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Design strategies for improving terahertz (THz) quantum cascade lasers (QCLs) in the 5-6 THz range are investigated numerically and experimentally, with the goal of overcoming the degradation in performance that occurs as the laser frequency approaches the Reststrahlen band. Two designs aimed at 5.4 THz were selected: one optimized for lower power dissipation and one optimized for better temperature performance. The active regions exhibited broadband gain, with the strongest modes lasing in the 5.3-5.6 THz range, but with other various modes observed ranging from 4.76 to 6.03 THz. Pulsed and continuous-wave (cw) operation is observed up to temperatures of 117 K and 68 K, respectively. In cw mode, the ridge laser has modes up to 5.71 THz - the highest reported frequency for a THz QCL in cw mode. The waveguide loss associated with the doped contact layers and metallization is identified as a critical limitation to performance above 5 THz.

Polaritons, i.e., hybrid quasi-particles of light and matter resonances, have been extensively investigated due to their potential to enhance light-matter interactions. Although polaritonic applications thrive in the mid-infrared range, their extension to the terahertz (THz) range remains limited. Here, we present paratellurite (α-TeO2) nanowires, a versatile material acting as a platform for different types of phonon polaritons. Utilizing synchrotron infrared nanospectroscopy from 10 to 24 THz, we uncover the polaritonic properties of α-TeO2 nanowires, showcasing their dual functionality as both a Fabry-Pérot cavity and a waveguide for surface phonon polaritons. Furthermore, near-field measurements with a free-electron laser as a THz source reveal a localized optical contrast down to 5.5 THz, an indication of hyperbolic bands. Our findings complement the repertoire of polaritonic materials, with significant implications for advancing THz technologies.

In this study, NaMg 1- x Mn x (PO 3) 3 (0.00 ≤ x ≤ 0.0625) ceramics with low dielectric constant were prepared via the solid-state reaction method. The effects of Mn2+ substitution on the microstructure and dielectric properties (at microwave and terahertz frequencies) of the NaMg(PO 3) 3 ceramics were studied by means of the Rietveld refinement, morphology, and ionic polarizability analysis. It is obtained that the NaMg 0.0975 Mn 0.025 (PO 3) 3 ceramic sintered at 880 °C for 4 h exhibited optimal dielectric properties at microwave frequencies, where ε r = 4.585, Q × f = 48,313 GHz (@17.228 GHz), and τ f = −40.83 ppm/°C. The Q × f and τ f values were changed by 74.2 % and 27.8 %, as compared to the values of 27,728 GHz and −56.52 ppm/°C for the pure NaMg(PO 3) 3 ceramic, respectively. For terahertz frequencies (0.2–1.0 THz), the ε r values were decreased to 3.79–3.98, and the tan δ values were between 0.0203 and 0.1152. All the experimental results indicate that this system exhibits great potentials both at microwave and terahertz frequencies. [ABSTRACT FROM AUTHOR]

ABSTRACT In this article, a compact triple‐band frequency‐tunable (FT) hexagonal‐shaped graphene antenna through a machine learning (ML) approach for terahertz (THz) application is presented. The proposed THz antenna is designed on a polyamide (∈r=3.5$$ {\in}_r=3.5 $$) substrate with a thickness of 10 μm, and graphene is used as an antenna radiator. The size of the substrate is 38 × 46 μm2. The FT is achieved by changing the chemical potential of graphene material. The performance of the proposed THz antenna has been investigated, and the impacts of several conducting materials like gold, aluminum, copper, and graphene and dielectric materials like Rogers RT/duroid 5880, polyamide, quartz, and SiO2 are explored. The proposed THz antenna provides three operating bands. The frequency of operation in Band‐1 is 2.51–5.05 THz, Band‐2 is 5.99–7.43 THz, and Band‐3 is 7.94–9.63 THz. The bandwidth in Band‐1, Band‐2, and Band‐3 are 2.54, 1.44, and 1.69 THz, respectively. The % of impedance bandwidth in Band‐1, Band‐2, and Band‐3 are 67.19%, 24.02%, and 21.28% respectively. The proposed antenna has a maximum peak gain of 5 dBi. The proposed antenna is optimized through various ML algorithms like random forest (RF), extreme gradient boosting (XGB), K‐nearest neighbor (KNN), decision tree (DT), and artificial neural network (ANN). The RF algorithm gives more than 99% accuracy compared to other ML algorithms and accurately predicts the S11 of the proposed antenna. The proposed THz antenna would be suitable for applications related to imaging, medical, sensing, and ultra‐speed short‐distance communication applications in the THz region. [ABSTRACT FROM AUTHOR]

On‐chip wavefront manipulation of terahertz surface plasmons is crucial for the miniaturization and practical application of terahertz technology. Recently, topological metasurfaces have emerged as a promising avenue for spin‐decoupled wavefront manipulation, leveraging the unique topological properties of non‐Hermitian matrices near their singular points. Despite their innovative phase control mechanisms, topological metasurfaces have not yet been explored for surface plasmon devices. Here, spin‐decoupled wavefront manipulation of terahertz surface plasmons is experimentally achieved using freestanding topological metasurfaces. This approach promises to expand the control methods of surface plasmons and broaden the application scenarios of topological metasurfaces, providing a new paradigm for the design of integrated on‐chip terahertz devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Spectral tunable technology has to meet the requirements of strong robustness and wide spectral range. We propose a method for the transmission and manipulation of infrared topological photonic crystal valley states based on tunable refractive index method that exhibits broad-spectrum and multi-band characteristics, along with a tunable emission angle. With this structure, different rotational directions of vortex light sources can independently excite the K valley and K′ valley within the frequency band ranging from 75.64 THz to 99.61 THz. At frequencies from 142.60 THz to 171.12 THz, it is possible to simultaneously excite both the K valley and K′ valley. The dual refractive index tunable design allows for the adjustment of the emission angle at a fixed frequency, enabling control over the independent excitation of either a single K valley or K′ valley, as well as their simultaneous excitation. This capability has significant implications for photonic computation and tunable filtering, offering enhanced operational flexibility and expanded functionality for future optical communications and integrated optical circuits. [ABSTRACT FROM AUTHOR]

