Spectral line-narrowing and saturation effects in fully resonant nondegenerate four wave mixing

Abstract

Pentacene doped benzoic acid crystals are used to study the line-narrowing capability of fully resonant four wave mixing for inhomogeneously broadened transitions. Both parametric and nonparametric nonlinear spectroscopies are used to scan the vibronic resonances of excited electronic states while maintaining resonance with a vibrational mode of the ground state and the vibrationless level of the excited state. The observed vibronic transitions correspond to modes that are coupled to the ground state mode. The spectra contain both lines that are narrowed and lines that have the full inhomogeneous width. Narrowing of the vibronic transitions is observed when site selection is performed on the vibrationless electronic transition in both parametric and nonparametric methods. Narrowing of the vibrational resonance is not observed under the same conditions showing that the vibrational and electronic inhomogeneous broadenings are not correlated. The shifts of the line positions and the changes in linewidths and relative intensities were measured across the inhomogeneous linewidth for both methods. The nonlinear signal intensity was also measured as a function of laser intensity. All of the results are consistent with a theoretical description where saturation effects are dominant.