Switchable terahertz metamaterial absorber with broadband absorption and multiband absorption

Tunable bandwidth terahertz perfect absorption device based on vanadium dioxide phase transition control

Utilizing the phase transition principle of VO2, this paper presents a tunable ultra-wideband terahertz perfect absorption device with simple structure and tunability. The proposed broadband terahertz perfect absorption device is a three-layer structure with a metal reflective layer, a silicon dioxide dielectric layer and a VO2 layer from bottom to top. It was found that the terahertz perfect absorption device's absorption could be dynamically adjusted from 1.2% to 99.9% when changing from an insulated to a metallic state. With the VO2 in the metallic state, the terahertz perfect absorption device has an absorption efficiency of more than 90% in 4.00 to 10.08 THz's ultra-broadband range and near-perfect absorption is achieved in the ranges of 4.71 THz to 5.16 THz and 7.74 THz to 8.06 THz. To explain the working principle of this terahertz perfect absorption device, this paper utilizes wave interference's principle, theory of impedance matching and electric field analysis. Compared to previously reported terahertz metamaterial devices, the vanadium dioxide device proposed in this paper is significantly optimized in terms of tunable range and absorption bandwidth. In addition, the terahertz perfect absorption device is polarization insensitive and maintains good absorptivity over a wide-angle incidence range. This tunable ultra-wideband terahertz perfect absorption device could have applications in the fields of modulation, stealth devices, and thermal emission devices.

A tunable ultra-broadband and ultra-high sensitivity far-infrared metamaterial absorber based on VO2 and graphene

We proposed a far-infrared tunable metamaterial absorber using vanadium dioxide (VO2) and graphene as controlling materials. The properties of the absorber are investigated theoretically using the finite-difference time-domain (FDTD) technique. It was found that when the Fermi energy level of graphene is fixed at zero, VO2 is in the insulated state, and the metasurface exhibits far-infrared broadband absorption performance, with absorptance exceeding 90% in the wavelength range of 12.6 μm to 23.2 μm. In addition, by elevating the Fermi energy level of graphene, the absorption bandwidth of the device is expanded continuously. When the VO2 is in the metallic state, the device can flexibly transform into a far-infrared narrowband absorber. The device also has the advantage of being insensitive to changes in polarization and incident angle. The origin of the absorption and the tuning principle of the device were analyzed and verified successfully by using an equivalent circuit model (ECM). Besides, we also studied the refraction index sensing characteristics of the absorber. Surprisingly, the absorber exhibits excellent sensing characteristics, and its sensitivity (S) reaches 14.108 μm per RIU and the figure of merit (FOM) is 6.13 per RIU.

Dynamically tunable multifunctional terahertz absorber based on hybrid vanadium dioxide and graphene metamaterials

In this work, in pursuit of a multifunctional device with a simple structure, high absorption rate, and excellent bandwidth, a tunable broadband terahertz (THz) absorber based on vanadium dioxide (V O 2) and graphene is proposed. Due to the phase transition of V O 2 and the electrically tunable properties of graphene, the structure realizes single broadband and dual-band absorption characteristics. When graphene is in the insulating state (E f=0e V) and V O 2 is in the metallic state, the developed system has more than 90% absorption and a wide absorption band from 1.36 to 5.48 THz. By adjusting the V O 2 conductivity, the bandwidth absorption can be dynamically varied from 23% to more than 90%, which makes it a perfect broadband absorber. When graphene is in the metallic state (E f=1e V), V O 2 is in the insulating state, and the designed device behaves as a tunable and perfect dual-band absorber, where the absorptivity of the dual-band spectrum can be continuously adjusted by varying the Fermi energy level of graphene. In addition, both the broad absorption spectrum and the dual-band absorption spectrum maintain strong polarization-independent properties and operate well over a wide incidence angle, and the designed system may provide new avenues for the development of terahertz and other frequency-domain tunable devices.

