Researchers at MIT and elsewhere have come up with an innovative way to enable much stronger interactions between photons and electrons, in a process that produces a hundredfold increase in light emission from a phenomenon called Smith-Purcell radiation. The finding are published today in the journal Nature & has potential implications for both commercial applications and basic science research, although it will require more years of research to become practical.
Using a combination of computer simulations and laboratory experiments, the team found that by using a beam of electrons combined with a specially designed photonic crystal a slab of silicon on an insulator etched with a series of nanometer-scale holes they could theoretically predict an emission many orders of magnitude stronger than would normally be possible in a conventional Smith-Purcell radiation. They also experimentally noted a hundredfold increase in radiation in their proof-of-concept measurements.
Unlike other approaches to producing sources of light or other electromagnetic radiation, the free electron method is fully tunable—it can produce emissions of any desired wavelength and simply adjusting the size of the photonic structure the speed of the electrons. This can be particularly valuable for creating emission sources at wavelengths that are difficult to produce efficiently or including terahertz waves, ultraviolet light, and X-rays.
So far, the team has demonstrated a hundredfold increase in emissions using a redesigned electron microscope that acts as an electron beam source. However, they say the basic principle could potentially allow for much greater improvements with devices specially adapted for this function.
This approach is based on a concept called flatbands, which has been widely explored in recent years for condensed matter physics and photonics, but has never been applied to affect the fundamental interaction of photons and free electrons. Basic principle involves transfer of momentum from an electron to a group of photons & vice versa. While conventional light-electron interactions rely on production of light single angle, the photonic crystal is tuned to allow the production range of angles.
The same process could also be used in reverse, using resonant light waves to propel electrons, increasing their speed in a way that could potentially be used to create miniaturized on-chip particle accelerators. These could eventually perform some of the functions that currently require giant underground tunnels, such as the 30-kilometer-wide Large Hadron Collider. “If you could actually build electron accelerators on a chip,” says Soljačić, “you could make much more compact accelerators for some of the interesting applications that would still produce very energetic electrons. That would be huge, of course”.
The new system potentially provide highly controllable X-ray beam for radiotherapy purposes, Roques-Carmes says. The system could be used to generate multiple entangled photons, a quantum effect that could be useful in building quantum-based computing or communication systems, the researchers say.
Much work remains to translate these new findings into practical devices, warns Soljačić. Developing the necessary interfaces between optical and electronic components and connecting them on a single chip can take several years, and developing the necessary on-chip electron source creating a continuous wavefront, among other things.
“The reason it’s exciting,” adds Roques-Carmes, “is that it’s quite a different type of resource.” While most technologies for generating light are limited to very specific ranges of colors or wavelengths, “it’s usually difficult to shift that emission frequency. Here it’s completely tunable. By simply changing the speed of the electrons, you can change the emission frequency.” This makes us excited about the potential of these resources. Because they are different, they offer new types of opportunities. But, concludes Soljačić, “for them to become truly competitive with other types of resources, I think it will require several more years of research. I would say that with a little bit of hard work, in two to five years, at least some areas of radiation could begin to compete”.
Reference: Yi Yang, et. at. Photonic flatband resonances for free-electron radiation. Nature, 2023; 613 (7942): 42 DOI: 10.1038/s41586-022-05387-5
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