CARS signals.

© 1995 Optical Society of America