An ultra-broadband and thermally tunable terahertz metamaterial absorber with high fabrication tolerance.

n a non-magnetic dielectric sphere of high-permittivity ( <20), effective magnetic response occurs as a result of the 1st Mie mode, known as the magnetic dipole resonance. This resonance produces a similar effect as split ring resonators, making it possible to use dielectric spheres as metamaterial components. In the terahertz (THz) part of the spectrum, where dielectrics with  ~100 can be found, all-dielectric metamaterials can potentially reduce absorption and provide isotropic and polarization-independent properties. In this contribution, we discuss TiO2 micro-spheres, ~1/10 of the wavelength in diameter. Such spheres are expected to support the magnetic and electric dipole resonances. To detect these resonances in a single TiO2 microsphere we use THz near-field microscopy with the sub-wavelength size aperture probe. This method allows detection of Mie resonances in single sub-wavelength spheres. Fano-type line-shape is observed in the near-field amplitude and phase spectra. The narrow line-width of the magnetic resonance and the subwavelength size of the TiO2 microspheres make them excellent candidates for realizing low-loss THz metamaterials. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Tunable broadband terahertz absorber based on graphene metamaterials and VO2

Terahertz (THz) metamaterials have attracted great attention due to their widely application potential in smart THz devices; however, most of them are fabricated on rigid substrate and thus limit the exploration of flexible THz electronics. In this paper, a flexible THz metamaterial absorber (MMA) incorporated with phase change material vanadium dioxide (VO2) is proposed. The simulation results indicate that two absorption peaks at around 0.24 THz (marked as A) and 0.46 THz (marked as B) can be observed by designing a I-shaped metamaterial combined with split ring structure. The strong absorption over 92% at 0.24 THz is bending-insensitive, but the absorption at 0.46 THz is bending-sensitive, across the bending angle in the range of 0–50 degrees. Moreover, dynamic modulation of the absorption can be achieved across the insulator-metal phase transition of VO2. Particularly, the absorption of the A-peak can be tuned from 99.4% to 46.9%, while the absorption of the B-peak can be tuned from 39.6% to 99.3%. This work would provide significance for the design of flexible THz smart devices.

The study develops a dual-tunable terahertz (THz) perfect absorber by utilizing optical pumping to alter the conductivity of photosensitive silicon (Si) and temperature-controlled conductivity of vanadium dioxide (VO2). The photogenerated carrier effect allows for the modulation of the Si's electrical conductivity when its photosensitive surface is illuminated with a specific wavelength pump light. This enables the dynamic switching of the absorber's properties from narrow-band to broadband absorption. Without pump light excitation, the absorber exhibits dual narrow-band absorption, with two distinct peaks at 12.3 THz and 14.2 THz. Notably, the absorption rate at 14.2 THz exceeds 99%, corresponding to a high quality Q-factor of 710. The application of the pump light leads to a significant increase in the Si's conductivity, which in turn switches the absorber to a broadband absorption mode. In this mode, an absorption bandwidth of 1.5 THz is achieved, with an average absorption rate of 96.2%. To understand the underlying mechanism, we employed three methods: impedance matching theory, electric field distribution analysis, and multipolar scattering decomposition. The narrow-band absorption without pump light is primarily attributed to electric dipole and multipole resonances generated by the combined action of Si and VO2, as well as specific EQ and TD mode resonances. The increased Si conductivity, when pump light is present, promotes broadband impedance matching between the device and free space. This effect also leads to the formation of new resonant cavities, which results in broadband absorption. The study systematically examined how structural parameters, incident angle, and the environmental refractive index affect the absorption performance. The results show that the system exhibits excellent wide-angle characteristics in the broadband mode with pump light, while it shows angle-sensitive characteristics in the narrowband mode. In the narrow-band absorption mode, it shows high sensitivity to changes in the environmental refractive index, confirming its potential for use as a THz sensor. These findings provide a novel design approach and a solid experimental foundation for creating THz functional devices that are high-performance, multifunctional, and tunable.

Terahertz (THz) broadband metamaterial absorbers with tunable absorption characteristics have garnered significant interest because of their high demand and application in cutting-edge optical systems. On top of that, the resurgence of graphene has presented researchers with new opportunities to advance high-performance metamaterial devices, particularly in the realm of absorption applications. This paper unveils a neoteric design for a broadband tunable graphene metamaterial absorber incorporating glass patterned on graphene in triple rectangular split ring and plus fashion. Systematically performing numerical simulations, our proposed metamaterial absorber (MMA) has been found to exhibit an absorption bandwidth of 4.26 THz, ranging from 0.69 THz to 4.96 THz, with absorptivity exceeding 90%. Through the manipulation of the chemical potential of graphene within the range of 0.1 eV to 0.7 eV, along with the adjustment of the dimensions of the structure and polarization angles, it becomes feasible to dynamically modify and improve the maximum absorption. A transmission line equivalent circuit model is used to calculate the impedance components of all layers, which is crucial for determining impedance. The optimized design configuration, relative bandwidth, and tunable capability of this absorber offer a promising avenue for the development of high-speed optical switches and there are numerous applications in the field of terahertz technology such as solar energy harvesting, stealth technology, imaging, and cloaking.

