Tunable Metamaterial Absorber Research Articles

Design of a tunable monolayer MoS2/BP based terahertz absorber with six absorption bands

A tunable six-band terahertz metamaterial absorber (MMA) was designed and simulated in this work, consisting of a MoS2 or BP monolayer as loss layer, a dielectric substrate and a metal-insulator phase change material VO2. By simultaneously adjusting the thickness and the conductivity of VO2 or the carrier concentration, the absorption spectrum can be dynamically tuned. The numerical simulation results showed that almost every absorption peak at six frequencies was above 90% under TE polarization when the loss layer was set as MoS2 or BP. Furthermore, the absorber with either of these two 2D materials as loss layer was insensitive to incident angles. Additionally, the influence on absorption peaks by regulating the structure parameters of the absorber was also briefly investigated. The proposed absorber, which exhibited superior absorption characteristics with either selected loss layer adopting the same simple physical unit structure, may provide a new direction for the design of tunable terahertz devices.

Derivation of a semi-analytical method for designing tunable metamaterial absorbers

Metamaterial perfect absorbers (MPAs) are usually designed through two main procedures in combination with an optimization method: numerical simulation and circuit modeling. In this article, we propose a new semi-analytical method to design the MPAs. A double-layer micro-strip resonator absorber with a broad bandwidth is designed and analyzed carefully using full-wave numerical simulation combined with a genetic algorithm. The mechanism of the absorption is analyzed carefully, and it is shown that the broadband absorption is caused by the coupling between surface plasmon and Fabry-Perot (FP) resonances in different modes. Also, the role of the localized surface plasmon in coupling FP resonances through subwavelength slits is clarified based on the special shape of the micro-strip resonators. Using the obtained results, a new semi-analytical method is proposed for designing MPAs. This new method is a combination of a theoretical method derived based on the analyzed absorber mechanism and a local search genetic algorithm. The generalization of this method is investigated for different dielectrics, metals, resonator layer numbers, and in different objective frequency ranges. Results show that the method can be used in designing both broadband and multiband absorbers. Also, the method is used to design absorbers using square resonators. These resonators are polarization insensitive and support the same resonances as the micro-strips. The designed micro-strip and square resonator absorbers show favorable performance for both TE and TM polarization. We also showed this method and the proposed theoretical formulas can shorten the design process of the absorbers.

A Terahertz Metamaterial Absorber-Based Temperature Sensor Having Nine Resonance Peaks

Design and investigation of a polarization-insensitive nine-band tunable metamaterial absorber at THz frequencies with equal to or more than 90% absorption ratio in all of the bands are reported. The tunable metamaterial absorber consists of four isosceles triangle patches with four U-shaped cut paths on top of an indium antimonide substrate, which has a fully metallic ground plane at the backside. Numerical analyses show that the metamaterial absorber has wide-angle characteristics under transverse-electric and transverse-magnetic modes. The permittivity of indium antimonide is highly dependent on temperature variations due to its temperature-dependent intrinsic carrier density, leading to shift of nine absorption peak frequencies upon change of environment temperature. Broadband switching of nine absorption peak frequencies with maximum 71.5% shift ratio between 190 K and 230 K is obtained. Temperature sensing performance of the metamaterial absorber is further evaluated and the sensitivities are found to be 11.5 GHz/K, 9.2 GHz/K, 8.3 GHz/K, 7.6 GHz/K, 7.0 GHz/K, 6.2 GHz/K, 5.3 GHz/K, 4.5 GHz/K and 4.2 GHz/K, from the first to ninth absorption band, respectively. Therefore, the proposed nine-band metamaterial absorber sensor has great potential in sensitive and accurate temperature measurement, absorption tuning in optoelectronic applications and as frequency selective thermal emitters.

