Hybrid Resonators and Highly Tunable Terahertz Metamaterials Enabled by Vanadium Dioxide (VO2)

Broadband switchable terahertz half-/quarter-wave plate based on metal-VO2 metamaterials

We propose a metal-vanadium dioxide (VO₂) metamaterial with broadband and functionality-switchable polarization conversion in the terahertz regime. Simulation results show that the function of the proposed metamaterial can be switched from a half-wave plate (HWP) to a quarter-wave plate (QWP) over a broad bandwidth of 0.66-1.40 THz, corresponding to a relative bandwidth of 71.8%. The HWP obtained when VO₂ is in the insulating state has reflection of 90% and linear polarization conversion ratio exceeding 98% over the bandwidth of 0.58-1.40 THz. By transiting the phase of VO₂ into the conducting state, the obtained QWP can convert the incident linearly-polarized wave to circularly-polarized wave with an ellipticity of 0.99 over 0.66-1.60 THz. Additionally, results show that the proposed broadband switchable HWP/QWP has a large angular tolerance. We expect that this broadband and switchable multi-functional wave plate will find applications in polarization-dependent terahertz systems including sensing, imaging, and telecommunications.

Switchable broadband and wide-angular terahertz asymmetric transmission based on a hybrid metal-VO2 metasurface

We propose a switchable broadband and wide-angular terahertz asymmetric transmission based on a spiral metasurface composed of metal and VO₂ hybrid structures. Results show that asymmetric transmission reaching up to 15% can be switched on or off for circularly polarized terahertz waves when the phase of VO₂ transits from the insulting state to the conducting state or reversely. Strikingly, we find that relatively high asymmetric transmission above 10% can be maintained over a broad bandwidth of 2.6-4.0 THz and also over a large incident angular range of 0^°-45^°. We further discover that as the incident angle increases, the dominant chirality of the proposed metasurface with VO₂ in the conducting state can shift from intrinsic to extrinsic chirality. We expect this work will advance the engineering of switchable chiral metasurfaces and promote terahertz applications.

A Thermal Tuning Meta-Duplex-Lens (MDL): Design and Characterization

Multifunctional metasurfaces play an important role in the development of integrated optical paths. However, some of the realizations of current multifunctional metasurface devices depend on polarization selectivity, and others change the polarization state of the outgoing light. Here, based on vanadium dioxide (VO₂) phase change material, a strategy to design a meta-duplex-lens (MDL) is proposed and numerical simulation calculations demonstrate that at low temperature (about 300 K), VO₂ behaves as a dielectric so that the MDL can act as a transmission lens (transmission efficiency of 87.6%). Conversely, when VO₂ enters the metallic state (about 355 K), the MDL has the ability to reflect and polymerize electromagnetic waves and works as a reflection lens (reflection efficiency of 85.1%). The dielectric waveguide and gap-surface plasmon (GSP) theories are used in transmission and reflection directions, respectively. In order to satisfy the coverage of the phase gradient in the range of 2π in both cases, we set the antenna as a nanopillar with a high aspect ratio. It is notable that, via symmetrical antennas acting in concert with VO₂ phase change material, the polarization states of both the incident light and the outgoing light are not changed. This reversible tuning will play a significant role in the fields of imaging, optical storage devices, communication, sensors, etc.

Temperature-dependent infrared ellipsometry of Mo-doped VO2 thin films across the insulator to metal transition

We present a spectroscopic ellipsometry study of Mo-doped VO₂ thin films deposited on silicon substrates for the mid-infrared range. The dielectric functions and conductivity were extracted from analytical fittings of Ψ and Δ ellipsometric angles showing a strong dependence on the dopant concentration and the temperature. Insulator-to-metal transition (IMT) temperature is found to decrease linearly with increasing doping level. A correction to the classical Drude model (termed Drude-Smith) has been shown to provide excellent fits to the experimental measurements of dielectric constants of doped/undoped films and the extracted parameters offer an adequate explanation for the IMT based on the carriers backscattering across the percolation transition. The smoother IMT observed in the hysteresis loops as the doping concentration is increased, is explained by charge density accumulation, which we quantify through the integral of optical conductivity. In addition, we describe the physics behind a localized Fano resonance that has not yet been demonstrated and explained in the literature for doped/undoped VO₂ films.