We investigate the terahertz (THz) scattering and emission properties of lunar regolith by modeling it as a random medium with rough top and bottom boundaries and a host medium situated beneath. The total scattering and emission arise from three sources: the rough boundaries, the volume, and the interactions between the boundaries and the volume. To account for these sources, we model their respective phase matrices and apply the matrix doubling approach to couple these phase matrices to compute the total emission. The model is then used to explore insights into lunar regolith scattering and emission processes. The simulations reveal that surface roughness is the primary contributor to total scattering, while dielectric contrasts between the volume and the boundaries dominate total emission. The THz emissivity is highly sensitive to the regolith dielectric constant, particularly its imaginary part, making it a promising alternative for identifying previously undetected water ice in the lunar polar regions. The THz emissivity model developed in this study can be readily applied to invert the surface parameters of lunar regolith using THz observations. [ABSTRACT FROM AUTHOR]

Herein, stacked layers of Nb2O5 structured with platinum nanosheets were fabricated to handle multifunctional electro-optic operations. The stacked layers of Nb2O5/Pt/Nb2O5 (NPN) were prepared by the ion coating technique and exhibited an amorphous structure. A remarkable increase in light absorption by more than 480% was achieved via the insertion of Pt nanosheets between layers of Nb2O5. Pt nanosheets with thicknesses of 50 nm and 100 nm successfully increased the electrical conductivity of the NPN layers by five and eight orders of magnitude, respectively, without losing transparency. This feature makes the NPN stacks suitable for transparent conducting oxide applications. The insertion of Pt nanosheets between layers of Nb2O5 also converted the conductivity from p- to n -type. In addition, NPN stacked layers exhibited highly enhanced dielectric properties, demonstrating an increase in the dielectric constant by 190%, making the NPN stacked layers suitable for use in the design of high-κ gate dielectric devices. Moreover, when treated as optical filters, Drude–Lorentz fittings of the dielectric spectra showed that Pt nanosheets increased the free carrier density and the plasmon frequency. The terahertz cutoff frequency of the NPN devices steadily increased with increasing Pt nanosheet thickness. The terahertz cutoff frequency for NPN optical filters comprising a 100-nm-thick layer of Pt displayed values of 7.5 THz and 5.0 THz in the infrared and visible light ranges. The NPN stacks have potential for use in terahertz applications. [ABSTRACT FROM AUTHOR]

It is expected that 6G communication technology will achieve a peak data rate capability of 1 Tbit/s. One way of reaching this goal is to employ THz frequencies for carrying wireless signals. THz frequencies have large continuous bandwidths which allow for high data rate signals. In order to support the development of THz communications and accompanying technologies, a 6G THz Communications Testbed has been set-up at the National Physical Laboratory. In this paper, the best signal performance for a variety of different baud rates and modulation schemes is explored for both a direct connection between the waveform generator and measuring oscilloscope and for several different Over-The-Air (OTA) path lengths up to 4 m specular Non-Line-of-Sight. For the direct connection, signals of up to 210 Gbit/s were achieved and data rates of up to 150 Gbit/s were achieved for OTA transmission. [ABSTRACT FROM AUTHOR]

In this study, terahertz time‐domain spectroscopy (THz‐TDS) is employed for the first time to explore the characteristics of mono‐, bi‐, and tri‐layer graphene coated on guided‐mode resonance filters (GMRFs). Owing to high quality‐factor (Q‐factor) resonances of GMRF, the proposed method significantly enhances the resonance depth variation by up to 9.3, 5.1, and 4.2 times at 0.58 THz in TE mode for mono‐, bi‐, and tri‐layer graphene, respectively, in contrast to conventional THz‐TDS methods relying on amplitude variation at 0.50 THz in TE mode. Excellent agreement is observed between experimental results and theoretical simulations using the Kubo formula and Drude model, even accounting for variations in sidelobes at an incident angle of 0.6 degrees. Through meticulous fitting process between measurements and simulations for the resonances formed by the GMRF and graphene, the study accurately determines the electrical and optical properties of mono‐, bi‐, and tri‐layer graphene, including frequency‐dependent sheet conductivity (σs(ω)), mobility (μ), carrier density (N), and Fermi velocity (vF). Furthermore, in the THz high‐frequency region, the observation reveals that as the number of graphene layers increases, the decrease in σs(ω) occurs more rapidly than in single‐layer graphene, attributed to the screening effect arising from electronic interactions between each graphene layer. [ABSTRACT FROM AUTHOR]

The ultrafast dynamics of photoexcited charge carriers are studied in micron‐scale crystals composed of the inorganic perovskite CsPbBr3 with time‐resolved terahertz spectroscopy. Exciting with photon energy close to the band edge, it is found that a fast (<10 ps) decay emerges in the terahertz photoconductivity with increasing pump fluence and decreasing temperature, dominating the dynamics at 4 K. The fluence‐dependent dynamics can be globally fit by a nonlinear recombination model, which reveals that the influence of different nonlinear recombination mechanisms in the studied pump fluence range depends on temperature. Whereas the Auger scattering rate decreases with decreasing temperature from 77 to 4 K, the radiative recombination rate increases by three orders of magnitude. Spectroscopically, the terahertz photoconductivity resembles a Drude response at all delays, yet an additional Lorentz component due to an above‐bandwidth resonance is needed to fully reproduce the data. [ABSTRACT FROM AUTHOR]

The terahertz frequency region of the electromagnetic spectrum is crucial for understanding the formation and evolution of galaxies and stars throughout the universe's history, as well as the process of planet formation. Detecting the unique spectral signatures of molecules and atoms requires terahertz spectrometers, which must be operated in space observatories due to water vapor absorption in the Earth's atmosphere. However, current terahertz spectrometers face challenges such as low resolution, limited bandwidth, large volume, and complexity. In this paper, the issues of size and weight are addressed by demonstrating a concept for a centimeter‐sized, low‐weight terahertz spectrometer using a metasurface. The design of the metasurface spectrometer is first discussed for the 1.85 to 2.4 THz range, followed by its fabrication. Next, an array of quantum cascade lasers operating at slightly different frequencies around 2.1 THz is utilized to characterize the spectrometer. Finally, a spectrum inversion method is applied to analyze the measured data, confirming a resolution R (λ/Δλ) of at least 273. This concept can be extended to other application areas, such as planetary observations and various wavelengths in the far‐infrared (FIR) and near‐infrared (NIR) ranges. [ABSTRACT FROM AUTHOR]

Metamaterials based on quasi‐bound states in the continuum (qBICs) with manipulable resonance quality (Q) factors have provided a standout platform for cutting‐edge terahertz (THz) sensing applications. However, most so far have been implemented as conventional metal patch structures with adjacent substrate layers, incurring the limitation of insufficient light‐matter interaction due to substrate effects. Here, qBIC‐driven metamaterials with substrate‐free metallic aperture structures for tailoring light‐matter interactions and exhibiting near‐ideal sensing performance is introduced. Specifically, it is incorporated ultrafast femtosecond laser processing technology to fabricate H‐type metallic aperture metamaterials with accessible high‐contrast Q factor resonances allowed by in‐plane symmetry breaking. Correspondingly, stronger light field energies are applied to the interactions due to completely eliminating the confinement of the substrate effect, enabling experimental sensitivity of up to 0.86 THz RIU−1 for the qBIC resonance, 1.9 times that of the conventional dipole resonance. Moreover, a high Q qBIC resonance achieved by optimized asymmetry parameter is exploited for detecting ultrathin layers of L‐proline molecules as low as 0.87 nmol. It is envisioned that this approach will deliver insights for real‐time, precise, and high‐performance detection of trace biomolecules, and open new perspectives for realizing ideal performance metadevices. [ABSTRACT FROM AUTHOR]

Unlike traditional group‐6 transition metal dichalcogenides (TMDs), group‐10 TMDs such as PtSe2 and PdTe2 possess highly tuneable indirect bandgaps, transitioning from semiconducting in the near‐infrared to semimetal behavior with a number of monolayers (MLs). This opens up the possibility of TMD‐based mid‐infrared and terahertz optoelectronics. Despite this large potential, the optical properties of such materials have shown an extremely large disparity between that predicted and measured. For example, simulations show that a few MLs is required for the semiconductor–semimetal transition, whilst tens of MLs is found experimentally. This is a result of widely used optical extrapolation methods to determine bandgaps, such as the Tauc plot approach, that are not adapted here owing to i) nearby direct transitions, ii) the material dimensionality and iii) large changes in the non‐parabolic bandstructure with MLs. Here, uniquely combining optical ellipsometry to determine the complex permittivity, terahertz time resolved spectroscopy for the complex conductivity and in‐depth density functional theory (DFT) simulations, it is shown that the optical properties and bandstructure can be determined reliably and demonstrate clearly that the semiconductor‐semimetal transition occurs for PtSe2 layers ≈5 MLs. The microscopic origins of the observed transitions and the crucial role of the Coulomb interaction for thin semiconducting layers, and that of interlayer van der Waals forces for multilayer semimetallic samples are also demonstrated. This work of combining complimentary experimental techniques and extensive simulations avoids the application of constrained extrapolation methods to determine the optical properties of group‐10 TMDs, and will be of importance for future mid‐infrared and terahertz applications. [ABSTRACT FROM AUTHOR]

Hybrid metasurfaces incorporated by active materials hold great promise for state‐of‐the‐art terahertz functional devices. However, it is still a major challenge to achieve ultrafast, dynamic, and multifunctional effective control of THz waves via hybrid metasurfaces. Herein, a modulator consisting of split rings and cut‐wires is first demonstrated, with an amplitude of −35.6 dB at 0.524 THz. By embedding semiconductor silicon into specified locations to form a hybrid metasurface, the ultrastrong connectivity of the silicon bridges leads to rapid optical molecularization. Under photoexcitation, the frequency tuning range is 26.7%, the phase shifting reaches 357.5°, and the maximal modulation depth is 94.54%. Taking advantage of the rapid relaxation of photocarriers in the silicon bridges, the ultrafast frequency switching is within 1400 ps. More interestingly, by changing the positions of the silicon bridges, the frequency tuning range is further promoted to 60%, the phase shifting is 353.5°, the modulation depth of 100% is achieved, and the full recovery time is 1600 ps. Furthermore, the underlying mechanism of the ultrafast tuning process is elucidated. This work demonstrates the feasibility of all‐optical‐controlled hybrid metasurface to achieve multifunctional dynamic modulation of THz waves, which has tremendous potential for applications in optical switching, signal processing, and frequency conversion. [ABSTRACT FROM AUTHOR]

