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A dual-tunable metamaterial absorption modulator is proposed based on a hybrid vanadium dioxide-graphene configuration. When vanadium dioxide is in the state of metal, the design serves as a narrowband absorber. It is composed of vanadium dioxide patch, topas spacer, and vanadium dioxide film. When vanadium dioxide is in the state of dielectric, the design serves as a broadband absorber. It includes vanadium dioxide patch, topas spacer, graphene layer, vanadium dioxide film, topas spacer, metal substrate. By adjusting Fermi level of graphene from 0.1 eV to 0.7 eV, absorptance of the proposed absorber can be dynamically tuned from 45.3% to 94.5% at 0.4 THz and from 31% to 96.3% at 1.0 THz, and absorption bandwidth gradually widens. Because of the unique dielectric to metal transition properties of vanadium dioxide, the tunable behavior of absorption frequency and intensity can be achieved through external stimuli. Thus our design can realize adjustable absorption of intensity and frequency in terahertz range. Besides, the designed narrowband and broadband absorbers are polarization-insensitive for small incident angle due to the symmetry, and absorptions of the design keep good performance within the angle range of 50 ° .

Graphene waveguide plays an important role in modulating optical signal. But it is hard to make a tradeoff between low propagating loss and high field confinement. Here, we propose a bilayer graphene waveguide in a thin topas film and a high refractive index material-germanium cladded with thin metallic film. The influences of structural parameter and chemical potential of graphene are studied to optimize the dimension and working mode. Simulation reveals that our structure can make a balance between high figure of merit (FOM) and low propagation loss, and it reaches a high modulation depth of $\text{2.5 dB}/ \mu\text{m}$ . The design can work with FOM over 200, propagation loss lower than $\text{0.2 dB}/\mu\text{m} $ , and propagation length beyond $\text{30}\;\mu\text{m}$ in a wide band from 6 THz to 18 THz. Compared with previous graphene waveguides, our structure operates from terahertz band to mid-infrared band, and it has longer propagation length due to the existence of bilayer graphene. Besides, benefiting from the thin metallic film, our structure can be integrated on chip.

Benefiting from excellent properties in wavefront control, germanium antimony telluride (GeSbTe)-based photonic devices provide new opportunities for manipulating electromagnetic wave. In this paper, Ge3Sb2Te6 meta-atoms are presented to realize polarization switching for mid-infrared wave through the state transition from the crystalline Ge3Sb2Te6 to the amorphous Ge3Sb2Te6. When the crystalline Ge3Sb2Te6 is involved, the proposed meta-atoms with 90° phase shift achieve high-efficiency polarization conversion and 360° phase coverage. As Ge3Sb2Te6 is changed to the amorphous state, phase coverage drops to only 36°, and most of the cross-polarized wave vanishes. Using these designed meta-atoms, three metasurfaces are implemented at 76.5 THz. Firstly, a gradient metasurface is constructed, and it dynamically switches between specular reflection and anomalous reflection. Next, a reflective metalens is proposed to realize switching between focusing and defocusing under different states of Ge3Sb2Te6. Lastly, a focused vortex beam is presented to reconstruct the mode of orbital angular momentum (OAM). All designs realize the switching between cross-polarization and co-polarization. Our work could have possible applications in fields such as mid-infrared switching, focusing, and wireless communication.

A vanadium dioxide-based multilayer metamaterial is proposed with bifunctional properties of absorption and polarization conversion. When vanadium dioxide is in the metallic state, the designed system behaves as a single-band absorber which is composed of a vanadium dioxide disk-shaped array, a silica spacer, and a vanadium dioxide continuous film. The performance of absorption can be tuned by changing either the diameter of disk or the thickness of silica. The design of this absorber is robust against incident polarization and incident angle. The proposed single-band absorber may be generally applied for plasmonic detection, cavity resonator, and optical band-stop filter. When vanadium dioxide is in the insulating state, the designed system behaves as a cross polarization converter which mainly consists of a one-dimensional metallic strip-shaped array, a silica spacer, and a metallic continuous film. The designed metamaterial can convert a linear plane wave into its corresponding cross-polarized wave with the efficiency of >90% in the frequency of 2.0-3.0 THz. The physical mechanism of polarization conversion is explained by a simple picture. The proposed metamaterial could be a potential candidate for the modern device of polarization control.

