Graphene detector reveals THz light's polarization

                                          

https://www.eurekalert.org/pub_releases/2020-10/miop-gdr100820.php

Physicists have created a broadband detector of terahertz radiation based on graphene. The device has potential for applications in communication and next-generation information transmission systems, security and medical equipment. The study came out in ACS Nano Letters.

The new detector relies on the interference of plasma waves. Interference as such underlies many technological applications and everyday phenomena. It determines the sound of musical instruments and causes the rainbow colors in soap bubbles, along with many other effects. The interference of electromagnetic waves is harnessed by various spectral devices used to determine the chemical composition, physical and other properties of objects -- including very remote ones, such as stars and galaxies.

Plasma waves in metals and semiconductors have recently attracted much attention from researchers and engineers. Like the more familiar acoustic waves, the ones that occur in plasmas are essentially density waves, too, but they involve 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. This is similar to how the pressure gradient of a sound wave impels the gas or liquid particles in an ever expanding region. However, 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.

Over the past 10 years, scientists have come a long way detecting THz radiation with graphene-based-devices. Researchers have explored the mechanisms of T-wave 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 T-rays. That said, devices sensitive to the waves' polarization would be of use in many applications. The study reported in this story experimentally demonstrated how detector response depends on the polarization of incident radiation. Its authors also explained why this is the case.

Study co-author Yakov Matyushkin from the MIPT Laboratory of Nanocarbon Materials commented: "The detector consists of a silicon wafer 4 by 4 millimeters across, and a tiny piece of graphene 2 by 5 thousandths of a millimeter in size. 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. 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 (fig. 1), with the two side contacts usually referred to as a source and a drain. The top contact is called a gate.

Terahertz radiation is a narrow band of the electromagnetic spectrum between microwaves and the far infrared light. From the applications standpoint, an important feature of T-waves is that they pass through living tissue and undergo partial absorption but cause no ionization and therefore do not harm the body. This sets THz radiation apart from X-rays, for example.

Accordingly, the applications traditionally considered for T-rays are medical diagnostics and security screening. THz detectors are also used in astronomy. Another emerging application is data transmission at THz frequencies. This means the new detector could be useful in establishing the 5G and 6G next-generation communication standards.

"Terahertz radiation is directed at an experimental sample, orthogonally to its surface. This generates photovoltage in the sample, which can be picked up by external measurement devices via the detector's gold contacts," commented study co-author Georgy Fedorov, deputy head of the MIPT Laboratory of Nanocarbon Materials. "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."

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.

Another co-author of the study, Maxim Rybin from MIPT and Prokhorov General Physics Institute of the Russian Academy of Sciences is the CEO of graphene manufacturer Rusgraphene, and he had this to say about the technology: "The fact that it was CVD graphene that we observed plasma wave interference in, means such graphene-based THz detectors are fit for industrial production. As far as we know, this is the first observation of plasma wave interference in CVD graphene so far, so our research has expanded the material's potential industrial applications."

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 the detector could prove useful in building communication and information transmission systems that operate at THz and sub-THz frequencies.

The study reported in this story was co-authored by researchers from the MIPT Laboratory of Nanocarbon Materials and their colleagues from Moscow State Pedagogical University, Ioffe Institute of the Russian Academy of Sciences, and the University of Regensburg, Germany. This research was supported by the Russian Foundation for Basic Research and the Russian Ministry of Science and Higher Education.

Graphene heterostructures with black phosphorus, arsenic layers enable new infrared detectors

Credit: Daria Sokol/MIPT

https://phys.org/news/2020-04-graphene-heterostructures-black-phosphorus-arsenic.html
by Moscow Institute of Physics and Technology

MIPT scientists and their colleagues from Japan and the U.S. have calculated the parameters of photodetectors comprised of layers of graphene and a combination of black phosphorus and black arsenic. These sensors are able to detect radiation with energy less than the band gap of the constituent layers without graphene. It is also easy to modify them in order to increase their sensitivity to the required wavelength of light. Such sensors could replace far-infrared and terahertz radiation detectors. The research findings were published in the journal Optics Express. 

