Theoretical design of a reconfigurable broadband integrated metamaterial terahertz device

Switchable and Tunable Terahertz Metamaterial Based on Vanadium Dioxide and Photosensitive Silicon

A switchable and tunable terahertz (THz) metamaterial based on photosensitive silicon and Vanadium dioxide (VO₂) was proposed. By using a finite-difference time-domain (FDTD) method, the transmission and reflective properties of the metamaterial were investigated theoretically. The results imply that the metamaterial can realize a dual electromagnetically induced transparency (EIT) or two narrow-band absorptions depending on the temperature of the VO₂. Additionally, the magnitude of the EIT and two narrow-band absorptions can be tuned by varying the conductivity of photosensitive silicon (PSi) via pumping light. Correspondingly, the slow-light effect accompanying the EIT can also be adjusted.

A dual ultra-broadband switchable high-performance terahertz absorber based on hybrid graphene and vanadium dioxide

A tunable dual broadband switchable terahertz absorber based on vanadium dioxide and graphene is proposed. The tunability of graphene and the phase transition properties of vanadium dioxide are used to switch broadband absorption between low-frequency and high-frequency, as well as the absorption rate tuning function. The simulation results indicate that when vanadium dioxide is in the insulating phase and the graphene Fermi energy is 0.7 eV, the absorber achieves low-frequency broadband absorption within the range of 2.6-4.2 THz with an absorptance greater than 90%; when vanadium dioxide is in the metallic phase and the graphene Fermi energy is 0 eV, the absorber achieves high-frequency broadband absorption within the range of 4.9-10 THz with an absorptance greater than 90%. Furthermore, the absorptance can be tuned by adjusting the conductivity of vanadium dioxide or the Fermi energy of graphene. Due to the central symmetry of the proposed structure, the absorber is completely insensitive to polarization. For TE and TM polarized waves, both low and high-frequency broadband absorption are maintained over a range of incident angles from 0° to 50°. The simple structure, tunable absorption rate, insensitivity to polarization angle and incident angle properties are advantages of our proposed absorber. It has broad application prospects in adjustable filters and electromagnetic shielding.

Triple-Band and Ultra-Broadband Switchable Terahertz Meta-Material Absorbers Based on the Hybrid Structures of Vanadium Dioxide and Metallic Patterned Resonators

A bifunctional terahertz meta-material absorber with three layers is designed. The surface of the bifunctional meta-material absorber is a periodically patterned array composed of hybrid structures of vanadium dioxide (VO₂) and metallic resonators; the middle layer is a nondestructive TOPAS film, and the bottom layer is a continuous metallic plane. Utilizing the phase-transition property of VO₂, the responses of the meta-material absorber could be dynamically switched between triple-band absorption and ultra-broadband absorption. When VO₂ is in the metallic state, an ultra-broadband absorption covering the bandwidth of 6.62 THz is achieved over the range from 4.71 THz to 11.33 THz. When VO₂ is in the di-electric state, three absorption peaks resonated at 10.57 THz, 12.68 THz, and 13.91 THz. The physical mechanisms of the bifunctional meta-material absorber were explored by analyzing their near-field distributions. The effects of varying structural parameters on triple-band and ultra-broadband absorption were investigated. It is revealed that by optimizing the structure parameters, the number of absorption peaks could be increased for a certain sacrifice of absorption bandwidth. FDTD Solutions and CST Microwave Studio were used to simulate the data of the absorber, and similar results were obtained.

