How fast can the thermometer drop? If you're a physicist armed with a graphene sandwich, it could be as fast as 30 quadrillionths of a seconds, at least if you're studying electrons. A new study, by researchers from MIT, University of California, Riverside, and the Barcelona Institute of Science and Technology, investigates how ultra-fast techniques can be used to measure the temperature change of electrons in graphene. In the material, as well as other semiconductors, electrons can be quickly heated by photons that hit them, but rapidly cool off by sharing the heat with nearby electrons as they collide with each other. That process can be over in less than 1 trillionth of a second.
Now, the team has created a new experimental set-up to accurately measure the temperature fluctuations. To do that, they constructed a sandwich that uses two sheets of graphene to surround several layers of insulating boron nitride — as shown in the image above. Here, electrons have two options when they're heated by light in the top layer of graphene: bounce around and hit each other within that layer, or migrate down through the boron nitride.
By tweaking the conditions of the set-up, like the intensity of the light and the voltage across the sandwich, the team found they could influence which path the electrons took. By measuring their propagation through the sandwich, the team could see the electrons cool off in time periods as short as 30 quadrillionths of a seconds. The research is published in Nature.
In fact, the team has ended up creating a new kind of ultra-thin photodetector. The sandwich, made up of just several atom-thick layers of carbon and boron nitride, measures just 10 nanometres top-to-bottom, making it an extremely small and efficient way to measure the presence of light. It could go on to be used in new types of miniature integrated circuits.