Tunable graphene-based plasmonic multispectral and narrowband perfect metamaterial absorbers at the mid-infrared region

Dual-band and spectrally selective infrared absorbers based on hybrid gold-graphene metasurfaces

In this paper, we propose a dual-band and spectrally selective infrared (IR) absorber based on a hybrid structure comprising a patterned graphene monolayer and cross-shaped gold resonators within a metasurface. Rooted in full-wave numerical simulations, our study shows that the fundamental absorption mode of the gold metasurface hybridizes with the graphene pattern, leading to a second absorptive mode whose properties depend on graphene's electrical properties and physical geometry. Specifically, the central operation band of the absorber is defined by the gold resonators whereas the relative absorption level and spectral separation between the two modes can be controlled by graphene's chemical potential and its pattern, respectively. We analyze this platform using coupled-mode theory to understand the coupling mechanism between these modes and to elucidate the emergence and tuning of the dual band response. The proposed dual-band device can operate at different bands across the IR spectrum and may open new possibilities for tailored sensing applications in spectroscopy, thermal imaging, and environmental monitoring.

Graphene electromagnetically induced transparent polarization-insensitive sensors in the mid-infrared frequency band

In this paper, a polarization-insensitive sensor based on graphene electromagnetically induced transparency (EIT) is proposed. The device consists of two graphene orthogonal T-shaped structures. This T-shaped resonator produces transparent windows that largely overlap under x and y polarizations, and the results demonstrate its good polarization insensitivity. The device can accomplish detection performance with sensitivity higher than 4960 nm/RIU and figure of merit (FOM) greater than 11.4. Meanwhile, when the Fermi energy level of graphene changes from 0.5 to 0.8 eV, it enables arbitrary modulation of the operating frequency over a wide frequency range of about 4.5 terahertz in the mid-infrared band. Our work has the potential to significantly advance the area of biological molecular detection.

Perfect Absorption of Fan-Shaped Graphene Absorbers with Good Adjustability in the Mid-Infrared

This paper presents a graphene metamaterial absorber based on impedance matching. A finite difference in time domain (FDTD) method is used to achieve a theoretically perfect absorption in the mid-infrared band. A basis is created for the multiband stable high absorption of graphene in the mid-infrared. The designed graphene absorber is composed of graphene, a dielectric layer, a gold plane, and a silicon substrate, separately. The incident source of mid-infrared can be utilized to stimulate multiband resonance absorption peaks from 2.55 to 4.15 μm. The simulation results show that the absorber has three perfect resonance peaks exceeding 99% at λ1 = 2.67 μm, λ2 = 2.87 μm, and λ3 = 3.68 μm, which achieve an absorption efficiency of 99.67%, 99.61%, and 99.40%, respectively. Furthermore, the absorber maintains an excellent performance with a wide incident angle range of 0°–45°, and it also keeps the insensitive characteristic to transverse electric wave (TE) and transverse magnetic wave (TM). The results above indicate that our perfect graphene absorber, with its tunability and wide adaptability, has many potential applications in the fields of biosensing, photodetection, and photocell.

Ultra-Broadband Tunable Terahertz Metamaterial Absorber Based on Double-Layer Vanadium Dioxide Square Ring Arrays

Based on the phase transition of vanadium dioxide(VO₂), an ultra-broadband tunable terahertz metamaterial absorber is proposed. The absorber consists of bilayer VO₂ square ring arrays with different sizes, which are completely wrapped in Topas and placed on gold substrate. The simulation results show that the absorption greater than 90% has frequencies ranging from 1.63 THz to 12.39 THz, which provides an absorption frequency bandwidth of 10.76 THz, and a relative bandwidth of 153.5%. By changing the electrical conductivity of VO₂, the absorption intensity can be dynamically adjusted between 4.4% and 99.9%. The physical mechanism of complete absorption is elucidated by the impedance matching theory and field distribution. The proposed absorber has demonstrated its properties of polarization insensitivity and wide-angle absorption, and therefore has a variety of application prospects in the terahertz range, such as stealth, modulation, and sensing.