Tunable wideband-narrowband switchable absorber based on vanadium dioxide and graphene

A functionally tunable and absorption-tunable terahertz (THz) metamaterial absorber based on vanadium dioxide (VO₂) and graphene is proposed and verified numerically. Based on phase transition properties of VO₂ and tunability of graphene, the switching performance between ultra-broadband and narrow-band near-perfect absorption can be achieved. We simulate and analyze the characteristics of the constructed model by finite element analysis. Theoretical calculations show that when VO₂ is in the metallic state and the graphene Fermi energy is 0 eV, the designed absorber can perform ultra-broadband absorption. The absorber achieves greater than 95% absorption in the 2.85 - 10THz range. When VO₂ is in the insulating state and the graphene Fermi energy is 0.7 eV, more than 99.5% absorption can be achieved at 2.3 THz. The absorption rate can be tuned by changing the conductivity of VO₂ and the Fermi energy of graphene. Moreover, the proposed absorber displays good polarization insensitivity and wide incident angle stability. The design may have potential applications in terahertz imaging, sensing, electromagnetic shielding and so on.

Tailoring dual-band electromagnetically induced transparency with polarization conversions in a dielectric-metal hybrid metastructure

Metastructure analogs of electromagnetically induced transparency (EIT) provide a new approach for engineering realizations of nonlinear optical manipulations regardless of harsh conditions; further can be employed in polarization conversions for its low-loss transmission and phase modulation. In this work, dual-band EIT in a dielectric-metal hybrid metasurface achieved via providing different coupling channels is theoretically investigated with a maximum group delay of 404 ps. The linear-to-circular polarization conversion (LCPC) behaviors are observed respectively holding the transmittance of 0.58 at 0.68 THz, 0.73 at 0.76 THz, 0.61 at 0.90 THz, 0.53 at 0.99 THz, owning to the asymmetric EIT responses in the transverse magnetic (TM) and transverse electric (TE) modes incidence. On the other hand, phase-transition VO₂ is doped to perturb the dark mode resonances. With its conductivity σ = 10⁵ S/m, dual transparency peaks transform into unimodal broadband transmission windows with relative bandwidths of 17.1% and 9.1% under the TE and TM excitations apart. Induced LCPC possesses a bandwidth of 10.4% centered at 0.76 THz attributed to the drastic dispersion. The as-proposed design exploits pattern asymmetry of EIT responses to realize LCPC, promising the wide prospect of reconfigurable multiplexings.

Dual-controlled tunable dual-band and ultra-broadband coherent perfect absorber in the THz range

This paper proposes a vanadium dioxide metamaterial-based tunable, polarization-independent coherent perfect absorber (CPA) in the terahertz frequency range. The designed CPA demonstrates intelligent reconfigurable switch modulation from an ultra-broadband absorber mode to a dual-band absorber mode via the thermally controlled of VO₂. The mode of ultra-broadband absorber is realized when the conductivity of VO₂ reaches 11850 S/m via controlling its temperature around T = 328 K. In this mode, the CPA demonstrates more than 90% absorption efficiency within the ultra-wide frequency band that extends from 0.1 THz to 10.8 THz. As the conductivity of VO₂ reaches 2×10⁵ S/m (T = 340 K), the CPA switches to a dual-band absorber mode where a relatively high absorption efficiency of 98% and 99.7% is detected at frequencies of 4.5 THz and 9.8 THz, respectively. Additionally, using phase modulation of the incident light, the proposed CPA can regulate the absorption efficiency, which can be intelligently controlled from perfect absorption to high pass-through transmission. Owing to the ability of the proposed CPA to intelligently control the performance of light, this study can contribute towards enhancing the performance of stealth devices, all-optical switches and coherent photodetectors.

Three-stimulus control ultrasensitive Dirac point modulator using an electromagnetically induced transparency-like terahertz metasurface with graphene

This letter presents a fabricated Dirac point modulator of a graphene-based terahertz electromagnetically induced transparency (EIT)-like metasurface (GrE & MS). Dynamic modulation is realized by applying three stimulus modes of optical pump, bias voltage, and optical pump-bias voltage combination. With increasing luminous flux or bias voltage, the transmission amplitude undergoes two stages: increasing and decreasing, because the graphene Fermi level shifts between the valence band, Dirac point, and conduction band. Thus, an approximate position of the Dirac point can be evaluated by the transmission spectrum fluctuation. The maximum modulation depth is measured to be 182% under 1 V. These findings provide a method for designing ultrasensitive terahertz modulation devices.