In the past decade, metamaterial absorber has been attracting great attention and making huge progress. Most of previous metamaterial absorbers are working with a narrow band. Some efforts to realize broadband absorber have been made from microwave, terahertz to optical frequencies, which often includes multiple resonators with different sizes or multilayer structure with different geometrical dimensions. The configurations of these designs are not simple, and they require some complicated fabrication procedures. Meanwhile, the position of absorption peak and working intensity are usually fixed once the structure of metamaterial absorber is fabricated. Here a new isotropic tunable absorber is presented with a simple vanadium dioxide structure in the terahertz region. By adjusting the geometrical size, simulation shows that our design under normal incidence can achieve broadband absorptance >90% with a central frequency of 1.815 THz in the frequency band of 1.08–2.55 THz. Working intensity of the proposed absorber can be tuned from 1.3% to near 100% by changing the conductivity of vanadium dioxide from $\text{0}\;{\Omega ^{ - 1}}\text{cm}^{ - 1}$ to $\text{2000}\;{\Omega ^{ - 1}}\text{cm}^{ - 1}$ . The obtained absorption spectrum is independent of incident polarization, and absorptance has a good performance when incident angle varies from ${\text{0}^\circ }$ to ${\text{45}^\circ }$ . The proposed design may be useful for promising applications in stealth technology, thermal emitter, and terahertz detection.

Metasurfaces are highly advantageous in realizing the phase discontinuity of an electromagnetic wave. Aiming at manipulating electromagnetic wavefront in terahertz band, a subwavelength multilayer meta-atom is designed, and it is composed of gold mesh, gold disk, and polyimide. By varying the diameter of the central gold disk, eight meta-atoms are prepared to provide full phase modulation with phase increase of 45°. A series of transmissive metasurfaces consisting of these meta-atoms are designed with different arrays. They achieve generations of deflection, single-focusing, dual-focusing, and orbital angular momentum with and . It is noteworthy that metasurfaces assembled by these isotropic meta-atoms are polarization-insensitive. Our models are numerically verified, and all results are consistent with theoretical predictions. Different from reflective metasurfaces that sometimes encounter feeding blockage, transmissive metasurfaces are feasible for the realization of a more compact terahertz system.

Due to the increasing complexity of modern communication systems, the requirements for electromagnetic wave are becoming more and more rigorous. Traditional natural materials are not easy to integrate into tunable systems. Metasurface is a two-dimensional array, and it has a more significant advantage in electromagnetic control and attracts great attention. Here, Ge3Sb2Te6-based metasurfaces are presented to control electromagnetic wavefront by changing the state of Ge3Sb2Te6 in the mid-infrared range. Eight meta-atoms are designed to form linear phase distribution with phase difference of 45° at 74 THz. When Ge3Sb2Te6 is in the amorphous state, the designed meta-atoms have low loss and high reflection. Ge3Sb2Te6 block is designed into a specific structure in a certain way, which realizes deflection of electromagnetic wavefront and vortex beam with orbital angular momentum l = 1 or l = 2. By means of convolution, anomalous reflection is combined with orbital angular momentum to form an anomalously deflected vortex beam. When the state of Ge3Sb2Te6 is crystalline, amplitudes of meta-atoms decrease and linear phase distribution is destroyed. Reflected wave becomes almost mirror reflection wave. Our design may open a pathway to novel types of metasurfaces.

A bifunctional metamaterial is proposed based on a hybrid graphene and vanadium dioxide (VO2) configuration, which can realize a dynamic switch between beam steering and broadband absorption. The structure consists of a VO2 square, graphene patch, topas spacer, VO2 film, topas spacer, and metal substrate. When VO2 is in the metallic state, the structure serves as a coding metamaterial. By engineering different sizes of the top VO2 square and adjusting the Fermi energy level of graphene, the incident wave is scattered in different patterns. When VO2 is in the dielectric state, the structure serves as a broadband absorber. By changing the Fermi energy level of graphene from 0.0 eV to 0.9 eV, absorptance can be gradually changed and working bandwidth widens. There is an absorption band with near 100% absorptance from 0.9 THz to 1.35 THz when the Fermi energy level is 0.73 eV. And the designed broadband absorber is polarization-insensitive within the incident angle of 50°. Our work may show great potential in applications such as terahertz switching and modulation.

