Search

Recent developments on the detector and the cryogenics for the terahertz photon detector system are discussed. The detector has been redesigned for improved performance, which exhibited optical efficiency of 6% at 530 GHz. Cryogenics for the detector system has been newly developed which employs a pulse-tube cooler with two originally developed compact 4He sorption fridges. Initial experiments demonstrated continuous cooling by altering operation of two sorption fridges, supported by a pulse-tube cooler with cooling power of 0.4 W. [ABSTRACT FROM AUTHOR]

We report on the effects of cosmic ray interactions with the kinetic inductance detector (KID)-based focal plane array for the terahertz intensity mapper (TIM). TIM is a NASA-funded balloon-borne experiment designed to probe the peak of the star formation in the Universe. It employs two spectroscopic bands, each equipped with a focal plane of four ∼ 900-pixel, KID-based array chips. Measurements of an 864-pixel TIM array show 791 resonators in a 0.5 GHz bandwidth. We discuss challenges with resonator calibration caused by this high multiplexing density. We robustly identify the physical positions of 788 (99.6 %) detectors using a custom LED-based identification scheme. Using this information, we show that cosmic ray events occur at a rate of 2.1 events / min / cm 2 in our array. 66 % of the events affect a single pixel, and other 33 % affect < 5 KIDs per event spread over a 0.66 cm 2 region (2 pixel pitches in radius). We observe a total cosmic ray dead fraction of 0.0011 % and predict that the maximum possible in-flight dead fraction is ∼ 0.124 %, which demonstrates our design will be robust against these high-energy events. [ABSTRACT FROM AUTHOR]

In this paper, a multilevel coordinate search (MCS) optimization technique with a kriging model is employed to optimize an ultrathin broadband terahertz metamaterial absorber (MMA). The reported design consists of a discontinuous circular gold resonator over a lossy polyimide dielectric substrate layer. The introduced optimization technique is linked with the finite element method to analyze and simulate the investigated MMA. The proposed design could achieve a high absorptivity (more than 90%) in the frequency range from 0.97 to 3.76 THz with a wide bandwidth of 2.79 THz and a high relative absorption bandwidth (RAB) of 117.97%. In addition, the reported MMA is polarization insensitive for both transverse electric (TE) and transverse magnetic (TM) polarized waves. Further, the absorptivity of the suggested design is greater than 80% over large incident angle range (up to 40 ∘ ) for the TE and TM polarizations. Therefore, the suggested optimization technique could be used efficiently for the design and optimization of the terahertz MMAs. [ABSTRACT FROM AUTHOR]

The article introduces an analytical model for light transmission through metal-insulator-metal cavity arrays featuring an in-plane mirror-symmetry plane. The model involves three intrinsic parameters: eigenfrequency, radiative and non-radiative decay rates, describing the basic dynamics of the leaky eigenmode. The off-resonant response of the structure is described by two additional parameters which account for the direct light scattering process, i.e. transmission without resonant excitation. In the regime of negligible inter-cavity coupling, the intrinsic parameters are shown to be related to geometrical parameters by simple relations, while the direct transmission is found to be mostly governed by the spacing between cavities. [ABSTRACT FROM AUTHOR]

Single or multi-layer graphene has wide applicability in microwave and THz radiating structure designs. Owing to wide frequency tunability with an external bias of graphene, this paper presents a graphene–metal hybrid (GMH) based microstrip patch antenna (MSA) design at terahertz (THz) frequencies. Eight antenna structures with rectangular patches made of copper and GMH are studied and compared. The proposed structures are designed at 0.835 THz for short-distance communication. Polytetrafluoroethylene (PTFE) substrate of the relative permittivity (εr) 2.1, loss tangent 0.0002 and thickness 40 μm is used. By placing the copper metamaterial (CMTM) as a parasitic element on either side of the antenna, adjacent to the microstrip feedline, the impedance matching bandwidth (BW) is improved from 0.761 THz to 1.115 THz with more than 95% of radiation efficiency throughout the range. In another similar MSA, CMTMs are replaced with graphene MTMs (GMTM) for improving the bandwidth by more than 50% along with reconfigurability. The antenna parameters such as bandwidth, gain, directivity, and radiation efficiency of the antennas with and without GMH patch are analysed. It is observed from the results that the structure with GMH gives higher bandwidth of 439 GHz, along with 8.14 dBi gain and 84.65% efficiency. By changing the chemical potential (μc) of graphene through external bias GMH based patch antenna is made reconfigurable. A numerical simulation of GMH and copper-based wide bandwidth, antenna for short distance communication with high data rates at THz frequency range is studied. [ABSTRACT FROM AUTHOR]

Water and graphene have been used separately to effectively broaden absorption bandwidth in the design of metamaterial absorbers (MAs). In this paper, a broad terahertz MA is presented, in which water and graphene are used together for improving absorption performance. The absorptivity of the presented MA exceeds 90% over the frequency range of 2.81–6.74 THz. It is found that the proposed MA has superior absorption performance to those using water or graphene separately. The impedance matching theory as well as the distribution of the field and power loss are used to provide a deep insight into the absorption mechanism. Furthermore, the proposed MA exhibits excellent thermal stability as well as good insensitivity to polarization and incident angle. This work provides a reference for designing broadband MA in terahertz frequency band. [ABSTRACT FROM AUTHOR]