Achieving broadband absorption and polarization conversion with a vanadium dioxide metasurface in the same terahertz frequencies

We present the bifunctional design of a broadband absorber and a broadband polarization converter based on a switchable metasurface through the insulator-to-metal phase transition of vanadium dioxide. When vanadium dioxide is metal, the designed switchable metasurface behaves as a broadband absorber. This absorber is composed of a vanadium dioxide square, silica spacer, and vanadium dioxide film. Calculated results show that in the frequency range of 0.52-1.2 THz, the designed system can absorb more than 90% of the energy, and the bandwidth ratio is 79%. It is insensitive to polarization due to the symmetry, and can still work well even at large incident angles. When vanadium dioxide is an insulator, a terahertz polarizer is realized by a simple anisotropic metasurface. Numerical calculation shows that efficient conversion between two orthogonal linear polarizations can be achieved. Reflectance of a cross-polarized wave can reach 90% from 0.42 THz to 1.04 THz, and the corresponding bandwidth ratio is 85%. This cross-polarized converter has the advantages of wide angle, broad bandwidth, and high efficiency. So our design can realize bifunctionality of broadband absorption and polarization conversion between 0.52 THz and 1.04 THz. This architecture could provide one new way to develop switchable photonic devices and functional components in phase change materials.

Terahertz bifunctional absorber based on a graphene-spacer-vanadium dioxide-spacer-metal configuration

A terahertz bifunctional absorber is presented with broadband and narrowband absorbing properties in a graphene-spacer-vanadium dioxide-spacer-metal configuration. Carrier relaxation time of graphene τ = 1.0ps (τ = 0.1ps) is chosen for narrowband (broadband) absorption. When vanadium dioxide is in the conducting state, the design behaves as a narrowband absorber, and it is composed of a square-shaped graphene, topas spacer, and metallic vanadium dioxide film. There is an absorption band with 100% absorptance at the frequency of 1.37 THz. Narrowband absorption is caused by the localized magnetic resonance. When vanadium dioxide is in the insulating state, the design behaves as a broadband absorber composed of a square-shaped graphene, topas layer, vanadium dioxide film, and metal film. It has a broadband absorption in the frequency range of 1.05-2.35 THz, and the corresponding absorptance is more than 90%. The merging of two resonances with overlapping region ensures broadband performance of the designed absorber. The working bandwidth and intensity of narrowband absorption and broadband absorption can be dynamically adjusted by changing the Fermi energy level of graphene. The influences of structure parameters are discussed on absorption performance. In addition, the designed absorber is not sensitive to incident angle. Because of the simple structure, our design can be applied to many promising fields in intelligent absorption and terahertz switch.

Simultaneous realizations of absorber and transparent conducting metal in a single metamaterial

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.

Terahertz switching between broadband absorption and narrowband absorption

A multilayer metamaterial with switchable functionalities is presented based on the phase-transition property of vanadium dioxide. When vanadium dioxide is in the metallic state, a broadband absorber is formed. Calculated results show that the combination of two absorption peaks enables absorptance more than 90% in the wide spectral range from 0.393 THz to 0.897 THz. Absorption performance is insensitive to polarization at the small incident angle and work well even at the larger incident angle. When vanadium dioxide is in the insulating state, the designed system behaves as a narrowband absorber at the frequency of 0.677 THz. This narrowband absorber shows the advantages of wide angle and polarization insensitivity due to the localized magnetic resonance. Furthermore, the influences of geometrical parameters on the performance of absorptance are discussed. The proposed switchable absorber can be used in various applications, such as selective heat emitter and solar photovoltaic field.

Active Switching of Extremely High-Q Fano Resonances Using Vanadium Oxide-Implanted Terahertz Metamaterials

In this paper, we demonstrate an active switching of extremely high Q-factor Fano resonances using vanadium oxide (VO2)-implanted THz asymmetric double C-shaped metamaterial (MM) structures. The simulation results indicate the highly temperature-sensitive nature of the double Fano resonances that can be switched at very low external thermal pumping (68 °C), which is only slightly higher than room temperature. We employ the surface current and electric field distributions of the structure to analyze the physical mechanism of the observed switching behavior in the thermally excited Fano MMs. More importantly, by optimizing the asymmetric parameter (offset length), the linewidth of the Fano resonance can reach only 0.015 THz and the Q-factor is as high as 98, which is one order of magnitude higher than that of the traditional MMs. The findings of this work would enable a thermally-induced high-Q Fano resonance MMs for ultra-sensitive sensors, modulators, low threshold switching in metadevices.

Integrated metamaterial with functionalities of absorption and electromagnetically induced transparency

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.

