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:
- They play an essential role in the formation of tails of comets (see above), as already suggested by Johannes Kepler in 1619.
- They can be used for trapping, cooling and manipulating particles, e.g. with optical tweezers or in levitating traps, in high-power lasers (where dust particles can be captured and pulled towards cavity mirrors), or in optical clocks.
- They can be used as intracavity “photon thrusters” which are seriously considered as a tool for stabilizing the relative positions of satellites within satellite arrays for application e.g. in astronomy, geophysical monitoring, mapping, imaging, and surveillance.
- Unwanted light forces occur in interferometers for gravitational wave detection, where they can disturb the test masses.
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