Actively Tunable THz Absorber for Switchable Operations Between Different Absorption Behaviors

In this paper, periodic arrays of concentric graphene disks and rings placed near a metallic ground coated by a dielectric substrate are proposed and analyzed by a circuit model approach (CMA) to predict the absorption of the structure in the terahertz (THz) region. As numerical examples, single-band absorbers composed of graphene disks or graphene rings and a dual-band absorber composed of both graphene disks and rings are designed, presented, and proposed. In order to validate the accuracy of the results achieved by the CMA, numerical simulations based on the finite integral technique are applied. One of the significant features of the proposed structures is the tunability of the position of the absorption peak by controlling the chemical potential of the graphene patterns. An additional advantage of the proposed absorbers is that the absorption peaks of the dual-band absorber are stable for both transverse magnetic and transverse electric polarization with incident wave angles up to 70°. The proposed absorber could be used as an essential element in real-time THz detection and sensing systems.

Terahertz tunable three band narrowband perfect absorber based on Dirac semimetal

We propose a magnetically tunable dual-band terahertz (THz) absorber by using an InAs substrate with a subwavelength zero-contrast germanium grating. The results demonstrate that the absorption peaks in this absorber can be dynamically tuned by changing the intensity and the rotation angle of the applied transverse magnetic field, which is achievable at a moderate order of magnitude of 0.1 Tesla. In addition, we investigate the distribution of magnetic field intensity and find that the magnetically tunable absorption originates from the combination of the magneto-optical effect and the guided-mode resonance effect, where the absorption peaks shift in different directions at normal incidence and oblique incidence. Furthermore, the absorption intensity of the proposed structure could reach 99% with an ultra-high Q-factor of 258. This work paves the way for actively adjustable high-resolution THz absorption or a nonreciprocal thermal emitter.

We experimentally demonstrate a dynamically tunable terahertz (THz) metamaterial absorber based on an electrostatic microelectromechanical systems (MEMS) actuator and electrical dipole resonator array. The absorption of the THz wave is mainly a result of the electrical dipole resonance, which shows a tunable performance on demand. By preforming the finite integral technique, we discovered that the central absorption frequency and the amplitude can be simultaneously tuned by the applied voltage U. Characterized by a white light interferometer and a THz time domain spectroscopy system, our THz absorber is measured to show a modulation of the central frequency and the amplitude to about 10% and 20%, respectively. The experimental results show good agreement with the simulation. This dynamically tunable absorber has potential applications on THz filters, modulators and controllers.

At present, the development of metamaterials is abundant. Scientists have developed plenty of metamaterials to meet the rich needs in medicine, military and so on. A temperature sensitive dual-band absorber made of strontium titanate(SrTiO3, STO) with stacked structure is designed, and the absorbance of double modes in the range of 0–0.25 THz exceeds 99%. The absorber is insensitive to polarization and could maintain good absorption efficiency at large angle incidence. As the temperature changes from 200 K to 400 K, the resonant frequency changes significantly, and the modulation depth exceeds 40%. The proposed thermal tunable absorber can be used in radiative heat measurement, biomedical and chemical sensing, photoelectric detection and other fields. It also can be used to make thermal tunable sensors, light modulators, Terahertz absorbers and other novel devices.

We theoretically and numerically demonstrate a tunable and wide-angle terahertz absorber, which is composed of multilayer graphene-dielectric grating and bottom metal substrate. Numerical simulation shows that the proposed absorber has the advantage of dynamically tunable range from 1.015 THz to 1.165 THz when the chemical potential of graphene increases from 10 meV to 150 meV. The absorption efficiency can reach a high value of 99%. To show the working mechanism of absorption, the near field distributions of magnetic components are presented at the absorption wavelength. We also demonstrate that the tunable range of absorption can be engineered by designing the geometry parameters. In addition, it is shown that the designed absorber can maintain the good performance of absorption over a wide incident angle from [Formula: see text] to [Formula: see text] under TM-polarization.

Tunable broadband terahertz absorber based on graphene metamaterials and VO2

In this paper, a broadband and tunable terahertz absorber based on a graphene metasurface in a sandwiched structure is introduced. A single-layered graphene patterned with hollow-out squares is applied in this design, which is continuously connected to provide convenience for electrical tuning and fabrication. Plasmonic coupling and hybridization inside the graphene pattern can significantly enhance the absorption bandwidth. Moreover, polarization-insensitive and omnidirectional performances are also guaranteed by the symmetrical design. Full wave simulations demonstrate that the absorber exhibits over 90% absorbance within 1.14∼3.31 THz with a fractional bandwidth up to 97.5%. The device reveals tunable absorbance from 14% to almost 100% by manipulating the graphene chemical potential from 0 to 0.9 eV. When the incident angle sweeps up to 55°, the absorbance remains more than 90% from 1.77 to 3.42 THz for TE polarization, while over 90% absorbance maintains around 3.3 THz for TM polarization. These superior abilities guarantee the applicability of the presented absorber in THz cloaking, tunable sensor and photovoltaic devices.

