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Raman spectroscopy in a continuous beam from a 2D MOT

Aaron Meldrum, Michael Manicchia, Frank A. Narducci, Jon P. Davis

Published Feb 22, 2018• 0 citations
Publisher

Abstract

Atom interferometers consist of light pulses designed to create coherent superpositions of atomic states (“π/2” or “beam splitting” pulses) and that coherently interchange states (“π” or “mirror” pulses). In this article, we investigate the effects of imperfect pulses for a geometry specific to our apparatus. Atoms emerge from a 2-dimensional magneto-optical trap (2D MOT) in a continuous beam and cross continuous laser beams that drive stimulated Raman transitions. We use the atoms’ transit time through the laser field as the “pulse” time. We describe the impact of various effects on the contrast of the Rabi cycling, specifically the longitudinal velocity spread, the laser beam diameter and the spacing between the laser beams.

DOI
10.1117/12.2299225

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Atom interferometers consist of light pulses designed to create coherent superpositions of atomic states (“π/2” or “beam splitting” pulses) and that coherently interchange states (“π” or “mirror” pulses). In this article, we investigate the effects of imperfect pulses for a geometry specific to our apparatus. Atoms emerge from a 2-dimensional magneto-optical trap (2D MOT) in a continuous beam and cross continuous laser beams that drive stimulated Raman transitions. We use the atoms’ transit time through the laser field as the “pulse” time. We describe the impact of various effects on the contrast of the Rabi cycling, specifically the longitudinal velocity spread, the laser beam diameter and the spacing between the laser beams.
In this article, we investigate both theoretically and experimentally the method of Ramsey interference applied to cold atoms being emitted from a 2-dimensional magneto-optical trap (2D-MOT) crossing continuous wave (cw) laser fields that drive Raman transitions between ground-state hyperfine sublevels. In our apparatus, the atomic beam divergence is large (56 mrad half-angle), necessitating the inclusion in our model of both the atomic divergence and the laser beam divergence as well as averaging over the transit time of the atoms through the laser beam. We discuss the effects of the divergence of the atomic and laser beams, and then we describe our apparatus and qualitatively compare our experimental results to the theoretical results. We find good agreement between our model and our experimental results.
Sodium atoms prepared in hyperfine ground-state 11 > are driven to the other ground-state hyperfine level 12> when two laser beams nearly resonant with an optical transition have a frequency difference equal to the ground-state hyperfine splitting. When the two laser beams are counterpropagating, conservation of momentum between the atom and the light field requires that the atom acquire a velocity kick ~22 ħk/m during the 11> and 12> transition (~6 cm/sec in our experiment). When the lasers are detuned far from the optical transition, spontaneous emission from the intermediate optical level becomes negligible, and the atom behaves as a two-level system coupled to a resonant rf driving field. We have created an interferometer by applying a π/2-π-π/2 Raman pulse sequence: a first π/2 pulse coherently splits the atomic wave packet by putting it in a superposition of states 11> and 12>, a second π pulse occurring a time Δt later redirects each wave packet and a final π/2 pulse recombines the wave packets at time 2Δt. We have observed wave packet interference for wave packet separations of 2.4 mm and, by using the interferometer as an accelerometer, have measured the atom’s acceleration due to gravity to 3 parts in 106.
In this paper we report on the first purely ``pump-probe'' nonlinear laser spectroscopy results in a slow atomic beam. We have observed Raman, Rayleigh, and recoil-induced resonances (RIR) in a continuous beam of slow and cold cesium atoms extracted from a two-dimensional (2D) magneto-optical-trap (MOT) with the moving molasses technique. The RIR enabled us to measure the velocity distribution, therefore the average speed (0.6--4 m/s) and temperature (50--500 \ensuremath{\mu}K) of the atomic beam. Compared to time of flight, this technique has the advantage of being local, more sensitive in the low-velocity regime (v<1m/s),(v<1 \mathrm{m}/\mathrm{s}),(v<1m/s), and it gives access to transverse velocities and temperatures. Moreover, it may be extended to measure atomic velocities in the 2D MOT source of the atomic beam.
We demonstrate a light-pulse atom interferometer based on the diffraction of free-falling atoms by a picosecond frequency-comb laser. More specifically, we coherently split and recombine wave packets of cold ^{87}Rb atoms by driving stimulated Raman transitions between the |5s ^{2}S_{1/2},F=1⟩ and |5s ^{2}S_{1/2},F=2⟩ hyperfine states, using two trains of picosecond pulses in a counterpropagating geometry. We study the impact of the pulses' length as well as the interrogation time onto the contrast of the atom interferometer. Our experimental data are well reproduced by a numerical simulation based on an effective coupling that depends on the overlap between the pulses and the atomic cloud. These results pave the way for extending light-pulse interferometry to transitions in other spectral regions and therefore to other species, for new possibilities in metrology, sensing of gravito-inertial effects, and tests of fundamental physics.