Incident THz Wave Research Articles

Active Control of Temperature‐Sensitive GaN‐Graphene van der Waals Heterojunctions Integrated Metasurfaces: A Platform for Multifunctional Micro–Nanophotonic Devices

AbstractVan der Waals (vdW) heterojunctions composed of GaN/graphene have high transmittance and excellent carrier transport properties. The combination of multidimensional hybrid heterojunctions with metasurfaces can open up many fascinating prospects for novel optical components over a broad range of the electromagnetic spectrum. This work experimentally demonstrates a multifunctional temperature‐sensitive meta‐device based on GaN/graphene vdW heterojunctions integrated with a metasurface. Notably, it is discovered that the conductivity of the vdW heterojunctions increases rapidly when it is excited by a thermal signal, resulting in a significant change in the relative phase retardation as well as amplitude modulation of an incident THz wave. Then a continuous wavelet transform is used instead of the traditional Fourier transform, and the two‐dimensional wavelet coefficient card is built to achieve fast detection over a wider range of temperature. Simultaneously, the variation of temperature dynamically controls the contributions of the multipoles, eventually determining the active switching of exotic anapoles with extreme non‐radiative confinement to highly radiative electric dipoles. This work offers the possibility of designing novel chip‐scale multifunctional thermal tuning devices and promotes the potential application of active micro‐nanophotonic devices in temperature sensors, terahertz modulators, and dynamic near‐field imaging.

Multi-mode coupling in a H-shaped metamaterial structure in terahertz frequency

Mode coupling can not only effectively control the frequency, amplitude and linewidth of the transmission spectrum, but also improve the Q-factor of the spectrum. Herein, we propose a H-shaped metamaterial, in which the dipole mode, LC mode and lattice mode can be excited selectively, and each mode frequency can be independently tuned by changing the polarization of incident THz wave as well as the lattice constant of the metamaterial structure, thus allowing greater degrees of freedom to customize the polarization components of different properties in the system. Under different polarization directions, the strong coupling between the lattice mode and the inductance-capacitance (LC) mode as well as the lattice mode and the dipole mode is realized, which makes the transmission resonance Q-factor of the hybrid state increase to 8 times that of the single resonance state, and the obvious anti-crossing phenomenon is observed. In addition, the LC mode and the dipole mode can be excited simultaneously, and the coupling between these two modes successfully excites a bound states in the continuum with an infinite Q-factor.

Dual-frequency switchable bandpass filter in the terahertz range based on enhanced trapped-mode resonances

We propose a high-quality and mechanically tunable dual-band narrow bandpass filter working in the THz range. Its high transmittance and modulation depth are verified both by simulations and experiments. The proposed design consists of two metasurfaces with concentric fabricated meta-atoms back-to-back on the two sides of a thin polyimide layer. When the polarization of the incident THz waves changes from y to × direction, the modulation depth of transmittance monotonically changes from 95.46% to a few percent at the resonance frequencies of 0.481 THz and inversely varies from a few percent to 97.62% at the resonance frequency of 0.931 THz. The analysis of in-plane and inter-planar distributions of induced surface current reveals that the narrow bandpass response is attributed to the excitation of “trapped mode”. The induced antiphase current excitation between the two metasurfaces not only lower the dipole moment but also weakly couple to external electromagnetic fields, consequently, leading to the high transmittance and narrow bandwidth. These features of such a simple device potentially can contribute to the design of much more advanced THz modulators and other THz applications in the future.