Magneto‐optical activity is a significant way to achieve asymmetric transmission for spin photons. However, it is severely limited by the weak light‐matter interaction in the terahertz (THz) region. In this work, a magnetic Moiré grating (MMG) is constructed by sandwiching an Yttrium iron garnet (YIG) single crystal in a pair of twisted 2D subwavelength gratings. The twisted angle between two gratings introduces a strong geometric Moiré chirality related to the Moiré pattern. When an external magnetic field is applied to the YIG layer, three different symmetries of spacial mirror image, time reversal, and spin conjugation are broken simultaneously under the chiral coupling between magneto‐optical activity and Moiré chirality, leading to both the enhanced magneto‐optical chiral transmission and the nonreciprocal spin isolation. Under the magnetic field varying from −0.26 to +0.26 T in the experiment, the MMG achieves an active modulation of circular dichroism from 39 to −37.5 dB and optical activity from −35° to 30°. Moreover, the maximum spin isolation over 32.6 dB is also obtained near 0.5 THz. This mechanism provide a new way to realize independent manipulation and active modulation of THz spin photons more flexibly. [ABSTRACT FROM AUTHOR]

Recent advances in metadevices, featuring complex subwavelength metastructures, have dramatically transformed the control and manipulation of electromagnetic waves. However, the inherently narrow operational bandwidth of these devices, stemming from their wavelength‐specific meta‐atoms, restricts their application in rapidly advancing fields such as the Internet of Things and advanced intelligent systems. Here, a novel hyperband synergistic metadevice is introduced, realized through a comprehensive multi‐scale meta‐atom architecture. The complementary metal‐oxide‐semiconductor (CMOS)‐compatible prototype integrates the distinct properties of double‐walled carbon nanotubes with advanced interlayer and intralayer coupling mechanisms, coherently combining nanoscale, microscale, and macroscale meta‐atoms. This prototype is thus adept at operating across a wide electromagnetic spectrum, spanning from the centimeter‐wavelength microwave band to the hundred nanometer‐wavelength visible and infrared optical band. Significantly, this singular device synergistically delivers three critical functionalities: selective microwave absorption, efficient terahertz beam steering, and enhanced optical transparency. These result signifies a breakthrough in hyperband electromagnetic device engineering, leading to compact, versatile, intelligent electromagnetic platforms. [ABSTRACT FROM AUTHOR]

Terahertz waves can be widely used for short‐range communication in complex indoor environments and non‐destructive object detection applications. Metasurfaces are widely used in terahertz sensing and communication devices because they can modulate terahertz waves in multiple dimensions. Metamaterial robot brain can utilize metasurfaces' powerful direct modulation ability to achieve sensing and communication functions. The metasurface devices realized based on Dynamic Heterogeneous Redundancy (DHR) architecture can improve the confidentiality and security of terahertz wave wireless communication. While the intrinsic ohmic loss and quality factor of usual metallic metamaterials are usually low, the concept of bound states in the continuum (BIC) has been proposed for stronger terahertz‐matter interactions. Among them, high‐order BICs are of interest because of their strong robustness to structural defects. Therefore, an aluminium‐graphene hybrid metasurface with high‐order BIC is proposed. We have the principle of excitation of high‐order BICs is investigated and creatively proposed with high robustness realized using the magnetical EIT effect. The robustness of the high‐order BIC is also utilized to design security hardware based on DHR architecture. The designed secure hardware can satisfy the demand for an intelligent robotic brain to the internal terahertz wave confidential wireless communication. [ABSTRACT FROM AUTHOR]

An additively stacked quasi-self-similarity Sierpinski Carpet-based fractal antenna embedded over a photonic band gap substrate is proposed in this paper. The proposed structure is designed for continuous in-vivo monitoring of cardiac Troponin I (cTnI) levels in the blood released from the heart muscles even with a trivial malfunction. The total size of the antenna biosensor is 0.4 λ0 × 0.33 λ0 × 0.025 λ0 and is assumed to be implanted over a leadless pacemaker. The substrate material used is Teflon (εr = 2.1) with gold as patch material to accomplish as bio-receptor. The frequency band of 0.1–3 Terahertz is considered for the design as per IEEE standard. Result validation without bio-model is performed through High Frequency Structural Simulator, Computer Simulation Technology, Advanced Design System, and COMSOL simulation software. The dielectric constant values of blood, myocardium, and other biological parameters are calculated based on the Cole–Cole model using Matrix Laboratory for further analysis. Those values are incorporated in the design to imitate a bio-model, to check the variation in frequency with regard to permittivity, in turn corresponding to troponin level change. [ABSTRACT FROM AUTHOR]

Breast cancer is a potentially life-threatening disease is one of the most common cancers specially for women. For fast and effective treatment early detection of the tumor is very important. But the frequent checkup is not possible for many people with the traditional way of detection due to their cost and availability. To detect the biomedical imaging terahertz (THz) frequency is a suitable range and using patch antenna in biomedical detection is cost effective compared to the traditional methods. So, to detect the early stage of breast tumor, this article has proposed a graphene based THz antenna. The primary goal of this antenna is to detect the breast tumor with the improvement of the performance and the secondary goal is to make the antenna very light weighted. The design of the antenna was done using computer simulation technology (CST) studio and the dimension of the antenna is kept 18×23 μm2 and a breast phantom model has been designed and the performance analysis of the antenna has been done for both free space and breast tissue. The return loss of the antenna in free space is -52.4422 dB at 6.96 THz. The details of the detection of tumor through the proposed patch antenna has been shown in this study. [ABSTRACT FROM AUTHOR]

A terahertz photonic crystal fiber with two sensing channels was designed. Graphene coated on the micro-grooves in the cladding was used as plasma material to introduce tunability. The dispersion relation, mode coupling, and sensing characteristics of the fiber were studied using the finite element method. Ultrahigh sensitivity of 2.014 THz/RIU and 0.734 GHz/°C were obtained for analytes with refractive index in the range of 1.33 to 1.4 and environment temperature in the range of 10–60 °C, respectively. Refractive index resolution can reach the order of 10−5. The dual parameter simultaneous detection, dynamic tunable characteristics, and working in the low-frequency range of terahertz enable the designed photonic crystal fiber to have application prospects in the field of biosensing. [ABSTRACT FROM AUTHOR]