Based on the phase-transition property of vanadium dioxide (VO2), a terahertz bifunctional absorber is proposed with switchable functionalities of broadband absorption and multiband absorption. When VO2 is metal, the system is regarded as a broadband absorber, which is composed of VO2 patch, topas spacer, and VO2 film with metallic disks inserted. The system obtains a broadband absorption with absorptance >90% from 3.25 THz to 7.08 THz. Moreover, the designed broadband absorber has a stable performance within the incident angle range of 50°. When VO2 is dielectric, multiband absorption with six peaks is realized in the designed system. Graphene and the metallic disk-shaped array play the dominant role in the mechanism of multiband absorption. Through changing the Fermi energy level of graphene, the performance of multiband absorption can be dynamically adjusted. Because of the switchable functionalities, the proposed design may have potential application in the fields of intelligent absorption and terahertz switch.

The design of a broadband tunable absorber is proposed based on a thin vanadium dioxide metasurface, which is composed of a simple array of vanadium dioxide and a bottom gold film. When the conductivity of vanadium dioxide is equal to 2000 Ω-1cm-1, simulated absorptance exceeds 90% with 71% bandwidth from 0.47 to 0.99 THz and full width at half maximum is 98% from 0.354 to 1.036 THz with center frequency of 0.695 THz. Simulated results show that absorptance peak can be tuned from 5% to 100% when the conductivity changes continually from 10 Ω-1cm-1 to 2000 Ω-1cm-1. The designed absorber may have useful applications in terahertz spectrum such as energy harvesting, thermal emitter, and sensing.

A tunable G e 3 S b 2 T e 6 metasurface is numerically introduced with near-unity absorption and large tuning range. It has a perfect absorptance of 100% for the crystalline state of G e 3 S b 2 T e 6 . Absorptance will have a low intensity for the amorphous state of G e 3 S b 2 T e 6 . The designed absorber is insensitive to polarization and robust against incident angle. This design may have potential applications in stealth technology, antiradar detection, and phase-sensitive biosensor.

A broadband tunable absorber is studied based on the silicon metasurface at terahertz frequencies. By tuning the conductivity of silicon, absorptance is more than 90% from 0.497 THz to 1.045 THz with central frequency of 0.771 THz when the conductivity is equal to 2500 S / m . Simulated results show that absorptance peak can be modulated from 1% to 100% when the conductivity continuously changes from 0 S / m to 2500 S / m . The central-symmetry structure leads to polarization independence. Our design may be used as optoelectronic modulator, tunable detector, and terahertz switch.

An active toroidal switch is numerically presented in a terahertz metamaterial. Transmission of toroidal dipole can be tuned by the conductivity of vanadium dioxide, and its modulation depth can be up to 32%. The resonance of toroidal dipole is verified by the distribution of electric current. Due to the unique field distribution of the toroidal mode, the proposed design may find potential applications in enhanced absorption and sensing.

By introducing vanadium dioxide film into a multilayer structure, the dual functionalities of perfect absorption and high transmission are presented using the insulator-to-metal phase transition of vanadium dioxide. When vanadium dioxide is in the conducting state, the designed system acts as a narrowband absorber. The proposed absorber is composed of the top metallic ring, silica spacer, and the vanadium dioxide film. The absorption peak is originated from localized magnetic resonance between metallic ring and vanadium dioxide film. When vanadium dioxide is in the insulating state, the designed system acts as a transparent conducting metal. The top metallic ring, the middle dielectric spacer, and the subwavelength metallic mesh are combined together to form an antireflection coating. The influences of incident angle and structure parameter on absorption and transmission are also discussed. This work has demonstrated a new route for developing vanadium dioxide-based switchable photonic devices in the fields of filter and modulator at terahertz frequencies.

A new type of terahertz switchable metamaterial is proposed with dual functions. By introducing vanadium dioxide possessing insulator-to-metal transition, the designed metamaterial can be switched from a dual-band absorber to an analog of electromagnetically induced transparency. When vanadium dioxide is in the conducting state, the designed structure works as a dual-band absorber with 100% absorptance at 1.02 THz and 1.71 THz. When vanadium dioxide is in the insulating state, the designed structure works as an analog of electromagnetically induced transparency. The performances of bifunctionality are omnidirectional and efficient in the incident angle range of 60° or 30° in two modes. The proposed design may enable advanced applications in the fields of modulator and filter.

