Terahertz Time-domain Spectroscopy Research Articles

Time-Domain-Filtered Terahertz Nanoscopy of Intrinsic Light-Matter Interactions.

Terahertz (THz) technology holds great potential across diverse applications, including biosensing and information communications, but conventional far-field techniques are limited by diffraction. Near-field optical microscopy overcomes this barrier through a sharp tip that concentrates incident THz waves into nanometric volumes, detecting scattered near-field to reveal nanoscale optical properties. However, owing to the large THz wavelengths, resonant surface waves arising on the tip and cantilever obscure the intrinsic response. Here we combine near-field microscopy with THz time-domain spectroscopy and implement time-domain filtering with an elongated cantilever to eliminate this artifact, achieving intrinsic nanospectroscopy and nanoimaging. By applying this technique, we distinguish and characterize historical pigments of an ancient sculpture, such as vermilion and red lead, on the nanoscale. We also unravel deep-subwavelength localized resonance modes in THz optical antennas, demonstrating capabilities for THz nanophotonics. Our work advances THz nanoimaging and nanospectroscopy techniques to probe intrinsic nanoscale THz light-matter interactions.

Developing Hydration Maps of Polymer Latex Film Formation Using Terahertz Time-Domain Spectroscopy.

The dynamics of the drying process of polymer latexes after casting as a wet film onto a substrate are important to track as they influence the physical and mechanical properties and performance of the dried polymer films. Current methods used to follow this drying process include gravimetric analysis, coupled with advanced techniques like GARField-NMR or optical coherence tomography. The latter two methods provide height and spatial information in the z-direction, normal to the substrate, and occasionally in the xz- or yz-planes. Terahertz time-domain spectroscopy (THz-TDS) is a welcome addition as it provides both the structural and spectroscopic information in the parallel xy-plane, filling the geometric gap. Herein, we utilize THz-TDS to study the drying and film formation process of various polymer latexes with a broad range of glass transition temperatures. We showcase the applicability of this technique in obtaining 2D parallel hydration maps of the drying dispersions, in the form of droplets, using latex-dependent calibration lines. Our findings display known phenomena arising from the drying of the colloidal dispersions.

Terahertz time-domain spectral hierarchical thickness measurement based on integer genetic algorithm

ABSTRACT Terahertz time-domain spectroscopy is a widely used non-contact detection technology with significant potential in the field of non-destructive testing. In this study, the transfer function of the coating system is derived based on the Rouard model and the refractive index of the coating within the transfer function is modelled using the Debye model. The hierarchical thickness of the multilayer composite material is fitted by random optimisation algorithm. Considering the complexity of the objective function and the practical accuracy requirements in engineering problems, the integer genetic algorithm is adopted to find the optimal parameters. The comparison of the ordinary genetic algorithm and the integer genetic algorithm reveals that the integer genetic algorithm requires a significantly smaller population size and computation time than the ordinary genetic algorithm. Furthermore, the predicted thickness obtained by the integer genetic algorithm is more repeatable, has higher computational efficiency, and exhibits smaller errors. Our results indicate that combining the integer genetic algorithm with the Rouard model holds potential applications in thickness measurement of multilayer composite materials.

Tuning direct-written terahertz metadevices with organic mixed ion-electron conductors

In the past decade, organic mixed ion-electron conductors have been successfully adopted in innovative bioelectronic, neuromorphic, and electro-optical technologies, as well as in multiple energy harvesting and printed electronics applications. However, despite the intense research efforts devoted to these materials, organic mixed conductors have not yet found application in electronic/photonic devices operating in key regions of the electromagnetic spectrum, such as the microwave (>5 GHz) and terahertz (0.1-10 THz) ranges. A possible reason for this technological gap is the widespread notion that organic electronic materials are unsuitable for high-frequency applications. In this work, we demonstrate for the first time the utility of high-performance polymer mixed conductors as electro-active tuning layers in reconfigurable terahertz metasurfaces, achieving modulation performances comparable with state-of-the-art inorganic and 2D semiconductors. Through time-domain terahertz spectroscopy, we show that the large conductivity modulations of these polymers, until now probed only at very low frequencies, are effectively preserved in the terahertz range, leading to optimal metadevice reconfigurability. Finally, we leverage the unique processability of organic materials to develop fully direct-written electrically tuneable metasurfaces onto both rigid and flexible substrates, opening new opportunities for the mass-scale realization of flexible and light-weight terahertz optics with unique mechanical characteristics and environmental footprint.

