## Tuesday, April 20, 2021

### Abstract-Switchable generation of azimuthally- and radially-polarized terahertz beams from a spintronic terahertz emitter

Hiroaki Niwa, Naotaka Yoshikawa, Masashi Kawaguchi, Masamitsu Hayashi, and Ryo Shimano

 (a) THz generation from a spintronic THz emitter. Transient spin current 𝐣s${\mathbf{j}}_{\mathrm{s}}$ generated upon photoexcitation converts into charge current 𝐣c${\mathbf{j}}_{\mathrm{c}}$ via the inverse spin Hall effect, which radiates THz electric field with the polarization perpendicular to the magnetization 𝐦̂ $\stackrel{^}{\mathbf{m}}$. (b) Schematic of the experimental setup. (c) and (d) Schematic presentation of generating azimuthal and radial polarization by converting HE21${\text{HE}}_{\text{21}}$ mode with different orientations, respectively.
https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-9-13331&id=450178

We propose and demonstrate a method of generating two fundamental terahertz cylindrical vector beams (THz-CVBs), namely the azimuthally- and radially-polarized THz pulses, from a spintronic THz emitter. We begin by presenting that the spintronic emitter generates the HE21 mode, a quadrupole like polarization distribution, when placed between two magnets with opposing polarity. By providing an appropriate mode conversion using a triangular Si prism, we show both from experiment and numerical calculation that we obtain azimuthal and radial THz vector beams. The proposed method facilitates the access of CVBs and paves the way toward sophisticated polarization control in the THz regime.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

## Monday, April 19, 2021

### Graphene: Everything under control in a quantum material

 The gated graphene sample device in which the graphene film acts as a channel between source and drain electrodes subjected to a constant potential difference of 0.2 mV. Image from Science Advances

https://www.sciencedaily.com/releases/2021/04/210408131501.htm

In a new study, a team of researchers demonstrates that graphene's nonlinearity can be very efficiently controlled by applying comparatively modest electrical voltages to the material.

How can large amounts of data be transferred or processed as quickly as possible? One key to this could be graphene. The ultra-thin material is only one atomic layer thick, and the electrons it contains have very special properties due to quantum effects. It could therefore be very well suited for use in high-performance electronic components. Up to this point, however, there has been a lack of knowledge about how to suitably control certain properties of graphene. A new study by a team of scientists from Bielefeld and Berlin, together with researchers from other research institutes in Germany and Spain, is changing this. The team's findings have been published in the journal Science Advances.

Consisting of carbon atoms, graphene is a material just one atom thick where the atoms are arranged in a hexagonal lattice. This arrangement of atoms is what results in graphene's unique property: the electrons in this material move as if they did not have mass. This "massless" behavior of electrons leads to very high electrical conductivity in graphene and, importantly, this property is maintained at room temperature and under ambient conditions. Graphene is therefore potentially very interesting for modern electronics applications.

It was recently discovered that the high electronic conductivity and "massless" behavior of its electrons allows graphene to alter the frequency components of electric currents that pass through it. This property is highly dependent on how strong this current is. In modern electronics, such a nonlinearity comprises one of the most basic functionalities for switching and processing of electrical signals. What makes graphene unique is that its nonlinearity is by far the strongest of all electronic materials. Moreover, it works very well for exceptionally high electronic frequencies, extending into the technologically important terahertz (THz) range where most conventional electronic materials fail.

In their new study, the team of researchers from Germany and Spain demonstrated that graphene's nonlinearity can be very efficiently controlled by applying comparatively modest electrical voltages to the material. For this, the researchers manufactured a device resembling a transistor, where a control voltage could be applied to graphene via a set of electrical contacts. Then, ultrahigh-frequency THz signals were transmitted using the device: the transmission and subsequent transformation of these signals were then analyzed in relation to the voltage applied. The researchers found that graphene becomes almost perfectly transparent at a certain voltage -- its normally strong nonlinear response nearly vanishes. By slightly increasing or lowering the voltage from this critical value, graphene can be turned into a strongly nonlinear material, significantly altering the strength and the frequency components of the transmitted and remitted THz electronic signals.

