Terahertz Modulator Research Articles

Reconfigurable EIT Metasurface with Low Excited Conductivity of VO2

The active materials-loaded reconfigurable metasurface is a potential platform for terahertz (THz) communication systems. However, the requirements of the modulation performance and the modulation rate put forward the opposite requirements on the excited conductivity of active materials. In this paper, we proposed a concept for a metal-doped active material switch that can produce an equivalent high excited conductivity while reducing the required threshold of the active material conductivity, thus balancing the conflict between the two mutual requirements. Based on it, we designed a reconfigurable electromagnetically induced transparency (EIT) metasurface driven by a low excited conductivity of vanadium dioxide VO2, which can achieve the amplitude modulation and amplitude coding under the control of light and electric. Simulation results validate the role of the metal-doped VO2 switch on the metasurface. This work provides a new scheme to mediate the contradiction between the modulation performance and the modulation rate in the requirement of active material’s excited conductivity, which facilitates the development of new terahertz modulators based on reconfigurable metasurfaces. In addition, the concept of a metal-doped active material switch will also provide a solution to the limitations of active material from the design layer.

All-optical broadband terahertz modulator based on CdS NWs/Si heterojunction and interface photoconductivity analysis

A high-performance, low-cost terahertz modulator is fabricated by spin-coating CdS nanowires onto silicon. This all-optical modulator achieves broadband modulation (0.4–1.6 THz) with a significant depth of 85% at a low power density (2 W cm−2), exceeding bare silicon by fourfold. THz-TDS confirms its terahertz amplitude modulation capability. Tests of the simple photonic switch further illustrate the modulator’s potential for carrying information on a terahertz transmission wave. The modulation mechanism is explained through energy band theory and photoconductivity data. This work also presents a novel method for probing heterostructure formation using THz-TDS with laser irradiation.

Perovskite polycrystal for optically tuning terahertz interference fringes

We optically tuned the terahertz interference fringe using a perovskite polycrystal. Specifically, we used CH3NH3PbI3 polycrystals with different morphologies that were annealed at different temperatures. The largest modulations depth (23 %) was obtained for the CH3NH3PbI3 polycrystal drop-coated at 130 °C; this was also greater than that of a spin-coated CH3NH3PbI3 thin film. We demonstrated that the combination of a drop-coated CH3NH3PbI3 polycrystal with a metasurface allows modulation of terahertz waves. Using the interference formed by partial-through measurement and the shift of the periodic peak in the terahertz time-domain spectroscopy system, we experimentally verified that the perovskite polycrystal can redshift the terahertz-band interference fringes by 50 GHz by changing the optical power density of the external excitation light source. This provides a new idea and possibility for the development of novel terahertz modulation devices.

Terahertz phase imaging of large-aperture liquid crystal modulator with ITO interdigitated electrode

Abstract We have fabricated and characterized a large-aperture electrooptic phase modulation device operating in the terahertz (THz) frequency range. The device consists of a 1.6 mm thick nematic liquid crystal placed between glass plates with a novel interdigitated electrode design. Using THz time-domain spectroscopy (THz-TDS) coupled with raster scanning imaging, we evaluated phase modulation across a 25 mm diameter LC device and mapped the spatial uniformity of phase shift. Our results confirm the functionality of the LC cell as a controllable quarter-wave plate at 0.26 THz and half-wave plate at 0.52 THz. This work contributes to the development of large-aperture and transmissive LC devices as low-cost phase plates for THz waves and paves the way for future applications in THz modulators.