A Simple Structure for an Independently Tunable Infrared Absorber Based on a Non-Concentric Graphene Nanodisk

Due to its unique electronic and optical properties, graphene has been used to design tunable optical absorbers. In this paper, we propose a plasmonic absorber consisting of non-concentric graphene nanodisk arrays, which is designed to operate in the mid-infrared spectral range and is capable of achieving nearly perfect absorption. Two perfect absorption peaks are produced due to the impedance of the structure, which matches that of the free space. The influences of the thicknesses of the dielectric layer, the size of graphene nanodisk, and the incident conditions on the absorption are studied. Moreover, the absorption intensity can be independently tuned by varying the Fermi levels of two graphene nanodisks. Furthermore, the polarization-independent absorbance of the absorber exceeds 95% under oblique incidence, and remains very high over a wide angle. This proposed absorber has potential applications in optical detectors, tunable sensors, and band-pass filters.

Polarization-sensitive and active controllable electromagnetically induced transparency in U-shaped terahertz metamaterials

Electromagnetically induced transparency (EIT) phenomenon is observed in simple metamaterial which consists of concentric double U-shaped resonators (USRs). The numerical and theoretical analysis reveals that EIT arises from the bright-bright mode coupling. The transmission spectra at different polarization angle of incident light shows that EIT transparency window is polarization sensitive. More interestingly, Fano resonance appears in the transmission spectrum at certain polarization angles. The sharp and asymmetric Fano lineshape is high valuable for sensing. The performance of sensor is investigated and the sensitivity is high up to 327 GHz/RIU. Furthermore, active control of EIT window is realized by incorporating photosensitive silicon. The proposed USR structure is simple and compact, which may find significant applications in tunable integrated devices such as biosensor, filters, and THz modulators.

Tunable dual-band mid-infrared absorber based on the coupling of a graphene surface plasmon polariton and Tamm phonon-polariton

We propose and demonstrate a tunable dual-band mid-infrared absorber structure based on the coupling effect of a surface plasmon polariton (SPP) and Tamm phonon-polariton (TPhP). The structure is composed of the distributed Bragg reflector (DBR), air layer, SiC and graphene ribbons. In the air layer, the graphene ribbons are embedded to realize the localized SPP (LSPP), which makes the structure support both the graphene LSPP (GLSPP) and TPhP. The absorption properties of the structure are investigated theoretically and numerically. It is found that strong coupling of the GLSPP and TPhP can be realized by choosing reasonable parameters, which causes a dual-frequency perfect absorption and makes the maximum Rabi splitting of the coupled mode reach 5.76 meV. Furthermore, the mode coupling and absorption intensity can be tuned by adjusting the thickness of the air layer and the Fermi level of the graphene ribbons. This work might provide new possibilities for the development of mid-infrared band sensors, filters and emitters based on the coupling of multiple modes.

Ultrafast and low power all-optical switching in the mid-infrared region based on nonlinear highly doped semiconductor hyperbolic metamaterials

Guided wave modes in the uniaxial anisotropic hyperbolic metamaterials (HMMs) based on highly doped semiconductor instead of metal in the mid-infrared region are investigated theoretically. The heavily doped semiconductor is used to overcome the restrictions of the conventional metal-based structures caused by the lake of tunability and high metal loss at mid-infrared wavelengths. The unit cells of our proposed metamaterial are composed of alternating layers of undoped InAs as a dielectric layer and highly doped InAs as a metal layer. We numerically study the linear and nonlinear behavior of such multilayer metamaterials, for different arrangements of layers in the parallel (vertical HMM) and perpendicular (horizontal HMM) to the input wave vector. The effect of doping concentration, metal to dielectric thickness ratio in the unit cell (fill-fraction), and the total thickness of structure on the guided modes and transmission/reflection spectra of the metamaterials are studied. Moreover, the charge redistribution due to band-bending in the alternating doped and undoped layers of InAs is considered in our simulations. We demonstrate that the guided modes of the proposed hyperbolic metamaterial can change by increasing the intensity of the incident lightwave and entering the nonlinear regime. Therefore, the transition from linear to the nonlinear region leads to high-performance optical bistability. Furthermore, the switching performance in the vertical and horizontal HMMs are inspected and an ultrafast, low power, and high extinction ratio all-optical switch is presented based on a vertical structure of nonlinear highly doped semiconductor hyperbolic metamaterials.