The new sensors will benefit many areas of science and technology. The far-infrared band is substantial both for household applications and for fundamental science. These waves are emitted by cosmic dust, whose study reveals the evolution of galaxies. Infrared light sensors are used in night vision equipment, remote controls, homing missiles, and heartbeat sensors. Terahertz radiation offers a less dangerous alternative to X-ray baggage scanners.

The researchers here considered far-infrared interband photodetectors based on a graphene monolayer. The graphene was surrounded by layers made of black phosphorus and black arsenic in varying proportions. By changing the ratio of these substances, it is possible to shift the working range of the photodetector. The energies inaccessible to electrons in black phosphorus and arsenic are different. The detector operates by registering an electron or hole entering the conduction band of black phosphorus or arsenic following a transition between two energy bands of graphene. However, temperature effects cause infrared and terahertz sensors to detect signal even "in the dark," in the absence of radiation. The layered structures examined in the study turned out to experience a dark current much lower than in those used today.

"We calculated the parameters of the light-sensitive elements for far-infrared detection based on a graphene monolayer. Such devices can replace almost any far-infrared and terahertz radiation sensors used today. The decreased dark current and the high photosensitivity significantly improve the signal-to-noise ratio even for low-intensity radiation. By applying a carefully calibrated voltage, the working range of the detectors can be changed without affecting signal reception quality. Such sensors could enhance the performance of infrared telescopes. According to calculations, at high temperatures the detectors will produce a much cleaner signal than the detectors used now," adds Victor Ryzhii, the head of the 2-D Materials and Nanodevices Laboratory.

Terahertz light pulses speed up spin switching

Researchers from the Moscow Institute of Physics and Technology and their colleagues from Germany and the Netherlands have achieved material magnetization switching on the shortest timescales, at a minimal energy cost. They have thus developed a prototype of energy-efficient data storage devices. CREDIT: @tsarcyanide/MIPT Press Office

https://physicsworld.com/a/terahertz-light-pulses-speed-up-spin-switching/

A new technique to rapidly reverse a magnet’s polarity in a way that all of its spins coherently rotate could be used to develop more energy-efficient data storage devices and superfast computers in the future. The technique, which works by applying ultrashort pulses of terahertz-frequency light to the magnet, does not produce any waste heat and requires very little energy – just one photon per spin flip.

Modern-day computer hard drives encode data as binary zeros and ones by orienting the spins in magnetic materials using magnetic field pulses created by an electrical current. This process dissipates huge amounts of energy though (and is relatively slow). Indeed, today’s data centres consume between 2 and 5% of the world’s electricity and produce waste heat that, in turn, requires even more power, to cool the servers down.

Researchers in Russia, Germany, the UK and the Netherlands have now exploited a novel, unprecedented strong interaction between the electric field of terahertz light pulses and magnetic spins. The effect, which they discovered in 2016 in the antiferromagnetic material thulium orthoferrite, makes the spins oscillate with large amplitude. This interaction, they found, is still not strong enough to switch the orientations of the spins, however, even using the most powerful THz radiation sources.

To be able to switch spins with THz light, the team designed and fabricated a special nanosized antenna (made of gold) and placed it on top of the thulium orthoferrite magnet. The antenna possesses plasmonic modes (collective oscillations of the metal’s conduction electrons) that increase the coupling between light and the antenna.

Enhancing the local light field

The device collects and focuses light at THz frequencies and enhances the local light electric field by more than 10 times. “This electric field is now strong enough to steer the magnetization of all the spins over a potential energy barrier and into a new orientation, in just picoseconds,” explains team member Rostislav Mikhaylovskiy, formerly of Radboud University in the Netherlands and now at Lancaster University in the UK. “This is because the photon energies of THz radiation are comparable to the energy needed to align the spins in the magnet.”

And that is not all: the temperature of the magnet does not increase at all during switching since the process requires the energy of just one quantum of the terahertz light – a single photon – per spin, he adds.nt

“The speed of purely electrical spin switching is typically limited to the GHz range by capacitances and inductances in electronic circuitry,” explains team member Christoph Lange of Regensburg University in Germany. “Most importantly, however, electronics inherently suffer from Ohmic energy losses, and subsequent heating. The flow of data in modern-day systems is now so intense that this waste heat is already restricting the performance of data centres and supercomputing facilities. Our approach avoids this problem by replacing electric current with light pulses.”

