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IEEE Journal of Quantum Electronics

Conical emissions and phase conjugation in atomic sodium vapor

John Pender, Lambertus Hesselink

Published Mar 1, 1989• 25 citations
Publisher

Abstract

Observations of a new type of conical emission in atomic sodium provide the motivation for a series of phase conjugation experiments. These experiments show large variations in the reflectivity as a function of probe angle, which are not explained by existing theory. Analysis which includes self-induced refractive index changes leads to numerical calculations, which agree well with the observations.>

DOI
10.1109/3.18554

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There Is an ongoing interest in the conical emission resulting from the passage of a laser light tuned in the vicinity of a resonant atomic transition(1–2). We also have observed such emission in high density sodium vapor in the range of 1015 cm−3, when an intense laser radiation is blue detuned with respect to any of the D lines. This conical emission shell has half angle of a few degrees around the laser axis, is red detuned from the respective D line, is spectrally broad (3A) and is obtained in laser intensity and detunings and sodium vapor density ranges where self-focusing occurs. No symmetric blue shifted component was observed. Harter and Boyd(1) have obtained symmetrical Rabi side-band emission in the forward direction at sodium vapor density of 1014 cm−3.

•Jun 18, 1984•0 citations
Observations of a new type of conical emission in atomic sodium provide the motivation for a series of phase conjugation experiments. These experiments show large variations in the reflectivity as a function of probe angle, which are not explained by existing theory. Analysis which includes self-induced refractive index changes leads to numerical calculations, which agree well with the observations.>
Degenerate conical emissions that appear in laser-pumped atomic sodium have been reported. The cones appear for both single- and double-pump geometries. The nonlinear medium is a small, high-density sodium-seeded flame. Earlier work that relates the cones to the process of degenerate phase conjugation is extended. The analysis for the single-pump phenomenon is formally related to amplified spontaneous emission. The double-pump case is analogous to a distributed-feedback laser. Theory and experiment are presented and discussed.
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.
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.
There Is an ongoing interest in the conical emission resulting from the passage of a laser light tuned in the vicinity of a resonant atomic transition(1–2). We also have observed such emission in high density sodium vapor in the range of 1015 cm−3, when an intense laser radiation is blue detuned with respect to any of the D lines. This conical emission shell has half angle of a few degrees around the laser axis, is red detuned from the respective D line, is spectrally broad (3A) and is obtained in laser intensity and detunings and sodium vapor density ranges where self-focusing occurs. No symmetric blue shifted component was observed. Harter and Boyd(1) have obtained symmetrical Rabi side-band emission in the forward direction at sodium vapor density of 1014 cm−3.