Terahertz generation through a photoconductive antenna is quite popular, mainly due to the advancement of nanotechnology and material research. Based on numerical modelling, a thorough study of the interdigitated photoconductive antenna (IPCA) in the terahertz (THz) frequency region has been conducted. To comprehend the capacitive behaviour of the gap between the interdigitated electrodes, an equivalent circuit model for IPCA based on its electrical perspective with matching expressions has been solved. This work explores the performance of IPCA with variations in structural parameters of their interdigitated electrode geometry in the active region. The interdigitated elemental periodicity, width and length of the IPCA are varied by keeping the interdigitated gap constant between the elements. The temporal behavior of the generated electric field and its corresponding spectral response of the IPCAs with variations in the interdigitated electrode geometry have been compared. Moreover, the coupling behavior of the elements has been observed in the generated radiation. Finally, the results prove that the THz amplitude, frequency and efficiency can be enhanced by optimizing the structural parameters of the interdigitated electrode elements in the active region. This analysis is very useful when doing practical experiments in spectroscopy, imaging and communication applications. [ABSTRACT FROM AUTHOR]

The diversity and practicability of terahertz metamaterials have experienced rapid development in the past decade due to the increasing demand for various devices. This topic has attracted significant interest from researchers. Among the key functional devices in terahertz metamaterial systems, the active control ability of terahertz metamaterials is highly valuable and captivating. This implies that the electromagnetic properties of metamaterials can be modulated over a wide dynamic range by external stimuli. This review categorizes the different types of tunable terahertz metamaterials based on the external stimuli to which they respond, namely, mechanical modulation, electrical modulation, magnetic modulation, and optical modulation. Mechanically modulated devices offer simple yet efficient modulation, while electrical and magnetic modulation provide effective active modulation through electrical mechanisms. Optical modulation, in contrast, focuses on incorporating various materials to achieve active modulation. [ABSTRACT FROM AUTHOR]

In this paper, we have proposed a novel photonic crystal fiber (PCF) design having a spider-web (SW) cladding. The finite element method (FEM)-based COMSOL Multiphysics software has been used to investigate the guiding properties and performance of the fiber in the terahertz radiation regime. Detailed numerical analyses ensure that proposed fiber design possesses a low confinement loss ( ∼ 10 - 17 cm - 1 ) ) and low effective material loss (EML). The high core power fraction of 99 % and the effective mode area of 1.06 × 10 - 5 m 2 make this fiber an exception. V-parameter confirms the single-mode behavior of fiber over 0.1–3.0-THz frequency range. Thus, we can say that the proposed fiber can be useful in various applications of terahertz propagation such as broadband transmission, biosensors, and chemical sensors. [ABSTRACT FROM AUTHOR]

In this paper, we propose an absorber with adjustable single-layer graphene multi-band perfect absorption, which has the advantages of simple structure, polarization insensitivity, tunability, multi-band absorption, and high sensitivity. The device can achieve four perfect absorption peaks at the same time, and the absorption rate of all absorption peaks is above 99%. The absorption effect of the absorber can be efficiently adjusted and controlled by adjusting the geometric parameters of the single-layer graphene array and the thickness of the dielectric layer. In addition, by changing the strength of the applied magnetic field, the Fermi level, and the relaxation rate of graphene, the absorption of the device can be dynamically adjusted, and high absorption can be maintained in the range of 0°–70° wide incidence angle. Finally, considering the potential sensing applications of the device, we measured maximum sensitivity (S) of 3.06 THz/RIU and a maximum figure of merit (FOM) of 54.6 when exposed to different ambient refractive indices, suggesting that the device can be used as a refractive index sensor. These results show that this study provides a new idea for the design of the tunable multi-band perfect metamaterial absorber based on graphene, which has great application value in many fields and provides a new reference for future research. [ABSTRACT FROM AUTHOR]

Tunability of properties is one of the most important features of 2D materials, among which graphene is attracting the most attention due to wide variety of its possible applications. Here, we demonstrated that the carrier concentration in graphene can be efficiently tuned by the material of the dielectric substrate on which it resides. To this end, we fabricated samples of CVD-grown graphene transferred onto silicon wafers covered with alumina, titanium dioxide, and silicon dioxide. We measured the transmission spectra of these samples using a time-domain terahertz spectrometer and extracted the Drude frequency-dependent graphene conductivity. We found that the sheet resistance of graphene is strongly affected by the underlying dielectric material, while the carrier scattering time remains the same. The carrier concentration value was found to range from 7 × 10 11 / c m 2 in the case of alumina and 4.5 × 10 12 / c m 2 in the case of titanium dioxide. These estimations are consistent with what can be extracted from the position of the G-peak in the Raman spectra of graphene. Our results show a way to control the graphene doping level in applications where it does not have to be adjusted. [ABSTRACT FROM AUTHOR]

