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Physical Review A

Collision- and stochastic-fluctuation-induced Hanle resonances in four-wave light mixing in samarium

Y. H. Zou, N. Bloembergen

Published Oct 1, 1986• 15 citations
Publisher

Abstract

A broadband multimode dye laser, detuned from the \ensuremath{\lambda}=570.68 nm resonance line of Sm atoms, was used to provide three incident beams in the standard phase-conjugate four-wave-mixing geometry. The phase-conjugate output intensity was measured as a function of polarization and external magnetic field configurations, and of the pressure of several buffer gases. We have confirmed the theoretical prediction that correlated stochastic fluctuations in the incident light field make a contribution to the four-wave-mixing signal. Collisional cross sections for reorientation among the m sublevels in the J=1 ground state of Sm have been obtained for He, Ar, and Xe. Differences in polarization characteristics between the collision-induced Hanle resonances in Na and Sm are pointed out.

DOI
10.1103/physreva.34.2968

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A broadband multimode dye laser, detuned from the \ensuremath{\lambda}=570.68 nm resonance line of Sm atoms, was used to provide three incident beams in the standard phase-conjugate four-wave-mixing geometry. The phase-conjugate output intensity was measured as a function of polarization and external magnetic field configurations, and of the pressure of several buffer gases. We have confirmed the theoretical prediction that correlated stochastic fluctuations in the incident light field make a contribution to the four-wave-mixing signal. Collisional cross sections for reorientation among the m sublevels in the J=1 ground state of Sm have been obtained for He, Ar, and Xe. Differences in polarization characteristics between the collision-induced Hanle resonances in Na and Sm are pointed out.
Collision-enhanced Hanle resonances in the 3 2S1/2^{2}S_{1/2}2S1/2​ electronic ground state of Na atoms have been studied in a phase-conjugate four-wave light-mixing geometry. Narrow resonances in zero magnetic field have been observed at very high inert-buffer-gas pressures, with a width of 21 mG, or less than 10 kHz, full width at half-maximum. The intensity and the width of these resonances has been studied as a function of buffer-gas pressure, of alkali-metal vapor pressure, of detuning from the D1{D}_{1}D1​ and D2{D}_{2}D2​ Na resonances, and of the intensity of the incident light beams. Saturation phenomena and power broadening are observed. The results are explained theoretically in terms of transversely pumped optical gratings. The equivalence of this description with collision-induced coherence gratings in the limit of low light intensities is demonstrated.
The effect of pump fluctuations on various coherent processes that arise in three-level systems interacting with two external fields is examined. Such coherent processes include the forward Hanle effect and various four-wave mixing effects such as the generation of phase-conjugate signals. A general formulation that enables one to calculate the influence of laser linewidth on the coherent signals produced in various directions is presented. Ensemble averages, over laser temporal fluctuations, of various physical quantities, such as atomic polarization, are calculated. The spectrum of polarization fluctuations is shown to consist of several new features which lead to coherent radiation in different directions depending on the resonant frequencies in the polarization fluctuations. The influence of pump linewidth on pressure-induced extra resonance (PIER) is treated in detail. The possibility of producing a laser-fluctuation-induced coherent signal at one of the atomic frequencies is examined. This new signal, which is produced in a direction different from that of the PIER signal, but has the same type of resonant character as PIER, is found to have significantly different pressure dependence than the PIER signal. The results of our numerical computations are qualitatively explained in terms of the convolutions of products of third-order susceptibilities and pump field-correlation functions.
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.
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.
We explore theoretically the signal generated by a fluctuating radiation field via a four-wave mixing process in an atomic vapor undergoing Markovian dephasing collisions. We consider specifically a broadband laser field well detuned from the atomic resonance and calculate the signal generated using counterpropagating pump fields as a function of the time delay of the incident probe field. We assume that the pulse duration is long compared to the correlation time of the incident fields. By considering the various time-ordered pathways and treating the incident-field amplitudes as random Gaussian variables, we find that the signal generated is dominated by two specific pathways that depend only on the incident-field bandwidth and the decay rate of the ground to excited state coherence. An interesting aspect of the far-impact-detuning case is that the signal is dominated by resonances involving spatially separated atoms. Our formalism provides a basis for describing the effects of mode correlations on four-wave-mixing signals generated by stochastic fields.