Coherent backscatter

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Coherent backscattering occurs when coherent radiation propagates in a medium that has a large number of scattering centers that are on the order of the radiation wavelength . An example is the propagation of laser light in milk or a thick cloud.

Paths of two rays of light in a disordered medium. Since the paths merge into one another through time inversion, they interfere constructively when the angle θ approaches zero.

When passing through the medium, the waves are scattered several times. Even for incoherent radiation there is a local maximum in the backscatter direction . In the case of coherent radiation, the intensity of this maximum doubles.

Backscatter is difficult to identify and measure for two reasons. The first reason is quite obvious as it is difficult to measure the reflected light without blocking the incoming light. However, this is a solvable problem. The second reason is that the distribution around the maximum at 180 degrees is usually very narrow, which requires a very high angular resolution in order to be able to recognize the maximum without averaging the intensity over surrounding solid angle areas in which the intensity can be greatly reduced . At other scattering angles, fluctuations called essentially random speckles occur.

Coherent backscattering is one of the most robust interference phenomena that survives multiple scattering and is seen as an aspect of the quantum mechanical phenomenon of weak localization . In the weak localization, the interference between the direct and inverse path leads to a reduction in the light transport in the forward direction. This phenomenon is characteristic of every wave that is scattered several times. Usually this is considered for light where it is similar to weak localization in disordered (semiconductors) and is often seen as a precursor to Anderson localization (or strong localization). The weak localization of light can be detected because it manifests itself in an increase in the light intensity in the backscatter direction. This gain is called the coherent backscatter cone.

Coherent backscatter originates from the interference between the direct and inverted path in the backscatter direction. If a medium with many scattering centers is illuminated by a laser beam, the intensity results from the superposition of the amplitudes of the various scattering paths; for a disordered medium, the interference terms are washed out when averaging over many configurations, with the exception of a narrow region around 180 degrees in which the mean intensity is increased. This effect is the accumulation of many two-wave interference patterns. The backscatter cone is the Fourier transformation of the spatial distribution of the scattered light on the sample surface when it is illuminated with a point light source. The increased backscatter is based on the constructive interference between two amplitudes which correspond to the two directions of a path.

See also

  • Opposition effect Astronomical phenomenon which is partly based on coherent backscattering

literature

Individual evidence

  1. ^ E. Akkermans, PE Wolf, R. Maynard: Coherent Backscattering of Light by Disordered Media: Analysis of the Peak Line Shape . In: Physical Review Letters . 56, No. 14, 1986, pp. 1471-1474. bibcode : 1986PhRvL..56.1471A . doi : 10.1103 / PhysRevLett.56.1471 . PMID 10032680 .