When optical beams, consisting of photons, travel through fibers, they cause vibrations that generate acoustic waves, consisting of phonons. The phenomenon, called Brillouin scattering, has been harnessed by researchers to optomechanically “couple” acoustic waves with light waves. This coupling allows information carried by photons to be transduced, or converted, to the phonons, which travel nearly a million times more slowly than light waves.
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Source: Phys.org