Broadband Terahertz Research Articles

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.

Ultra-broadband composite terahertz absorber prediction based on K-nearest neighbor

Broadband Terahertz (THz) absorbers with characteristics of thin thickness, wide absorption bandwidth, and tunability can play a significant role in the field of electromagnetic (EM) wave shielding. However, when designing absorbers, it is necessary to analyze a large number of variable structural parameters to obtain the desired broadband absorption spectrum. Therefore, it is difficult to obtain the optimal structure efficiently and accurately by artificially designing the broadband absorbers because a large amount of structural parameter optimization work is required. This study uses the K-nearest neighbor (KNN) algorithm to predict the design of composite multilayer terahertz absorber. The prediction obtained an ultra-large effective absorption bandwidth of 5.493 THz, with an absolute percentage error (APE) of only 0.69%. In addition, the proposed absorber has functions such as polarization insensitivity, stable performance for large incident angles, and absorption intensity tunability. This study will provide a new approach for the structure design of ultra-broadband terahertz absorbers in the future.

Light-guiding-light-based temporal integration of broadband terahertz pulses in air

The next generation of all-optical computation platforms prefers the light-guiding-light (LGL) scheme inside a medium that envisions circuitry-free and rapidly reconfigurable systems powered by dynamic interactions between light beams. Currently, suitable LGL materials and corresponding mechanisms are in urgent need. In this work, we proposed ubiquitous air as a restorable LGL signal manipulation medium with transient air-plasma waveguide circuits. Briefly, by focusing femtosecond laser beams in free space, the created atmospheric plasma filament array via photoionization was able to guide terahertz (THz) pulses along its epsilon-near-zero zone with a 1/f-profile spectral response. Consequently, this achieved a time-domain integration of the THz pulse in broad bandwidth. When the pumping laser was sequentially turned off and on, this air-plasma multi-filament structure was erased and rebuilt within nano- and femto-seconds, respectively, allowing rapid and repeated rearrangements of the all-optical stage. Furthermore, this air-based LGL information processing approach is promising to pave the way toward all-optical calculations during free-space directional transmission of THz waves, in which way the delivered THz signal can be remotely controlled.

Terahertz magneto-optical metadevice for active spin-selective beam steering and energy distribution with nonreciprocal isolation

Active beam steering and energy distribution have important applications for terahertz (THz) communication, radar, and imaging. However, the spin-conjugated mirror symmetry of a passive Pancharatnam–Berry (PB) metasurface limits the active energy distribution. Here, we prepared a low dispersion, low loss, and high magneto-optical coefficient La:YIG single crystal, ±45∘ Faraday rotation angle that can be actively tuned by a weak magnetic field in broadband THz range at room temperature. Based on this functional material, a THz magneto-optical (MO) spin-modulated metadevice has been demonstrated, which is integrated with a La:YIG, an anisotropic metasurface, and a PB metasurface layer. The spin-conjugated mirror symmetry is broken by the MO chirality effect and photonic spin Hall effect in this metadevice so that spin-selective beam steering within the range of ±27∘−±41∘ is dynamically manipulated by external magnetic fields and power distribution can be tuned with the max modulation depth of 91.6%. More importantly, the MO crystal introduces nonreciprocal phase shift between two spin states so that the device not only achieves active THz beam steering but also provides isolation for reflected waves with the max isolation of 23 dB. This mechanism of THz nonreciprocal spin beam steering provides multiple functions for THz point-to-point networking communication, anti-shielding, anti-interference, and power distribution.

