An active controllable wide-angle and ultra-wideband terahertz absorber/reflector based on VO2 metamaterial.

The use of metamaterials in the design of optics is an important strategy for controlling light fields. Numerous terahertz metamaterial devices have been recently designed; however, their performance is relatively limited. Here, the thermally induced phase change characteristics of vanadium dioxide (VO2) were harnessed to design a perfect wide-angle and ultra-wideband switchable terahertz absorber/reflector with a simple structure and three layers from top to bottom (VO2, SiO2, and Au). The absorption mechanism based on the impedance matching theory and electric field distribution was investigated, and the influence of structural parameters on the absorption rate and performance of the absorber in a wide wave vector range were analyzed. The study findings showed that the device perfectly absorbed a bandwidth of over 6.0 THz (absorption >90%). The absorption (reflection) was modulated from 0.01 to 0.999 with the change of the background temperature. More importantly, the device could switch between complete ultra-wideband reflection and perfect absorption over a wide angle range. This study provides important insights into the design of terahertz functional devices.

Controlling the spin current around the rectangular cavities in two-dimensional topological insulators.

Controlling spin current in topological insulators (TIs) is a crucial requirement for applications in quantum computing and spintronics. Using the non-equilibrium Keldysh Green's function formalism, we demonstrate that such control can be established around rectangular cavities of two-dimensional TIs by breaking their time reversal symmetry via exchange magnetic fields and magnetic defects. In the presence of magnetic defects with xy symmetry or Ising symmetry, the density of states is localized, and the spin current forms a current loop around the rectangular cavity in TIs interfacing with two ferromagnetic stripes. We also observe that the spin direction of the traveling electrons is inverted under the reversal of bias and gate voltages. The change in the spin-polarized current around the cavities is predicted by varying the strength of Rashba spin-orbit coupling. This result allows for the creation and control of nearly fully spin-polarized currents with various spatial patterns around the cavities in TIs, and the design of tunable spin diodes for highly integrated spintronics.

Terahertz absorber based on vanadium dioxide with high sensitivity and switching capability between ultra-wideband and ultra-narrowband.

The terahertz perfect absorber can be applied in the control, sensing and modulation of optical fields in micro- and nanostructures. However, they are only single function, complex device structure and low sensing sensitivity. Based on this, by introducing the bound state in the continuum (BIC) with infinite quality factor and field enhancement effect, and taking advantage of the phase transition characteristics of vanadium dioxide (VO2), we designed a terahertz perfect absorber device which can actively switch between ultra-wideband and ultra-narrowband. The absorption mechanism is explained by multipole analysis theory, impedance matching theory and electromagnetic field distribution. The broadband absorption is mainly due to the electric dipole resonance on metallic VO2 materials, and the absorption is more than 99% across 3.64-6.96 THz, and it has excellent characteristics such as robustness. Ultra-narrowband perfect absorption has a quality factor greater than 2200 due mainly to the implementation of symmetrically protected BIC with a sensing sensitivity of 2.575 THz per RIU. Therefore, this research could be widely used in the fields of integrated optical circuits, optoelectronic sensing and perceptual modulation of energy, as well as providing additional design ideas for the design of terahertz multifunctional devices.

Structural Design of Dual-Type Thin-Film Thermopiles and Their Heat Flow Sensitivity Performance.

Aiming at the shortcomings of the traditional engineering experience in designing thin-film heat flow meters, such as low precision and long iteration time, the finite element analysis model of thin-film heat flow meters is established based on finite element simulation methods, and a double-type thin-film heat flow sensor based on a copper/concentrate thermopile is made. The influence of the position of the thermal resistance layer, heat flux density and thickness of the thermal resistance layer on the temperature gradient of the hot and cold ends of the heat flow sensor were comprehensively analyzed by using a simulation method. When the applied heat flux density is 50 kW/m2 and the thermal resistance layer is located above and below the thermopile, respectively, the temperature difference between the hot junction and the cold junction is basically the same, but comparing the two, the thermal resistance layer located above is more suitable for rapid measurements of heat flux at high temperatures. In addition, the temperature difference between the hot and cold contacts of the thin-film heat flux sensor increases linearly with the thickness of the thermal resistance layer. Finally, we experimentally tested the response-recovery characteristics of the sensors, with a noise of 2.1 µV and a maximum voltage output of 15 µV in a room temperature environment, respectively, with a response time of about 2 s and a recovery time of about 3 s. Therefore, the device we designed has the characteristic of double-sided use, which can greatly expand the scope of use and service life of the device and promote the development of a new type of heat flow meter, which will provide a new method for the measurement of heat flow density in the complex environment on the surface of the aero-engine.

