Strengthening Electron-Triggered Light Emission

Massachusetts Institute of Technology, Cambridge, MA

Researchers have found a way to create much stronger interactions between photons and electrons, in the process producing a hundred-fold increase in the emission of light from a phenomenon called Smith-Purcell radiation. (Image: Courtesy of the researchers/MIT)

A wavelength of visible light is about 1,000 times larger than an electron, so the way the two affect each other is limited by that disparity. Now, researchers have come up with a way to make much stronger interactions between photons and electrons possible — in the process producing a hundred-fold increase in the emission of light from a phenomenon called Smith-Purcell radiation.

The findings, reported in the journal Nature, show that using a beam of electrons in combination with a specially designed photonic crystal — a slab of silicon on an insulator, etched with nanometer-scale holes — could theoretically predict stronger emission by many orders of magnitude than would ordinarily be possible in conventional Smith-Purcell radiation.

Unlike other such approaches, the free-electron-based method is fully tunable; it can produce emissions of any wavelength by adjusting the size of the photonic structure and the speed of the electrons — ideal for making sources of emission at wavelengths that are difficult to produce efficiently (e.g., terahertz waves, ultraviolet light, and X-rays).

According to the team, the basic principle involved could potentially enable far greater enhancements using devices specifically adapted for this function.

The approach is based on a concept called flatbands, and the underlying principle involves the transfer of momentum from the electron to a group of photons — or vice versa. Whereas conventional light-electron interactions rely on producing light at a single angle, the photonic crystal is tuned in such a way that it enables the production of a whole range of angles.

“If you could actually build electron accelerators on a chip,” said Professor Marin Soljačić, “you could make much more compact accelerators for some of the applications of interest, which would still produce very energetic electrons. That obviously would be huge. For many applications, you wouldn’t have to build these huge facilities.”

The new system could potentially provide a highly controllable X-ray beam for radiotherapy purposes, said MIT postdoc Charles Roques-Carmes. Also, the system could be used to generate multiple entangled photons — a quantum effect that could be useful in the creation of quantum-based computational and communications systems.

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