Constrained tandem neural network assisted inverse design of metasurfaces for microwave absorption

Designing microwave absorbers with customized spectrums is an attractive topic in both scientific and engineering communities. However, due to the massive number of design parameters involved, the design process is typically time-consuming and computationally expensive. To address this challenge, machine learning has emerged as a powerful tool for optimizing design parameters. In this work, we present an analytical model for an absorber composed of a multi-layered metasurface and propose a novel inverse design method based on a constrained tandem neural network. The network can provide structural and material parameters optimized for a given absorption spectrum, without requiring professional knowledge. Furthermore, additional physical attributes, such as absorber thickness, can be optimized when soft constraints are applied. As an illustrative example, we use the neural network to design broadband microwave absorbers with a thickness close to the causality limit imposed by the Kramers-Kronig relation. Our approach provides new insights into the reverse engineering of physical devices.

Terahertz broadband tunable chiral metamirror based on VO2-metal hybrid structure

Aiming at the problems of narrow working bandwidth, low efficiency, and complex structure of existing terahertz chiral absorption, we propose a chiral metamirror composed of C-shaped metal split ring and L-shaped vanadium dioxide (VO₂). This chiral metamirror is composed of three layers of structure, a gold substrate at the bottom, the first polyethylene cyclic olefin copolymer (Topas) dielectric layer and VO₂-metal hybrid structure as the top. Our theoretical results led us to show that this chiral metamirror has a circular dichroism (CD) value greater than 0.9 at 5.70 to 8.55 THz and has a maximum value of 0.942 at f = 7.18 THz. In addition, by adjusting the conductivity of VO₂, the CD value can be continuously adjustable from 0 to 0.942, which means that the proposed chiral metamirror supports the free switching of the CD response between the on and off states, and the CD modulation depth exceeds 0.99 in the range of 3 to 10 THz. Moreover, we discuss the influence of structural parameters and the change of incident angle on the performance of the metamirror. Finally, we believe that the proposed chiral metamirror has important reference value in the terahertz range for constructing chiral light detectors, CD metamirrors, switchable chiral absorbers and spin-related systems. This work will provide a new idea for improving the terahertz chiral metamirror operating bandwidth and promote the development of terahertz broadband tunable chiral optical devices.

Switchable broadband/narrowband absorber based on a hybrid metasurface of graphene and metal structures

This paper proposes a switchable broadband/narrowband absorber based on a hybrid metasurface comprising graphene and metal in the millimeter-wave regime. The designed absorber achieves broadband absorption when the surface resistivity of graphene R_s = 450 Ω/◻ and narrowband absorption when R_s = 1300 Ω/◻ and 2000 Ω/◻. The physical mechanism behind the graphene absorber is explored by analyzing the distributions of power loss, electric field, and surface current densities. An equivalent circuit model (ECM) based on transmission-line theory is derived to theoretically investigate the performance of the absorber, with ECM results in good agreement with simulation results. Furthermore, we fabricate a prototype and evaluate its reflectivity by applying various biasing voltages. The results obtained from the experiment are also consistent with those obtained from the simulation. When the external bias voltage is changed from +1.4 V to -3.2 V, the proposed absorber has an average reflectivity ranging from -5 dB to -33 dB. The proposed absorber has potential applications in radar cross-section (RCS) reduction, antenna design, electromagnetic interference (EMI) shielding, and EM camouflage techniques.

Silicon-based asymmetric dimer-resonator grating for narrowband perfect absorption and sensing

In this work, a method for designing an ultra-narrowband absorber platform is presented with asymmetric silicon-based dimer-resonators grating. Within the infrared range of 3000 ∼ 4000 nm, two narrowband absorption peaks with absorptivity greater than 99% are produced by the absorber. Moreover, during the optical sensing, such an absorber platform shows high-performance sensitivity factors for the absorption wavelengths at λ₁ = 3468 nm (S = 3193 nm/RIU, FOM = 532) and at λ₂= 3562 nm (S = 3120 nm/RIU, FOM = 390). Strong scattering coupling and the magnetic resonances supported in this silicon based grating produce the high absorption. Otherwise, additional methods such as the polarization and incident angles are used to further tune the absorption responses in the intensity and wavelengths, indicating the feasibility for artificial manipulations. The achieved ultra-sharp perfect absorption and the related sensitive response hold the silicon based resonant scheme with wide applications in bio-sensing, spectral filtering and other fields.

