Temperature-Controlled Asymmetric Transmission of Electromagnetic Waves

Using Thin Films of Phase-Change Material for Active Tuning of Terahertz Waves Scattering on Dielectric Cylinders

The scattering of electromagnetic waves by isotropic dielectric cylinders can be dramatically modified by means of vanadium dioxide (VO2) thin-film coatings. Efficient dynamic control of scattering is achieved due to the variations in material parameters realizable by means of external biasing. In this paper, we study the scattering of terahertz waves in a case where the coating shells are made of VO2, a phase-change material, whose thin films may work rather as electromagnetic phase screens in the insulator material phase, but as lossy quasi-metallic components in the metallic material phase. The shells that uniformly cover the dielectric cylinders are investigated. Attention will be paid to the demonstration of the potential of VO2 in the external control of diverse scattering regimes of the dielectric-VO2 core-shell scatterer, while conductivity of VO2 corresponds to rather insignificant variations in temperature. In line with the purposes of this work, it is shown that the different resonant and nonresonant regimes have different sensitivity to the variations in VO2 conductivity. Both the total scattering cross section and field distributions inside and around the core are studied, as well as the angle-dependent scattering cross section.

Terahertz Metasurfaces Exploiting the Phase Transition of Vanadium Dioxide

Artificially designed modulators that enable a wealth of freedom in manipulating the terahertz (THz) waves at will are an essential component in THz sources and their widespread applications. Dynamically controlled metasurfaces, being multifunctional, ultrafast, integrable, broadband, high contrasting, and scalable on the operating wavelength, are critical in developing state-of-the-art THz modulators. Recently, external stimuli-triggered THz metasurfaces integrated with functional media have been extensively explored. The vanadium dioxide (VO2)-based hybrid metasurfaces, as a unique path toward active meta-devices, feature an insulator-metal phase transition under the excitation of heat, electricity, and light, etc. During the phase transition, the optical and electrical properties of the VO2 film undergo a massive modification with either a boosted or dropped conductivity by more than four orders of magnitude. Being benefited from the phase transition effect, the electromagnetic response of the VO2-based metasufaces can be actively controlled by applying external excitation. In this review, we present recent advances in dynamically controlled THz metasurfaces exploiting the VO2 phase transition categorized according to the external stimuli. THz time-domain spectroscopy is introduced as an indispensable platform in the studies of functional VO2 films. In each type of external excitation, four design strategies are employed to realize external stimuli-triggered VO2-based THz metasurfaces, including switching the transreflective operation mode, controlling the dielectric environment of metallic microstructures, tailoring the equivalent resonant microstructures, and modifying the electromagnetic properties of the VO2 unit cells. The microstructures' design and electromagnetic responses of the resulting active metasurfaces have been systematically demonstrated, with a particular focus on the critical role of the VO2 films in the dynamic modulation processes.

Multi-functional device: manipulating linear and circular-polarization conversion in a terahertz chiral metamaterial

We propose a terahertz chiral metamaterial as a multi-functional device to manipulate asymmetry transmission of linear polarized waves, linear-to-elliptical polarization conversion and circular dichroism in transmission mode while asymmetry reflection of circular polarized waves. For incidence of linear polarized waves, dual-band asymmetry transmission is shown around 0.42 THz and 1.04 THz where asymmetry transmission factors reach up to two peak values: ∼0.51 and ∼0.55, respectively. Intense linear-to-elliptical polarization conversion occurs at 0.81 THz and 0.97 THz, respectively. For incidence of circular polarized waves, a strong circular dichroism appears at 0.36 THz where circular dichroism parameter reaches to ∼0.64 and asymmetry reflection is displayed around 0.36 THz with the maximum of asymmetry reflection factors approaching to 0.55.

