Tunable Terahertz Absorber Research Articles

Multi-band circuit model of tunable THz absorber based on graphene sheet and ribbons

Terahertz (THz) absorbers due to the high potential of providing reliable applications in modern devices and technology are increasingly being investigated. Recently, a dual-band THz absorber based on a graphene sheet and ribbons has been proposed in which the results were obtained using Full-wave simulations and its authors have not provided any specific design method. In this work, we have proposed and investigated that tunable THz absorber by its circuit model. A developed transmission line method beside the analytical circuit model of graphene continues sheet and periodic arrays of graphene ribbons are used to obtain analytical expressions for the circuit model. Here, first of all, we have compared the results of the proposed method with the results of that paper and secondly, using impedance matching concept we have proposed multi-band tunable THz absorber with near-unity absorption by the same structure. Then, we have compared the results of our circuit method with the Full-wave simulations. In either case, our proposed method in addition to excellent performance in terms of runtime and memory sources, has an acceptable agreement with the results of the Full-wave simulations. The proposed method is general and can be applied to design and simulate the other sub-wavelength structures.

Tunable ultra-wideband terahertz absorber based on graphene disks and ribbons

Terahertz frequency band has become one of the most interesting fields of research these days and the necessity of various modern applications in this area is THz absorbers. In this paper, we have proposed and investigated a novel ultra-wideband terahertz absorber by using periodic arrays of graphene disks and ribbons. Analytical circuit model of graphene arrays besides a developed transmission line theory is used to obtain analytical expressions for the input impedance of the proposed structure. The input impedance is adjusted to be closely matched to the free space impedance in a wide frequency range. Using three layers of patterned graphene, bandwidth of 90% absorption reached to 131% of central frequency. Regarding excellent performance of the proposed method in terms of computation time (showing more than 5 orders of magnitude reduction in runtime) and memory sources, this method with producing results with an acceptable agreement (with an error less than 5%) compared to the results of full-wave numerical modeling can be utilized in design of other graphene-based sub-wavelength devices.

Multi-mode tunable absorber based on graphene metamaterial

In this paper, a novel design of metamaterial absorber (MA) with multi modes and tunability characteristics in terahertz (THz) range is presented. The MA consists of patterned gold-graphene on the top layer, graphene sheet and gold ground separated by SiO2. The chemical potential of patterned graphene and graphene sheet can be tuned independently. Thus, the regulation flexibility of MA is greatly enhanced. For demonstration, the proposed MA is investigated using circuit analogy method and the distributions of the electric field are also discussed in detail to explore underlying mechanism. Numerical calculations are stimulated in a commercial full-wave solver and indicate that the proposed MA possesses excellent absorption properties. There are three perfect absorption (≥99.2%) modes at 2.4 THz, 4.2 THz and 7.2 THz respectively. Under the regulation of chemical potential, the proposed MA could serve as single-frequency or dual-frequency absorber without change of physical structure. When working as single-frequency absorber, the MA could be regulated between different modes during a large frequency range and the absorption peaks could also be regulated within the same mode during a relative smaller frequency range. Furthermore, the MA has two absorption peaks at 2.4 THz and 7.2 THz (≥95%) simultaneously as dual-frequency absorber. The proposed MA shows promising application potentials due to the flexible modulation and functions and this work provides a new perspective for the design of tunable THz absorbers based on graphene.

Plasmonic absorption characteristics based on dumbbell-shaped graphene metamaterial arrays

In this paper, we proposed a theoretical model in the far-infrared and terahertz (THz) bands, which are dumbbell-shaped graphene metamaterial arrays with a combination of graphene nanobelt and two semisphere-suspended heads. We report a detailed theoretical investigation on how to enhance localized electric field and the absorption in the dumbbell-shaped graphene metamaterial arrays. The simulation results show that absorption characteristics can be changed by changing the geometrical parameters of the structure and the Fermi level of graphene. Furthermore, we have discovered that the resonant wavelength is insensitive to TM polarization. In addition, we also find that the double-layer graphene arrays have better absorption characteristics than single-layer graphene arrays. This work allows us to achieve tunable terahertz absorber and may also provide potential applications in optical filter and biochemical sensing.

Tunable multi-band terahertz absorber based on a one-dimensional heterostructure containing semiconductor

The absorption properties of a simple layered heterostructure composed of semiconductor and dielectric are analyzed by transfer matrix method. The numerical results indicate that such a structure can serve as a tunable multi-band terahertz perfect absorber. This phenomenon is due to the photon localization in the semiconductor layer at particular wavelengths and the presence of the damping factor of semiconductor layer. The number of the absorption peaks can be tuned by changing the thickness of the semiconductor layer. Moreover, the influences coming from the incident angle and the temperature of semiconductor on the absorption properties are also numerically studied.

