Advanced Terahertz Research Articles

Switchable Bifunctional Chiral Metasurface Based on Vanadium Dioxide for Terahertz Waves

A switchable chiral metasurface (CM) with high polarization conversion ratio (PCR) and absorptivity based on vanadium dioxide (VO2) is proposed and numerically demonstrated in the terahertz (THz) regime. The proposed design consists of VO2–Au‐mixed resonant pattern layer, Si dielectric layer, and Au ground plate, which can achieve tunable bifunctional linear polarization conversion (LPC) and absorption (ABS) by varying the conductivity of VO2 (). For = 10 S m−1, the designed CM possesses a function of broadband LPC. More than 90% PCR is obtained in 1.79–2.46 THz, and the corresponding relative bandwidth (RBW) is 31.28%. For = 2 × 105 S m−1, the designed CM can efficiently absorb incident electromagnetic (EM) waves in 1.54–2.76 THz with the absorptivity exceeding 90% (RBW = 56.47%). The mechanisms of LPC and ABS are analyzed. The effects of geometric and EM parameters, incident angle, and polarization angle on its bifunctional characteristics are also studied. The LPC and ABS performances are maintained within a certain range of incident angles. Furthermore, the proposed CM exhibits polarization insensitivity. The developed switchable bifunctional CM has great potential application value in advanced THz research and smart device.

Graphene-Integrated Microbolometer Array Imaging System: A Novel Approach for Fast and Sensitive Terahertz Detection in Biomedical Applications.

Existing biomedical imaging modalities are often restricted by their substantial size, high costs, and potential risks associated with ionizing radiation exposure. Given these challenges, there is an urgent need for innovative imaging systems that not only excel in detection performance but are also compact, cost-effective, and ensure safety for biomedical applications. In response to these requirements, our research introduces an advanced terahertz (THz) microbolometer array imaging system (MAIS), specifically engineered for biomedical detection. At the core of this system is our novel microbolometer, which is distinguished by its unique structure and integration with graphene; its innovative design and strategic material composition substantially enhance the MAIS's efficacy. This graphene-integrated microbolometer demonstrates outstanding performance within the 1-5 THz operational bandwidth, achieving an average response time of 0.246 s, a peak responsivity of 8.95 × 105 V W-1, and an optimum detectivity of 5.97 × 108 cm Hz1/2 W-1. These exceptional metrics significantly extend our MAIS's applicability in nonionizing and noninvasive imaging, providing a robust solution for fast, sensitive, and accurate detection in biomedical contexts. This innovative study constitutes a considerable advancement in THz detection, with the potential to substantially transform the field of biomedical imaging.

Ultrabroadband THz probing of anisotropic optical conductivity and plasmonic damping in graphene nanostructures

Abstract Graphene nanostructures have garnered significant attention in the terahertz (THz) range due to their exceptional plasmonic properties. Exploring the THz conductivity of graphene nanostructures is crucial for optimizing their performance in advanced devices, necessitating comprehensive research into their optical responses across a broad frequency spectrum. In this study, we experimentally investigate the optical conductivity of graphene nanostructures using ultra-broadband THz time-domain spectroscopy. Extracted from the time-dependent THz echo signals, an order-of-magnitude enhancement of localized surface plasmon resonances (LSPRs) in graphene nanostructures was observed. This led to the anisotropic behavior in optical conductivity over the broadband THz range. In addition, the analyzed results show that LSPRs in graphene nanostructures experience damping loss from the intraband transition, resulting in energy dissipation into the substrate through the dielectric space layer. The induced redshift of the LSPR and linewidth broadening, in turn, modifies the optical conductivity of the graphene nanostructures. These findings provide valuable insights for developing advanced tunable THz devices based on graphene nanostructures.

