Abstract-Ultra-broadband terahertz bandpass filter with dynamically tunable attenuation based on a graphene–metal hybrid metasurface

 

Wenli Huang, Xiaoqing Luo, Yuanfu Lu, Fangrong Hu, and Guangyuan Li

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-60-22-6366

We propose an ultra-broadband terahertz bandpass filter with dynamically tunable attenuation based on a graphene–metal hybrid metasurface. The metasurface unit cell is composed of two metal stripes enclosed with a graphene rectangular ring. Results show that when the metasurface is normally illuminated by a terahertz wave polarized along the metal stripes, it can act as an ultra-broadband bandpass filter over the spectral range from 1.49 THz to 4.05 THz, corresponding to a fractional bandwidth of 92%. Remarkably, high transmittance above 90% covering the range from 1.98 THz to 3.95 THz can be achieved. By changing the Fermi level of graphene, we find that the attenuation within the passband can be dynamically tuned from 2% to 66%. We expect that the proposed ultra-broadband terahertz bandpass filter with tunable attenuation will find applications in terahertz communication and detection and sensing systems.

© 2021 Optical Society of America

Abstract-Strong slow-light effect for a hexagonal graphene coupled metasurface in terahertz

 

Wei Cui, Yixuan Wang, Zhihui He, Hui He, 

https://www.sciencedirect.com/science/article/pii/S2211379721004824

Exploring higher group index has great benefits in the application of plasmonic slow-light devices. A hexagonal graphene coupled metasurface is proposed for realizing dual plasma induced transparency(dual-PIT) in terahertz through finite-difference time-domain (FDTD) method. The results show that dual-PIT can be effectively tuned by Fermi levels, carrier mobilities, and polarization angles. Moreover, the group index can be as high as ∼2553. These results may pave the way for the design of slow-light devices.

Using micro-sized cut metal wires, team forges path to new uses for terahertz waves

 

Researchers from Tokyo University of Agriculture and Technology successfully tested reflectionless, highly refractive index metasurfaces that may eventually be used in practical applications to send, receive, and manipulate light and radio waves in the terahertz waveband (THz). Credit: Takehito Suzuki, Tokyo University of Agriculture and Technology

by Tokyo University of Agriculture and Technology

https://phys.org/news/2021-05-micro-sized-metal-wires-team-forges.html

Japanese researchers successfully tested reflectionless, highly refractive index metasurface that may eventually be used in practical applications to send, receive, and manipulate light and radio waves in the terahertz waveband (THz). THz is measured in millionths of a meter, known as micrometers. The metasurface, an artificial two-dimensional flat material, was made of micro-sized cut metal wires of silver paste ink placed on both the front and back of a polyimide film. The team, led by Takehito Suzuki, Associate Professor at the Tokyo University of Agriculture and Technology (TUAT) Institute of Engineering, published their findings on April 29, 2021 in Optics Express.

Such flat metasurfaces represent a leap forward in the study of THz optics, because they may be flexible, adaptable to a much wider array of potential uses, and far smaller than the present generation of THz optics which rely upon naturally occurring materials that have fixed indices of refraction in the THz waveband, such as cyclo-olefin polymer, magnesium oxide, and silicon. An index of refraction of a material shows that how slow electromagnetic waves travel in the material compared to a vacuum.

A greater ability to receive, transmit, control, and manipulate electromagnetic waves above 1.0 THz is necessary to unlock their potential, which remains largely untapped, according to Suzuki. "The reflectionless metasurface with a high refractive index above 1.0 THz can offer an accessible platform for terahertz flat optics such as 6G wireless communications and other possible commercial applications," Suzuki said. "In addition to vastly faster wireless data transfer speeds, a better ability to manipulate THz waves using metasurfaces may greatly advance technology in the areas of wavefront shaping, beam forming, polarization control, and optical vortices—subjects of great interest to the scientific and communication communities."

Suzuki's research team set out to support the greater scientific community's goal of replacing conventional three-dimensional bulky optical components with two-dimensional flat ones, a feat that would free up space and allow the development of smaller, more adaptable scientific and communication instruments, as well as more advanced security cameras.