Switchable trifunctional terahertz absorber for both broadband and narrowband operations

In this paper, we proposed a multilayer terahertz absorber composed of hybrid graphene and vanadium dioxide (VO2). Based on electrical controlling of graphene and thermal tuning of VO2, three different switchable absorption states are achieved in one structure. When VO2 is in the metal phase and the Fermi level of graphene is set as 0eV, high-frequency broadband (bandwidth, 5.45THz) absorption from 4.5 to 9.95THz is demonstrated. While VO2 is switched to the insulator state, absorption states depend on the Fermi energy of graphene. As the Fermi level changes from 1eV to 0eV, the absorption can be switched from low-frequency broadband (bandwidth, 2.86THz) to dual-frequency absorption. The effect of geometric parameters and fabrication tolerance on the robustness of the absorption properties is explored. The proposed absorber has three switchable states through modulation of graphene and VO2, which is expected to realize potential applications in modulating, filtering, detecting, and other fields.

Tunable wideband-narrowband switchable absorber based on vanadium dioxide and graphene

A functionally tunable and absorption-tunable terahertz (THz) metamaterial absorber based on vanadium dioxide (VO₂) and graphene is proposed and verified numerically. Based on phase transition properties of VO₂ and tunability of graphene, the switching performance between ultra-broadband and narrow-band near-perfect absorption can be achieved. We simulate and analyze the characteristics of the constructed model by finite element analysis. Theoretical calculations show that when VO₂ is in the metallic state and the graphene Fermi energy is 0 eV, the designed absorber can perform ultra-broadband absorption. The absorber achieves greater than 95% absorption in the 2.85 - 10THz range. When VO₂ is in the insulating state and the graphene Fermi energy is 0.7 eV, more than 99.5% absorption can be achieved at 2.3 THz. The absorption rate can be tuned by changing the conductivity of VO₂ and the Fermi energy of graphene. Moreover, the proposed absorber displays good polarization insensitivity and wide incident angle stability. The design may have potential applications in terahertz imaging, sensing, electromagnetic shielding and so on.

Dynamically electrical/thermal-tunable perfect absorber for a high-performance terahertz modulation

We present a high-performance functional perfect absorber in a wide range of terahertz (THz) wave based on a hybrid structure of graphene and vanadium dioxide (VO₂) resonators. Dynamically electrical and thermal tunable absorption is achieved due to the management on the resonant properties via the external surroundings. Multifunctional manipulations can be further realized within such absorber platform. For instance, a wide-frequency terahertz perfect absorber with the operation frequency range covering from 1.594 THz to 3.272 THz can be realized when the conductivity of VO₂ is set to 100000 S/m (metal phase) and the Fermi level of graphene is 0.01 eV. The absorption can be dynamically changed from 0 to 99.98% and in verse by adjusting the conductivity of VO₂. The impedance matching theory is introduced to analyze and elucidate the wideband absorption rate. In addition, the absorber can be changed from wideband absorption to dual-band absorption by adjusting the Fermi level of graphene from 0.01 eV to 0.7 eV when the conductivity of VO₂ is fixed at 100000 S/m. Besides, the analysis of the chiral characteristics of the helical structure shows that the extinction cross-section has a circular dichroic response under the excitation of two different circularly polarized lights (CPL). Our study proposes approaches to manipulate the wide-band terahertz wave with multiple ways, paving the way for the development of technologies in the fields of switches, modulators, and imaging devices.

Design of an Optically Transparent Microwave Absorber Based on Coding Metasurface

In this paper, a metamaterial absorber with a checkerboard patterned ITO (indium tin oxide) film as the surface is obtained by using flexible and optically transparent wave-absorbing material ITO–PET (polyethylene terephthalate), and a coding arrangement of two basic coding units based on the APS-PSO (Array Pattern Synthesis -Particle Swarm Optimization) algorithm. The surface structure of the absorber consists of ITO rectangular patch structures and ITO circular patch structures (110 Ω/sq). The ITO rectangular patch structures and ITO circular patch structures are symmetrical. The middle layer is made up of two layers of PET and one layer of PMMA, and the bottom surface is covered with a layer of low square resistance ITO film (8 Ω/sq). The experimental results, which are consistent with the simulation results, show that the absorber has superior performance: over 90% absorptance in the 5.06–9.01 GHz band, high transmittance, and a −10 dBsm RCS (radar cross-section) reduction in the 5.3–8.7 GHz band. This design also has polarization insensitivity and angular stability.