Hybrid dual-mode tunable polarization conversion metasurface based on graphene and vanadium dioxide

We present and numerically verify a functionally hybrid dual-mode tunable polarization conversion metasurface based on graphene and vanadium dioxide (VO₂). The tunable polarization converter consists of two patterned graphene layers separated by grating which is composed of gold and VO₂. Due to the existence of phase change material VO₂, the polarization conversion mode can be switched flexibly between the transmission and reflection modes. Theoretical calculations show the proposed polarization conversion metasurface can obtain giant asymmetric transmission (AT) at 0.42 and 0.77 THz when VO₂ is in the insulating state. Conversely, when VO₂ is in the metallic state, the converter switches to the reflection mode, demonstrating broadband polarization conversion for both forward and backward incidences. Furthermore, the conductivity of graphene can be modulated by changing the gate voltage, which allows dynamic control polarization conversion bandwidth of the reflection mode as well as the AT of the transmission mode. The robustness of the metasurface has also been verified, the high polarization conversion efficiency and AT can be maintained over wide incidence angles up to 65° for both the xoz plane and yoz plane. These advantages make the proposed hybrid tunable polarization conversion metasurface a promising candidate for THz radiation switching and modulation.

Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅, V₂O₂, V₆O₁₃, and V₄O₉. We begin with a tutorial on the phase diagram of the V-O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

VO2-enabled transmission-reflection switchable coding terahertz metamaterials

Coding metamaterials have offered unprecedented degrees of freedom to manipulate electromagnetic waves in time and frequency domains in terms of various coding sequences, however, it is still challenging to realize dynamic coding metamaterials in the terahertz range. Here, we propose VO₂-enabled transmission-reflection switchable coding terahertz metamaterials consisting of multilayered gold and VO₂ patterns. The insulator-to-metal transition of VO₂ leads to switch between the refractive and reflective scattering beams by changing the temperature. The four 2-bit elements are used to construct coding metasurface-based OAM generator with l = 1. Remarkably, the transmission-reflection switching functionality of the coding metasurface can be achieved at different frequencies. In addition, the novel designs in our work can achieve EM waves manipulation and provide a useful method to dynamically switch transmission-reflection response in the THz frequency regime.

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.

Transmissive terahertz metasurfaces with vanadium dioxide split-rings and grids for switchable asymmetric polarization manipulation

Metasurfaces containing arrays of thermally tunable metal-free (double-)split-ring meta-atoms and metal-free grids made of vanadium dioxide (VO[Formula: see text]), a phase-change material can deliver switching between (1) polarization manipulation in transmission mode as well as related asymmetric transmission and (2) other functionalities in the terahertz regime, especially when operation in the transmission mode is needed to be conserved for both phases of VO[Formula: see text]. As the meta-atom arrays function as arrays of metallic subwavelength resonators for the metallic phase of VO[Formula: see text], but as transmissive phase screens for the insulator phase of VO[Formula: see text], numerical simulations of double- and triple-array metasurfaces strongly indicate extreme scenarios of functionality switching also when the resulting structure comprises only VO[Formula: see text] meta-atoms and VO[Formula: see text] grids. More switching scenarios are achievable when only one meta-atom array or one grid is made of VO[Formula: see text] components. They are enabled by the efficient coupling of the geometrically identical resonator arrays/grids that are made of the materials that strongly differ in terms of conductivity, i.e. Cu and VO[Formula: see text] in the metallic phase.

Active beam manipulation and convolution operation in VO2-integrated coding terahertz metasurfaces

Coding metasurfaces have received tremendous interest due to their unprecedented control of beams through the flexible design of coding sequences. However, realizing tunable coding metasurfaces with scattering-pattern shifts in the terahertz range is still challenging. Here, we propose a VO₂-integrated coding metasurface to realize a thermally controlled scattering-pattern shift by convolution operation. The required phase profiles and high amplitudes of 1-bit and 2-bit coding metasurfaces are easily obtained only by changing the length of the VO₂ cut-wires. The insulator-metal phase transition of the VO₂ cut-wires leads to an ultrafast switching effect between multiple deflected scattering beams and one normally reflected beam. In particular, the VO₂ phase transition contributes to dynamical convolution operations of the 2-bit coding metasurface. The proposed VO₂-integrated coding metasurfaces are important for realizing tunable terahertz beam manipulation as well as arbitrary required scattering beams.

Terahertz dual-band asymmetric transmission for a single cross-polarized linear wave

The reported dual-band asymmetric transmission is usually an effect of mutual polarization conversion, where one polarized wave is converted to its cross-polarization in the first band while the other polarized wave is converted to its cross-polarization in the second band. In this work, we experimentally demonstrate a dual-band asymmetric transmission effect only for one-polarized linear wave in the terahertz band. It is measured that the cross-polarization transmission coefficient T_yx reaches two peaks of 0.715 and 0.548 at the frequency of 0.74 THz and 1.22 THz, respectively. While the transmission coefficient T_xy is lower than 0.2 in the wide-band from 0.5 THz to 1.5 THz. Firstly, the multiple interference model is used to discuss the physical mechanism of the dual-band asymmetric transmission. However, the second band of the calculated spectrum is offset due to the strong near field coupling between the two metal layers. The coupled-mode theory is then introduced and the fitting result of the coupled-mode theory is in good agreement with that of the experiment in the two bands. This research would provide new theoretical instructions in designing and analyzing multiband asymmetric transmission in the terahertz, microwave or the optical bands.

Temperature-dependent circular conversion dichroism from chiral metasurfaces patterned in Dirac semimetal Cd3As2

Chiral metasurfaces patterned with L-shaped holes in a thin film of Dirac semimetal Cd3As2 are designed. The impact of temperature T on circular conversion dichroism, mainly characterized by circular polarization differential transmittance (CPDT), is studied by rigorous coupled-wave analysis. The results show that decreasing T will give rise to the appearance of much more narrow CPDT peaks and dips, and the maximum differential transmittance between two opposite circularly polarized light can reach above 0.60 by optimizing the structural parameters at 80 K. As the T increases, the differential transmittance gradually decreases, and the CPDT peak and dip values exhibit variation tendencies of 'Z' and 'S' types, respectively. Two simple formulae of CPDT extreme values with respect to T are derived, predicting that the decreasing tendency will reach saturation when T ≥ 500 K. Differing from the wavelength-independent variation trend of differential transmittance, CPDT extremum positions mainly show a blueshift (redshift) tendency at the wavelength λ > 10 μm (λ < 5 μm) as the T increases. Moreover, evolutions of CPDT with various factors including the thickness of Cd3As2, incident and azimuth angles are also clearly unveiled.

Ultra-wideband, wide angle, asymmetric transmission based chiral metasurface for C and X band applications

A multi-layered chiral device manifesting asymmetric transmission (AT) facilitating one-way channeling of electromagnetic (EM) waves, based on the inherent polarization is presented. The designed metasurface depicts a high transmission contrast with an efficiency above 80% for an ultra-wide operational range of 6.3–12.3 GHz, constituting a fractional bandwidth of 64%. As an additional feature, the reported metasurface yields robustness against oblique incidences up to 45\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^\circ $$\end{document}∘ while maintaining high transmission efficiency. This report also introduces a unique analogy of the AT based communication system with logic-gates by formulating its truth-table and logic circuit. Furthermore, new insights of AT magnitude’s dependence to oblique incidences are presented on the account of surface impedance mismatch due to TE and TM polarization with varying incidence angle. Moreover, avoidance of grating lobes and the associated transmission deterioration through utilization of electrically small periodic metasurface is presented. The results have been numerically and practically validated yielding state-of-the-art features. Operating within C and X band, the reported work is an ideal candidate for practical AT applications.

Switchable and Dual-Tunable Multilayered Terahertz Absorber Based on Patterned Graphene and Vanadium Dioxide

In this paper, a switchable and dual-tunable terahertz absorber based on patterned graphene and vanadium dioxide is proposed and analyzed. By controlling the Fermi level of graphene and the temperature of vanadium dioxide, the device’s function can be switched and its absorbing properties can be tuned. When the vanadium dioxide is in an insulator state, the device can be switched from near-total reflection (>97%) to ultra-broadband absorption (4.5–10.61 THz) as the Fermi level of graphene changes from 0 to 0.8 eV. When the vanadium dioxide is changed to a metal state, the device can act as a single-band absorber (when the Fermi level of graphene is 0 eV) and a dual-band absorber with peaks of 4.16 THz and 7.3 THz (when the Fermi level of graphene is 0.8 eV). Additionally, the absorber is polarization-insensitive and can maintain a stable high-absorption performance within a 55° incidence angle. The multilayered structure shows great potential for switchable and tunable high-performance terahertz devices.

Bi-functional tunable reflector/high-Q absorber design using VO2 assisted graphene-coated cylinder array

In this paper, a bi-functional tunable reflector/absorber device using an assembly of graphene-coated cylindrical wires, backed by a thermally controlled phase change material, is proposed. The reflection coefficient of the graphene-coated wire-grating manifests multiple resonances, originating from the hybridized excitation of localized surface plasmons in the graphene shells. The first plasmonic resonance (with the order of two), in the free-standing configuration, shows tunable near-perfect reflection while the second plasmonic resonance (with the order of three), in the reflector-backed array, exhibits near-perfect absorption. Because of the metal-insulator transition in the phase change material, it is feasible to switch between these two functionalities using a VO₂ back layer. Moreover, the high-quality factor of the absorption band (Q ∼ 128.86) is due to its Fano line shape, leading to a narrow bandwidth. Thus, the absorbing mode can be possibly used for refractive index sensing with the sensitivity of S ∼ 9000 nm/RIU (refractive index unit) and figure of merit of FOM ∼ 104 RIU^-1. In the proposed structure, different optical, material, and geometrical parameters affect the optical response of the operating bands, offering a flexible design.

Switchable dual-band to broadband terahertz metamaterial absorber incorporating a VO2 phase transition

We design and demonstrate a thermally switchable terahertz metamaterial absorber consisting of an array of orthogonal coupled split-ring metal resonators involving a VO₂ phase transition. Numerical results indicate that the active metamaterial always absorbs the TE wave in dual-band regardless of insulating and metallic VO₂, while the insulator-to-metal phase transition enables a switchable effect between dual-band and broadband absorption of the TM wave with the resonant frequency tunability of 33%. Especially under the metallic VO₂ state, the absorption properties are polarization-dependent and exhibit a switching effect between dual-band and broadband absorption with the increase of the polarization angle. The tunable absorption mechanism can be explained by effective impedance theory and electric energy density distributions. The proposed dual-band to broadband metamaterial switching absorber may have broad applications in sensors, imaging and emitters.

Reprogrammable meta-hologram for optical encryption

Meta-holographic encryption is a potentially important technique for information security. Despite rapid progresses in multi-tasked meta-holograms, the number of information channels available in metasurfaces is limited, making meta-holographic encryption vulnerable to some attacking algorithms. Herein, we demonstrate a re-programmable metasurface that can produce arbitrary holographic images for optical encryption. The encrypted information is divided into two matrices. These two matrices are imposed to the incident light and the metasurface, respectively. While the all-dielectric metasurface is static, the phase matrix of incident light provides additional degrees of freedom to precisely control the eventual functions at will. With a single Si metasurface, arbitrary holographic images and videos have been transported and decrypted. We hope that this work paves a more promising way to optical information encryption and authentication.

Here, the authors demonstrate a re-programmable metasurface that can produce arbitrary holographic images for optical encryption. The encrypted information is divided into two matrices and defined to the incident laser to produce arbitrary holographic images.

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.