The team, Harumi Asada, Kota Endo, and Takehito Suzuki, created their experimental metasurface using silver paste ink and a very thin polyimide film. Cut metal wires with a silver paste ink laid onto the film by a super-fine ink-jet printer (SIJ Technology, Inc.) capable of drawing lines in the order of 10 micrometers in width, yielded the result they had hoped for: The metasurface, which was made of 80,036 pairs of cut metal wires with silver paste ink on both the front and back of 6x6 square millimeters (roughly an infant's thumbnail) plot of a polyimide film, has a high refractive index and low reflection at 3.0 THz.

Suzuki and his collaborating scientists plan to further investigate the potential of flat optics for use in the THz waveband, with the hope of finding scalable, commercially viable materials suitable for a wide array of future uses.

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Metasurfaces for manipulating terahertz waves

                                              
LIGHT PUBLISHING CENTER, CHANGCHUN INSTITUTE OF OPTICS, FINE MECHANICS AND PHYSICS, CAS

The associated THz responses include focusing, holograms, polarization modulation, special beams and active controlling.

CREDIT

by Xiaofei Zang, Bingshuang Yao, Lin Chen, Jingya Xie, Xuguang Guo, Alexei V. Balakin, Alexander P. Shkurinov, and Songlin Zhuang

https://www.eurekalert.org/pub_releases/2021-03/lpcc-mfm032221.php

THz waves have a plethora of applications ranging from biomedical and medical examinations, imaging, environment monitoring, to wireless communications, because of the abundant spectral information, low photon energy, strong penetrability, and shorter wavelength. THz waves with technological advances not only determined by the high-efficiency sources and detectors but also decided by a variety of the high-quality THz components/functional devices. However, traditional THz devices should be thick enough to realize the desired wave-manipulating functions, hindering the development of THz integrated systems and applications. Although metamaterials have been shown groundbreaking discoveries due to the tunable electric permittivity and magnetic permeability of a meta-atom, they are limited to technical challenges of fabrication and high loss of the metal-based unit cell.

In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Songlin Zhuang from Terahertz Technology Innovation Research Institute, University of Shanghai for Science and Technology, and co-workers have summarized the recent advancements of metasurfaces for the manipulation of THz waves. These ultra-compact devices with unusual functionalities render metasurface devices very attractive for applications such as imaging, encryption, information modulation and THz communications.

Actually, metasurfaces typically consist of planar antennas that enable predesigned EM responses. The antennas are made by metals or traditional high-refractive index dielectrics that can be easily fabricated based on the standard fabrication process. In addition, metasurfaces with the functionality in manipulating EM waves are dependent on the abrupt phase changes at planar antenna interfaces, and thus the thickness of metasurfaces is much thinner than the incident wavelength. Metasurfaces can locally control the wavefront of EM waves at subwavelength resolution, leading to various practical applications such as metalens, waveplates, vortex beam generators, beam steering and holograms. The ultrathin nature of metasurfaces, the ease of fabrication, and the subwavelength resolution in manipulating of EM waves make metasurfaces ideal candidates for THz device miniaturization (ultra-compact THz devices) and system integration.

The metasurface-based approach for manipulatig THz waves enables remarkable contributions in designing ultra-thin/ultra-compact and tunable THz components. The main advantages/contributions of THz metasurfaces can be concluded as follows: (1) THz components with reduced size: The functionalities of focusing, OAM, and polarization conversion realized by metasurfaces can be traditionally obtained by using a THz lens, helical phase plate, and half-wave (or quarter-wave) plate, respectively; (2) THz components with multiple functions: The traditional THz devices, e.g. THz lenses, waveplates, etc..., are always show a single function. Metasurfaces not only provide a flexible platform to realize ultra-thin/ultra-compact THz devices with single function, but also enable the unprecedented capability in designing multifunctional THz devices. (3) THz components with tunable function: Metasurfaces combined with VO2, graphene, etc, open a new avenue for designing THz components with active functions.