Exploring effects of plasma surface engineering on cellulose nanofilms via broadband THz spectroscopy

For the extended ultra-wideband THz band up to 10 THz and beyond, materials with different functional properties are required. In addition, the THz shielding devices require the desired thickness for different shielding effects, which can be tailored by the surface functionalities. Here, a plasma-assisted soft chemistry approach was used to tailor the cellulose nanofibrils (CNFs), and the effects of plasma surface treatment were investigated using non-destructive terahertz time-domain spectroscopy (THz-TDS). CNFs with three different thicknesses were investigated as a function of mild and strong plasma treatment. The ultra-broadband THz-TDS evaluation of CNF films overcomes the challenge by surpassing the conventional low-frequency THz-TDS measurements and reveals many characteristic THz features of CNF at 2.1, 3.1, 5.1, 7.0, 10.3 and 10.9 THz. THz-TDS analysis indicates thinning of CNF after plasma treatment due to the plasma etching effect. At the same time, the microcrystalline properties of the CNF samples remain intact even after significant surface modification under plasma carbonisation conditions. Apart from this, the studies proposed the required minimum thickness for THz shielding applications based on the optical parameters and THz properties derived from the plasma-treated CNF structures, which could be used to define the thickness of sustainable shielding materials for THz devices.

Temperature-dependent optical and dielectric properties of polyvinyl chloride and polystyrene blends in terahertz regime

Terahertz time-domain spectroscopy (THz-TDS) has been used to study the temperature-dependent refractive index, absorption coefficient and dielectric constant of polyvinyl chloride/polystyrene (PVC/PS) blends in frequency range of 0.2–1.8 THz. Moreover, the Sellmeier and thermo optic coefficients of PVC/PS blends with different weight ratios have been explored in the temperature range of 25–80 °C. These parameters are used to evaluate the dispersion properties of these blends in the observed frequency range. A clear indication of temperature dependence on the values of refractive index and the real dielectric constant of these blends have been observed. Their values decrease linearly with increasing temperature up to 80 °C. Whereas, no noticeable change has been observed in the imaginary dielectric constant and the absorption coefficient. These results provide a database of temperature-dependent optical and dielectric parameters of PVC/PS polymer blends for their efficient utilization for device fabrication in THz technology.

Optical Properties of Millimeter-Size Metal-Organic Framework Single Crystals Using THz Techniques

Metal-organic frameworks (MOFs) crystals are promising emerging materials for terahertz (THz) photonics i.e., for THz wave generation through difference frequency or optical rectification and electrooptic detection, including optical components for THz beam steering. The present work reports optical properties of three different non-centrosymmetric single MOF crystals, grown by an innovative solvo-thermal technique with tunable morphology, termed MOF [Zn(3-ptz)2]n (MIRO-101). THz time-domain spectroscopy (TTDS) in the range of 0.25 – 1.5 THz has been used for the measurement of the transfer function, H(ω) of these MOF crystals. Through this experimental function Hexp(ω), optical parameters such as refractive index, nMOF(ω) and absorption coefficient,αMOF(ω) have been calculated for the analysis of the optical properties of this crystal. The results indicate that this MOF crystal offers opportunities for long-term exploration of properties toward the creation of novel nonlinear THz photonics materials, as a THz radiation emitter via Different Frequency Generation (DFG) or Optical Rectification (OR) and Electro-optic (EO) detection via optical sampling, including for its use in optoelectronics, and materials science.

Donepezil hydrochloride fingerprint spectral and formation mechanism analysis.

Donepezil hydrochloride is a widely used medication for treating Alzheimer's disease. This study utilized terahertz time-domain spectroscopy to analyze the fingerprint spectra of donepezil hydrochloride, identifying five characteristic absorption peaks at 1.65, 2.44, 2.56, 3.31, and 3.75THz. The vibrational spectrum of the donepezil hydrochloride crystal was further examined using solid-state density functional theory. Based on simulation calculations, the characteristic peaks were identified and analyzed in detail, focusing on long-range ordering and weak interaction networks. The results demonstrate that terahertz spectroscopy is an effective tool for studying intermolecular interactions in drug crystals.

