Graphene detector reveals polarization of terahertz light
Physicists from the Moscow Institute of Physics and Technology (MIPT) have created a broadband detector of terahertz radiation based on graphene that can detect circularly polarized light via helicity- and phase-sensitive conversion into DC photovoltage.1 Based on interference of plasmons, the device has potential for applications in communications and next-generation information transmission systems, security, and medical equipment.
Plasmons, which occur in metals, are essentially density waves involving charge carriers: electrons and holes. Their local density variation gives rise to an electric field, which nudges other charge carriers as it propagates through the material. Plasma waves die down rapidly in conventional conductors; that said, two-dimensional conductors enable plasma waves to propagate across relatively large distances without attenuation. It therefore becomes possible to observe their interference, yielding much information about the electronic properties of the material in question. The plasmonics of 2D materials has emerged as a highly dynamic field of condensed matter physics.
New property: polarization sensitivity
Over the past 10 years, scientists have come a long way detecting terahertz radiation with graphene-based-devices. Researchers have explored the mechanisms of terahertz interaction with graphene and created prototype detectors, whose characteristics are on par with those of similar devices based on other materials. However, studies have so far not looked at the details of detector interaction with distinctly polarized terahertz radiation. That said, devices sensitive to the waves’ polarization would be of use in many applications. The study reported in this story experimentally demonstrates how detector response depends on the polarization of incident radiation. Its authors also explain why this is the case.
The detector consists of a 4 x 4 mm silicon wafer and a piece of graphene 2 x 5 μm in size, says study co-author Yakov Matyushkin from the MIPT Laboratory of Nanocarbon Materials. “The graphene is connected to two flat contact pads made of gold, whose bow tie shape makes the detector sensitive to the polarization and phase of incident radiation," he explains. “Besides that, the graphene layer also meets another gold contact at the top, with a nonconductive layer of aluminum oxide interlaid between them.”
In microelectronics, this structure is known as a field transistor (see figure, with the two side contacts usually referred to as a source and a drain. The top contact is termed a gate.
“Terahertz radiation is directed at an experimental sample, orthogonally to its surface,” adds study co-author Georgy Fedorov, deputy head of the MIPT Laboratory of Nanocarbon Materials. “This generates photovoltage in the sample, which can be picked up by external measurement devices via the detector’s gold contacts. What’s crucial here is what the nature of the detected signal is. It can actually be different, and it varies depending on a host of external and internal parameters: sample geometry, frequency, radiation polarization and power, temperature, etc.”
Type of graphene is easy to make
Notably, the new detector relies on the kind of graphene already produced industrially. Graphene comes in two types: The material can either be mechanically exfoliated or synthesized by chemical vapor deposition. The former type has a higher quality, fewer defects and impurities, and holds the record for charge carrier mobility, which is a crucial property for semiconductors. However, it is CVD graphene that the industry can scalably manufacture already today, making it the material of choice for devices with an ambition for mass production.
The team showed that the nature of the new detector’s photoresponse has to do with plasma wave interference in the transistor channel. Wave propagation begins at the two opposite ends of the channel, and the special geometry of the antenna makes the device sensitive to the polarization and phase of the detected radiation. These features mean that the detector could prove useful in building communication and information transmission systems that operate at terahertz and subterahertz frequencies.
Source: Moscow Institute of Physics and Technology
REFERENCE:
1. Yakov Matyushkin et al., ACS Nano Letters (2020); https://doi.org/10.1021/acs.nanolett.0c02692.
John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.