Tunable Terahertz Absorber Research Articles

Total resonant terahertz absorption enabled by a large-scale adjustable admittance inverse matching in a metal groove with graphene

Abstract We propose a tunable terahertz (THz) perfect absorber based on a metal groove with graphene-loaded dielectric resonator and theoretically study its basic properties. The proposed absorber allows switching between the regimes of perfect absorption at the Fabry-Perot resonance excited near the cut-off frequency of the metal groove and almost total reflection away from the resonance by changing the Fermi energy in graphene. For this purpose, we propose a “bottom up” approach which is based on tuning the admittance of input line (the metal groove in our case) instead of the structure admittance in order to reach the perfect admittance matching condition. We demonstrate that this effect can be realized at arbitrary selected frequency in the entire THz range due to the dispersion of incoming wave in metal groove, which ensures a large-scale tunability of its characteristic admittance. As a result, the total absorption can be realized in the Fabry-Perot resonance even in a simple graphene-loaded dielectric cavity for any admittance of graphene layer, which is advantageous as compared to majority of existing THz absorbers with more complicated design.

[formula omitted] assisted temperature tunable wideband terahertz absorber as a biosensor

In this article, vanadium dioxide (V O2) based tunable wideband terahertz absorber is proposed. It consist of a gold plated ground plane and V O2 based concentric ring resonating geometry on the backside and top surfaces of dielectric material Silicon dioxide (SiO2). Its unit cell has dimension of 30 × 30×9.235μm3. It achieves polarization insensitive absorption A≥90% from 2.279 to 5.699 THz with average absorption >92% and Full Width at Half Maxima (FWHM) of 4.6131 THz. It shows incident angle stability up to θ=20° for A≥90% and up to θ=45° for A≥80%. The proposed absorber’s peak absorption can be tuned from 3% to 94% by changing V O2 conductivity dependent on temperature. As a sensor, it has high sensitivity of 697.7 GHz/RIU for RI sensing, 586.096 GHz/RIU for adrenal gland cell cancer detection and around 650.0 GHz/RIU for urine concentration detection.

Tunable wideband terahertz absorber based on single-layer patterned graphene

In this study, we propose and validate a broadband terahertz perfect absorber. The absorber has a simple structure, which consists of a gold (Au) substrate, a TOPAS dielectric layer and a patterned graphene layer. By the simulations, a broad absorption band can be obtained with a high absorption above 90% between 3.31 THz and 7.15 THz, and the relative bandwidth reaches 73.4%. Meanwhile, the absorption higher than 99% is between 3.81 THz and 6.6 THz with a relative bandwidth of 53.6%. The mechanism of the surface excited plasma (SPP) and electromagnetic dipole resonance are responsible for its almost perfect broadband absorption. By changing the Fermi energy level of the graphene layer, the flexible broadband absorption spectra can be acquired. By varying the geometrical parameters of the structure, a wider spectral range of absorption wavelengths can be realized. For both TE and TM polarization, the absorption can remain above 90% over a wide range of incidence angles. Due to the independent tunability and high absorption of the proposed device, it has great potential applications in terahertz imaging, smart absorption, detectors and communications.

Polarization-independent tunable multi-band terahertz absorber based on graphene structure to design the ultra-high sensitive biosensors

In this paper, we investigated the heterogeneous structure of a multi-band perfect absorber based on graphene in the terahertz range, benefiting from polarization independence. The proposed structure comprises three layers: copper, silicon dioxide, and an inhomogeneous graphene structure with an analyte. By altering the sub-layers dimensions and the graphene slices' geometric shape, we can modify the number of bands, quality, and absorption levels. Additionally, adjusting the chemical potential of graphene enables the customization of absorption frequencies as needed. The application of this structure in biological sensors extends to the detection of proteins, viruses, and cancer cells, as well as filtering telecommunication waves and imaging. Through geometrically shaping the graphene cuts at frequencies of 4.89 THz, 9.14 THz, and 10.76 THz, absorption values of 99.54%, 99.64%, and 98.3% have been achieved, respectively. Introducing the analyte to the biosensor structure causes a shift in absorption frequency values due to varying refractive index values in different materials. This property has been utilized for biosensor design. Within the refractive index range of biological analytes (e.g., 1.3), the first band achieved a sensitivity value of 2700 GHz/RIU and FoM = 13.08, while the second band achieved a sensitivity value of 2200 GHz/RIU and FoM = 14.02. An important characteristic of this structure is its insensitivity to polarization. Simulations were conducted using Computer Simulation Technology (CST) Microwave Studio Suite 2023.