Advanced Terahertz Graphene Biosensor for Enhanced Breast Cancer Diagnosis

Advanced Terahertz Graphene Biosensor for Enhanced Breast Cancer Diagnosis

Sparse-deconvolution terahertz near-field microprobe tomography enabling non-destructive inspection of small solid dosage forms

Terahertz (THz) pulsed imaging is a powerful tool for investigating solid dosage forms. However, traditional far-field systems struggle with physically small samples and strongly bent surfaces due to inherently limited lateral resolution. The present study introduces a novel approach using photo-conductive near-field microprobes (PC-NFMs) with a THz time-domain spectroscopy module to overcome the limitations of far-field setups concerning their achievable lateral resolution. In addition, a modified sparse deconvolution algorithm for advanced THz signal processing is presented, enabling the reconstruction of fainting interfaces in scattering media. This approach is particularly valuable for specialized dosage forms with extremely thick coatings, such as the investigated controlled-release dosage form, aiding as a worst-case scenario. While most pharmaceutical coatings are <100 µm thick, our method’s ability to analyze thicker coatings up to nearly 400 µm at an extraordinarily high spatial resolution of 87 µm in the x-direction and 175 µm in the y-direction sets it apart from approaches presented so far. This versatility makes the approach relevant for standard pharmaceutical products and niche, specialized dosage forms, including mini tablets.

Ultra-Low Threshold Resonance Switching by Terahertz Field Enhancement-Induced Nanobridge.

Ongoing efforts spanning decades aim to enhance the efficiency of optical devices, highlighting the need for a pioneering approach in the development of next-generation components over a broad range of electromagnetic wave spectra. The nonlinear transport of photoexcited carriers in semiconductors at low photon energies is crucial to advancements in semiconductor technology, communication, sensing, and various other fields. In this study, ultra-low threshold resonance mode switching by strong nonlinear carrier transport beyond the semi-classical Boltzmann transport regime using terahertz (THz) electromagnetic waves are demonstrated, whose energy is thousands of times smaller than the bandgap. This is achieved by employing elaborately fabricated 3D tip structures at the nanoscale, and nonlinear effects are directly observed with the THz resonance mode switching. The nanotip structure intensively localizes the THz field and amplifies it by more than ten thousand times, leading to the first observation of carrier multiplication phenomena in these low-intensity THz fields. This experimental findings, confirmed by concrete calculations, shed light on the newly discovered nonlinear behavior of THz fields and their strong interactions with nanoscale structures, with potential implications and insights for advanced THz technologies beyond the quantum regime.

Manipulating terahertz guided wave excitation with Fabry-Perot cavity–assisted metasurfaces

Metasurfaces are emerging as powerful tools for manipulating complex light fields, offering enhanced control in free space and on-chip waveguide applications. Their ability to customize refractive indices and dispersion properties opens up new possibilities in light guiding, yet their efficiency in exciting guided waves, particularly through metallic structures, is not fully explored. Here, we present a new method for exciting terahertz (THz) guided waves using Fabry-Perot (FP) cavity-assisted metasurfaces that enable spin-selective directional coupling and mode selection. Our design uses a substrate-free ridge silicon THz waveguide with air cladding and a supporting slab, incorporating placed metallic metasurfaces to exploit their unique interaction with the guided waves. With the silicon thin layer and air serving as an FP cavity, THz waves enter from the bottom of the device, thereby intensifying the impact of the metasurfaces. The inverse-structured complementary metasurface could enhance excitation performance. We demonstrate selective excitation of TE00 and TE10 modes with directional control, confirmed through simulations and experimental validations using a THz vector network analyzer (VNA) system. This work broadens the potential of metasurfaces for advanced THz waveguide technologies.

Tunable VO2 metasurface for reflective terahertz linear and circular polarization wavefront manipulation at two frequencies independently

In this work, a tunable VO2 metasurface (MS) is proposed and numerically demonstrated in the terahertz (THz) region, which can independently manipulate reflected linearly polarized (LP) and circularly polarized (CP) wavefronts at two frequencies independently. The unit-cell of the proposed VO2 MS consists of a metallic circular ring (CR) structure, rectangular-shaped-patch (RSP) and circular-double-split-ring (CDSR) structure VO2 adhered on a dielectric substrate by a bottom metallic ground plane. The proposed MS structure enables the reflected cross-polarization conversion of LP waves at lower frequencies (fL = 1.34 THz) and CP waves at higher frequencies (fH = 2.53 THz), respectively. By changing the external temperature, the conductivity of the VO2 RSP and CDSR structures can be adjusted dynamically. By varying the conductivity of the VO2, the proposed MS can achieve a reflected LP conversion with efficiency from 0% to 98.86% at lower frequency of 1.34 THz and the reflected CP conversion with efficiency from 0% to 81.86% at higher frequency of 2.53 THz. Moreover, the corresponding modulation depths of this MS are as high as 98.86% and 81.86% at two different frequencies, respectively. Moreover, a 2π phase shift can be realized by the transmission phase and geometric phase principles at lower and higher frequencies, respectively. Finally, two VO2 MSs have been constructed to achieve the reflection deflection for both CP and LP waves, and reflective CP focusing and LP vortex beam generation, respectively. These findings are favorable for expanding the range of temperature-controlled VO2 MS for wavefront manipulation, thereby opening up new possibilities for the development of advanced, tunable, and multifunctional THz devices that can handle both LP and CP waves.

