New T-wave detector uses waves of the electronic sea in graphene

Waves in graphene. Credit: @tsarcyanide/MIPT

https://phys.org/news/2018-12-t-wave-detector-electronic-sea-graphene.html

A team of researchers from Russia, Great Britain, Japan and Italy has created a graphene-based terahertz detector. The study was published in Nature Communications.

Any system for wireless data transfer relies on electromagnetic wave sources and detectors, but they are not available for every kind of waves. The existing sources of terahertz radiation, which occupies 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.

Detector layout. The transistor channel, made of bilayer graphene (BLG), is sandwiched between two crystals of hexagonal boron nitride (hBN). This structure is placed on an oxidized silicon substrate (shown in gray). The two sleeves of a terahertz antenna are connected between the source and the top gate — that is, the left and top electrodes shown in gold. Signal voltage is read between the source and the drain terminals — the right and left electrodes. Credit: @tsarcyanide/MIPT

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.

The authors of the research presented a solution for the long-standing problem of resonant T-wave detection. They created a photodetector (figure 1) made of bilayer 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 bilayer 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.

"Our 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. We packed the same functionality into a dozen micrometers," said co-author Dmitry Svintsov, who heads the Laboratory of 2-D Materials for Optoelectronics at the Moscow Institute of Physics.

 Explore further: Dispute about terahertz photoresponse in graphene results in a draw

More information: Denis A. Bandurin et al. Resonant terahertz detection using graphene plasmons, Nature Communications (2018). DOI: 10.1038/s41467-018-07848-w

Abstract-Investigation of antenna-coupled Nb5N6 microbolometer THz detector with substrate resonant cavity

Xuecou Tu, Chengtao Jiang, Peng Xiao, Lin Kang, Shimin Zhai, Zhou Jiang, Run Feng Su, Xiaoqing Jia, Labao Zhang, Jian Chen, and Peiheng Wu

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-7-8990

Fabricating resonant cavities with conventional methods to improve the coupling efficiency of a detector in the terahertz (THz) region is difficult for the wavelength is too long. Here, we propose a solution by using the substrate cavity effect given that the substrate wavelength and thickness of the preparation device are in the same order. The planar dipole antenna-coupled Nb5N6 microbolometers with different substrate thicknesses were fabricated. The interference effect of the substrate cavity on the optical voltage response of the detector is analyzed experimentally and theoretically. The experimental results show that the optical response of the detector is determined by the length of the substrate cavity. Thus, the THz devices with different detection frequencies can be designed by changing the substrate cavity length. Furthermore, on the basis of this substrate cavity effect, an asymmetric coupled Fabry-Pérot (FP) cavity is constituted by simply placing a movable metallic planar mirror at the backside of the Si substrate. The incident THz radiation on the Nb5N6 microbolometer can be effectively manipulated by changing the substrate-mirror distance to modulate the phase relation between the reflect wave and the incident wave. The distinct radiation control can be observed, and the experiments can be well explained by numerically analyzing the responsivity dynamics that highlights the role of the FP cavity effect during radiation. All of the results discussed here can be extended to a broad range of frequency and other type of THz detectors.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Harnessing the rattling motion of oxygen ions to convert T-rays to visible light

Scientists have shown that terahertz rays can be converted to light visible to the human eye. The finding is a breakthrough for functional materials research and could lead to the development of a new kind of terahertz detector.

Top: A schematic representation of the nanoscale cages. The oxygen anions randomly occupy one sixth of the cages. Bottom: A photograph of the emission of visible light at a terahertz radiation level of 0.21 and output power of 50 W.

Credit: ACS Nano

https://www.sciencedaily.com/releases/2017/11/171128230400.htm

Scientists have successfully visualized terahertz radiation, popularly known as T-rays, using a crystal called mayenite (Ca12Al14O33). Their method cleverly utilizes the rattling motion caused by the vibration of oxygen ions inside the cage-like structures of the crystal.

In recent years, there has been growing interest in developing practical devices based on terahertz technology. With wavelengths longer than infrared light, T-rays are considered safer than conventional imaging systems. They are already used, for example, at airport security checkpoints, and are starting to be used more widely in areas such as medical screening, food inspection and analysis of artworks. The visualization of terahertz light itself, however, has so far proved challenging.

Now, Hideo Hosono of Materials Research Center for Element Strategy, Tokyo Tech and co-workers in Japan, Ukraine and the US have devised a simple approach to convert T-rays to bright, visible light. Their findings have been published in ACS Nano.

First, the study involved beaming T-rays onto the mayenite crystal using a gyrotron[1]. This led to the vibration of oxygen anions[2], which collide with the inside walls of the cages within the crystal. Each cage has an inner diameter of 0.4 nanometers and an outer diameter of 0.7 nanometers.

"The rattling of oxygen ions within the cages promotes upward energy conversion," Hosono explains. "Strong and frequent collisions of the oxygen ions induce electron transfer to neighboring empty cages. The excitation of the oxygen ions is key to the emission of visible light."

Spectroscopy measurements confirmed that the visible light originated from vibrations caused by the free-moving oxygen anions. The researchers took care to rule out the possibility of other sources such as black body radiation and surface polarization as reasons behind the production of visible light.