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by the deposition of amyloid-β (Aβ), neurofibrillary tangles, neuroinflammation, and neurodegeneration in the brain. In recent years, considering the unsatisfied benefits of pharmacological therapies, non-pharmacological therapy has become a research hotspot for AD intervention. Terahertz (THz) waves with a range between microwave and infrared regions in the electromagnetic spectrum and high permeability to a wide range of materials have great potential in the bioengineering field. However, its biological impacts on the central nervous system, under either physiological or pathological conditions, are poorly investigated. In this study, we first measured the 0.14 THz waves penetration across the skull of a C57BL/6 mouse and found the percentage of THz penetration to be ~70%, guaranteeing that THz waves can reach the relevant brain regions. We then exposed the APPSWE/PS1DE9 mouse model of AD to repeated low-frequency THz waves on the head. We demonstrated that THz waves treatment significantly improved the cognitive impairment and alleviated AD neuropathology including Aβ deposition and tau hyperphosphorylation in the AD mice. Moreover, THz waves treatment effectively attenuated mitochondrial impairment, neuroinflammation, and neuronal loss in the AD mouse brain. Our findings reveal previously unappreciated beneficial effects of THz waves treatment in AD and suggest that THz waves may have the potential to be used as a novel therapeutic intervention for this devastating disease. [ABSTRACT FROM AUTHOR]

Terahertz (THz) spectroscopy has demonstrated significant potential for substance detection due to its low destructiveness and due to the abundance of molecular fingerprint absorption signatures that it contains. However, there is limited research on the fingerprint detection of substances at different temperatures. Here, we propose a THz metamaterial slit array sensor that exploits localized surface plasmons to enhance the electric field within the slit. The transmission peak frequency can be modulated via temperature adjustments. This method enables the detection of molecular absorption characteristics at multiple spectral frequency points, thereby achieving a specific and highly sensitive detection of characteristic analyte fingerprint spectra. Additionally, the sensor supports the detection of substances at multiple temperatures and sensitively identifies changes in their absorption properties as a function of temperature. Our research has employed temperature variation to achieve a highly sensitive and specific detection of trace analytes, offering a new solution for THz molecular detection. [ABSTRACT FROM AUTHOR]

The metasurfaces that support high‐Q resonances can provide ultra‐sharp resonance and achieve strong light‐matter interaction, which promotes their application in various fields such as sensing, lasing, and imaging. Bound modes, unable to radiate into free space, can be transformed into quasi‐bound modes and coupled with external radiation, achieving high‐Q resonances. For periodic structures, by introducing appropriate periodic perturbations, the guided modes (GMs) below the light line can be exposed above the light line, forming the high‐Q quasi‐guide modes (QGMs). Herein, a metallic metasurface is designed that supports the QGM in the continuum by introducing the Brillouin zone folding strategy. The simulation and experiment demonstrate that the GM below the light line can be transformed into the QGM in the continuum by tuning the distance parameter, and a typical "electromagnetic‐induced transparency‐like (EIT‐like)" spectrum is observed. The design unlocks additional degrees of freedom for generating high‐Q metallic metasurface. The advantage of avoiding the production of asymmetric defects contributes to releasing the strict requirement on manufacturing precision. The combination of the metallic metasurface supporting QGM resonance with high‐Q factors and the flexible substrate can improve the performance of the flexible devices and broaden its application in terahertz optoelectronic devices. [ABSTRACT FROM AUTHOR]

A tunable material black phosphorus (BP) terahertz (THz) half‐ring Fano resonator is proposed, exhibiting enhanced sensitivity, tunable frequency parameters, and the flexible sensing range. A half‐ring is positioned above the main channel, while a groove is excavated beneath it to produce the Fano resonance. The discrete mode of the half‐ring is coupled with the continuous mode of the groove, leading to a significantly enhanced sensitivity. This sensor can pick up subtle changes in the surrounding environment. Additionally, the incorporation of BP into the half‐ring positioned above the channel enables the flexible adjustment of the Fano resonator's resonant frequency. This adjustment is achieved through the manipulation of the electron doping concentration of the BP material. At the third‐order resonance around 5.81 THz, the frequency shift margin can reach 160 GHz. Adjusting the structural parameters of the Fano resonator, such as the radius of its outer ring, the distance of this ring to the main channel, and the groove's height, significantly affects its transmission spectrum. The Fano resonator demonstrates its considerable potential for applications in the field of integrated electronics. It not only provides an innovative design perspective, but also lays the foundation for the study of THz systems. [ABSTRACT FROM AUTHOR]

To detect and differentiate two essential amino acids (L-Valine and L-Phenylalanine) in the human body, a novel asymmetrically folded dual-aperture metal ring terahertz metasurface sensor was designed. A solvent mixture of water and glycerol with a volume ratio of 2:8 was proposed to reduce the absorption of terahertz waves by reducing the water content. A sample chamber with a controlled liquid thickness of 15 μm was fabricated. And a terahertz time-domain spectroscopy (THz-TDS) system, which is capable of horizontally positioning the samples, was assembled. The results of the sensing test revealed that as the concentration of valine solution varied from 0 to 20 mmol/L, the sensing resonance peak shifted from 1.39 THz to 1.58 THz with a concentration sensitivity of 9.98 GHz/mmol∗L−1. The resonance peak shift phenomenon in phenylalanine solution was less apparent. It is assumed that the coupling enhancement between the absorption peak position of solutes in the solution and the sensing peak position amplified the terahertz localized electric field resonance, which resulted in the increase in frequency shift. Therefore, it could be shown that the sensor has capabilities in performing the marker sensing detection of L-Valine. [ABSTRACT FROM AUTHOR]

