Terahertz Metasurfaces Research Articles

Ionic terahertz metasurface in ultrathin-layered conductive hydrogel by ultrafast laser tailoring

The ion-interactive semi-conductive hydrogel emerged as a promising optoelectronic material due to its desirable electrical conductivity, optical clarity, and switchable dielectric properties beyond incumbent inorganic materials. However, developing the actively modulated terahertz (THz) devices via hydrogel remained barely studied at an early stage. Herein, a scalable high-throughput picosecond laser system rapidly tailored hydrogel into ion-interactive dielectric metasurfaces (IDMs). The IDMs exhibited minus permittivity (ɛ < 0 F/m) not belonging to hydrogel, tunable time/frequency modulation, and high-consistency geometries for specific THz wave manipulations. The interplay between anions and the π-conjugated thiophene ring of hydrogel dynamically changed the dielectric properties and electrical conductivity through mixed electron/hole transportation, making IDMs change their microscale electric fields anomalously. Other as-prepared IDMs (split resonators, interleaved antennas, and their array) further confirmed the native capability of generating orbital angular momentum, spatially manipulating THz waves, or tuning frequencies. This work ushers in a new platform to customize and deploy non-metal THz meta devices in batches.

Multi-polarization selective absorber based on terahertz metasurface

Tunable absorbers attract much attention due to their wide applications, such as integrated optical sensors, multiplexing far-field imaging and so on. In this paper, we design a multi-polarization selective absorber based on terahertz metasurface, whose geometric symmetry is tunable to satisfy the requirements of linear and circular polarization selective absorbers. At the ON state, the metasurface selectively absorbs y-polarized wave at 1.0 THz, with the linear dichroism of 0.84. At the OFF state, the metasurface selectively absorbs left circularly polarized wave at 1.16 THz, with the circular dichroism of 0.81. The physical mechanism of the metasurface is explained by the Jones calculus and the electromagnetic field distributions. In addition, the multi-polarization selective behavior is insensitive to the incident angle. Finally, we design a sample to verify the potential application of the metasurface absorber, which displays multiplexing imaging of ‘C’, ‘mirrored C’, ‘O’ and blurred images under different polarized incidences at different states. Such a metasurface, switching between linear polarization selective absorber and circular polarization selective absorber, may achieve stable applications in multifunction integrated absorbers, detectors, and image encryptions.

Genetic algorithm-enhanced design of ultra-broadband tunable terahertz metasurface absorber

In this paper, we demonstrate an ultra-broadband tunable metasurface absorber and investigate the automation of its design process, avoiding the traditional manual trial-and-error process. Here, automation is achieved by controlling the random coding structure of the VO2 layer by genetic algorithm. Subsequently, the bandwidth of the metasurface absorber is enlarged by a structural design forming a Fabry-Perot cavity. The genetic algorithm-enhanced metasurface absorber absorption bandwidth ranges from 1.84 to 8.36 THz, with a relative bandwidth of up to 127.8 %. The maximum absorption can be tuned from 4 % to 100 % by controlling the conductivity of VO2 through temperature. Our results may provide guidance for the automated design studies of ultra-broadband tunable terahertz metasurface absorbers.

Graphene and gold nanoparticles integrated terahertz metasurface for improved sensor sensitivity

Terahertz metamaterial technology, as an efficient nondestructive testing method, has shown great development potential in the detection of biological samples. In this study, we have proposed a graphene-integrated gold nanoparticles (AuNPs) terahertz (THz) biosensor and have analyzed the sensing performance of four IC-type resonators (ICSRs) for biosensors: bare ICSRs, ICSRs-integrated AuNPs, ICSRs-integrated graphene, and ICSRs-integrated graphene and AuNPs. The AuNPs generate charge accumulation and induce local electric field enhancement due to the tip effect, and enhance the interaction between the THz wave and the analyte, thus improving the sensitivity of the sensor. The combination of AuNPs and ICSRs-integrated graphene results in sensors with more pronounced amplitude changes and higher sensitivity. Due to the Fermi energy level of graphene being close to the Dirac point, the sensor is ultra-sensitive to external stimuli with a limit of detection of 10.48 fg/ml for aspartic acid solutions. This biosensor has the potential to detect trace amino acids with high sensitivity, contributing to early disease detection and environmental detection for early diagnosis and ecological balance.

