An international group of researchers has created a graphene-based terahertz detector. Any system for wireless data transfer relies on electromagnetic wave sources and detectors, but they are not available for all kinds of waves. The existing sources of terahertz radiation, which occupy a middle ground between microwaves and infrared light, consume too much power or require intense cooling. Yet T-waves could potentially enable faster Wi-Fi, new methods of medical diagnostics, and studies of space objects using radio telescopes.
The reason for the inefficiency of existing terahertz detectors is the mismatch between the size of the detecting element, the transistor—about one-millionth of a meter—and the typical wavelength of terahertz radiation, which is some 100 times greater. This results in the wave slipping past the detector without any interaction.
In 1996, it was proposed that to address this issue, the energy of an incident wave could be compressed into a volume comparable to the size of the detector. For this purpose, the detector material should support "compact waves" of a special kind, called plasmons. They represent the collective motion of conduction electrons and the associated electromagnetic field, not unlike the surface sea waves moving together with the wind as a storm sets in. In theory, the efficiency of such a detector is further increased under wave resonance. Implementing such a detector proved harder than anticipated. In most semiconductor materials, plasmons undergo rapid damping—that is, they die down—due to electron collisions with impurities. Graphene was seen as a promising way out, but until recently, it was not clean enough.
According to the new study that was published in Nature Communications, the researchers - hailing from Russia, Great Britain, Japan, presented a solution for the long-standing problem of resonant T-wave detection. They created a photodetector made of bi-layer graphene encapsulated between crystals of boron nitride and coupled to a terahertz antenna. In this sandwich structure, impurities are expelled to the exterior of the graphene flake, enabling plasmons to propagate freely. The graphene sheet confined by metal leads forms a plasmon resonator, and the bi-layer structure of graphene enables wave velocity tuning in a wide range.
In fact, the team has developed a compact terahertz spectrometer, several microns in size, with the resonant frequency controlled via voltage tuning. The physicists have also shown the potential of their detector for fundamental research: By measuring the current in the detector at various frequencies and electron densities, plasmon properties can be revealed. According to the team, the device doubles up as a sensitive detector and a spectrometer operating in the terahertz range, and it's also a tool for studying plasmons in two-dimensional materials. All of these things existed before, but they took up a whole optical table. The researchers packed the same functionality into a dozen micrometers.
Paper: Resonant Terahertz Detection Using Graphene Plasmons