Fiber-Coupled THz TDS System With mW-Level THz Power and up to 137-dB Dynamic Range

Fiber-Coupled THz TDS System With mW-Level THz Power and up to 137-dB Dynamic Range

Enhanced Terahertz Characterization of Multilayer Graphene on Guided‐Mode Resonance Filter: Boosting Sensitivity and Precision in Electrical and Optical Characteristics

AbstractIn 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.

Terahertz spectroscopy study of oridonin and ponicidin in the anticancer Chinese herbal medicine Rabdosia rubescens.

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.

Generation of a coherent longitudinal optical phonon plasmon coupled mode in an n-type InSb single crystal in the absence and/or strong suppression of the photo-Dember effect confirmed with the use of terahertz time-domain spectroscopy

We explored the feasibility of generating a coherent longitudinal optical (LO) phonon–plasmon coupled mode in an n-type InSb single crystal with the condition that the photo-Dember effect is strongly suppressed and/or eliminated. We systematically performed terahertz time-domain spectroscopic measurements of the n-type InSb single crystal and undoped InSb crystal. The terahertz electromagnetic wave originating from the photo-Dember effect was extinguished in the n-type InSb single crystal, while the clear signal from the photo-Dember effect was detected in the undoped InSb single crystal. Meanwhile we detected the clear signal of the terahertz wave from the coherent LO-phonon–plasmon coupled mode in the n-type InSb single crystal, indicating that the photo-Dember effect is not dominant in the generation of the coherent LO-phonon–plasmon coupled mode although the photo-Dember effect has been attributed to the dominant generation mechanism of the coherent LO-phonon–plasmon coupled mode so far. We explain the present phenomenon, taking account of the fact that the impulsive stimulated Raman scattering process has a possibility of generating coherent LO-phonon–plasmon coupled modes in light-absorbing solids [Nakamura et al., Phys. Rev. B 99, 180301(R) (2019)].

Single pulse shaping for higher harmonic demodulation in terahertz time-domain spectroscopy

We present a simple, highly stable, low noise, and rapid detection scheme using higher harmonic demodulation applied to terahertz time domain spectroscopy (THz-TDS). The presence of higher harmonics in the detected periodic signal is because of the non-sinusoidal shape of a single pulse, which is controlled by an ultrafast current pre-amplifier. Instead of using external signal modulators, the use of the inherent repetition rate (frep) of the femtosecond laser and its harmonics as reference for the lock-in amplifier simplifies the setup, while allows rapid and low noise detection owing to the megahertz modulation frequencies. Unlike the signal detected at the fundamental frep, signals detected at higher harmonics have much lower offset and are unaffected by perturbations in the environment present during measurements, which is an essential characteristic for an analytical tool. Our proposed technique can be readily integrated to existing THz-TDS systems and is applicable to scans with rapid acquisition times and to scans that require long periods of time (e.g., hyperspectral imaging).

A Series of Ultra-low Permittivity ALaP4O12 (A = Li, Na, K) Metaphosphate Microwave Dielectric Ceramics for Ultra-wideband Dielectric Resonant Antenna Application.

The employment of ultra-low permittivity materials in the configuration of antennas has been demonstrated to augment the antenna bandwidth and diminish signal delay effectively. This study presents three ultra-low permittivity metaphosphate microwave dielectric ceramics (MWDCs). The ALaP4O12 (A = Li, Na, K) metaphosphate ceramics, which all belong to the monoclinic crystal system, exhibit extremely low permittivity (εr ≈ 5) and excellent quality factor (Q·f > 10,000 GHz) at a low sintering temperature (T < 950 °C). Terahertz time-domain spectroscopy indicates that the εr of ALaP4O12 at terahertz frequencies is comparable to that observed in the microwave band and its value remains stable over an extensive frequency range. Furthermore, the relationship between the crystal structure and the dielectric properties of ALaP4O12 has been analyzed through the lens of chemical bond theory. The highest Q·f value observed for LiLaP4O12 can be attributed to the high chemical bond strength and stability of its crystal structure. The lowest ionic polarizability per unit volume is exhibited by NaLaP4O12, which results in the lowest pore-corrected permittivity. The thermal expansion of the chemical bonds within KLaP4O12 is considerable, resulting in the highest coefficient of thermal expansion. Finally, the performance of a LiLaP4O12-based dielectric resonator antenna (DRA) excited by slot-coupled microstrip lines was designed and optimized by using different ceramic radius-to-height (RH) ratios. It was found that when the RH ratio of DRA reached 1.85, both the fundamental mode (HEM11δ) and the higher-order mode (HEM12δ) of DRA were simultaneously excited. The two modes overlap significantly, resulting in an ultra-wideband (UWB) of 46.8% (bandwidth = 7.93 GHz). Concurrently, the maximum radiation efficiency and gain of the DRA, obtained from the simulation, are 97.9% and 7.92 dBi, respectively. The findings of this study may inform the investigation of ultra-low permittivity phosphorus-based MWDCs and UWB DRAs.