Dynamically Temperature-Voltage Controlled Multifunctional Device Based on VO2 and Graphene Hybrid Metamaterials: Perfect Absorber and Highly Efficient Polarization Converter

Vanadium dioxide (VO₂) is a temperature phase change material that has metallic properties at high temperatures and insulation properties at room temperature. In this article, a novel device has been designed based on the dielectric metasurface consisting of VO₂ and graphene array, which can achieve multiple functions by adjusting temperature and voltage. When the temperature is high (340 K), the device is in the absorption state and its absorptivity can be dynamically controlled by changing the temperature. On the other hand, the device is in the polarization state under room temperature, and the polarization of electromagnetic waves can be dynamically controlled by adjusting the voltage of graphene. This device can achieve a broadband absorber (the maximum absorptance reaches 99.415% at wavelengths ranging from 44 THz to 52 THz) and high polarization conversion efficiency (>99.89%) in the mid-infrared range, which has great advantages over other single-function devices. Our results demonstrate that this multifunctional device may have widespread applications in emitters, sensors, spatial light modulators, IR camouflages, and can be used in thermophotovoltaics and wireless communication.

Thermally switchable terahertz wavefront metasurface modulators based on the insulator-to-metal transition of vanadium dioxide

Active use of phase transition phenomena for reversibly tuning the properties of functional materials in devices currently is an attractive research area of materials science. We designed and fabricated two kinds of metasurface modulators for dynamically controlling the wavefront of terahertz (THz) radiation based on the temperature-induced insulator-to-metal phase transition of vanadium dioxide (VO₂). The modulators designed are based on the C-shaped slot antenna array. The slot antennas are made of the VO₂ films on c-sapphire substrates. The C-shaped slot antennas are active only when the VO₂ is in its metallic phase, i.e. at temperatures T > T_C ∼68 °C. At T > T_C, the first kind acts as a THz multi-focus lens which converges an incident THz plane wave into four focal spots and the second kind as an Airy beam generator. We characterized the function of two THz wavefront modulators over a broad frequency range, i.e. from 0.3 to 1.2 THz. Such thermally switchable THz wavefront metasurface modulators with a capability of dynamically steering THz fields will be of great significance for the future development of THz active devices.

Vanadium dioxide based broadband THz metamaterial absorbers with high tunability: simulation study

With their unprecedented flexibility in manipulating electromagnetic waves, metamaterials provide a pathway to structural materials that can fill the so-called "THz gap". It has been reported that vanadium dioxide (VO₂) experiences a three orders of magnitude increase in THz electrical conductivity when it undergoes an insulator-to-metal transition. Here, we propose a VO₂ based THz metamaterial absorber exhibiting broadband absorptivity that arises from the multiple resonances supported by a delicately balanced doubly periodic array of VO₂ structures and numerically demonstrate that the corresponding absorption behavior is highly dependent on the VO₂'s THz electrical properties. Considering the phase transition induced dramatic change in VO₂'s material property, the proposed metamaterial absorbers have the potential for strong modulation and switching of broadband THz radiation.

Switchable multifunctional terahertz metasurfaces employing vanadium dioxide

In this paper, we design a type of switchable metasurfaces by employing vanadium dioxide (VO₂), which possess tunable and diversified functionalities in the terahertz (THz) frequencies. The properly designed homogeneous metasurface can be dynamically tuned from a broadband absorber to a reflecting surface due to the insulator-to-metal transition of VO₂. When VO₂ is in its insulating state, the metasurface can efficiently absorb the normally incident THz wave in the frequency range of 0.535-1.3 THz with the average absorption of ~97.2%. Once the VO₂ is heated up and switched to its fully metallic state, the designed metasurface exhibits broadband and efficient reflection (>80%) in the frequency range from 0.5 to 1.3 THz. Capitalizing on such meta-atom design, we further extend the functionalities by introducing phase-gradients when VO₂ is in its fully metallic state and consequently achieve polarization-insensitive beam-steering and polarization-splitting, while maintaining broadband absorption when VO₂ is in insulating state.

Terahertz toroidal metamaterial with tunable properties

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.

Dynamically Tunable Resonant Strength in Electromagnetically Induced Transparency (EIT) Analogue by Hybrid Metal-Graphene Metamaterials

In this paper, a novel method to realize a dynamically tunable analogue of EIT for the resonance strength rather than the resonance frequency is proposed in the terahertz spectrum. The introduced method is composed of a metal EIT-like structure, in which a distinct EIT phenomenon resulting from the near field coupling between bright and dark mode resonators can be obtained, as well as an integrated monolayer graphene ribbon under the dark mode resonator that can continuously adjust the resonance strength of transparency peak by changing the Fermi level of the graphene. Comparing structures that need to be modulated individually for each unit cell of the metamaterials, the proposed modulation mechanism was convenient for achieving synchronous operations for all unit cells. This work demonstrates a new platform of modulating the EIT analogue and paves the way to design terahertz functional devices which meet the needs of optical networks and terahertz communications.