Terahertz Modulation Properties Based on ReS2/Si Heterojunction Films

Low cost, low power consumption and high performance are urgent needs for the application of terahertz modulation devices in the 6G field. Rhenium disulfide (ReS2) is one of the ideal candidate materials due to its unique direct band gap, but it lacks in-depth research. In this work, a highly stable ReS2 nanodispersion was prepared by liquid-phase exfoliation, and a uniform, dense and well-crystallized ReS2 film was prepared on high-resistivity silicon by drop casting. The morphological, optical and structural properties of the ReS2/Si heterojunction film were characterized by OM, SEM, AFM, XRD, RS and PL. The terahertz performance was tested by using a homemade THz-TDS instrument, and the influence of different laser wavelengths and powers on the terahertz modulation performance of the sample was analyzed. The modulation depth of the sample was calculated based on the transmission curve, and the changes in the refractive index and conductivity of the sample with frequency at the corresponding laser power were calculated. The results show that the fabricated ReS2/Si heterojunction terahertz modulator can stably achieve 30% broadband modulation in the range of 0.3~1.5 THz under the low-power pumping of 1555 mW/cm2, and the maximum conductivity is 3.8 Ω−1m−1.

High-Q resonances based on BIC in graphene-dielectric hybrid double-T metasurfaces for terahertz modulation and sensing

In recent years, there has been a growing interest in bound states in the continuum (BIC) in metasurfaces. One particular area of focus is achieving high-quality (Q) factor resonance, as this is crucial for enhancing the performance of refractive index sensors. In this study, a graphene-dielectric hybrid metasurface that supports the bound states in the continuum is proposed. By varying the width of the dielectric rectangle, quasi-BIC resonances with a high Q factor can be excited, and the Q factor can reach 752724.95 and 272004.759 respectively. The analysis of multipole decomposition reveals that the two quasi-BIC resonances are predominantly influenced by the electric quadrupole and magnetic dipole, respectively. Moreover, the transmittance of the resonance point can be changed rapidly with the change of the chemical potential of graphene, so the function of modulation can be realized by changing the chemical potential of graphene. Based on these findings, we have designed a terahertz wave modulator, which exhibits modulation depths of 98.1% and 99.9% at the two resonance peaks, respectively. The corresponding chemical potential shifts are 50 meV and 0.5 eV. Additionally, we have investigated the sensing performance of the metasurface. By analyzing the magnitude of the frequency shifts of the quasi-BIC resonance peaks at different gas refractive indexes, we have determined sensitivities of 740 GHz RIU−1 and 630 GHz RIU−1 at the two resonance peaks. The maximum figure of merit (FOM) values are 132911.39 RIU−1 and 45000 RIU−1, respectively. This research serves as a valuable reference for the design of dynamic optical modulators and sensors operating in the terahertz band.

Dual-Tuned Terahertz Absorption Device Based on Vanadium Dioxide Phase Transition Properties.

In recent years, absorbers related to metamaterials have been heavily investigated. In particular, VO2 materials have received focused attention, and a large number of researchers have aimed at multilayer structures. This paper presents a new concept of a three-layer simple structure with VO2 as the base, silicon dioxide as the dielectric layer, and graphene as the top layer. When VO2 is in the insulated state, the absorber is in the closed state, Δf = 1.18 THz (absorption greater than 0.9); when VO2 is in the metallic state, the absorber is open, Δf = 4.4 THz (absorption greater than 0.9), with ultra-broadband absorption. As a result of the absorption mode conversion, a phenomenon occurs with this absorber, with total transmission and total reflection occurring at 2.4 THz (A = 99.45% or 0.29%) and 6.5 THz (A = 90% or 0.24%) for different modes. Due to this absorption property, the absorber is able to achieve full-transmission and full-absorption transitions at specific frequencies. The device has great potential for applications in terahertz absorption, terahertz switching, and terahertz modulation.

Triple-frequency terahertz modulator based on electronically controllable metamaterial from the coupling effect

This paper proposes a triple-frequency terahertz amplitude modulator that utilizes an I-shaped strip and four U-shaped metal patches within a common metal-substrate configuration. The top metal layer consists of an I-shaped strip and four U-shaped metal patches, while the bottom substrate layer is made of polyimide. Amplitude modulation is achieved through adjusting the plasma frequency of the high-electron-mobility transistors, resulting in a modulation depth of nearly 93% at resonance frequencies of 0.26 and 0.49 THz. At 0.6 THz, the modulation depth reaches 65%, demonstrating excellent performance. Resonance frequencies are determined by electric field and surface current distribution. The triple-frequency terahertz amplitude modulator is applicable in various fields, including terahertz communications and imaging.