Design of a Penta-Band Graphene-Based Terahertz Metamaterial Absorber with Fine Sensing Performance.

This paper presents a new theoretical proposal for a surface plasmon resonance (SPR) terahertz metamaterial absorber with five narrow absorption peaks. The overall structure comprises a sandwich stack consisting of a gold bottom layer, a silica medium, and a single-layer patterned graphene array on top. COMSOL simulation represents that the five absorption peaks under TE polarization are at fI = 1.99 THz (95.82%), fⅡ = 6.00 THz (98.47%), fⅢ = 7.37 THz (98.72%), fⅣ = 8.47 THz (99.87%), and fV = 9.38 THz (97.20%), respectively, which is almost consistent with the absorption performance under TM polarization. In contrast to noble metal absorbers, its absorption rates and resonance frequencies can be dynamically regulated by controlling the Fermi level and relaxation time of graphene. In addition, the device can maintain high absorptivity at 0~50° in TE polarization and 0~40° in TM polarization. The maximum refractive index sensitivity can reach SV = 1.75 THz/RIU, and the maximum figure of merit (FOM) can reach FOMV = 12.774 RIU-1. In conclusion, our design has the properties of dynamic tunability, polarization independence, wide-incident-angle absorption, and fine refractive index sensitivity. We believe that the device has potential applications in photodetectors, active optoelectronic devices, sensors, and other related fields.

Multifunctional Plasmon-Induced Transparency Devices Based on Hybrid Metamaterial-Waveguide Systems.

In this paper, we design a multifunctional micro-nano device with a hybrid metamaterial-waveguide system, which leads to a triple plasmon-induced transparency (PIT). The formation mechanisms of the three transparent peaks have their own unique characteristics. First, PIT-I can be switched into the BIC (Friedrich-Wintge bound state in continuum), and the quality factors (Q-factors) of the transparency window of PIT-I are increased during the process. Second, PIT-II comes from near-field coupling between two bright modes. Third, PIT-III is generated by the near-field coupling between a low-Q broadband bright mode and a high-Q narrowband guide mode, which also has a high-Q transparent window due to the guide mode. The triple-PIT described above can be dynamically tuned by the gate voltage of the graphene, particularly for the dynamic tuning of the Q values of PIT-I and PIT-III. Based on the high Q value of the transparent window, our proposed structure can be used for highly sensitive refractive index sensors or devices with prominent slow light effects.

Multi-mode surface plasmon resonance absorber based on dart-type single-layer graphene.

In this paper, a multi-mode surface plasmon resonance absorber based on dart-type single-layer graphene is proposed, which has the advantages of polarization independence, tunability, high sensitivity, high figure of merit, etc. The device consists of a top layer dart-like patterned single-layer graphene array, a thicker silicon dioxide spacer layer and a metal reflector layer, and has simple structural characteristics. The numerical results show that the device achieves the perfect polarization-independent absorption at the resonance wavelengths of λ I = 3369.55 nm, λ II = 3508.35 nm, λ III = 3689.09 nm and λ IV = 4257.72 nm, with the absorption efficiencies of 99.78%, 99.40%, 99.04% and 99.91%, respectively. The absorption effect of the absorber can be effectively regulated and controlled by adjusting the numerical values such as the geometric parameters and the structural period p of the single-layer graphene array. In addition, by controlling the chemical potential and the relaxation time of the graphene layer, the resonant wavelength and the absorption efficiency of the mode can be dynamically tuned. And can keep high absorption in a wide incident angle range of 0° to 50°. At last, we exposed the structure to different environmental refractive indices, and obtained the corresponding maximum sensitivities in four resonance modes, which are S I = 635.75 nm RIU-1, S II = 695.13 nm RIU-1, S III = 775.38 nm RIU-1 and S IV = 839.39 nm RIU-1. Maximum figure of merit are 54.03 RIU-1, 51.49 RIU-1, 43.56 RIU-1, and 52.14 RIU-1, respectively. Therefore, this study has provided a new inspiration for the design of the graphene-based tunable multi-band perfect metamaterial absorber, which can be applied to the fields such as photodetectors and chemical sensors.