Programmable VO2 metasurface for terahertz wave beam steering

Programmable vanadium dioxide (VO₂) metasurface is proposed at THz frequencies. The insulating and metallic states of VO₂ can be switched via external electrical stimulation, resulting in the dynamical modulation of electromagnetic response. The voltages of different columns of the metasurface can be controlled by the field-programmable gate array, and thus the phase gradients are realized for THz beam steering. In 1-bit coding, we design periodic and nonperiodic 24 × 24 coding sequences, and achieve wide-angle beam scanning with the deflection angles from −60° to +60°. In 2-bit coding, we use two different meta-atoms to design 18 × 18 coding sequences. Compared with 1-bit coding, 2-bit coding has more degree of freedom to control the optical phase, and 3 dB diffraction efficiency is improved by generating a single deflection angle. The proposed programmable metasurfaces provide a promising platform for manipulating electromagnetic wave in 6G wireless communication.

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The reversible phase-transition material VO₂ is integrated into the metasurface

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Programmable VO₂ metasurfaces are proposed to achieve THz beam steering

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Wide-angle beam scanning from −60° to +60° is realized in the digitalized metasurface

Switchable Multifunctional Terahertz Metamaterials Based on the Phase-Transition Properties of Vanadium Dioxide

Currently, terahertz metamaterials are studied in many fields, but it is a major challenge for a metamaterial structure to perform multiple functions. This paper proposes and studies a switchable multifunctional multilayer terahertz metamaterial. Using the phase-transition properties of vanadium dioxide (VO2), metamaterials can be controlled to switch transmission and reflection. Transmissive metamaterials can produce an electromagnetically induced transparency-like (EIT-like) effect that can be turned on or off according to different polarization angles. The reflective metamaterial is divided into I-side and II-side by the middle continuous VO2 layer. The I-side metamaterials can realize linear-to-circular polarization conversion from 0.444 to 0.751 THz when the incident angle of the y-polarized wave is less than 30°. The II-side metamaterials can realize linear-to-linear polarization conversion from 0.668 to 0.942 THz when the incident angle of the y-polarized wave is less than 25°. Various functions can be switched freely by changing the conductivity of VO2 and the incident surface. This enables metamaterials to be used as highly sensitive sensors, optical switches, and polarization converters, which provides a new strategy for the design of composite functional metamaterials.

Multitasking device with switchable and tailored functions of ultra-broadband absorption and polarization conversion

We present a multitasking tailored device (MTD) based on phase change material vanadium dioxide (VO₂) and photoconductive semiconductor (PS) in the terahertz (THz) regime, thereby manipulating the interaction between electromagnetic waves and matter. By altering the control multitasking device, its room temperature, or pump illumination, we switch the function of absorption or polarization conversion (PC) on and off, and realize the tuning of absorptivity and polarization conversion rate (PCR). Meanwhile, the construction of cylindrical air columns (CACs) in the dielectric provides an effective channel to broaden the absorption bandwidth. For the MTD to behave as a polarization converter with VO₂ pattern in the insulating phase (IP), exciting the PS integrated to the proposed device via an optical pump beam, the PCR at 0.82-1.6 THz can be modulated continuously from over 90% to perfectly near zero. When the PS conductivity is fixed at 3×10⁴ S/m and VO₂ is in the metal phase (MP) simultaneously, the MTD switched to an absorber exhibits ultra-broadband absorption with the absorptivity over 90% at 0.68-1.6 THz. By varying the optical pump power and thermally controlling the conductivity of VO₂, at 0.68-1.6 THz, the absorbance of such a MTD can be successively tuned from higher than 90% to near null. Additionally, the influences of the polarization angle and incident angle on the proposed MTD are discussed. The designed MTD can effectively promote the electromagnetic reconfigurable functionalities of the present multitasking devices, which may find attractive applications for THz modulators, stealth technology, communication system, and so on.

