The way that electrons interact with photons of light is a key part of many modern technologies, from lasers to solar panels and LEDs. But the interaction is inherently weak due to a large mismatch in scale: a wavelength of visible light is about 1000 times larger than an electron, so how the two things affect each other is limited by that. disparity.
Now, researchers at MIT and elsewhere have devised 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. The finding has potential implications for both commercial applications and fundamental scientific research, although it will require more years of research to be practical.
The findings are reported today in the journal Naturein a paper by MIT postdocs Yi Yang (now an assistant professor at the University of Hong Kong) and Charles Roques-Carmes, MIT professors Marin Soljačić and John Joannopoulos, and five others at MIT, Harvard University, and the Institute of Technology Technion-Israel.
In a combination of computer simulations and laboratory experiments, the team discovered that by using an electron beam in combination with a specially designed photonic crystal – a slab of silicon on an insulator, etched with a series of nanometer-scale holes – theoretically they could predict emission many orders of magnitude more intense than would normally be possible in conventional Smith-Purcell radiation. They also experimentally recorded a hundred-fold increase in radiation in their proof-of-concept measurements.
Unlike other approaches to produce light sources or other electromagnetic radiation, the free-electron method is fully 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 can make it especially valuable for creating emission sources at wavelengths that are difficult to produce efficiently, including terahertz waves, ultraviolet light, and X-rays.
The team has so far demonstrated the 100-fold improvement in emission using an electron microscope repurposed to function as an electron beam source. But they say the basic principle involved could potentially allow for much greater improvements using devices specifically tailored for this function.
The approach is based on a concept called flat bands, which have been widely explored in recent years for photonics and condensed matter physics, but have never been applied to affect the basic interaction of photons and free electrons. The underlying principle involves the transfer of momentum from the electron to a group of photons, or vice versa. While conventional light-electron interactions are based on the production of light at a single angle, the photonic crystal is tuned in such a way that it allows the production of a wide range of angles.
The same process could also be used in the opposite direction, using resonant light waves to drive electrons, increasing their speed in a way that could be harnessed to build miniaturized particle accelerators on a chip. Ultimately, these could perform some functions that currently require giant underground tunnels, such as the 30-kilometre-wide Large Hadron Collider in Switzerland.
“If you could actually build electron accelerators on a chip,” Soljačić says, “you could make much more compact accelerators for some of the applications of interest, which would still produce very energetic electrons. That would obviously be huge. For many applications, you would not have to build these huge facilities.”
The new system could also potentially provide a highly controllable X-ray beam for radiotherapy purposes, says Roques-Carmes.
And the system could be used to generate multiple entangled photons, a quantum effect that could be useful in creating quantum-based computing and communications systems, the researchers say. “You can use electrons to couple many photons together, which is a fairly difficult problem using a purely optical approach,” says Yang. “That’s one of the most exciting future directions for our work.”
Much work remains to be done to translate these new findings into practical devices, warns Soljačić. It may take a few years to develop the necessary interfaces between optical and electronic components and how to connect them on a single chip, and to develop the necessary on-chip electron source to produce a continuous wavefront, among other challenges.
“The reason this is exciting,” adds Roques-Carmes, “is because it’s quite a different type of source.” While most technologies for generating light are restricted to very specific ranges of color or wavelength, it is “generally difficult to move that emission frequency. Here it is completely tunable. By simply changing the speed of the electrons, you can change the frequency of emission. … That excites us about the potential of these sources. Because they are different, they offer new kinds of opportunities.”
But, concludes Soljačić, “for them to be really competitive with other types of sources, I think it will take a few more years of research. I would say that with a serious effort, in two to five years they could start to compete in at least some areas of radiation.”
The research team also included Steven Kooi at MIT’s Institute for Soldier Nanotechnologies, Haoning Tang and Eric Mazur at Harvard University, Justin Beroz at MIT, and Ido Kaminer at the Technion-Israel Institute of Technology. The work was supported by the US Army Office of Research through the Institute for Soldier Nanotechnologies, the US Air Force Office of Scientific Research, and the US Office of Naval Research.
