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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Dynamical behaviour of the temperature in Cs optical molasses

L. Marmet, Marie-Claude Gagné, J.-S. Boulanger

Published Nov 27, 2002• 0 citations
Publisher

Abstract

We measured the temperature of Cs atoms in an optical molasses after a sudden intensity reduction of the cooling beams. The temperature exhibits an interesting temporal behaviour in that the decay follows a series of exponential curves which can include a few damped oscillations. We present the results of a one-dimensional model which reproduces the features observed experimentally. The complex decay of the temperature is understood as a result of frictional forces in the molasses and collective oscillatory motion of the atoms. This is interpreted as showing a spatial localization of the atoms in the optical quasi-lattice formed in a six-beam molasses.

DOI
10.1109/freq.1998.717878

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We measured the temperature of Cs atoms in an optical molasses after a sudden intensity reduction of the cooling beams. The temperature exhibits an interesting temporal behaviour in that the decay follows a series of exponential curves which can include a few damped oscillations. We present the results of a one-dimensional model which reproduces the features observed experimentally. The complex decay of the temperature is understood as a result of frictional forces in the molasses and collective oscillatory motion of the atoms. This is interpreted as showing a spatial localization of the atoms in the optical quasi-lattice formed in a six-beam molasses.
Optical molasses is a three-dimensional (3-D) configuration of laser beams used to laser-cool and to viscously confine neutral atoms. Atoms laser cooled in optical molasses reach temperatures much lower than the limit given by the original theories of laser cooling based on the Doppler effect. This cooling below the Doppler-cooling limit is now seen as being due to new laser cooling mechanisms not considered in the original theories. The dependence of the atomic temperature on parameters such as laser intensity and detuning shows good agreement between calculations performed in 1-D and experiments performed in 3-D. For cooling of Na and Cs atoms, the lowest observed temperatures correspond to rms velocities between three and four times the single photon recoil velocity. For Cs the temperature is 2.5 ± 0.6 μK and is the lowest temperature ever measured for 3-D cooling.
We have measured the temperature of laser-cooled Rb atoms in optical molasses as a function of laser intensity and detuning. For both 85 Rb and 87 Rb, cooled on the F = 3 → F ' = 4 and F = 2 → F ' = 3 transitions, respectively, the temperatures are proportional to the ratio of laser power and detuning for a wide range of these parameters. We observe a small but significant difference between the two isotopes. We also show the results of three-dimensional semi-classical numerical calculations. Our results favor a model which includes atomic localization in optical standing waves.
We have measured the spring constant, friction parameter, and temperature of the motion of caesium atoms in a magneto-optical trap. The results are very different from those predicted by models based on the scattering force, which have been used up to now. We show that the behaviour can be understood in terms of the sub-Doppler cooling mechanisms which have been observed in molasses, involving atomic motion through a polarization gradient. These mechanisms can operate in a trap because the atoms congregate in a region of low magnetic field. The lowest temperature measured was (30 ± 15) μK.
In two recent experiments Verkerk et al. and our group have observed quantization of the center-of-mass motion of laser cooled atoms in sub-wavelength sized potential wells formed by the light field of a 1-dimensional optical molasses. Our 1-dimensional optical molasses consists of a pair of counterpropagating laser beams of orthogonal, linear polarization, creating a series of potential wells separated by {lambda}/4. Polarization gradient laser cooling reduces the thermal energy of the atoms until they are trapped in well resolved vibrational levels near the bottom of the wells. Transitions between these vibrational levels are seen in a high resolution spectrum of fluorescence, where they give rise to a Rayleigh peak centered at the laser frequency, and well resolved Raman sidebands shifted by a frequency {plus_minus}{omega}, typically on the order of 100 kHz. We find quantitative agreement between the measured level separation h{omega} and the results of a calculation of the bandstructure in the periodic optical potential. Based on the observed ratio of the strength of the blue to the red sideband we determine the temperature of the atoms, and find agreement with theoretical predictions. Based on the level spacing and temperature we find the atoms to be localized to {approx}/15, withmore » up to 50-60% of the population of the trapped atoms in the vibrational ground state. We observe intriguing differences between fluorescence radiated by localized atoms, and the fluorescence radiated by non-localized atoms in a 1-dimensional optical molasses composed of a pair of counterpropagating laser beams of opposite circular polarization. The prospect of observing quantum motion and spatial order in 2 and 3 dimensions, and the consequences for the spectrum of fluorescence, will be discussed.« less
We have studied the expansion of a bright rubidium three-dimensional optical molasses realized with an intense laser light. In this regime, in addition to the laser friction force, the atoms strongly repel each other due to the radiation trapping force, which is analogous to the Coulomb force. The experimental observations show that the cloud volume increases linearly in time and that, simultaneously, the cloud temperature decreases following the law $(1+t/\ensuremath{\tau}{)}^{\ensuremath{-}4/3}.$ These time evolutions are explained using a physical picture of Coulomb explosion in a viscous medium.