Coherent spin switching

To prove that they had indeed observed coherent switching of all the spins in the magnet, the researchers monitored the spin orientation using the polarization rotation imprinted on a short optical pulse that is delayed relative to the THz pulse. “If the initial spin deflection is not sufficient for synchronous spin switching, we observe a sinusoidal, oscillating signal in the polarization rotation,” explains Mikhaylovskiy. “If, on the other hand, the spins are switched, we observe a ‘beating’ signature on top of the oscillations, which is the characteristic ‘fingerprint’ of the spins deflecting over a potential barrier into a neighbouring local potential energy minimum.”

Our work is a major milestone in the worldwide research effort towards complete control of spins by THz pulses, Mikhaylovskiy tells Physics World. “The technology we have developed could enable highly energy-efficient data storage at greatly increased speeds as compared to existing technology. What is more, the coherent dynamics made possible by the extremely low energy dissipation may even allow for quantum information processing based on solid-state spins at THz clock rates.”

The team, which also includes Stefan Schlauderer and Rupert Huber from Regensburg University, Alexey Kimel of Radboud University and Anatoly Zvezdin from the Russian Academy of Sciences, now plans to continue its research at the new ultrafast laser at Lancaster University and accelerators at the Cockroft Institute. These facilities are able to generate intense pulses of THz light and the new experiments will allow the researchers to determine the practical and fundamental speed and energy limits of magnetic recording using THz light pulses.

The research is detailed in Nature 10.1038/s41586-019-1174-7.

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

The dispute about the origins of terahertz photoresponse in graphene results in a draw

CAPTION Photoresponse in graphene.

 CREDIT Lion_on_helium, MIPT press office

MOSCOW INSTITUTE OF PHYSICS AND TECHNOLOGY

https://www.eurekalert.org/pub_releases/2018-04/miop-tda042418.php

Physicists at MIPT and their British and Russian colleagues revealed the mechanisms leading to photocurrent in graphene under terahertz radiation. The paper published in Applied Physics Letters not only puts a period to a long-lasting debate about the origins of direct current in graphene illuminated by high-frequency radiation but also sets the stage for the development of high-sensitivity terahertz detectors. Such detectors are highly demanded in medical diagnostics, wireless communications and security systems.

In 2005 MIPT alumni Andre Geim and Konstantin Novoselov experimentally studied the behavior of electrons in graphene, a flat honeycomb lattice of carbon atoms. They found that electrons in graphene respond to electromagnetic radiation with an energy of quantum, whereas the common semiconductors have an energy threshold below which the material does not respond to light at all. However, the direction of electron motion in graphene exposed to radiation has long remained a point of controversy, as there is a plenty of factors pulling it in different directions. The controversy was especially stark in the case of the photocurrent caused by terahertz radiation.

What sets terahertz radiation apart is its unique set of properties. As an example, it easily passes through many dielectrics without ionizing them: this is of particular value to medical diagnostic or security systems. A terahertz camera can "see" the weapons concealed under a person's clothes, and a medical scanner can detect skin diseases at early stages by the spectral lines ("fingerprints") of characteristic biomolecules in the terahertz range. Finally, raising the carrier frequency of Wi-Fi devices from several to hundreds of gigahertz (into the sub-terahertz range) will proportionally increase the bandwidth. But all these applications need a sensitive and low-noise terahertz detector which is simple in fabrication.

A terahertz detector designed by researchers at MIPT, MSPU and the University of Manchester (the place where graphene was first discovered) is a graphene sheet (colored green in figures 1 and 2) sandwiched between dielectric layers of boron nitride and electrically coupled to a terahertz antenna--a metal spiral about a millimeter in size. As radiation impinges on the antenna, it rocks electrons on one side of the graphene sheet, while the resulting direct current is measured on the other side. It is the "packing" of graphene into boron nitride that enables record-high electric characteristics, giving the detector a sensitivity that is a cut above the earlier designs. However, the main result of the research is not a better-performing instrument; it is the insight into the physical phenomena responsible for the photocurrent.

