Conical fluorescence emission from sodium vapor excited with tunable femtosecond light pulses

An experimental and theoretical investigation is made of the angular spectrum of a near-resonant femtosecond laser pulse propagating through sodium vapor. A directional fluorescence emission corresponding to single-photon sodium transition 3S→3P1 at ∼sol∼ at 2,3 at ∼sol∼ at 2 has been registered for the pump wavelength ranging from 540 to 605 nm. It is found that such emission can be detected under the conditions of single-photon (enhanced by two-photon) resonance of pump frequency with relevant atomic states. Theoretical treatment of the field-matter interaction by means of the nonperturbative (semiclassical) approach revealed conical components of the cylindrically symmetric beam spectrum originating both from the fluorescence induced by coherent atomic polarization and four-wave mixing