Role of losses and coupling in bright-dark metasurfaces mimicking non-Hermitian parity-time symmetric systems

All real systems are essentially non-conservative in nature, which is non-Hermitian in the fundamental premises of the quantum realm. Such open systems obeying parity-time (PT) symmetry can offer intriguing physics, which has sparked much attention in recent years. The thoughtful interplay of loss and gain in PT-symmetric systems can tune the eigenstates from real to complex space passing through a singular point known as an exceptional point (EP) with concurrently coalescing the discrete eigenstates. In this regard, artificially engineered near-field coupled metasurfaces provide a remarkable podium to introduce as well as control loss and coupling strength simultaneously, manifesting a practical channel to attain EP. Motivated by this notion, we have theoretically explored near-field coupled dark-bright-mode assisted terahertz metasurfaces imitating a typical non-Hermitian PT-symmetric system. Structurally, such metasurfaces are realized by utilizing a pair of orthogonally twisted near-field coupled split-ring resonators (bright-dark-resonators). We have meticulously examined such a system with two distinctive approaches, i.e., the loss parameter and inter-resonator coupling strength. Eventually, our exploration demonstrates that judicious modulation of the loss parameter can invoke an undemanding route to approach the other wisely elusive exceptional point. We believe that this comprehensive study of non-Hermitian quantum systems in a photonic context can lead to the development of accurate and precise ultra-sensitive sensors besides other futuristic photonic devices.

A simple structured and homo-frequency point mode switchable THz full-space metasurface based on temperature-controlled vanadium dioxide

Terahertz (THz) full-space metasurfaces have attracted great attention in the field of modern communication systems due to their inherent advantages in electromagnetic (EM) wave manipulation. However, nearly all the reported full-space metasurfaces are realized by multilayer dielectric cascade structures. Furthermore, for mode switchable metasurfaces, reflection and transmission modes are always switched at different frequency points, which limits their application prospects. In this paper, we propose a full-space THz metasurface that can switch between reflection and transmission modes at the same frequency point only by controlling a single substrate layer of vanadium dioxide. The reflection and transmission properties can be independently controlled by rotating the optimized meta-atoms on the metasurface independently. To demonstrate its application potential for multifunctional devices, full-space focusing vortex beam generator and near-field imaging are practiced. A mode switchable focusing vortex beam generator using right circularly polarized (RCP) light at 1.312 THz is designed. The mode purity of the vortex beam under both the transmission and reflection directions is above 85 %. Then a full-space near-field imaging of letters “UPC” at 200 μm from the reference plane is achieved both in the reflection and transmission direction. The transmission and reflection modes can be switched simply by regulating the temperature of the vanadium dioxide substrate through a thermistor. Our suggested design concept of homo-frequency points and simple structure is more conducive to future practical engineering applications of metasurface and provides a worthwhile direction for future research of full-space functional devices.

Modulo-addition operation enables terahertz programmable metasurface for high-resolution two-dimensional beam steering.

Electrically controlled terahertz (THz) beamforming antennas are essential for various applications such as wireless communications, security checks, and radar to improve coverage and information capacity. The emerging programmable metasurface provides a flexible, cost-effective platform for THz beam steering. However, scaling such arrays to achieve high-gain beam steering faces several technical challenges. Here, we propose a pixelated liquid crystal THz metasurface with a crossbar structure, thereby increasing the array scale to more than 3000. The coding pattern on the programmable device is generated by the modulo-addition of the coding sequences on the top and bottom layers. We experimentally demonstrate the programmable liquid crystal metasurface capable of active beam deflection in the upper half-space. This scale-up of programmable devices opens exciting opportunities in pencil beamforming, high-speed information processing, and optical computing.

