Quantum Tunneling in Graphene Advances the Age of High Speed Terahertz Wireless Communications

 

Quantum tunneling. Credit: Daria Sokol/MIPT Press Office

https://scitechdaily.com/quantum-tunneling-in-graphene-advances-the-age-of-high-speed-terahertz-wireless-communications/

Scientists from MIPT, Moscow Pedagogical State University and the University of Manchester have created a highly sensitive terahertz detector based on the effect of quantum-mechanical tunneling in graphene. The sensitivity of the device is already superior to commercially available analogs based on semiconductors and superconductors, which opens up prospects for applications of the graphene detector in wireless communications, security systems, radio astronomy, and medical diagnostics. The research results are published in a high-rank journal Nature Communications.

Information transfer in wireless networks is based on the transformation of a high-frequency continuous electromagnetic wave into a discrete sequence of bits. This technique is known as signal modulation. To transfer the bits faster, one has to increase the modulation frequency. However, this requires a synchronous increase in carrier frequency. A common FM-radio transmits at frequencies of hundred megahertz, a Wi-Fi receiver uses signals of roughly five gigahertz in frequency, while the 5G mobile networks can transmit up to 20 gigahertz signals.

This is far from the limit, and a further increase in carrier frequency admits a proportional increase in data transfer rates. Unfortunately, picking up signals with hundred gigahertz frequencies and higher is an increasingly challenging problem.

A typical receiver used in wireless communications consists of a transistor-based amplifier of weak signals and a demodulator that rectifies the sequence of bits from the modulated signal. This scheme originated in the age of radio and television, and becomes inefficient at frequencies of hundreds of gigahertz desirable for mobile systems. The fact is that most of the existing transistors aren’t fast enough to recharge at such a high frequency.

An evolutionary way to solve this problem is just to increase the maximum operation frequency of a transistor. Most specialists in the area of nanoelectronics work hard in this direction. A revolutionary way to solve the problem was theoretically proposed in the beginning of 1990’s by physicists Michael Dyakonov and Michael Shur, and realized, among others, by the group of authors in 2018. It implies abandoning active amplification by transistor, and abandoning a separate demodulator. What’s left in the circuit is a single transistor, but its role is now different. It transforms a modulated signal into bit sequence or voice signal by itself, due to non-linear relation between its current and voltage drop.

In the present work, the authors have proved that the detection of a terahertz signal is very efficient in the so-called tunneling field-effect transistor. To understand its work, one can just recall the principle of an electromechanical relay, where the passage of current through control contacts leads to a mechanical connection between two conductors and, hence, to the emergence of current. In a tunneling transistor, applying voltage to the control contact (termed as ‘’gate’’) leads to alignment of the energy levels of the source and channel. This also leads to the flow of current. A distinctive feature of a tunneling transistor is its very strong sensitivity to control voltage. Even a small “detuning” of energy levels is enough to interrupt the subtle process of quantum mechanical tunneling. Similarly, a small voltage at the control gate is able to “connect” the levels and initiate the tunneling current.

“The idea of ​​a strong reaction of a tunneling transistor to low voltages is known for about fifteen years,” says Dr. Dmitry Svintsov, one of the authors of the study, head of the Laboratory of 2D Materials for Optoelectronics at the MIPT center for Photonics and 2D materials. “But it’s been known only in the community of low-power electronics. No one realized before us that the same property of a tunneling transistor can be applied in the technology of terahertz detectors. Georgy Alymov (co-author of the study) and I were lucky to work in both areas. We realized then: if the transistor is opened and closed at a low power of the control signal, then it should also be good in picking up weak signals from the ambient surrounding. ”

The created device is based on bilayer graphene, a unique material in which the position of energy levels (more strictly, the band structure) can be controlled using an electric voltage. This allowed the authors to switch between classical transport and quantum tunneling transport within a single device, with just a change in the polarities of the voltage at the control contacts. This possibility is of extreme importance for an accurate comparison of the detecting ability of a classical and quantum tunneling transistor.

The experiment showed that the sensitivity of the device in the tunneling mode is few orders of magnitude higher than that in the classical transport mode. The minimum signal distinguishable by the detector against the noisy background already competes with that of commercially available superconducting and semiconductor bolometers. However, this is not the limit – the sensitivity of the detector can be further increased in “cleaner” devices with a low concentration of residual impurities. The developed detection theory, tested by the experiment, shows that the sensitivity of the “optimal” detector can be a hundred times higher.

