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Self-focusing and self-defocusing of fs laser light in rubidium vapor

We studied the emission from dense rubidium vapor under 100-fs pulsed laser excitation in the 730–820 nm range. Self-focusing along with conical emission (CE) was observed for the excitation in the blue wing of D2 atomic resonance. This effect was earlier observed for fs-pulsed excitation in Cs2 molecules. For the far blue-wing excitation, the CE consists of a bright ring without central spot. The magnitude of the cone angle as a function of detuning from the resonance obeys ~∆-1.5 dependence (∆ being frequency detuning from the resonance) which is in accordance with the theory of self-focusing. The evidence of self-focusing along with the spectral broadening leads to the conclusion that the self-phase modulation generates observed phenomena. Self-defocusing occured for the red-wing excitation. When the laser wavelength is tuned at two-photon allowed transition at 778 nm, 5s1/2 → 5d3/2, the output pattern shrinks into a small spot whose wavelength is around 762 nm. This can be explained by two-photon resonantly enhanced FWM.

self-focusing; self-phase modulation; conical emission; rubidium

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