Active terahertz frequency conversion on silicon-based time-varying metasurface

Terahertz (THz) metasurfaces have emerged as the preferred material for active devices, with numerous potential applications. However, due to a lack of materials with an ultrafast response, functional devices with fascinating time-varying phenomena remain to be explored. In this study, we experimentally investigated a time-varying THz metasurface with silicon–gold hybrid structures. The results show that the transmission spectra undergo a remarkable transformation when an optical pump is used. A transient time boundary is introduced to study the time-varying response using the sudden change in silicon conductivity in the presence of an optical pump. We observed the generation of new spectral components, and the magnitude of the converted wave is linear with the incident power. The proposed silicon-based THz time-varying metasurface paves the way for developing THz sources and nonlinear optical devices.

Tunable terahertz multi-functional hybrid-patterned metasurface arrays based on vanadium dioxide

This study investigates a nine-layer multi-functional periodic array with active broadband tuning in the terahertz (THz) band. The device comprises symmetrical vanadium dioxide (VO2) films and polypropylene (PP) layers, along with silicon dioxide (SiO2) layers, hybrid-patterned metasurfaces, and a central VO2 layer. Through detailed analyses of the electric field distribution, equivalent circuit, and effective impedance, we have performed a thorough investigation of the resonance modes present in the device and meticulously optimized various parameters. Leveraging the insulator-to-metal transition of VO2, a remarkable device capable of seamlessly switching between extraordinary terahertz transmission and bi-directional perfect absorption was obtained. These characters exhibit limited susceptibility to incident angle of the incoming wave. By incorporating bow-tie apertures within the “vacuum region” of the hybrid-patterned metasurfaces, a significant improvement in field enhancement has been achieved, all while effectively eliminating any adverse effects on transmission and absorption performance. This device presents a novel and effective approach in the development of adjustable and multifunctional THz metasurface devices.

Design, fabrication, and sensing applications of MWCNT/PET terahertz metasurface

In this paper, the periodic rectangular aperture arrays have been etched on a multi-walled carbon nanotube (MWCNT) film prepared by vacuum filtrating to fabricate a terahertz (THz) metasurface on a polyethylene terephthalate (PET) substrate. The transmission properties and the underlying resonance physical mechanism of the metasurface were investigated in detail. An obvious resonant peak located at 0.48 THz can be observed, the results of the scattering powers for various multipole moments show that the enhanced transmission peak is dominated by the radiation contribution of the electric dipole ultimately. To demonstrate the sensing capability, the fabricated metasurface was used to detect different concentrations of 2,4-dichlorophenoxyacetic acid (2,4 - D) solutions. The experimental results reveal that the transmission amplitude decreases linearly as the concentrations increase, and the amplitude sensitivity can achieve 1.69 × 10−2/ppm. This study demonstrates the MWCNT/PET metasurfaces can be applied for trace analytes sensing and improve the sensitivity and accuracy, while the processing technology can also be extended to the preparation and various applications of CNT-based metasurfaces, which provides a beneficial reference for the extended applications of carbon-based metamaterial devices in the THz band.

Reconfigurable three multi-mode terahertz metasurface.

We designed a three reconfigurable multi-mode terahertz metasurface based on a concentric elliptical ring structure. The proposed unit cell is a concentric elliptical ring composed of copper, vanadium oxide and photosensitive silicon from the inside ring to the outside ring. The conductivity of photosensitive silicon and vanadium oxide can be adjusted by changing the external operating temperature and pump light intensity. The same unit cell can reconstruct three kinds of states with different properties, and they have completely different transmission characteristics in various terahertz bands. By encoding the arrangement, through changing external stimulus and operating frequencies, the reconfigurable terahertz metasurface can achieve multiple functions including terahertz focusing with adjustable focal length, vortex beam with different topological charge, and near-field imaging with different patterns. It provides what we believe to be a new idea for the field of information security and the design of multifunctional and multifrequency terahertz devices.

