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

A dual functional tunable terahertz metamaterial absorber based on vanadium dioxide

A dual-functional switchable metamaterial absorber (MMA) based on vanadium dioxide (VO2), which achieves flexible switching between broadband absorption and four-band absorption by adjusting the VO2 conductivity, was proposed. The device has a broadband absorption function when VO2 is in the metal phase, and the conductivity is 3 × 105 S m-1. Numerical simulation shows that the absorption rate of the device reaches over 90% in the frequency range of 3.36-6.98 THz. The absorber exhibits polarization insensitivity and wide-angle absorption to transverse electric (TE) and transverse magnetic (TM) waves. When VO2 is in the insulator phase, and the conductivity is 3 × 102 S m-1, the device switches to a narrowband absorber with a band-efficient absorption function. Numerical simulation shows that the device has an absorption rate of 99.7% at 2.39 THz, 98.3% at 2.83 THz, 95.6% at 3.84 THz, and 96.1% at 4.61 THz. It can be used as a sensor with high sensitivity. In addition, to verify the absorption mechanism of the absorber, we introduced impedance matching theory to analyze the device. Finally, the influence of structural parameters on the performance of resonators was investigated. Through the joint action of multi-layer structures, the proposed MMA concentrates broadband and narrowband absorption functions on one device, achieving flexible switching between tasks without changing the structure. The switchable metamaterial absorber designed through simple tuning methods has broad application prospects in stealth technology and thermal emitters. It provides a wide range of ideas for the design of terahertz functional devices.

Phase distribution and circular dichroism switchable terahertz chiral metasurface

Chiral metasurfaces have many applications in the terahertz (THz) band, but they still lack modulation flexibility and functionality expansion. This paper presents a terahertz chiral metasurface with switchable phase distribution and switchable circular dichroism (CD). The metasurface unit consists of a metallic inner ring embedded in vanadium oxide and a vanadium oxide outer ring, state switching by thermal control of vanadium oxide and a change in the frequency of the incident wave. Based on the switchable phase distribution, we designed a focusing vortex beam generator with adjustable focal lengths through simulation. Based on the switching CD capability, we simulate its mode switching in near-field imaging using numerical simulation, and innovatively propose an optical encryption method. Utilizing the chiral property, we also designed dual-channel switchable holographic imaging in the same frequency band, which combined with the state change of VO2 can realize a total of 4 holograms switching. Our proposed metasurface is expected to provide new ideas for the study of optical encryption and wavefront modulation of dynamics.

Grid composite meta-surface absorber with thermal isolation structure for terahertz detection

This paper specifically focuses on the absorber, the critical component responsible for the detector's response performance. The meta-surface absorber combines two resonant structures and achieves over 80% absorptance around 210 GHz, resulting in a broad operating frequency range. FR-4 is selected as the dielectric layer to be compatible with standard printed circuit board (PCB) technology, which reduces the overall fabrication time and cost. The absorbing unit and array layout are symmetrically designed, providing stable absorptance performance even under incident waves of different polarization angles. The polarization-insensitive absorptance characteristic further enhances the compatibility between the absorber and the detector in the application scenario. Furthermore, the thermal insulation performance of the absorber is ensured by introducing thermal insulation gaps. After completing fabrication through PCB technology, testing revealed that the absorber maintained excellent absorptance performance within its primary operating frequency range. This performance consistency closely matched the simulation results.

Structures, principles, and properties of metamaterial perfect absorbers

Metamaterials are a kind of artificial material with special properties, showing huge potential for applications in fields such as infrared measurement, solar cells, optical sensors, and optical stealth. A metamaterial perfect absorber (MPA) is designed based on a metamaterial, featuring strong absorption, small volume, light weight, ultra-bandwidth, tunability and other characteristics. This paper introduces the absorption mechanism of MPAs from microwave to optical wave band, and four directions of absorber design are elaborated. Equivalent impedance matching, plasma resonance and interference effect are the main absorption mechanisms of MPA. Multiband perfect absorption, ultra-wideband and ultra-narrowband perfect absorption, polarization and angle insensitive absorption, and dynamically controllable tunable absorption are the main design aspects. Among them, the proposal of a dynamically tunable absorber realizes the dynamic absorption. Finally, the problems and challenges of metamaterial perfect absorber design are discussed.

Parity-time symmetry transition and exceptional points in terahertz metal-graphene hybrid metasurface with switchable transmission and reflection characteristics

Non-Hermitian metasurfaces provide an excellent platform for the study of parity-time (PT) symmetry transition. The exceptional point (EP) in the transition process exhibits peculiar physical phenomena and enriches the development of metasurfaces. In this study, a terahertz metal-graphene hybrid metasurface that can study PT symmetry transition and EP in transmission and reflection polarization channels is designed by using the phase transition characteristics of VO₂. The tunable asymmetric loss and PT symmetry transition can be actively controlled by changing the Fermi energy of the graphene strip. Interestingly, owing to the special chirality of the structure, the original metasurface, and the mirrored metasurface degenerate into a circularly polarized state with opposite rotations at the same Fermi energy. The π-phase mutation at EP is observed following the interaction of circularly polarized waves and the metasurface and is expected to have good application prospects in environmental monitoring and gas sensing.