Design of a Tunable Ultra-Broadband Terahertz Absorber Based on Multiple Layers of Graphene Ribbons

We propose and numerically demonstrate an ultra-broadband graphene-based metamaterial absorber, which consists of multi-layer graphene/dielectric on the SiO2 layer supported by a metal substrate. The simulated result shows that the proposed absorber can achieve a near-perfect absorption above 90% with a bandwidth of 4.8 Thz. Owing to the flexible tunability of graphene sheet, the state of the absorber can be switched from on (absorption > 90%) to off (reflection > 90%) in the frequencies range of 3–7.8 Thz by controlling the Fermi energy of graphene. Moreover, the absorber is insensitive to the incident angles. The broadband absorption can be maintained over 90% up to 50°. Importantly, the design is scalable to develop broader tunable terahertz absorbers by adding more graphene layers which may have wide applications in imaging, sensors, photodetectors, and modulators.

Tunable THz perfect absorber with two absorption peaks based on graphene microribbons

Perfect absorption is characterised by the complete suppression of incident and reflected electromagnetic wave, and complete dissipation of the incident energy. A tunable perfect terahertz (THz) absorber with two absorption peaks based on graphene is presented. The proposed structure consists of graphene microribbons sandwiched between aluminium oxide strips and polymethyl methacrylate slab on a silicon dioxide (SiO 2 ) layer which is covered by periodic gold (Au) ribbons, and the SiO 2 layer is supported by the Au substrate. The influence of the structural parameters on the absorption performance was analysed too. Especially, graphene can be used to design tunable THz devices due to its tunability of sheet conductivity which could be controlled by applying a bias voltage or chemical doping. A single-band perfect absorption can be achieved when the chemical potential is set to 0.1 eV. Interestingly, when the chemical potential changed to 0.6 eV, a dual-band absorption spectrum is obtained with absorption efficiency of the two peaks more than 90%. These results imply that the characteristics of graphene-based surface plasmon could be used to design novel THz devices in future.

Hybridization-induced broadband terahertz wave absorption with graphene metasurfaces

Electromagnetic (EM) wave absorption plays a vital role in photonics. While metasurfaces are proposed to absorb EM waves efficiently, most of them exhibit limited bandwidth and fixed functionalities. Here, we propose a broadband and tunable terahertz (THz) absorber based on a graphene-based metasurface, which is constructed by a single layer of closely patterned graphene concentric double rings and a metallic mirror separated by an ultrathin SiO2 layer. Plasmonic hybridization between two graphene rings significantly enlarges the absorption bandwidth, which can be further tuned by gating the graphene. Moreover, the specific design also makes our device insensitive to the incident angle and polarization state of impinging EM waves. Our results may inspire certain wave-modulation-related applications, such as THz imaging, smart absorber, tunable sensor, etc.

Broadband tunable terahertz absorber based on vanadium dioxide metamaterials.

An active absorption device is proposed based on vanadium dioxide metamaterials. By controlling the conductivity of vanadium dioxide, resonant absorbers are designed to work at wide range of terahertz frequencies. Numerical results show that a broadband terahertz absorber with nearly 100% absorptance can be achieved, and its normalized bandwidth of 90% absorptance is 60% under normal incidence for both transverse-electric and transverse-magnetic polarizations when the conductivity of vanadium dioxide is equal to 2000 Ω-1cm-1. Absorptance at peak frequencies can be continuously tuned from 30% to 100% by changing the conductivity from 10 Ω-1cm-1 to 2000 Ω-1cm-1. Absorptance spectra analysis shows a clear independence of polarization and incident angle. The presented results may have tunable spectral applications in sensor, detector, and thermophotovoltaic device working at terahertz frequency bands.

Hybrid Genetic Programming for the Development of Metamaterials Designs With Improved Characteristics

The expansion of a hybrid genetic program's (HGP) functionality to allow for the development of broadband two-dimensional metamaterials designs with advanced characteristics is presented. Artificial magnetic conductor (AMC) ground planes and a tunable terahertz absorber generated by the HGP are compared with designs available in the literature. The HGP is shown to produce human-competitive results. The AMC ground planes synthesized by the HGP were found to produce improved results including wider bandwidths and larger reflection coefficient magnitudes than that of the human designs. For one of the designed AMCs, the HGP's bandwidth is 73.3% larger and the minimum reflection magnitude is 1.0% larger than the reference AMC. Similarly, the absorber synthesized by the HGP has larger bandwidths than that of a recently published absorber optimized by random hill climbing. Three bias voltages were tested with the tunable absorber. The bandwidths of the HGP absorber are 23.1%, 37.6%, and 400% larger than the reference absorber, for biases of 4, 2, and 0.5 V/nm, respectively. Four example designs are discussed together with comparative results to illustrate the advantages of the developed HGP-enabled design method.

