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 k ⃗ a and k ⃗ b wave-vectors, and detected simultaneously in the 2 k ⃗ a − k ⃗ b and 2 k ⃗ b − k ⃗ a 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.

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 ka and k b wave-vectors, and detected simultaneously in the 2 ka − k b and 2 k b − ka directions. In each direction, we observe a single peak when the two beams are tuned on the closed transition 85 Rb 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 obtained only if coherent effects such as electromagnetically induced absorption are included.

Two symmetrical forward four-wave mixing processes in rubidium vapor present an opposing frequency shift. We model the effect with an anomalous refractive index due to the nonlinear interaction of the incident fields in the medium.