Dual-band wide-angle perfect absorber based on the relative displacement of graphene nanoribbons in the mid-infrared range

In this paper, a novel graphene-based dual-band perfect electromagnetic absorber operating in the mid-infrared regime has been proposed. The absorber has a periodic structure which its unit cell consists of a sliver substrate and two graphene nanoribbons (GNRs) of equal width separated with a dielectric spacer. Two distinct absorption peaks at 10 and 11.33 µm with absorption of 99.68% and 99.31%, respectively have been achieved due to a lateral displacement of the GNRs. Since graphene surface conductivity is tunable, the absorption performance can be tuned independently for each resonance by adjusting the chemical potential of GNRs. Also, it has been proved that performance of the proposed absorber is independent of the incident angle and its operation is satisfactory when the incident angle varies from normal to ±75°. To simulate and analyze the spectral behavior of the designed absorber, the semi-analytical method of lines (MoL) has been extended. Also, the finite element method (FEM) has been applied in order to validate and confirm the results.

Electrically switchable capabilities of conductive polymers-based plasmonic nanodisk arrays

The electrically dynamic regulation of plasmonic nanostructures provides a promising technology for integrated and miniaturized electro-optical devices. In this work, we systematically investigate the electrical regulation of optical properties of plasmonic Au nanodisk (AuND) arrays integrated with different conductive polymers, polypyrrole (PPy), polyaniline (PANI), and poly(3,4-ethylenedioxythiophene) (PEDOT), which show their respective superiority of electrical modulation by applying the appropriate low voltages. For the hybrid structure of polymer-coated AuND arrays, its reflection spectrum and corresponding structural color are dynamically modulated by altering the complex dielectric function of the covering nanometer-thick conductive polymers based on the electrically controlled redox reaction. Due to the distinct refractive index responses of different polymers on the external voltage, polymer-coated AuND arrays exhibit different spectral variations, response time, and cycle stability. As a result, the reflection intensity of PPy-coated AuND arrays is mainly tailored by increasing optical absorption of the PPy polymer over a broad spectral range, which is distinguished from the wavelength shift of the resonance modes of AuND arrays induced by the other two polymers. Additionally, AuND arrays integrated with both PANI and PEDOT polymers exhibit a rapid switching time of less than 50 ms, which is 5 times smaller than the case of the PPy polymer. Most importantly, PPy-coated AuND arrays exhibit excellent cycle stability over 50 cycles compared to the other two polymers integrated devices. This work demonstrates a valuable technique strategy to realize high-performance polymer-coated dynamically tunable nanoscale electro-optical devices, which has especially significance for smart windows or dynamic display applications.

Ultra-broadband and completely modulated absorption enhancement of monolayer graphene in a near-infrared region

Achieving ultra-broadband and completely modulated absorption enhancement of monolayer graphene in near-infrared region is practically important to design graphene-based optoelectronic devices, however, which remains a challenge. In this work, by spectrally designing multiple magnetic plasmon resonance modes in metamaterials to be adjacent to each other, near-infrared light absorption in monolayer graphene is greatly improved to have an averaged absorption efficiency exceeding 50% in a very broad absorption bandwidth of about 800 nm. Moreover, by exerting an external bias voltage on graphene to change Fermi energy of graphene, the ultra-broadband absorption enhancement of monolayer graphene exhibits an excellent tunability, which has a nearly 100% modulation depth and an electrical switching property. This work is promising for applications in near-infrared photodetectors, amplitude modulators of electromagnetic waves, etc.

Dual-controlled tunable dual-band and ultra-broadband coherent perfect absorber in the THz range

This paper proposes a vanadium dioxide metamaterial-based tunable, polarization-independent coherent perfect absorber (CPA) in the terahertz frequency range. The designed CPA demonstrates intelligent reconfigurable switch modulation from an ultra-broadband absorber mode to a dual-band absorber mode via the thermally controlled of VO₂. The mode of ultra-broadband absorber is realized when the conductivity of VO₂ reaches 11850 S/m via controlling its temperature around T = 328 K. In this mode, the CPA demonstrates more than 90% absorption efficiency within the ultra-wide frequency band that extends from 0.1 THz to 10.8 THz. As the conductivity of VO₂ reaches 2×10⁵ S/m (T = 340 K), the CPA switches to a dual-band absorber mode where a relatively high absorption efficiency of 98% and 99.7% is detected at frequencies of 4.5 THz and 9.8 THz, respectively. Additionally, using phase modulation of the incident light, the proposed CPA can regulate the absorption efficiency, which can be intelligently controlled from perfect absorption to high pass-through transmission. Owing to the ability of the proposed CPA to intelligently control the performance of light, this study can contribute towards enhancing the performance of stealth devices, all-optical switches and coherent photodetectors.

