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Physical Review A

Sub-Doppler-broadened magnetic field resonances in the resonant stimulated electronic Raman scattering of multimode laser light

Animesh Sharma, W. Happer, Yukai Lu

Published Feb 1, 1984• 4 citations
Publisher

Abstract

Continuous-wave lasing without the use of any feedback mirrors is observed when atomic cesium is optically pumped to its second excited resonance state. Resonant enhancement of the lasing intensity is observed at certain magnetic fields when the atoms are pumped by multimode laser light. The resonances, with widths of only a few gauss, are shown to be due to frequency crossings in the peak gains of certain velocity groups of excited atoms.

DOI
10.1103/physreva.29.749

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Soviet Journal of Quantum Electronics•Feb 28, 1989•0 citations
Continuous-wave lasing without the use of any feedback mirrors is observed when atomic cesium is optically pumped to its second excited resonance state. Resonant enhancement of the lasing intensity is observed at certain magnetic fields when the atoms are pumped by multimode laser light. The resonances, with widths of only a few gauss, are shown to be due to frequency crossings in the peak gains of certain velocity groups of excited atoms.
The fluorescence light emitted by atoms (Ne) optically pumped with a multimode laser beam exhibits a resonance each time the Zeeman splitting is equal to the frequency difference between two modes. The experimental study shows that these resonances are very sensitive to the relative phase of modes and that their width is approximately equal to the Hanle effect width. A theoretical interpretation is given with the help of a perturbation calculation. Two phenomena produce resonances : 1) A Population Effect (PE) which is resonant when the σ + component of one mode and the σ- component of another interact with atoms of same velocity (hole crossing). The width of these resonances is 2 Γ'ab (natural width broadened by collisions). 2) A Zeeman Coherence Effect (ZCE) : the action of a pair of modes (ν, σ +) and (μ, σ-) produces (at 2nd order) transverse alignment modulated at the beat frequency ων — ω μ and resonant when the Zeeman splitting is equal to that frequency ; by interaction with a second pair of modes (λ, σ-) and (κ, σ+) (ων —ω μ + ωλ — ωκ = 0), this alignment produces unmodulated longitudinal quantities (4th order) which exhibit, as the magnetic field is scanned, a resonance whose width is close to that of the Hanle curve (Γ b(2)). In our experimental conditions (close modes : Γ'ab≳ Δω > Γ b(2)), the resonances of the first kind are not resolved and the observed resonances are only of the second type's which arevery sensitive to the phases of modes. We also discuss the possibility of using these resonances to measure Landé g factors.
The passage of strong monochromatic laser radiation through a resonance medium consisting of three-level atoms is investigated. It is shown that if collisions are taken into account in the scattered waves the resonances at combination frequencies occur to first order in the laser field intensity. A numerical analysis of the dependence of the stimulated electronic Raman scattering (SERS) components' gain factors on the pump frequency is carried out. The dispersion curves of the SERS components' gain (absorption) factors for fixed values of pump frequency and intensity are plotted.
We have observed a new type of sub-Doppler laser cooling with neither polarization gradients nor magnetic fields. All known sub-Doppler laser cooling methods require one of these two to resdistribute the atomic population among the energy levels. This allows the irreversible processes of optical pumping vels. This allows the irreversible processes of optical pumping (OP) to manipulate the conservative force of light shifts (e.g. resonant exchange between the two beams comprising a standing wave) into a damping force. Our scheme employs stimulated Raman transitions between ground state hyperfine levels of Rb with a bichromatic standing wave, accompanied by a momentum change of {Delta}p = 2{h_bar}k. OP, with {Delta}p = 1{h_bar}k, creates a population imbalance allowing repetition of the Raman process. When the detuning of the light from Raman resonance {delta} is properly chosen, the net momentum exchange damps the atomic motion. Changing {delta} by only {gamma}/6(1 MHz) replaces cooling by heating. This process separates the velocity selective Raman resonance and OP processes differently from others, and helps clarify the role of these different mechanisms.
The optical pumping of atoms by a multimode gas laser, in the presence of a magnetic field, is studied theoretically. The atoms are described by their density matrix, which is expanded on an irreducible tensorial set. The atomic relaxation is assumed to be isotropic. In order to avoid the usual perturbation theory, we use the so-called "Broad-Line approximation" (BLA). BLA is valid when the width of the "Bennett hole" created by one laser mode in the velocity distribution of the excited atoms is comparable to the spacing between modes; under these conditions, the atomic response does not depend on the velocity and the multimode laser irradiation is equivalent to a broad-line excitation. For the atomic system, BLA leads to a set of coupled equations which is valid at arbitrary intensities of the laser field and which depends on this field through a unique parameter, $\ensuremath{\gamma}$. This parameter can be understood as a laser-induced transition probability. In this article, we analyze the nonlinear effects which can be deduced from this theory, for any value of the angular momenta involved in the laser transition: Hanle-effect broadening, alignment coupling, saturation resonances on the populations are shown. The exact calculations, valid at arbitrary laser intensities for $J=1\ensuremath{\leftrightarrow}J=0$, $J=1\ensuremath{\leftrightarrow}J=1$, and $J=1\ensuremath{\leftrightarrow}J=2$ transitions, and the corresponding experimental results will be presented in forthcoming papers.
Experimental and theoretical investigations were made of four-wave frequency mixing of the ?4? = ?1 ? ?2 ? ?3 type in cesium vapor enhanced by a Raman resonance (the field at the frequency ?2 is formed as a result of electronic stimulated Raman scattering of the pump field ?1). A strong dependence of the mixing characteristics on the conditions for excitation of electronic stimulated Raman scattering was identified and analyzed. Recommendations are given on the choice of optimal conditions for efficient frequency mixing in Raman resonance systems.