Researchers discover breakthrough method for stronger photon-electron interactions

Scientists from MIT and other institutions have developed a groundbreaking technique to significantly enhance interactions between photons and electrons, leading to a hundredfold increase in light emission. 

This breakthrough, known as Smith-Purcell radiation, has broad implications for both commercial applications and fundamental scientific research, according to a recent report by MIT News. However, further research and development are needed before the method can be implemented practically. Published in the journal Nature, the research paper details the work of MIT postdocs Yi Yang — now an assistant professor at the University of Hong Kong— and Charles Roques-Carmes, along with MIT professors Marin Soljačić and John Joannopoulos, and collaborators from Harvard University and the Technion-Israel Institute of Technology. 

In combining computer simulations with laboratory experiments, the team discovered that by utilizing a beam of electrons in conjunction with a specially designed photonic crystal — a silicon slab etched with nanometer-scale holes — they could achieve much stronger emission than traditional Smith-Purcell radiation. The researchers recorded a hundredfold increase in radiation during their proof-of-concept measurements, the report states.

According to the report, what sets this free-electron-based method apart from other approaches, which are limited to specific color or wavelength ranges, is its tunability. By changing the velocity of electrons, the emission frequency can be adjusted, unlocking new opportunities and applications and making it able to produce emissions at any desired wavelength by adjusting the size of the photonics structure and the speed of the electrons.

While Soljačić emphasizes that further research and effort are required to make these sources competitive with existing technologies, with dedicated work, he believes that within two to five years they may begin to rival other sources in some radiation applications. This versatility will make it particularly valuable for generating emission sources at wavelengths that are typically challenging to produce efficiently, such as terahertz waves, ultraviolet light and X-rays. 

The team has demonstrated a significant enhancement in emission using a repurposed electron microscope as an electron beam source. However, they believe that by developing devices specifically tailored for this purpose, even greater enhancements could be achieved. The underlying concept behind this breakthrough lies in a concept called flatbands, which have been extensively explored in condensed matter physics and photonics but have never been applied to photon-electron interactions until now. The principle involves the transfer of momentum between electrons and groups of photons. Unlike conventional light-electron interactions that rely on producing light at a single angle, the specially tuned photonic crystal enables the production of a range of angles.

Moreover, this process could also be employed in reverse, using resonant light waves to accelerate electrons. This acceleration could potentially lead to the development of miniaturized particle accelerators on a chip, replacing the need for large underground tunnels like the Large Hadron Collider, the report states. 

This comes as the implications of this research extend beyond photon-electron interactions. With this, the system could offer a highly controllable X-ray beam for radiotherapy applications and has the potential to generate multiple entangled photons, a quantum effect that could revolutionize quantum-based computational and communications systems. 

While the findings are promising, researchers caution that a large degree of work remains before practical devices can be realized. Developing interfaces between optical and electronic components, integrating them onto a single chip and creating an on-chip electron source are among the challenges that need to be addressed.

According to the report, despite the obstacles, the team is enthusiastic about the potential of these sources.  The research team also included collaborators Steven Kooi from MIT's Institute for Soldier Nanotechnologies, Haoning Tang and Eric Mazur from Harvard University, Justin Beroz from MIT, and Ido Kaminer from the Technion-Israel Institute of Technology. The study received support from the U.S. Army Research Office, the U.S. Air Force Office of Scientific Research, and the U.S. Office of Naval Research.