A Photoexcited Switchable Dual-Function Metamaterial Absorber for Sensing and Wideband Absorption at THz Band

Based on the tunable conductivity of silicon as a function of incident pump power, a photoexcited switchable dual-function metamaterial absorber for sensing and wideband absorption at the THz band is designed in this paper. The absorber has an absorption peak at 2.08 THz with the absorption up to 99.6% when the conductivity of silicon is 150 Sm−1, which can be used for sensing. The refractive index sensitivity of the absorption peak is up to 456 GHz/RIU. A wideband absorption is generated from 3.4 THz to 4.5 THz with the bandwidth of 1.1 THz as the conductivity σsi = 12,000 Sm−1. The generation mechanism of the sensing absorption peak and wideband absorption is explained by monitoring the surface current, electric, and magnetic field distribution at some absorption frequencies. It has the advantages of being simple and having a high sensitivity, and wideband absorption with wide application prospects on terahertz communication, electromagnetic stealth, and biochemical detection.

Tunable Dual-Broadband Terahertz Absorber with Vanadium Dioxide Metamaterial

A dual broadband terahertz bifunction absorber that can be actively tuned is proposed. The optical properties of the absorber were simulated and numerically calculated using the finite-difference time-domain (FDTD) method. The results show that when the conductivity of vanadium dioxide is less than σ0=8.5×103 S/m, the absorptance can be continuously adjusted between 2% and 100%. At vanadium dioxide conductivity greater than σ0=8.5×103 S/m, the absorption bandwidth of the absorber can be switched from 3.4 THz and 3.06 THz to 2.83 THz and none, respectively, and the absorptance remains above 90%. This achieves perfect modulation of the absorptance and absorption bandwidth. The physical mechanism of dual-broadband absorptions and perfect absorption is elucidated by impedance matching theory and electric field distribution. In addition, it also has the advantage of being polarization insensitive and maintaining stable absorption at wide angles of oblique incidence. The absorber may have applications in emerging fields such as modulators, stealth and light-guided optical switches.

Thermal tuning of terahertz metamaterial absorber properties based on VO2

We present a novel, structurally simple, multifunctional broadband absorber. It consists of a patterned vanadium dioxide film and a metal plate spaced by a dielectric layer. Temperature control allows flexible adjustment of the absorption intensity from 0 to 0.999. The modulation mechanism of the absorber stems from the thermogenic phase change properties of the vanadium dioxide material. The absorber achieves total reflection properties in the terahertz band when the vanadium dioxide is in the insulated state. When the vanadium dioxide is in its metallic state, the absorber achieves near-perfect absorption in the ultra-broadband range of 3.7 THz-9.7 THz. Impedance matching theory and the analysis of electric field are also used to illustrate the mechanism of operation. Compared to previous reports, our structure utilizes just a single cell structure (3 layers only), and it is easy to process and manufacture. The absorption rate and operating bandwidth of the absorber are also optimised. In addition, the absorber is not only insensitive to polarization, but also very tolerant to the angle of incidence. Such a design would have great potential in wide-ranging applications, including photochemical energy harvesting, stealth devices, thermal emitters, etc.

Transmissive terahertz metasurfaces with vanadium dioxide split-rings and grids for switchable asymmetric polarization manipulation

Metasurfaces containing arrays of thermally tunable metal-free (double-)split-ring meta-atoms and metal-free grids made of vanadium dioxide (VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2), a phase-change material can deliver switching between (1) polarization manipulation in transmission mode as well as related asymmetric transmission and (2) other functionalities in the terahertz regime, especially when operation in the transmission mode is needed to be conserved for both phases of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2. As the meta-atom arrays function as arrays of metallic subwavelength resonators for the metallic phase of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2, but as transmissive phase screens for the insulator phase of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2, numerical simulations of double- and triple-array metasurfaces strongly indicate extreme scenarios of functionality switching also when the resulting structure comprises only VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 meta-atoms and VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 grids. More switching scenarios are achievable when only one meta-atom array or one grid is made of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 components. They are enabled by the efficient coupling of the geometrically identical resonator arrays/grids that are made of the materials that strongly differ in terms of conductivity, i.e. Cu and VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 in the metallic phase.

