The best way electrons interact with photons of sunshine is a key a part of many modern technologies, from lasers to solar panels to LEDs. However the interaction is inherently a weak one due to a serious mismatch in scale: A wavelength of visible light is about 1,000 times larger than an electron, so the way in which the 2 things affect one another is restricted by that disparity.
Now, researchers at MIT and elsewhere have give you an modern method to make much stronger interactions between photons and electrons possible, in the method producing a hundredfold increase within the emission of sunshine from a phenomenon called Smith-Purcell radiation. The finding has potential implications for each business applications and fundamental scientific research, although it should require more years of research to make it practical.
The findings are reported today within the journal , in a paper by MIT postdocs Yi Yang (now an assistant professor on the University of Hong Kong) and Charles Roques-Carmes, MIT professors Marin Soljačić and John Joannopoulos, and five others at MIT, Harvard University, and Technion-Israel Institute of Technology.
In a mixture of computer simulations and laboratory experiments, the team found that using a beam of electrons together with a specially designed photonic crystal — a slab of silicon on an insulator, etched with an array of nanometer-scale holes — they may theoretically predict stronger emission by many orders of magnitude than would ordinarily be possible in conventional Smith-Purcell radiation. In addition they experimentally recorded a one hundredfold increase in radiation of their proof-of-concept measurements.
Unlike other approaches to producing sources of sunshine or other electromagnetic radiation, the free-electron-based method is fully tunable — it could actually produce emissions of any desired wavelength, just by adjusting the dimensions of the photonic structure and the speed of the electrons. This will make it especially invaluable for making sources of emission at wavelengths which can be difficult to supply efficiently, including terahertz waves, ultraviolet light, and X-rays.
The team has to date demonstrated the hundredfold enhancement in emission using a repurposed electron microscope to operate as an electron beam source. But they are saying that the fundamental principle involved could potentially enable far greater enhancements using devices specifically adapted for this function.
The approach is predicated on an idea called flatbands, which have been widely explored in recent times for condensed matter physics and photonics but have never been applied to affecting the fundamental interaction of photons and free electrons. The underlying principle involves the transfer of momentum from the electron to a gaggle of photons, or vice versa. Whereas conventional light-electron interactions depend on producing light at a single angle, the photonic crystal is tuned in such a way that it enables the production of an entire range of angles.
The identical process may be utilized in the other way, using resonant light waves to propel electrons, increasing their velocity in a way that would potentially be harnessed to construct miniaturized particle accelerators on a chip. These might ultimately give you the chance to perform some functions that currently require giant underground tunnels, similar to the 30-kilometer-wide Large Hadron Collider in Switzerland.
“When you could actually construct electron accelerators on a chip,” Soljačić says, “you may make way more compact accelerators for among the applications of interest, which might still produce very energetic electrons. That obviously can be huge. For a lot of applications, you wouldn’t need to construct these huge facilities.”
The brand new system could also potentially provide a highly controllable X-ray beam for radiotherapy purposes, Roques-Carmes says.
And the system may very well be used to generate multiple entangled photons, a quantum effect that may very well be useful within the creation of quantum-based computational and communications systems, the researchers say. “You should use electrons to couple many photons together, which is a considerably hard problem if using a purely optical approach,” says Yang. “That’s some of the exciting future directions of our work.”
Much work stays to translate these recent findings into practical devices, Soljačić cautions. It might take some years to develop the obligatory interfaces between the optical and electronic components and easy methods to connect them on a single chip, and to develop the obligatory on-chip electron source producing a continuous wavefront, amongst other challenges.
“The explanation that is exciting,” Roques-Carmes adds, “is because this is sort of a unique variety of source.” While most technologies for generating light are restricted to very specific ranges of color or wavelength, and “it’s often difficult to maneuver that emission frequency. Here it’s completely tunable. Just by changing the rate of the electrons, you possibly can change the emission frequency. … That excites us in regards to the potential of those sources. Because they’re different, they provide recent kinds of opportunities.”
But, Soljačić concludes, “to ensure that them to develop into truly competitive with other kinds of sources, I feel it should require some more years of research. I might say that with some serious effort, in two to 5 years they could start competing in at the very 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 Technion-Israel Institute of Technology. The work was supported by the U.S. Army Research Office through the Institute for Soldier Nanotechnologies, the U.S. Air Force Office of Scientific Research, and the U.S. Office of Naval Research.