Reconfigurable multi-band water-graphene cascade metamaterial perfect absorbers loaded with vanadium dioxide

We demonstrate the perfect trapping of electromagnetic fields over multi-band frequencies through all-dielectric terahertz absorbers using water graphene cascade metamaterials. More specifically, the coating water layer greatly enhances the higher-order Fabry-Pérot resonant absorbing modes and can achieve more than 8 absorbing peaks with the absorptions exceeding 99% in the spectrum below 3 THz. Especially such multiple perfect absorbing bands can readily be reset when the proposed water-graphene metamaterial absorbers integrate with thermal controlled vanadium dioxide. Such a perfect absorbing capacity would also be valid for the wide angular illuminations with different polarizations, and the reconfigurable characteristics of graphene can also enable the dynamically tuning of the absorbing frequencies, offering great freedom of extensive applications in energy harvesting and wave manipulation.

Realization of absorption, filtering, and sensing in a single metamaterial structure combined with functional materials

In this paper, a hybrid vanadium dioxide (VO₂)-graphene-based bifunctional metamaterial is proposed. The realization of the different functions of perfect transmission and high absorption is based on the insulator-metal phase transition of VO₂ material. The Fermi energy level of graphene can be treated to dynamically tune the absorption and transmission rates of the metamaterial structure. As a result, when VO₂ is in the insulating state, the designed metamaterial can be used as a filter providing three adjustable passbands with center frequencies of 1.892 THz, 1.124 THz, and 0.94 THz, and the corresponding transmittances reach 93.11%, 98.62%, and 90.01%, respectively. The filter also shows good stopband characteristics and exhibits good sensing performance at the resonant frequencies of 1.992 THz and 2.276 THz. When VO₂ is in metal state, the metamaterial structure acts as a double-band absorber, with three absorption peaks (>90%) in the range of 0.684 THz to 0.924 THz, 2.86 THz to 3.04 THz, and 3.28 THz to 3.372 THz, respectively. The designed structure is insensitive to the polarization of vertically incident terahertz waves and still maintains good absorption performances over a large range of incidence angles. Finally, the effects of geometric parameters on the absorption and transmission properties of the hybrid bifunctional metamaterials have also been discussed. The switchable metamaterial structures proposed in this paper provide great potential in terahertz application fields, such as filtering, smart sensing, switching, tunable absorbers, and so on.

Broadband terahertz tunable multi-film absorber based on phase-change material

Based on the impedance matching method, we have numerically demonstrated a broadband tunable multilayer structure in a terahertz (THz) regime. The switchable functional characteristics of the absorber can be achieved by utilizing the phase transition property of vanadium dioxide (VO₂). When VO₂ is in the metallic state, the designed device behaves as a broadband absorber with an absorbance greater than 90% under normal incident from a 4.5 to 10 THz range. When VO₂ is in the insulating state, the absorption in this band is down to near 0%. Moreover, this high absorption band shows a good polarization insensitive property and can be maintained over a range of incident angles up to 45°. Our proposed device exhibits the merits of wideband reconfigure absorbance in THz, and the absorber can be easily fabricated without involving any lithographic process, both of which are very attractive to potential THz applications such as sensing, camouflaging, and modulation of THz waves.

Terahertz Reconfigurable Intelligent Surfaces (RISs) for 6G Communication Links

The forthcoming sixth generation (6G) communication network is envisioned to provide ultra-fast data transmission and ubiquitous wireless connectivity. The terahertz (THz) spectrum, with higher frequency and wider bandwidth, offers great potential for 6G wireless technologies. However, the THz links suffers from high loss and line-of-sight connectivity. To overcome these challenges, a cost-effective method to dynamically optimize the transmission path using reconfigurable intelligent surfaces (RISs) is widely proposed. RIS is constructed by embedding active elements into passive metasurfaces, which is an artificially designed periodic structure. However, the active elements (e.g., PIN diodes) used for 5G RIS are impractical for 6G RIS due to the cutoff frequency limitation and higher loss at THz frequencies. As such, various tuning elements have been explored to fill this THz gap between radio waves and infrared light. The focus of this review is on THz RISs with the potential to assist 6G communication functionalities including pixel-level amplitude modulation and dynamic beam manipulation. By reviewing a wide range of tuning mechanisms, including electronic approaches (complementary metal-oxide-semiconductor (CMOS) transistors, Schottky diodes, high electron mobility transistors (HEMTs), and graphene), optical approaches (photoactive semiconductor materials), phase-change materials (vanadium dioxide, chalcogenides, and liquid crystals), as well as microelectromechanical systems (MEMS), this review summarizes recent developments in THz RISs in support of 6G communication links and discusses future research directions in this field.

