RP Photonics logo
RP Photonics
Technical consulting services on lasers, nonlinear optics, fiber optics etc.
Profit from the knowledge and experience of a top expert!
Powerful simulation and design software.
Make computer models in order to get a comprehensive understanding of your devices!
Success comes from understanding – be it in science or in industrial development.
The famous Encyclopedia of Laser Physics and Technology – available online for free!
The ideal place for finding suppliers for many photonics products.
Advertisers: Make sure to have your products displayed here!
… combined with a great Buyer's Guide!
VLib part of the

Light Forces

<<<  |  >>>

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

German: Lichtkräfte

Categories: methods, quantum optics

How to cite the article; suggest additional literature

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.

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:


[1]T. W. Hänsch and A. L. Schawlow, “Cooling of gases with laser radiation”, Opt. Commun. 13, 68 (1975)
[2]D. J. Wineland and W. M. Itano, “Laser cooling of atoms”, Phys. Rev. A 20 (4), 1521 (1979)
[3]J. Ye et al., “Trapping of single atoms in cavity QED”, Phys. Rev. Lett. 83 (24), 4987 (1999)
[4]C. Savage, “Introduction to light forces, atom cooling, and atom trapping”, http://arxiv.org/abs/atom-ph/9510004
[5]D. Van Thourhout and J. Roels, “Optomechanical device actuation through the optical gradient force” (review article), Nature Photon. 4, 211 (2010)

(Suggest additional literature!)

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

If you like this article, share it with your friends and colleagues, e.g. via social media: