Terahertz Time-domain Spectroscopy Research Articles

Terahertz time-domain spectroscopy imaging combined with convolutional long short-term memory neural network for In situ moisture reduction prediction of particle foods during pulsed fluidized bed drying

Fluidized bed drying (FBD), as a novel dehydration method, has been widely applied in drying various heat-sensitive high-value food products, in which the pulsed fluidized bed drying (PFBD) technique addresses the ineffective fluidized limitation of the traditional FBD when dealing with cohesive products. However, achieving in situ real-time monitoring of dynamic product moisture content (MC) change during both FBD and PFBD without sampling is still challenging. This paper presents an innovative integration of a whisk-broom terahertz time-domain spectroscopy (THz-TDS) imaging system with a fluidized bed dryer, facilitating in situ, real-time monitoring of MC reduction of products without process pausing for sampling. The reliability, and robustness of the established system and prediction models were tested by drying peppercorn and sweetcorn under various drying settings, including different temperatures, flow patterns, and durations. Implementing the convolutional neural networks (CNN) and long short-term memory (LSTM) for correlating terahertz time-domain image data with reference datasets in this research provided reliable MC reduction prediction results for the mixture dataset from peppercorn and sweetcorn drying. The successful application of this combined sensing and analytical method presents a promising approach for enhancing industrial processing control and expands the utility of THz-TDS in the agriculture and food industrial fields.

Cross-correlation technique for phase error correction in reflection mode terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy in reflection mode geometry provides valuable surface and subsurface information, making it suitable for a layer analysis, coating, and non-destructive testing applications. The exchanging of the sample and reference’s position introduces a phase error when the position or alignment of the sample is not precisely maintained during measurements. This micrometer order of pitch error (Δx) between the reference and the sample could lead to an inherent error in the phase spectrum of the sample. In the present work, a novel approach, to the best of our knowledge, based on the cross-correlation with an envelope technique, has been demonstrated to reduce the uncertainty in the phase and reveal the hidden characteristic features of the given sample in THz TDS spectroscopy. In conjunction with experimental verification, we have employed a finite element analysis in COMSOL Multiphysics to simulate a misplacement error between a lossy dielectric medium (n=1.2 to 3.0 and k=0 to 0.9) and a reference. We have investigated the impact of varying properties of the lossy dielectric medium on delay measurements using a cross-correlation with an envelope analysis. We illustrated and demonstrated the advantage of our approach by measuring the optical properties of Teflon, quartz, and RDX by correcting the misalignment of the 15.75, 17.55, and 20.70 µm ranges, respectively.

Terahertz emission mechanisms in low-temperature-grown and semi-insulating gallium arsenide photoconductive antenna devices excited at above- and below-bandgap photon energies

In this work, the terahertz (THz) time-domain spectroscopy was employed in studying the carrier dynamics in low-temperature grown (LT-) and semi-insulating (SI-) gallium arsenide (GaAs) photoconductive antenna (PCA) at above- ( λ= 780 nm, Eg= 1.59 eV) and below- ( λ= 1.55 µm, Eg 0.80 eV) bandgap excitation. We measured the excitation power dependence of the LT-GaAs (SI-GaAs) THz emission. Then, the equivalent circuit model which considers the (i) photogeneration, (ii) screening effects, and (iii) transport of carriers was utilized in analyzing the THz radiation mechanisms in the above- and below-bandgap excitation of the two substrates. In simulating the above-bandgap THz emission of both PCAs, we employed the direct bandgap excitation model which takes into account the band-to-band transitions of photoexcited carriers. Meanwhile, to simulate the LT-GaAs (SI-GaAs) THz emission at below-bandgap excitation we utilized the two-step photoabsorption facilitated by the mid-gap states. In this model the photoexcited carriers jump from the valence band to the mid-gap states and then to the conduction band. Results suggest that the THz emission from LT-GaAs and SI-GaAs at above- and below-bandgap excitation occur due to band-to-band transitions, and two-step photoabsorption process via midgap states, respectively.

Terahertz time-domain transmission spectroscopy of water and hydrogel thin films: Extraction of optical parameters and application to agarose gel characterization

Terahertz time-domain spectroscopy (THz-TDS) is an emerging optical technique that has potential applications in the characterization of (bio)materials. However, the complicated extraction of optical parameters from multi-layered and optically thin samples is a barrier towards its acceptance by applied scientists. Therefore, the aim of this work is to provide a straightforward approach for the extraction of the THz absorption coefficient and index of refraction profiles of aqueous thin films in a window-sample-window configuration, which is ubiquitous in many laboratories (i.e., sample in a cuvette). A numerical approach-based methodology that accounts for multiple layers, Fabry–Pérot effect, and sample thickness is elaborated which involves an optical interference model based on a tri-layer structure and a simple thickness estimation technique. This method was validated on water samples where a good agreement was found with the THz optical parameters of water reported in the literature, while the use of a commercial software resulted in erroneous optical parameters estimates when used without due regard to its limitations. A case study was then performed to demonstrate the ability of the proposed method to characterize agarose hydrogels with varying degree of sulfation. It was demonstrated that THz-TDS can provide insight into the hydration state of the agarose hydrogels, including the relative number of the hydrogen bonds between the hydroxyl moieties of water and the polysaccharide network which is perturbed by the presence of sulfate. The trend in the index of refraction profiles suggested microstructural differences between the agarose hydrogels, which were confirmed by visualizing the agarose network morphology using cryo-SEM imaging.

