A wavelength of visible light is about 1000 times larger than an electron. Because of the difference in scale, the interactions between photons and electrons are inherently weak. MIT researchers have found that photonic flatbands can overcome the dimensionality mismatch between photons and electrons and significantly strengthen light-electron interactions. Their experimental results support the use of flatbands as test beds for developing strong light-electron interactions and efficient, compact, free-electron light sources. The underlying principle of flatbands 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, a structure — in this case a photonic crystal — is tuned in such a way that it enables the production of a whole range of angles. Although the use of flatbands has been explored for physics and photonics, flatbands have never been used to influence the interactions between photons and electrons. The researchers worked with colleagues at Harvard University and Technion-Israel Institute of Technology to design flatband resonances in a silicon-on-insulator photonic crystal slab, with the goal of controlling and enhancing the associated free-electron radiation. The researchers observed a two-order increase in emission, compared with conventional, diffraction-enabled Smith-Purcell radiation. They recorded a hundredfold increase in radiation in their proof-of-concept measurements. The enhancement enabled polarization shaping of free-electron radiation and characterization of photonic bands through electron-beam measurements. To demonstrate the enhancement in emission, the team repurposed an electron microscope to function as an electron beam source. According to the researchers, devices specifically adapted to function as an electron beam source could potentially enable far greater enhancements. The researchers’ free-electron-based approach to producing light is fully tunable. It can be tuned to produce light at a range of angles. Unlike most technologies for generating light, it can produce emissions of any desired wavelength and frequency. “It’s usually difficult to move that emission frequency,” researcher Charles Roques-Carmes said. “Here it’s completely tunable. Simply by changing the velocity of the electrons, you can change the emission frequency.” Researchers have created much stronger interactions between photons and electrons, while producing a hundredfold increase in the emission of light from a phenomenon called Smith-Purcell radiation in the process. Courtesy of Yi Yang et al. When the process of transferring momentum is used in the opposite direction (from photon to electron), resonant lightwaves are used to propel the electrons. This approach to increasing the electrons’ velocity could potentially be harnessed for building miniaturized particle accelerators on a chip, the researchers said. Ultimately, miniature particle accelerators could perform some functions that currently require facilities like the Large Hadron Collider in Switzerland. “If you could actually build electron accelerators on a chip, you could make much more compact accelerators for some of the applications of interest, which would still produce very energetic electrons,” professor Marin Soljacic said. “That obviously would be huge. For many applications, you wouldn’t have to build these huge facilities.” The tunability of the new approach could be useful for generating emission sources at wavelengths that are difficult to produce efficiently, including terahertz waves, ultraviolet light, and x-rays. The approach could also be used to generate multiple entangled photons for creating quantum computing and communications systems, the researchers said. “You can use electrons to couple many photons together, which is a considerably hard problem if using a purely optical approach,” professor Yi Yang said. It will take time to translate the approach to generating free-electron radiation into practical devices, Soljacic said. The necessary interfaces between the optical and electronic components coexisting on a single chip will need to be developed, as well as an on-chip electron source for producing a continuous wavefront. Soljacic believes that with serious effort, in two to five years the new approach to sourcing light could be competitive with some other areas of radiation. “The reason this is exciting is because this is quite a different type of source,” Roques-Carmes said. The research was published in Nature (www.doi.org/10.1038/s41586-022-05387-5).