Measurement of stress optical coefficients for GFRP based on terahertz time-domain spectroscopy

Glass Fiber-Reinforced Polymer (GFRP) finds extensive applications in the high-end equipment manufacturing industry owing to its advantages of light weight, high strength, and corrosion resistance. Since the residual stress in GFRP builds up during the curing process and affect its mechanical properties and service life, the characterization of the residual stress in GFRP is crucial. In this study, we establish a theoretical model based on the anisotropic stress-optics law for the orthorhombic crystalline system to describe the terahertz-elasticity of GFRP and calibrate the stress optical coefficients of GFRP. First, the residual stress in GFRP at different curing temperatures are measured by fiber Bragg grating sensors. Then, the refractive index of GFRP with different residual stress are obtained based on transmission-type THz-TDS. Finally, based on the proposed photoelastic model of GFRP, the stress optical coefficients of GFRP are measured by combining the measurement results of residual stress and refractive index. The experimental results show that the refractive index of GFRP decreases with the increase of residual stress; the stress optical coefficients of GFRP are determined as q11 = −5.612 × 10−9 Pa−1, q12 = −2.548 × 10−9 Pa−1, q21 = −1.305 × 10−8 Pa−1, q22 = −1.408 × 10−9 Pa−1. The modeling of terahertz photoelasticity in GFRP and the determination of stress optical coefficients provide a basis for characterizing residual stress in GFRP by THz-TDS.

Experimental Analysis of Terahertz Wave Scattering Characteristics of Simulated Lunar Regolith Surface

This study investigates terahertz (THz) wave scattering from a simulated lunar regolith surface, with a focus on the Brewster feature, backscattering, and bistatic scattering within the 325 to 500 GHz range. We employed a generalized power-law spectrum to characterize surface roughness and fabricated Gaussian correlated surfaces from Durable Resin V2 using 3D printing technology. The complex dielectric permittivity of these materials was determined through THz time-domain spectroscopy (THz-TDS). Our experimental setup comprised a vector network analyzer (VNA) equipped with dual waveguide frequency extenders for the WR-2.2 band, transmitter and receiver modules, polarizing components, and a scattering chamber. We systematically analyzed the effects of root-mean-square (RMS) height, correlation length, dielectric constant, frequency, polarization, and observation angle on THz scattering. The findings highlight the significant impact of surface roughness on the Brewster angle shift, backscattering, and bistatic scattering. These insights are crucial for refining theoretical models and developing algorithms to retrieve physical parameters for lunar and other celestial explorations.

Excitation of high-quality quasi-BIC toroidal mode in a lattice perturbed terahertz metasurface

The bound state in continuum (BIC) is a phenomenon that describes the existence of nonradiative modes (dark modes) embedded in the continuum frequency range. However, an ideal BIC cannot be detected experimentally. The BIC can be transformed into a quasi-BIC by establishing a leaky channel to the radiation continuum. In this study, instead of the conventional asymmetric split ring resonator structure, a sharp quasi-BIC mode is excited in a symmetric split ring resonator (SRR) metasurface by the perturbation of the lattice constant of the unit cell via changing the interspacing distance between two adjacent SRRs. The quality factor of the quasi-BIC mode can be tuned by varying the interspacing of two SRRs, while the resonance frequency of the quasi-BIC mode remains stable. An eigenmode analysis confirms the presence of the quasi-BIC mode, while the ab initio Fano theory and a coupled oscillator model elucidate the radiative and nonradiative coupling mechanisms. The influence of geometric perturbations on the quasi-BIC mode is quantitatively assessed through the extracted fitting parameters, providing insights into the transition from the dark mode (ideal BIC) to the quasi-BIC mode. The terahertz time domain spectroscopy measurement demonstrates a signature of the quasi-BIC resonance mode as a result of the band folding in the first Brillouin zone induced by the doubling of the lattice constant.