Two directional THz wave duplexer and filter based on graphene disks and rings

AbstractCoating both sides of a dielectric spacer with graphene patterns, leads to a tunable meta surface which acts as a THz duplexer. Two separate channels, reflection and transmission are considered as outputs ports while the aim is to minimize absorption ration. The presented structure consists of periodic arrays of graphene patterns on both sides of silicon dioxide as substrate. Moreover, the patterns include disks and rings are biased differently compared to conventional patterns which makes it possible to achieve tunable behaviors versus external bias. utilizing equivalent circuit models (ECM) for graphene patterns and dielectric, the whole structure is modeled by passive RLC branches. According to the simulation outputs, the presented structure can transmit and reflect incident THz waves at desired frequencies in 0.1 THz to 30 THz which makes it an ideal candidate for manipulating THz waves in terms of transmission and reflection. The THz duplexer can transmit and reflect incident waves at 1–3.3 THz as wide‐band and 1.05 THz as narrow‐band operation. Such a tunable device is in great demand for indoor communications, security, less harmful medical imaging and space related researches.

Triple-Mode Switchable Terahertz Metamaterial Absorber with Tunable Absorption Characteristics

Dynamically tunable terahertz metamaterial absorbers integrating with active materials have been widely explored. However, there are still problems that need to be urgently solved, such as modulation depth deficiency, a lack of multiparameter-tunable characteristics, and so on. In this paper, a multiparameter-tunable terahertz absorber composed of two concentric double-opening resonant rings is proposed. Semiconductor silicon and germanium are introduced to fill the openings, so that the absorber possesses three different absorption modes. Regulating semiconductor conductivities using different pump lasers allows dynamic switching among the three absorption modes to be realized. Adjusting the polarization angle of incident THz waves through device rotation facilitates easy and convenient modulation of the absorption amplitude. Calculation results show that the maximum modulation range for amplitude is 0 to 90.1%. Thus, due to the existence of two regulatory degrees of freedom, absorption mode switching and amplitude modulation are realized simultaneously. Most importantly, continuous modulation of the absorption amplitude is obtained at every resonant point in all three absorption modes without frequency drift. This scheme provides a new perspective for exploring future terahertz absorbers.

Polarization-State Modulation in Fano Resonant Graphene Metasurface Reflector

Although Polarization Modulation offers a high spectral efficiency, it has been not exploited much in modern communication systems because of the involvement of high switching speeds and complexity of associated hardware. We propose a dynamically controllable graphene metasurface capable to switch the polarization state of incident THz waves in real-time. The metasurface is designed by patterning two-dimensional graphene sheets with Fano-resonant chiral unit cells. Since the reflectance spectrum is characterized with co and cross polarized fields that bear asymmetric Fano line-shapes, several combinations of linear and elliptical polarization states are observed in a narrowband spectrum. At a fixed frequency, the electrostatic tunability property of graphene allows to switch between different polarization states by varying the chemical potential between 500 and 700 meV. The polarization state modulation with the proposed chiral graphene based metasurface is applied to implement a quaternary modulated digital communication system. Full-wave simulation results show the strong possibility of polarization modulation in THz communication systems. Graphene metasurfaces on account of the high electron mobility and discrete Fermi levels offer high switching speeds and seamless integration with digital systems.

A graphene-based THz wave duplexer and filter: Switching via gate biasing

This work introduces a Graphene-based multi-layer reconfigurable device as a wave duplexer in the THz frequency range. Adjusting transmitting and reflecting parts of incident waves alongside controlling absorption provides the interesting capability to select target waves in different frequencies. The proposed device includes periodic graphene patterns on both sides of silicon dioxide as substrate. Additionally, the patterns are biased differently compared to conventional patterns which makes it possible to achieve two distinct behaviors versus frequency. Exploiting equivalent circuit models (ECM) for graphene and dielectric, the whole device is modeled by passive RLC circuits. According to simulation results, the proposed device can transmit and reflect incident THz waves at desired frequencies in 0.1–30 THz which makes it an ideal candidate for manipulating THz waves in terms of transmission and reflection.