Tunable terahertz absorber based on quasi-BIC supported by a graphene metasurface

In this paper, a tunable terahertz (THz) absorber operating at a quasi-bound state in the continuum (quasi-BIC) mode supported by a graphene metasurface is proposed. There are two graphene strips and a fully covered graphene layer in one unit cell. By breaking the symmetrical arrangement of graphene stripes, the symmetry-protected BIC transforms into a quasi-BIC mode. The reflective configuration results in high-Q absorption of the metadevice at the quasi-BIC mode with the equivalent impedance matching the impedance in free space. The change in the Fermi level of graphene can cause a frequency shift in the position of the absorption peak at the quasi-BIC mode. Benefiting from the high Q-value and narrow linewidth of the quasi-BIC, the frequency shift of the absorption peak can easily exceed its linewidth. At this time, the designed THz absorber can be used as a switch, and the “on” and “off” states are achieved by tuning the Fermi level of graphene. Under normal incidence, the modulation depth of the absorption type THz switch can reach up to 99% with the insertion loss only 0.062 dB. Within the range of incident angle inclination approaching 10°, the absorption type THz switch can still achieve more than 90% modulation depth and insertion loss below 0.1 dB. Due to the characteristics of large modulation depth, low insertion loss, and wide angle incidence, the designed tunable THz absorber has great application prospects in fields such as THz communication and THz wavelength division multiplexing.

A mid-infrared ultra-wideband polarization-independent tunable perfect absorber based on Dirac metal materials

A mid-infrared ultra-wideband tunable terahertz absorber based on bulk Dirac semimetal (BDS) is presented. It has a simple three-layer structure: a top BDS metal layer, a middle dielectric layer, and a bottom reflective metal layer. The BDS layer was designed by creating a square cavity and a long rectangular cavity in the center of the BDS rectangle. The long rectangle was then rotated by 90° to form a centrosymmetric cavity. Using CST Studio Suite software, we numerically simulate the absorption characteristics. The simulation results indicate that the absorber achieves a high absorption (>90 %) of about 47.59 THz in the range of 37.5–90 THz when the Fermi energy level is 70 meV. The average absorption exceeds 95 %. In addition, adjusting the Fermi energy level of the BDS alters the absorption bandwidth. The centrosymmetric design of the structure ensures the absorber exhibits insensitivity to different polarization modes and angles of incidence, as well as excellent absorption stability. The designed shock absorber also exhibits excellent tolerance in manufacturing, reducing fabrication challenges and enabling practical applications. In addition, our design possesses the unique ability to modulate light in the mid-infrared band. These remarkable properties position our findings with significant potential in fields such as spectral analysis, optical biosensing technology, infrared sensing, and related applications.

Dynamically tunable terahertz multi-band perfect absorber based on photosensitive silicon

A tunable narrowband terahertz absorber is proposed based on the photosensitive characteristics of silicon. When silicon is insulating without the pump beam, the absorber realizes three-frequency absorption at 0.731 THz, 1.145 THz, and 1.546 THz with absorptivity of 99.43%, 99.99%, and 99.98%, respectively. When the silicon is excited by the pump beam, it is conducting, and the absorber realizes double-frequency absorption at 0.852 THz, 1.536 THz, with 99.99% and 99.31%. The impedance matching theory explains the perfect absorption, and the electric field and surface current distributions are further discussed to elaborate the physical mechanisms. In addition, the effect of geometric parameters on the absorptivity is discussed. The absorber exhibits wide-angle absorption characteristics when light is polarized along the y-direction, and the absorptivity exhibits weak dependence on the polarization angle. The proposed absorber has promising applications in electromagnetic cloaking, narrow-band thermal radiation, and optoelectronic detection.

Adaptive impedance matching in microwave and terahertz metamaterial absorbers using PIN diodes and GaN HEMTs

