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Moxibustion, a traditional Chinese medicine practice, employs Moxa Wool , derived from Artemisia argyi. Flavonoids, the key pharmacological constituents in Moxa Wool , are known for their anti-inflammatory and analgesic properties. The purity of Moxa Wool , particularly its flavonoid content, directly influences the efficacy of moxibustion treatments. However, quantifying these bioactive flavonoids accurately and non-destructively has been a challenge. This study introduces terahertz spectroscopy as a non-destructive optical detection method for qualitative detection and quantitative analysis of flavonoids in Moxa Wool. By establishing a mathematical model between spectral signals and clinical efficacy, a reliable correlation between flavonoid concentration and the therapeutic effect of moxibustion can be established, providing a potential predictive model for the treatment outcomes of rheumatoid arthritis. We adopted terahertz spectroscopy technology and combined it with terahertz metamaterial biosensors to achieve rapid, efficient, and non-destructive testing of the quality of Moxa Wool. This method reduces the detection time from hours to minutes while lowering the sample detection limit, overcoming the limitations of traditional detection methods in pharmacological research. Through terahertz metamaterial biosensors, rapid detection of the purity of Moxa Wool has been achieved. A combination of molecular simulation and terahertz spectroscopy was used to quantitatively analyze the flavonoid content in different purities of Moxa Wool. To ensure accuracy, the quantitative results of flavonoids obtained by terahertz spectroscopy were validated using high-performance liquid chromatography (HPLC). In addition, moxibustion treatment was performed on rats with rheumatoid arthritis using Moxa Wool , and medical indicator information was recorded. A mathematical analysis model was established to evaluate the correlation between flavonoid content and analgesic and anti-inflammatory effects. Terahertz spectroscopy analysis shows that there is a direct correlation between the flavonoid content in moxibustion and the absorption peak intensity. The maximum R2 in the model analysis is 0.98, indicating a high accuracy in predicting the purity of Moxa Wool. These results were also validated by HPLC. In a rat model, the purity of 30:1 Moxa Wool samples showed a 50 % decrease in TNF-α, IL-1β, and IL-6 levels during treatment compared to low-purity samples, significantly reducing inflammation markers and pain symptoms. Meanwhile, The PLS prediction model established a correlation between terahertz-detected flavonoid levels and treatment outcomes (PWL and IL-1β). The maximum R2 in the model is 0.91, indicating a high correlation between flavonoid levels and the anti-inflammatory and analgesic effects of moxibustion treatment. This study not only demonstrates the effectiveness of terahertz spectroscopy in the pharmacological quantification of bioactive compounds but also establishes a novel predictive model for the efficacy of moxibustion in rheumatoid arthritis treatment. It underscores the potential of integrating traditional medicine insights with advanced technology to enhance therapeutic strategies in pharmacology. [Display omitted] [ABSTRACT FROM AUTHOR]

Toroidal excitation manifests as currents flowing on the surface of a torus along its meridians that are known as poloidal currents. Several intriguing phenomena such as nonradiating anapole excitation, dynamic Aharonov–Bohm effect, and vector potential in the absence of electromagnetic fields occur in the presence of toroidal excitations. Toroidal resonances in metamaterials also offer a platform to probe strong light–matter interactions in its proximity due to the nonradiating confinement of photons. Here, a dual‐torus toroidal flexible metasurface containing an array of overlapped split ring resonators is experimentally demonstrated, which results in strong field confinement for enhanced sensing of polar liquid analytes. Microfluidic integrated dual‐torus toroidal resonance experimentally shows significantly higher sensitivity of 124.3 GHz per refractive index unit compared to a single torus toroidal resonance. Flexible metasurfaces supporting dual torus toroidal resonance can find a wide range of applications in the development of ultrasensitive terahertz molecular sensors for probing real time monitoring of chemicals and biomolecules. [ABSTRACT FROM AUTHOR]

In this work we perform ultrafast optical pump‐optical probe (OPOP) and optical pump terahertz (THz) emission (OPTE) studies on the ultrafast excitation dynamics in <110> ZnTe crystals. Ultrafast two‐photon absorption and coherent phonon are revealed in OPOP measurements. Pump‐power‐ and polarization‐dependent phonon dynamics are characterized in time‐resolved transmission, reflection, and Kerr rotation using OPOP. The phonon polariton‐induced THz emission is directly observed in the time domain of OPTE dynamics. It is clear that the transverse optical phonon at ~3.7 THz and phonon polariton at ~2.6 THz are evident in OPOP measurement while OPTE only reveals part of the polariton dynamics. [ABSTRACT FROM AUTHOR]