Utilizing the phase transition characteristic of vanadium dioxide, we present a metamaterial configuration to achieve both reflective and transmissive cross-polarization converters. When vanadium dioxide is metal, the design behaves as a reflective cross-polarization converter. It consists of metallic grating, topas spacer, and vanadium dioxide film. Polarization conversion ratio is more than 90% in the frequency range from 4.80 THz to 13.13 THz. When vanadium dioxide is insulator, the design behaves as a transmissive cross-polarization converter using cascaded metallic gratings with rotation angle 45°. High-efficiency broadband cross-polarization wave conversion is achieved in the frequency band of 0.50–4.75 THz. Effect of oblique incidence is studied on polarization conversion. Results tell that cross-polarization conversion is better when incident angle is in the range of 0°–40°. The designed metamaterial may have a certain inspiration for the research of terahertz multifunctional polarization converter.

We study multipole plasmons in graphene nanoellipses under the quasi-static approximation. The graphene is characterized by a homogeneous surface conductivity, and two coupled differential and integral equations are solved self-consistently to investigate the plasmonic modes in nanoellipses with a fixed area. With respect to the major axis, the symmetric and antisymmetric modes originally doubly degenerate in nanodisks will show different behavior as the semi-major axis increases. The eigen frequencies of the symmetric modes decrease, while those of the antisymmetric modes increase. At the edges, the phase changes of the symmetric dipole modes are linear and independent on structural changes; the phase changes of antisymmetric modes deviate from linear relationship, and the deviation depends on the semi-major axis. As a very large aspect ratio, they exhibit sharp peaks at the endpoints of the minor axis and zero phase changes at the endpoints of the major axis. The non-degenerate breathing mode shows its hot spots at the endpoints of the minor axis, and its eigen frequency gradually increases as the semi-major axis increases.

The analogs of electromagnetically induced transparency (EIT) based on the concept of metamaterials (MMs) bring the primitive quantum phenomena into the scope of classical optics. However, most of the previous designs are anisotropic. Here, in this letter, a tunable isotropic classical analog of EIT is studied based on the wide-angle MM with a layer of vanadium dioxide underneath it. The tunable conductivity of vanadium dioxide is the key point to realize the dynamic modulation of EIT. The numerical calculation tells that the transmission peak of EIT can be tuned from 66% to 12% when the conductivity of vanadium dioxide is changed from 10 to $2000\;\Omega ^{-1}$ cm−1. Furthermore, such design possesses a nearly insensitive characteristic of transmission on polarization and incident angle. Although the simple planar structure presented here is a specific design to manipulate EIT-like phenomenon in terahertz MMs, it can be easily scaled to other electromagnetic spectra. This letter could provide one possible way for the plasmonic filter and sensing application.

As a key component of on-chip interconnection, optical modulator with large modulation depth and tiny footprint has always been studied. Profiting by high carrier mobility and flexible adjustability of graphene, numerous graphene modulators at optical communication band are proposed to overcome inherent flaws of traditional semiconductor waveguide modulators. Here, a terahertz waveguide modulator combing noble metal and graphene is presented. When Fermi level changes from 0 eV to 1 eV, intensity distribution of electric field becomes dispersed. Interaction area of graphene and wave increases, which results in larger propagation loss. On the premise of the existence of the allowed mode, the size of metal and the thickness of dielectric should be small. Besides, modulation capability of this device can also be improved by multilayer graphene with relaxation time of 0.1 ps. After optimizing structure parameters, the designed graphene waveguide modulator obtains modulation depth of 6.1 dB μm−1 at the frequency of 5 THz, and keeps effective mode area below 10−5. With the increase of frequency, modulation depth decreases. Modulation depth of 1.5 dB μm−1 is achieved at 10 THz, but the corresponding effective mode area remains in an ideal range. Because the allowed mode is confined in a tiny room, cross-sectional area of device is less than 4 μm2.

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

© 1963-2012 IEEE. A virtual active element pattern (AEP) expansion method is presented in which each AEP in an unequally spaced array is considered to be the pattern radiated by a subarray of some equally spaced virtual elements. With the help of this method, the pattern of an unequally spaced array including mutual coupling can be efficiently evaluated by fast Fourier transform (FFT). By incorporating this idea into the iterative Fourier transform procedure, we develop a novel iterative synthesis method, which can apply the iterative FFT to efficiently synthesize unequally spaced arrays including mutual coupling. Different excitation constraints, such as phase-only control and amplitude-phase optimization with a prescribed dynamic range ratio, can be easily added into the proposed synthesis procedure. A set of synthesis examples for different antenna arrays with pencil and shaped beam patterns are provided to validate the effectiveness and advantages of the proposed method.