Thermally-stable graphene metamaterial absorber with excellent tunability for high-performance refractive index sensing in the terahertz band

Graphene plasmons have attracted great interest for constructing high-performance functional metamaterial devices, among which graphene-based narrowband metamaterial absorbers are valuable in numerous applications. In this paper, a graphene-dielectric-metal hybrid structure is proposed to achieve dual-band excellent absorption and high-performance refractive index sensing. Numerical simulations show that the designed structure exhibits a high Q factor of 277.8 at 1.945 THz, which is higher than most previously-reported results. The sensitivity of the proposed structure can be up to 1.84 THz/RIU when the analyte thickness is 80 μm, and it exhibits promise for sensing thin-film biomolecule and thick biologic tissues. The electromagnetic field distributions show that the electromagnetic energy is highly confined in the structure at the higher order plasmon resonance, which plays an important role in achieving the excellent characteristics of the proposed structure. The tunability of the proposed structure is realized via changing the gate voltage applied to the graphene and the angles of the incident THz wave. In addition, we reveal that the proposed structure can be switched between single-band, dual-band, and 3-band absorption by tuning the incident angle under TM and TE polarization. Further results demonstrate that the proposed device possesses good thermal stability, polarization insensitivity, and outstanding fabrication tolerance. We believe that the proposed dual-band narrowband absorber can serve as a good example for other devices based on plasmons, such as selective sensors, modulators, and optical detectors.

Multi-band multi-tunable perfect plasmon absorber based on L-shaped and double-elliptical graphene stacks

In this paper, a multi-band multi-tunable graphene absorber structure is proposed. The absorber is made by “+” intersecting double-ellipse graphene, L-shaped graphene layer, medium layer, the metal layer, and Si substrate. By using this structure, we investigated the possibility of graphene surface plasmon resonance in the incident far-infrared wave and THz wave range. For this structure, we use the FDTD Solution of finite time domain difference software to conduct theoretical research on it. The results show that the absorbing device in the range of the infrared in λ1 = 10.78 μm, λ2 = 12.20 μm and λ3 = 21.74 μm show perfect absorption peak of absorption rate is as high as 99.97%、99.81% and 99.91%, respectively. By controlling the Fermi level or the geometric parameters of the graphene layer can dynamically adjust the resonant wavelength of absorbing device, and polarization insensitivity characteristics. And in TE or TM mode, it can maintain good absorption performance in a wide range of 0° ~ 60°. We believe that our research results will open new doors for the use of metamaterial absorbers with multi-band, multi-tunable, polarization-independent and insensitive to large-angle oblique incidence. In the future, it can be used in photoelectric detection, photovoltaic cells, photoelectric sensors, photoelectric thermal radiation and other fields.

Terahertz metamaterial resonator with tunable Fano-resonance characteristic

We present two types of tunable terahertz metamaterial (TTM) resonator. They are denoted as TTM-1 and TTM-2 for TTM resonators composed of a moveable metal frame carrying with inner disc and inner ring on Si substrates, respectively. The electromagnetic responses of TTM resonators are caused by the inductance-capacitance coupling effect between the incident THz wave into TTM structure. There is a Fano-resonance with a reflection intensity of 99% excited by electromagnetic wave within TTM-1 at 2.62 THz. By actively moving the metal frame in a specific direction, it enables a modulation of resonant frequency. The tuning ranges of TTM-1 and TTM-2 are 0.19 THz and 0.79 THz, respectively. Furthermore, TTM devices have the ability of anti-inference by moving metal frame transversely. It means TTM devices exhibit stable characteristic used in dynamic motion. Such designs of TTM configurations provide a high-efficiency THz resonator in the THz-waves applications such as filtering, switching, polarizing, sensing, and imaging fields.