3D Graphene Straintronics for Broadband Terahertz Modulation

AbstractThe increasing utilization of terahertz (THz) bandwidth in both industrial and private sectors highlights the significance of efficient terahertz shielding and absorption devices. These devices play a crucial role in safeguarding electronic components from disruptive effects and rendering objects less detectable by radar systems. However, the limited availability of materials and devices hinders progress in this field. In this study, a strain engineering route is presented for the active control of terahertz shielding and absorption properties in 3D graphene through the application of mechanical strain. A straintronic modulator based on 3D graphene is demonstrated, capable of modulating absorption and reflection of THz radiation in real‐time over a wide range of 0.1–3 THz. The modulator can be tuned to exhibit either shielding capability with a specific shielding effectiveness of 0.3 × 105 dB cm2 g−1 or stealth characteristics with an average reflection loss of 25 dB and 99.4% absorption. These findings open new avenues for leveraging 2D materials in their 3D porous form, where strain‐induced changes in interlayer interactions enable control over the properties of these materials. This discovery unveils vast unexplored physical phenomena with immense potential for advanced THz imaging, radar, and electromagnetic applications.

Active control of polarization-dependent terahertz surface plasmonic wave excitation using coupled graphene-metal hybrid metasurfaces

Active control of terahertz surface plasmonic wave (SPW) intensity in the propagation direction holds substantial significance for advanced terahertz functional devices. In this study, we propose a graphene-metal hybrid split-ring slit resonator (SRSR) array metasurfaces and employ the concept of electromagnetically induced transparency (EIT) to achieve excitation of asymmetric SPWs under various polarization states. By individually integrating two graphene ribbons into the two split-ring slit gaps and applying different bias voltages, we observed a gradual transition in the excitation behavior of asymmetric terahertz SPWs, ultimately resembling that of a single SRSR. Near-field simulations reveal that this phenomenon is attributed to the short-connection effect of graphene. Our proposed graphene-metal active hybrid metasurface introduces a novel approach to realize active SPWs devices, holding potential applications in terahertz on-chip communication.

Sub-terahertz excitations in a synthetic antiferromagnet with perpendicular anisotropy

In this paper, micromagnetic simulations are employed to investigate terahertz (THz) magnetic excitations in a spin torque nano-oscillator (STNO) with a perpendicularly magnetized synthetic antiferromagnetic (SAF) free layer. The magnetization precession of the free layer can be finely tuned into the sub-THz range without the necessity of external magnetic fields. The excited frequency exhibits two distinctive regions, namely region-I and region-II, depending on the applied current strength. In region-I, characterized by relatively small currents, the two ferromagnetic layers are stabilized at two separate precession orbits. The frequency in this region decreases with current strength, exhibiting similar features as the Néel vector change observed in antiferromagnets. In contrast, region-II is defined by currents where the two ferromagnetic layers synchronize into the same precession orbit. The frequency increases with current, correlating with the variation in the net magnetization of the SAF layer. An analytical model is developed through the canonical transformation of Lagrange’s equation, which can describe the frequency dependence on both the applied current and the antiferromagnetic interlayer coupling strengths. The simulations and the analytical model show good agreement, offering a more profound understanding of the magnetic excitation properties in STNOs with ultrathin SAF free layers. These insights are crucial for the design of advanced terahertz spintronic devices.