Graphene can support surface plasmon polaritons (SPPs) in the terahertz band, and graphene SPP sensors are widely used in the field of terahertz micro- and nano-optical devices. In this paper, we propose an H-shaped graphene metasurface and investigate the plasmon-induced transparency (PIT) phenomenon in the proposed structure using the finite-difference time-domain (FDTD) method. Our results show that the Fermi energy levels, as well as certain shape parameters, can effectively modulate the PIT phenomenon in the proposed structure. Interestingly, changing some of these shape parameters can excite two dips into three. In terms of sensing performance, the maximum values of sensitivity and figure of merit (FOM) are 1.4028 THz/RIU and 17.97, respectively. These results offer valuable guidance for the use of terahertz optical graphene SPP sensors. [ABSTRACT FROM AUTHOR]

A new type of anti-resonant hollow-core fiber for terahertz waveguides is proposed. By introducing central support pillars and an elliptical structure, the fiber achieves high birefringence while maintaining low confinement loss and low material absorption loss. The fiber structure is optimized through simulation using the finite element method. The optimized fiber exhibits a birefringence of up to 1.22 × 10−2 at a frequency of 1 THz, with a confinement loss of 8.34 × 10−6 dB/cm and a material absorption loss of 7.17 × 10−3 dB/cm. Furthermore, when the bending radius of the fiber is greater than 12 cm, the bending loss of the anti-resonant optical fiber at 1 THz is less than 1.36 × 10−4 dB/cm, demonstrating good bending resistance and high practical value. It is expected to play a significant role in optical communication systems. [ABSTRACT FROM AUTHOR]

Metasurfaces represent a new frontier in materials science paving for unprecedented methods of controlling electromagnetic waves, with a range of applications spanning from sensing to imaging and communications. For pulsed terahertz (THz) generation, metasurfaces offer a gateway to tuneable thin emitters that can be utilized for large‐area imaging, microscopy, and spectroscopy. In literature, THz‐emitting metasurfaces generally exhibit high absorption, being based either on metals or on semiconductors excited in highly resonant regimes. Here, the use of a fully dielectric semiconductor exploiting morphology‐mediated resonances and inherent quadratic nonlinear response is proposed. This system exhibits a remarkable 40‐fold efficiency enhancement compared to the unpatterned at the peak of the optimized wavelength range, demonstrating its potential as a scalable emitter design. [ABSTRACT FROM AUTHOR]

A triple-band absorber with typical three-layer structure is proposed in the terahertz region. Pattern layer consists of two concentric rings with different sizes, and photosensitive silicon is put into the symmetric double gaps of the inner one. The conductivity of silicon is taken as one regulating method. Under the action of a suitable pump laser, dynamic switching of the resonance frequency is realized through manipulating the conducting state of photosensitive silicon. Concretely, without excitation, there are three resonance absorption peaks at 1.000 THz (f1), 1.550 THz (f2), and 2.090 THz (f3). While pumped by an excitation laser, peak f2 generates a red shift to 1.500 THz and peak f3 to 2.075 THz. Taking polarization angle of the incident THz wave as another regulating method, we can modulate the absorption amplitude with or without excitation by device rotation. Thus, we obtain the characteristics of frequency switching and amplitude modulation simultaneously. It is even more crucial that continuous modulation of the absorption amplitude is achieved at each fixed resonant point without any frequency shift. The maximum value of amplitude modulation depths is up to 100%. The proposed device provides a new perspective to explore future multifunctional terahertz absorbers. [ABSTRACT FROM AUTHOR]

Quasi-bound state in the continuum (QBIC) can effectively enhance the interaction of terahertz (THz) wave with matter due to the tunable high-Q property, which has a strong potential application in the detection of low-concentration biological samples in the THz band. In this paper, a novel THz metamaterial sensor with a double-chain-separated resonant cavity structure based on QBIC is designed and fabricated. The process of excitation of the QBIC mode is verified and the structural parameters are optimized after considering the ohmic loss by simulations. The simulated refractive index sensitivity of the sensor is up to 544 GHz/RIU, much higher than those of recently reported THz metamaterial sensors. The sensitivity of the proposed metamaterial sensor is confirmed in an experiment by detecting low-concentration lithium citrate (LC) and bovine serum albumin (BSA) solutions. The limits of detection (LoDs) are obtained to be 0.0025 mg/mL (12 μM) for LC and 0.03125 mg/mL (0.47 μM) for BSA, respectively, both of which excel over most of the reported results in previous studies. These results indicate that the proposed THz metamaterial sensor has excellent sensing performances and can well be applied to the detection of low-concentration biological samples. [ABSTRACT FROM AUTHOR]

Growth of 4-N-N-dimethylamino-4 ′ -N ′ -methyl-stilbazolium tosylate (DAST), 4-chloro-N-methyl 4-stilbazolium tosylate (CMST) and 4-bromo-N-methyl 4-stilbazolium tosylate (BMST) crystals by slope nucleation technique using mixed solvents are reported. Investigation on compositional, structural, thermal, luminescent and dielectric properties of these crystals were studied. Purity and the crystal quality are analysed using NMR and XRD. The thermal stability of the crystals is studied in detail from the TG-DTA. Emission characteristics are examined by photoluminescence spectra. Finally, 800 nm wavelength of 140 and 50 fs duration of pulses obtained from Ti:sapphire oscillator and amplifier were used for the measurement of refractive indices and absorption coefficients and generation of terahertz (THz) radiation. The conversion efficiency of DAST, CMST and BMST with respect to incident laser power of 150 mW were of the order of 0.67 × 10 - 4 , 0.826 × 10 - 4 and 0.947 × 10 - 4 , respectively. Finally, the significance of effective polarisability in THz generation has been demonstrated in terms of rotational angle θ . [ABSTRACT FROM AUTHOR]

