Proposed optical terahertz graphene transistor

https://www.nanowerk.com/nanotechnology-news2/newsid=54715.php

(Nanowerk News) Researchers at the Center for Theoretical Physics of Complex Systems (PCS), within the Institute for Basic Science (IBS, South Korea) have proposed a transistor made of graphene and a two-dimensional superconductor that amplifies terahertz (THz) signals.

This research was conducted in collaboration with colleagues from the Micro/Nano Fabrication Laboratory Microsystem and Terahertz Research Center (China), the A. V. Rzhanov Institute of Semiconductor Physics (Russia), and Loughborough University (UK) and was published in Physical Review Letters ("Optical Transistor for Amplification of Radiation in a Broadband Terahertz Domain").

Graphic representation of the proposed terahertz (THz) amplification system with a graphene sheet and a two-dimensional superconductor. The amplification is due to the coordinated oscillatory behavior of the electrons at the interface between the two layers, powered by a light source or a battery, which results in stronger THz radiation, as shown with the reflected yellow arrow. (Image: IBS)

The growing interest in the THz frequency range can be easily explained by its various potential applications. This region of the electromagnetic spectrum, between radio waves and infrared light, is suited for extremely high-resolution images, non-invasive tumor detection, biosecurity, telecommunications, and encryption-decryption procedures, among others.

However, practically, finding a powerful source of rays in this frequency range is so challenging, that researchers commonly refer to this problem as the “Terahertz gap.”

In this work, the researchers proposed a novel strategy to amplify THz radiation from weak and non-uniform signals, which are common in, for instance, biological samples.

Illustration of the mechanism of THz waves amplification. (Image: IBS)

The device consists of a graphene sheet positioned in the vicinity of a two-dimensional superconductor and is connected to a power source, which provides enough energy to excite the electrons of the superconductor.

The THz signal amplification is explained by the collective oscillatory behavior of electrons in both of the two materials plus the quantum capacity of graphene.

“This work demonstrates the application-oriented perspectives of systems characterized purely by quantum effects. Light-matter interaction in these hybrid systems not only represent fundamental interest, but it can become a basis for future devices, such as terahertz logic gates, which are currently in high demand,” explains Ivan Savenko, the leader of the Light-Matter Interaction in Nanostructures (LUMIN) team at PCS IBS.

Graphene amplifier unlocks hidden frequencies in the electromagnetic spectrum

Light in the THz frequencies hits the ‘sandwich’ and is reflected with additional energy. Credit: Loughborough University

https://phys.org/news/2020-02-graphene-amplifier-hidden-frequencies-electromagnetic.html

by Peter Warzynski, Loughborough University

Researchers have created a unique device which will unlock the elusive terahertz wavelengths and make revolutionary new technologies possible.

Terahertz waves (THz) sit between microwaves and infrared in the light frequency spectrum, but due to their low-energy scientists have been unable to harness their potential.

The conundrum is known in scientific circles as the terahertz gap.

Being able to detect and amplify THz waves (T-rays) would open up a new era of medical, communications, satellite, cosmological and other technologies.

One of the biggest applications would be as a safe, non-destructive alternative to X-rays.

However, until now, the wavelengths—which range between 3mm and 30μm—have proved impossible to utilise due to relatively weak signals from all existing sources.

A team of physicists has created a new type of optical transistor—a working THz amplifier—using graphene and a high-temperature semiconductor.

The physics behind the simple amplifier replies on the properties of graphene, which is transparent and is not sensitive to light and whose electrons have no mass.

It is made up of two layers of graphene and a superconductor, which trap the graphene massless electrons between them, like a sandwich.

The device is then connected to a power source.

When the THz radiation hits the graphene outer layer, the trapped particles inside attach themselves to the outgoing waves giving them more power and energy than they arrived with—amplifying them.

Professor Fedor Kusmartsev, of Loughborough's Department of Physics, said: "The device has a very simple structure, consisting of two layers of graphene and superconductor, forming a sandwich (as shown above).

"As the THz light falls on the sandwich it is reflected, like a mirror.

A graphene amplifier. Credit: Loughborough University

"The main point is that there will be more light reflected than fell on the device.

"It works because external energy is supplied by a battery or by light that hits the surface from other higher frequencies in the electromagnetic spectrum.

"The THz photons are transformed by the graphene into massless electrons, which, in turn, are transformed back into reflected, energised, THz photons.

"Due to such a transformation the THz photons take energy from the graphene—or from the battery—and the weak THz signals are amplified."

The breakthrough—made by researchers from Loughborough University, in the UK; the Center for Theoretical Physics of Complex Systems, in Korea; the Micro/Nano Fabrication Laboratory Microsystem and THz Research Center, in China and the AV Rzhanov Institute of Semiconductor Physics, in Russia—has been published in Physical Review Letters.

The team is continuing to develop the device and hopes to have prototypes ready for testing soon.

Prof Kusmartsev said they hope to have a working amplifier ready for commercialisation in about a year.

He added that such a device would vastly improve current technology and allow scientists to reveal more about the human brain.

"The Universe is full of terahertz radiation and signals, in fact, all biological organisms both absorb and emit it.

"I expect, that with such an amplifier available we will be able to discover many mysteries of nature, for example, how chemical reactions and biological processes are going on or how our brain operates and how we think.

"The terahertz range is the last frequency of radiation to be adopted by humankind.

"Microwaves, infrared, visible, X-rays and other bandwidths are vital for countless scientific and technological advancements.

"It has properties which would greatly improve vast areas of science such as imaging, spectroscopy, tomography, medical diagnosis, health monitoring, environmental control and chemical and biological identification.

"The device we have developed will allow scientists and engineers to harness the illusive bandwidth and create the next generation of medical equipment, detection hardware and wireless communication technology."