In conclusion, metasurfaces with planar structures can locally modify the wavefront of THz waves at subwavelength resolution. Metasurfaces not only provide an ultra-compact platform for manipulating the wavefront of THz waves, but also generate a plethora of applications that are difficult to achieve with conventional functional devices. As an overview, the recent developments of metasurfaces for manipulating THz waves were presented in this paper, and this progress report may open a new avenue to design ultra-thin or ultra-compact THz functional devices and systems.

Abstract-Metasurfaces for Terahertz Wavefront Modulation: a Review

Jingwen He, Tao Dong, Baihong Chi, Yan Zhang,

https://link.springer.com/article/10.1007/s10762-020-00677-3

Metasurface is an artificial material composed of a series of subwavelength structure units and has unique electromagnetic characteristics. Based on the ability of manipulating the phase, amplitude, and polarization of electromagnetic wave, various kinds of metasurfaces are designed to realize wavefront manipulations, such as beam focusing, beam steering, vector beams generating, and holographic imaging. This paper reviews the design methods of metasurfaces for wavefront modulation and evolution of the metasurfaces designed for wavefront manipulation in the terahertz (THz) region. The metasurfaces can be divided into two categories: passive and active metasurfaces. For the passive metasurfaces, the single-functional metasurfaces, multifunctional metasurfaces, and high diffraction efficient metasurfaces designed for various THz wavefront shaping, such as focusing, imaging, and special beams generating, are reviewed. For the active metasurfaces, the metasurfaces with fixed structure and all-optical metasurfaces without fixed structure for THz wavefront modulation are summarized. Furthermore, a comparison on the performance of different kinds of metasurfaces for THz wavefront modulation is presented and the development direction and challenges of the THz wavefront modulation metasurfaces in the future are discussed.

New Metasurface Helps Make the Switch to Terahertz Frequencies

Image adapted with permission from Siday et al., “Terahertz detection with perfectly-absorbing photoconductive metasurface.” Nano Letters, 19, 2888-2896 (2019). Copyright 2020 American Chemical Society.

Graphic depicting a photoconductive metasurface. Each unit cell supports resonance of light on two axes of symmetry (labeled Hx and Hz) to enable perfect light absorption. The inset image shows a scanning electron micrograph of a terahertz detector.

A new photoconductive metasurface paves the way for smaller, more efficient terahertz detectors.

Researchers are finding promising new applications for terahertz radiation (between microwaves and infrared light). Terahertz radiation could lead to new capabilities in imaging, wireless communications, and other areas. To expand its use, we need new devices designed for terahertz frequencies. Those needs include switches that work in less than a thousandth of a second, have a high contrast between “off” and “on” states, and efficiently carry electrical charges. Researchers have developed a new metasurface—an artificial material only billionths of a meter thick—that does all three.

The Impact

The new highly efficient metasurface switch needs no metal or backing material. This means detectors using the switch would be smaller, thinner, more efficient, and require less power. That makes them lower in cost and easier to integrate into terahertz detectors for communications and imaging. Terahertz radiation has potential safety and other benefits compared to other frequencies.

Summary

Terahertz radiation is in the band of frequencies higher than microwaves but lower than infra-red light. Terahertz radiation is very safe, and technologies such as wireless communications and imaging systems may eventually use this frequency range. Researchers are therefore developing devices that can efficiently generate, detect, and modulate terahertz waves. A key technology for terahertz devices is an ultrafast photoconductive switch,a switch that would use light to change electrical conductivity between a highly resistive “off” state and a highly conductive “on” state in less than a picosecond (one trillionth of a second). To achieve this performance, these switches need high contrast in conductivity between the “on” and “off” states. Until recently, researchers have not been able to meet this requirement.

Now, researchers at the Center for Integrated Nanotechnologies, a Department of Energy Nanoscale Science Research Center, have solved that challenge. The researchers developed a perfectly absorbing ultrathin photoconductive metasurface. The metasurface is a network of tiny repeating structures called resonators. These connected resonators support magnetic dipole modes and can efficiently absorb light at a desired wavelength. They achieve perfect light absorption using two overlapping resonances with the correct symmetry. The resulting metasurface is a tenth of the thickness of the original photoconductive switch. The thinner the device, the greater the contrast between “off” and “on” states. The metasurface could be applied to applications such as efficient fast modulators, a critical component of modern communication systems, and smaller, more efficient terahertz detector arrays.