There are three main effects leading to the electric current flowing in graphene exposed to terahertz radiation. The first one, the photothermoelectric effect, is due to the temperature difference between the antenna terminal and the sensing terminal. This sends electrons from the hot terminal to the cold one, like air rising up from a warm radiator up to cold ceiling. The second effect is the rectification of current at the terminals: it turns out that the edges of graphene let through only the high-frequency signal of a certain polarity. The third and most interesting effect is called plasma wave rectification. We can think of the antenna terminal as stirring up "waves in the electronic sea" of the graphene strip, while the sensing terminal registers the average current associated with these waves.

"Earlier attempts to explain the photocurrent in such detectors used only one of these mechanisms and excluded all the others," says Dmitry Svintsov, head of the Laboratory of 2d Materials' Optoelectronics at MIPT. "In reality, all three of them are at play, and our study found which effect dominates at which conditions. Thermoelectric effects dominate at low temperatures, while plasmonic rectification prevails at high temperatures and in longer-channel instruments. And the main thing is that we figured out how to make a detector in which the different photoresponse mechanisms will not cancel each other, but rather reinforce each other"

These experiments will help choose the best design for terahertz detectors and bring us closer to remote detection of dangerous substances, safe medical diagnostics, and high-speed wireless communications.

###

The work was supported by the Russian Science Foundation, the Ministry of Education and Science of the Russian Federation, the Leverhulme Trust (Great Britain) and the Russian Foundation for Basic Research

Solid-state physics offers insights into dielectric properties of biomaterials

MOSCOW INSTITUTE OF PHYSICS AND TECHNOLOGY

Some of the experimental data were obtained using a terahertz spectrometer based on backward-wave oscillators. (Credit MIPT Press Office)

https://www.eurekalert.org/pub_releases/2018-01/miop-spo010918.php

A team of Russian, Czech and German researchers gained a new perspective on the properties of three materials of biological origin. Besides two reference materials with well-studied properties -- serum albumin and cytochrome C -- the researchers looked at the extracellular matrix of the Shewanella oneidensis MR-1 bacterium, which is used in biofuel cells. The team measured the materials' dynamic conductivity and dielectric permittivity in a wide range of frequencies and temperatures. To interpret their findings, the researchers used theoretical approaches and concepts from condensed matter physics. The paper detailing the study was published in the journal Scientific Reports.

"So far, the formalism of condensed matter physics has only found limited use in classical biochemistry and biophysics. As a result, certain interesting effects evade our attention," says Konstantin Motovilov, senior research scientist at the Laboratory of Terahertz Spectroscopy at Moscow Institute of Physics and Technology (MIPT). "When we do make use of this language, we acquire new ways of modeling observed phenomena and describing biological structures. In our paper, we characterize the behavior of proteins, considered as classical amorphous semiconductors, with the help of the formalism of condensed matter physics."

Before discussing the study, here is a quick example of how solid-state physics explains the electrical properties of different materials.

There are in fact multiple mechanisms of electrical conductivity. For each, there is a corresponding theory that describes the properties of certain materials. For example, the conductivity in metals is adequately explained by the Drude theory. In the theory, there is no interaction between the conduction electrons, which are assumed to only occasionally collide with crystal lattice, impurities, and defects. Electrical conductivity is the inverse of electrical resistivity. Conductivity indicates how easy it is for an electric current to pass through a given material. Within the Drude model, this property does not depend strongly on frequency up to the frequency of the collisions between charge carriers and lattice or impurities. However, there is a large group of conductive materials that do not fit this description. Yet their behavior in an external electromagnetic field is quite interesting. Among them are glasses, ionic conductors, and amorphous semiconductors.

To qualitatively describe the electrical properties of such materials, another theory was proposed about 40 years ago by Andrzej Karol Jonscher, an English physicist. According to his theory, charge carriers -- electrons, for example -- can adequately be considered as free at room temperature, provided the alternating current frequency does not exceed several megahertz. Under these conditions, the Drude model is applicable and conductivity is nearly constant, i.e., it does not depend on the frequency of the external field. If, however, the frequency is higher, this description is no longer valid and there is an increase in conductivity proportional to a certain power -- which is close to 0.8 -- of frequency. The same effect is observed for materials that are gradually cooled, even if the frequency is kept constant.