Monocrystalline titanium dioxide (TiO2) micro‐spheres support two orthogonal magnetic dipole modes at terahertz (THz) frequencies due to strong dielectric anisotropy. For the first time, we experimentally detected the splitting of the first Mie mode in spheres of radii m through near‐field time‐domain THz spectroscopy. By fitting the Fano lineshape model to the experimentally obtained spectra of the electric field detected by the sub‐wavelength aperture probe, we found that the magnetic dipole resonances in TiO2 spheres have narrow linewidths of only tens of gigahertz. Anisotropic TiO2 micro‐resonators can be used to enhance the interplay of magnetic and electric dipole resonances in the emerging THz all‐dielectric metamaterial technology. image

Terahertz (THz) absorbers with broad bandwidth and high absorption are significant interest for applications in electromagnetic shielding and photonic devices. In this work, we propose a broadband THz metamaterial absorber consists of square-pyramid structure with optical transmittance, composed of liquid water, polyethylene terephthalate, and indium tin oxide. The continuously varying square-pyramid configuration promotes multiple reflections and electromagnetic field superposition, in combination with the strong intrinsic THz absorption of water, which leads to efficient THz absorption. As a result, the proposed absorber achieves absorption exceeding 90% from 0.5 to 10 THz, over 95% from 0.65 THz to 10 THz, and over 99% from 1.2 THz to 10 THz. In addition, the absorber exhibits polarization-insensitive behavior and maintains a broadband absorption bandwidth of 7 THz with absorptance above 90% for incident angles up to 70°. These features highlight the proposed design as a promising platform for broadband THz absorption in practical applications where visible-band optical transmittance is desirable.

We present a multitasking tailored device (MTD) based on phase change material vanadium dioxide (VO2) and photoconductive semiconductor (PS) in the terahertz (THz) regime, thereby manipulating the interaction between electromagnetic waves and matter. By altering the control multitasking device, its room temperature, or pump illumination, we switch the function of absorption or polarization conversion (PC) on and off, and realize the tuning of absorptivity and polarization conversion rate (PCR). Meanwhile, the construction of cylindrical air columns (CACs) in the dielectric provides an effective channel to broaden the absorption bandwidth. For the MTD to behave as a polarization converter with VO2 pattern in the insulating phase (IP), exciting the PS integrated to the proposed device via an optical pump beam, the PCR at 0.82-1.6 THz can be modulated continuously from over 90% to perfectly near zero. When the PS conductivity is fixed at 3×104 S/m and VO2 is in the metal phase (MP) simultaneously, the MTD switched to an absorber exhibits ultra-broadband absorption with the absorptivity over 90% at 0.68-1.6 THz. By varying the optical pump power and thermally controlling the conductivity of VO2, at 0.68-1.6 THz, the absorbance of such a MTD can be successively tuned from higher than 90% to near null. Additionally, the influences of the polarization angle and incident angle on the proposed MTD are discussed. The designed MTD can effectively promote the electromagnetic reconfigurable functionalities of the present multitasking devices, which may find attractive applications for THz modulators, stealth technology, communication system, and so on.

To solve the problems of single absorption function and the complex structure of terahertz absorbers, this study proposes a terahertz (THz) absorber based on vanadium dioxide (VO2) driven by electric dipole resonance, which can achieve wideband and narrowband absorption conversion. Simulation results indicate that in the narrowband absorption mode, two narrowband absorption peaks were observed at 14.6 THz and 16.8 THz respectively, and the maximum absorption exceeds 99% at the 14.6 THz frequency. In the broadband absorption mode, the absorber achieves a perfect absorption bandwidth (≥99%) of 4.6 THz, while the bandwidth with absorption exceeding 90% extends to 6.9 THz. The average absorption within the 6.9 THz range is approximately 98.29%. The investigation demonstrated that the perfect absorption effect originates from polarization resonance along with surface plasmon excitation on the VO2 surface under the impact of incident waves. Additionally, the symmetric design of our absorber ensures polarization insensitivity under normal incidence conditions and maintains excellent absorption performance over a wide range of incident angles. Following an analysis of the impact of the air refractive index on the absorber, our findings reveal that the absorber demonstrates both high refractive index sensitivity and remarkable tuning capabilities. This design has broad application prospects and can be utilized in thermal absorbers, terahertz sensors, and detectors.