We analyzed fluid fill height sensing supported by terahertz spoof surface plasmon polaritons (SPPs). The existence of spoof SPPs was proved experimentally and was discussed from perspective of dispersion and field distribution. For the application, we designed a fluid fill height sensor based on spoof SPPs, which is much more sensitive than previous work based on parallel plate waveguide (PPWG). For example, resonant shift of spoof SPPs is almost four times larger than the shift of PPWG at the height of 250 μm. This fluid fill height sensor calibrated the fluid volume in grooves, which contributes to improving the accuracy of high refractive index sensing. The experiment is in good agreement with simulation. Such proposed fluid height sensor has another advantage of being suitable for the fluid with refractive index higher than prism, which further benefits spoof SPPs refractive index sensing, such as blood detection in future. [ABSTRACT FROM PUBLISHER]

Terahertz reflection behaviors of metallic-grating-dielectric-metal (MGDM) microcavity with a monolayer graphene embedded into the dielectric layer are theoretically investigated. A tunable wideband reflection dip at about the Fabry–Pérot resonant frequency of the structure is found. The reflectance at the dip frequency can be electrically tuned in the range of 96.5% and 8.8%. Because of the subwavelength distance between the metallic grating and the monolayer graphene, both of the evanescent grating slit waveguide modes and the evanescent Rayleigh modes play key roles in the strong absorption by the graphene layer. The dependence of reflection behaviors on the carrier scattering rate of graphene is analyzed. A prototype MGDM-graphene structure is fabricated to verify the theoretical analysis. Our investigations are helpful for the developments of electrically controlled terahertz modulators, switches, and reconfigurable antennas based on the MGDM-graphene structures. [ABSTRACT FROM AUTHOR]

A polarization sensitive frequency selective surface (FSS) composed of connected electric LC resonators is proposed as the terahertz wave filter. The experimental results match the simulation results well and show that the FSS has good capability of polarization control and wavelength selection. The tunability of FSS is also studied by investigating the relation between the conductivity of the joint area of bent-lines and the resonant peak position/strength. Such a device can be applied as a flexible polarization modulator. [ABSTRACT FROM PUBLISHER]

We applied the harmonic oscillator model combined with the transfer matrix method to study the polarization conversion for transmitted waves in metallic grating/plasmon-excitation layer/metallic grating structure in the terahertz (THz) region. By comparing the calculated spectra and the simulated (by the finite-difference-time-domain method) ones, we found that they correspond well with each other. Both methods show that the Drude background absorption and the excited plasmon resonances are responsible for polarization conversion. The transmission is close to 0 when the distance between the top/bottom metallic gratings and gated graphene is an integer multiple of half the wavelength of the incident wave (in the dielectrics), at which points the plasmon resonances are greatly suppressed by the destructive interference between the backward/forward electromagnetic waves and that reflected by the top/bottom metallic gratings. Away from these points, the transmission can be higher than 80%. The electron density and the excitation efficiency of the plasmon-excitation layer were found to be important for the bandwidth of the polarization conversion window, while the scattering rate was found to influence mainly the polarization conversion rate. Multi-broadband polarization conversion is realized by exciting plasmon modes between the 0 transmission points in the THz region. [ABSTRACT FROM AUTHOR]

Terahertz (THz) fingerprint metasensing is an effective method to identify chemical substances in a rapid and non-destructive way. Currently, two main principles are used in THz metasensing: the change of the real part of permittivity causing the dip resonance frequency deviation, and the fingerprint peak of the imaginary part of permittivity causing the dip resonance splitting (absorption induced transparency, AIT). Most previous work investigated AIT detection for only single chemical substance. The suitable AIT metasensor structure are still required for simultaneously measurement of multiple and mixture chemical substances. In this manuscript, we proposed the N-order concentric rings metasensor for specific recognition multiple and mixed chemical substances based on AIT fingerprint enhancement. The structure has broadband multiple plasmonic resonance dips which are generated by near field dipole resonances. The equivalent circuit model was built to realize the reconfigurable function. Then, 5-order concentric rings structure was designed and fabricated for simultaneously specific recognition of four chemical substances (α-lactose, benzoic acid, vitamin B2 and 2, 5-dichloroanline). The influence of the real and imaginary part of the chemical substances' permittivity on AIT effect had discussed in details. Simulation results indicated that the frequency-deviation of the resonance dip can be stabilized and will not be changed when the concentration of chemical substances is over 20 mg/mL. As shifted plasmonic resonance peaks match the chemical substances' imaginary part of permittivity fingerprint spectra, the perfect AIT effect can be realized. The metasensor can simultaneously and non-destructively conduct a specific detection of α-lactose, benzoic acid, vitamin B2 and 2,5-dichloroanline, and their mixture. The limit of detections of α-lactose, benzoic acid, vitamin B2 and 2,5-dichloroanline are 8.61 mg/mL, 6.96 mg/mL, 7.54 mg/mL and 8.35 mg/mL, respectively. Also, the sensitivity of the metasensor can reach 0.00211, 0.00208, 0.00211 and 0.00219 (unit: 1/mg/mL), respectively. By utilizing one-way analysis of variance method, the possibility of recognition error for each chemical substance is lower than 0.001. Our metasensor provides a novel and accurate platform for THz fingerprint sensing. [Display omitted] • A N-order concentric rings patten has been investigated for simultaneous multiple fingerprint detection. • Different dipole resonance mode comes from different diameters of the concentric rings. • Four dipole modes of 5-order plasmonic metasensor was optimized to perfectly match the fingerprint absorption spectra of four chemical substances by considering frequency-shift compensation. • The plasmonic metasensor can simultaneously distinguish fingerprints of four chemical substances according to AIT effect. [ABSTRACT FROM AUTHOR]

• An ultrasensitive THz biosensor with a Q factor up to 503 is proposed for the first time. • The wide-area and intense light-matter interaction supported by QBIC is realized and proved. • The limit of detection for trace molecules is about 40 times higher than that of traditional THz biosensor. A quasi-bound state in the continuum (QBIC) has unique attraction in optical switch, nonlinearity, communication, and sensing due to its ultrahigh radiation quality (Q) factor. The QBIC observed in metasurfaces also provides a feasible platform to achieve in-plane strong light-matter interaction, as well as to develop ultrasensitive biosensor. However, the existing metasurface designs are difficult to realize highly efficient excitation and high-performance sensing of QBIC in terahertz (THz) band. Here, we manipulate the interference coupling between electric quadrupole and magnetic dipole by introducing an asymmetry α into the metallic metasurface structure, which excites ultrahigh quality QBIC resonance with Q factor of up to 503. Correspondingly, light field energy constrained by the metasurface and effective sensing area achieved enormous increases of about 400% and 1300%, respectively, which greatly expands the spatial extent and intensity of light-matter interaction. Simulations and experiments show that the proposed QBIC metasurface deliver a high refractive index sensitivity reaching 420 GHz/RIU, where RIU is the refractive index unit, and its direct limit of detection (LoD) for trace homocysteine (Hcy) molecules is 12.5 pmol/μL. Its performance is about 40-times better than that of the classical Dipole mode. This work provides a new avenue to achieve rapid, precise, and nondestructive sensing of trace molecules, and has potential applications in the fields of biochemical reaction monitoring, photocatalysis and photobiomodulation. [ABSTRACT FROM AUTHOR]

A polarization dependent dual-band EIT-like effect (0.35 THz and 0.7 THz) has been applied to optimize the FSS-based terahertz devices. The simulated and experimental results show that such devices can act as a polarization dependent narrowband THz wave filter. Under modulation signal, the optical controllable design can be adopted as a flexible polarization modulator. These results can be further applied in polarization controllable terahertz source for nonlinear terahertz optics. [ABSTRACT FROM AUTHOR]

We fabricated several reverse conical holes on high-resistivity silicon substrate with different power and pulse number of femtosecond laser, and investigated their patterns and features by using scanning electron microscope (SEM). Then, we chose one of the experimental parameters prepared a reverse conical anti-reflection structure sample with period of 90 μm. Terahertz Time-domain Spectroscopy (THz-TDS) was used to test its properties. Compared with the nonstructural high-resistivity silicon, the transmission of structural high-resistivity silicon increases by the maximum of 14% in the range 0.32–1.30 THz. Furthermore, we simulated the sample by finite integral method (FIM). The simulated results show good consistency with experimental results. The transmission effect of the reverse conical holes were optimized via simulation. Results show that the related transmission effect can be improved by increasing the pulse numbers and decreasing the spot size of the femtosecond laser. The different transmission window can also be tuned by changing the reverse conical structure of different periods. [ABSTRACT FROM AUTHOR]

