Photoinduced Broad-band Tunable Terahertz Absorber Based on a VO2 Thin Film

Design of terahertz metamaterial absorbers with switchable absorption functions utilizing thermal and electrical dual-modulation strategies

This work demonstrates a dual-functional tunable terahertz metamaterial absorber based on thermally controllable vanadium dioxide (VO2) and electrically tunable graphene. The switchable absorption functions could be obtained in the same metamaterial, which consists of alternating stacked cross-cut graphene disks (CGDs) and VO2 square rings (VSRs) separated by an ultra-thin dielectric film placed on a continuous gold mirror. The metallic state of VSRs is the dominant factor for the broadband absorption function, resulting in a broadband absorption of 4.746 THz. Based on this, the Fermi energy level of CGDs increases to 0.7 eV, which could broaden the absorption bandwidth to 5.398 THz. When the VSRs are in the insulating state, CGDs dominate the absorption, and the suggested device switches to the dual-band absorption function. These two absorption peaks appear to be larger than 97% and their frequencies could be dynamically controlled by the Fermi energy level of CGDs. In addition to the excellent absorption characteristics of dynamic switching of two different functions, polarization insensitivity and large-angle tolerance are also advantages of this work, which could provide new insights and guidance for the study of dynamically tunable metamaterial absorbers.

Terahertz rewritable wavefront modulator based on indium oxide and DMSO-doped PEDOT:PSS

An optically rewritable and electrically erasable terahertz (THz) wavefront modulator based on indium oxide (In2O3) and DMSO-doped PEDOT:PSS is proposed. The modulator has a three-layer structure of In2O3/PEDOT:PSS/quartz, which can weaken the THz transmission under the action of light excitation. Optically written THz Fresnel plates, which can focus the input Gaussian beam into a point, were realized. After optical excitation, the function of the device reduces slowly if it is stored in the room environment. However, the function can be stored for a long time if it is encapsulated in the nitrogen environment. If a bias voltage of 22 V is applied on the device, the function of the device can be erased in 10 seconds. The new function can be written into the device after wiping. Experiments on THz rewritable holographic devices are carried out to show the validity of this approach. This method can provide new devices for THz wavefront modulation and develop tunable optical imaging elements.

Flexible Metasurface for Microwave-Infrared Compatible Camouflage via Particle Swarm Optimization Algorithm

Metamaterials have the powerful ability to freely control multiband electromagnetic (EM) waves through elaborately designed "artificial atoms" and are hence in the limelight in various fields. Typically, camouflage materials manipulate wave-matter interactions to achieve the desired optical properties, in particular, various techniques are used for multiband camouflage materials in both infrared (IR) and microwave (MW) ranges to overcome the scale difference between these bands. However, in the context of components required for microwave communications, simultaneous control of IR emission and MW transmission is required, which is challenging owing to differences in the wave-matter interactions in these two bands. Herein, the state-of-the-art concept of flexible compatible camouflage metasurface (FCCM) is demonstrated, which can manipulate IR signatures while maintaining MW selective transmission simultaneously. For achieving maximum IR tunability and MW selective transmission, it is performed optimization using the particle swarm optimization (PSO) algorithm. Consequently, the FCCM exhibits compatible camouflage performance with both IR signature reduction and MW selective transmission is demonstrated, with 77.7% IR tunability and 93.8% transmission achieved for a flat FCCM. Furthermore, the FCCM reached the 89.8% IR signature reduction effect even in curved situations.

Multi-Band High-Efficiency Multi-Functional Polarization Controller Based on Terahertz Metasurface

Electromagnetic metasurfaces with excellent electromagnetic wave regulation properties are promising for designing high-performance polarization control devices, while the application prospect of electromagnetic metasurfaces is limited because of the current development situations of the complex structure, low conversion efficiency, and narrow working bandwidth. In this work, we design a type of reflective terahertz metasurface made of a simple structure that can achieve multiple polarization modulation with high efficiency. It is shown that the presented metasurface can realize ultra-broadband, cross-polarization conversion with the relative working bandwidth reaching 94% and a conversion efficiency of over 90%. In addition, the proposed metasurface can also efficiently accomplish different polarization conversion functions, such as linear-to-linear, linear-to-circular, or circular-to-linear polarization conversion in multiple frequency bands. Due to the excellent properties, the designed metasurface can be used as a high-efficiency multi-functional polarization modulation device, and it has important application value in terahertz imaging, communication, biological detection, and other fields.