Adjustable multichannel terahertz resonator

Most reported terahertz devices are relatively simple to control by means of external applied electricity, light, heat, magnetism, etc. In the practical application of a terahertz system, the devices with a variety of adjustable methods are promising. In this paper, we design a rectangular-ambulatory-plane metal groove structure on apolymer polydimethylsiloxane (PDMS) substrate with graphene embedded in the metal grooves. The resonance performance of the proposed structure can be controlled by changing the external force and graphene chemical potential. Three electromagnetic-induced reflection peaks of the structure are achieved at 0.54, 0.60, and 0.63 THz with reflectivity of 89.6%, 77.1%, and 75.7%, respectively. Under the change of the applied bias voltage and external force, the reflectivity of the three reflection peaks of the resonator can be regulated freely. In addition, the proposed structure is polarization-insensitive and maintains stable terahertz reflection peaks with incident angles up to 25°.

Terahertz absorber with dynamically switchable dual-broadband based on a hybrid metamaterial with vanadium dioxide and graphene

An absorber based on hybrid metamaterial with vanadium dioxide and graphene has been proposed to achieve dynamically switchable dual-broadband absorption property in the terahertz regime. Due to the phase transition of vanadium dioxide and the electrical tunable property of graphene, the dynamically switchable dual-broadband absorption property is implemented. When the vanadium dioxide is in the metallic phase, the Fermi energy level of graphene is set as zero simultaneously, the high-frequency broadband from 2.05 THz to 4.30 THz can be achieved with the absorptance more than 90%. The tunable absorptance can be realized through thermal control on the conductivity of the vanadium dioxide. The proposed device acts as a low-frequency broadband absorber if the vanadium dioxide is in the insulating phase, for which the Fermi energy level of graphene varies from to 0.1 eV to 0.7 eV. The low-frequency broadband possesses high absorptance which is maintained above 90% from 1.10 THz to 2.30 THz. The absorption intensity can be continuously adjusted from 5.2% to 99.8% by electrically controlling the Fermi energy level of graphene. The absorption window can be further broadened by adjusting the geometrical parameters. Furthermore, the influence of incidence angle on the absorption spectra has been investigated. The proposed absorber has potential applications in the terahertz regime, such as filtering, sensing, cloaking objects, and switches.

Tunable and polarization-sensitive perfect absorber with a phase-gradient heterojunction metasurface in the mid-infrared

Inspired by the growing family of Van der Waals materials, hBN supported phonon polaritons have attracted much attention due to their inherent hyperbolic dispersion properties in the mid-infrared. However, the lack of tunability imposes a severe restriction on the diversified, functional and integrated applications. Here, we propose a phase-gradient heterostructure metasurface to realize a dynamically tunable and polarization-sensitive perfect absorber in the mid-infrared through combining hBN and phase change VO₂. Narrow-band perfect absorption at 7.2 µm can be switched to broadband around 11.2 µm through controlling the temperature of VO₂. The governed physics of the bandwidth and absorption differences are demonstrated. Phonon polaritons in hBN phase-gradient configurations and plasmon polaritons in periodic VO₂ blocks are respectively excited. We also investigate the absorption dependence on the polarization states of designed absorber. The method of engineering the absorption through controlling the temperature and polarization states opens up a new avenue for tunable applications such as data storage and integrated optical circuits.