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

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 ${\vec{k}}_a$ and ${\vec{k}}_b$ wave-vectors, and detected simultaneously in the $2{\vec{k}}_a-{\vec{k}}_b$ and $2{\vec{k}}_b-{\vec{k}}_a$ directions. In each direction, we observe a single peak when the two beams are tuned on the closed transition ${}^{85}$Rb$5S_{1\mathrm{/}2}(F=3)\to 5P_{3\mathrm{/}2}(F=4)$. 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.