Analyzing broadband tunable metamaterial absorbers by using the symmetry model

In this paper, a broadband tunable absorber based on bulk Dirac semimetal (BDS) in the far-infrared regime is studied. By optimizing the Femi energy and geometric size, the structure can achieve absorption levels greater than 90% in the 8.11-13.94 THz range, with a total thickness of 5.1 µm. Further, the bandwidth of this proposed absorber can be dynamically controlled by changing the Femi energy of the BDS instead of geometry. Meanwhile, the polarization and oblique incident angles in the TE and TM electromagnetic waves are also investigated. Finally, a new symmetry model is adopted to analyze the absorption mechanism, which is the innovation of this paper. This research result may provide potential applications for all types of optical photovoltaic devices and tunable biological chemistry sensors.

Tunable ultra-broadband terahertz perfect absorber based on vanadium oxide metamaterial

A dynamically tunable ultra-broadband terahertz perfect metamaterial absorber based on vanadium oxide (VO2) is proposed and numerically demonstrated. The excellent absorption bandwidth of greater than 90% absorptance is as wide as 5.10 THz from 3.03 to 8.13 THz under normal incidence. By changing the conductivity of VO2 from 200 S/m to 2×105 S/m, the absorption intensity can be dynamically tuned from 1.47% to 100%. The ultrabroad bandwidth and flexibility are dramatically improved compared with previously reported VO2 based absorbers. The physical mechanism of the ultra-wideband absorption is discussed based on the interference cancellation, impedance matching theory, and field distributions. The influences of structure parameters on perfect absorption are also discussed. In addition, the absorber has the advantages of insensitivity to polarization and incident angle. Such a tunable ultra-broadband absorber may have promising potential in the applications of modulating, cloaking, switching, and imaging technology.

Tunable terahertz metamaterial wideband absorber with liquid crystal

In this paper, a tunable metamaterial absorber (MA) based on liquid crystal (LC) with wideband absorption is presented. The design and absorption principle of the absorber is introduced, and the simulation analysis is conducted. The results show that the center resonant frequency of the proposed absorber is 130.0 GHz without a bias voltage. When the bias voltage increases to saturation, the center resonant frequency of the absorber is tuned to 119.9 GHz, with a frequency tunability of 7.8%. Moreover, the tunable absorber exhibits wideband absorption characteristics and maintains absorption above 90% in the frequency range of 127.7 GHz to 132.3 GHz with no bias voltage, with a relative absorption bandwidth of 3.5%. While under the saturation voltage, absorption greater than 90% can be achieved from 117.9 to 121.8 GHz, with a relative absorption bandwidth of 3.3%. The wideband absorption effect of the proposed LC-based tunable MA makes it a promising candidate for applications such as electromagnetic shielding and stealth.

A compact metamaterial broadband THz absorber consists of graphene crosses with different sizes

A compact tunable graphene-based broadband metamaterial absorber in the terahertz (THz) frequency band was presented in this paper. The proposed absorber is the classic sandwich structure, which is composed of graphene cross resonators of different sizes placed on the top of a dielectric spacer backed with a gold ground at the bottom. It is shown that the absorber can achieve a broad absorption band from 2.81 THz to 4.67 THz with absorption up to 97 % under the normal incident. Meanwhile, the absorption property can be kept to a moderate level when the incident angle is up to 45° for both TE and TM incident light, and the absorption is polarization insensitive. By increasing the chemical potential of graphene from 0.40 eV to 0.60 eV, the broad absorption band can be tuned to shift blue with slightly changing maximum absorption rate. What's more, the absorption bandwidth can be further extended to over 2.34 THz with absorption larger than 97 % by adding another graphene cross layer. Based on the optimal and novel characteristics, we hope that the designed absorber may have potential applications in THz absorbing, sensing, switching, modulating, stealthy and so on.

Research on Perfect and Tunable Metamaterial Absorber Based on Crosshair-shaped Graphene

Graphene, as a new nano-material, according to the physical properties of electric field localization and selective absorption on light of surface plasmon resonance (SPR), a tunable, multi-band and wide-angle perfect absorber based on crosshair-shaped graphene is devised by using the Finite Difference in Time Domain (FDTD) method. In this paper, the effects of chemical potential, relaxation time, and incident angle of light on the absorptivity of graphene are systematically discussed. The simulation experiment shows that there are two absorption peaks with perfect absorption rate appeared in the study range, and the maximum modulation index can be obtained by changing the relaxation time. Finally, it proves that the absorber is insensitive to wide-angle of light. Thus, it is able to be concluded that the absorber has a great reference value to sensor, wireless communication, biomedical and other fields.

