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Journal of the Optical Society of America B

Spectral and spatial evolution of a conical emission in Na vapor

Alexander A. Dreischuh, Vladimir P. Kamenov, Stoyan Dinev, Ursula Reiter-Domiaty, Dieter Gruber, Laurentius Windholz

Published Jan 1, 1998• 9 citations

Abstract

A detailed study on yellow conical emission observed in dense sodium vapor following near-resonant one-photon excitation of the Na 42P levels was performed. The origins of conical emission are suggested to be due to Raman resonant four-wave parametric mixing processes. In order to describe spectral and spatial features of off-axis emission, we introduce self-phase modulation and induced-phase modulation as important additional nonlinear processes, explaining the nonmonotonic detuning of the wavelength of the conical radiation with the exiting laser wavelength in the vicinity of resonance transitions. The spatial evolution of emission generated via four-wave mixing is modeled by solving the two-dimensional nonlinear Schrödinger equation describing the spatial evolution of light beams in a nonlinear media. Experimental and theoretical results are presented and discussed.

DOI: 10.1364/josab.15.000034

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A detailed study on yellow conical emission observed in dense sodium vapor following near-resonant one-photon excitation of the Na 42P levels was performed. The origins of conical emission are suggested to be due to Raman resonant four-wave parametric mixing processes. In order to describe spectral and spatial features of off-axis emission, we introduce self-phase modulation and induced-phase modulation as important additional nonlinear processes, explaining the nonmonotonic detuning of the wavelength of the conical radiation with the exiting laser wavelength in the vicinity of resonance transitions. The spatial evolution of emission generated via four-wave mixing is modeled by solving the two-dimensional nonlinear Schrödinger equation describing the spatial evolution of light beams in a nonlinear media. Experimental and theoretical results are presented and discussed.

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 $3S\ensuremath{\rightarrow}3{P}_{1∕2,3∕2}$ has been registered for the pump wavelength ranging from

540\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}605\phantom{\rule{0.3em}{0ex}}\mathrm{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.

Two types of parametric four-wave mixing are observed when a single intense laser beam propagates through Na vapor near the D(1) and D(2) resonance lines. The first is related to three-photon scattering, while the second results from the coherent parametric interaction of the laser with excited-state Raman scattering. Identification of the two processes was greatly facilitated by the use of an optical multichannel detector system to observe the total emission spectrum for each individual laser pulse. The experimental conditions that determine whether stimulated emission or one or both of the four-wave mixing processes dominates the emission spectrum are described.

In our experiment conical emission (CE) has no wavelength shift and the same polarization as the original beam. Varying laser power, detuning, and atomic density, the images of CE were captured and analyzed. Typical far-field images are presented. Under the blue detuning condition, manifest CE patterns were observed, and under red detuning, original beam blurs due to not only to self-defocusing but also the conical pattern were observed. By comparing laser power and atomic density dependence of cone angle and theoretical curve, we could estimate spectroscopic data such as nonlinear refractive index. From the density dependence data, we get the fact that cone angle is proportional to atomic density which supports the self-phase-modulation (SPM) model. In conclusion, we observed and analyzed CE from near-resonant Sm vapor. With this simple setup, we could measure spectroscopic data.