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Physical Review A

Measurements of intensity correlations of scattered light from laser-cooled atoms

Samir Bali, Dominikus Hoffmann, José Simân, Thad G Walker

Published May 1, 1996• 33 citations
Publisher

Abstract

We report measurements of the intensity correlations of scattered light from atoms in optical molasses. For small numbers of atoms, the observations are consistent with recent models of the Rayleigh and Raman contributions to the frequency spectrum. Magnetic fields on the order of 100 mG significantly broaden the spectrum. Radiation trapping results in reduction of the size of the correlations as well as broadening of the spectrum. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.53.3469

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We report measurements of the intensity correlations of scattered light from atoms in optical molasses. For small numbers of atoms, the observations are consistent with recent models of the Rayleigh and Raman contributions to the frequency spectrum. Magnetic fields on the order of 100 mG significantly broaden the spectrum. Radiation trapping results in reduction of the size of the correlations as well as broadening of the spectrum. \textcopyright{} 1996 The American Physical Society.
In this thesis, the effect of radiation trapping on the photon statistics of the light scattered from optical molasses is experimentally investigated.It is demonstrated that the intensity correlation function is sensitive to the presence of radiation trapping at an on-resonance optical depth, OD, as low as 0.1.It is also shown that it is possible by introducing a mild anisotropy in the trapped cloud to change the number density of a trapped sample of laser-cooled atoms in a dilute magnetooptical trap by over an order of magnitude, from 1.9 × 10 9 /cm 3 to 1.6 × 10 8 /cm 3 , while keeping the temperature constant.
Using three orthogonal pairs of counterpropagating red-detuned lasers and a magnetic field gradient, we have constructed a trap of 85 Rb atoms at temperatures reduced to the order of 100 microKelvins and densities of 10 9 atoms/cm 3 .When the atoms interact with the light, they are absorbing and spontaneously emitting photons.The emitted photons from atoms can be absorbed by neighboring atoms.This phenomenon, called radiation trapping, limits the density and temperature of the trap and also destroys the atomic coherence.We can noninvasively measure the effects of radiation trapping by measuring the intensity correlation function of the light radiated from the trap.We have found that theoretically, the effects of radiation trapping can be detected in dilute atomic samples with an on-resonance optical depth as low as 0.1, more than an order of magnitude less than seen before.Experimental progress is reported.
In this paper, we calculate the effect of radiation trapping on the photon statistics of the light scattered from optical molasses. We propose that an intensity correlation function measurement may be sensitive to the presence of radiation trapping at an on-resonance optical depth as low as 0.1, more than an order of magnitude less than where effects of multiple scattering in cold-atom clouds have been previously observed [T. Walker, D. Sesko, and C. Wieman, Phys. Rev. Lett. 64, 408 (1990); D. Sesko, T. Walker, and C. Wieman, J. Opt. Soc. Am. B. 8, 946 (1991)].
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 measured light shifts, also known as AC Stark shifts, as a function of laser intensity in cold Rubidium atoms by observing sub-natural linewidth gain and loss features in the transmission spectrum of a weak probe beam passing through the atomic sample. The observed energy-level shifts for atoms in a magneto-optical trap (MOT) are found to be consistently higher than that obtained in optical molasses (i.e., when the magnetic field gradient in the MOT is turned off). Using a simple model of a multilevel Rubidium atom interacting with pump and probe beams, we have calculated the theoretical light shift as a function of intensity. A comparison of these calculated values with the light shift data obtained for molasses reveals good agreement between experiment and theory. Further, our model elucidates the role of the Zeeman shifts arising from the magnetic field gradient in the observed probe transmission spectrum for the MOT.
We present experimental evidence that the intensity correlations of light scattered from a cold-atom cloud are sensitive to the presence of small amounts of radiation trapping in an atomic sample of density 6 x 10(8)/cm3, with an optical depth (for a resonant light beam) of 0.4. This density and optical depth are approximately an order of magnitude less than the density and on-resonance optical depth at which effects of multiple scattering in cold-atom clouds have been previously observed [Phys. Rev. Lett. 64, 408 (1990)].