Coherent Backscattering

So what's Coherent Backscattering?

Coherent Backscattering (CBS) is a phenomenon of randomly scattered light, where photons traveling in the exact backscattered direction interfere constructively with themselves resulting in an enhancement factor of about 2 in the intensity observed....huh?

Basically, when you shine light at a bunch of particles that randomly scatter the light, the light that's scattered back almost exactly 180 degrees has a sort of speckled intensity distribution. If the particles are moving, or the sample is tilted and more images of the backscattered light are recorded and averaged, an intensity cone appears. This cone results from constructive interference between the individutal scattered photon and its time-reversed (momentum-reversed) path.

CBS has many implications in non-invasive biomedical techniques, because these methods (like optical coherence tomography) are dependant on examining how light interacts with and scatters inside biological tissue. Also, CBS has been linked with localization of photons, which is basically a photon being trapped within the scattering media.