“The current characteristics give rise to great hopes for the creation of fast and sensitive detectors for wireless communications,” says the author of the work, Dr. Denis Bandurin. And this area is not limited to graphene and is not limited to tunnel transistors. We expect that, with the same success, a remarkable detector can be created, for example, based on an electrically controlled phase transition. Graphene turned out to be just a good launching pad here, just a door, behind which is a whole world of exciting new research. ”

The results presented in this paper are an example of a successful collaboration between several research groups. The authors note that it is this format of work that allows them to obtain world-class scientific results. For example, earlier, the same team of scientists demonstrated how waves in the electron sea of ​​graphene can contribute to the development of terahertz technology. “In an era of rapidly evolving technology, it is becoming increasingly difficult to achieve competitive results.” – comments Dr. Georgy Fedorov, deputy head of the Laboratory of Nanocarbon Materials, MIPT, “Only by combining the efforts and expertise of several groups can we successfully realize the most difficult tasks and achieve the most ambitious goals, which we will continue to do.”

Reference: “Tunnel field-effect transistors for sensitive terahertz detection” by I. Gayduchenko, S. G. Xu, G. Alymov, M. Moskotin, I. Tretyakov, T. Taniguchi, K. Watanabe, G. Goltsman, A. K. Geim, G. Fedorov, D. Svintsov and D. A. Bandurin, 22 January 2021, Nature Communications.
DOI: 10.1038/s41467-020-20721-z

The work was supported by Russian Science Foundation (grant # 16-19-10557)  and Russian Foundation for Basic Research (grant # 18-29-20116 mk).

Abstract-Tight-Binding Terahertz Plasmons in Chemical-Vapor-Deposited Graphene

Andrey Bylinkin, Elena Titova, Vitaly Mikheev, Elena Zhukova, Sergey Zhukov, Mikhail Belyanchikov, Mikhail Kashchenko, Andrew Miakonkikh, and Dmitry Svintsov

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.11.054017

Transistor structures comprising graphene and subwavelength metal gratings hold great promise for plasmon-enhanced terahertz detection. Despite considerable theoretical effort, little experimental evidence for terahertz plasmons in such structures has been found so far. Here we report an experimental study of plasmons in graphene-insulator-grating structures using Fourier-transform spectroscopy in the 5–10-THz range. The plasmon resonance is clearly visible above the Drude absorption background even in chemical-vapor-deposited graphene with low carrier mobility of approximately ${10}^3$ ${\mathrm{cm}}^2/$V s. We show that the plasmon lifetime exceeds the transport relaxation time extracted from dc mobility, and argue that the former is weakly sensitive to scattering by grain boundaries and macroscopic defects inherent in chemical-vapor-deposited samples. We find that a grating coupler close to graphene strongly modifies the plasmon spectrum, which is determined by metal stripe width but not by grating period. We present a simple theory of grating-coupled two-dimensional plasmons, akin to the tight-binding theory of electrons in solids, that reproduces the observed resonant frequencies without fitting parameters. Our results demonstrate the prospect of large-area commercially available graphene for resonant terahertz detectors.

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Abstract-Resonant terahertz detection using graphene plasmons

Denis A. Bandurin, Dmitry Svintsov, Igor Gayduchenko, Shuigang G. Xu, Alessandro Principi, Maxim Moskotin, Ivan Tretyakov, Denis Yagodkin, Sergey Zhukov, Takashi Taniguchi, Kenji Watanabe, Irina V. Grigorieva, Marco Polini, Gregory N. Goltsman, Andre K. Geim,  Georgy Fedorov

https://www.nature.com/articles/s41467-018-07848-w

Plasmons, collective oscillations of electron systems, can efficiently couple light and electric current, and thus can be used to create sub-wavelength photodetectors, radiation mixers, and on-chip spectrometers. Despite considerable effort, it has proven challenging to implement plasmonic devices operating at terahertz frequencies. The material capable to meet this challenge is graphene as it supports long-lived electrically tunable plasmons. Here we demonstrate plasmon-assisted resonant detection of terahertz radiation by antenna-coupled graphene transistors that act as both plasmonic Fabry-Perot cavities and rectifying elements. By varying the plasmon velocity using gate voltage, we tune our detectors between multiple resonant modes and exploit this functionality to measure plasmon wavelength and lifetime in bilayer graphene as well as to probe collective modes in its moiré minibands. Our devices offer a convenient tool for further plasmonic research that is often exceedingly difficult under non-ambient conditions (e.g. cryogenic temperatures) and promise a viable route for various photonic applications.

