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Physical Review A

Strong optical forces in frequency-modulated light

M. Cashen, Olivier Rivoire, V. I. Romanenko, Леонід Петрович Яценко, H. Metcalf

Published Nov 16, 2001• 18 citations
Publisher

Abstract

Multifrequency light can be exploited to produce different forms of optical forces on atoms. Here, we demonstrate that weakly modulated light can produce forces stronger than the radiative force limit $\ensuremath{\Elzxh}k\ensuremath{\gamma}/2$ that can be used either for cooling or for deflection. We present various models of these forces and compare them with our measurements.

DOI
10.1103/physreva.64.063411

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Multifrequency light can be exploited to produce different forms of optical forces on atoms. Here, we demonstrate that weakly modulated light can produce forces stronger than the radiative force limit \ensuremath\Elzxhk\ensuremathγ/2\ensuremath{\Elzxh}k\ensuremath{\gamma}/2\ensuremath\Elzxhk\ensuremathγ/2 that can be used either for cooling or for deflection. We present various models of these forces and compare them with our measurements.
We have demonstrated that coherent control of the momentum exchange between a light field and atoms can be implemented in frequency-modulated light. The optical parameters are chosen to drive the atoms between ground and excited states by adiabatic rapid passage in counterpropagating modulated light beams. We apply the optical force transversely to a thermal atomic beam, and the deflection provides a measure of the force. The modulation period is 2?/?s<<?, and each half-cycle exchanges momentum 2k, so the resulting force is much greater than the ordinary radiative force. Moreover, the velocity range of the force in such a light field is much greater than that of the radiative force. This constitutes another example of the huge range of capabilities of optical forces in non-monochromatic light.
The spontaneous decay rate $\ensuremath{\gamma}$ of excited atomic states limits the radiative force on atoms to $\ensuremath{\Elzxh}k\ensuremath{\gamma}/2.$ This bottleneck in the dissipative exchange of momentum between atoms and light can be overcome by coherent control of the momentum exchange interactions. The bichromatic force provides such control by using stimulated instead of spontaneous emission to produce the force, while at the same time exploiting spontaneous emission to enable a dissipative interaction. It is implemented with two counterpropagating light beams, each containing two frequencies, whose phases, amplitudes, and frequency difference are carefully chosen. We have made precise measurements of the extremely large magnitude and velocity range of this bichromatic force, and have shown that its velocity dependence near the edge of its range is suitable for atomic beam slowing and laser cooling. Our measurements have corroborated various models and calculations of this bichromatic force.
One of the fundamental processes in physics is the interaction of matter with light. One learns early in quantum mechanics how electrons in a hydrogen atom absorb light with well-defined energy or frequency and make transitions between different quantized energy levels. However, the physical behavior is rather different when an atom interacts with multiple light beams with different frequencies because the optical forces change tremendously. This Colloquium discusses this and its consequences in atom cooling.
We have recently begun to explore the very strong optical forces that can be exerted on atoms by polychromatic light. Unlike the radiative force that is limited to be less than ℏkγ/2 in monochromatic light or the dipole force that alternates on a wavelength scale in a monochromatic standing wave, limiting its utility, these new forces have very attractive features. Not only is their strength far greater than Frad, but also their velocity range is far greater than γ/k, so that beam slowing can be accomplished without Doppler compensation in distances measured in millimeters instead of meters. These forces have been implemented by modulation of a single-frequency laser or by combination of the light of two different single-frequency lasers.
The limit of $\ensuremath{\Elzxh}k\ensuremath{\gamma}/2$ imposed on the magnitude of radiative forces by the spontaneous decay rate $\ensuremath{\gamma}$ of the excited state can be overcome by coherent control of the momentum exchange between atoms and the light field. This can be implemented with two light beams, each containing two frequencies whose phases, amplitudes, and frequency difference are carefully chosen. We have made precise measurements of the extremely large magnitude and velocity range of this bichromatic force, and have shown that its velocity dependence near the edge of its range is suitable for atomic beam slowing and laser cooling. Our measurements have corroborated various models and calculations of this bichromatic force.