Tunable bifunctional terahertz metamaterial device based on Dirac semimetals and vanadium dioxide

A tunable bifunctional terahertz (THz) metamaterial device based on Dirac semimetal films (DSFs) and VO₂ is presented. The insulator-to-metal phase transition of VO₂ enables bifunctional asymmetric transmission and dual-directional absorption to be switched in the THz range. When VO₂ serves as a dielectric, tunable broadband asymmetric transmission of linearly polarized THz waves can be achieved. When VO₂ is in a metallic state, the proposed device acts as a tunable dual-directional absorber with perfect absorption in both illumination directions. In each case, the response can be tuned by varying the Fermi energy of the DSFs. This offers a new pathway for the development of tunable multifunctional THz metamaterial devices.

Electromagnetically induced transparency in terahertz metasurface composed of meanderline and U-shaped resonators

Mimicking the quantum phenomena of electromagnetically induced transparency using metasurfaces has drawn continuous interest in recent years owing to its potential in realizing optical switching, slow-light, nonlinear enhancement, and sensing devices with much reduced working conditions. Various kinds of structures have been proposed through designing the internal coupling effect among the unit cell. In this work, we theoretically and experimentally propose a new type of coupled resonant structures composed of meanderline and U-shaped resonators in the terahertz regime, which can exhibit strong behavior of electromagnetically induced transparency. The introduction of the meanderline structure provides an effective manner for realizing electrically controlled electromagnetically induced transparency devices due to its continuous connection feature, making it convenient to serve as an integrated electrode. Such ability is verified by simulations where vanadium dioxide structures are further integrated. The proposed design opens new avenues to realize compact and tunable slow-light devices.

Experimental verification of a broadband asymmetric transmission metamaterial in the terahertz region

In this work, a broadband terahertz asymmetric transmission metamaterial is experimentally demonstrated for a linearly polarized wave. The measured transmission coefficient T_yx is larger than 0.6 from 0.55 to 0.82 THz, and reaches a peak value of 0.714 at 0.62 THz, while the transmission coefficient T_xy is lower than 0.2 from 0.4 to 0.9 THz. The calculated asymmetric transmission parameter of the measurement ranges from 0.53 to 0.84 THz for magnitudes over 0.4. The peak value reached 0.65 at the frequency of 0.78 THz. The physical mechanism of the polarization conversion was also analyzed from the distributions of the surface currents and electric fields.

In this work, a broadband terahertz asymmetric transmission metamaterial is experimentally demonstrated for a linearly polarized wave.

Controllable broadband asymmetric transmission of terahertz wave based on Dirac semimetals

We present a dynamic metamaterial based on Dirac semimetals and capable of realizing broadband and tunable asymmetric transmission in the terahertz region. The Dirac semimetal resonators have a chiral structure patterned with double-T resonators that results in partial polarization conversion of waves incident upon the material, leading to asymmetric transmission across a wide frequency range. We show how the gradual shift of the semimetal Fermi energy permits a method of control over the asymmetric total transmission.

Spin-dependent switchable metasurfaces using phase change materials

Metasurfaces have been widely investigated for various applications enabled by their strong light manipulation capabilities. Their monolithic designs offer the convenience to incorporate novel natural materials in order to realize advanced electromagnetic (EM) functionalities. Here, based on the usage of the phase change material vanadium dioxide (VO₂), a switchable metasurface that could work at two different working states is proposed. With insulating VO₂, we show that helicity-dependent metasurface could be rigorously designed by adopting two phase variables, i.e., initial phase and Pancharatnam-Berry (P-B) phase, which is verified by showing an asymmetric photonic spin Hall effect (APSHE). When VO₂ goes into the metallic phase (e.g., by raising the operating temperature above ~341K), the loss factor of the unit cell will be enhanced, and in this case with the assistance of multi-mode resonances, the metasurface will turn into a perfect broadband circular-polarization-insensitive EM absorber. Based on these, switchable beam splitters and focus-lenses have been designed and discussed in the paper. The method proposed here may pave a new way to pursue active and multifunctional optical devices.