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中国激光(英文版)

Velocity Measurement of Laser cooled Atoms by FM Spectroscopy TechniqueSupported by the National Science Foundation of China, No.19834060, and the Department of National Science and Technology of China, No.95 Yu 34.

Xinqi Wang

Published Jan 1, 1999• 0 citations
OpenAlex

Abstract

It is suggested an improved method for measuring the velocity distribution of cold atoms, using FM technique to derive the signal from the recoil induced resonance. The method is capable to measure velocity distribution of atoms cooled below the Doppler limit of laser cooling in a compact space with high S/N ratio.

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It is suggested an improved method for
measuring the velocity distribution of cold atoms, using FM technique to derive the signal from
the recoil induced resonance. The method is capable to measure velocity distribution of atoms
cooled below the Doppler limit of laser cooling in a compact space with high S/N ratio.
We report on an experimental study of recoil-induced resonances as a method of velocimetry for cold atomic samples. We present a refined experimental method that greatly improves the sensitivity of the measurement over previous experiments. Using frequency-modulation (FM) spectroscopy techniques we achieve a sensitivity that approaches the shot noise limit. In addition, we present a novel approach to deriving the line shape of the observed signal, based on the concept of quantum transport and tunnelling in motional Bloch bands.
We describe our use of recoil-induced resonances or stimulated optical Compton scattering to measure the velocity distribution of laser-cooled cesium atoms in one dimension. The technique works well at velocities near 1 cm/s, is insensitive to the effects of light shifts or stray fields, requires only a few mm3{\mathrm{mm}}^{3}mm3 of work space, and may be completed in 1 ms. It may be extended to allow the measurement of velocity distributions in three dimensions and for in situ measurements in optical molasses, traps, or lattices. Our measurements are compared with traditional ballistic measurements in the same apparatus.
A low noise time-of-flight detection system for laser cooled atoms has been constructed and incrementally optimized. Here, a thorough description of the construction is presented along with an analysis of the capabilities of the system. The quality of the detection (the resolution, the reproducibility, the sensitivity, etc.) is crucial for, e.g., the ability to see details in the velocity distribution profile, which is of interest for fundamental studies of statistical physics and of the laser cooling processes, and for detection of small initial velocities of an atomic cloud, important, e.g., when studying small drifts induced by Brownian motors and ratchets. We estimate the signal-to-noise ratio of our signal to be better than 1000:1 for a typical single shot, and we discuss the effect of the initial atomic cloud size, the probe size, and the effects of the wave packet spread during the fall time on the measured quantities. We show that the shape of the velocity distribution is well conserved during the mapping done in the detection, i.e., in the convolution with the probe beam, and that velocities as small as a few percent of the single photon recoil velocity can be resolved.
The transverse velocity distribution of a sodium atomic beam, laser cooled in one dimension, has been measured with sub-recoil resolution. A velocity-selective detection system was employed to separate the longitudinal and transverse velocity components to provide a direct measure of the transverse velocity distribution. The effects of varying the cooling laser intensity and detuning on the derived transverse temperature are presented, and show a remarkable insensitivity to the laser intensity. Atomic velocities within a factor of two of the recoil velocity are
To decrease the Doppler broadening and frequency shift,reducing the atomic velocity by laser cooling is necessary.Noting that through the first stage cooling,atoms can only be slowed down to the temperature at the mK level,which is too high to be loaded to the optical lattice,We made further efforts to slow down the velocity of the atoms,and the second stage cooling was implemented.When the first stage cooling stopped,using the 689 nm laser corresponding to(5s2)1S0→(5s5p)3P1further cooling the atoms could attain the temperature at as low asμK level.Using the timing sequence accurately controlling the time of interaction between the electromagnetic field and atoms.calculated the temperature through timeof-flight method.Dropping time of cold atoms in the MOT was accurately controlled,and the atomic photos were taken at 0ms and 20 ms.By analyzing the data,the atomic temperature after the second stage cooling is 4.39μK,uncertainty is only 0.19μK and the number of the atoms 1.2×107.The temperature and number of the Sr atoms offer the important reference for estimating the SNR of signal,which is also the foundation for achieving the high-precision standard of time and frequency.
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.