Vortex‐vector beam generators can effectively improve the channel capacity and communication accuracy, but they face the problems of narrow operating frequency band, low efficiency, and fixed function. Here, a terahertz liquid crystal cascaded metadevice is demonstrated, including a liquid crystal layer, an asymmetric metasurface layer, and a helical phase metasurface layer. The combination of liquid crystal's anisotropy and structural anisotropy from the asymmetric metasurface realizes the dynamic chirality construction. Then, the tunable chiral fields are effectively coupled with the helical geometric phase of the last metasurface layer to realize an active control between symmetric and asymmetric spin‐orbit angular momentum coupling, leading to the two types of beam generation and conversion: 1) spin‐dependent conversion and energy distribution between the vortex beams with ±2 topological charges; 2) the conversions between the 2‐order vortex beam and the cylindrical vector beam. The experimental results show that the excitation efficiency of vortex beams can reach over 92.1%, and the intensity modulation depth between different types of beams is more than 72.4%. This THz beam dynamic conversion mechanism and device scheme increase the degree of freedom of the device and have potential applications in terahertz multi‐channel wireless communication, polarization multiplexing imaging, and chiral spectrum detection. [ABSTRACT FROM AUTHOR]

We present novel PIN photodiode (PD) continuous wave (cw) terahertz (THz) emitters with an increased responsivity and reduced substrate thickness compared to the state-of-the-art. Our improved devices feature up to 4 dB higher output power below 500 GHz with maximum power of -0.53 dBm at 115 GHz and strongly reduced THz absorption of the substrate for frequencies above 3 THz. The latter enables us to measure coherent cw THz spectra with a record bandwidth of 5.5 THz, for the first time, which is 1 THz (22%) more than the state-of-the-art. [ABSTRACT FROM AUTHOR]

It has been suggested that 'transient surface currents' caused by multi-MeV fast electrons can be responsible for the emission of low frequency radiation (e.g. in THz range) from ultra-intense laser–solid interactions. This mechanism has been analyzed, and analytic upper bounds on the intensity, electric field amplitude, and normalized vector potential have been developed and tested against 1D EM Particle-in-Cell simulations. The 'transient surface current' mechanism is effective and sufficiently efficient to fully account for all radiation that has been emitted in experiments so far. [ABSTRACT FROM AUTHOR]

Recently, with the development of nanotechnology and the emergence of new materials, Wireless Nanosensor Networks (WNSNs) have been presented. To address the problems of link instability in WNSNs and poor adaptability of the EBCNF framework to network topology changes, a reliable multi-path routing based on link stability called RMRLS is proposed. Based on the EBCNF framework, this protocol introduces a path similarity judgment model to judge the similarity between paths. In the route establishment process, the link stability evaluation model is used to measure the stability of each path and as a criterion for path selection. Then alternative route is selected through the routing similarity judgment model. Optimal path and backup path are two routing paths to cope with changes in the network topology. The simulation results show that the RMRLS protocol has advantages in energy efficiency, data packet transmission success rate and average throughput, which are 28.84% lower, 53.8% higher, and more than double compared to EBCNF at a distance of 10 mm, respectively. These findings proved the RMRLS protocol's capacity to significantly boost the stability and reliability of the network. [ABSTRACT FROM AUTHOR]

We propose an original design for a HgCdTe-based terahertz vertical-cavity surface-emitting laser with twenty 5 nm HgTe quantum wells. Feasibility of laser generation at 9 THz and a lattice temperature of 8 K is shown. The estimate of the threshold pump intensity using laser radiation at a wavelength of 5 μm is 3 W/cm2, which makes it possible to expect continuous wave (CW) mode lasing. Such a low required pump intensity will make it possible to create a very compact system of a laser pumped by CW mid-infrared quantum cascade laser. [ABSTRACT FROM AUTHOR]

In this study, a rapid water content detection method for donkey-hide gelatin was established based on terahertz time-domain spectroscopy in reflection mode. Notably, the effective medium theory, informed by the double-Debye relaxation for free water, was employed to predict the water content in the sample. The water content obtained through effective medium theory was lower than that of the traditional thermogravimetric method due to the existence of bound water. Bound water is invisible in THz spectroscopy due to its slower dynamics. In the frequency range of 0.22–0.48 THz, the offset in measured values between the thermogravimetric method and the THz-based method was fitted by a polynomial function. The results of THz-based method calibrated by polynomial fitting were consistent with the findings of the thermogravimetric method. The water content detection results derived by these two methods demonstrated high correlation values (R2) of 0.9860, 0.9160, and 0.9784, and small root mean square errors (RMSE) of 4.5584, 4.4887, and 5.3657 for three samples, respectively. Moreover, the proposed water content detection method needs to be calibrated only once by the thermogravimetric method. Overall, this study provides a rapid water content detection method and demonstrates the potential of real-time water content detection in donkey-hide gelatin. [ABSTRACT FROM AUTHOR]

It is crucial to study the materials that could effectively facilitate the inter‐conversion between charge, spin, and orbital degrees of freedom. In this work, the conversion among these three types of degrees of freedom in Pt/CoFeB/rare‐earth (RE, represents Nd, Gd, and Ho)/Ti multilayers is manipulated. Through terahertz (THz) emission measurements, it is found that the spin current induced by a femtosecond laser is converted into an orbital current via the spin‐orbit coupling of the RE layer. Notably, the light RE (Nd) and heavy RE (Gd and Ho) induce the orbital current with opposite polarization directions, ultimately leading to a weakening or enhancement of the THz emission intensity, respectively. Moreover, the fast Fourier transform reveals that Gd exerts the most significant influence on increasing the whole THz spectrum within the Pt/CoFeB/RE/Ti structure. The findings of RE‐modulated spin‐to‐orbital conversion provide valuable insights into the fundamental transport mechanism of the orbital current. [ABSTRACT FROM AUTHOR]