Broadband terahertz metasurface with three switchable functions based on complementary structure

In this work, we propose a switchable multifunctional metasurface based on complementary VO2-graphene structure. Tuning the state of VO2 and the Fermi level of graphene, the metasurface can act as broadband half-wave plate (HWP), quarter-wave plate (QWP) or absorber in the same frequency range of 0.65–1.27THz, respectively. In the V1G0 state, the metasurface behaves as a broadband HWP to realize linear-to-linear polarization conversion with the polarization conversion ratio >0.9 in the range of 0.64–1.37THz. In the V0G1 state, the metasurface acts as a broadband QWP achieving linear-to-circular polarization conversion with the ellipticity below −0.9 in the range of 0.65–1.27THz. In the V1G1 state, the metasurface is a broadband absorber with the absorption over 0.9 in the 0.63–1.32THz range. In addition, the physical mechanisms of the metasurface in different states are analyzed through the field and current distributions. Such a switchable multi-functional metasurface can simultaneously achieve high efficiency polarization conversion and absorption, which has potential applications in the integrated terahertz devices.

Accurate inverse design for high-efficiency and broadband terahertz devices by co-simulation with genetic algorithms

In this study, we combined MATLAB with the rigorous electromagnetic field simulation software Computer Simulation Technology to perform a co-simulation method for inverse design of high-efficiency and broadband THz metasurface devices. In the proposed design method, genetic algorithm (GA) is embedded to realize automatic and inverse design. Aiming toward the different requirements of high-efficiency and broadband THz metasurface devices, different objective functions are set to optimize the design of different types of THz metasurface devices. Based on the rigorous electromagnetic simulation and genetic algorithm, the proposed design method can realize automatic and inverse design with high reliability, compared to the theoretical model based on catenary e-field theory. This study provides an important guiding role and an efficient method for designing and optimizing required metasurface devices with practical applied value.

Tunable broadband terahertz absorption and shielding of bioderived graphitic carbon microspheres

The rapid advancement of Terahertz (THz) science and technology has led to a growing interest in electromagnetic interference (EMI) shielding in the THz frequency region. There is an urgent need for broadband and tunable devices capable of absorbing terahertz (THz) waves to effectively eliminate undesired THz interference. In this regard, this work demonstrates the facile synthesis of graphitic carbon microspheres from turpentine oil (a natural carbon source) and its broadband absorption and shielding properties in the THz frequency region from 0.2 THz to 1.6 THz. Advanced characterization techniques like XRD, FESEM, RAMAN, and XPS have been employed to explore the structural, microstructural, and elemental properties of graphitic carbon microspheres (GCMs). Herein, an effort has been made to discuss the underlying mechanism for THz absorption by GCMs. The minimum shielding efficacy reaches −27.5 dB covering the broad frequency region from 0.2 to 1.6 THz with a matching thickness of ∼3.75 μm, signifying that more than 99% of the THz radiations can be shielded. Also, GCMs are exhibiting exceptional terahertz absorption (∼86%) spanning from 0.2 to 1.6 THz for the same sample thickness. Our findings reveal valuable insights into the THz shielding properties of graphitic carbon thin films without any complex element doping or functionalization, shedding light on their suitability for future THz-based devices.

Design and performance evaluation of a novel metamaterial broadband THz filter for 6G applications

Terahertz (THz) radiation, which has applications in the imaging of objects, non-destructive testing, satellite communication, medical diagnostics, and biosensing, has generated a great deal of attention due to its remarkable properties. This paper proposes a novel broadband filter for THz applications. The main idea is to overcome the insertion loss and bandwidth issues by modeling a frequency-domain finite difference method and guided-mode resonance (GMR). The optimal design scheme of the wideband pass filter based on the circular resonant ring is discussed by comparing the transmission parameters under various parameters. This scheme overcomes the restriction of the narrow passband bandwidth of the prior THz filters and achieves approximately 3 dB bandwidth of 0.54 THz. The proposed THz filter paper also has the advantages of a straightforward structure, low processing costs, and ease of conformal with other structures, and it can be used for stealth fighters, new communication technology, and precise instruments. In addition, when compared to existing models, the suggested filter offers higher 3 dB BW operation, increased transmittance, low insertion loss, and stable performance at various oblique angles.