Near-field imaging and spectroscopy of terahertz resonators and metasurfaces [Invited

Terahertz (THz) metasurfaces have become a key platform for engineering light-matter interaction at THz frequencies. They have evolved from simple metallic resonator arrays into tunable and programmable devices, displaying ultrafast modulation rates and incorporating emerging quantum materials. The electrodynamics which govern metasurface operation can only be directly revealed at the scale of subwavelength individual metasurface elements, through sampling their evanescent fields. It requires near-field spectroscopy and imaging techniques to overcome the diffraction limit and provide spatial resolution down to the nanoscale. Through a series of case studies, this review provides an in-depth overview of recently developed THz near-field microscopy capabilities for research on metamaterials.

Terahertz metasurface for independent modulation of amplitude and phase in multi-channels

It is necessary to achieve simultaneous and independent modulation of the amplitude, polarization and phase of lights utilizing metasurface, which is also a great challenge. In this paper, an all-dielectric metasurface is proposed for simultaneously controlling the amplitude and phase of orthogonal linearly polarized lights in multi-channels. Three pairs of complex amplitude information are encoded in three independent channels by elaborately selecting and combining the meta-atoms. To verify the performance of proposed metasurface in the terahertz (THz) band, three examples are designed, including producing multiple vortex beams and arbitrary orbital angular momentum (OAM) superposition states, producing multiple foci with controllable intensity ratios, and producing multiple foci with controllable intensity ratios and focal lengths. The simulative results agree with experimental results. The proposed design based on single-layer structure has advantages of controllable focusing, compact structure, and simple fabrication, providing an alternative way for the preparation of integrated multifunctional and miniaturized optical devices.

Identification of Chemical and Structural Characteristics of Acrylic Paint Layer Using Terahertz Metasurfaces.

The precise investigation and monitoring of the internal structural change within complex layered systems are crucial, as the emergence of undesirable defects or formation of secondary internal structures significantly exerts a profound influence on the overall properties of the system. We demonstrate an advanced sensing platform utilizing terahertz metasurfaces, allowing chemical detection and precise identification within an acrylic paint layer with a noticeable sensitivity, reaching down to several hundreds of nanometers, in nondestructive and noncontact manners. The identification of solid and mixed paint samples was achieved by analyzing their optical properties, including the refractive index and absorption coefficient. Notably, the presence of internal pore defects within the mixed acrylic paint led to geometric distortions, affecting the state of the overall system. Intriguingly, even in cases where acrylic paint exhibited identical colors perceptible under visible light, distinct discrimination and identification of chemical compositions were successfully proposed.

Broadband terahertz metasurface with three switchable functions based on complementary structure

In this work, we propose a switchable multifunctional metasurface based on complementary VO2-graphene structure. Tuning the state of VO2 and the Fermi level of graphene, the metasurface can act as broadband half-wave plate (HWP), quarter-wave plate (QWP) or absorber in the same frequency range of 0.65–1.27THz, respectively. In the V1G0 state, the metasurface behaves as a broadband HWP to realize linear-to-linear polarization conversion with the polarization conversion ratio >0.9 in the range of 0.64–1.37THz. In the V0G1 state, the metasurface acts as a broadband QWP achieving linear-to-circular polarization conversion with the ellipticity below −0.9 in the range of 0.65–1.27THz. In the V1G1 state, the metasurface is a broadband absorber with the absorption over 0.9 in the 0.63–1.32THz range. In addition, the physical mechanisms of the metasurface in different states are analyzed through the field and current distributions. Such a switchable multi-functional metasurface can simultaneously achieve high efficiency polarization conversion and absorption, which has potential applications in the integrated terahertz devices.