Refractive index sensing characteristics of electromagnetic metamaterial absorber in terahertz band

<sec>Terahertz metamaterial (THz MM) absorber, as an important type of MM functional device, can not only achieve perfect absorption of incident THz waves, but also act as a refractive index sensor to capture and monitor changes in the information about surrounding environment. Generally, the sensing characteristics of the THz MM absorber can be improved by optimizing the structure of the surface metal resonance unit and changing the material and shape of the dielectric layer. In order to further study the influence of the intermediate dielectric layer on the sensing characteristics of the THz MM absorber, in this paper we implement three THz MM absorbers with continuous dielectric layer, discontinuous dielectric layer and microcavity structure based on the metallic split-ring resonator array, and conduct in-depth study of their sensing characteristics and sensing mechanism. </sec><sec>The THz MM absorber with continuous dielectric layer and metallic split-ring resonator array can be used as a refractive index sensor to realize the sensing detection of analytes coated on its surface with different refractive indexes. However, it can be seen from its corresponding refractive index frequency sensitivity and FOM value that the detection sensitivity of this sensor is limited, and its sensing performance still needs improving. The main reason is that most of the resonant electromagnetic (EM) field of the THz MM absorber is tightly bound in the intermediate dielectric layer, and only the fringe field extending to the surface of the MM absorber resonant unit array can interact with the analyte to be measured, and the intensity of this part of the field directly determines the sensitivity of the sensor. In order to further improve the refractive index frequency sensitivity of the THz MM absorber, reduce the restriction of the intermediate dielectric layer to the resonant EM field, and enhance the interaction between the resonant EM field and the analyte to be measured, a THz MM absorber with discontinuous dielectric layer is proposed and studied. Compared with the THz MM absorber with continuous dielectric layer, the THz MM absorber based on discontinuous dielectric layer can be used as a refractive index sensor to realize higher-sensitivity sensing and detection of the analyte coated on the surface. In order to further enhance the interaction between the resonant EM field and the analyte to be measured, and improve the refractive index frequency sensitivity of the THz MM absorber, a THz MM absorber with a microcavity structure is proposed. For this THz MM absorber, the analyte to be measured filled in the microcavity structure can serve as the intermediate dielectric layer of the THz MM absorber, and when the metallic split-ring resonator array is completely immersed in the analyte to be measured, the resonant EM field originally confined in the intermediate dielectric layer and the analyte to be measured completely overlap in space. Therefore, compared with the first two THz MM absorbers, THz MM absorber with a microcavity structure achieves the tightly and fully contacting the resonant EM field, thereby greatly improving its sensitivity as a sensor. </sec><sec>The results show that in order to improve the sensing characteristics of the THz MM absorber, such as the refractive index sensitivity and the maximum detection range, in addition to using the materials with lower relatively permittivity as the intermediate dielectric layer, the morphology of the intermediate dielectric layer can be changed, thereby reducing the restraint of the intermediate dielectric layer on the resonant field and enhancing the coupling between the resonant field and the analyte to be measured. Compared with the conventional THz MM absorber with continuous dielectric layer, the MM absorber with discontinuous dielectric layer and microcavity structure have many superior sensing characteristics, and can be applied to the high-sensitivity and rapid detection of analytes to be measured, and has a broader application prospect in the future sensing field. </sec>

Active meta polarizer for terahertz frequencies

Active meta polarizers based on phase-change materials have recently led to emerging developments in terahertz devices and systems for imaging, security, and high-speed communications. Existing technologies of adaptive control of meta polarizers are limited to the complexity of external stimuli. Here, we introduce an active terahertz polarizer consisting of a single layer of large array patterns of vanadium dioxide material integrated with metallic patch matrix to dynamically reconfigure the polarization of the terahertz waves. The proposed active polarizer is simple in structure and can independently manipulate the polarization of the incident THz waves in two orthogonal directions. In addition, the device can also be performing as a highly efficient reflector at the same frequencies. We demonstrate that efficient and fast polarization changes of THz waves can be achieved over a wide operating bandwidth. Compared with other active polarizers using mechanical, optical and thermal controls, it can be conveniently manipulated with DC bias without any external actuators, intense laser source or heater. Therefore, with the advantages of high efficiency, compact size, low loss, low cost and fast response, the proposed polarizer can be highly integrative and practical to operate within adaptive terahertz circuits and systems.