Millimetre-scale rhombohedral PCBM crystals were fabricated on multiple substrates, including metalised glass and polymer. The multi-substrate fabrications granted the ability of characterising the crystals composition, magnetically, electrically, and mechanically. Colossal crystals were fabricated with or without P3HT, elucidating novel capabilities of the crystals for flexible and wearable electronics applications. The lone PCBM crystals were characterised for their optical and mechanical properties, whereas the P3HT coated crystals have notable magnetic and conductive performances. The substrate type played a crucial role in property-map of this organic framework, including crystal growth and segregation. The PCBM crystals combined with P3HT hold a promising future for organic photovoltaics and magnetic systems. [ABSTRACT FROM AUTHOR]

Steganography technology which conceals a message in a carrier to make it invisible is critical for information security. Conventional optical image steganography using diffractive optical components or spatial light modulators suffers from less encoding channel and bulky volume. The emergence of multifunctional metasurface that can manipulate abundant physical dimensions of optical fields allows the multi‐channel image steganography in a compact volume. Here, the image hiding in a metasurface is demonstrated by modulating the amplitude, phase, and polarization states of terahertz (THz) waves completely. Especially, the phase channel can decouple from the amplitude channel based on a Fresnel‐diffraction‐based algorithm. By directly measuring the phase distribution using the homemade THz focal plane imaging system, the number of transmission channels can expand from N to 2N. As a proof of concept, it is shown that the secret images can encode in the phase channel and subsequently extract by using different keys, such as polarization states, detection distances, and its combinations. Moreover, different hiding strategies with different attacker behaviors are also demonstrated. The decoupling of the phase and amplitude channels with a single metasurface may open an avenue toward innovative optoelectronic devices for optical image steganography, data storage, and terahertz communication. [ABSTRACT FROM AUTHOR]

The Terahertz (THz) band is seen as one of the potentially essential technologies to meet the data throughput requirements of terabits per second (Tbits) in future networks. Considering vehicular communication in particular, the current vehicular communication technologies cannot cope with the rapidly increasing data traffic. To deliver increased capacity and data rate with reduced latency, new technologies and methodologies are therefore required. It is recognized that one possible technology to meet these needs is THz technology. Since the change in distance between vehicles due to their speed affects THz communication, channel modeling was carried out for single-lane and multi-lane vehicular communication scenarios in this study. The system's performance was also examined. Performance metrics were determined as path loss, signal-to-noise ratio (SNR), capacity and bit error rate (BER). While performing these analyses, the absorption of THz waves is taken into account and the High-Resolution Transmission Molecular Absorption Database (HITRAN) was used to model it. As a result, the most appropriate circumstances for successful vehicular communication have been identified and studied. [ABSTRACT FROM AUTHOR]

Cadmium sulfide thin films are deposited using the thermal evaporation technique and coated with 50 nm thick Sb nanosheets. Both the coated and uncoated films undergo structural, morphological, and optical investigations to explore potential modifications resulting from the Sb coating. The antimony nanosheets successfully increase the crystallite sizes from 28 to 34 nm and decrease the defect concentration from 4.36 × 1011 lines/cm2 to 3.02 × 1011 lines/cm2. The deposition of Sb nanosheets on CdS films induces the formation of nanowires with a length of 3.0 μm. Sb nanosheet coatings improve visible light absorption by more than nine times, redshift the energy band gap, suppress free carrier absorption in the infrared (IR) range, and increase the optical conductivity of CdS films. Additionally, as optical filters and waveguides, Sb-coated CdS films exhibit a terahertz cutoff frequency range of 0.47–32.06 THz as light energy increases from the IR to ultraviolet ranges. It is also observed that Sb coating alters the third-order nonlinear susceptibility of CdS, making it tunable, more polarizable, and suitable for nonlinear optical applications. Photocurrent measurements showed that Sb nanosheets improved the light responsivity by 330% and significantly enhanced the response time. The enhanced features of CdS achieved through Sb nanosheet coatings position CdS in the preferred group for nonlinear optical waveguides applicable in terahertz and nonlinear optical applications. [ABSTRACT FROM AUTHOR]

Chikungunya virus (CHIKV) poses a significant public health threat due to its capacity to cause widespread and debilitating outbreaks. The virus is responsible for CHIKV fever, a disease characterized by severe joint pain, sudden onset of fever, headache, muscle pain, and rash. The virus has been reported in various regions globally, with outbreaks occurring in parts of Africa, Asia, the Americas, and the Indian subcontinent. Consequently, the scientific community expends substantial efforts in developing dependable, rapid, highly sensitive, and cost-effective techniques in order to identify the CHIKV virus. In this study, an innovative biomedical sensor using photonic crystal fiber technology enables precise detection of the CHIKV virus through uric acid, normal and infected plasma, red blood cells, and platelets in the blood. The introduced sensor identifies those kinds with extremely increased relative sensitivity and minimal losses in contrast to alternative photonic crystal fiber-based biosensors. The introduced sensor showcases a minimal confinement loss of 2.25 × 10− 13 cm− 1, a relative sensitivity of 99.37%, an effective area of 1.36 × 105 µm2, with a minimal effective material loss of 0.001966 cm–1, a numerical aperture of 0.1874, and low dispersion of 0.06. Also, the demonstrated sensor is able to function within the terahertz spectrum, covering a substantial span from 0.8 to 2.6 THz. Furthermore, an extensive comparison analysis is performed between the showcased sensor and related literature on photonic crystal fibers to verify the reliability and effectiveness of the introduced structure. [ABSTRACT FROM AUTHOR]