Tunable broadband THz absorber using vanadium dioxide metamaterials

Optical properties of VO2 spherical nanoparticles

Plasmonic and phase transition has been blended to gain the infrared radiative switching which is tunable with temperature or voltage supply. This is applied via vanadium dioxide, tungsten trioxide, and molybdenum trioxide as transition metal oxides (TMO). The metallic phase at high temperature or colored state contributes in magnetic polariton (MP) excitation, producing broad absorptance. The TMO-based sub-layer is integrated underneath the grating fully supporting MP resonance. In contrast, this underlayer leads to producing the narrowband absorptance originated from concept of zero contrast grating (ZCG). The zero gradient in refractive index at the output plane of the grating cause transmission of light in broad wavelength range. With introduction of reflective silver underlayer, those transmitted through the grating are reflected back. However, there exists the near-zero narrowband transmission peaks in ZCG. This undergoes transformation to narrowband absorptance. In addition, another absorptance peak can be induced due to phonon modes at insulating phase. The MP resonance at metallic phase is characterized with inductor-capacitor (LC) circuit and the narrowband absorptance peaks are characterized with phase shift from the Fabry–Perot round trip (FP-RT) eigenequation from high contrast grating (HCG). The work expands the usage of transition metal oxides in infrared region with larger contrast.

The evolution of sixth-generation wireless communications demands advanced metasurfaces capable of precise beam steering and adaptive electromagnetic (EM) wave absorption to enhance signal quality and mitigate interference. In this work, we present a programmable and reconfigurable digital THz-metasurface, engineered through the integration of metal patch, graphene wire grid, and a vanadium dioxide (VO2)-based square ring to switch between dynamic beam steering (ON-state) and EM wave absorption (OFF-state). In the former state, without physically altering the patch dimensions to create a required phase gradient in the metasurface, here the idea of digitally increasing the patch dimensions only by changing the crystallization level of the VO2 ring is proposed. Consequently, a programmable and reconfigurable coding metasurface by digitally changing the arrangement of coding sequence is achieved that can shift the mainlobe of the reflected EM wave from 0° to 40°. Remarkably, in the OFF-state, a dual-band EM wave absorption is observed based on a dual mechanism including tunable crystallinity level of the VO2 ring, and the Fermi energy of the patterned graphene. In addition, in the OFF-state, the performance of the absorber is investigated by increasing the angle of incidence. The results of detailed full-wave simulations are compared and verified by analytical modeling based on an array antenna theory and an equivalent electric circuit model. The presented structure is a strong candidate for integration into future Reconfigurable Intelligent Surfaces platforms, sensing and imaging systems in the THz band.

We report the design, characterization, and experimental demonstration of an infrared dual-band metamaterial absorber composed of simple periodically patterned structures. Experimental results show that two distinct absorption peaks of 74% and 96% are obtained, which are in reasonable agreement with the simulations. We demonstrate two absorption resonances that are derived from the mixture of magnetic and electric plasmon resonances. The dual-band absorber is polarization insensitive and the absorption peaks remain high with large angles of incidence for both transverse electric and transverse magnetic polarizations, which provide more efficient absorptions for nonpolarized or oblique incident beams.

An infrared (IR) absorber based on the metamaterial structure is proposed theoretically and numerically. The near-unity absorption can be achieved at a certain wavelength by optimizing geometrical parameters of the structure. Moreover, we can switch a single-band absorber to dual-band absorber by decreasing the thickness of top metallic layer which is perforated by an air-filled ribbon. At the same time, we confirm that the mechanism of this two absorption bands is completely different. The simultaneous effects of the magnetic resonance and the cavity resonance occur at our proposed structure. Besides according to the control of polarization direction, the absorption peaks occur at the two constant wavelengths, and the superposed value of this two absorption peaks is always close to a constant. Based on this phenomenon, a simple dual-band absorber is designed when the thickness of top shaped metallic film is relatively large. The cavity response is not the existence in this condition. These results that we obtain may provide some promising applications such as sensors, thermal imagers, and IR detectors.

Phase coexistence in the metal–insulator transition of a VO 2 thin film