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In recent years, absorbers related to metamaterials have been heavily investigated. In particular, VO2 materials have received focused attention, and a large number of researchers have aimed at multilayer structures. This paper presents a new concept of a three-layer simple structure with VO2 as the base, silicon dioxide as the dielectric layer, and graphene as the top layer. When VO2 is in the insulated state, the absorber is in the closed state, Δf = 1.18 THz (absorption greater than 0.9); when VO2 is in the metallic state, the absorber is open, Δf = 4.4 THz (absorption greater than 0.9), with ultra-broadband absorption. As a result of the absorption mode conversion, a phenomenon occurs with this absorber, with total transmission and total reflection occurring at 2.4 THz (A = 99.45% or 0.29%) and 6.5 THz (A = 90% or 0.24%) for different modes. Due to this absorption property, the absorber is able to achieve full-transmission and full-absorption transitions at specific frequencies. The device has great potential for applications in terahertz absorption, terahertz switching, and terahertz modulation. [ABSTRACT FROM AUTHOR]

As graphene-related technology advances, the benefits of graphene metamaterials become more apparent. In this study, a surface-isolated exciton-based absorber is built by running relevant simulations on graphene, which can achieve more than 98% perfect absorption at multiple frequencies in the MWIR (MediumWavelength Infra-Red (MWIR) band as compared to the typical absorber. The absorber consists of three layers: the bottom layer is gold, the middle layer is dielectric, and the top layer is patterned with graphene. Tunability was achieved by electrically altering graphene's Fermi energy, hence the position of the absorption peak. The influence of graphene's relaxation time on the sensor is discussed. Due to the symmetry of its structure, different angles of light source incidence have little effect on the absorption rate, leading to polarization insensitivity, especially for TE waves, and this absorber has polarization insensitivity at ultra-wide-angle degrees. The sensor is characterized by its tunability, polarisation insensitivity, and high sensitivity, with a sensitivity of up to 21.60 THz/refractive index unit (RIU). This paper demonstrates the feasibility of the multi-frequency sensor and provides a theoretical basis for the realization of the multi-frequency sensor. This makes it possible to apply it to high-sensitivity sensors. [ABSTRACT FROM AUTHOR]

This paper presents a new theoretical proposal for a surface plasmon resonance (SPR) terahertz metamaterial absorber with five narrow absorption peaks. The overall structure comprises a sandwich stack consisting of a gold bottom layer, a silica medium, and a single-layer patterned graphene array on top. COMSOL simulation represents that the five absorption peaks under TE polarization are at fI = 1.99 THz (95.82%), fⅡ = 6.00 THz (98.47%), fⅢ = 7.37 THz (98.72%), fⅣ = 8.47 THz (99.87%), and fV = 9.38 THz (97.20%), respectively, which is almost consistent with the absorption performance under TM polarization. In contrast to noble metal absorbers, its absorption rates and resonance frequencies can be dynamically regulated by controlling the Fermi level and relaxation time of graphene. In addition, the device can maintain high absorptivity at 0~50° in TE polarization and 0~40° in TM polarization. The maximum refractive index sensitivity can reach SV = 1.75 THz/RIU, and the maximum figure of merit (FOM) can reach FOMV = 12.774 RIU−1. In conclusion, our design has the properties of dynamic tunability, polarization independence, wide-incident-angle absorption, and fine refractive index sensitivity. We believe that the device has potential applications in photodetectors, active optoelectronic devices, sensors, and other related fields. [ABSTRACT FROM AUTHOR]