Interestingly, different materials exhibit quite similar behavior in that regard. Moreover, if you restate the dependences -- say, talk about the ratio between direct current (static) conductivity and alternating current conductivity, as opposed to conductivity as such -- the relations for all materials turn out to be identical, revealing the so-called Universal Dielectric Response (UDR). This curious phenomenon was thoroughly investigated in a study that examined the conduction in glasses and other amorphous materials, offering new insights into their structure and properties.

The authors of the paper showed that Jonscher's law for conductivity applies to three organic materials. Among them, two are well-known reference proteins: bovine serum albumin and bovine heart cytochrome C. Their structural, physical, and chemical properties have been investigated in detail, so the researchers used them as reference materials.

In addition, they examined the extracellular matrix and filaments (EMF) of the Shewanella oneidensis MR-1 bacterium, which can produce electricity in biological fuel cells. S. oneidensis has been used in many studies with a focus on alternative energy sources, so its electrical properties are of interest to both researchers and engineers. In 2010, a team of researchers based in the United States and Canada showed that the bacterium's extracellular appendages behave a lot like p-type semiconductors. The electrical properties of S. oneidensis MR-1 have nevertheless not been studied in detail. The recently published paper is an attempt to remedy that.

The authors measured the conductivity of the materials, as well as the energy losses in a frequency range from 1 hertz to 1.5 terahertz, or trillion hertz, for temperatures from -260 to 40 degrees Celsius. (Strictly speaking, the energy losses are given by the imaginary part of the complex dielectric permittivity.) Next, the researchers measured the direct current conductivity of EMF for temperatures from zero to 40 C, as well as the temperature dependence of their heat capacity. For each of the three materials, water content and ion concentration were also determined.

To do this, the researchers pressed the substances into pellets using a 1-centimeter mold. They then applied electrodes to the faces of the pellets to pass alternating current through them in order to measure the electrical conductivity and dielectric permittivity of the materials in the 1-300 million hertz range. For higher frequencies, this approach does not work, so for the 30-1,500 gigahertz, or billion hertz, range, the team obtained the spectra of complex dielectric permittivity using quasioptical terahertz spectroscopy. No measurements were made in the intermediate frequency range.

It turned out that at room temperature, EMF conductivity is nearly constant, and when the frequency is increased above several million hertz, or several megahertz, the conductivity is proportional to a certain power -- which is close to 1 -- of the frequency. Cytochrome C did not exhibit such behavior unless the frequency was low and the temperature high. In the case of albumin, it was not observed at all. This suggests that different conductivity mechanisms are at play in these materials. It is likely that EMF has nearly free charges at room temperature -- just like in the Drude model -- whereas albumin does not have them and cytochrome C is a mixed bag.

The dependence observed by the researchers can be explained in terms of the individual properties of the materials. Both cytochrome C and albumin are regular proteins. Although these materials do have some free charges, these are not nearly as many as it would be necessary to justify the Drude model. Comparing the conductivity in EMF to that in metals (conductors) is more realistic, as free charges are more easily generated in these molecules. However, a comparison even more valid would be that with a solution of table salt, which has a high concentration of free ions.

Naturally, a complete description is more complex and would require us to take the water content of materials and other factors into account. For instance, because EMF contains significant amounts of loosely bound water, its conductivity grows quadratically at temperatures of about -250 C and frequencies on the order of 100 billion hertz (sub-terahertz terahertz range). Temperatures that low cause the bulk water in the material to freeze, and high frequencies mean that the dielectric properties resulting from water dipole dynamics become non-negligible. The other materials, too, exhibit deviations from Jonscher's predictions, but they are not as dramatic.

The authors have thus clearly shown the powerful methodology and instrumentation of condensed matter physics to be effective for fundamental research into the electrodynamics of biological objects. The next step could involve the application to biomaterials research of the wide range of other theories and models that have been effectively used by the physics community for many decades.