Sensitive terahertz-wave detector responses originated by negative differential conductance of resonant-tunneling-diode oscillator

We present an experimental characterization of frequency- and bias-dependent detector responses in a resonant-tunneling-diode (RTD) terahertz (THz)-wave oscillator. By tuning the incident THz-wave frequency and the bias voltage applied to the RTD device, the origins of detection signals are identified to be two distinct detection modes. One is based on square-law detection near the peak and valley points of the negative differential conductance (NDC) region, with the detection bandwidth determined by an integrated slot antenna. The other is based on detectable current changes induced by injection locking within the NDC region when the frequency of the incident THz-wave radiation to be detected is coincident with that of the bias-dependent RTD self-oscillation between 0.74 and 0.81 THz, resulting in a minimum noise equivalent power (NEP) of 7.7 pW/√Hz at 0.78 THz at room temperature. Our conclusions demonstrate that an RTD oscillator can be used as a sensitive THz-wave detector within and around the NDC region.

Investigation of graphene supported terahertz elliptical metamaterials

Abstract Based on a graphene asymmetric sdouble ellipse (ADE) structure, the tunable Fano resonance characteristics in the terahertz range have been symmetrically studied, including the effects of graphene Fermi levels, asymmetrical degrees, operation frequencies and structural parameters. The results show that due to the strong coupling between the incident THz waves and graphene ADE structures, a strong Fano peak can be observed by arranging double ellipse asymmetrically. The maximum peak of Fano resonance reaches 0.918, and its Q-factor is about 20. As the asymmetrical degree of the ADE structure increases, the Fano peak value increases, and the amplitude modulation depth reaches about 75%. As graphene level increases, the amplitude and frequency modulation depths reaches more than 31% and 11%, respectively. These results are very helpful to design novel tunable graphene devices with high Q-factor devices in the future, such as sensors, modulators, and filters.

Highly Sensitive THz Gas-Sensor Based on the Guided Bloch Surface Wave Resonance in Polymeric Photonic Crystals

THz waves have interesting applications in refractive index sensing. A THz gas sensor based on the guided Bloch surface wave resonance (GBSWR) in a one-dimensional photonic crystal (1DPhC), which consists of periodic polycarbonate (PC) layers and polyvinylidene fluoride (PVDF) layers, has been proposed. Numerical results based on finite element method (FEM) show that the photonic band gap that confines Bloch surface waves (BSWs) lies in the regime of 11.54 to 21.43 THz, in which THz wave can transmit in both PC and PVDF with the ignored absorption. The calculated sensitivity of hazardous gas HCN in angle is found to be 118.6°/RIU (and the corresponding figure of merit (FOM) is 227) and the sensitivity in frequency is 4.7 THz/RIU (the corresponding FOM is 301.3). The proposed structure may also be used for monitoring hazardous gases which show absorption to the incident THz wave. Further results show that for N2O gas, the maximum sensitivity goes up to 644 (transmittance unit/ one unit of the imaginary part of the refractive index). The proposed design may find applications in the detection of dangerous gases.

Theoretical study on the modulation characteristics of THz wave by two-dimensional black phosphorus

Using the Delude model. we theoretically calculate the dispersion of conductivity with frequency in the orthogonal direction of the two-dimensional black phosphorus (2D BP) <i>x</i> and <i>y</i> direction in the THz band. We find that the conductivity in the <i>x</i> direction is more sensitive to the electron doping concentration. The difference between 2D BP conductivities in both directions leads to the difference in dielectric constant which in turn can modulate light in different polarization directions. Using 2D BP to polarize the THz wave, the 2D BP-SiO<sub>2</sub> periodic sandwich structure is designed. The three-dimensional electromagnetic field simulation software CST Microwave Studio can be used to calculate the regulation characteristics of this structure to THz wave. It is found that this structure has different polarization directions, and the incident THz wave has different absorption. By changing the thickness of the underlying SiO<sub>2</sub> layer in the structure it is found that the absorption rate of this structure also changes accordingly. When the polarization direction of the THz pulse is parallel to the <i>x</i> axis, the absorption rate first increases and then decreases. When <i>d</i><sub>5</sub> = 9.5 μm, the absorption rate reaches 93% near 3.86 THz; when the polarization direction of the THz pulse is parallel to the <i>y</i> axis, the absorption rate gradually increases. The absorption peak has a significant red shift.

