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<title>Coherent backscattering</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Coherent backscattering</span></span>

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<p>In physics, <b>coherent backscattering</b> is observed when <a href="Coherence_(physics)" title="Coherence (physics)">coherent</a> radiation (such as a <a href="Laser" title="Laser">laser</a> beam) propagates through a medium which has a large number of <a href="Scattering" title="Scattering">scattering</a> centers (such as milk or a thick cloud) of size comparable to the wavelength of the radiation.

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<p>The waves are scattered many times while traveling through the medium. Even for incoherent radiation, the scattering typically reaches a <a href="Maxima_and_minima" class="mw-redirect" title="Maxima and minima">local maximum</a> in the direction of <a href="Backscattering" class="mw-redirect" title="Backscattering">backscattering</a>. For coherent radiation, however, the peak is two times higher.

</p><p>Coherent backscattering is very difficult to detect and measure for two reasons. The first is fairly obvious, that it is difficult to measure the direct backscatter without blocking the beam, but there are methods for overcoming this problem. The second is that the peak is usually extremely sharp around the backward direction, so that a very high level of <a href="Angular_resolution" title="Angular resolution">angular resolution</a> is needed for the detector to see the peak without averaging its intensity out over the surrounding angles where the intensity can undergo large dips. At angles other than the backscatter direction, the light intensity is subject to numerous essentially random fluctuations called <a href="Speckle_pattern" class="mw-redirect" title="Speckle pattern">speckles</a>.

</p><p>This is one of the most robust <a href="Interference_(wave_propagation)" class="mw-redirect" title="Interference (wave propagation)">interference</a> phenomena that survives multiple scattering, and it is regarded as an aspect of a <a href="Quantum_mechanics" title="Quantum mechanics">quantum mechanical</a> phenomenon known as <a href="Weak_localization" title="Weak localization">weak localization</a> (Akkermans et al. 1986). In weak localization, interference of the direct and reverse paths leads to a net reduction of light transport in the forward direction. This phenomenon is typical of any coherent wave which is multiple scattered. It is typically discussed for light waves, for which it is similar to the weak localization phenomenon for electrons in disordered semi-conductors and often seen as the precursor to <a href="Anderson_localization" title="Anderson localization">Anderson</a> (or strong) localization of light. Weak localization of light can be detected since it is manifested as an enhancement of light intensity in the backscattering direction. This substantial enhancement is called the cone of coherent backscattering.

</p><p>Coherent backscattering has its origin in the interference between direct and reverse paths in the backscattering direction. When a multiply scattering medium is illuminated by a laser beam, the scattered intensity results from the interference between the amplitudes associated with the various scattering paths; for a disordered medium, the interference terms are washed out when averaged over many sample configurations, except in a narrow angular range around exact backscattering where the average intensity is enhanced. This phenomenon is the result of many sinusoidal two-wave interference patterns which add up. The cone is the <a href="Fourier_transform" title="Fourier transform">Fourier transform</a> of the spatial distribution of the intensity of the scattered light on the sample surface, when the latter is illuminated by a point-like source. The enhanced backscattering relies on the constructive interference between reverse paths. One can make an analogy with a <a href="Young's_interference_experiment" title="Young's interference experiment">Young's interference experiment</a>, where two diffracting slits would be positioned in place of the "input" and "output" scatterers.

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<ul><li><a href="Back_scattering_alignment" title="Back scattering alignment">Back scattering alignment</a> (BSA), a coordinate system most commonly used in radar</li>

<li><a href="Forward_scattering_alignment" title="Forward scattering alignment">Forward scattering alignment</a> (FSA), a coordinate system primarily used in optics</li>

<li><a href="Opposition_surge" title="Opposition surge">Opposition surge</a>, an astronomical phenomenon caused by the coherent backscatter effect</li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFAkkermansP._E._WolfR._Maynard1986" class="citation journal cs1">Akkermans, E.; P. E. Wolf; R. Maynard (1986). "Coherent Backscattering of Light by Disordered Media: Analysis of the Peak Line Shape". <i><a href="Physical_Review_Letters" title="Physical Review Letters">Physical Review Letters</a></i>. <b>56</b> (14): <span class="nowrap">1471–</span>1474. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1986PhRvL..56.1471A">1986PhRvL..56.1471A</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1103%2FPhysRevLett.56.1471">10.1103/PhysRevLett.56.1471</a>. <a href="PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/10032680">10032680</a>.</cite></li></ul></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2025-03-24" href="https://en.wikipedia.org/wiki/?title=Coherent_backscattering&oldid=1282101041">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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