Tunable broadband terahertz beam splitting using gated graphene metasurfaces [Invited

Active control of split ratios in terahertz (THz) beam splitters holds substantial potential for applications in imaging and spectroscopy. In this study, we introduce an approach for electrically controlling THz beam splitting with near non-dispersive characteristics, using the metasurface hybridized with single-layer graphene. Specifically, the resonant frequency of the metasurface is meticulously chosen to provide an enhanced effective conductivity that is almost frequency-independent. Furthermore, the split ratio can be manipulated by adjusting the gate voltage applied to graphene, thereby altering the effective conductivity of the hybrid metasurface without affecting its non-dispersive transmission and reflection characteristics. The feasibility of our approach is confirmed by the near non-dispersive split ratio across a wide operating frequency range, from 0.5 to 1.5 THz. The demonstration of this efficient, broadband, and tunable beam splitting ability validates the potential of gated graphene metasurfaces for advanced THz applications.

Enhancement of spintronic terahertz emission enabled by increasing Hall angle and interfacial skew scattering

Spintronic terahertz (THz) emitters (STEs) based on magnetic heterostructures have emerged as promising THz sources. However, it is still a challenge to achieve a higher intensity STE to satisfy all kinds of practical applications. Herein, we report a STE based on Pt0.93(MgO)0.07/CoFeB nanofilm by introducing dispersed MgO impurities into Pt, which reaches a 200% intensity compared to Pt/CoFeB and approaches the signal of 500 μm ZnTe crystal under the same pump power. We obtain a smaller spin diffusion length of Pt0.93(MgO)0.07 and an increased thickness-dependent spin Hall angle relative to the undoped Pt. We also find that the thickness of a Pt layer leads to a drastic change in the interface role in the spintronic THz emission, suggesting that the underlying mechanism of THz emission enhancement is a combined effect of enhanced bulk spin hall angle and the interfacial skew scattering by MgO impurities. Our findings demonstrate a simple way to realize high-efficiency, stable, advanced spintronic THz devices.

Broadband All-Optical THz Modulator Based on Bi2Te3/Si Heterostructure Driven by UV-Visible Light.

All-optical terahertz (THz) modulators have received tremendous attention due to their significant role in developing future sixth-generation technology and all-optical networks. Herein, the THz modulation performance of the Bi2Te3/Si heterostructure is investigated via THz time-domain spectroscopy under the control of continuous wave lasers at 532 nm and 405 nm. Broadband-sensitive modulation is observed at 532 nm and 405 nm within the experimental frequency range from 0.8 to 2.4 THz. The modulation depth reaches 80% under the 532 nm laser illumination with a maximum power of 250 mW and 96% under 405 nm illumination with a high power of 550 mW. The mechanism of the largely enhanced modulation depth is attributed to the construction of a type-II Bi2Te3/Si heterostructure, which could promote photogenerated electron and hole separation and increase carrier density dramatically. This work proves that a high photon energy laser can also achieve high-efficiency modulation based on the Bi2Te3/Si heterostructure, and the UV-Visible control laser may be more suitable for designing advanced all-optical THz modulators with micro-level sizes.

Tutorial: Real-time coherent terahertz imaging of objects moving in one direction with constant speed

Pulsed terahertz (THz) electric fields enable various coherent THz imaging modes, such as reflection tomography, spectral imaging, and computed tomography (CT) for nondestructive inspection, quality control, and material characterization. The extension of coherent THz imaging modes to moving objects has been regarded as key to their social implementation. This Tutorial focuses on two-dimensional spatiotemporal (2D-ST) THz imaging of objects moving in one direction with constant speed as a promising means of enabling real-time coherent THz imaging. In 2D-ST THz imaging, the temporal waveform and line image of the THz pulse are simultaneously acquired without the need for mechanical scanning of the time delay and sample position using a combination of non-collinear 2D free-space electro-optic sampling with THz line-imaging optics. This 2D-ST THz imaging boosts the imaging rates of THz reflection tomography, THz spectral imaging, and THz CT to levels that are applicable to moving objects. The advanced THz reflection tomography and THz spectral imaging that result from the assistance of 2D-ST THz imaging achieve real-time line imaging of cross sections and spectral signatures, respectively. Subsequently, this enables in-line total inspection of objects moving on a translation stage or a conveyor belt. A THz CT system using real-time line projection of a THz beam is effectively applied to a 2D spectral cross section of a continuously rotating object. 2D-ST THz imaging enables the functional THz imaging of moving objects in various practical applications.

