Two symmetric four-wave mixing signals generated in a medium with anomalous refractive index

Abstract

We report experimental and theoretical results of two symmetrical signals of degenerate four-wave mixing generated in rubidium vapor. Both nonlinear signals are induced by two almost copropagating laser beams, with and wave-vectors, and detected simultaneously in the and directions. In each direction, we observe a single peak when the two beams are tuned on the closed transition 85Rb 5S 1/2(F = 3) → 5P 3/2(F = 4). The excitation spectra reveal a small frequency separation between the two peaks, which is explained when propagation effects are taken into account. Furthermore, our theoretical analysis shows that a correct description of the frequency position of each peak is achieved with a variable refractive index for both lasers, in which the scanning laser experiences an anomalous window in the refractive index near resonance.