The way electrons interact with photons of light is an essential part of many modern technologies, from lasers to solar panels to LED lights. But the interaction is inherently weak because of a huge mismatch in scale: The wavelength of visible light is about 1,000 times greater than that of an electron, so the way the two things affect each other is limited by that mismatch.
Now, researchers at MIT and elsewhere have come up with an innovative way to make much stronger interactions between photons and electrons possible, in the process producing a hundredfold increase in light emission from a phenomenon called Smith-Purcell radiation. This discovery has potential implications for both commercial applications and basic scientific research, though it will require many more years of research to make it practical.
The results are reported today in the journal naturein a paper written by MIT postdoctoral scholars Yi Yang (now assistant professor at the University of Hong Kong) and Charles Rock Karms, MIT professors Marin Soljic and John Jonopoulos, and five others at MIT, Harvard University, and the Technion-Israel Institute of Technology.
In a combination of computer simulations and lab experiments, the team found that using a beam of electrons with a specially designed photonic crystal — a slab of silicon on an insulator, etched with an array of nanometer holes — they could theoretically predict an emission several orders of magnitude stronger than would normally be possible. In traditional Smith-Purcell radiation. They also experimentally recorded a hundredfold increase in radiation in proof-of-concept measurements.
Unlike other methods for producing sources of light or other electromagnetic radiation, the free-electron-based method is completely tunable – it can produce emissions of any desired wavelength, simply by adjusting the size of the photonic structure and the speed of the electrons. This may make it particularly valuable for creating emission sources at wavelengths that are difficult to reproduce efficiently, including terahertz waves, ultraviolet light, and X-rays.
So far the team has shown a hundredfold enhancement in the emission by using a recombinant electron microscope to act as the electron beam source. But they say the basic principle involved could enable much greater improvements with devices specifically adapted for the function.
This approach is based on a concept called flat bands, which have been explored extensively in recent years for condensed matter physics and photonics but have never been applied to affect the fundamental interaction of photons and free electrons. The basic principle involves the transfer of momentum from an electron to a group of photons, or vice versa. Whereas conventional light-electron interactions rely on light being produced at a single angle, the photonic crystal is tuned in such a way as to allow a full range of angles to be produced.
The same process can also be used in reverse, using resonant light waves to propel electrons, increasing their speed in a way that can be harnessed to build miniature particle accelerators on a chip. These may eventually be able to perform some of the functions that currently require giant underground tunnels, such as the 30-kilometre-wide Large Hadron Collider in Switzerland.
“If you can actually build electron accelerators on a chip,” Soljačić says, “you can make more compact accelerators for some important applications, which will still produce very energetic electrons. Obviously, that would be huge. For many applications, you wouldn’t have to build these huge facilities.” .
The new system could also deliver a highly controllable beam of X-rays for radiotherapy purposes, Roques Karms says.
The system can be used to generate multiple entangled photons, a quantum effect that could be useful for creating quantum-based computational and communication systems, the researchers say. “You can use electrons to bind many photons together, which is a pretty difficult problem if you’re using a purely optical approach,” Yang says. “This is one of the most exciting future directions for our work.”
There is still a lot of work to do to translate these new findings into practical tools, Soljačić warns. It can take a few years to develop the necessary interfaces between optical and electronic components and how to connect them on a single chip, and develop the necessary on-chip electron source to produce a continuous wave interface, among other challenges.
“The reason this is exciting is because it’s a completely different kind of source,” adds Roux-Karms. While most light-generating technologies are limited to very specific ranges of color or wavelength, and it’s usually hard to move that emission frequency. About the potential of these sources. Because they are different, they offer new types of opportunities.”
But Soljačić concludes, “In order for them to become truly competitive with other types of sources, I think it will take a few more years of research. I would say that with some serious effort, they may start to compete in at least some areas of radiation within two to five years “.
The research team also included Stephen Cui at MIT Nanotechnologies Soldier, Huning Tang and Eric Mazur at Harvard University, Justin Peroz at MIT, and Edo Kaminer at the Technion Israel Institute of Technology. The work was supported by the US Army Research Office through the Institute for Soldier Nanotechnologies, the US Air Force Office of Scientific Research, and the US Office of Naval Research.