Editorial: Advanced Terahertz spectrum and metamaterials for biochemical sensing and detection

Editorial: Advanced Terahertz spectrum and metamaterials for biochemical sensing and detection

Forward-wave enhanced radiation in the terahertz electron cyclotron maser

Based on the principle of electron cyclotron maser (ECM), gyrotrons are among the most promising devices to generate powerful coherent terahertz (THz) radiation and play a vital role in numerous advanced THz applications. Unfortunately, THz ECM systems using a conventional high-Q cavity were theoretically and experimentally demonstrated to suffer from strong ohmic losses, and, accordingly, the wave output efficiency was significantly reduced. A scheme to alleviate such a challenging problem is systematically investigated in this paper. The traveling-wave operation concept is employed in a 1-THz third harmonic gyrotron oscillator, which strengthens electron-wave interaction efficiency and reduces the ohmic dissipation, simultaneously. A lossy belt is added in the interaction circuit to stably constitute the traveling-wave interaction, and a down-tapered magnetic field is employed to further amplify the forward-wave (FW) component. The results demonstrate that the proportion of ohmic losses is nearly halved, and output efficiency is nearly doubled, which is promising for further advancement of high-power continuous-wave operation of the ECM-based devices.

Launching Coherent Acoustic Phonon Wave Packets with Local Femtosecond Coulomb Forces.

Generation and manipulation of coherent acoustic phonons enables ultrafast control of solids and has been exploited for applications in various acoustic devices. We show that localized coherent acoustic phonon wave packets can be launched by ultrafast Coulomb forces in a scanning tunneling microscope (STM) using tip-enhanced terahertz electric fields. The wave packets propagate at the speed of the longitudinal acoustic phonon, creating standing waves up to 0.26THz for a 6.4nm thin Au film on mica. The ultrafast lattice displacement can be as large as 5pm and is precisely controlled by varying the tip-sample distance. This nonthermal femtosecond Coulomb-force-based excitation mechanism is applicable in nano-optomechanics for advanced terahertz engineering and opens new perspectives in exploiting coherent phonons at the atomic scale.

Vanadium dioxide-assisted switchable multifunctional metamaterial structure.

A multifunctional design based on vanadium dioxide (VO2) metamaterial structure is proposed. Broadband absorption, linear-to-linear (LTL) polarization conversion, linear-to-circular (LTC) polarization conversion, and total reflection can be achieved based on the insulator-to-metal transition (IMT) of VO2. When the VO2 is in the metallic state, the multifunctional structure can be used as a broadband absorber. The results show that the absorption rate exceeds 90% in the frequency band of 2.17 - 4.94 THz, and the bandwidth ratio is 77.8%. When VO2 is in the insulator state, for the incident terahertz waves with a polarization angle of 45°, the structure works as a polarization converter. In this case, LTC polarization conversion can be obtained in the frequency band of 0.1 - 3.5 THz, and LTL polarization conversion also can be obtained in the frequency band of 3.5 - 6 THz, especially in the 3.755 - 4.856 THz band that the polarization conversion rate is over 90%. For the incident terahertz waves with a polarization angle of 0°, the metamaterial structure can be used as a total reflector. Additionally, impacts of geometrical parameters, incidence angle and polarization angle on the operating characteristics have also been investigated. The designed switchable multifunctional metasurfaces are promising for a wide range of applications in advanced terahertz research and smart applications.

A Biomedical Perspective in Terahertz Nano-Communications—A Review

Terahertz (THz)-band (0.1~10 THz) communications are celebrated to be a crucial enabling technology for sixth generation (6G) wireless systems that fulfil the stringent requirements of future healthcare scenarios. Broadband THz sources have drawn the attention of researchers by virtue of their prominent detectability and their noninvasive and non-ionization properties. More importantly, the most advanced wideband THz sources enable THz communication in vivo with many appealing properties, including potential link capacities (terabit-per-second), miniature transceivers, and high energy efficiency. Compared to conventional devices, nano-scale devices will be potentially pivotal in subsequent medical diagnostics and treatment technologies, by virtue of the non-ionizing property of THz light and its high sensitivity in conveniently reaching delicate body sites. Thereby, real-time, label-free detection methods are expected to perform a crucial effect in clinical practice; however, difficulties such as unknown biological safety still need to be overcome. With channel modeling progress of the THz frequencies, we have considered both the radiation of the medium and the molecular absorption from the transmitted signal, and envisoned the possibility to solve the challenges such as the spectrum scarcity and capacity limitation.