Herein two stacked nanosheets of lanthanum and silver coated by the thermal evaporation technique under a vacuum pressure of 10–5 mbar are used to form Ag2La compound. Ag2La thin films exhibited crystalline orthorhombic structure with lattice parameters, crystallite sizes and defect densities of a = 6.670 Å, b = 10.386 Å, c = 7.485 Å, 29 nm and 3.99 × 1011 lines/cm2, respectively. The films showed an indirect band gap of 3.25 eV. The band gap contained energy band tails of widths of 0.75 eV. In addition, the dielectric dispersion analyses on the Ag2La film have shown their excellent performance as high k dielectric materials displaying high dielectric constant value of 9.5 at incident light energy of 2.12 eV. On the other hand the optical conductivity and terahertz cutoff frequency spectral analyses have shown that Ag2La films have potentials applications in terahertz technology. The terahertz cutoff frequency varied in the range of 2–9 THz based on the driving light signal energy. As optical resonators they showed high sensitivity to visible and ultraviolet light signals. A respective high drift mobility of ~ 1.5 cm2/Vs and 44 cm2/Vs are estimated for the semitransparent Ag2La terahertz optical filters. The simplicity of producing Ag2La compound, the high dielectric constant values and the semiconducting characteristics of this compound make it attractive for high k-dielectric devices and for terahertz technology applications. [ABSTRACT FROM AUTHOR]

This paper proposed an accurate series resistance model tailored for Schottky diode‐based terahertz multipliers. Compared to the conventional electrothermal model (E‐T model) only considering thermal effects, this model comprehensively accounts for both thermal and frequency effects of the series resistor components, including the temperature‐dependent epilayer resistance (Repi) and the temperature‐frequency‐dependent spreading resistance (Rspreading). The evaluation of thermal effects relies on steady‐state thermal simulation and the corresponding electrothermal model. Notably, the frequency‐dependent spreading resistance part is derived from the fitting conductivity of doped buffer layers and extracted by auxiliary three‐dimensional electromagnetic simulations. Based on this model, a balanced 225–300 GHz frequency tripler with AlN substrate has been designed and manufactured. By introducing this model, a significant improvement in the consistency between simulated and measured results has been achieved compared to the single E‐T model, regardless of whether the input power is low (80 mW) or high (160 mW). [ABSTRACT FROM AUTHOR]

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

With advances in terahertz technology, achieving high and nearly constant birefringence over a wide frequency range plays an extreme role in many advanced applications. In the past decade, significant research efforts have been devoted to creating new systems or elements with high birefringence. To our knowledge, the maximum birefringence attainable using artificial crystals, intrinsic liquid crystals or fiber-based systems has been less than unity. More importantly, the birefringence created in previous studies has exhibited a strong frequency dependence, limiting their practical applications. In this work, we propose a novel approach to achieve extraordinarily high birefringence over a broad terahertz frequency band (>100 GHz). To address the limitation of frequency dependence, we combined the principle of metamaterials with the form birefringence concept. First, we designed a metamaterial with an exceptionally high refractive index, thoroughly characterizing it using simulations and analytical analysis. Next, we systematically investigated the form birefringence concept, exploring its frequency response, geometric limitations, and complex refractive index differences between constituent elements. Finally, we designed a hybrid material system, combining the strengths of both metamaterials and form birefringence. Our results demonstrate the feasibility of achieving a birefringent medium exceeding three orders of magnitude higher than previous reports while maintaining a time-invariant frequency response in the sub-terahertz regime. [ABSTRACT FROM AUTHOR]

A perfect metamaterial absorber (MMA) using a SiO2 substrate has been designed. The suggested absorber model was created by implementing graphene as a metallic layer for the structured patch and copper in the ground plane of the design. The geometry of the prescribed absorber was finalized by cutting the rectangular and circular slots and loading the open-ended dual rings on the structure. Furthermore, the approach based on equivalent circuit modeling (ECM) with mathematical analysis for the proposed MMA is thoroughly discussed. The simulation outcome (S11) obtained with the help of CST software has been compared with the ECM analysis result. The proposed absorber shows a perfect absorbance of 99.97% at the operating frequency of 6.15 THz. The absorption characteristics of the suggested structure have been parametrically investigated as a function of oblique incidence and polarization angles. Furthermore, the designed structure has been analyzed by using various substrates and conducting materials to achieve the optimal value of the absorption peak as a percentage. Overall, the compact, ultra-thin, simple geometry of the prescribed MMA with a high absorption peak and polarization-insensitive nature makes it a suitable choice for utilization in sensing, imaging, and detection in the terahertz regime. [ABSTRACT FROM AUTHOR]

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

Terahertz (THZ) spectroscopy has emerged as a superior label-free sensing technology in the detection, identification, and quantification of biomolecules in various biological samples. However, the limitations in identification and discrimination sensitivity of current methods impede the wider adoption of this technology. In this article, a meticulously designed metasurface is proposed for molecular fingerprint enhancement, consisting of a periodic array of lithium tantalate triangular prism tetramers arranged in a square quartz lattice. The physical mechanism is explained by the finite-difference time-domain (FDTD) method. The metasurface achieves a high quality factor (Q-factor) of 231 and demonstrates excellent THz sensing capabilities with a figure of merit (FoM) of 609. By varying the incident angle of the THz wave, the molecular fingerprint signal is strengthened, enabling the highly sensitive detection of trace amounts of analyte. Consequently, cinnamoylglycine can be detected with a sensitivity limit as low as 1.23   μ g · c m − 2 . This study offers critical insights into the advanced application of THz waves in biomedicine, particularly for the detection of urinary biomarkers in various diseases, including gestational diabetes mellitus (GDM). [ABSTRACT FROM AUTHOR]