"This is a significant step forward towards implementation of graphene in electrical signal processing and signal modulation applications," says Prof. Dmitry Turchinovich, a physicist at Bielefeld University and one of the heads of this study. "Earlier we had already demonstrated that graphene is by far the most nonlinear functional material we know of. We also understand the physics behind nonlinearity, which is now known as thermodynamic picture of ultrafast electron transport in graphene. But until now we did not know how to control this nonlinearity, which was the missing link with respect to using graphene in everyday technologies."

"By applying the control voltage to graphene, we were able to alter the number of electrons in the material that can move freely when the electrical signal is applied to it," explains Dr. Hassan A. Hafez, a member of Professor Dr. Turchinovich's lab in Bielefeld, and one of the lead authors of the study. "On one hand, the more electrons can move in response to the applied electric field, the stronger the currents, which should enhance the nonlinearity. But on the other hand, the more free electrons are available, the stronger the interaction between them is, and this suppresses the nonlinearity. Here we demonstrated -- both experimentally and theoretically -- that by applying a relatively weak external voltage of only a few volts, the optimal conditions for the strongest THz nonlin-earity in graphene can be created."

"With this work, we have reached an important milestone on the path towards to using graphene as an extremely efficient nonlinear functional quantum material in devices like THz frequency converters, mixers, and modulators," says Professor Dr. Michael Gensch from the Institute of Optical Sensor Systems of the German Aerospace Center (DLR) and the Technical University of Berlin, who is the other head of this study. "This is extremely relevant because graphene is perfectly compatible with existing electronic ultrahigh-frequency semiconductor technology such as CMOS or Bi-CMOS. It is therefore now possible to envision hybrid devices in which the initial electric signal is generated at lower frequency using existing semiconductor technology but can then very efficiently be up-converted to much higher THz frequencies in graphene, all in a fully controllable and predictable manner."

## Friday, April 16, 2021

### Abstract-Diversified functions for a terahertz metasurface with a simple structure

Wei-Mang Pan and Jiu-Sheng Li

 (a) Three-dimensional schematic diagram of terahertz metasurface with diversified functions, (b) Designed unit cell with the relevant geometric parameters
https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-9-12918&id=450048

Here, we propose a new encoded metasurface with diﬀerent predesigned coding sequences to dynamic manipulate terahertz wavefront and realize various functionalities including beam splitting, anomalous beam deflection, vortex beam generation, angle controlled single-beam deflection, angle controlled multi-beam deflection, angle-controlled vortex beam generation and multi-vortex beam generation. The far-ﬁeld scattering patterns obtained by CST Microwave Studio demonstrate the behavior of the terahertz wave in each case and shows a high consistency with our theoretical prediction results. Due to the excellent properties of the diversified functionalities in a single structure at terahertz frequencies, the proposed encoded metasurface provides promising applications in terahertz multiple-input, multiple-output (MIMO) communication.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

## Thursday, April 15, 2021

### Abstract-Tunable and multifunctional terahertz devices based on one-dimensional anisotropic photonic crystals containing graphene and phase-change material

Xiangfei Gao, Zebin Zhu, Jing Yuan, and Liyong Jiang

 Real part (a) and imaginary part (b) of the photonic bands of the graphene-Si 1D APC when kx = 0 and different chemical potentials are considered. (c) Corresponding transmission (T), reflection (R) and absorption (A) spectra of the graphene-Si 1D APC. The total layer number of 1D APC is 20. The green area represents the metallic band. The insets show the zoomed-in view for selected band gaps.
https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-9-13314&id=450177

In the past few years, designing tunable and multifunctional terahertz devices has become a hot research area in terahertz science and technology. In this work, we report a study on one-dimensional anisotropic photonic crystals (1D APCs) containing graphene and phase-change material VO2. We numerically demonstrate the band-pass filtering, perfect absorption, comb-shaped extraordinary optical transmission and Fano-like resonance phenomenon in pure 1D APCs and 1D APCs with a VO2 defect layer under different conditions of a tangential wave vector. The performance of these phenomena in the terahertz region can be modulated by changing the chemical potential of graphene. The band-pass filter and perfect absorber functions of 1D APCs with a VO2 defect layer can be freely switched by changing the phase of VO2. We employ the equivalent-permittivity model and dispersion-relation equation to give reasonable explanations on these behaviors.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