Photonic bandgap engineering in (VO2)n/(WSe2)nphotonic superlattice for versatile near- and mid-infrared phase transition applications

The application of the photonic superlattice in advanced photonics has become a demanding field, especially for two-dimensional and strongly correlated oxides. Because it experiences an abrupt metal-insulator transition near ambient temperature, where the electrical resistivity varies by orders of magnitude, vanadium oxide (VO₂) shows potential as a building block for infrared switching and sensing devices. We reported a first principle study of superlattice structures of VO₂as a strongly correlated phase transition material and tungsten diselenide (WSe₂) as a two-dimensional transition metal dichalcogenide layer. Based on first-principles calculations, we exploit the effect of semiconductor monoclinic and metallic tetragonal state of VO₂with WSe₂in a photonic superlattices structure through the near and mid-infrared (NIR-MIR) thermochromic phase transition regions. By increasing the thickness of the VO₂layer, the photonic bandgap (PhB) gets red-shifted. We observed linear dependence of the PhB width on the VO₂thickness. For the monoclinic case of VO₂, the number of the forbidden bands increase with the number of layers of WSe₂. New forbidden gaps are preferred to appear at a slight angle of incidence, and the wider one can predominate at larger angles. We presented an efficient way to control the flow of the NIR-MIR in both summer and winter environments for phase transition and photonic thermochromic applications. This study's findings may help understand vanadium oxide's role in tunable photonic superlattice for infrared switchable devices and optical filters.

Active and Smart Terahertz Electro-Optic Modulator Based on VO2 Structure

Modulating terahertz (THz) waves actively and smartly through an external field is highly desired in the development of THz spectroscopic devices. Here, we demonstrate an active and smart electro-optic THz modulator based on a strongly correlated electron oxide vanadium dioxide (VO2). With milliampere current excitation on the VO2 thin film, the transmission, reflection, absorption, and phase of THz waves can be modulated efficiently. In particular, the antireflection condition can be actively achieved and the modulation depth reaches 99.9%, accompanied by a 180° phase switching. Repeated and current scanning experiments confirm the high stability and multibit modulation of this electro-optic modulation. Most strikingly, by utilizing a feedback loop of "THz-electro-THz" geometry, a smart electro-optic THz control is realized. For instance, the antireflection condition can be stabilized precisely no matter what the initial condition is and how the external environment changes. The proposed electro-optic THz modulation method, taking advantage of strongly correlated electron material, opens up avenues for the realization of THz smart devices.

A Review: The Functional Materials-Assisted Terahertz Metamaterial Absorbers and Polarization Converters

When metamaterial structures meet functional materials, what will happen? The recent rise of the combination of metamaterial structures and functional materials opens new opportunities for dynamic manipulation of terahertz wave. The optical responses of functional materials are greatly improved based on the highly-localized structures in metamaterials, and the properties of metamaterials can in turn be manipulated in a wide dynamic range based on the external stimulation. In the topical review, we summarize the recent progress of the functional materials-based metamaterial structures for flexible control of the terahertz absorption and polarization conversion. The reviewed devices include but are not limited to terahertz metamaterial absorbers with different characteristics, polarization converters, wave plates, and so on. We review the dynamical tunable metamaterial structures based on the combination with functional materials such as graphene, vanadium dioxide (VO2) and Dirac semimetal (DSM) under various external stimulation. The faced challenges and future prospects of the related researches will also be discussed in the end.