Electronically Controlled Broadband Terahertz Amplitude Modulator Based on Ionic Liquid/PEDOT:PSS:DMSO Composite Structure

Electronically controlled modulation devices operating in the terahertz (THz) band offer a crucial foundation for the integration and minimization of THz systems. In this work, we offer a broadband THz amplitude modulator that can be electrically controlled. The modulator is based on a composite structure made of ionic liquid and polymer mixture poly (3-ethoxythiophenyl): poly (4-phenylsulfonate) (PEDOT: PSS: DMSO). By applying merely +4 V excitation and -4 V turn-off voltage, a 40% amplitude modulation depth may be produced in the frequency region of 0.1-1.5 THz due to the joint action of electrons in the polymer and ions in the ionic liquid. The measurement of the resistance change of the polymer surface provides an explanation and confirmation of the mechanism of polymer action in this occurrence. This work provides a material selection for a simple preparation, inexpensive, and effective modulation of THz amplitude for an ionic liquid gate electronic controlled THz modulator. It is also expected that this work can be helpful for the miniaturization and integration development of THz modulators.

Broadband terahertz modulation in symmetric gate-controlled graphene photonic crystals

Innovative research in terahertz (THz) communications is urgently needed to meet the evolving demands of the industry. In the past, the absence of optoelectronic devices made it difficult to control THz waves effectively. In this study, a symmetric gate-controlled graphene photonic crystal (PC) is proposed, which achieves a modulation depth (MD) of more than 99 %, an ultralow insertion loss (IL) of 0.08, and a wide operation bandwidth (BW) simultaneously. Additionally, even in graphene samples with low carrier mobilities, the high-performance MD can be achieved with acceptable IL in the broadband THz range. Furthermore, our proposed one-dimensional graphene PC system enhances manufacturing convenience. Through the adoption of the proposed symmetric structure, highly efficient THz communications can be achieved through photonic systems that utilize graphene.

Electrically driven cavity plasmons in Au nanowire over Au film

Light emission via inelastic tunneling electrons is appealing for integrated optoelectronic devices due to its femtosecond time scale that can in principle allow terahertz modulation bandwidth. It has gained renewed interest since 2015 due to the improved quantum efficiency, highly tunable emission wavelength, linewidth, or directionality once the electrodes are designed as a plasmonic nanocavity. However, efficient construction of stable tunnel junctions with desired plasmonic resonances is still technically challenging because of the subnanometer precision required in the electrical and optical design. Here, we demonstrate an easily accessible electrically driven cavity plasmon in metal-insulator-metal (MIM) tunnel junctions, comprised by a Au nanowire (NW) across two separate ultrasmooth Au electrodes. Two layers of self-assembled thiol molecule defines a reliable tunneling barrier. The contribution from the localized cavity plasmons to the total light emission is found to be dominant over that from the propagating surface plasmon polariton in the MIM waveguide, different from the traditional explanations. This work introduces a simplified method for constructing electrically driven cavity plasmons using crystalline metals, which holds promise for applications in in situ chemical or biosensing and the development of flexible light-emitting metasurfaces.

Revealing the Properties of Electrically Driven Optical Antennas via Conductive Atomic Force Microscope.

Light emission from ultracompact electrically driven optical antennas (EDOAs) has garnered significant attention due to its terahertz modulation bandwidth. Typically, the EDOAs are fixed and nonadjustable once fabricated, thus hindering the attempts to investigate the influence of structural geometry on light emission properties. Here, we propose and demonstrate that the EDOAs can be constructed by carefully manipulating the gold-coated tips of atomic force microscopy operated in conductive mode into contact with the optical antennas covered by insulating film, where the position of the tunnel junction on the antenna surface can be controlled with high accuracy and flexibility. Taking gold nanorod antennas covered by HfO2 film as an example, we found that the highest light generation efficiency is obtained when the tunnel junction is located at the shoulder edge of the nanorod antenna, where the bonding dipolar surface plasmon mode in the junction is spectrally and spatially coupled with the longitudinal radiation mode of the EDOAs. Besides, position variation of the tunnel junction on the nanorod surface also strongly influences the far-field radiation angular distribution and emission spectrum. Numerical simulations are in good agreement with the experimental results. Our findings offer fundamental insights into the influence of structural parameters on the light emission performance of EDOAs, thus leading to better design of EDOAs with high light generation efficiency and powerful functionality.