Funding

This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and by Britain’s Engineering and Physical Sciences Research Council. Fabrication, optical, and some terahertz experiments were performed at the Center for Integrated Nanotechnologies (CINT), a Department of Energy Office of Science user facility.

Abstract-The investigation of the electromagnetic coupling effect in terahertz toroidal metasurfaces and metamaterials

Shuang Wang, Xiaoli Zhao, Song Wang, Quan Li, Jianyu Zhu,  Lei Han



https://www.sciencedirect.com/science/article/pii/S2238785419321970

We proposed and fabricated toroidal dipole (TD) metasurfaces(MSs) with a metamolecule of two coplanar U-shaped split ring resonators(USRRs) fabricated on polyimide substrate, and TD metamaterials (MMs) were the stacks of two TD MSs layers, whose frequencies, electromagnetic (EM) distributions and Q factor can be efficiently affected by the EM coupling effect in TD MSs/MMs. It was found that the resonances frequencies of TD MMs were shifted to lower frequencies due to the increase of inductance by the stacks of metallic layer. Meanwhile, the high-frequency TD resonance in TD MMs would be tailored by the periodicity. Considering the relation between TD resonances and the scattering power of TD (Ty), the Q factor depended highly on the value of Ty in the same metamolecule structure. The optimization in excitation of TD provide opportunity to further increase the Q-factor of metamaterial and pave a way for potential applications in terahertz sensor and other functional devices.

Abstract-Tunable THz Graphene Filter Based on Cross-In-Square-Shaped Resonators Metasurface

Anton Zaitsev, ,Alexander Grebenchukov, Mikhail Khodzitsky,

https://www.mdpi.com/2304-6732/6/4/119

The tunable terahertz (THz) Fano-resonant filter based on hybrid metal-graphene metamaterial was proposed. The optical parameters of metasurface with unit cell in the form of a cross-shaped graphene sheet in the center of a square gold ring were simulated by the finite element method using a surface conductivity model of a graphene monolayer. The narrowband modulation of the transmission by varying the Fermi level of the graphene and the position of graphene cross inside the metal ring was demonstrated. Simulation results were well explained theoretically using a three-coupled oscillator model. The proposed device can be used as a narrowband filter in wireless THz communication systems and sensing applications.

Abstract-Improvement of Wide-Angle Response for Terahertz Carpet Cloaking by Using a Metasurface with Multilayer Microstructure

Meng Li, Shuang Han, Haiyong Gan, Chenxia Li, Jianjun Liu, Zhi Hong, Xufeng Jing, 

https://link.springer.com/article/10.1007%2Fs10762-019-00617-w

A metasurface provides a completely new path to realize a cloaking effect by using phase compensation. However, the traditional invisible cloaks by using metasurfaces are limited at the narrow bandwidth and angle response. Here, we propose a multilayer metasurface to construct a cloak device, and more geometric freedom ensures the broadband and wide-angle response of our designed cloak. The invisible effect is revealed by near-field and far-field distribution for the ground plane, bare bump, and cloak, respectively. Our designed cloaking with the metasurface composed of five-layer unit cell structure has a good invisible effect at the broadband range from 0.7 to 0.8 THz and has a wide-angle response from 35 to 50°.

Abstract-Nonreciprocal Metasurface with Space-Time Phase Modulation

Xuexue Guo, Yimin Ding, Yao Duan, Xingjie Ni

https://arxiv.org/abs/1905.10316

Creating materials with time-variant properties is critical for breaking reciprocity that imposes fundamental limitations to wave propagation. However, it is challenging to realize efficient and ultrafast temporal modulation in a photonic system. Here, leveraging both spatial and temporal phase manipulation offered by an ultrathin nonlinear metasurface, we experimentally demonstrated nonreciprocal light reflection at wavelengths around 860 nm. The metasurface, with traveling-wave modulation upon nonlinear Kerr building blocks, creates spatial phase gradient and multi-terahertz temporal phase wobbling, which leads to unidirectional photonic transitions in both momentum and energy spaces. We observed completely asymmetric reflections in forward and backward light propagations within a sub-wavelength interaction length of 150 nm. Our approach pointed out a potential means for creating miniaturized and integratable nonreciprocal optical components.