## Wednesday, April 14, 2021

### Abstract-Electrical tunability of terahertz nonlinearity in graphene

Sergey Kovalev,  Hassan A. Hafez, Klaas-Jan Tielrooij, Jan-Christoph Deinert, Igor Ilyakov, Nilesh Awari,  David Alcaraz. Karuppasamy Soundarapandian, David Saleta, Semyon Germanskiy, Min Chen, Mohammed Bawatna, Bertram Green, Frank H. L. Koppens, Martin Mittendorff, Mischa Bonn, Michael Gensch, Dmitry Turchinovich,

Graphene is conceivably the most nonlinear optoelectronic material we know. Its nonlinear optical coefficients in the terahertz frequency range surpass those of other materials by many orders of magnitude. Here, we show that the terahertz nonlinearity of graphene, both for ultrashort single-cycle and quasi-monochromatic multicycle input terahertz signals, can be efficiently controlled using electrical gating, with gating voltages as low as a few volts. For example, optimal electrical gating enhances the power conversion efficiency in terahertz third-harmonic generation in graphene by about two orders of magnitude. Our experimental results are in quantitative agreement with a physical model of the graphene nonlinearity, describing the time-dependent thermodynamic balance maintained within the electronic population of graphene during interaction with ultrafast electric fields. Our results can serve as a basis for straightforward and accurate design of devices and applications for efficient electronic signal processing in graphene at ultrahigh frequencies.

## Monday, April 12, 2021

### Terahertz Imaging of Graphene Paves the Way to Optimization and Industrialization

Graphene Flagship researchers have developed a new measurement standard for the analysis of graphene and layered materials that could accelerate production and optimize device fabrication. Credit: Graphene Flagship

Graphene Flagship researchers have developed a new measurement standard for the analysis of graphene and layered materials that could accelerate production and optimize device fabrication.

X-ray scans revolutionized medical treatments by allowing us to see inside humans without surgery. Similarly, terahertz spectroscopy penetrates graphene films allowing scientists to make detailed maps of their electrical quality, without damaging or contaminating the material. The Graphene Flagship brought together researchers from academia and industry to develop and mature this analytical technique, and now a novel measurement tool for graphene characterization is ready.

The effort was possible thanks to the collaborative environment enabled by the Graphene Flagship European consortium, with participation by scientists from Graphene Flagship partners DTU, Denmark, IIT, Italy, Aalto University, Finland, AIXTRON, UK, imec, Belgium, Graphenea, Spain, Warsaw University, Poland, and Thales R&T, France, as well as collaborators in China, Korea and the US.

Graphene is often ‘sandwiched’ between many different layers and materials to be used in electronic and photonic devices. This complicates the process of quality assessment. Terahertz spectroscopy makes things easier. It images the encapsulated materials and reveals the quality of the graphene underneath, exposing imperfections at critical points in the fabrication process. It is a fast, non-destructive technology that probes the electrical properties of graphene and layered materials, with no need for direct contact.

The development of characterization techniques like terahertz spectroscopy is fundamental to accelerating large-scale production, as they guarantee that graphene-enabled devices are made consistently and predictably, without flaws. Quality control precedes trust. Thanks to other developments pioneered by the Graphene Flagship, such as roll-to-roll production of graphene and layered materials, fabrication technology is ready to take the next step. Terahertz spectroscopy allows us to ramp up graphene production without losing sight of the quality.

