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Conical emission due to four-wave mixing enhanced by the ac Stark effect in self-trapped filaments of light.

Experimental evidence is presented that shows that the conical emission often found surrounding a near-resonant laser beam after it passes through an atomic vapor is due to nondegenerate four-wave mixing enhanced by the ac Stark effect. We demonstrate self-trapping of a pulsed dye-laser beam in sodium vapor for wavelengths 0.04 to 10 A shorter than that of the D(2) resonance and show that the four-wave mixing process leading to conical emission occurs within these self-trapped filaments.