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Physical Review Letters

Collision-Induced Hanle Resonances of Kilohertz Width in Phase-Conjugate Four-Wave Light Mixing

N. Bloembergen, Y. H. Zou, Lewis J. Rothberg

Published Jan 21, 1985• 27 citations
Publisher

Abstract

Hanle resonances for coherent four-wave mixing in the ground state of Na are reported whose intensity increases and whose linewidth decreases with increasing inert buffer gas pressure. In 10-mTorr Na vapor with 4 atm of argon, resonances of less than 35 mG full width at half maximum are observed. The high resolution permits direct observation of the fundamental homogeneous broadening mechanisms for the Zeeman coherences responsible for four-wave mixing.

DOI
10.1103/physrevlett.54.186

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Optics Letters•Jul 1, 1981•54 citations
Hanle resonances for coherent four-wave mixing in the ground state of Na are reported whose intensity increases and whose linewidth decreases with increasing inert buffer gas pressure. In 10-mTorr Na vapor with 4 atm of argon, resonances of less than 35 mG full width at half maximum are observed. The high resolution permits direct observation of the fundamental homogeneous broadening mechanisms for the Zeeman coherences responsible for four-wave mixing.
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Near-resonant four-wave light mixing in Na vapor with inert buffer gases reveals collision-induced coherences when the difference frequency between two incident light beams equals the hyperfine splitting or the Zeeman splitting between two states of the 32S3^{2}S32S ground-state manifold. The Doppler broadening of the corresponding resonances decreases with increasing buffer-gas pressure. The width of the resonances becomes significantly narrower than the spontaneous linewidth of the near-resonant intermediate state and is determined by spin-exchange collisions between ground-state atoms. The dependence of the intensity of these collision-induced resonances on buffer-gas pressure and detuning from one-photon resonances is measured. Selection rules for various polarizations of the incident light fields and directions of an externally applied magnetic field are established.
Optical four-wave mixing resonances in sodium vapor induced by collisions with inert buffer gases (He, Ar, Xe, N2{\mathrm{N}}_{2}N2​) are characterized at high resolution ( 5 MHz) using lasers tuned near the DDD lines of Na. These resonances are interpreted in terms of coherent scattering from a population grating established by collisional absorption of the incident radiation. The roles of homogeneous and inhomogeneous contributions to the resonance line shapes are clarified. The line shapes exhibit structure sharper than the predicted linewidth, that characteristic of the 3P3P3P-to-3S3S3S transition rate. We show that this is caused by collision-induced optical pumping which introduces the ground-state lifetime into the grating relaxation physics. These long-lived gratings are exemplified dramatically by experiments in the presence of N2{\mathrm{N}}_{2}N2​ which collisionally quenches the Na 3P3P3P states. Similar results are obtained in Xe where it is argued that excimer formation with the 3P3P3P states plays a role in the population dynamics. Model calculations of the effects of sodium level degeneracy on the grating dynamics are presented to support these conclusions. Finally we present data illustrating interference between the collision-induced population grating resonances detailed here and collision-induced coherent Raman resonances between Zeeman sublevels of the ground state.
First, some historical comments on the Hanle effect and Kastler-type optical pumping are made. Next the recent history of the phenomenon of collision-induced coherences in optical four-wave light mixing experiments is briefly reviewed New experimental results on collision-enhanced Hanle-type resonances in the 3 2S1/2 ground state configuration of Na atoms in a buffer gas at high pressure are presented. The relationship between these optical fourwave experiments and Kastler-type optical pumping is pointed out.