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Light Forces

Definition: forces associated with absorption, reflection or scattering of light

German: Lichtkräfte

Categories: quantum optics, methods

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Photons carry not only energy, but also momentum. As a consequence, momentum is transferred (i.e., a force is exerted) when a particle absorbs light or alters its direction (e.g. by refraction). Such light forces are responsible for e.g. the tails of comets, which always occur on the side opposite to the Sun and consist of dust particles dragged away by the light.

Light forces have been first considered by the astronomer Johannes Kepler. James Clerk Maxwell, the creator of the famous Maxwell equations, has later substantially developed further the understanding of light forces. They have later been experimentally proven by the Russian physicist Pyotr Lebedev in 1900 [1].

In simple cases, a light force is described as radiation pressure, exerting a force in the propagation direction of the absorbed light. However, the effective force can also have a direction which is different from the propagation direction. For example, refraction of light in a transparent sphere can redirect the light so that a force perpendicular to a (e.g. Gaussian) laser beam occurs, which pulls the sphere into the region of highest optical intensity.

When calculating light forces on atoms, it is often more convenient to derive the light forces from the gradient of a potential which arises from the interaction of the atoms with the light field. Such conservative forces arise e.g. from the dipole interaction with a laser beam the frequency of which is lower or higher than that of a resonant transition of the atom; the force is then directed towards locations with higher or lower optical intensity, respectively. A red-detuned laser beam can be used for trapping.

Non-conservative (dissipative) light forces are also important for trapping, because they serve to dampen the motion of a trapped particle. This is also called laser cooling.

Light forces are relevant in a number of very different situations:

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Bibliography

[1]P. Lebedev, “Untersuchungen über die Druckkräfte des Lichtes”, Annalen der Physik 311 (11), 433 (1901)
[2]T. W. Hänsch and A. L. Schawlow, “Cooling of gases with laser radiation”, Opt. Commun. 13, 68 (1975), doi:10.1016/0030-4018(75)90159-5
[3]D. J. Wineland and W. M. Itano, “Laser cooling of atoms”, Phys. Rev. A 20 (4), 1521 (1979), doi:10.1103/PhysRevA.20.1521
[4]J. Ye et al., “Trapping of single atoms in cavity QED”, Phys. Rev. Lett. 83 (24), 4987 (1999), doi:10.1103/PhysRevLett.83.4987
[5]C. Savage, “Introduction to light forces, atom cooling, and atom trapping”, http://arxiv.org/abs/atom-ph/9510004
[6]D. Van Thourhout and J. Roels, “Optomechanical device actuation through the optical gradient force” (review article), Nature Photon. 4, 211 (2010), doi:10.1038/nphoton.2010.72

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See also: radiation pressure, optical tweezers, laser cooling, The Photonics Spotlight 2006-10-22
and other articles in the categories quantum optics, methods

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