Abstract

A nonlinear medium interacting with two counterpropagating pump beams is potentially unstable. This instability is related to a mirrorless four-wave mixing oscillation.¹ Recent experiments done in vapors² show that the behavior is strongly different according to the sign of the frequency detuning from resonance. When the laser frequency ω is larger than the atomic resonance frequency ω₀, the oscillation appears as a sharp conical emission, the oscillation frequency being ω. In some cases the cone is fragmented into a hexagon. When ω < ω₀, the oscillation is more difficult to achieve and appears as a very broad cone.

© 1989 Optical Society of America