Artificial neural network-based inverse design of metasurface absorber with tunable absorption window

Although the development of graphene broadband terahertz (THz) absorbers is relatively mature, studies focusing on modulating the absorption window remain limited. Existing research has mainly concentrated on achieving absorption window switching via the phase transition of materials like vanadium dioxide (VO2). However, this method typically allows for switching within two specific windows, lacking the capacity for arbitrary and flexible regulation. This study introduces a novel approach involving a bilayer graphene metasurface absorber (BGMA) to address this limitation, which allows arbitrary manipulation of the absorption windows. To facilitate rapid and accurate selection of the structural parameters, an artificial neural network (ANN)-based inverse design method is employed, which achieves a tunable absorption bandwidth of 6.70 THz with an error amount to 0.59%. Through in-depth mechanistic analyses, it is demonstrated that the BGMA can achieve tunable absorption between a low-frequency broadband absorption (L-FBA) mode and a high-frequency broadband absorption (H-FBA) mode. This tunability arises from the transition between graphene localized surface plasmon resonance and graphene surface plasmon resonance. Overall, the outcomes of this study underscore the remarkable tunable performance of the BGMA achieved through ANN-based inverse design. This innovation holds significant promise for applications in security detection, and object stealth.

Improved Numerical Modeling of Terahertz Wave Propagation in Epoxy Coating with the Finite-Difference Time-Domain Method

Electromagnetic theoretical analysis was usually used to investigate the pulsed terahertz wave interaction with the medium. For epoxy coatings, the material dispersion of the coating was often simplified or ignored in the traditional propagation model. It is difficult to elaborate on the propagation mechanism and to differentiate the coating deterioration as serving time increases. An improved propagation model based on the finite-difference time domain was established to characterize the propagation in the epoxy protective coating under broad-band terahertz radiation. Either an intact or defective coating structure was investigated, and the simulated results were compared with the experimental test. The dissipation mechanism was considered in the proposed model. The results indicated that the terahertz reflections varied with the change in optical and dielectric properties caused by coating aging, which influences the intrinsic impedance of the medium. Moreover, it could well elucidate the propagation mechanism of pulsed terahertz waves in rusted, defective coating structures.

New Soliton Regime of Generation of Broadband Terahertz Radiation by Laser Pulses with Tilted Wave Fronts

A new soliton-like regime of generation of terahertz radiation by optical pulses with tilted wave fronts is analyzed. It has been shown that the diffraction of an optical pulse is of fundamental importance for the formation of optical–terahertz soliton. A nonsoliton broadband terahertz component is generated synchronously with the soliton component of radiation. Two matching conditions called “super-Cherenkov” and “anti-Cherenkov” have been revealed under which generation is the most efficient. In the former and latter cases, the optical terahertz soliton propagates ahead and behind the nonsoliton terahertz component, respectively.

A triple band local oscillator coupling system based on a phase grating for five-meter Dome-A terahertz explorer telescope.

For heterodyne receivers, broadband and integrated local oscillator (LO) coupling schemes are crucial. In this paper, we present a novel terahertz broadband receiver system based on a one-pixel phase grating. We designed a phase grating that can simultaneously couple the LO signals at 690, 850, and 1300GHz to a superconducting hot-electron bolometer mixer at the incident angles of 25.7°, 20.6°, and 13.3°, respectively. At these three frequencies, the measured diffraction efficiencies of the phase grating are as high as 70%, 73%, and 65%, and the total power coupling efficiencies of the LO coupling system are 22%, 23%, and 23%, in good agreement with the simulated results. This efficient and highly integrated multiband receiver system has potential applications in the proposed Five-meter Dome-A Terahertz Explorer telescope.