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-Nonlinear response of infrared photodetectors based on van der Waals heterostructures with graphene layers

Victor Ryzhii, Maxim Ryzhii, Dmitry Svintsov, Vladimir Leiman, Vladimir Mitin, Michael S. Shur, and Taiichi Otsuji

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-5-5536&origin=search

We report on the device model for the infrared photodetectors based on the van der Waals (vdW) heterostructures with the radiation absorbing graphene layers (GLs). These devices rely on the electron interband photoexcitation from the valence band of the GLs to the continuum states in the conduction band of the inter-GL barrier layers. We calculate the photocurrent and the GL infrared photodetector (GLIP) responsivity at weak and strong intensities of the incident radiation and conclude that the GLIPs can surpass or compete with the existing infrared and terahertz photodetectors. The obtained results can be useful for the GLIP design and optimization.

© 2017 Optical Society of America

Abstract-Ultra-compact injection terahertz laser using the resonant inter-layer radiative transitions in multi-graphene-layer structure

Alexander A. Dubinov, Andrey Bylinkin, Vladimir Ya. Aleshkin, Victor Ryzhii, Taiichi Otsuji, Dmitry Svintsov

(Submitted on 29 Nov 2016)

https://arxiv.org/abs/1611.09523

The optimization of laser resonators represents a crucial issue for the design of terahertz semiconductor lasers with high gain and low absorption loss. In this paper, we put forward and optimize the surface plasmonic metal waveguide geometry for the recently proposed terahertz injection laser based on resonant radiative transitions between tunnel-coupled grapheme layers. We find an optimal number of active graphene layer pairs corresponding to the maximum net modal gain. The maximum gain increases with frequency and can be as large as ~ 500 cm-1 at 8 THz, while the threshold length of laser resonator can be as small as ~ 50 mkm. Our findings substantiate the possibility of ultra-compact voltage-tunable graphene-based lasers operating at room temperature.

Researchers clear the way for fast plasmonic chips

Schematic of the electrically pumped active hybrid plasmonic waveguide and energy density distribution of the surface plasmon field. Credit: Moscow Institute of Physics and Technology

http://phys.org/news/2015-08-fast-plasmonic-chips.html#jCp

Researchers from the Laboratory of Nanooptics and Plasmonics at the MIPT Center of Nanoscale Optoelectronics have developed a new method for optical communication on a chip, which will could decrease the size of optical and optoelectronic elements and greatly increase computer performance. According to an article published in Optics Express, they have proposed a way to completely eliminate energy loss of surface plasmons in optical devices.
"Surface plasmon polaritons have previously been proposed as information carriers for optical communication, but the problem is that the signal is rapidly attenuated propagating along plasmonic waveguides. Now, we have come very close to the complete solution of this problem. Our approach clears the way for the development of a new generation of high-performance optoelectronic chips," says Dmitry Fedyanin, the head of the research.

Modern electronics is based on the use of electrons as information carriers, but they have ceased to meet the contemporary requirements: Standard electrical copper wires and channels on chips cannot transfer information with speeds sufficient for modern microprocessors. This currently hinders microprocessor performance growth; hence, the implementation of new groundbreaking technologies is required to maintain Moore's law.

Transition from electrical to optical pulses can solve the problem. The high frequency of light waves (hundreds of terahertz) allows transferring and processing more data, and offers the possibility of increasing performance. Fiber optic technologies are widely used in communication networks, but the use of light in microprocessors and logical elements faces the problem of diffraction limit, since the size of waveguides and other optical elements cannot be significantly smaller than the light wavelength. These are micrometers for near-infrared radiation used in optical communications, which don't meet the requirements of contemporary electronics. Logical elements of standard contemporary processors are dozens of nanometers in size. "Optical electronics" can become competitive only if light is "compressed" to this scale.

Nanoscale plasmonic waveguides under the scanning electron microscope. Credit: Moscow Institute of Physics and Technology

Overcoming the diffraction limit is possible by using surface plasmon polaritons, which are collective excitations that emerge due to interaction between photons and electron oscillations on the boundary between a metal and an insulator. They are also called quasi-particles, because they are quite similar to standard particles such as photons or electrons. Unlike three-dimensional light waves, surface polaritons "hold on" the boundary between two media. This offers the possibility of switching from conventional three-dimensional optics to two-dimensional optics.

"Roughly speaking, a photon occupies a certain volume in space, which is of the order of the light wavelength. We can 'compress' it, transforming it into a surface plasmon polariton. Using this approach, we can improve the integration density and reduce the size of optical elements. Unfortunately, this brilliant solution has its flip side. For the surface plasmon polariton to exist, a metal, or more specifically, an electron gas in the metal, is needed. This leads to excessively high Joule losses similar to those observed when current is passed through metal wires or resistors," says Dr. Fedyanin.

According to Dr. Fedyanin, the surface plasmon energy drops a billion times at distances of around one millimeter due to absorption in the metal, which makes the practical implementation of surface plasmons pointless.

