Calculation of saturation line shapes and intensities in coherent anti-Stokes Raman scattering spectra of nitrogen

Abstract

Line shapes and intensities for coherent anti-Stokes Raman scattering (CARS) at saturation laser intensities are determined for nitrogen vibrational Q-branch lines by solving the time-dependent density-matrix equations numerically. We have previously performed measurements of saturated CARS line shapes in pure nitrogen by using nearly Fourier-transform-limited pump and Stokes lasers. The experimental laser pulse shapes, Stark effects, Doppler broadening, and the nonresonant background are incorporated in the numerical calculations. The numerical results show good agreement with the high-resolution measurements of saturated CARS line shapes. The lines show prominent saturation dips, and agreement between theory and experiment is excellent in terms of the depth and the width of the dips. The numerical results indicate that the Doppler effect tends to broaden and to deepen the dip in highly saturated lines, an effect that cannot be explained by a steady-state theory.