Metamaterial absorbers allow electromagnetic waves to be converted into heat energy based on impedance matching. However, passive metamaterial absorbers exhibit fixed absorption characteristics, limiting their flexibility. This work demonstrates tunable microwave and terahertz absorbers by integrating adjustable resistors into the metamaterial units. First, a microwave absorber from 1 to 5 GHz was designed by embedding PIN diodes with voltage-controlled resistance. Calculations, simulations, and measurements verified two separate absorption peaks over 90% when optimized to a resistance of 250 Ω. The absorption frequencies shifted based on the resistor tuning. Building on this, a terahertz absorber was modeled by substituting gallium nitride high electron mobility transistors (GaN HEMTs) as the adjustable resistor component. The GaN HEMTs were controlled by an integrated gate electrode to modify the two-dimensional electron gas density, allowing resistance changes without external voltage terminals. Simulations revealed two absorption peaks exceeding 90% absorption at 0.34 THz and 1.06 THz by adjusting the equivalent resistance from 180 Ω to 380 Ω, and the tunable resistance is verified by DC measurement of single GaN HEMT in the unit. This work demonstrates how integrating adjustable resistors enables dynamic control over the absorption frequencies and bandwidths of metamaterial absorbers. The proposed geometries provide blueprints for tunable microwave and terahertz absorbers.

Broadband terahertz absorber based on patterned slotted vanadium dioxide

This study aims to explore broadband terahertz absorbers based on metamaterials by introducing a design featuring patterned slotted vanadium dioxide. This absorber structure includes a vanadium dioxide resonator layer, an intermediate Topas dielectric layer, and an underlying metal reflector layer. To enhance the absorption bandwidth, the vanadium dioxide resonant layer is strategically patterned to optimize impedance matching. Simulation outcomes reveal that, under positive incidence, the absorption rate exceeds 90 % within the frequency range of 1.5–4.2 THz. The absorber demonstrates a substantial absorption bandwidth of 2.7 THz, centered at 2.85 THz, corresponding to a relative bandwidth of 94.7 %. Additionally, through modulation of the conductivity of vanadium dioxide, the absorption peak can be finely tuned, spanning from approximately 2.5 %–99 %. In addition, it's angularly symmetrical and exhibits absorption properties for wide-angle incidence. The tunable terahertz absorber designed in this paper simultaneously achieves high absorption over a wide frequency range, is dynamically tunable, and has a simple structure. From the application point of view, it can provide a way to realize detection, and imaging in the terahertz range.

Graphene-based magnetically tunable multi-band terahertz absorber with switchable frequency

Graphene-based magnetically tunable multi-band terahertz absorber with switchable frequency

Adjustable graphene disk‐based THz absorber for biomedical sensing: Theoretical description

AbstractAs a basic building block, the THz wave absorber has intense imaging, sensing, and nondestructive testing applications. There are several methods for tuning THz absorbers, including electricity modulation, light modulation, mechanical tuning, using phase change materials, liquid crystal, flexible materials, MEMS technology, and thermally tuning vanadium dioxide. The choice of tuning method depends on the specific application and the desired performance characteristics of the THz absorber. In this work, we report a theoretical description of mechanically tunable THz absorber based on overlapping periodic arrays of graphene nano‐disks. The basis of this work is based on the movement of a dielectric surface covered on both sides with graphene disks. This surface moves on a fixed plane while the distance between these two surfaces is free space. Also, the fixed surface consists of a relatively thick layer of gold at the bottom, dielectric on it, and graphene disk patterns on the dielectric. Now, by moving the movable surface in the horizontal direction, it is possible to adjust the amount of absorption in different frequencies of the terahertz (THz) band. Additionally, an equivalent RLC circuit model is developed and theoretical results match with simulated data. The proposed mechanically tunable THz absorber can be exploited in various emerging applications such as sensing due to its capability of covering all of the THz gap and beyond with multiple absorption peaks.

Dynamically tunable multifunctional terahertz absorber based on hybrid vanadium dioxide and graphene metamaterials.

In this work, in pursuit of a multifunctional device with a simple structure, high absorption rate, and excellent bandwidth, a tunable broadband terahertz (THz) absorber based on vanadium dioxide (V O 2) and graphene is proposed. Due to the phase transition of V O 2 and the electrically tunable properties of graphene, the structure realizes single broadband and dual-band absorption characteristics. When graphene is in the insulating state (E f=0e V) and V O 2 is in the metallic state, the developed system has more than 90% absorption and a wide absorption band from 1.36 to 5.48THz. By adjusting the V O 2 conductivity, the bandwidth absorption can be dynamically varied from 23% to more than 90%, which makes it a perfect broadband absorber. When graphene is in the metallic state (E f=1e V), V O 2 is in the insulating state, and the designed device behaves as a tunable and perfect dual-band absorber, where the absorptivity of the dual-band spectrum can be continuously adjusted by varying the Fermi energy level of graphene. In addition, both the broad absorption spectrum and the dual-band absorption spectrum maintain strong polarization-independent properties and operate well over a wide incidence angle, and the designed system may provide new avenues for the development of terahertz and other frequency-domain tunable devices.