Secondary Spin Current Driven Efficient THz Spintronic Emitters

AbstractFemtosecond laser‐induced photoexcitation of ferromagnet (FM)/heavy metal (HM) heterostructures has attracted attention by emitting broadband terahertz frequencies. The phenomenon relies on the formation of an ultrafast spin current, which is primarily attributed to the direct photoexcitation of the FM layer. However, during the process, the FM layer also experiences a secondary excitation led by the hot electrons from the HM layer that travel across the FM/HM interface and transfer additional energy in the FM. Thus, the generated secondary spins enhance the total spin current formation and lead to amplified spintronic terahertz emission. These results emphasize the significance of the secondary spin current, which even exceeds the primary spin currents when FM/HM heterostructures with thicker HM are used. An analytical model is developed to provide deeper insights into the microscopic processes within the individual layers, underlining the generalized ultrafast superdiffusive spin‐transport mechanism.

All-Metal Coding Metasurfaces for Broadband Terahertz RCS Reduction and Infrared Invisibility

With the rapid advancement of modern technology and radar detection systems, electromagnetic (EM) stealth technology has become increasingly significant, particularly in aircraft stealth and military radar applications. In this work, an all-metal metasurface is designed for broadband terahertz radar cross-section (RCS) reduction and infrared invisibility. The all-metal metasurface possesses extremely low infrared emissivity and high polarization conversion in the terahertz band. Through the joint simulation of MATLAB and CST, a genetic algorithm is used to optimize the random phase distribution of 2, 3, and 4-bit metasurfaces, so that the reflected wave is scattered to achieve broadband terahertz RCS reduction. Simulation results show that the metasurface can simultaneously achieve broadband terahertz RCS reduction in 3–5 THz and infrared invisibility in 24–38 THz (8–12.5 μm). The RCS reduction of the coding metasurface is greater than 10 dB compared to the metal plate, and the maximum RCS reduction of the 4-bit metasurface can reach 21.1 dB. Compared to the traditional design method, the proposed method can reduce time consumption and find the optimal result to achieve high performance. We believe the proposed method can provide significant guidance for surface coating in camouflage applications and opens up new possibilities for improving the information capacity of coding metasurfaces.

Ultra-sensitive terahertz introduced harmonic emission towards terahertz detection

The sixth-generation (6G) technology has garnered significant interest from researchers due to its improved spectral and energy efficiency. However, a major challenge lies in the strong attenuation of terahertz waves in the air, necessitating the development of ultra-sensitive detectors. In recent years, there has been gradual progress in the development of harmonic signal detection methods, offering a new solution for terahertz wave detection with a low detection threshold and high sensitivity. In our study, we proposed a highly sensitive detection approach involving a 1040 nm femtosecond laser and broadband terahertz waves incident on SnSe2 nanosheets with a few layers, obtained through mechanical exfoliation. Utilizing confocal microscopy, the distinct harmonic wave can be observed, with the intensity of the harmonic signal varying based on the polarization angle difference between the laser and terahertz waves. Our estimation suggests that this detection sensitivity could be around ten times higher than that of existing detectors. This approach, which involves THz-induced harmonic generation without the need for high voltage or intense THz/laser exposure, holds immense potential for weak signal detection applications and carries significant importance for the advancement of 6G technology.

Recording the angular dispersion of a terahertz beam into its frequency spectrum for fast measurements.

The frequency-dependent divergence angle of terahertz (THz) beams is a crucial aspect in understanding the generation and transmission of broadband THz waves. However, traditional beam profiling methods, such as 1D or 2D translation/rotation scanning detection, are time-consuming and wasteful of THz energy, making them unsuitable for fast measurement applications, such as single-shot THz generation and detection. Here, we proposed a simple solution that involves passing the THz beam through a core-anti-resonant reflective (CARR) cavity (e.g., a paper tube). The spatial information of the beam is then recorded into its frequency spectrum, which can be easily detected by a following traditional THz time-domain spectroscopy (TDS) system or a single-shot sampling setup. Our method enables the acquisition of the angular dispersion without repetitive measurements, and represents a significant step forward in fast and efficient achievement of spatial properties of broadband THz beams.