Design of triple-band polarization controlled terahertz metamaterial absorber

A kind of triple-band polarization tunable terahertz absorber based on a metallic mirror and a metallic patch structure with two indentations spaced by an insulating medium layer is presented. Results prove that three near-perfect absorption peaks with average absorption coefficients of 98.25% are achieved when the polarization angle is equal to zero, and their absorptivities gradually decrease (and even disappear) by increasing the angle of polarization. When the polarization angle is increased to 90°, three new resonance modes with average absorption rates of 96.59% can be obtained. The field distributions are given to reveal the mechanisms of the triple-band absorption and the polarization tunable characteristics. Moreover, by introducing photosensitive silicon materials (its conductivity can be changed by the pump beam) in the indentations of the patch structure, the number of resonance peaks of the device can be actively tuned from triple-band to dual-band. The presented absorbers have potential applications, such as controlling thermal emissivity, and detection of polarization direction of the incident waves.

Complementary graphene metamaterial with independently tunable dual absorption bands at terahertz frequency

We design an independently tunable dual-band terahertz absorber consisting of graphene ribbons patterned with a combination of two complementary crosses of different sizes within a unit-cell. The Fermi energy of the interdigitated graphene ribbons can be tuned independently via applied gate voltages. Our simulations indicate that the absorption at two resonant bands can be ~75% under normal incidence. In addition, our design allows independent tuning of the resonant frequencies by changing the Fermi energy of graphene via controlling the applied bias voltages. Our results may find application in developing frequency tunable terahertz modulators, sensors, and detectors.

Ultrathin tunable terahertz absorber based on MEMS-driven metamaterial

The realization of high-performance tunable absorbers for terahertz frequencies is crucial for advancing applications such as single-pixel imaging and spectroscopy. Based on the strong position sensitivity of metamaterials’ electromagnetic response, we combine meta-atoms that support strongly localized modes with suspended flat membranes that can be driven electrostatically. This design maximizes the tunability range for small mechanical displacements of the membranes. We employ a micro-electro-mechanical system technology and successfully fabricate the devices. Our prototype devices are among the best-performing tunable THz absorbers demonstrated to date, with an ultrathin device thickness (~1/50 of the working wavelength), absorption varying between 60% and 80% in the initial state when the membranes remain suspended, and fast switching speed (~27 μs). The absorption is tuned by an applied voltage, with the most marked results achieved when the structure reaches the snap-down state. In this case, the resonance shifts by >200% of the linewidth (14% of the initial resonance frequency), and the absolute absorption modulation measured at the initial resonance can reach 65%. The demonstrated approach can be further optimized and extended to benefit numerous applications in THz technology.

Broadband, wide-angle and tunable terahertz absorber based on cross-shaped graphene arrays.

Tunable terahertz absorbers composed of periodically cross-shaped graphene arrays with the ability to achieve near-unity absorbance are proposed and studied. Our results demonstrate that the bandwidth of absorption rate above 90% can reach up to 1.13 terahertz by use of a single layer of cross-shaped graphene arrays. By simply stacking the double layer cross-shaped graphene with careful design, the working bandwidth can be broadened compared with the single-layer graphene-based absorber. The proposed absorbers have the properties of being polarization insensitive and having large angle tolerance, and the tunability of the Fermi level in graphene allows us to realize tunable terahertz absorbers with potential interest in integrated terahertz optoelectronic devices.

Evolutionary Optimization of Graphene-Metal Metasurfaces for Tunable Broadband Terahertz Absorption

Tunable terahertz absorbers based on graphene frequency selective surfaces are designed using well-established evolutionary optimization techniques. Randomly initialized hill climbing (RHC) algorithm is applied to find the design with the broadest bandwidth. The effects of three factors including the substrate loss, structuring the substrate and using double layer patches are examined. The optimum structure with broadest operation bandwidth consists of a perforated lossy substrate with a single layer patch and offers 1.95 THz absorption bandwidth around 2 THz central frequency leading to 100% fractional bandwidth.

Staked Graphene for Tunable Terahertz Absorber with Customized Bandwidth

Terahertz (THz) absorber with dynamically tunable bandwidth possesses huge application value in the fields of switches, sensors, and THz detection. However, the perfect absorbers based on photonic crystals and metamaterials are not intelligent enough to capture the electromagnetic wave in a tunable way. In this paper, we utilized only patterned graphene to tune the absorption positions and the bandwidth in the terahertz regime. More distinguished than some dynamic absorbers proposed before, the performances with peak frequency relative tuning range of 68 % and nearly unity absorbance are obtained by a single cross-shaped graphene layer. Additionally, the working bandwidth can be broadened with stacked structured graphene. The almost perfect absorption shifted from 2.36∼3.2 to 3.26∼3.99 THz continuously via changing the chemical potential of graphene.