Tunable terahertz perfect absorber using vanadium dioxide-based metamaterial for sensing applications

An actively reconfigurable broadband terahertz (THz) metamaterial functional device based on the phase-change material vanadium dioxide (VO2) and two-dimensional graphene material is theoretically proposed and demonstrated. The device has excellent tolerance under oblique incidence. When the VO2 is in the metallic state, and the Fermi energy of graphene is fixed at 0.1 eV, the designed device acts as a broadband THz absorber in the transverse magnetic (TM) polarization mode. The absorptance bandwidth exceeds 0.55 THz with a complete absorption intensity of more than 90%. In this state, the absorber operates as a broadband modulator with the total modulation depth exceeding 91.5% as the continually decreased conductivity of VO2 from 200000 S/m to 10 S/m. In the transverse electric (TE) polarization process, the structure behaves as a dual-band absorber with two perfect absorption peaks. When the conductivity of VO2 is changed, the tunable absorber can also be regarded as an absorptance modulator, with a maximum modulation intensity of 92.1%. Alternatively, when VO2 behaves as an insulator at room temperature in the TE polarization mode, a strong broadband electromagnetically induced transparency (EIT) window is obtained, with a bandwidth exceeding 0.42 THz in the transmittance spectrum. By varying the Fermi energy of graphene from 0 to 0.9 eV, the EIT-like window or broadband transmission spectrum (in TM mode) can be switched. The results indicate that the device can also be operated as a modulator in the transmission mode. The impedance matching theory is used, and electric field distributions are analyzed to quantify the physical mechanism. An advantage of the manipulation of the polarization angle is that the modulation performance of the proposed multi-functional THz device can be regulated after fabricated.

Multi-mode tunable absorber based on graphene metamaterial

This paper presents a graphene-based tunable polarization-insensitive metamaterial absorber (MMA) at terahertz (THz) frequencies. The absorber consists of top patterned gold (Au) layer followed by single layer of graphene, dielectric spacer, and Au layer at bottom. The proposed MMA demonstrates multi-band absorption with the characteristics of both broad- and dual-band absorption by optimizing dimensions (parametric analysis). Broad-band absorption reaches over 90% for the range of 4.57–6.45 THz with the relative absorption bandwidth of 34%, and the absorption peak at 6.86 and 7.20 THz having 98.9 and 95.2% absorption. The normalized impedance and constitutive electromagnetic parameters of the MMA are calculated using the Nicolson–Ross–Weir (NRW) method to validate the absorption rate. Furthermore, proposed absorber is polarization-insensitive upto 90° for transverse electric wave. The tunable characteristic of MMA is achieved by tuning the Fermi energy of graphene with the application of bias voltage. Accordingly, the proposed multi- and broad-band absorbers find its potential applications in spectroscopy detection, imaging, and sensing.

In recent years, a great deal of effort has been made to create terahertz (THz) devices with metamaterials (MM). Among these devices, perfect absorbers and modulators are highly desired in THz system such as communication, remote sensing, imaging, and astronomy. MM absorbers have a variety of potential applications including thermal emitters, detector, stealth technology, phase imaging. High-speed modulators, which allow actively control the spatial transmission (reflection) of an incident THz wave, is the key components of an advanced communication system. In this presentation, we firstly introduce the basic structure and work principle of MM devices, and then we report our recent progress in the design, fabrication and and measurement of the quite a few new MM devices. And as a close to this presentation, we demonstrate the application of our devices in Terahertz communication system.

Terahertz (THz) has the characteristics of non ionization, penetration, water absorption, high resolution, etc. It has shown an important application prospect in many fields, such as non-destructive testing, imaging and communication. However, THz is in the transition frequency band ranges from macro-electronics to micro-photonics, so, it belongs to the interdisciplinary field, forming the “terahertz gap” in electromagnetic wave. In recent years, with the continuous development and improvement of THz radiation source and detection technology, the THz modulation technology has gradually aroused the interest of researchers. Metamaterials with many properties that natural materials do not possess provide a common way to control THz. The two-dimensional structure of a metamaterial is called a metasurface. The coding metasurface encodes the phase digitally so that the electromagnetic wave can be regulated. It is proposed that it is first in the microwave band and then extended to the THz band. In the microwave band, the number, direction and amplitude of the far-field beams can be changed dynamically by programming, which is connected with the integrated circuit such as FPGA by using diodes, but due to the limitation of diode size and micro-nano manufacturing technology, the programmable metasurface in microwave band cannot be well used in THz band. In order to improve the flexibility of THz coding metasurface, in this paper we choose the phase change material vanadium dioxide (VO<sub>2</sub>) to active modulation coding metasurface. In this paper, we analyze the VO<sub>2</sub>’s insulating state before the phase transformation and metallic state after the phase transformation. Then designing an active control 1 bit coding metasurface by using the influence of the two states on the amplitude and phase of the unit structure, which is composed of VO<sub>2</sub>, polyimide and aluminum, can not only realize the basic function of coding metasurface adjusting the electromagnetic wave beams, but alsoimplement the switching of two kinds of far-field beams at 1.1 THz for the same coding sequence by thermal stimulated VO<sub>2</sub>. The coding metasurface also realizes the switching between two near-field focal points at 1.1 THz for the same coding sequence. Based on the effect of VO<sub>2</sub> on the phase, this coding metasurface provides a new way to adjust and control the THz wave flexibly, and will have a great application prospect in THz transmission, imaging and communication.