Abstract-Multi-mode graphene based terahertz amplitude modulation enhanced by hollow cross H-structured metasurface

Physica Scripta

Liangping Xia, Yixuan Zou, Man Zhang, Wenjuan Yan, Suihu Dang, Songbai Li, Shaoyun Yin,  Hongliang Cui

https://iopscience.iop.org/article/10.1088/1402-4896/ab1bfe/pdf

A graphene-metasurface based THz amplitude modulator enhanced with the hollow cross H array in a metal film is proposed. Based on the transmission line theory, the equivalent analysis model of the modulator is built. With the model, the multi-mode resonation excited by the metasurface is analyzed and the amplitude modulations of the THz transmission are discussed. By applying modulation voltage between the metallic metasurface and graphene film in the experiment, three enhanced modes in the transmission spectrum are observed and the maximum modulation depth is enhanced to 49.3% in the band of 0.1-1.8THz. The experimental results exhibit a good agreement with the theory analysis.

Tuning terahertz transmission

A mounted device including the new tunable metasurface developed by Ding, Teng and co-workers. (right) When terahertz radiation hits the surface of interlinked p-type and n-type semiconducting silicon fingers, the amount of radiation reflected and transmitted can be controlled precisely using an applied voltage. Credit: A*STAR Institute of Materials Research and Engineering

https://phys.org/news/2019-04-tuning-terahertz-transmission.html

The ability to manipulate light on a subwavelength-scale could lead to a revolution in photonic devices such as antennas, solar panels, and even cloaking devices. Nanotechnology advances have made this possible through the development of metasurfaces, materials covered in features smaller than the wavelength of the light.

Now, a team led by A*STAR researchers has produced a highly promising metasurface that can be precisely controlled using a conventional electrical circuit so that it reflects and transmits different amounts of radiation. It can even reach the condition of 'perfect antireflection' where it reflects no radiation at all. Specifically, the surface works with broadband terahertz radiation, which is found at the far end of the infrared spectrum and has many potential uses, particularly in security or medical fields.

"Terahertz radiation can penetrate a wide variety of non-conducting materials, but is blocked by liquid water or metals," explains Lu Ding, who led the work with Jinghua Teng at the A*STAR Institute of Materials Research and Engineering (IMRE). "This means that terahertz beams can be used for material characterization, layer inspection, and producing high-resolution images of the interior of solid objects. It is non-ionizing radiation, and safer than X-rays."

Previous metasurfaces have been designed to manipulate the reflection of terahertz radiation. However, their application has been limited, as Ding explains: "Conventional terahertz antireflection surfaces are passive and often employ an ultrathin metal coating that, once fabricated, becomes fixed and you can't actively tune its performance."

"An electrically tunable metasurface would produce more versatile devices and render more flexibility in system design," adds Teng. "It is the breakthrough the community is looking for."

Ding and Teng, along with coworkers at the A*STAR Institute of Microelectronics (IME), Nanyang Technological University, National University of Singapore and Jilin University in China, fabricated their new metasurface on a silicon wafer, using a process entirely compatible with the complementary metal–oxide semiconductor (CMOS) technologies that underpin most electronics.

The exposed metasurface contains stripes of semiconducting silicon, doped with other elements. These stripes are alternately n-type, in which the moving charge carriers are electrons, and p-type, in which the carriers are positively-charged 'holes' in the electron structure. When the voltage supplied to the p-n junctions is changed, the reflection and transmission of the radiation also change.

The team realized that the reflection coefficient increased in response to a temperature rise caused by the applied voltage. Meanwhile, the transmission showed a more complex response depending on the voltage polarity, which affected the type of charge carrier that became dominant. Using terahertz time-domain spectroscopy, the team showed that certain voltage conditions caused the echo pulse from the metasurface to vanish, representing complete antireflection.

As well as providing this unprecedented control over reflection and transmission, the metasurface has the benefit of being almost entirely flat at an atomic level. This makes it ideal for building up smooth layers in more complex devices.