A dynamically tunable terahertz metamaterial absorber based on an electrostatic MEMS actuator and electrical dipole resonator array

We experimentally demonstrate a dynamically tunable terahertz (THz) metamaterial absorber based on an electrostatic microelectromechanical systems (MEMS) actuator and electrical dipole resonator array. The absorption of the THz wave is mainly a result of the electrical dipole resonance, which shows a tunable performance on demand. By preforming the finite integral technique, we discovered that the central absorption frequency and the amplitude can be simultaneously tuned by the applied voltage U. Characterized by a white light interferometer and a THz time domain spectroscopy system, our THz absorber is measured to show a modulation of the central frequency and the amplitude to about 10% and 20%, respectively. The experimental results show good agreement with the simulation. This dynamically tunable absorber has potential applications on THz filters, modulators and controllers.

Tunable terahertz absorber based on complementary graphene meta-surface

Recently, metamaterials have attracted considerable attention because of their unique properties and potential applications in many areas, such as in bio-sensing, imaging, and communication. Among these researches, the metamaterial absorber has aroused much interest of researchers. The metamaterial absorber is important due to a broad range of potential application to solar energy, sensing, coatings for reducing the reflection, and selective thermal emitters. As a two-dimensional honeycomb structure composed of a single layer carbon atom, graphene is a promising candidate for tuning metamaterials and plasmonic structures due to its unique properties which differ substantially from those of metal and semiconductors. In this paper, we propose a tunable terahertz absorber based on graphene complementary metamaterial structure by removing periodic cut-wires on the graphene meta-surface. On the basis of the tunability of graphene conductivity, the absorber possesses a frequency tunable characteristic resulting from the change of graphene Femi level by altering the applied voltage. Here, we mainly study the influences of Fermi level of graphene and the size of the structure on the absorption characteristic of this metamaterial absorber. We finally obtain the corresponding Femi level and structural size under the perfect absorption condition. In addition, we utilize the multiple reflection theory to explore the physical mechanism, and verify the feasibility of the simulation method at the same time. The research indicates that the absorber can achieve 99.9% perfect absorption at 1.865 THz when the graphene Femi level is 0.6 eV, the thickness of substrate is 13 m, and the length and width of slit are 2.9 m and 0.1 m, respectively. When graphene Femi level increases from 0.4 eV to 0.9 eV, the resonance frequency of the absorber is blue-shifted from 1.596 THz to 2.168 THz. Meanwhile, the absorption rate increases from 84.68% at 0.4 eV to a maximum value of 99.9% at 0.6 eV, then gradually decreases to 86.63% at 0.9 eV. The maximum modulation of the absorption rate is 84.55% by varying the Femi level. When the thickness of substrate increases, the resonant frequency is red-shifted. The resonant frequency is blue-shifted when both the width and the length of the cut-wire on graphene increase. On the basis of the proposed graphene meta-surface absorber, one can gain different resonant frequencies by adjusting the structure geometric size and graphene Femi level. The graphene complementary structure can also be designed into different patterns to achieve the purpose of practical application.

Tunable terahertz absorption in graphene-based metamaterial

One of the most important advantages of graphene is the capability of dynamically tuning its conductivity by means of chemical doping or gate voltage. Based on this property, we proposed a tunable graphene-based terahertz absorber composed of a periodically patterned graphene structure and a metal ground plane spaced by a thin SiO2 dielectric layer. Our calculated results show that a perfect absorption can be achieved by using a single layer of graphene-based metamaterial structure at a fixed Fermi energy level. Moreover, the calculated electric field and power loss distributions enable us to reveal the absorption mechanism of the designed absorber. More importantly, we found that the absorption peak can be dynamically controlled over a broadband frequency range by adjusting the gate voltage without re-optimizing or re-fabricating the physical structure. This work may provide a further step in the development of compact tunable devices, such as tunable sensors and absorbers, switches, and slow light devices.

Design of broadband and tunable terahertz absorbers based on graphene metasurface: equivalent circuit model approach

This study presents an effective method to model, analyse and design graphene metasurface-based terahertz (THz) absorbers using equivalent circuit model approach. Broadband and tunable absorbers consisting of graphene metasurface and metal-backed dielectric layer have been designed based on the formulas derived from this approach and verified by full-wave electromagnetic simulation. By properly constructing the graphene metasurface, broadband absorption over 70% fraction bandwidth has been achieved, showing that graphene can provide a wideband absorption in the low THz spectrum. Furthermore, tunability of the graphene metasurface has also been investigated. It is demonstrated that the absorption peak frequencies can be tuned while maintaining the peak absorption unchanged, which is highly desirable for THz sensing applications.