Terahertz spectroscopy penetrates graphene films allowing scientists to make detailed maps of their electrical quality, without damaging or contaminating the material. Credit: Peter Bøggild (Graphene Flagship / DTU)

“This is the technique we needed to match the high-throughput production levels enabled by the Graphene Flagship,” explains Peter Bøggild from Graphene Flagship partner DTU. “We are confident that terahertz spectroscopy in graphene manufacturing will become as routine as X-ray scans in hospitals,” he adds. “In fact, thanks to terahertz spectroscopy you can easily map even meter-scale graphene samples without touching them, which is not possible with some other state-of-the-art techniques.” Furthermore, the Graphene Flagship is currently studying how to apply terahertz spectroscopy directly into roll-to-roll graphene production lines, and speed up the imaging.

Collaboration was key to this achievement. Graphene Flagship researchers in academic institutions worked closely with leading graphene manufacturers such as Graphene Flagship partners AIXTRON, Graphenea and IMEC. “This is the best way to ensure that our solution is relevant to our end-users, companies that make graphene and layered materials on industrial scales,” says Bøggild. “Our publication is a comprehensive case study that highlights the versatility and reliability of terahertz spectroscopy for quality control and should guide our colleagues in applying the technique to many industrially relevant substrates such silicon, sapphire, silicon carbide, and polymers.” he adds.

Setting standards is an important step for the development of any new material, to ensure it is safe, genuine and will offer a performance that is both reliable and consistent. That is why the Graphene Flagship has a dedicated work-group focused on the standardization of graphene, measurement and analytical techniques and manufacturing processes. The newly developed method for terahertz spectroscopy is on track to become a standard technical specification, thanks to the work of the Graphene Flagship Standardisation Committee. “This will undoubtedly accelerate the uptake of this new technology, as it will outline how analysis and comparison of graphene samples can be done in a reproducible way,” explains Peter Jepsen from Graphene Flagship Partner DTU, who co-authors the study. “Terahertz spectroscopy is yet another step to increase the trust in graphene-enabled products,” he concludes.

Amaia Zurutuza, co-author of the paper and Scientific Director at Graphene Flagship partner Graphenea, says: “At Graphenea, we are convinced that terahertz imaging can enable the development of quality control techniques capable of matching manufacturing throughput requirements and providing relevant graphene quality information, which is essential in our path towards the successful industrialization of graphene.”

Thurid Gspann, the Chair of the Graphene Flagship Standardisation Committee, says: “This terahertz [spectroscopy] technique is expected to be widely adopted by industry. It does not require any particular sample preparation and is a mapping technique that allows one to analyze large areas in a time efficient way.”

Marco Romagnoli, Graphene Flagship Division Leader for Electronics and Photonics Integration, adds: “The terahertz spectroscopy tool for wafer-scale application is a state-of-the-art, high TRL system to characterize multilayer stacks on wafers that contain CVD graphene. It works in a short time and with good accuracy, and provides the main parameters of interest, such as carrier mobility, conductivity, scattering time and carrier density. This high-value technical achievement is also an example of the advantage of being part of a large collaborative project like the Graphene Flagship.”

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: “Yet again, Graphene Flagship researchers are pioneering a new characterization technique to facilitate the development of graphene technology. This helps us progress steadily on our innovation and technology roadmap and will benefit the industrial uptake of graphene in a wide range of applications.”

Reference: “Case studies of electrical characterisation of graphene by terahertz time-domain spectroscopy” by Patrick R Whelan, Binbin Zhou, Odile Bezencenet, Abhay Shivayogimath, Neeraj Mishra, Qian Shen, Bjarke S Jessen, Iwona Pasternak, David M A Mackenzie, Jie Ji, Cunzhi Sun, Pierre Seneor, Bruno Dlubak, Birong Luo, Frederik W Østerberg, Deping Huang, Haofei Shi, Da Luo, Meihui Wang, Rodney S Ruoff, Ben R Conran, Clifford McAleese, Cedric Huyghebaert, Steven Brems, Timothy J Booth, Ilargi Napal, Wlodek Strupinski, Dirch H Petersen, Stiven Forti, Camilla Coletti, Alexandre Jouvray, Kenneth B K Teo, Alba Centeno, Amaia Zurutuza, Pierre Legagneux, Peter U Jepsen and Peter Bøggild, 17 February 2021, 2D Materials.