Hexagonal boron nitride as a natural hyperbolic material has attracted lots of attention recently. Here, we investigate numerically the optical spectrum properties of hexagonal boron nitride from the perspective of optical transition. After careful data analysis, hexagonal boron nitride at the epsilon-near-zero point of permittivity either turns from a hyperbolic material to an effective dielectric for transverse magnetic-polarized wave or from an effective metal to an effective dielectric for transverse electric-polarized wave. The results in this work may pave the way for potential applications of hexagonal boron nitride in the field of metamaterials.

We present a terahertz classical analog of electromagnetically induced transparency by means of single-layer metamaterial consisting of two closed square ring. The narrow transparency peak is mainly caused by the excitation of antiparallel currents in different rings. Because of symmetry configuration, the designed metamaterial possesses stable polarization-independent and wide-angle transmission spectral features. Moreover, the design has a simple structure ideally suitable for the current planar micro- and nano-fabrications.

A switchable metamaterial with bifunctionality of absorption and electromagnetically induced transparency is proposed based on the phase-transition characteristic of phase change material-vanadium dioxide. When vanadium dioxide is in the metallic state, an isotropic narrow absorber is obtained in the terahertz region, which consists of a top metallic cross, a middle dielectric layer, and a bottom vanadium dioxide film. By adjusting structure parameters, perfect absorption is realized at the frequency of 0.498 THz. This designed narrow absorber is insensitive to polarization and incident angle. Absorptance can still reach 75% for transverse electric polarization and transverse magnetic polarization at the incident angle of 65∘. When vanadium dioxide is in the insulating state, the top metallic cross will interact with the bottom split ring resonator, and the interaction between them will lead to the appearance of electromagnetically induced transparency. The behavior of electromagnetically induced transparency works well for transverse electric polarization and transverse magnetic polarization at the small incident angle. The designed hybrid metamaterial opens possible avenues for achieving switchable functionalities in a single device.

We present a tunable metamodulator to work at terahertz frequencies by employing the dependency of toroidal dipolar resonance on the conductivity of vanadium dioxide. Numerical results show that toroidal dipolar resonance in the proposed planar structure can be observed around 0.288 THz in transmission spectrum. From the distribution of the anti-phase current flowing in the symmetric split ring resonator, the formation of toroidal dipole is validated. Our design may have potential applications in advanced terahertz devices, such as filter, plasmonic sensor, and fast switch.

The efficient electromagnetic absorber is highly demanded in device applications. A big challenge is to design the absorber of very thin thickness and broad bandwidth, which is even more difficult at terahertz regime. In this article, we propose and numerically (finite element method, FEM) demonstrate a high-performance broadband absorber with near-perfect absorption over terahertz band based on multilayer graphene squares. The absorber consists of four graphene layers in which the width of the square is gradually increased, and a bottom gold layer separated by a dielectric spacer. The proposed structure can achieve above 90% wave absorption for the frequencies from 1.45 THz to 4.35 THz, with a bandwidth of 2.9 THz and the bandwidth fraction of 100%. Because of symmetry of the structure, this terahertz absorber is polarization insensitive and can work at a large angle range of oblique incidence, for instance absorptance can be maintained nearly unchanged up to 40° for transverse electric polarization and up to 45° for transverse magnetic polarization. Except for the layer stacking, our design is very easy to be realized in experiment, since only the simple square shape and a uniform 0.4 eV Fermi level are used in our design, which may play an important role in future terahertz device applications.

A novel method of decoupling two closely placed antennas is proposed based on tunable metamaterials. The experimental results show that this method has more than 110-dB isolation ratio, frequency and bandwidth reconfigurable ability, and environmental adaptivity. These features are realized by placing multiple varactor-based tunable resonant structures between two antennas within the near-field region and by using artificial intelligent algorithm to optimize the transmission coefficient between the antennas. A case of suppressing mutual coupling between two orthogonally polarized arrays is given to demonstrate the versatility of this method.

In this paper, a wafer-scale uniform silver nanocave array is fabricated by soft ultraviolet nanoimprint lithography. We investigate its plasmonic effects using far-field and near-field experimental approaches and illuminate the physics inside by theoretical analysis and computational simulation. The array shows robust multispectral features for various surrounding media and possesses the sensitivity up to 514.7 nm/RIU in the visible range, which is promising for the mass production of high-performance plasmonic refractive index sensors.