When surface plasmon resonance (SPR) occurs, the incident light is absorbed by the surface of the SPR structure, thus minimizing the intensity of the reflected light. Therefore, the SPR method is adopted in this paper to achieve perfect absorption of the absorbent. In this paper, we first propose a multi-frequency broadband absorber structure based on graphene SPR, which uses the continuous resonance of patterned graphene surface plasmon in the frequency spectrum to form a multi-frequency broadband absorption. In this simulation, a sandwich-stack structure was adopted, whereby the patterned graphene is situated on top of the SiO2 layer and the metal layer. The broad-band absorption bands of the absorber were obtained as 4.14–4.38 THz, 5.78–6.36 THz, and 7.87–8.66 THz through the analog simulation of finite-difference time-domain method (FDTD) solutions. Then, based on the multi-layer resonant unit structure, through the superposition and combination of absorbing units responding to different frequency bands, the perfect absorption of ultra-wideband is achieved. The data results illustrate that the total absorption bandwidth of the absorber is 2.26 THz, and the relative absorption bandwidth Bw is equal to 28.93%. The electric field in X-Y direction of the absorber in the perfect absorption band is analyzed, respectively, and the dynamic tunability of the absorber is studied. Finally, we studied whether the absorbing structure still has efficient absorption characteristics for the two polarization modes when the incident angle is changed from 0° to 70°. The structure model proposed has potential value for application in terahertz photoelectric detection, filtering, and electromagnetic shielding. [ABSTRACT FROM AUTHOR]

This paper introduces a novel metamaterial absorber based on surface plasmon resonance (SPR). The absorber is capable of triple-mode perfect absorption, polarization independence, incident angle insensitivity, tunability, high sensitivity, and a high figure of merit (FOM). The structure of the absorber consists of a sandwiched stack: a top layer of single-layer graphene array with an open-ended prohibited sign type (OPST) pattern, a middle layer of thicker SiO2, and a bottom layer of the gold metal mirror (Au). The simulation of COMSOL software suggests it achieves perfect absorption at frequencies of fI = 4.04 THz, fII = 6.76 THz, and fIII = 9.40 THz, with absorption peaks of 99.404%, 99.353%, and 99.146%, respectively. These three resonant frequencies and corresponding absorption rates can be regulated by controlling the patterned graphene's geometric parameters or just adjusting the Fermi level (EF). Additionally, when the incident angle changes between 0~50°, the absorption peaks still reach 99% regardless of the kind of polarization. Finally, to test its refractive index sensing performance, this paper calculates the results of the structure under different environments which demonstrate maximum sensitivities in three modes: SI = 0.875 THz/RIU, SII = 1.250 THz/RIU, and SIII = 2.000 THz/RIU. The FOM can reach FOMI = 3.74 RIU−1, FOMII = 6.08 RIU−1, and FOMIII = 9.58 RIU−1. In conclusion, we provide a new approach for designing a tunable multi-band SPR metamaterial absorber with potential applications in photodetectors, active optoelectronic devices, and chemical sensors. [ABSTRACT FROM AUTHOR]

The paper proposes an ultra-narrow band graphene refractive index sensor, consisting of a patterned graphene layer on the top, a dielectric layer of SiO2 in the middle, and a bottom Au layer. The absorption sensor achieves the absorption efficiency of 99.41% and 99.22% at 5.664 THz and 8.062 THz, with the absorption bandwidths 0.0171 THz and 0.0152 THz, respectively. Compared with noble metal absorbers, our graphene absorber can achieve tunability by adjusting the Fermi level and relaxation time of the graphene layer with the geometry of the absorber unchanged, which greatly saves the manufacturing cost. The results show that the sensor has the properties of polarization-independence and large-angle insensitivity due to the symmetric structure. In addition, the practical application of testing the content of hemoglobin biomolecules was conducted, the frequency of first resonance mode shows a shift of 0.017 THz, and the second resonance mode has a shift of 0.016 THz, demonstrating the good frequency sensitivity of our sensor. The S (sensitivities) of the sensor were calculated at 875 GHz/RIU and 775 GHz/RIU, and quality factors FOM (Figure of Merit) are 26.51 and 18.90, respectively; and the minimum limit of detection is 0.04. By comparing with previous similar sensors, our sensor has better sensing performance, which can be applied to photon detection in the terahertz band, biochemical sensing, and other fields. [ABSTRACT FROM AUTHOR]