Multifunctional active terahertz metasurface with electromagnetically induced transparency, perfect absorption, and circular dichroism

Navigated by the continued demand for design integration and miniaturization, incorporating multiple diversified operations into a single metasurface hybridized with the tunable metasurface is highly demanding at the terahertz (THz) range. However, up to now, because of the limitation of the reconfigurable metasurface at the terahertz range, most metasurfaces only feature a single function or process similar functionalities at a single frequency. A multi-functional terahertz metasurface based on vanadium dioxide (VO2) is proposed in this paper, which consists of a gold layer, a VO2 bridge, a SiO2 spacer, and a VO2 layer on the back of the spacer. Under different conductivity of different VO2 layers, the proposed metasurface achieves various terahertz device functions, including single-band perfect terahertz absorption, circular dichroism (CD) for the reflected and transmitted fields, and electromagnetically induced transparency (EIT) effect. Furthermore, the proposed metasurface exhibits robust response against oblique incidences. The mechanisms of perfect absorption, chiral-selective absorption and EIT are illustrated by impedance matching theory, surface current and electric field distributions, respectively. The function-switching ability of proposed metasurface provides more options for the design of terahertz devices in the future.

Perovskite and Graphene-Based Meta-Devices for Effective Optical Modulation of THz Waves

Active and effective control over terahertz (THz) waves is essential for THz applications, and the incorporation of dynamic materials into metasurfaces offers opportunities to realize highly efficient THz manipulation. Here, optically controlled THz metasurface devices (meta-devices) based on either perovskite or graphene are experimentally demonstrated, each exhibiting ultra-sensitive THz modulation under extremely low external optical power. We achieve a maximum modulation depth of 237.5% in the THz transmission amplitude of a metal-perovskite hybrid meta-device when the power density is as low as 13.7483 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Moreover, a metal-graphene hybrid meta-device also shows ultra-sensitive optically induced characteristics, with a maximum modulation depth of 403.36% under the extremely low power density of 11.1846 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . These devices offer a meaningful guide for the design of tunable terahertz meta-devices that will find utility in applications that require the ultra-sensitive optical modulation of terahertz waves.

Observation of the bound states in the continuum supported by mode coupling in a terahertz metasurface.

Metasurface supporting bound states in the continuum (BIC) provides a unique approach for the realization of intense near-field enhancement and high quality factor (Q-factor) resonance, which promote the advancement of various applications. Here we experimentally demonstrate a Friedrich-Wintgen BIC based on the mode coupling in the terahertz metasurface, which produces BIC by the coupling of the LC mode and dipole mode resonances. The transition from ideal BIC to quasi-BIC is caused by the mismatch of the coupling, and the mode decay rate during this process is analyzed by temporal coupled mode theory. The Q-factor and the electric field enhancement of the quasi-BIC resonance are significantly increased, which provides enormous potential in sensing, nonlinear optics, and topological optics.

3-bit reconfigurable THz metasurface based on structured light illumination for vortex beams and holographic imaging

Metasurfaces have attracted widespread attention due to their excellent control ability of terahertz wavefronts. Terahertz metasurfaces have made rapid progress in areas such as biological detection, high-speed communication, and imaging. However, terahertz metasurfaces currently have a single function, and the primary goal of this research is to create a reconfigurable and multifunctional terahertz metasurface. In this work, we designed a novel eight-gaps-splits-ring resonator (EGSRR). The EGSRR is filled with photosensitive semiconductor material, and the metasurface unit's amplitude-phase characteristics are controlled by different encodings of structured light. Finally, under the incidence of right-handed circularly polarized (RCP) waves, 3-bit phase encoding of its reflected wave with cross-polarization is achieved. To verify the ability of metasurface to control terahertz wavefronts, the 3-bit metasurface units are assembled into an array to achieve different-order vortex beams and reconfigurable holographic images. The novel optically controlled reconfigurable metasurface not only has promising applications in generating vortex beams and holographic imaging, but it also represents a novel approach to designing multifunctional and programmable terahertz metamaterials.

Vortex beam generator based on exceptional topological phase in terahertz all-dielectric metasurface

In this study, an all-dielectric metasurface with an F-shaped chiral structure in the terahertz region has been proposed. The coupling between the electric and magnetic dipole modes supported by the metasurface can be adjusted by varying the structural parameters. In the parameter space composed of l l and s, any closed path around exceptional point in the circular polarization conversion channel t rl generates a topologically protected exceptional topological phase. A vortex beam generator with a unit topological charge was proposed to provide a novel concept for designing terahertz metasurface wavefront modulation devices.