Fast Electrons Radiate Vortex Photons

UV radiation from a relativistic electron beam is diffracted by a double-slit. In contrast to the normal light (left), the diffraction shows a deformation in the central part (right), indicating the existence of the phase singularity, which is a definite evidence of the vortex nature. (Source: IMS, NINS)

Researchers at the National Insti­tutes of Natural Sciences in Japan (NINS) and their coworkers have shown theore­tically and experi­mentally that a high energy electron in circular/spiral motion radiates vortex photons in the entire wave­length range from the radio-wave to the gamma-rays. This greatly broadens appli­cation spectra of the vortex photons in the field of physical science. Moreover, the finding indi­cates that vortex photons are ubi­quitous in the universe. It paves a way to a completely new research field, natural vortex photon science.

Normally, light has a plane wave front. In contrast, optical vortex possesses helical wave front and carries orbital angular momentum. Existence of such peculiar photons was theore­tically predicted about 25 years ago. Nowadays, such photons can be readily produced in labora­tories by using special optical devises, although the wave­lengths are limited in the vici­nity of the visible light. Researchers are exploring their appli­cations in nano­technology, imaging and infor­mation/communication tech­nologies. On the other hand, a natural elementary process which is capable of producing vortex photons has not been known, except for a few theo­retical proposals that normal photons may be converted to vortex photons in a strong gravi­tational field around a rotating black hall or as passing through inhomo­geneous interstellar media.

About 10 years ago, it was theore­tically predicted that an undulator, used in modern synchro­tron light sources, is capable of producing vortex photons in the X-ray range. This was experi­mentally confirmed several years later at a German synchro­tron. However, a majority of significant charac­teristics of optical vortex generation has remained unverified. NINS-Researchers and their coworkers theore­tically investi­gated this process and have found that it is based on a more general and elementary process. They have shown that an electron in circular or spiral motion radiates vortex photons.

Since this process is the basis of various important radiation processes in the astro­physics or the plasma physics, such as cyclotron radia­tion, synchro­tron radiation or Compton scattering, it has been described in many textbooks or research papers. However, so far, there has been no discus­sion on the vortex nature of this radiation. The wave­length of this radiation extends from the radio wave to the gamma-rays, depending on the physical condition. This new finding indi­cates that vortex photons are produced in various situations in the universe, in the entire wave­length range.

Further­more, the researchers succeeded in a precise experi­mental obser­vation of the undulator radiation from a synchro­tron light source, UVSOR-III, for the first time. They observed the UV light emitted from an electron beam in spiral motion, and indicated that a phase singu­larity exists in the center of the photon beam from the undulator. Moreover, higher energy photons have been shown to carry larger angular momenta. These measure­ments defi­nitely support the theo­retical prediction. Masahiro Katoh says: “Vortex photons should be ubiquitous in the universe. What kind of role do they play in nature? Our achieve­ment opens up a totally new research field”. (Source: NINS)

Reference: M. Katoh et al.: Helical Phase Structure of Radiation from an Electron in Circular Motion, Sci. Rep. 7, 6130 (2017); DOI: 10.1038/s41598-017-06442-2

Link: Inst. for Molecular Science, National Institutes of Natural Sciences NINS, Okazaki, Japan

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