"Another big advantage is for our research looking into how 2-D materials interact with 2-D metamaterials or metasurfaces, a topic in our project in A*STAR's 2-D Semiconductors Pharos Program," says Teng. "The atomically smooth surface makes the transfer and formation of 2-D-Si heterostructures much easier than the patterned surfaces of nano-sized pillars or disks seen on conventional metasurfaces."

"We could further exploit this type of metasurface by independently biasing the p-n junctions or designing modular functions, meaning that we would have pre-programmable metamaterials," says Ding. Teng adds that the same platform could be used for studying promising 2-D materials like molybdenum disulfide, which exhibits impressive electronic and optical properties for use in new flexible circuits.

Abstract-Reconfigurable Terahertz Quarter-Wave Plate for Helicity Switching Based on Babinet Inversion of an Anisotropic Checkerboard Metasurface

Yosuke Nakata, Kai Fukawa, Toshihiro Nakanishi, Yoshiro Urade, Kunio Okimura, and Fumiaki Miyamaru

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.11.044008

Dynamic helicity switching by utilizing metasurfaces is challenging because it requires deep modulation of polarization states. To realize such helicity switching, this paper proposes a dynamic metasurface functioning as a switchable quarter-wave plate, the fast axis of which can be dynamically rotated by ${90}^{\circ }$. The device is based on the critical transition of an anisotropic metallic checkerboard, which realizes the deep modulation and simultaneous design of the switchable states. After verifying the functionality of the ideally designed device in a simulation, we tune its structural parameters to realize practical experiments in the terahertz frequency range. By evaluating the fabricated sample with vanadium dioxide, the conductivity of which can be controlled by temperature, its dynamic helicity switching function is demonstrated.

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Abstract-Reconfigurable Terahertz Metasurface Pure Phase Holograms

Jinying Guo, Teng Wang,  Huan Zhao, Xinke Wang,  Shengfei Feng, Peng Han, Wenfeng Sun,   Jiasheng Ye, Guohai Situ, Hou‐Tong Chen,Yan Zhang

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201801696?af=R

Terahertz (THz) radiation has many potential applications. However, comparing with the rapid development of THz sources and detectors, functional devices for THz modulation, especially the spatial modulation devices, are still insufficient. Here, a novel approach for generating arbitrary wavefronts of a THz beam is presented. By dynamically creating metasurface structures through illuminating a thin silicon wafer with femtosecond laser, which is spatially modulated, an array of reconfigurable subwavelength resonators is generated. The wavefront of the THz beam is then determined by forming spatial profiles of the Pancharatnam–Berry scattering phase by dynamically controlling the resonator orientation. Proof‐of‐concept experiments demonstrate that streaming holographic images and lenses of variable focal length can be realized in real time. The reconfigurable scheme demonstrated here is convenient and fast, and may lead to advances in a host of THz applications.

Abstract-Terahertz wave front manipulation based on Pancharatnam-Berry coding metasurface

Li Shao-he, Li Jiu-sheng, and Sun Jian-Zhong

Fig. 1. Design of the basic coding particle. (a) Schematic diagram of the basic coding particle. Reflection magnitude of the basic coding particle under normal incidence of LCP (b) and RCP (c) waves. Here, RLL(RRR) and RRL(RLR) are the reflection magnitude with co-polarization and cross-polarization under normally incident LCP (RCP) waves, respectively. (d) Reflection phase and amplitude values under normal incidence of x- and y linearly polarized waves, respectively.

