Influence of polariton propagation on spectra of CARS from polaritons

Abstract We analyzed the features of CARS from polaritons caused by the ability of polaritons to propagate over macroscopic distances in a crystal. The polariton propagation is taken into account by retaining both a driven and a free polariton wave in complete solution of the wave equation for polaritons. It is shown that the polariton propagation can broaden CARS spectra. This broadening originates from the fact that polaritons can escape from the excitation region and the value of the broadening depends on the ratio L θ / L p of the dimension L θ of the excitation region along the coherently excited polariton direction and the mean path of coherent polaritons L p . We also find that when the phase‐matching Δ k a = 0 for the overall anti‐Stokes generation process is not fulfilled, an additional peak in the spectrum of CARS may appear even when the polariton damping is appreciable. These predictions are corroborated by our experimental investigations of CARS from polaritons in a BeO crystal. Copyright © 2002 John Wiley & Sons, Ltd.