Dynamic Manipulation of THz Waves Enabled by Phase-Transition VO2 Thin Film

A Metamaterials-Based Absorber Used for Switch Applications with Dynamically Variable Bandwidth in Terahertz Regime

A broadband absorber based on metamaterials of graphene and vanadium dioxide (VO2) is proposed and investigated in the terahertz (THz) regime, which can be used for switch applications with a dynamically variable bandwidth by electrically and thermally controlling the Fermi energy level of graphene and the conductivity of VO2, respectively. The proposed absorber turns 'on' from 1.5 to 5.4 THz, with the modulation depth reaching 97.1% and the absorptance exceeding 90% when the Fermi energy levels of graphene are set as 0.7 eV, and VO2 is in the metallic phase. On the contrary, the absorptance is close to zero and the absorber turns 'off' with the Fermi energy level setting at 0 eV and VO2 in the insulating phase. Furthermore, other four broadband absorption modes can be achieved utilizing the active materials graphene and VO2. The proposed terahertz absorber may benefit the areas of broadband switch, cloaking objects, THz communications and other applications.

Recent Advances in Tunable Metasurfaces and Their Application in Optics

Metasurfaces can be opportunely and specifically designed to manipulate electromagnetic wavefronts. In recent years, a large variety of metasurface-based optical devices such as planar lenses, beam deflectors, polarization converters, and so on have been designed and fabricated. Of particular interest are tunable metasurfaces, which allow the modulation of the optical response of a metasurface; for instance, the variation in the focal length of a converging metalens. Response tunability can be achieved through external sources that modify the permittivity of the materials constituting the nanoatoms, the substrate, or both. The modulation sources can be classified into electromagnetic fields, thermal sources, mechanical stressors, and electrical bias. Beside this, we will consider optical modulation and multiple approach tuning strategies. A great variety of tunable materials have been used in metasurface engineering, such as transparent conductive oxides, ferroelectrics, phase change materials, liquid crystals, and semiconductors. The possibility of tuning the optical properties of these metamaterials is very important for several applications spanning from basic optics to applied optics for communications, depth sensing, holographic displays, and biochemical sensors. In this review, we summarize the recent progress on electro-optical magnetic, mechanical, and thermal tuning of metasurfaces actually fabricated and experimentally tested in recent years. At the end of the review, a short section on possible future perspectives and applications is included.

Temperature-controlled optical switch metasurface with large local field enhancement based on FW-BIC

Introduction: Many researchers have explored the bound states in the continuum (BICs) as a particular bound wave state which can be used to achieve a very high Q-factor. High-Q factor devices, typically based on the bound states in the continuum (BICs), are well used in the fields of hypersensitive biochemical sensors, non-linear effects enhancement, plasmon lasers, and hi-performance filtering. However, symmetrical-protected BIC is difficult to achieve experimentally high-Q factor because it strongly depends on the geometry and can be destroyed by any slight disturbance in the potential well.

Methods: Therefore, we proposed a parameter-adjusted Friedrich-Wintergen BIC based on the analysis model of time-coupled model theory, where the target system parameters can be tuned to achieve high-Q excitation.

Results: Moreover, considering the tunability and flexibility of the components in various practical applications, we integrate active materials into metasurface arrays with the help of external stimuli to achieve modulation of high-Q resonances. Our results demonstrate that an optical resonator based on FW-BIC can modulate the BIC state by changing the intermediate gap.

Discussion: The BIC state and the high-Q factor Fano resonance can be dynamically tuned by adding temperature-sensitive VO2 material.

Terahertz phase modulator based on a metal-VO2 reconfigurable metasurface

Actively controlling the phase of a terahertz (THz) wave is of great significance for beaming, tunable focusing, and holography. We present a THz phase modulator based on an electrically triggered vanadium dioxide (V O ₂) reconfigurable metasurface. The unit cell of the device consists of two split-ring resonators embedded with a V O ₂ ribbon. By electrically triggering the insulator-to-metal transition of V O ₂, the resonance mode and resonance intensity of the unit cell can be dynamically controlled. The simulation results show that the structure can achieve a phase shift of about 360° in the range of 1.03-1.13 THz, and the reflection amplitude can reach 80%. The device has potential applications in THz imaging, radar, broadband wireless communications, and array phase control.

Multifield-Controlled Terahertz Hybrid Metasurface for Switches and Logic Operations

Terahertz (THz) meta-devices are considered to be a promising framework for constructing integrated photonic circuitry, which is significant for processing the upsurge of data brought about by next-generation telecommunications. However, present active metasurfaces are typically restricted by a single external driving field, a single modulated frequency, fixed switching speed, and deficiency in logical operation functions which prevents devices from further practical applications. Here, to overcome these limitations, we propose a hybrid THz metasurface consisting of vanadium dioxide (VO₂) and germanium (Ge) that enables electrical and optical tuning methods individually or simultaneously and theoretically investigate its performance. Each of the two materials is arranged in the meta-atom to dominate the resonance strength of toroidal or magnetic dipoles. Controlled by either or both of the external excitations, the device can switch on or off at four different frequencies, possessing two temporal degrees of freedom in terms of manipulation when considering the nonvolatility of VO₂ and ultrafast photogenerated carriers of Ge. Furthermore, the "AND" and "OR" logic operations are respectively achieved at two adjacent frequency bands by weighing normalized transmission amplitude. This work may provide an auspicious paradigm of THz components, such as dynamic filters, multiband switches, and logical modulators, potentially promoting the design and implementation of multifunctional electro-optical devices in future THz computing and communication.

Terahertz Chiral Metamaterials Enabled by Textile Manufacturing

Easy-to-fabricate, large-area, and inexpensive microstructures that realize control of the polarization of terahertz (THz) radiation are of fundamental importance to the development of the field of THz wave photonics. However, due to the lack of natural materials that can facilitate strong THz radiation-matter interactions, THz polarization components remain an undeveloped technology. Strong resonance-based responses offered by THz metamaterials have led to the recent development of THz metadevices, whereas, for polarization control devices, micrometer-scale fabrication techniques including aligned photolithography are generally required to create multilayer microstructures. In this work, leveraging a two-step textile manufacturing approach, a chiral metamaterial capable of exhibiting strong chiroptical responses at THz frequencies is demonstrated. Chiral-selective transmission and pronounced optical activity are experimentally observed. In sharp contrast to smart-clothing-related devices (e.g., textile antennas), the investigated chiral metamaterials gain their THz properties directly from the yarn-twisting enabled microhelical strings. It is envisioned that the interplay between meta-atom designs and textile manufacturing technology will lead to a new family of metadevices for complete control over the phase, amplitude, and polarization of THz radiation.

Active terahertz spin Hall effect in vanadium dioxide metasurfaces

Insulator-to-metal transition induces large material property variations in vanadium dioxide (VO₂) over a broad frequency band. VO₂, therefore, has been introduced into metallic resonating structures to realize reconfigurable metadevices from microwave to optical wavelengths. Beyond enabling metal/VO₂ hybrid meta-atoms, in the THz regime metallic-phase VO₂ micro-structures can support strong electromagnetic resonances, offering great potential in active manipulation of THz radiation. In this paper, we show that VO₂ dipole antennas can be used to realize geometric phase coded metasurfaces for wave-front shaping and polarization rotation of THz waves. Moreover, we demonstrate that the corresponding efficiency of the THz spin Hall effect is closely related to VO₂'s THz electrical conductivity. In light of the dispersionless nature of the geometric phase, our study indicates that metasurfaces constructed by VO₂ subwavelength resonators are good candidates for active control of broadband THz radiation.