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.

Organic Electro-Optic Materials with High Electro-Optic Coefficients and Strong Stability.

The preparation of high-performance electro-optical materials is one of the key factors determining the application of optoelectronic communication technology such as 5G communication, radar detection, terahertz, and electro-optic modulators. Organic electro-optic materials have the advantage of a high electro-optic coefficient (~1000 pm/V) and could allow the utilization of photonic devices for the chip-scale integration of electronics and photonics, as compared to inorganic electro-optic materials. However, the application of organic nonlinear optical materials to commercial electro-optic modulators and other fields is also facing technical bottlenecks. Obtaining an organic electro-optic chromophore with a large electro-optic coefficient (r33 value), thermal stability, and long-term stability is still a difficulty in the industry. This brief review summarizes recent great progress and the strategies to obtain high-performance OEO materials with a high electro-optic coefficient and/or strong long-term stability. The configuration of D-π-A structure, the types of materials, and the effects of molecular engineering on the electro-optical coefficient and glass transition temperature of chromophores were summarized in detail. The difficulties and future development trends in the practical application of organic electro-optic materials was also discussed.

Graphene-based polarization insensitive structure of ultra-wideband terahertz wave absorber

Graphene, a new smart material, has gained a lot of attention recently because of its tunable band structure and strong light-matter interaction. In comparison with typical absorbers, graphene absorbers have emerged as a highly attractive substitute for various types of applications. To broaden the absorption bandwidth, a two-dimensional graphene-based ultra-wideband and polarization-insensitive novel terahertz (THz) absorber with absorption over 95 % from 3.0 THz to 8.5 THz is proposed, with peak absorption of 99.9 % at 6.6 THz and average absorption of 98.4 % for the 5.5 THz bandwidth. The surface plasmon increased by the gold-graphene structure, the Mei resonance, and the interference cancellation between metasurfaces collectively contribute to the ultra-wideband absorption. To obtain nearly perfect broadband terahertz absorption, a single layer of graphene is used. The finite difference time domain (FDTD) approach is used for absorber analysis. In addition, the absorber is insensitive to the polarization angles and incidence angles of incident electromagnetic waves. The proposed graphene-based absorber dominates previously reported wideband THz absorbers in terms of over 95 % bandwidth, absorption peak value, and overall volume. This research will present a novel idea for the design of an ultra-wideband absorber, which can be considered a suitable candidate for different potential applications in terahertz imaging, sensors, photodetectors, and modulators.

Color coded metadevices toward programmed terahertz switching

Terahertz modulators play a critical role in high-speed wireless communication, non-destructive imaging, and so on, which have attracted a large amount of research interest. Nevertheless, all-optical terahertz modulation, an ultrafast dynamical control approach, remains to be limited in terms of encoding and multifunction. Here we experimentally demonstrated an optical-programmed terahertz switching realized by combining optical metasurfaces with the terahertz metasurface, resulting in 2-bit dual-channel terahertz encoding. The terahertz metasurface, made up of semiconductor islands and artificial microstructures, enables effective all-optical programming by providing multiple frequency channels with ultrafast modulation at the nanosecond level. Meanwhile, optical metasurfaces covered in terahertz metasurface alter the spatial light field distribution to obtain color code. According to the time-domain coupled mode theory analysis, the energy dissipation modes in terahertz metasurface can be independently controlled by color excitation, which explains the principle of 2-bit encoding well. This work establishes a platform for all-optical programmed terahertz metadevices and may further advance the application of composite metasurface in terahertz manipulation.

Self-Powered Terahertz Modulators Based on Metamaterials, Liquid Crystals, and Triboelectric Nanogenerators.