With the rapid development of terahertz radar technology, the electromagnetic device for terahertz radar cross-section (RCS) reduction is worth investigating. However, the existing research concentrates on the RCS reduction metasurface with fixed performance working in the microwave band. This paper proposes a terahertz dynamic coding metasurface integrated with vanadium dioxide (VO2) for dual-polarized, dual-band, and wide-angle RCS reduction. The simulation result indicates that by switching the state of the VO2 between insulator and metal, the metasurface can realize the effective RCS reduction at 0.18 THz to 0.24 THz and 0.21 THz to 0.39 THz under the left-handed and right-handed circularly polarized incident waves. When the polar and azimuth angles of the incident wave vary from 0° to 40° and 0° to 360° respectively, this metasurface can maintain a 10 dB RCS reduction. This work has potential value in the terahertz stealth field. [ABSTRACT FROM AUTHOR]

Terahertz Technology is a promising newer technology for various applications in wireless and radar communication namely tracking and detecting radar targets. The challenging aspect of radar transmitters in the target detection process is spurious harmonic signals that affect the communication path between radar transceivers. The spurious signal can be neglected by a strong filtering method. Filtering is vital in radar transmission to avoid high spurious emission level signals. Low pass filtering at terahertz frequency range (LPFT) in microstrip structure defined in the chapter analysis to avoid the harmonics above the cut-off frequency. In this chapter, the analysis part of microstrip structured LPFT is implemented under finite difference time domain analysis at (0.3 THz to 0.5 THz) cut-off frequency. Finite difference time domain (FDTD) is the three-dimensional approach commonly used for the analysis in higher frequency applications. In this FDTD method, Maxwell equation's partial derivatives are centred to finite frequency by discretization. LPFT 3D-plot is characterized by the signal factors of the input signal, reflected signal, and passed signal concerning time. Scattering parameters|s11| and|s21| are characterized by frequency and magnitude plots with an insertion loss of 0.3 dB. Full-wave analysis of LPFT is compared with Chebyshev and Butterworth filter at terahertz cut-off range is implemented. The comparison plot of attenuation versus relative frequency and characteristic impedance versus dielectric constant is shown with FDTD results with good agreement. [ABSTRACT FROM AUTHOR]

In this letter, by using vanadium dioxide (VO2) with metal–insulator transition (MIT) temperatures of 60 ℃ and 72 ℃, respectively, and analyzing the effects of the two states of VO2 before (insulator state) and after (metal state) MIT on the amplitude and phase of the double MIT VO2 (DMITV) unit structure, constituting the terahertz beam reconfigurable phase gradient metasurface of VO2 based on different MIT temperatures, which achieves flexible regulation of terahertz beam. The structure is composed of two different MIT temperature VO2, polytetrafluoroethylene (PTFE) and metal. By changing the external temperature, the structure has different beam deflection angles at different temperatures. At 1.4 THz, when the temperature is below 60 ℃, the beam deflection angle is 0°, when the temperature is between 60 ℃ and 72 ℃, the beam deflection angle is 36° and when the temperature is above 72 ℃, the beam deflection angle is 17°. This terahertz phase gradient metasurface based on VO2 with different MIT temperatures provides a new way to flexibly control terahertz beams, and will have great application prospects in terahertz transmission, imaging, wireless communication, or other fields. [ABSTRACT FROM AUTHOR]

LoS (Line of Sight) MIMO (Multiple Input Multiple Output) is considered the best way to deliver high-capacity channels for terahertz communications due to the severe attenuation suffered by reflected components. Unfortunately, terahertz links are easily blocked by any obstruction resulting in link breakage. Therefore, it is necessary to provide alternative paths via reflectors. A problem shared by LoS paths and reflected paths (via polished reflectors) is that the channel matrix is rank 1 in the far field. As a result, the achieved capacity is lower than what can theoretically be achieved in a rich multi-path environment. In this work, we simultaneously solve the channel rank problem and the coverage problem by using static reflective surfaces which provide limited scattering of the incident signal in a way that minimizes signal loss but provides multiple paths to the receiver with varying phase. We construct such a surface and characterize the received signal using a terahertz testbed. We show that using our surface, we can improve channel capacity for 2 × 2 LoS MIMO. We also develop a theoretical model for the received signal and show that the reflected capacity matches the measured capacity well. [ABSTRACT FROM AUTHOR]

Terahertz (THz) backward wave oscillators (BWOs) hold immense potential for a broad range of industrial and military applications. This study presents a comparative analysis of 0.34 THz sheet beam (SB) and circular beam (CB) folded waveguide (FWG) BWOs. We examined the design, simulation (CST MWS and PS, HFSS), and performance, revealing that sheet beam BWO outperforms circular beam BWO in terms of interaction impedance, power, efficiency, and bandwidth. Under 20 kV beam voltage and 10 mA beam current conditions, sheet beam BWO achieves 0.47 Ω (0.34 THz) interaction impedance, 0.65 W output power, and a bandwidth of approximately 12 GHz, surpassing the 0.18 Ω , 0.23 W, and 8.5 GHz values of circular beam BWO, respectively. Furthermore, this study encompasses the fabrication and thorough characterization of the sheet beam BWO's slow-wave structure. Experimental validation confirms its effectiveness, with measured S 11 demonstrating reflection below - 10 dB and S 21 exhibiting transmission above - 2 dB. [ABSTRACT FROM AUTHOR]