Far-field radiative thermal rectifier based on nanostructures with vanadium dioxide

Radiative thermal rectifiers capable of realizing asymmetric heat flux transfer have attracted a lot of research interests recently, mainly focusing on the engineering of the emissivity spectra. In this Letter, we propose a far-field radiative thermal rectifier utilizing the phase change material vanadium dioxide (VO₂). The thermal rectifier consists of a metamaterial infrared absorber and a two-layer thin-film structure acting as the active and the passive components, respectively. Numerical optimization has been carried out to control the emissivity spectra of both parts and maximize the overall rectification effect. A large thermal rectification factor of 3.5 is predicted at a temperature bias of ΔT=100  K.

Electrically controllable THz asymmetric split-loop resonator with an outer square loop based on VO2

In this paper, we propose an asymmetric split-loop resonator with an outer square loop (ASLR-OSL) based on vanadium dioxide (VO₂) which can actively control the transmission characteristics of a terahertz wave while maintaining a high quality factor of the asymmetric split-loop resonator (ASLR) by adding an outer square loop. The proposed ASLR-OSL demonstrated transmission characteristics similar to those of ASLR, and the transmission characteristics of ASLR-OSL were successfully controlled by directly applying a bias voltage. These results show a simple method for imposing active properties on a common metamaterial having a high quality factor by adding a loop structure.

Reflective grating-coupled structure improves the detection efficiency of THz array detectors

A reflective grating-coupled structure on the silicon substrate was designed to improve the detection efficiency of terahertz detectors for the frequency ranging from 0.26 THz to 0.36 THz. By using finite difference time domain (FDTD) solutions, the simulation and optimized design of the grating-coupled structure were carried out. The results showed that the signal was effectively reflected and diffracted by the reflective grating-coupled structure which significantly enhanced the electric field in the place of the detector. The maximum electric field can be increased by 2.8 times than that of the Fabry-Perot resonator. To verify the design results, the reflective grating-coupled structure was applied in the preparation of the Nb₅N₆ array detector chip and compared with the Nb₅N₆ array detector chip with the F-P resonator. The results showed that the maximum voltage responsivity of the Nb₅N₆ detector with the reflective grating-coupled structure was 2 times larger than the Nb₅N₆ detector with the F-P resonator. It indicates that the reflective grating-coupled structure can efficiently improve the detection efficiency of THz detectors.

Broadband tunable terahertz absorber based on vanadium dioxide metamaterials

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

Thermally Tunable Ultra-wideband Metamaterial Absorbers based on Three-dimensional Water-substrate construction

Distilled water has frequency dispersive characteristic and high value of imaginary part in permittivity, which can be seen as a good candidate of broadband metamaterial absorbers(MAs) in microwave. Here, an interesting idea based on the combination of water-substrate and metallic metamaterial in the three-dimensional construction is proposed, which can achieve outstanding broadband absorption. As a proof, the distilled water is filled into the dielectric reservoir as ultra-thin water-substrate, and then the water-substrates are arranged on the metal backplane periodically as three-dimensional water-substrate array(TWA). Simulation shows that the TWA achieves broadband absorption with the efficiency more than 90% from 8.3 to 21.0 GHz. Then, the trigonal metallic fishbone structure is introduced here between the water-substrate and the dielectric reservoir periodically as three-dimensional water-substrate metamaterial absorber(TWMA). The proposed TWMA could achieve ultra-broadband absorption from 2.6 to 16.8 GHz, which has increase by 64.8% in relative absorption bandwidth. Meanwhile, due to the participation of distilled water, the thermally tunable property also deserves to be discussed here. In view of the outstanding performance, it is worth to expect a wide range of applications to emerge inspired from the proposed construction.

Active Terahertz Chiral Metamaterials Based on Phase Transition of Vanadium Dioxide (VO2)

Compared with natural materials, chiral metamaterials have been demonstrated with orders of magnitude stronger chiroptical response, which provides the basis for applications such as ultracompact polarization components and plasmonic-enhanced biosensing. Terahertz chiral metamaterials that allow dynamic polarization control of terahertz waves are of great practical interest, but remain extremely rare. Here, we show that hybrid metamaterials integrated with vanadium dioxide (VO2) exhibiting phase transition can enable dynamically tunable chiroptical responses at terahertz frequencies. In particular, a circular dichroism of ~40° and a maximum polarization rotation of ~200°/λ are observed around 0.7 THz. Furthermore, our study also reveals that the chiroptical response from the proposed metamaterials is strongly dependent on the phase transition of VO2, leading to actively controllable polarization states of the transmitted terahertz waves. This work paves the way for the development of terahertz metadevices capable of enabling active polarization manipulation.