"Our idea is to compensate the surface plasmon propagation losses by pumping extra energy to surface plasmon polaritons. It should be also noted that, if we want to integrate plasmonic waveguides on a chip, we can use only electrical pumping," explains the researcher.

Operating principle of the proposed electrical pumping scheme. Credit: Moscow Institute of Physics and Technology

Together with his colleagues Dmitry Svintsov and Aleksey Arsenin from the Laboratory of Nanooptics and Plasmonics, he has developed a new method of electric pumping of plasmonic waveguides based on the metal-insulator-semiconductor (MIS) structure and carried out its simulations. The results show that the passage of relatively weak pump currents through the nanoscale plasmonic waveguides can fully compensate the surface plasmon propagation losses. This means that it is possible to transmit a signal over long distances (in chip standards) with no losses. At the same time, the integration density of such active plasmonic waveguides is an order of magnitude higher than that of photonic waveguides.

"Working in optoelectronics, we always need to find a compromise between optical and electrical properties, whereas in plasmonics, it is almost impossible, since the choice of metals is limited to three or four materials. The main advantage of the proposed pumping scheme is that it doesn't depend on the properties of the metal-semiconductor contact. For each semiconductor, we can find an appropriate insulator, which achieves the same efficiency level as in double-heterostructure lasers. At the same time, we are able to maintain the typical plasmonic structure size at a level of 100 nanometers," says Fedyanin.

The researches note that their results are awaiting an experimental verification, but the key difficulty has been eliminated.

 Explore further: IMEC reports method to integrate plasmonic technology with state-of-the-art ICs

More information: Svintsov, D. A., Arsenin, A. V., & Fedyanin, D. Y. (2015). Full loss compensation in hybrid plasmonic waveguides under electrical pumping. Optics Express, 23(15), 19358. DOI: 10.1364/oe.23.019358

Read more at: http://phys.org/news/2015-08-fast-plasmonic-chips.html#jCp

Abstract-Carrier-carrier scattering and negative dynamic conductivity in pumped graphene

Dmitry Svintsov, Victor Ryzhii, Akira Satou, Taiichi Otsuji, and Vladimir Vyurkov  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-17-19873

Optics Express, Vol. 22, Issue 17, pp. 19873-19886 (2014)
http://dx.doi.org/10.1364/OE.22.019873

We theoretically examine the effect of carrier-carrier scattering processes on the intraband radiation absorption and their contribution to the net dynamic conductivity in optically or electrically pumped graphene. We demonstrate that the radiation absorption assisted by the carrier-carrier scattering is comparable with Drude absorption due to impurity scattering and is even stronger in sufficiently clean samples. Since the intraband absorption of radiation effectively competes with its interband amplification, this can substantially affect the conditions of the negative dynamic conductivity in the pumped graphene and, hence, the interband terahertz and infrared lasing. We find the threshold values of the frequency and quasi-Fermi energy of nonequilibrium carriers corresponding to the onset of negative dynamic conductivity. The obtained results show that the effect of carrier-carrier scattering shifts the threshold frequency of the radiation amplification in pumped graphene to higher values. In particular, the negative dynamic conductivity is attainable at the frequencies above 6 THz in graphene on SiO₂ substrates at room temperature. The threshold frequency can be decreased to markedly lower values in graphene structures with high-κ substrates due to screening of the carrier-carrier scattering, particularly at lower temperatures.

© 2014 Optical Society of America

Abstract-Carrier-carrier scattering and negative dynamic conductivity in pumped graphene

Dmitry Svintsov, Victor Ryzhii, Akira Satou, Taiichi Otsuji, Vladimir Vyurkov

http://arxiv.org/abs/1405.4338

We theoretically examine the effect of carrier-carrier scattering processes (electron-hole and electron-electron) on the intraband radiation absorption and their contribution to the net dynamic conductivity in optically or electrically pumped graphene. We demonstrate that the radiation absorption assisted by the carrier-carrier scattering can be stronger than the Drude absorption due to the carrier scattering on disorder. Since the intraband absorption of radiation effectively competes with its interband amplification, this can substantially affect the conditions of the negative dynamic conductivity in the pumped graphene and, hence, the interband terahertz and infrared lasing. We find the threshold values of the frequency and quasi-Fermi energy of nonequilibrium carriers corresponding to the onset of negative dynamic conductivity. The obtained results show that the effect of carrier-carrier scattering shifts the threshold frequency of the radiation amplification in pumped graphene to higher values. In particular, the negative dynamic conductivity is attainable at the frequencies above 6 THz in graphene on SiO2 substrates at room temperature. The threshold frequency can be decreased to markedly lower values in graphene structures with high-k substrates due to screening of the carrier-carrier scattering, particularly at lower temperatures.