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-9-3-1118

We design a coding metasurface based on Pancharatnam-Berry (PB) phase to manipulate terahertz waves, which is simple and flexible. Compared with the previous design of the coding metasurface, the present coding particles can be obtained by using a same size meta-particle with various orientations instead of designing multiple structures or changing specific size parameters. The PB coding metasurfaces composed of U-shaped particles with pre-designed coding sequences can generate multi-bit coding in the terahertz frequencies and control the reflected terahertz waves to the various directions. Both simulation and theoretical calculation scattering patterns of the designed PB coding metasurfaces demonstrate the expected manipulations. Additionally, the bandwidth of radar cross section (RCS) reduction of approaching −15 dB is 1.05THz (range from 0.9THz to 1.95THz). We believe that the proposed design provides a more flexible way for the manipulation of reflected terahertz waves.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Multi-functional Device with Switchable Functions of Absorption and Polarization Conversion at Terahertz Range

Lin Peng, Xing Jiang, Si-min Li,

https://nanoscalereslett.springeropen.com/articles/10.1186/s11671-018-2811-z

Terahertz electromagnetic (EM) wave components usually have a single function, such as they can only convert the polarization state of an incident wave or absorb the incident energy, which would be a limitation for their applications. To make a breakthrough, a multi-functional device (MFD) is proposed in this paper, and it is capable of switching between absorption mode and polarization conversion mode. The device has a low-profile and simple structure, and it is constructed by graphene-based absorbing metasurface (AM) and gold-based polarization conversion metasurface (PCM). By controlling the chemical potential (μc) of the graphene, the leading role is transferred between the AM and the PCM, which leads to steerable absorption and polarization conversion (PC) modes. For the PC mode, the simulated polarization conversion ratio (PCR) is larger than 0.9 in the 2.11–3.63-THz band (53.0% at 2.87 THz). For the absorption mode, the simulated absorptivity is larger than 80% in the 1.59–4.54-THz band (96.4% at 3.06 THz). The physical mechanisms and operating characteristics of the MFD are discussed. This research has potential applications in terahertz imaging, sensors, photodetectors, and modulators.

U-M researchers develop small device that bends light to generate new radiation

https://news.umich.edu/u-m-researchers-develop-small-device-that-bends-light-to-generate-new-radiation/

ANN ARBOR—University of Michigan physicists have led the development of a device the size of a match head that can bend light inside a crystal to generate synchrotron radiation in a lab.

When physicists bend very intense beams of charged particles in circular orbits near the speed of light, this bending throws off bits of light, or X-rays, called synchrotron radiation. The U-M-led researchers used their device to bend visible light to produce light with a wavelength in the terahertz range. This range of wavelength is considerably larger than that of visible light, but much smaller than the waves your microwave produces—and can penetrate clothing.

A research team led by University of Michigan physicists have developed a way to generate synchrotron using a device the size of a match head. Typically, synchrotron radiation is generated at facilities the size of several football fields. Image credit: Austin Thomason, Michigan Photography

Synchrotron radiation is usually generated at large-scale facilities, which are typically the size of several football stadiums. Instead, U-M researchers Roberto Merlin and Meredith Henstridge’s team developed a way to produce synchrotron radiation by printing a pattern of microscopic gold antennae on the polished face of a lithium tantalate crystal, called a metasurface. The U-M team, which also included researchers from Purdue University, used a laser to pulse light through the pattern of antennae, which bent the light and produced synchrotron radiation.

Microscopic device that bends light. Image credit: Austin Thomason, Michigan Photography

“Instead of using lenses and spatial light modulators to perform this kind of experiment, we figured out by simply patterning a surface with a metasurface, you can achieve a similar end,” said Merlin, professor of physics and electrical engineering and computer science. “In order to get light to curve, you have to sculpt every piece of the light beam to a particular intensity and phase, and now we can do this in an extremely surgical way.”

Anthony Grbic, U-M professor of electrical engineering and computer science, led the team that designed the metasurface with former doctoral student Carl Pfeiffer developing the metasurface.

The metasurface is composed of roughly 10 million tiny boomerang-shaped antennae. Each antenna is considerably smaller than the wavelength of the impinging light, said Henstridge, lead author of the study. The researchers use a laser that produces “ultrashort” bursts or pulses of light which last for one trillionth of a second. The array of antennae causes the light pulse to accelerate along a curved trajectory inside the crystal.

The light pulse creates a collection of electric dipoles—or, a group of positive and negative charge pairs. This dipole collection accelerates along the curved trajectory of the light pulse, resulting in the emission of synchrotron radiation, according to Henstridge, who earned her doctoral degree at U-M and is now a postdoctoral scientist at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany.