Terahertz unipolar polarimetry by second-harmonic generation in air

Femtosecond laser-based terahertz (THz) sources have gained attention for their potential in various applications. As for any electromagnetic radiation, the wave polarization is a critical parameter, which needs to be under control in a wide class of those applications. However, characterizing the polarization of broadband THz pulses remains challenging due to their limited efficient optics. THz air-biased coherent detection has emerged as a promising candidate. The technique employed is heterodyne detection, utilizing second-harmonic generation induced by THz radiation in laser-induced air plasmas. This approach provides exceptional spectral bandwidth and an unbounded power detection limit, rendering it highly suitable for ultra-broadband and high-power THz sources. It enables a very efficient polarization measurement too. However, recent findings have revealed that the laser-induced air plasma generated in this technique can exhibit birefringence, which in turn adds systematic errors to the polarization-state determination. In this Letter, we propose a simplified approach that utilizes a weak probe beam and avoids high-voltage DC bias-fields. Unlike the terahertz (THz) air-biased coherent detection scheme, our approach yields a unipolar, intensity-proportional signal for second-harmonic generation. The experimental results reported in this Letter show the absence of induced birefringence in air and, hence, demonstrate accurate measurements of the polarization state of ultra-broadband THz pulses. Therefore, our technique may provide valuable results in applications where the polarization state, and not the full electric field waveform, is required for analysis or characterization. Finally, we discuss a possible application of our method to the emergent field of THz singular optics.

Space-Based THz Radar Fly-Around Imaging Simulation for Space Targets Based on Improved Path Tracing

Aiming at the space target detection application of a space-based terahertz (THz) radar, according to the imaging mechanism of broadband THz radars, a THz radar imaging simulation method based on improved path tracing is proposed. Firstly, the characterization method of THz scattering characteristics based on Kirchhoff’s approximation method is introduced. The multi-parameter THz bidirectional reflectance distribution function (THz-BRDF) models of aluminum (Al), white-painted Al, and polyimide film at 0.215 THz are fitted according to the theoretical data, with fitting errors below 4%. Then, the THz radar imaging simulation method based on improved path tracing is presented in detail. The simulation method utilizes path tracing to simulate parallelized THz radar echo signal data, considering multi-path energy scattering based on the THz-BRDF model. Finally, we conducted THz radar imaging simulation experiments. The influences in the imaging process of different fly-around motions are analyzed, and a comparison experiment is conducted with the fast-physical optics (FPO) method. The comparative results indicate that the proposed method exhibits richer and more realistic features compared with the FPO method. The simulation experiments results demonstrate that the proposed method can provide a data source for ground algorithm testing of THz radars, particularly in evaluating the target detection and recognition algorithm based on deep learning, providing strong support for the application of space-based THz radars in the future.

Nondepleted Time-Domain Method for Simulating Difference Frequency Generation From Metallic Metasurfaces

Difference frequency generation (DFG) is an essential phenomenon for generating far-infrared (FIR) and broadband terahertz (THz) waves, which cannot be generated by either alternating current flow-driven antennas for microwave source or pumped materials for higher frequency lights. Metallic nonlinear metasurfaces (NLMSs) as a new platform for generating THz sources were intensively developed. In this work, a nondepleted time-domain model derived from the Maxwell-hydrodynamic model is proposed where linear and nonlinear responses in metallic structures under an intense excitation are separated. By using the proposed model, only linear and low-frequency DFG signals will be considered, which makes this model suitable for unconditionally stable time-domain algorithms. A finite-difference time-domain (FDTD) method is employed to solve a set of multiphysics equations based on the proposed model. The stability of the algorithm is apparent. We validate our method by simulating THz generations from metallic NLMSs and benchmark the approach with previous numerical studies as well as our recent experimental results. In addition, the implementation of solving the DFG response can be remarkably accelerated by using the unconditionally stable algorithm.

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.