Quantitatively characterization of rare earth ore by terahertz time-domain spectroscopy

The Bayan Obo deposit is the world’s largest polymetallic associated minerals deposit of rare earths, iron and niobium, and the rarity of its physical properties restrict the knowledge and understanding of its laws. Taking the high-grade mixed rare earth concentrate of Bayan Obo as the research object, terahertz time-domain spectroscopy (THz-TDS) has been adopted for the systematic investigation of high-grade rare earth concentrate base on the traditional X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric-differential thermal analysis (TG-DTA). The absorption coefficient and refractive index of high-grade rare earth ores and their associated minerals of fluorite and dolomite, are all investigated by terahertz time-domain spectroscopy. The terahertz spectral response is affected by the type of mineral and its content. The acquired rare earth terahertz spectral data are processed by correlation analysis. Three machine learning algorithms, Partial Least Squares Regression (PLSR), Random forest (RF) and Multilayer Perceptron (MLP), are used to achieve quantitative detection of their concentrations and components with the coefficient of determination R2 of the absorption coefficient of the optical parameter reaching up to 0.975, 0.992 and 0.984, respectively. This work promotes the growing understanding of terahertz transmission spectroscopy of rare earth-bearing minerals, which can be used to help guide the search for minerals, and to detect, identify as well as quantify them in geology. Terahertz time-domain spectroscopy supplies a new method for study of rare earth resources, and the comprehensive development and utilization of resources in the Bayan Obo deposit.

THz-TDS characterization of stress evolution of EB-PVD TBCs under thermal cycling

The failure of thermal barrier coatings (TBCs) during service is a critical issue affecting aeroengine efficiency. In this study, electron beam physical vapor deposition (EB-PVD) TBCs were subjected to thermal cycling at 1100 °C. Terahertz time-domain spectroscopy (THz-TDS) was employed to characterize the relationship between the evolution of characteristic frequency and the number of thermal cycles, along with the development of stress in the ceramic layer. In situ tensile test revealed a linear correlation between the characteristic frequency and micro strain derived from THz-TDS data, with a proportional coefficient of −9.94 between the characteristic frequency and the slope of the refractive index curve. The results show that the stress evolution can be effectively monitored by recording the trend of characteristic frequency. During the non-failure stage, the characteristic frequency followed a parabolic trend as thermal cycles increased, influenced by the growth of thermally grown oxide (TGO), ceramic layer sintering, and thermal mismatch. Approaching the failure stage, stress evolution was dominated by the expansion of vertical cracks in the ceramic layer and the widening of transverse cracks at the interface, which corresponded to a sudden change in the characteristic frequency.

Generation of femtosecond spin-polarized current pulses at Fe/MgO interface by quasi-static voltage

The generation of short spin-current pulses is essential for fast spintronic devices. So far, spin current pulses are generated by femtosecond laser pulses which drive spins from a ferromagnetic metal layer. However, the need for miniaturization, simplicity and energy efficiency favour electric-field control of spintronic devices over optic or thermal control. Here, we combine ab initio calculations of electronic density of states at MgO/Fe interface with continuous model for charge transport to investigate the dynamics of the spin-dependent potential. We demonstrate that the voltage-driven instability of the electronic band structure due to the electronic resonant states at the Fe/MgO interface results in the generation of the femtosecond spin-polarized current pulse with the spin polarization up to P=7 00 % that propagates from the interface to the bulk. The dynamics of the current pulses driven by the Stoner instability depends neither on the dielectric relaxation time nor on the details of how the instability is achieved by changing the voltage, i.e. as long as the voltage changes are slow (quasi-static) with respect to the time determined by the spin diffusion constant, being of the order of fs. The presence of the instability can be detected by THz time-domain spectroscopy or pump-probe techniques.