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Physical Review A

Pump-probe spectroscopy and velocimetry of cold atoms in a slow beam

G. Domenico, G. Mileti, P. Thomann

Published Sep 1, 2001• 16 citations
Publisher

Abstract

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 $(v<1 \mathrm{m}/\mathrm{s}),$ 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.

DOI
10.1103/physreva.64.043408

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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 present experimental observation of recoil-induced resonance (RIR) in an operating cesium magneto-optical trap (MOT) by both pump–probe absorption and four-wave mixing spectra simultaneously. We investigate the dependence of amplitudes of these two spectra on pump beam intensity and frequency. The measurement results agree well with the recoil-induced theory with modifications of Raman transition effect and atomic number. The systematical study on RIR spectra is meaningful for the diagnostic measurement of cold atoms in an operating MOT.
By the moving molasses technique we have extracted laser-cooled cesium atoms in a continuous way directly from an optical molasses. The mean launching velocity is precisely tunable from 1 to 12 m/s. The atomic beam has a flux of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{8} \mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}/\mathrm{s}$ at 7 m/s and a longitudinal temperature of $70 \ensuremath{\mu}\mathrm{K},$ which represents the highest flux and lowest velocity spread obtained so far in a continuous beam of cold atoms. These features makes it well suited for atomic fountains. The atomic flux can be slightly increased in a two-dimensional magneto-optical trap operation (+40(+40%).(+40 A simple model accounts for the observed dependence of the flux with the magnetic-field gradient.
Starting from a 2D magneto-optical trap where cesium atoms are permanently subjected to 3D sub-Doppler cooling and 2D magneto-optical trapping, we have produced a beam of cold atoms continuously extracted along the trap axis. The simplest extraction mechanism, presently used, is the drift velocity induced by a constant magnetic field. We have used this continuous beam of atoms to produce Ramsey fringes in a microwave cavity as a first demonstration of an atomic resonator operating continuously with laser cooled atoms. The shape of the resonance pattern allows an estimate of the axial temperature, typically 200 μK. The average velocity can be adjusted from 0.7 to 3 m/s; the trap-to-atomic-beam conversion efficiency is close to one.
We describe the design and performance of a slow atom source based on a 2D magneto-optical trap (MOT) that uses an innovative simple optical configuration. Metal-coated retro-reflecting prisms replace mirrors and quarter-wave plates so the optical power of the cooling laser beam is recycled. This source has been characterised for three different configurations: with and without transverse magnetic field gradient, and with a pusher beam to obtain a 2D+-MOT. The longitudinal velocity is of the order of 25 m·s−1, with a transverse velocity spread ≤1 m·s−1, while the typical atomic flux density obtained is up to 1.3×1014 at ·s−1·m−2 for a cesium vapour pressure of ~4×10−8 mbar in the source. We use this slow atom beam, instead of cesium vapour, to load a 3D moving optical molasses that feeds a continuous cold atom fountain. We obtain a gain of a factor ~20 in the atomic flux launched by the fountain.
We report on the first expeimental observation of recoil-induced resonances. These resonances, which can be considered as the atomic analog of the grain mechanism of the free-electron laser, are observed on the transmission spectrum of a probe beam making a small angle with a one-dimensional lin\ensuremath{\perp}lin cesium optical molasses. They are interpreted either as stimulated Raman transitions between differently populated velocity groups (the internal atomic state being unchanged), or as stimulated Rayleigh resonances originating from a bunching of the atomic density. Their shape provides information about the transverse velocity distribution of the atoms.
To achieve a primary frequency standard based on a continuous fountain of laser cooled cesium atoms, we report on the production of a bright, slow and cold continuous atomic beam. Atoms are cooled from a thermal vapor in a 45/spl deg/ laser geometry and launched directly from the optical molasses by the moving molasses technique. The effects of the source fluorescence on the continuous beam (average velocity, temperature and flux) have been observed, and ways to optimize simultaneously the atomic capture and the continuous beam are presented. Flux measurements rise to 2/spl times/10/sup 8/ at/s from a 2D magneto-optical trap at 7 m/s. The initial velocity is easily tunable from 1 to 10 m/s. If the longitudinal temperature is about 70 /spl mu/K, additional transverse cooling even reduces the transverse temperature in one dimension below 30 /spl mu/K. By extrapolating the obtained atomic flux to the continuous fountain under construction, we expect a short-term stability of 7/spl times/10/sup -14//spl tau//sup -1/2/. Other applications of continuous beams of cold atoms are suggested.