Tunable ultranarrow-band metamaterial perfect absorber based on electromagnetically induced transparency structure

A tunable ultranarrow-band metamaterial perfect absorber (MPA) based on electromagnetically induced transparency (EIT) is demonstrated theoretically and experimentally. An EIT structure (EITS) with a full width at half-maximum (FWHM) of 0.12 nm is first studied, then the transmission peak is converted into an absorption peak by adding an Ag substrate beneath the EITS. After optimization for the structure and parameters, an ultranarrow-band MPA with a FWHM of 0.52 nm is obtained. Theoretical results show the ultranarrow-band absorption spectra of the proposed MPA can be tuned by rotating the incident angle. Moreover, the MPA exhibits obvious spectral shift as the refractive index of the surrounding medium increases. Finally, a MPA sample is fabricated to verify the theoretical results, and the measured spectra are consistent with the theoretical ones.

Quad-band tunable graphene-based metamaterial absorber operating at THz frequencies

Quad-band tunable graphene-based metamaterial absorber operating at THz frequencies

A tunable hybrid graphene-metal metamaterial absorber for sensing in the THz regime

In this paper, a graphene-based metamaterial absorber is proposed and investigated numerically, in which the interaction between a split ring resonator (SRR) and graphene results in a high-Q absorption. To make a better understanding of the resonance mechanism, the electric and the magnetic fields, and the surface currents at the resonance frequency are investigated. In order to ease the analysis of the structure, an equivalent circuit model is introduced using the transmission line theory, and the accuracy of the proposed model is verified by the full-wave simulation. Finally, different aspects of the designed metamaterial are discussed as a potential label-free sensor for chemical and biomedical sensing. It is shown that by using this structure, a sensor with a sensitivity of 597 GHz/RIU can be achieved.

Multiband tunable perfect metamaterial absorber realized by different graphene patterns

In this paper, the absorption performance of a proposed metamaterial (MM) absorber based on a three-layer graphene structure working in the terahertz (THz) frequency band is studied. By using different types of combined graphene patterns, dual-band, tri-band, and quad-band absorption can be achieved. In the case of tri-band absorption, three absorption peaks with absorption rates of 99.7%, 99.9%, and 99.9% can be found at frequencies of 4.64 THz, 6.45 THz, and 9.71 THz, respectively. In addition, the proposed structure is polarization independent and has the absorption characteristic of wide incident angles. The frequency and the intensity of the absorption peaks can be adjusted by changing the chemical potential and the relaxation time of the graphene and the structural parameters. Therefore, we believe that the proposed graphene MM absorber structure provides flexible design ideas for a multibandwidth MM perfect absorber, and the proposed absorber also can be applied to subwavelength integrated sensors and optoelectronic devices in the terahertz range.

Tunable dual-band metamaterial absorber at deep-subwavelength scale

A tunable dual-band metamaterial absorber composed of a gold square ring and a metal film separated by a strontium titanate dielectric is investigated. Two resonance peaks at frequencies 0.162 THz and 0.329 THz with over 99% absorbance are realized at room temperature. The maximum ratio between the lattice constant and the resonance wavelength is approximately 8.2%, which is much smaller than that of the formerly reported absorber. When the temperature changes from 150 K to 350 K, the two resonance absorption peaks at approximately 66.2% and 67.8% of the modulation depths are obtained. The effective impedance method and the LC equivalent circuit modelling are used to analyze the absorption mechanism of the proposed absorber. The proposed absorber is not affected by the change in polarization angles at normal incidence. The proposed structure adopts a single patterned square ring, which can be manufactured easily, and extended to other frequencies in applications such as biosensors, monitoring and imaging.