In this paper, we come up with a wavelength tunable absorber which is made up of periodically elliptical graphene arrays in the far-infrared and terahertz regions. Through simulation, we find that we can increase the length of long axis of the ellipse, raise the incidence angles of TM- and TE-polarization (TM- and TE-polarization indicate the direction of the incident electric field along the direction of the x and the y axis, respectively.) within certain limits, and increase the chemical potential of graphene, so as to enhance the absorption of light in the elliptical graphene arrays. We also compare the absorption spectra of the original structure and the complementary structure, and find that the absorption of the original structure is higher than that of the complementary structure. In the end, we study the changes in the absorption rate of the double layer structure of the elliptical array with the increase in the thickness of SiO2. The elliptical array structure can be applied to tunable spectral detectors, filters and sensors at far-infrared and terahertz wavelengths. [ABSTRACT FROM AUTHOR]

In this paper, we design and present a graphene-based "ancient coin"-type dual-band perfect metamaterial absorber, which is composed of a silver layer, silicon dioxide layer, and a top "ancient coin" graphene layer. The absorption performance of the absorber is affected by the hollowed-out square in the center of the graphene layer and geometric parameters of the remaining nano disk. The optical properties of graphene can be changed by adjusting the voltage, to control the absorption performance of the absorber dynamically. In addition, the centrally symmetric pattern structure greatly eliminates the polarization angle dependence of our proposed absorber, and it exhibits good angular polarization tolerance. Furthermore, the proposed "ancient coin"-type absorber shows great application potential as a sensor or detector in biopharmaceutical, optical imaging, and other fields due to its strong tunability and high refractive index sensitivity. [ABSTRACT FROM AUTHOR]

By means of critical coupling and impedance matching theory, we have numerically simulated the perfect absorption of monolayer graphene. Through the critical coupling effect and impedance matching, we studied a perfect single-band absorption of the monolayer graphene and obtained high quality factor (Q-factor = 664.2) absorption spectrum which has an absorbance close to 100% in the near infrared region. The position of the absorption spectrum can be adjusted by changing the ratio between the radii of the elliptic cylinder air hole and the structural period. The sensitivity of the absorber can be achieved S = 342.7 nm/RIU (RIU is the per refractive index unit) and FOM = 199.2 (FOM is the figure of merit), which has great potential for development on biosensors. We believe that our research will have good application prospects in graphene photonic devices and optoelectronic devices. [ABSTRACT FROM AUTHOR]

We propose a graphene nanoribbon-ring cross structure. A multi-band absorber is obtained by a simple combination of graphene nanoribbons and and graphene ring. We carefully analyzed each resonance wavelength, and change the thickness of the substrate, the effect of chemical potential and the angle of incident light of this absorber. The FoM of resonance wavelengths are not low. So the absorber has certain advantages in sensing of refractive index. • The multi-band absorber support two perfect absorption resonance wavelengths. • We carefully analyzed the source of each resonance wavelength. • The absorber can be tune by Femi level and the absorber is insensitive to the angle of incident light. • The FoM of absorber is not low, has a certain application in the field of refractive index sensing. In this paper, we propose a graphene nanoribbon-ring cross structure. A multi-band absorber is obtained by a simple combination of graphene nanoribbons and and graphene ring. We obtain that the theoretical absorption of three resonance wavelengths are 99.8%, 98.4% and 65.7%, respectively. We carefully analyzed each resonance wavelength, and the compact structure makes that small changes in the position of graphene nanoribbons and graphene ring have a great influence on the number of resonance wavelength. Meanwhile, the small changes in the chemical potential of graphene can tune the position of resonance wavelengths in the spectrum. At last, we discuss the sensing properties of this structure. The FoM (figure of merit) values of the three resonance wavelengths are high, and the FoM of the three peaks can reach 4.1, 3.679 and 12.66. The absorbers have great adjustability and great application potential in sensing. [ABSTRACT FROM AUTHOR]

