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Čerenkov radiation

Light from ripples

Charged particles passing through a dielectric medium faster than the speed of light in that medium produce radiation at wavelengths from ultraviolet to visible — a kind of sonic boom for light, known as Čerenkov radiation. First detected by Pavel Čerenkov in 1934, the origin of this light was explained theoretically by Igor Tamm and Ilya Frank, and all three shared the Nobel Prize in Physics in 1958. Now Čerenkov-radiation detectors have applications in nuclear engineering, astrophysics and particle-physics experiments, such as Super-Kamiokande in Japan, which uses 50,000 tons of water and 13,000 photomultiplier tubes to pick up Čerenkov radiation from neutrinos.

Credit: © ISTOCKPHOTO/THINKSTOCK

Shenggang Liu and colleagues have, however, uncovered a new type of Čerenkov radiation (Phys. Rev. Lett., in the press). On the surface of a nanometre-thick metal film on a dielectric substrate, a uniformly moving bunch of electrons can excite electromagnetic surface waves — so-called surface polaritons — which ripple along the metal/dielectric interface. The surface polaritons propagate faster than the speed of light in the dielectric substrate and so Čerenkov radiation is emitted, its frequency adjustable by tuning the intensity of the electron beam.

Liu et al. suggest that this Čerenkov radiation from surface polaritons could be exploited as a tunable miniature light source. They calculate that radiation from ultraviolet to visible wavelengths and with a high power density could be generated, depending on the electron beam energy and current density. The choice of dielectric material and metal film is also important, and the authors have investigated two different structures: a dielectric material sandwiched between two thin metal films, and a cylindrical dielectric core surrounded by a thin metal film, which generates two-colour light.

A surface-polariton Čerenkov-radiation light source would have potential for on-chip integration, and hence widespread application. It is a welcome surprise, indeed, that the same physical phenomenon could prove valuable for both detectors and sources of light.

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Georgescu, I. Light from ripples. Nature Phys 8, 704 (2012). https://doi.org/10.1038/nphys2447

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