The researchers’ device produces synchrotron radiation that contains many terahertz frequencies because the light pulses travel just a fraction of a circle. But they hope to refine their device so that the light pulse revolves continuously along a circular path, producing synchrotron radiation at a single terahertz frequency.

The scientific community uses single-frequency terahertz sources to study the behavior of atoms or molecules within a given solid, liquid or gas. Commercially, terahertz sources are used to scan items hidden in clothing and packaging crates. Drugs, explosive and toxic gases all have unique “fingerprints” in the terahertz range that could be identified using terahertz spectroscopy.

The device’s uses aren’t limited to the security industry.

“Terahertz radiation is useful for imaging in the biomedical sciences,” Henstridge said. “For instance, it has been used to distinguish between cancerous and healthy tissue. An on-chip, single-frequency terahertz source, such as a tiny light-driven synchrotron such as our device, can allow for new advancements in all of these applications.”

The study was published in the journal Science. Fellow Purdue researchers are Vladimir Shalaev, Di Wang and Alexandra Boltasseva.

Reporters can contact the Science press package team at 202-326-6440 or scipak@aaas.org to receive a copy of the study. More information, including a copy of the study, can be found online at the Science press package webpage at EurekAlert (ID and password required to access).

Abstract-Active optical modulator based on a metasurface in the terahertz region

Yue Tian, Jie Ji, Siyan Zhou, Hu Wang, Zhichao Ma, Furi Ling, and Jianquan Yao

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-57-27-7778

The characteristics of the electromagnetically induced transparency (EIT) analog exposed under different illumination powers have been investigated theoretically and experimentally. The EIT analog is composed of a fixed aluminum structure fabricated on the silicon substrate. It was found that the resonance degree of the transparent window displayed a decreasing trend, and a blueshift phenomenon emerged by increasing the powers of the laser. Similarly, the properties of the time delay under different illumination powers have also been researched. The realization of the tuning effect may provide a possible choice for the modulation of the slow light devices.

© 2018 Optical Society of America

Abstract-Enhanced High Performance of a Metasurface Polarizer Through Numerical Analysis of the Degradation Characteristics

Hiroyuki Kurosawa, Bongseok Choi, Masanobu Iwanaga,

https://link.springer.com/article/10.1186/s11671-018-2627-x

This study focuses on the experimental and numerical investigations for the degradation characteristics of a metasurface polarizer. The metasurface has a stacked complementary structure that exhibits a high extinction ratio of the order of 10,000 in the near-infrared region. However, its performance has significantly degraded over time. To clarify the origin of this degradation, the effects of surface roughness and metallic loss are investigated numerically. The degradation is mainly attributed to increase in the loss. These numerical calculations also reveal that the extinction ratio is enhanced by adjusting the thicknesses of the complementary structures to different values. This study paves a way to realize a metasurface polarizer that has a low sensitivity to the time degradation and has a high extinction ratio.

Abstract-Metasurface for multi-channel terahertz beam splitters and polarization rotators

XiaoFei Zang, HanHong Gong, Zhen Li, JingYa Xie, QingQing Cheng,   Lin Chen,   Alexander P. Shkurinov, YiMing Zhu, SongLin Zhuang,

https://aip.scitation.org/doi/abs/10.1063/1.5028401

Terahertz beam splitters and polarization rotators are two typical devices with wide applications ranging from terahertz communication to system integration. However, they are faced with severe challenges in manipulating THz waves in multiple channels, which is desirable for system integration and device miniaturization. Here, we propose a method to design ultra-thin multi-channel THz beam splitters and polarization rotators simultaneously. The reflected beams are divided into four beams with nearly the same density under illumination of linear-polarized THz waves, while the polarization of reflected beams in each channel is modulated with a rotation angle or invariable with respect to the incident THz waves, leading to the multi-channel polarization rotator (multiple polarization rotation in the reflective channels) and beam splitter, respectively. Reflective metasurfaces, created by patterning metal-rods with different orientations on a polyimide film, were fabricated and measured to demonstrate these characteristics. The proposed approach provides an efficient way of controlling polarization of THz waves in various channels, which significantly simplifies THz functional devices and the experimental system.