Passive tunable and polarization-insensitive fan-like metamaterial absorber in the visible spectrum

We numerically study a fan-like metamaterial absorber (FMMA) composed of an A u - T i O 2 - A u multilayer with a 3-D finite-difference-time-domain method (FDTD) and verify it experimentally. Our proposed design shows high absorbance, > --> 98 % numerically, in the visible spectrum, which is due to the excitation of both localized surface plasmons (LSPs) and propagating surface plasmons (PSPs). In particular, the T i O 2 dielectric spacer sandwiched between the top fan-like nanopatterned structure and the bottom planar Au-film is identified as playing a crucial role in the spectral response of the FMMA. Its introduction excites the PSP mode, which first enhances the absorbance of the FMMA and subsequently redshifts the spectral absorbance with a further increase in the thickness. Moreover, the proposed FMMA, numerically and experimentally, also shows polarization and angle insensitivity in the visible spectrum under normal and selected oblique incidence. These results offer unique potential in sensing applications and devices requiring selective absorption in the visible spectral region.

Tunable quad-band perfect metamaterial absorber on the basis of monolayer graphene pattern and its sensing application

In this paper, a classic three-layer metamaterial absorber (MA) is designed to achieve quad-band perfect absorption. The unit cell of the top graphene pattern layer is made up of a periodically arranged graphene split-ring nested with a cross-like graphene structure formed by four L-shaped strips. The middle dielectric spacer layer is SiO2 and the ground layer is gold. Simulation results show that four absorption bands with near unity peak absorption rate will occur in the interested frequency range of 3–11 THz. The absorption mechanism and the parameter dependency are discussed at first, which will provide guiding significances for the actual fabrication. Meanwhile, when the Fermi level of graphene is increased, the four absorption bands will have different degrees of blue-shift, and when the relaxation time of graphene is increased, the four absorption peaks will increase first and then decreases. As a result, dynamical tunability of the absorption properties can be achieved by changing the Fermi level or the relaxation time of the graphene without changing the geometry of the proposed MA. Further investigation indicates that the proposed absorber is independent of the polarization of the input electromagnetic (EM) wave and can maintain well absorption properties when the incident angle is changed within a broader range. Considering the potential sensing applications, it is shown that the proposed MA can be operated as a type of refractive index sensor with highest sensitivity of 2.97THz/RIU, and the maximum figure of merit (FOM) is 47.5. In terms of these unique performances, the proposed MA has broad application prospects in multispectral optical stealth, optical filtering, integrated photo detecting, sensing and so on.

Actively tunable dual-broadband graphene-based terahertz metamaterial absorber* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11504006 and 61805072) and the Key Scientific Research Project of Colleges and Universities in Henan Province, China (Grant No. 22A140001).

A tunable metamaterial absorber (MA) with dual-broadband and high absorption properties at terahertz (THz) frequencies is designed in this work. The MA consists of a periodic array of flower-like monolayer graphene patterns at top, a SiO2 dielectric spacer in middle, and a gold ground plane at the bottom. The simulation results demonstrate that the designed MA has two wide absorption bands with an absorption of over 90% in frequency ranges of 0.68 THz–1.63 THz and 3.34 THz–4.08 THz, and the corresponding relative bandwidths reach 82.3% and 20%, respectively. The peak absorptivity of the absorber can be dynamically controlled from less than 10% to nearly 100% by adjusting the graphene chemical potential from 0 eV to 0.9 eV. Furthermore, the designed absorber is polarization-insensitive and has good robustness to incident angles. Such a high-performance MA has broad application prospects in THz imaging, modulating, filtering, etc.