Equivalent circuit model of graphene chiral multi-band metadevice absorber composed of U-shaped resonator array

In this study, we have designed an equivalent circuit model (ECM) by use of a simple MATLAB code to analyze a single-layered graphene chiral multi-band metadevice absorber which is composed of U-shaped graphene resonator array in terahertz (THz) region. In addition, the proposed metadevice absorber is analyzed numerically by the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed device which is the first tunable graphene-based chiral metadevice absorber can be used in polarization sensitive devices in THz region. It is single-layered, tunable, and it has strong linear dichroism (LD) response of 94% and absorption of 99% for both transverse electric (TE) and transverse magnetic (TM) electromagnetic waves. It has four absorption bands with absorption >50% in 0.5-4.5 THz : three absorption bands for TE mode and one absorption band for TM mode. Proposed ECM has good agreement with the FEM simulation results. ECM analysis provides a simple, fast, and effective way to understand the resonance modes of the metadevice absorber and gives guidance for the analysis and design of the graphene chiral metadevices in the THz region.

Switchable broadband terahertz spatial modulators based on patterned graphene and vanadium dioxide

We numerically demonstrate a switchable broadband terahertz spatial modulator composed of ginkgo-leaf-patterned graphene and transition material vanadium dioxide (VO₂). The phase transition property of VO₂ is used to switch the spatial modulator from absorption mode to transmission mode, and the graphene behaves as dynamically adjustable material for a large scale of absorption and transmittance modulation. When VO₂ is in the metallic state and the Fermi energy of graphene is set as 0.8 eV, the proposed modulator behaves as a broadband absorber with the absorbance over 85% from 1.33 to 2.83 THz. By adjusting the graphene Fermi level from 0 to 0.8 eV, the peak absorbance can be continuously tuned from 24.3% to near 100% under the absorption mode, and the transmittance at 2.5 THz can be continuously tuned from 87% to 35.5% under the transmission mode. To further increase the bandwidth, a three-layer-patterned-graphene is introduced into a new modulator design, which achieves a wide bandwidth of 3.13 THz for the absorbance over 85%. By the combination of the tunability of graphene and VO₂, the proposed modulators not only can flexibly switch between dual-functional modulation modes of absorption and transmission but also possess deep modulation depth. Benefitting from the excellent modulation performance, the proposed switchable dual-functional spatial modulators may offer significant potential applications in various terahertz smart optoelectronic devices.

Graphene-based hyperbolic metamaterial as a switchable reflection modulator

A tunable graphene-based hyperbolic metamaterial is designed and numerically investigated in the mid-infrared frequencies. Theoretical analysis proves that by adjusting the chemical potential of graphene from 0.2 eV to 0.8 eV, the reflectance can be blue-shifted up to 2.3 µm. Furthermore, by modifying the number of graphene monolayers in the hyperbolic metamaterial stack, we are able to shift the plasmonic resonance up to 3.6 µm. Elliptic and type II hyperbolic dispersions are shown for three considered structures. Importantly, a blue/red-shift and switching of the reflectance are reported at different incident angles in TE/TM modes. The obtained results clearly show that graphene-based hyperbolic metamaterials with reversibly controlled tunability may be used in the next generation of nonlinear tunable and reversibly switchable devices operating in the mid-IR range.

Modeling and analysis of high-sensitivity refractive index sensors based on plasmonic absorbers with Fano response in the near-infrared spectral region

In this paper, we present various optical metamaterial nanoabsorbers with the aim of improving the refractive index sensitivity using the Fano response. The proposed absorbers consist of various parasitic elements such as single cross, broken cross, Jerusalem cross, and also single L and double L models. We numerically study their absorption and reflection using the three-dimensional finite-difference time-domain method and calculate the sensitivity and figure of merit (FOM) in every absorption peak (reflection dip). Our simulations reveal that the Fano resonance at longer wavelengths can be used for increasing the sensitivity and FOM. The proposed absorbers have been coated with an external material with a maximum thickness of 100 nm and refractive indices in the range of 1.05-1.2. We also study and compare the FOM for these structures. They are modified for 900 and 1200-1300 nm. The maximum FOM is achieved around 2400  RIU^-1 for the double L nanoabsorber coated with 1.05 indexed material, while its sensitivity is 473 nm/RIU. This absorber is an appropriate component for the design of highly sensitive optical sensors.