The study is an example of strategic research on functional materials under the Element Strategy initiative supported by Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT) and the Japan Science and Technology Agency (JST).

"The crystal in our study is just composed of calcium, aluminium and oxygen, all of which are in the top five of the most abundant elements," says Hosono. "So, it's one of the most inexpensive materials, at around 15 cents per kilogram."

Despite its simplicity, Hosono says that the crystal has many exciting properties due to its nanostructure. Drawing on 20 years of research, his group has already succeeded in demonstrating that the material has excellent catalytic properties for ammonia synthesis and superconductivity.

Best known for his pioneering work on iron-based superconductors, Hosono says that the current study marks a new research direction. "Our group has been concentrating on the cultivation of new functionalities using abundant elements, but it's the first time for me to focus on ionic motion -- this is completely new," he says.

The findings could lead to the development of a T-ray detector, as no such conventional detector has yet been designed.

Hosono adds: "Right now, our material is good at detecting strong terahertz radiation. The challenge will be how to adjust the sensitivity."

His group has also reported that the oxygen anions can be substituted with gold or hydrogen anions inside the cages. By making use of these different anions, it may be possible to develop detectors that emit different-colored light in future.

Technical terms

[1]Gyrotron: A device capable of generating high-power terahertz radiation. The gyrotron used in this study (called Gyrotron FU CW IV) was developed at the Research Center for Development of Far-Infrared Region, University of Fukui, Japan.

[2]Anions: Negatively charged ions. Together with their positively charged counterparts (cations), they hold together the structure of ionic crystals.

Abstract-Size effect on the performance of Nb5N6 bolometer for THz detection

 S. M. Zhai, X. C. Tu, C. T. Jiang,  P. Xiao,  X.Q. Jia, L. Kang,  J. Chen, P. H. Wu

http://ieeexplore.ieee.org/document/8066950/

Nb5N6 bolometer is a high sensitive terahertz detector at room temperature. The core part of traditional Nb5N6 bolometer is microbridge with a typical size of 3 μm × 12 μm. We have recently proposed a method using electron beam lithography (EBL) instead of UV lithography on fabrication of Nb5N6 micro bridge, which will reduce the size of the micro-bridge one to two orders of magnitude, the minimum up to 100 nm. We studied the relationship between the size of the micro bridge and the performance of the device, for further improving the sensitivity and the speed of the device and expanding the applications of this kind of detector.

Abstract-Terahertz detector with series connection of asymmetric gated transistors

D M Yermolaev1, K M Marem'yanin2, N A Maleev4, V E Zemlyakov1, V I Gavrilenko2, V V Popov3 and S Yu Shapoval1

yermolayev.denis@gmail.com

1 Institute of Microelectronic Technology and High-Purity Materials, Russia
2 Institute for Physics of Microstructures, Russia
3 Kotelnikov Institute of Radio Engineering and Electronics (Saratov Branch), Russia
4 Ioffe Physical Technical Institute, Russia 

http://iopscience.iop.org/1742-6596/486/1/012016;jsessionid=590E7E5B537BB812B7FF197D7794D0D8.c2

Terahertz (THz) detection by a one-dimensional array of series connected field-effect transistors (FETs) is studied experimentally. Such terahertz detector demonstrates greatly enhanced voltaic responsivity up to 2 kV/W. Asymmetrical position of the gate contact in each FET in the array enables strong photovoltaic response.

Panasonic Develops A Gallium Nitride (GaN) Terahertz Detector with High Sensitivity

June 23, 2010

Panasonic today announced the development of a new terahertz (THz) detector using a Gallium Nitride (GaN) transistor. The detector exhibits the world highest sensitivity at room temperature. The GaN-based THz detector is applicable to a variety of security or analyzing systems, which will greatly help the wide-spread use of such THz applications in the future.

The GaN detector forms a so-called plasma wave of the electrons, in which the electron density is fluctuated as a wave. The plasma wave resonates with the incident THz wave, which is detected as an electric signal at the GaN transistor. The use of GaN with high electron velocity effectively increases the amplitude of the plasma wave and the extracted electric signal.
The detector uses the gate electrode itself as a dipole antenna free from the loss in the transmission lines. In addition, the source and the drain electrodes of the GaN transistor are designed to work as parasitic elements for the antenna, which effectively confine the incident THz wave in the vicinity of the gate. Note that the employed metal-oxide-semiconductor (MOS) gate structure reduces the gate leakage current suppressing the leakage of the plasma wave around the gate antenna. The choice of the material together with a novel antenna structure successfully increases the sensitivity of the THz detector.
The fabricated THz detector using Panasonic's proprietary GaN technologies achieves a very high sensitivity of 1100 V/W at room temperature, while a conventional detector utilizing thermal conversion requires cooling of the device down to -270°C to maintain high enough sensitivity. The developed GaN-based THz detector free from such cooling systems can make the THz systems very compact keeping high sensitivity.
Applications for 9 domestic and 1 overseas patents have been filed. These research and development results have been presented at 68th Device Research Conference, held in South Bend, Indiana, U.S. from June 21 to 23, 2010.
Source: Panasonic