Metasurface absorber with ultra-thin thickness designed for a terahertz focal plane array detector

Terahertz (THz) refers to electromagnetic waves with frequency from 0.1 to 10 THz, which lies between millimeter waves and infrared light. This paper proposes an ultra-thin metasurface absorber which is perfectly suited to be the signal coupling part of terahertz focal plane array (FPA) detector. The absorptance of the proposed metasurface is higher than 80% from 4.46 to 5.76 THz (25.4%) while the thickness is merely 1.12 µm (0.018 λ). Since the metasurface absorber will be applied to terahertz FPA detector which requires planar array formation, it is divided into meta-atoms. Each meta-atom consists of the same unit cell layout, and air gaps are introduced between adjacent meta-atoms to enhance the thermal isolation, which is crucial for FPA detector to obtain desired imaging results. Due to the symmetrical layout of meta-atoms, absorptance keeps stable for different polarized waves, moreover, good absorptance could also be achieved for incidence angles range of ± 30 °. Spectral measurements show good agreement with the simulation. As a result, features of ultra-thin thickness, polarization insensitivity, and high absorptance make the proposed metasurface absorber well suited to highly efficient coupling of terahertz signals in FPA detector.

Multidimensional engineered metasurface for ultrafast terahertz switching at frequency-agile channels

The ability to actively manipulate free-space optical signals by using tunable metasurfaces is extremely appealing for many device applications. However, integrating photoactive semiconductors into terahertz metamaterials still suffers from a limited functionality. The ultrafast switching in picosecond timescale can only be operated at a single frequency channel. In the hybrid metasurface proposed here, we experimentally demonstrate a dual-optically tunable metaphotonic device for ultrafast terahertz switching at frequency-agile channels. Picosecond ultrafast photoswitching with a 100% modulation depth is realized at a controllable operational frequency of either 0.55 THz or 0.86 THz. The broadband frequency agility and ultrafast amplitude modulation are independently controlled by continuous wave light and femtosecond laser pulse, respectively. The frequency-selective, temporally tunable, and multidimensionally-driven features can empower active metamaterials in advanced multiplexing of information, dual-channel wireless communication, and several other related fields.

Dynamically switchable broadband and triple-band terahertz absorber based on a metamaterial structure with graphene

This paper proposes a terahertz absorber with a simple four-layered structure that can be dynamically switched between broadband and triple-band by controlling the chemical potential of graphene. The proposed absorber owns broadband absorption in the frequency range from 5.28 THz to 7.86 THz with the corresponding absorption efficiency above 90%, when the chemical potential of graphene is 150 meV. By increasing the chemical potential of graphene to 550 meV, the broadband absorption splits into triple-band absorption, with the peak locating at 5.39 THz, 7.01 THz and 8.1 THz, respectively. Detailed investigation shows that the broadband absorption should originate from magnetic resonance, Fabry-Pérot cavity resonance and surface plasmon polariton. The triple-band absorption should arise from the combination of Fabry-Pérot cavity resonance and surface plasmon polariton. Additionally, both broadband absorption and triple-band absorption are insensitive to the incident polarization. This tunable and bifunctional metamaterial structure shows a great potential in terahertz applications, such as detectors, modulators and sensors.

Morphology engineering of a hybrid perovskite for active terahertz memory modulation

Morphology engineering was investigated for hybrid perovskites CH₃NH₃PbI₃:Ag/Poly(3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) that were fabricated in both air and nitrogen environments for active terahertz (THz) memory modulation. Under low optical excitation or an applied bias, THz amplitude modulation or rapid restore in both CH₃NH₃PbI₃:Ag/PEDOT:PSS hybrid structures were demonstrated. The recovery time of the modulated THz wave in the sample fabricated in air was considerably longer than that of the sample fabricated in nitrogen because of defect states induced by a high degree of roughness. THz transmissions were used as coded pixel units and were programmed to store a 4×4 image or a multi-order signal. Hence, active THz memory modulation was demonstrated. It also has potential applications as a visible to near-infrared broad-spectrum light detector.