Graphene ribbon based tunable terahertz metalens for dual polarization incidences

We propose a graphene ribbon based metalens that can achieve the function of tunable metalens in THz region. Different from these graphene ribbon based metalens that have been reported before, the proposed metalens can focus the x- and y-polarized incident THz wave separately by reconfiguring the Fermi energy distribution of the graphene ribbons. The proposed metalens can also focus the incident wave at different spatial positions and different working frequencies. The simulated results are in good agreement with the designed values. Our findings are beneficial in designing new active terahertz devices.

Optimizing the electro-optic detection of terahertz waves by ZnTe at 780 and 1560-nm probe-beam wavelengths

Investigations were made for optimizing ZnTe as a THz detector at 1560-nm probe beam wavelength, and was compared with its performance at 780[Formula: see text]nm. High-sensitive THz detection was achieved when phase matching was maintained for long coherence lengths between the probe beam and the incident THz waves. A 1-mm thick ZnTe crystal enabled detection bandwidths of THz radiation up to 2 and 3 THz with probe wavelengths at 1560 and 780[Formula: see text]nm, respectively. Theoretical calculations showed coherence length of less than 50[Formula: see text][Formula: see text]m in ZnTe between the THz wave components above 4 THz and the optical waves at 780 and 1560[Formula: see text]nm.

Plasmonic resonances in sub-terahertz fishnet metamaterial based on complementary hexagonal resonator

Exploration of the perfect combination between metamaterials (MTMs) and plasmonics has paved the way of the new innovations in electromagnetic and communication engineering, especially in sub-THz spectroscopy. In order to contribute to such developments, plasmonic resonances in sub-THz fishnet MTM on the basis of complementary hexagonal resonator structure is investigated in this paper. The design, in which method of removing hexagonal structure is applied, has been performed based on finite simulation technique. Five distinct plasmonic resonances are determined at some of the specific frequency spots where interactions between electron plasmas and incident electromagnetic waves can be observed. The proposed material can be employed in several subtopics of electromagnetic engineering such as high pass filters and antenna applications. Furthermore, thanks to the THz wave confinement property of this plasmonic structure, incident THz waves can be controlled efficiently and therefore, the proposed MTM can be tailored at THz spectroscopy and imaging, localization of the THz wave and energy, sensors and photonics applications, effectively.

Metasurface for multi-channel terahertz beam splitters and polarization rotators

Terahertz beam splitters and polarization rotators are two typical devices with wide applications ranging from terahertz communication to system integration. However, they are faced with severe challenges in manipulating THz waves in multiple channels, which is desirable for system integration and device miniaturization. Here, we propose a method to design ultra-thin multi-channel THz beam splitters and polarization rotators simultaneously. The reflected beams are divided into four beams with nearly the same density under illumination of linear-polarized THz waves, while the polarization of reflected beams in each channel is modulated with a rotation angle or invariable with respect to the incident THz waves, leading to the multi-channel polarization rotator (multiple polarization rotation in the reflective channels) and beam splitter, respectively. Reflective metasurfaces, created by patterning metal-rods with different orientations on a polyimide film, were fabricated and measured to demonstrate these characteristics. The proposed approach provides an efficient way of controlling polarization of THz waves in various channels, which significantly simplifies THz functional devices and the experimental system.