Equivalent circuit models (ECMs) are widely used to assess the photoconductive antennas (PCAs) performances in terahertz (THz) systems. However, an existing version, which has been vastly used as a reference in recent works, exhibits some inconsistencies. In this paper, we have investigated its physical behavior according to the antenna theory. Besides, we have demonstrated that the circuit element expressions are not accurate and can have an impact on the PCA evaluation based on this model. Moreover, we have illustrated that the direct deduction of antenna parameters (exciting current and radiated voltage) from the electrical circuit is not in line with the antenna theory. Then, a straightforward model for an accurate THz photoconductive antenna evaluation that is simple, clear, and without ambiguities, has been developed. The proposed ECM design models the physical phenomena in the PCA with direct lumped elements to make the interpretation easier. Additionally, an innovative direct relationship between the space charge screening voltage and the gap voltage has been demonstrated. Consequently, the developed differential equation for time-dependent gap voltage is simple to apply, and other circuit parameters are determined directly without any ambiguity. Furthermore, we have evaluated the PCA performance based on the proposed model. Finally, the proposed model results accuracy has been examined with comparison versus measurement results obtained from the analyzed model. Our proposed model provides a good agreement with experimental measurements, demonstrating its accuracy and effectiveness in assessing photoconductive antenna performances. Our findings contribute efficiently to the improvement of a more precise and reliable model for THz photoconductive antenna evaluation, presenting a valuable and simple model for future PCA designs and assessments. [ABSTRACT FROM AUTHOR]

A novel dual band tunable terahertz (THz) circularly polarized dielectric resonator antenna (CPDRA) is designed and numerically studied. Hybrid Silicon-Graphene materials are employed as radiator with cross-slot shaped aperture coupled feed, enabling tunable linearly and circularly polarized radiation. Silicon is a good material as dielectric resonator with its operation in the THz regime due to lack of absorption, cheaper to grow and machine. Cross shaped slots of appropriate dimensions are engraved in the centre of the ground plane to split the fundamental mode of the rectangular DR into two near degenerate orthogonal modes having quadrature phase difference. As a result, circular polarization is obtained in the upper passband. Introducing cross slot also helped to improve the impedance matching and dual band operation is achieved with linearly polarized response in lower band and circularly polarized response in the upper band. By adjusting the chemical potential of graphene, frequency tunability in the both the operating bands is obtained, without changing the physical geometry of the antenna. The proposed antenna operates with 10 dB impedance bandwidth of 4.24 \% (2.31–2.41 THz) and 12.1 \% (3.11–3.51 THz) and 3 dB axial ratio bandwidth of 3.54 \% (3.33–3.45 THz). The antenna parameters are calculated and gain of 6.11 dBi and 5.8 dBiC in lower and upper band, respectively and radiation efficiency of 96.5 \% in both operating bands is obtained. This hybrid radiator-based THz CPDRA shows great potential in the development of tunable THz devices and could pave a path for new researchers. [ABSTRACT FROM AUTHOR]

Metasurface devices have demonstrated powerful electromagnetic wave manipulation capabilities. By adjusting the shape and size parameters of the metasurface microstructure, we can control the resonance between spatial electromagnetic waves and the metasurface, which will trigger wave scattering at a specific frequency. By utilizing these characteristics, we design a metasurface device with a bandpass filtering function and a unit cell of the metasurface consisting of a double-layer pinwheel-shaped metal structure and high resistance silicon substrate (forming metal–silicon–metal configuration). A bandpass filter operating in the terahertz band has been implemented, which achieves a 36 GHz filtering bandwidth when the transmission amplitude decreases by 3 dB and remains effective in a wave incidence angle of 20°. This work uses an equivalent RC resonance circuit to explain the formation of bandpass filtering. In addition, the photosensitive properties of silicon enable the filtering function of the device to have on/off tuned characteristics under light excitation, which enhances the dynamic controllability of the filter. The designed device may have application prospects in 6G space communication. [ABSTRACT FROM AUTHOR]

In this work, we introduce a novel coherent perfect absorber, accentuating its novelty by emphasizing the broad bandwidth, reduced thickness, tunable property, and straightforward design achieved through the use of an asymmetric graphene metasurface. This design incorporates both square and circular graphene patches arranged on either side of a silicon substrate. With an optimized structural design, this absorber consistently captures over 90% of incoming waves across the frequency range of 1.65 to 4.49 THz, with a graphene Fermi level of 0.8 eV, and the whole device measures just 1.5 um thick. This makes our absorber significantly more effective and compact than previous designs. The absorber's effectiveness can be significantly enhanced by combining the metasurface's geometric design with the graphene Fermi level. It is anticipated that this ultrathin, wideband coherent perfect absorption device will play a crucial role in emerging on-chip THz communication technologies, including light modulators, photodetectors, and so on. [ABSTRACT FROM AUTHOR]