Design and Implementation of a Flexible Electromagnetic Actuator for Tunable Terahertz Metamaterials.

Actuators play a crucial role in microelectromechanical systems (MEMS) and hold substantial potential for applications in various domains, including reconfigurable metamaterials. This research aims to design, fabricate, and characterize structures for the actuation of the EMA. The electromagnetic actuator overcomes the lack of high drive voltage required by other actuators. The proposed actuator configuration comprises supporting cantilever beams with fixed ends, an integrated coil positioned above the cantilever's movable plate, and a permanent magnet located beneath the cantilever's movable plate to generate a static magnetic field. Utilizing flexible polyimide, the fabrication process of the EMA is simplified, overcoming limitations associated with silicon-based micromachining techniques. Furthermore, this approach potentially enables large-scale production of EMA, with displacement reaching up to 250 μm under a 100 mA current, thereby expanding their scope of applications in manufacturing. To demonstrate the function of the EMA, we integrated it with a metamaterial structure to form a compact, tunable terahertz absorber, demonstrating a potential for reconfigurable electromagnetic space.

Theoretical research on a broadband terahertz absorber for thermally controlled radiation emission based on the epsilon-near-zero mode.

In this paper, a tunable and ultra-broadband terahertz (THz) absorber is proposed. The absorber, which is built upon the conventional metal-dielectric-metal tri-layer configuration, incorporates a KCl thin film within the dielectric gap situated between the top resonator and the middle dielectric layer. The simulation indicates that the absorber effectively captures more than 90% of terahertz waves between 3.6 and 7.3 THz, achieving absorption of over 99% within the 5.8-6.9 THz range. This unique broadband absorber is enabled by the interaction of plasmon and epsilon-near-zero (ENZ) modes. Additionally, due to the utilization of VO2 in the top resonator, the designed absorber holds potential to function as a thermally controlled radiation emitter, exhibiting a high emissivity of 90.5% at high temperatures while maintaining a low emissivity of 8.2% at low temperatures. The absorber is uncomplicated and adjustable, offering great potential for use in thermal management, terahertz camouflage, and engineering insulation.

A tri-functional, independently tunable terahertz absorber based on a vanadium dioxide-graphene hybrid structure.

This paper proposes a simulated design for a versatile terahertz absorber that can be actively tuned. The absorber utilizes the unique tuning capabilities of graphene and vanadium dioxide, enabling it to alternate between ultra-broadband absorption, broadband absorption, and almost complete reflection. In the metallic phase of vanadium dioxide, coupled with a graphene Fermi level at 0 eV, the absorber achieves ultra-broadband absorption. This spans an extensive frequency range from 3.85 THz to 9.73 THz, exhibiting an absorption rate surpassing 90%. As we shift to the insulating phase of vanadium dioxide and adjust the graphene Fermi level to 1 eV, the absorber operates in a broadband absorption mode. This mode spans 2.98 THz to 4.63 THz, demonstrating an absorption rate exceeding 90%. In the insulating state of vanadium dioxide with a graphene Fermi level at 0 eV, the absorber metamorphoses into a nearly total reflector. Its maximum absorption rate is a mere 0.52%. The unique adjustability of vanadium dioxide and graphene independently enables the fine-tuning of absorption rates for both ultra-broadband and broadband absorption without encountering interference. Additionally, thanks to the central symmetry inherent in the proposed structure, the absorber exhibits insensitivity to alterations in polarization angles and remains stable under a broad range of incident angles. With these benefits, the absorber shows promising potential for applications in electromagnetic stealth, wireless communication, and so on.

A tunable terahertz broadband absorber based on patterned vanadium dioxide

A patterning vanadium dioxide (VO2) tunable broadband terahertz absorber is designed, which consists of a resonant layer of VO2 and a metallic reflector layer separated by a dielectric layer of Topas. VO2 is a phase-changing material that behaves as a metallic phase at high temperatures and an insulating phase at low temperatures. Different from conventional metal-dielectric-metal three-layer structure, the innovation of this paper is that the resonant layer adopts vanadium dioxide, which is a phase change material, not only does it increase the operating bandwidth but it also allows active tuning of the absorption amplitude by varying the temperature and hence the vanadium dioxide conductivity. By optimizing the width of the vanadium dioxide resonant layer square ring and the thickness of the dielectric layer, broadband absorption is achieved in the frequency range of 0.72–1.92 THz. Numerical simulation results show that VO2 with low conductivity then behaves as an insulating phase, and its peak absorption in the corresponding broadband absorption band is only 3%, so it can be used for the reflector. When VO2 has a high conductivity, its absorption is greater than 90% in the absorption bandwidth up to 1.2 THz. In addition, the proposed absorber is polarization insensitive under vertical incidence conditions because of the symmetry of the structure. The absorber can also maintain good absorption performance by changing the angle of incidence and polarization state. In the terahertz wave sector, the tunable terahertz absorber developed in this article has several potential applications, including detectors, sensors, stealth technologies, etc.