Terahertz spoof plasmon propagation in a vacuum region sandwiched by a planar and corrugated parallel metal plate has been analytically and numerically investigated. Metallic corrugations in the form of long rectangular grooves in transverse direction support confined surface plasma wave propagation in the THz regime. The parallel metal surface in close proximity ensures the propagation of spoof plasmons in a highly confined manner. A theoretical dispersion relation is obtained corresponding to the waveguide design using the effective medium approximation method which confirms plasmonic behavior. Further, the technique of finite difference time domain method is used to study the terahertz mode propagation along the waveguide. The attenuation coefficient and propagation length are calculated to understand the guided wave capability of the waveguide configuration. This study is helpful to analytically understand the role of corrugations in parallel plate configuration in guiding the terahertz modes and hence can be significant in designing plasmonic devices for terahertz photonics [ABSTRACT FROM AUTHOR]

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

The plasmon excitation in two-dimensional electron gases is a significant way of achieving micro-nanoscale terahertz (THz) devices. Here, we establish a kinetic simulation model to study the THz plasmons amplification in a semiconductor double-quantum-well system with counter-streaming electron drift velocities. By comparing the simulation results with theoretical dispersion relations, we confirm two competing mechanisms of negative damping suitable for THz amplification: Cherenkov-type two-stream instability and a new non-Cherenkov mechanism called kinetic relaxation instability. The former is caused by the interlayer coupling of two slow plasmon modes and only exists when the drift velocities are much greater than the fermi velocities. The latter is a statistical effect caused by the momentum relaxation of electron-impurity scattering and predominates at lower drift velocities. We show that an approximate kinetic dispersion relation can accurately predict the wave growth rates of the two mechanisms. The results also indicate that the saturated plasmonic waves undergo strong nonlinearities such as wave distortion, frequency downshift, wave-packet formation, and spectrum broadening. The nonlinear evolution can be interpreted as the merging of bubble structures in the electron phase-space distribution. The present results not only reveal the potential mechanisms of the plasmonic instabilities in double-layer 2DEGs, but also provide a new guideline for the design of on-chip THz amplifiers. [ABSTRACT FROM AUTHOR]

Due to the limitation of spraying conditions, the microstructure of Thermal Barrier Coatings (TBCs) is not homogeneous, so it is important to extract the peak information of the reflected signal efficiently and accurately using Terahertz (THz) non-destructive detection technique for online thickness measurement. To this end, a hybrid model peak information extraction method based on a convolutional neural network (CNN) and a gated recurrent unit (GRU) is proposed. Firstly, a theoretical model of the THz signal is used to generate the simulated signal; secondly, a 1D-CNN is used to extract features adaptively from the temporal signal input, and a GRU is constructed to learn the temporal information between feature vectors to complete the extraction of peak information; finally, a calibration strategy is employed to determine the highest value around the original localisation result in order to eliminate localisation error and achieve peak location. The proposed CNN-GRU model predicts evaluation metrics that are 2–4 times smaller than those of other methods, and the running time is reduced to 45.71% compared to networks with close prediction accuracy. The method can reduce manual involvement and time expenses while maintaining prediction accuracy and that it provides a new intelligent way for online thickness measuring. [ABSTRACT FROM AUTHOR]

We present two ideas to simplify the measurement and analysis of terahertz time-domain spectroscopic ellipsometry data of ultrathin films. The measurement is simplified by using a specially designed sample holder with mirrors, which can be mounted on a cryostat. It allows us to perform spectroscopic ellipsometry by simply inserting the holder into a conventional terahertz spectroscopy system for measurements in transmission geometry. The analysis of the obtained data is simplified by considering a single interface with a certain sheet conductivity σ s (since the film thickness is significantly smaller than the wavelength of the terahertz light). We demonstrate the application of these ideas by evaluating the sheet conductivities of two perovskite rare-earth nickelate thin films in the temperature range 78–478 K. The use of this particular analytical method and the sample holder design will help to establish terahertz time-domain spectroscopic ellipsometry as a characterization technique for ultrathin films. [ABSTRACT FROM AUTHOR]

A multifunctional terahertz metamaterial is proposed for achieving asymmetric transmission (AT) and broadband absorption, leveraging the phase transition property of vanadium dioxide (VO2). By manipulating the phase state of VO2, the metamaterial demonstrates switchable functionalities across distinct frequency ranges. When VO2 is in its insulating state, within the frequency range of 1.15–1.88 THz and the incident angle range of 0–12°, the AT efficiencies are all above 0.6, exhibiting wideband AT characteristics. Additionally, polarization conversion is achieved during transmission in the terahertz regime. When VO2 transitions to its conducting state, the metamaterial operates as a broadband perfect absorber, achieving absorptance exceeding 90% for both y- and x-polarized waves within the frequency range of 2.5–3.3 THz. The electric field and surface current distributions are presented to elucidate the underlying physical mechanisms. Moreover, the influence of structural parameters and angular tolerance have been comprehensively investigated. This proposed metamaterial offers a promising platform for developing multifunctional terahertz devices. [ABSTRACT FROM AUTHOR]

Terahertz (THz) waves have exhibited promising prospects in 6G/7G communications, sensing, nondestructive detection, material modulation, and biomedical applications. With the development of high-power THz sources, more and more nonlinear optical effects at THz frequency and THz-induced nonlinear optical phenomena are investigated. These studies not only show a clear physics picture of electrons, ions, and molecules but also provide many novel applications in sensing, imaging, communications, and aerospace. Here, we review recent developments in THz nonlinear physics and THz-induced nonlinear optical phenomena. This review provides an overview and illustrates examples of how to achieve strong THz nonlinear phenomena and how to use THz waves to achieve nonlinear material modulation.