Highly selective four-wave mixing of low-intensity radiation in a degenerate two-level atomic system

Quasi-degenerate four-wave mixing (FWM) of low-intensity radiation (with an intensity less than 1 mW cm^-2) was observed for the 5S_1/2(F = 2) – 5P_3/2 (F' = 3) transition in an optically transparent ⁸⁷Rb vapour under conditions of electromagnetically induced absorption. The efficiency of frequency conversion was no less than 1% with respect to the intensity of pump waves. Nonlinear FWM is highly selective relative to the frequency detuning of pump waves (the FWM full width at half maximum was less than 50 kHz).