We demonstrate a dual-band plasmonic perfect absorber (PA) based on graphene metamaterials. Two absorption peaks (22.5 μm and 74.5 μm) with the maximal absorption of 99.4% and 99.9% have been achieved, respectively. We utilize this perfect absorber as a plasmonic sensor for refractive index (RI) sensing. It has the figure of merit (FOM) of 10.8 and 3.2, and sensitivities of about 5.6 and 17.2 μm/RIU, respectively. Hence, the designed dual-band PA-based RI sensor exhibits good sensing performance in the infrared regime, which offers great potential applications in various biomedical, tunable spectral detecting, environmental monitoring and medical diagnostics. [ABSTRACT FROM AUTHOR]

• Various geometric parameters can realize the selectivity of resonance frequency. • Simple single graphene pattern, instead of stacked or patched structures, has perfect absorption properties in dual-band. • The resonance range can be adjusted by changing the Fermi level and relaxation time of graphene. • Absorption characteristics possess the strong independence between incident angle θ and polarization. In this paper, we propose a tunable dual-band perfect absorber, which makes the thicker SiO 2 dielectric as a spacer to separate the gold layer from the periodic cross-cross monolayer graphene array layer which locates the roof of whole absorber. The simulation results indicate that transforming the periodic p , the geometrical parameters of array and the thickness of SiO 2 medium can make the structure achieve perfect absorption in dual-band. Moreover, the resonance wavelength and absorption peak can be flexibly selected by Fermi level and relaxation time. In addition, the absorber is insensitive to incident angle, whether TM polarization or TE polarization. Therefore, the study provides a novel inspiration for the design of graphene-based tunable multi-band perfect THz absorber, which can be applied to photodetectors, chemical sensors and other optoelectronic devices. [ABSTRACT FROM AUTHOR]

• The plasmonic metamaterial absorber can achieve three-band perfect absorption. • The absorber is insensitive to the incident angle under two different polarization states (TE & TM). • The absorber has a great potential in infrared and terahertz light detection. In this paper, a new graphene-based tunable triple-band plasmonic perfect metamaterial absorber which is composed of two groups of graphene elliptical ring resonators and a gold plate separated by a dielectric layer is presented. The two sets of graphene elliptical ring resonators are connected to each other, so their internal parameters can be easily adjusted by voltage, which can dynamically control the absorptive characteristics of graphene metamaterials. In addition, the proposed absorber shows a very good angle-polarization-tolerance because of the high symmetry of the structure configuration. The results show that this kind of symmetrical double ellipse structure can produce three highly effective absorption peaks which are respectively at around 38.7 μm, 20.2 μm and 12.8 μm and the value of peaks can even reach more than 99%. Moreover, due to the strong tunability, our absorber has a great potential in infrared and terahertz detection. [ABSTRACT FROM AUTHOR]

In the present study, we design a tunable plasmonic refractive index sensor with nanoring-strip graphene arrays. The calculations prove that the nanoring-strip have two transmission dips. By changing the strip length L of the present structure, we find that the nanoring-strip graphene arrays have a wide range of resonances (resonance wavelength increases from 17.73 μm to 28.15 μm). When changing the sensing medium refractive index nmed, the sensitivity of mode A and B can reach 2.97 μm/RIU and 5.20 μm/RIU. By changing the doping level ng, we notice that the transmission characteristics can be tuned flexibly. Finally, the proposed sensor also shows good angle tolerance for both transverse magnetic (TM) and transverse electric (TE) polarizations. The proposed nanoring-strip graphene arrays along with the numerical results could open a new avenue to realize various tunable plasmon devices and have a great application prospect in biosensing, detection, and imaging. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel tunable THZ absorber consisting of a patterned graphene layer and a metal plate separated by a vanadium dioxide dielectric layer. The absorbent has a simple structure, is easy to make graphene patterns, and has a dual regulatory function. The ability to double regulate comes from the fact that the state of vanadium dioxide changes with the temperature between the insulating phase and the metal phase, resulting in a change in its conductivity. When vanadium dioxide is in the insulating phase, the total reflection characteristics of the absorber are >7 THz, and when vanadium dioxide is in the metallic phase, the absorber achieves near-perfect absorption in the ultra-wideband range of 3 THz to 12.4 THz (9.4 THz). Local surface plasmon resonance explains the perfect absorption. Compared with previous absorbers, our proposed absorbers are not only simple in structure, the graphene pattern is also easy to process, but also have double adjustment and perfect absorption in the ultra-wideband range, are not sensitive to polarized light, and have large tolerances for the Angle of incident. The further development and enrichment of terahertz metamaterials and metasurface devices will also have beneficial implications for terahertz wave applications in sensing, communications, and radar. Due to their significant application value in stealth, detection, and communication, metamaterial absorbers have become a research hotspot in the field of metamaterials. [Display omitted] • The absorber can realize ultra-wideband absorption from terahertz band to near infrared band. • This device achieves dual control by changing temperature and Fermi level. [ABSTRACT FROM AUTHOR]