Tunable Graphene-Based Plasmon-Induced Transparency Based on Edge Mode in the Mid-Infrared Region

A monolayer-graphene-based concentric-double-rings (CDR) structure is reported to achieve broadband plasmon-induced transparency (PIT) on the strength of edge mode in the mid-infrared regime. The theoretical analysis and simulation results reveal that the structure designed here has two plasmonic resonance peaks at 39.1 and 55.4 THz, and a transparency window with high transmission amplitude at the frequency of 44.1 THz. Based on the edge mode coupling between neighbor graphene ribbons, PIT phenomenon is produced through the interference between different (bright and dark) modes. The frequency and bandwidth of the transparency window and slow light time could be effectively adjusted and controlled via changing geometrical parameters of graphene or applying different gate voltages. Additionally, this structure is insensitive to the polarization and incident angle. This work has potential application on the optical switches and slow light modulators.

Implementation of Atomically Thick Graphene and Its Derivatives in Electromagnetic Absorbers

To reduce the radar cross section at microwave frequencies, it is necessary to implement electromagnetic (EM) absorbing devices/materials to decrease the strength of reflected waves. In addition, EM absorbers also find their applications at higher spectrum such as THz and optical frequencies. As an atomic-thick two-dimensional (2D) material, graphene has been widely used in the development of EM devices. The conductivity of graphene can be electrostatically or chemically tuned from microwave to optical light frequencies, enabling the design of reconfigurable graphene EM absorbers. Meanwhile, the derivatives of graphene such as reduced graphene oxide (rGO) also demonstrate excellent wave absorbing properties when mixed with other materials. In this article, the research progress of graphene and its derivatives based EM absorbers are introduced and the future development of graphene EM absorbing devices are also discussed.

Graphene-Based Perfect Absorption Structures in the Visible to Terahertz Band and Their Optoelectronics Applications

Graphene has unique properties which make it an ideal material for photonic and optoelectronic devices. However, the low light absorption in monolayer graphene seriously limits its practical applications. In order to greatly enhance the light absorption of graphene, many graphene-based structures have been developed to achieve perfect absorption of incident waves. In this review, we discuss and analyze various types of graphene-based perfect absorption structures in the visible to terahertz band. In particular, we review recent advances and optoelectronic applications of such structures. Indeed, the graphene-based perfect absorption structures offer the promise of solving the key problem which limits the applications of graphene in practical optoelectronic devices.

Graphene-based dual-band independently tunable infrared absorber

In this paper, we theoretically demonstrate a dual-band independently tunable absorber consisting of a stacked graphene nanodisk and graphene layer with nanohole structure, and a metal reflector spaced by insulator layers. This structure exhibits a dipole resonance mode in graphene nanodisks and a quadrupole resonance mode in the graphene layer with nanoholes, which results in the enhancement of absorption over a wide range of incident angles for both TE and TM polarizations. The peak absorption wavelength is analyzed in detail for different geometrical parameters and the Fermi energy levels of graphene. The results show that both peaks of the absorber can be tuned dynamically and simultaneously by varying the Fermi energy level of graphene nanodisks and graphene layer with nanoholes structure. In addition, one can also independently tune each resonant frequency by only changing the Fermi energy level of one graphene layer. Such a device could be used as a chemical sensor, detector or multi-band absorber.