Dynamically tunable broadband absorber/reflector based on graphene and VO2 metamaterials

We propose a difunctional tunable broadband absorber/reflector consisting of a periodic cross-shaped graphene array and a vanadium dioxide (VO₂) layer. When VO₂ reflects the properties of metal, the proposed dual-function device is used as a reflector; when VO₂ reflects the nature of the dielectric, the difunctional device will be used as an absorber. The simulation results indicate that more than 90% absorption bandwidth can be available in the absorber in the frequency range of 56.1-59.0 THz, up to 100%. Moreover, over 80% absorption can be achieved over the frequency range of 88.5 to 90.2 THz. In addition, the bandwidth and absorption of the metamaterial absorber can be dynamically changed because of the Fermi energy level in graphene and the temperature tunability of VO₂. The proposed device can be applied to manufacturing infrared spectrophotometers, on-dispersive infrared photometers, and Fourier transform infrared spectrometers. Therefore, it has potential application in the field of environmental monitoring.

Dynamically switchable broadband and triple-band terahertz absorber based on a metamaterial structure with graphene

This paper proposes a terahertz absorber with a simple four-layered structure that can be dynamically switched between broadband and triple-band by controlling the chemical potential of graphene. The proposed absorber owns broadband absorption in the frequency range from 5.28 THz to 7.86 THz with the corresponding absorption efficiency above 90%, when the chemical potential of graphene is 150 meV. By increasing the chemical potential of graphene to 550 meV, the broadband absorption splits into triple-band absorption, with the peak locating at 5.39 THz, 7.01 THz and 8.1 THz, respectively. Detailed investigation shows that the broadband absorption should originate from magnetic resonance, Fabry-Pérot cavity resonance and surface plasmon polariton. The triple-band absorption should arise from the combination of Fabry-Pérot cavity resonance and surface plasmon polariton. Additionally, both broadband absorption and triple-band absorption are insensitive to the incident polarization. This tunable and bifunctional metamaterial structure shows a great potential in terahertz applications, such as detectors, modulators and sensors.

Dynamically controlled nanofocusing metalens based on graphene-loaded aperiodic silica grating arrays

A new plasmonic nanofocusing metalens based on aperiodic silica grating arrays was designed and investigated. Assisted by the graphene surface plasmon, the infrared polarized light can be focused in a nanospot with a dynamically controlled focal length by varying the dielectric strip width or the graphene Fermi level E_f. For instance, with λ₀ = 8 µm and E_f at 0.3, 0.6 and 0.9 eV, focal lengths of 4.5, 3.8 and 3.5 µm with its corresponding FWHM of 64, 232 and 320 nm, respectively, can be realized. The variation of the focusing efficiency with respect to the incident wavelength and the Fermi level were also investigated. The results of theoretical analysis based on light differential equations agree well with the finite element analysis simulation, which further validate the model.

Graphene-based terahertz metamirror with wavefront reconfiguration

As an emerging material, graphene has been widely applied in the field of active metasurface. Recently, researchers employed graphene to achieve dynamical control of electromagnetic wavefront. In this work, graphene-based reflective metasurface is presented to realize dynamical wavefront reconfiguration for terahertz wave. Using a hybrid structure of cross-shaped graphene and metal patch, the designed metasurface has 360° phase modulation capability. Its wavefront is reconfigurable and can realize multiple functions. In order to verify this, three examples are designed to demonstrate the phenomenon of wavefront reconstruction. They are gradient metasurface, vortex beam generator, and focusing mirror, respectively. First of all, Fermi level of graphene is used to reconstruct the reflected wavefront of gradient metasurface, and then realize switching between positive and negative reflections. Secondly, a vortex beam generator is implemented, and it can reconstruct the mode number of orbital angular momentum through Fermi level. Finally, a reflective lens is proposed and verified, whose focus can appear or disappear with the tuning of Fermi level. The proposed functions have potential applications in the fields of terahertz switching, communication, and focusing.