In this paper, we report a numerical investigation of tunable dual-band metamaterial perfect absorber (MMPA) consisting of two-intersecting graphene nanorings arrays. The optical absorption performance of absorber is dominated by the inner/outer radius of nanorings and center distance between the two nanorings. For the resonance modes λ 1 and λ 2 , the sensitivity of probing the surrounding refractive index can reach 7.97 μm/RIU and 13.27 μm/RIU respectively. The absorption performance of the under-study graphene nanorings can be flexibly adjusted by modulating the Fermi level of graphene with voltage. Due to excellent wavelength selectivity and high refractive-index sensitivity, the presented tunable dual-band MMPA has great application prospects as sensors in biopharmaceutical, environmental monitoring. • The proposed nanostructure have two obvious perfect absorption peaks at 31.6 μm and 57.5 μm. • The proposed nanostructure absorption performance can be flexibly adjusted by modulating the Fermi level of graphene. • The sensitivity of the presented MMPA structure are 7.97 μm/RIU and 13.27 μm/RIU. • The figure of merits (FOM) of the presented MMPA structure can reach 7.6872 and 3.1489. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In recent years, as an important plasmonic nanomaterial, graphene has become the focus of research. In our research, we design a tunable plasmonic absorber composed of periodic H-shaped graphene arrays in the far-infrared and terahertz (THz) bands. We numerically investigate the structure parameters and the chemical potential of graphene that influence the optical properties of the periodic H-shaped graphene arrays. Compared with periodic H-shaped graphene arrays, under the same TM polarization, the complementary structure has higher absorption. In addition, the polarization sensitivity of the H-shaped graphene arrays is also investigated. Our research not only investigates the plasmonic resonance absorption characteristics, but also implies important potential applications in integrated optical circuits such as optical storage, ultra-fast plasmonic switches, high performance filters and modulators. [ABSTRACT FROM AUTHOR]

In this paper, we present a dual-band metamaterial absorber for graphene surface plasmon resonance at terahertz frequency. We use the finite difference time domain (FDTD) method to study the absorption characteristics of the homocentric graphene ring and disk nanostructure. These simulation results show that the change of the geometrical parameters and the substrate thickness of the nanostructure can change the absorption characteristics and the emergence of dual-band absorption peaks. Moreover, we study the field distribution of nanodisks with different radius in detail. By changing the Fermi level of graphene, the wavelength of their absorption peaks can be adjusted flexibly. In addition, the proposed dual-band absorber also shows a good angle tolerance for both TE and TM polarizations. By calculation the surface-filled water (n = 1.332) and 25% aqueous glucose solution (n = 1.372) for the metamaterial absorber, the sensitivities of mode I and mode II are 5.0 μm/RIU and 15.0 μm/RIU. These research results will have broad application prospects for sensing and spatial light modulators. We present a dual-band metamaterial absorber for graphene surface plasmon resonance at terahertz frequency. We find the nanostructure absorption peak wavelength is flexible and adjustable by changing the graphene Fermi level E F. Image 1 • The absorption enhancement of the nanostructure can be achieved by changing the geometric parameters. • By changing the Fermi level, the absorption characteristics can be tuned. • As the relaxation time increases, the absorption maximum also increases, while the resonance peak wavelength remains unchanged. • For TM and TE polarizations, the absorbers are insensitive to the incident angle. [ABSTRACT FROM AUTHOR]