Near-infrared absorbers based on the heterostructures of two-dimensional materials

Although the conductance and dielectric function of graphene can be tuned by applying external voltage, the tunability is less than 3%. Hybridizing graphene with other two-dimensional transition metal dichalcogenides (TMDs) can improve the adjustability and tunability of the optical properties of graphene-based structures at near-infrared frequencies. In this paper, we theoretically compute the dielectric function of graphene-MoTe₂-graphene and graphene-MoTe₂-graphene heterostructures utilizing the quantum electrostatic heterostructure (QEH) model, which is an ab-initio method. Utilizing the QEH results, we propose a hyper crystal (HC) absorber at near-infrared frequencies. Hence, we use the transfer matrix method to investigate our proposed absorber analytically. Moreover, we simulate the graphene-TMD-graphene (G-TMD-G) absorbers by the numerical finite difference time domain method. The results of the numerical solution are consistent with those of the analytical method. Due to the dependency of the Fermi level of graphene on the direct bandgap of the TMDs, the dielectric function of the G-TMD-G heterostructure can be tuned and enhanced further by changing the number of TMD layers. Finally, we demonstrate that the full absorption of the heterostructures can be achieved at different frequencies for transverse magnetic polarization. Since the thicknesses of the layers in the HC are lower than the wavelength of the light, no diffracted bands are ubiquitous, and the absorption can be observed for a wide range of incidence angles and bandwidths at near-infrared frequencies. Because of utilizing graphene-based HCs, in addition to the feasibility of design compared to the complex metasurfaces, the absorption bandwidth is significant for a wide range of incidence angles. This kind of HC absorber can be used in the design of sensitive optical devices, such as tunable filters, detectors, and photovoltaic applications.

Graphene-based tunable ultra-narrowband mid-infrared TE-polarization absorber

A graphene-based tunable ultra-narrowband mid-infrared TE-polarization absorber is proposed. The simulation results show that, the absorption peak can be tuned from 5.43896µm to 5.41418µm, by tuning the Fermi level of graphene from 0.2eV to 1.0eV. The simulation results also show that the absorption bandwidth is less than 1.0nm and the absorption rate is more than 0.99 for TE-polarization (electric field is parallel to grating grooves) in the tuning wavelength range. The ultra-narrowband absorption mechanism is originated from the low power loss in the guided-mode resonance. The tuning function is mainly attributed to the change of the real part of the graphene's permittivity. This tunable ultra-narrowband mid-infrared absorber has potential applications in the tunable filtering and tunable coherent emission of thermal source.

Tunable graphene plasmonic Y-branch switch in the terahertz region using hexagonal boron nitride with electric and magnetic biasing

A tunable graphene plasmonic Y-branch switch at THz wavelengths is proposed. The effects of magnetic and electric biasing are studied to harness the transmission of the transverse electric and magnetic guided mode resonances. In the structure, hexagonal boron nitride is utilized as a substrate for graphene. The application of hexagonal boron nitride, with the advantages of high mobility and ultralow ohmic loss, introduces a promising alternative substrate for graphene. Analytical and numerical results show that, by slight variation of the doping level in graphene through magnetic and electric biasing, the characteristics of the propagation of the guided mode resonances can be manipulated. A large extinction ratio of 40 dB at a wavelength of 60 μm is obtained. Besides, the proposed switch shows a low insertion loss of about 1 dB and a relatively large optical bandwidth of 1 μm. The electric biasing is of the order of 0.1 mV. Additionally, with the presence of magnetic biasing, a compact switch with a size of 25 μm is achieved. Showing a high extinction ratio, low insertion loss, and compact size, the proposed switch can find potential applications in graphene plasmonics integrated devices.

Toroidal resonance based optical modulator employing hybrid graphene-dielectric metasurface

In this paper, we demonstrate the combination of a dielectric metasurface with a graphene layer to realize a high performance toroidal resonance based optical modulator. The dielectric metasurface consists of two mirrored asymmetric silicon split-ring resonators (ASSRRs) that can support strong toroidal dipolar resonance with narrow line width (~0.77 nm) and high quality (Q)-factor (~1702) and contrast ratio (~100%). Numerical simulation results show that the transmission amplitude of the toroidal dipolar resonance can be efficiently modulated by varying the Fermi energy EF when the graphene layer is integrated with the dielectric metasurface, and a max transmission coefficient difference up to 78% is achieved indicating that the proposed hybrid graphene/dielectric metasurface shows good performance as an optical modulator. The effects of the asymmetry degree of the ASSRRs on the toroidal dipolar resonance are studied and the efficiency of the transmission amplitude modulation of graphene is also investigated. Our results may also provide potential applications in optical filter and bio-chemical sensing.