Dynamically tunable terahertz quadruple-function absorber based on a hybrid configuration of graphene and vanadium dioxide

A quadruple-function dynamically tunable terahertz absorber that uses a hybrid configuration of graphene and vanadium dioxide is proposed in this paper. The absorber achieves dynamic conversion of four functions in one structure: ultra-broadband, broadband, single-frequency narrowband and dual-frequency narrowband, by utilizing the electrical control properties of graphene and the phase-shifting properties of vanadium dioxide. Furthermore, the paper also reveals the physical mechanism of the proposed absorber through the electric field distribution and impedance matching theory. In addition, the influences of the Fermi energy level of graphene and the electrical conductivity of vanadium dioxide on the absorption spectra are investigated, demonstrating the structure’s dynamic tunability. Due to the above features, the designed absorber is expected to have potential applications in terahertz imaging, modulation and filtering.

Mechanically tunable multi-band terahertz absorber based on overlapping graphene nanoribbon arrays

With promising applications in broadband communication, terahertz (THz) imaging, biochemical sensing, nondestructive testing and other fields, THz absorbers have become a hot research topic in recent years. Although a number of graphene-based multi-band THz absorbers with tunable performances have been reported, the tuning methods are mostly limited to electrical tuning. Here, we report a mechanically tunable multi-band THz absorber, which is based on overlapping graphene nanoribbon arrays. By adjusting the horizontal position of the upper cover plate with graphene nanoribbon array, the frequencies of the absorption peaks can be tuned in a wide range. In addition to the absorption peaks originating from even-order lateral Fabry-Pérot resonance (LFPR) modes, there are also absorption peaks originating from inductor-capacitor (LC) oscillations. An equivalent LC circuit model is developed and theoretical results match well with simulated values. The proposed mechanically tunable THz absorber may provide a reference for tunable THz absorbers that can be applied in various fields.

A Tunable Terahertz Absorber Based on Double-Layer Patterned Graphene Metamaterials.

Graphene is widely used in tunable photonic devices due to its numerous exotic and exceptional properties that are not found in conventional materials, such as high electron mobility, ultra-thin width, ease of integration and good tunability. In this paper, we propose a terahertz metamaterial absorber that is based on patterned graphene, which consists of stacked graphene disk layers, open ring graphene pattern layers and metal bottom layers, all separated by insulating dielectric layers. Simulation results showed that the designed absorber achieved almost perfect broadband absorption at 0.53-1.50 THz and exhibited polarization-insensitive and angle-insensitive characteristics. In addition, the absorption characteristics of the absorber can be adjusted by changing the Fermi energy of graphene and the geometrical parameters of the structure. The above results indicate that the designed absorber can be applied to photodetectors, photosensors and optoelectronic devices.

A tunable broadband polarization-independent metamaterial terahertz absorber based on VO2 and Dirac semimetal

A broadband tunable metamaterial absorber with polarization and angle-insensitiveness is proposed for terahertz frequencies. The metamaterial design consists of a dielectric layer separating a periodic array of wheel-shaped vanadium dioxide (VO2) inclusions and a Dirac semimetal (DS) backplane. Numerical simulations show that the absorption performance can be flexibly adjusted from 4.3% to nearly 100% by changing the conductivity of VO2, the Fermi energy of DS, the permittivity of the dielectric spacer and the structure’s parameters. A 5.37 THz ultra-wideband absorptance over 90% can be achieved from 4.04 THz to 9.41 THz when the permittivity of the dielectric is 1.34 under normal incidence. The impedance matching theory, the Fabry–Pérot interference, and the electric field distributions are introduced to discuss the physical mechanisms of the broadband absorption. The upper edge of the absorption band is sensitive to the refractive index of the analyte, and a high sensitivity of 1.94 THz/RIU has been obtained around 4.5 THz. The proposed absorber possesses the advantages of polarization-independence and wide-angle tolerance both for TE and TM waves. The terahertz tunable broadband absorber has potential applications in terahertz energy harvesting, sensing, and modulation.