Tunable Terahertz Perfect-Absorbers With Dual Peak Based on Reverse Graphene Patch Metamaterials

In this paper, we proposed a tunable graphene-based metamaterial absorber (MA) at dual terahertz band to achieve perfect absorption. The electric field intensity distributions of demonstrated structure is illustrated to explain the origin of two absorption peaks. Simulation result shows the proposed MA owning polarization-insensitive property and wide incident angle tolerance. An inversion algorithm is used to extract the equivalent impedance and explore the physical mechanism of multiple-peak absorption. The equivalent impedance is explored through algorithm when the Fermi energy level alters, consequently the variation of resonant frequency of corresponding absorption peak is determined. Simultaneously, the resonant frequencies of absorption peaks can be adjusted by changing the geometric parameters of the graphene. Furthermore, we adapt the proposed structure into reversely designed pattern, achieving similar characteristic of dual-band absorption peaks and tunability. Thus, our work provides new insight and useful design guidance to various tunable multiple band terahertz metamaterial perfect-absorbers.

Thermally tunable polarization-insensitive ultra-broadband terahertz metamaterial absorber based on the coupled toroidal dipole modes.

In this paper, we propose a thermally tunable ultra-broadband polarization-insensitive terahertz (THz) metamaterial absorber (MMA) excited by the toroidal dipole moments. Due to the destructive interference resulting from two anti-parallel toroidal dipole moments, which depends on the twelve-fold trapezoidal metallic loops rotated by the axis parallel to the z-axis, the proposed MMA can achieve the absorption over 0.9 in a wide band of 2.38-21.13 THz, whose relative absorption band is 159.5%, at the temperature of 340 K. Meanwhile, by virtue of tuning the conductivity of vanadium dioxide (VO2) controlled by temperature, the tunability of absorption, maximum reaching 0.57, in the above band can be attained. On the other hand, the MMA is insensitive to the polarization angle owing to its symmetric configuration and can simultaneously keep the absorption above 0.9 in the high-frequency band from 15 to 25 THz under the incidence with a large angle of nearly 60°. In this study, a new way to enhance the absorption in a wide band which is based on the toroidal dipole modes is presented. Such a metamaterial can assist in further understanding the underlying mechanism with respect to the toroidal dipole electromagnetic responses.

Dynamically Switchable Polarization-Independent Triple-Band Perfect Metamaterial Absorber Using a Phase-Change Material in the Mid-Infrared (MIR) Region.

A tunable metamaterial absorber (MMA) by reversible phase transitions in a mid-infrared regime is theoretically investigated. The absorber is composed of a molybdenum (Mo)-germanium-antimony-tellurium (Ge2Sb2Te5, GST)-Mo nanodisk structure superimposed on the GST-Al2O3 (aluminum oxide)-Mo film. Studies have shown that the combination of the inlaid metal-medium dielectric waveguide mode and the resonant cavity mode and the excitation of the propagating surface plasmon mode are the main reasons for the formation of the triple-band high absorption. Additionally, through the reversible phase change, the transition from high absorption to high reflection in the mid-infrared region is realized. The symmetry of the absorber eliminates the polarization dependence, and the near unity absorption efficiency can be maintained by incidence angles up to 60°. The presented method will enhance the functionality of the absorber and has the potential for the applications that require active control over light absorption.

Triple-band tunable terahertz perfect absorber based on graphene metamaterial

To further reduce the structural complexity of high-performance modulation devices in the terahertz region, we propose and demonstrate a triple-band tunable metamaterial perfect absorber, which consists of a patterned graphene and Au ground plane spaced by Si dielectric layer. The graphene surface plasmon resonance at the terahertz region is used to couple the patterned graphene with the electric field and provide electric dipole resonance to form multiple absorption peaks. Numerical results indicate that the amplitude of the absorption peaks is large than 99.9% at 0.489 THz, 1.492 THz, and 2.437 THz, respectively. In addition, due to the amplitude of the absorption peaks that can be controlled by the Fermi level via externally applied bias voltage, it means that we are able to switch the structure between reflector and absorber at their corresponding absorption bands. However, the symmetric unit cell structure of the proposed absorber is the inherent reason for the excellent polarization-insensitive, and the broadband absorption of the absorber maintains excellent absorption performance over a wide range of incident angles. Therefore, the designed graphene-based terahertz metamaterial function device has great potential in modulation and sensing.