An optically controllable terahertz wave modulator based on the electromagnetically induced transparency-like effect (EIT-like) of the metamaterial structures is proposed and demonstrated. A modulation depth of 63% was measured at 0.33 THz in our study. The modulation action arises from the destructive interference between the resonators composed of high-resistivity silicon and gold resonators. Utilizing terahertz time domain spectrometer (THz-TDS), we show that the transmission properties of the structures can be tuned by an externally applied pump beam. By comparing the modulation depth with and without the structures producing EIT-like behavior, and the bare silicon's modulation depth, it is found that the modulation performance can be significantly improved with employment of EIT effect. Our study provides an alternative route to facilitate potential applications in terahertz range. [ABSTRACT FROM AUTHOR]

We propose an ultrathin and flexible dual band absorber operated at terahertz frequencies based on metamaterial. The metamaterial structure consists of periodical split ring resonators with two asymmetric gaps and a metallic ground plane, separated by a thin-flexible dielectric spacer. Particularly, the dielectric spacer is a free-standing polyimide film with thickness of 25 µm, resulting in highly flexible for our absorber and making it promising for non-planar applications such as micro-bolometers and stealth aircraft. Experimental results show that the absorber has two resonant absorption frequencies (0.41 THz and 0.75 THz) with absorption rates 92.2% and 97.4%, respectively. The resonances at the absorption frequencies come from normal dipole resonance and high-order dipole resonance which is inaccessible in the symmetrical structure. Multiple reflection interference theory is used to analyze the mechanism of the absorber and the results are in good agreement with simulated and experimental results. Furthermore, the absorption properties are studied under various spacer thicknesses. This kind of metamaterial absorber is insensitive to polarization, has high absorption rates (over 90%) with wide incident angles range from 0° to 45° and the absorption rates are also above 90% when wrapping it to a curved surface. [ABSTRACT FROM AUTHOR]

We experimentally and numerically demonstrate dual-mode resonances of electrically unconnected and connected U-shaped resonators array on flexible polyimide substrate in terahertz (THz) region. The unit cell consisting of two back-to-back U-shaped resonators (TUR) shows strong induction-capacitance resonance for TM mode and dipole resonance for TE mode. Then we clarify the transmission spectra characteristics of fourfold U-shaped resonators (FURs), which are composed of two twisted TUR at 90°. Distinct characteristic transmission responses are observed in varied coupled distances, specially, an “ L -shaped” resonance is observed at connected FUR which does not exist in unconnected FUR. The polarization tunable property of TUR and polarization-independent characteristics of FUR are adequately clarified. Some applications such as polarization-tunable modulator (peak-to-peak value is about 0.4 for 90° polarization rotation) and polarization-independent dual-channel filter (full width with half maximum (FWHMs) nears to 85 GHz) is proposed and verified experimentally, which may benefit to broaden novel ultrathin flexible THz devices. [ABSTRACT FROM AUTHOR]

Heavy metal pollution in water seriously affects human health. The disadvantages of traditional metal ion detection methods involve long and cumbersome chemical pretreatment in the early stage, and large volume of samples. In this study, microalgae were used as the medium, and terahertz spectroscopy technology was employed to collect the changes of material components in it, so as to deduce the types and concentrations of heavy metal pollution in water. Through the partial least square(PLS), we establish the prediction model of heavy metal concentration, and the results show that the best detection time for Pb2+ is 6 h and Ni2+ is 18 h. The principal component analysis(PCA) shows that β-carotene is the most affected substance. Afterward we collect five real surface waters in East China and verify that the judgment accuracy of Pb2+ and Ni2+ are 100% and 93.2% respectively. The results indicate that the time is shorter than the traditional pretreatment time from more than 20–6 h, the sample volume is reduced from 50 mL to 10 mL, the detection accuracy is improved from 10 ng/mL to 1 ng/mL. In a word, we provide a new fast and real-time method for biological monitoring of heavy metal pollution in water. [Display omitted] • Detection of heavy metal pollution in water by terahertz spectroscopy combined with microalgae. • The accuracy of detecting the concentration of Pb2+ and Ni2+ is 0.1 ng/mL. • The prediction accuracy of Pb2+ is 100%, The prediction accuracy of Ni2+ is 93.2%. • The prediction time of Pb2+ is 6 h. • The consumed sample size is 10 mL. [ABSTRACT FROM AUTHOR]