Wafer-scale freestanding vanadium dioxide film

Vanadium dioxide (VO₂), with well-known metal-to-insulator phase transition, has been used to realize intriguing smart functions in photodetectors, modulators, and actuators. Wafer-scale freestanding VO₂ (f-VO₂) films are desirable for integrating VO₂ with other materials into multifunctional devices. Unfortunately, their preparation has yet to be achieved because the wafer-scale etching needs ultralong time and damages amphoteric VO₂ whether in acid or alkaline etchants. Here, we achieved wafer-scale f-VO₂ films by a nano-pinhole permeation-etching strategy in 6 min, far less than that by side etching (thousands of minutes). The f-VO₂ films retain their pristine metal-to-insulator transition and intrinsic mechanical properties and can be conformably transferred to arbitrary substrates. Integration of f-VO₂ films into diverse large-scale smart devices, including terahertz modulators, camouflageable photoactuators, and temperature-indicating strips, shows advantages in low insertion loss, fast response, and low triggering power. These f-VO₂ films find more intriguing applications by heterogeneous integration with other functional materials.

Terahertz Metamaterial Modulator Based on Phase Change Material VO2

In this paper, a new type of terahertz (THz) metamaterial (MM) modulator has been presented with bifunctional properties based on vanadium dioxide (VO2). The design consists of a VO2 resonator, polyimide substrate, frequency selective surface (FSS) layer, and VO2 film. Based on the metal-insulator transition (MIT) of VO2, this structure integrated with VO2 material can achieve the dynamic modulation on both transmission and reflection waves at 2.5 THz by varying the electrical conductivity value of VO2. Meanwhile, it also exhibits adjustable absorption performance across the whole band from 0.5–7 THz. At the lower conductivity (σ = 25 S/m), this structure can act as a bandpass FSS, and, at the high conductivity (σ = 2 × 105 S/m), it behaves like a wideband absorber covering 2.52–6.06 THz with absorption A > 0.9, which realizes asymmetric transmission. The surface electric field distributions are illustrated to provide some insight into the physical mechanism of dynamic modulation. From the simulated results, it can be observed that this design has the capability of controlling tunable manipulation on both transmission/reflection responses at a wide frequency band. This proposed design may pave a novel pathway towards thermal imaging, terahertz detection, active modulators, etc.

Ultrawideband Terahertz Absorber with Dielectric Cylinders Loaded Patterned Graphene Structure

In this paper, we theoretically designed and numerically analyzed an ultrabroadband meta-absorber with near unity absorptivity that works in terahertz spectrum. A wideband meta-absorber composed of bilayer patterned graphene and dielectric cylinder array with high symmetry was proposed. The wideband absorption mechanism benefited from two aspects. The first one was enhanced surface plasmons based on bilayer patterned graphene. And the second one was the coupling of continuous resonant modes within Fabry-Perot cavities to the enhanced surface plasmons in the graphene. An ultrawide bandwidth with absorptivity over 90% were obtained from 3.2 THz to 9.4 THz. Simulated results showed that the proposed ultra-wideband absorbing structure also possessed high performance of polarization independence, flexible tunability, large incident angle insensitivity, and compact fabrication.

3D Printed Sub-Terahertz All-Dielectric Lens for Arbitrary Manipulation of Quasi-Nondiffractive Orbital Angular Momentum Waves

Terahertz (THz) vortex waves carrying orbital angular momentum (OAM) hold great potential in dealing with the capacity crunch in wireless high-speed communication systems. Nevertheless, it is quite a challenge for the widespread applications of OAM in the THz regime due to the beam divergence and stringent alignment requirement. To address this issue, an all-dielectric lens (ADL) is proposed for the arbitrary manipulation of quasi-nondiffractive THz OAM waves (QTOWs). On the basis of the concept of the optical conical lens and the multivorticity metasurface, the beam number, the topological charge (TC), and the deflection angle as well as the nondiffractive depth of the generated THz OAM waves are controllable. For proof-of-concept, two ADLs are 3D printed to create single and dual deflected QTOWs, respectively. Remarkably, measured by a THz imaging camera, the desired QTOWs with high mode purity are observed in predesigned directions with a nondiffractive depth predefined theoretically. The proposed designs and experiments, for the first time, verified that the QTOWs could be achieved with a nondiffractive range of 55.58λ_g (λ_g = wavelength at 140 GHz) and large deflection angles of 30° and 45°.