Fabriction of ZnO Nanorods with Strong UV Absorption and Different Hydrophobicity on Foamed Nickel under Different Hydrothermal Conditions.

ZnO nanorods have been grown on the surface of foamed nickel by a two-step method. Firstly, a layer of ZnO seed is sputtered on the surface of the foamed nickel by magnetron sputtering, and then the hydrothermal method is used to grow ZnO nanorods at different conditions (solution concentration, reaction time and reaction temperature). The results show that the morphology of ZnO nanorods is closely related to the solution concentration, reaction time, and reaction temperature. The energy band structure formed by the foamed nickel and ZnO seed layers and the growth mechanism of ZnO nanorods are discussed. The samples are characterized by Energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and Raman spectroscopy. The absorption characteristics of samples to light are characterized by ultraviolet-to-visible (UV⁻VIS) absorption. The hydrophilicity of the samples is characterized by the static contact angle. By analyzing the performance characteristics of the samples at different conditions, we finally obtained the optimal growth parameters. At the optimal parameters, the morphology of the grown nanorods is regular, the ultraviolet band has strong absorption, and the surface of the samples forms a superhydrophobic surface.

Tunable Graphene-based Plasmonic Perfect Metamaterial Absorber in the THz Region.

The optical performance of a periodically tunable plasma perfect metamaterial absorber based on a square-square-circle array we propose in the terahertz region is analyzed in this work by the finite difference time domain (FDTD) method. We not only discuss the impact of various parameters such as period a, length L, radius R, and incident angle θ under transverse magnetic (TM)- and transverse electric (TE)-polarization on the absorption spectra of the absorber but also study the effect of the Fermi energy EF and relaxation time τ. Finally, we simulate the spectra as the surrounding refractive index n changes to better evaluate the sensing performance of the structure, producing a sensitivity S of the structure of up to 15006 nm/RIU. On account of this research, we find that the absorber is beneficial to sensors and detectors in the terahertz region.

Dual-Band Plasmonic Perfect Absorber Based on Graphene Metamaterials for Refractive Index Sensing Application.

We demonstrate a dual-band plasmonic perfect absorber (PA) based on graphene metamaterials. Two absorption peaks (22.5 µm and 74.5 µm) with the maximal absorption of 99.4% and 99.9% have been achieved, respectively. We utilize this perfect absorber as a plasmonic sensor for refractive index (RI) sensing. It has the figure of merit (FOM) of 10.8 and 3.2, and sensitivities of about 5.6 and 17.2 µm/RIU, respectively. Hence, the designed dual-band PA-based RI sensor exhibits good sensing performance in the infrared regime, which offers great potential applications in various biomedical, tunable spectral detecting, environmental monitoring and medical diagnostics.

Plasmonic Absorption Enhancement in Elliptical Graphene Arrays.

In this paper, we come up with a wavelength tunable absorber which is made up of periodically elliptical graphene arrays in the far-infrared and terahertz regions. Through simulation, we find that we can increase the length of long axis of the ellipse, raise the incidence angles of TM- and TE-polarization (TM- and TE-polarization indicate the direction of the incident electric field along the direction of the x and the y axis, respectively.) within certain limits, and increase the chemical potential of graphene, so as to enhance the absorption of light in the elliptical graphene arrays. We also compare the absorption spectra of the original structure and the complementary structure, and find that the absorption of the original structure is higher than that of the complementary structure. In the end, we study the changes in the absorption rate of the double layer structure of the elliptical array with the increase in the thickness of SiO2. The elliptical array structure can be applied to tunable spectral detectors, filters and sensors at far-infrared and terahertz wavelengths.