Metasurfaces can realize the flexible manipulation of electromagnetic waves, which have the advantages of a low profile and low loss. In particular, the coding metasurface can flexibly manipulate electromagnetic waves through controllable sequence encoding of the coding units to achieve different functions. In this paper, a three-layer active coding metasurface is designed based on vanadium dioxide ( V O 2 ), which has an excellent phase transition. For the designed unit cell, the top patterned layer is composed of two split square resonant rings (SSRRs), whose gaps are in opposite directions, and each SSRR is composed of gold and V O 2 . When V O 2 changes from the dielectric state to the metal state, the resonant mode changes from microstrip resonance to LC resonance, correspondingly. According to the Pancharatnam-Berry (P-B) phase, the designed metasurface can actively control terahertz circularly polarized waves in the near field. The metasurface can manipulate the order of the generated orbital angular momentum (OAM) beams: when the dielectric V O 2 changes to metal V O 2 , the order l of the OAM beams generated by the metasurface changes from −1 to −2, and the purity of the generated OAM beams is relatively high. It is expected to have important application values in terahertz wireless communication, radar, and other fields. [ABSTRACT FROM AUTHOR]

A slotted patch antenna operating at 118 GHz is proposed to address challenges in the terahertz (THz) frequency band for wireless communication systems. The antenna design, utilizing a Rogers RO3003 substrate, which has a dielectric constant of er = 3 and tan δ = 0.001, strategically incorporates slots to enhance key performance parameters. Copper is employed for the ground and radiating patch, and a microstrip feeding method powers the antenna. High frequency structure simulator (HFSS) software is used for design and simulation, revealing resonance at 0.118 THz with a reflection coefficient of -42.41 dB and an impedance bandwidth of 4.42 GHz (115.84-120.26 GHz). At the operating frequency, the antenna exhibits a gain of 7.36 dB, maximum directivity of 7.38 dB, the voltage standing wave ratio (VSWR) of 1.01, and 99.75% radiation efficiency, all within a compact size of 1.5×1.3×0.1 mm³. The suggested antenna outperforms recent counterparts, making it suitable for applications like security screening and wireless communication systems (5G). Future efforts will target bandwidth expansion, gain enhancement, and further size reduction to enhance overall performance. [ABSTRACT FROM AUTHOR]

A novel terahertz polarization isolator using a two-dimensional square lattice tellurium rod array is numerically investigated at the interesting band of 0.22 THz in this short paper. The isolator is designed by inserting six hexagonal tellurium rods into a fully polarized photonic crystals waveguide with high efficiency of −0.34 dB. The TE and TM photonic band gaps of the 7 × 16 tellurium photonic crystals are computed based on the plane wave expansion method, which happen to coincide at the normalized frequency domain from 0.3859(a/λ) to 0.4033(a/λ), corresponding to the frequency domain from 0.2152 to 0.2249 THz. The operating bandwidth of the tellurium photonic crystals waveguide covers 0.2146 to 0.2247 THz, calculated by the finite element method. The six hexagonal tellurium rods with smaller circumradii of 0.16a serve to isolate transverse electric waves and turn a blind eye to transverse magnetic waves. The polarization isolation function and external characteristic curves of the envisaged structure are numerically simulated, which achieves the highest isolation of −33.49 dB at the central frequency of 0.2104 THz and the maximum reflection efficiency of 98.95 percent at the frequency of 0.2141 THz. The designed isolator with a unique function and high performance provides a promising approach for implementing fully polarized THz devices for future 6G communication systems. [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of Inorganic Analytical Chemistry / Zhongguo Wuji Fenxi Huaxue is the property of Beijing Research Institute of Mining & Metallurgy Technology Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Southeast University (English Edition) is the property of Journal of Southeast University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A novel, highly efficient terahertz fully polarized transmission line is designed by two-dimensional tellurium photonic crystals consisting of square lattice rod arrays with a complete photonic bandgap. The TE and TM photonic bandgaps of the tellurium photonic crystals, which are computed by plane wave expansion, happen to coincide, and the complete photonic bandgap covers from 2.894 to 3.025 THz. The function of the designed waveguide is simulated by the finite element method, and the transmission characteristics are optimized by accurately adjusting its structural parameters. The transmission efficiency of the waveguide for TE mode achieves a peak value of −0.34 dB at a central frequency of 2.950 THz and keeps above −3 dB from 2.82 THz to 3.02 THz, obtaining a broad relative bandwidth of about 6.84 percent. The operating bandwidth of the tellurium photonic crystals' waveguide for TM mode is narrower than that of TE mode, whose relative bandwidth is about 4.39 percent or around 2.936 THz above −5 dB. The designed terahertz photonic crystals' waveguide can transmit both TE and TM waves, and not only can it be used as a high-efficiency transmission line, but it also provides a promising approach for implementing fully polarized THz devices for future 6G communication systems. [ABSTRACT FROM AUTHOR]

In this study, we analyze a thin-size metasurface-based multiband terahertz (THz) absorber with a top layer comprised of nickel-made circled plus-shaped resonators. The geometric structure of the proposed absorber consists of subwavelength size and periodically arranged nickel resonators at the top followed by substrate SiO2 film, and the silver layer at the bottom features several high absorption bands within the 1–5-THz operating range. The proposed multiband THz absorber shows excellent absorption characteristics with perfect absorptivity, 100% at 1.5 THz, 98% at 3.2 THz, 96% at 3.72 THz, and 100% at 4.26 THz, respectively. The symmetry in the top-layer design of the unit cell shows persistence to incident waves with different polarization and makes this device independent of variation in the polarization of the waves. Besides that, surface current density analysis of the absorber illustrates that high absorption bands are achieved due to the existence of strong electric resonance in the unit cell structure. It is believed that the proposed multiband terahertz absorber with high absorption characteristics and polarization-independent behavior can be used in the field of THz shielding, THz detectors and emitters, THz sensing, and thermal imaging. [ABSTRACT FROM AUTHOR]