6G communication mainly occurs in the THz band (0.1-10 THz), which can achieve excellent performance. Self-powered THz modulators are essential for achieving better conduction, modulation, and manipulation of THz waves. Herein, a self-powered terahertz modulator, which is based on metamaterials, liquid crystals (LCs), and rotary triboelectric nanogenerators (R-TENGs), is proposed to realize the driving of different array elements. The corresponding designs can achieve an integrated design and preparation method for dynamic spectrum-reconfigurable liquid crystal metamaterials. In addition, for the type of cross-structure metamaterial liquid crystal box, a phase modulation of 1 GHz is achieved at frequencies of 0.117 and 0.161 THz with modulation depths of 13 and 11%, respectively. Because the R-TENG with a multifan blade and circular electrodes can generate 18 peaks of electric output in every rotation, it can successfully provide sufficient frequency alternating-current electric energy to drive the terahertz modulator and achieve a self-powered function. Our findings lay a solid theoretical foundation for further building self-powered THz communication systems and promote the development of a theoretical system for LC-driving spectrum-reconfigurable devices in the THz domain.

MEMS-actuated terahertz metamaterials driven by phase-transition materials

The non-ionizing and penetrative characteristics of terahertz (THz) radiation have recently led to its adoption across a variety of applications. To effectively utilize THz radiation, modulators with precise control are imperative. While most recent THz modulators manipulate the amplitude, frequency, or phase of incident THz radiation, considerably less progress has been made toward THz polarization modulation. Conventional methods for polarization control suffer from high driving voltages, restricted modulation depth, and narrow band capabilities, which hinder device performance and broader applications. Consequently, an ideal THz modulator that offers high modulation depth along with ease of processing and operation is required. In this paper, we propose and realize a THz metamaterial comprised of microelectromechanical systems (MEMS) actuated by the phase-transition material vanadium dioxide (VO2). Simulation and experimental results of the three-dimensional metamaterials show that by leveraging the unique phase-transition attributes of VO2, our THz polarization modulator offers notable advancements over existing designs, including broad operation spectrum, high modulation depth, ease of fabrication, ease of operation condition, and continuous modulation capabilities. These enhanced features make the system a viable candidate for a range of THz applications, including telecommunications, imaging, and radar systems.Graphical

Simulation of ageing and wear effect on graphene THz passive components using finite element method

In the growing scenario of 2D material-based metamaterials and metasurfaces for Terahertz (THz) applications, assessing the impact of ageing and wear due to environmental stressors on the components’ performance is becoming mandatory to understand the long-term reliability of novel technologies. This paper introduces approaches to assess the ageing and wear effects on THz passive components through numerical simulations. For this purpose, common techniques for introducing 2D materials and thin metal layers in numerical models are discussed. As a case study, this work explores the effects of graphene degradation and reflective metal ageing on the electromagnetic response of a graphene-enhanced reflective grating for THz absorption and modulation by finite element (FE) analysis. The developed FE model is validated against experimental data obtained through THz Time-Domain Spectroscopy. By computing the device’s transmission, reflection, and absorption spectra for degrading graphene and metal conductive properties, this work provides insights into the influence of ageing and wear on THz passive components.

Si-CMOS compatible epsilon-near-zero metamaterial for two-color ultrafast all-optical switching

Driven by the escalating demands of advanced technologies, developing integration strategies has kept pace with the realization of ultrafast components during the past two decades. Ultrafast all-optical switches enabled by artificial materials are considered at the forefront of the next generation of photonic integration for communications and high-volume data processing. Encouraged by these advancements, applications, and interest have increased toward all-optical switches based on epsilon-near-zero (ENZ) materials. However, some all-optical switches lack CMOS compatibility, require high energy activation, and are limited in switching speed and working wavelength. Here, we propose and demonstrate a multilayered ENZ metamaterial utilizing Si-compatible titanium nitride and indium-tin-oxide materials with two effective working wavelengths in the visible and near-infrared spectrum. This device enables switching time down to a few hundred femtoseconds utilizing minimal energy at the corresponding ENZ regions induced by intraband pumping. Our approach can enhance the adaptability of designing ENZ metamaterials for new hybrid integrated photonic components for low-power ultrafast all-optical terahertz modulation.