We theoretically and experimentally carried out an inspired type of toroidal dipole (TD) metasurfaces, which composed of a metamolecule of symmetric aluminium semicircles with a bar in the middle fabricated on polyimide substrate in the terahertz (THz) regime. It was found that the three resonances show red-shift tendency due to the increase of inductance and capacitance with the increase of semicircle's outer radius. Meanwhile, both the TD resonances and the current flowing in the metallic bar can generate the head-to-tail magnetic field distribution, which is the most prominent feature of TD phenomenon. The generation of this phenomenon is discussed deeply via the power of the multipoles, which are calculated according to the volume current density distribution data extracted from the simulations. The low frequencies (∼0.5 THz) head-to-tail magnetic field distribution is mainly attributed to TD resonance generated via the mutual effect between the two semicircles, while the same phenomenon at high frequencies (∼0.8 THz) is mainly attributed to the current flowing the middle metallic bar. The enhancement of head-to-tail magnetic field distribution leads to the increase of quality (Q) factor, and the Q factor of fabricated sample is as high as 24.5. Moreover, the electromagnetic properties in TD metasurfaces could be adjusted by the metal bar's width. The optimization of TD resonances provides opportunity to design the high Q-factor metasurfaces, and it opens up potential applications in terahertz high sensitivity devices.

Rabdosia rubescens, a Chinese herbal medicine with anticancer properties, contains two active ingredients: oridonin and ponicidin. Both compounds exhibit antitumor effects by inducing tumor cell apoptosis and autophagy and inhibiting tumor cell proliferation. To further explore the differences in molecular structure and pharmacological properties between the two substances, this study employs Terahertz Time-Domain Spectroscopy (THz-TDS) to investigate the spectral characteristics of oridonin and ponicidin in the frequency range of 0.1 to 2.3 THz. The crystal structures of the two substances are simulated using Materials Studio software and Density Functional Theory (DFT), yielding their spectra and molecular vibration modes, which elucidate the mechanism underlying the peak generation. The consistency between experimental and simulation results confirms the reliability of the experimental findings. Thus, THz spectroscopy can effectively distinguish between these two substances. Finally, a THz detection is performed on Rabdosia rubescens capsules purchased from the market, revealing the presence of absorption peaks for both substances in their absorption spectra. This provides a new approach for detecting active ingredients in Chinese herbal medicines.

In this paper, an efficiency tunable reflective metasurface (MS) consisting of a dielectric substrate sandwiched between hollow Z-shaped (HZS) structure graphene and a metallic ground plane is proposed for single/dual-focusing effects based on Pancharatnam-Berry (PB) in terahertz (THz) region. Numerical simulations demonstrate that the designed HZS graphene can achieve a circular polarization (CP) conversion with efficiency of approximately 98 % at a Fermi energy level (EF) of 1.0 eV. Moreover, by adjusting the rotation angle of the HZS graphene, a full 0-2π phase coverage can be achieved. Of note, the simulation results also reveal that the reflective CP conversion efficiency is highly dependent on the value of the EF. By carefully designing the spatial phase distribution of the graphene MS, tunable reflective single/dual-focusing effects can be realized, with focusing efficiency controlled by the EF. It is anticipated that the proposed tunable graphene MS will have broad applications in communications, imaging, and others in THz domains.

Abstract A comprehensive modeling approach for elucidating the intricacies of electro‐optic (EO) sampling is presented, which fully encapsulates the EO sampling process by accounting for all linear and second‐order nonlinear optical effects. The developed approach implements a multi‐step procedure, involving a vectorial‐field modeling of the second‐order nonlinear interactions occurring within the EO crystal, followed by a theoretical evaluation of the subsequent optical EO sampling components. To assess the efficacy of the approach, it is used for the sampling of terahertz electric fields (60 THz) within the bulk crystals of ZnTe and LiNbO3, as well as a ZnTe‐based waveguide. Nonetheless, this versatile method enables the investigation of EO sampling within any crystal incorporated within any geometry. This model provides a powerful tool for the in‐depth exploration of EO sampling, paving the way for advancements in various applications.

Vanadium dioxide (VO2) has become a reliable phase change material (PCM) and is increasingly utilized in multifunctional metasurface design. This study introduces designs of bilayer anisotropic VO2 meta-atoms in simulation for multifunctional wavefront modulation in the terahertz band. By leveraging the phase change property of VO2 and incident wave polarization, a reconfigurable metasurface composing anisotropic meta-atoms is proposed with four distinct functions. When VO2 is in its insulating state, the metasurface serves as a hologram generator for the x-polarized wave and a focusing reflector for the y-polarized wave. Upon the transition of VO2 into the metallic state, the metasurface functions as a generator of vortex beam for the x-polarized wave, while as an anomalous reflector for the y-polarized wave. The proposed metasurface showcases an outstanding broadband performance, delivering impressive four-channel wavefront modulation within a frequency range of 2.2–3.0 THz. Depending on the compact structure and integrated multi-functionalities, our design may have immense potential across a wide range of terahertz applications, including communication exchange and multiplexing utilization.