Solitons, bifurcation and chaotics in a bidirectional mamyshev oscillator

Bidirectional mode-locked fiber lasers are able to deliver two outputs of pulses in the clockwise (CW) and counterclockwise (CCW) directions, having important applications in dual-comb spectroscopy, asynchronous optical sampling and terahertz time-domain spectroscopy. In this paper, we have experimentally demonstrated an all-fiber Er/Yb-doped bidirectional passively mode-locked Mamyshev oscillator (MO) without an external seed. Two stable mode-locked states with remarkable different characteristics are observed by adjusting the pump power. The transition process between the two mode-locked states shows chaotic behaviors, which are further investigated with numerical simulations based on the Ginzburg-Landau equation. The bidirectional MO proposed in this work is not only a good laser source, but also a good platform to study soliton dynamics, system bifurcation, and chaos in ultrafast fiber laser systems.

Quantitative differentiation of normal and wound healed mice skin by terahertz time domain spectroscopy

Quantitative differentiation of normal and wound healed mice skin by terahertz time domain spectroscopy

Simulation of ageing and wear effect on graphene THz passive components using finite element method

In the growing scenario of 2D material-based metamaterials and metasurfaces for Terahertz (THz) applications, assessing the impact of ageing and wear due to environmental stressors on the components’ performance is becoming mandatory to understand the long-term reliability of novel technologies. This paper introduces approaches to assess the ageing and wear effects on THz passive components through numerical simulations. For this purpose, common techniques for introducing 2D materials and thin metal layers in numerical models are discussed. As a case study, this work explores the effects of graphene degradation and reflective metal ageing on the electromagnetic response of a graphene-enhanced reflective grating for THz absorption and modulation by finite element (FE) analysis. The developed FE model is validated against experimental data obtained through THz Time-Domain Spectroscopy. By computing the device’s transmission, reflection, and absorption spectra for degrading graphene and metal conductive properties, this work provides insights into the influence of ageing and wear on THz passive components.

Wide-range resistivity characterization of semiconductors with terahertz time-domain spectroscopy

Resistivity is one of the most important characteristics in the semiconductor industry. The most common way to measure resistivity is the four-point probe method, which requires physical contact with the material under test. Terahertz time domain spectroscopy, a fast and non-destructive measurement method, is already well established in the characterization of dielectrics. In this work, we demonstrate the potential of two Drude model-based approaches to extract resistivity values from terahertz time-domain spectroscopy measurements of silicon in a wide range from about 10−3 Ωcm to 102 Ωcm. One method is an analytical approach and the other is an optimization approach. Four-point probe measurements are used as a reference. In addition, the spatial resistivity distribution is imaged by X-Y scanning of the samples to detect inhomogeneities in the doping distribution.

High mobility graphene field effect transistors on flexible EVA/PET foils

Monolayer graphene is a promising material for a wide range of applications, including sensors, optoelectronics, antennas, EMR shielding, flexible electronics, and conducting electrodes. Chemical vapor deposition (CVD) of carbon atoms on a metal catalyst is the most scalable and cost-efficient method for synthesizing high-quality, large-area monolayer graphene. The usual method of transferring the CVD graphene from the catalyst to a target substrate involves a polymer carrier which is dissolved after the transfer process is completed. Due to often unavoidable damage to graphene, as well as contamination and residues, carrier mobilities are typically 1000–3000 cm2(Vs)−1 , unless complex and elaborate measures are taken. Here, we report on a simple scalable fabrication method for flexible graphene field-effect transistors that eliminates the polymer interim carrier, by laminating the graphene directly onto office lamination foils, removing the catalyst, and depositing Parylene N as a gate dielectric and encapsulation layer. The fabricated transistors show field- and Hall-effect mobilities of 7000–10 000 cm2(Vs)−1 with a residual charge-carrier density of 2×1011 1 cm−2 at room temperature. We further validate the material quality by terahertz time-domain spectroscopy and observation of the quantum Hall effect at low temperatures in a moderate magnetic field of ∼5 T. The Parylene encapsulation provides long-term stability and protection against additional lithography steps, enabling vertical device integration in multilayer electronics on a flexible platform.

Non-invasive inspection for a hand-bound book of the 19th century: Numerical simulations and experimental analysis of infrared, terahertz, and ultrasonic methods

Due to fungal growth and mishandling in the book, there are various types of defects as they age such as foxing, tears, and creases. It is important to develop novel non-invasive inspection techniques and defect recognition algorithms. In this work, three non-invasive inspection techniques, including infrared thermography (IRT), terahertz time-domain spectroscopy (THz-TDS), and air-coupled ultrasound (ACU), were employed for the detection of defects in an ancient book cover. To improve the image quality and defect contrast, principal component analysis, fast Fourier transform, and partial least squares regression algorithms are used as the post-processing methods. Furthermore, the YOLOv7 network is deployed for defect automatic detection. Finite element analysis and finite-difference time-domain methods were employed for generating training dataset of YOLOv7 network. Experimental results demonstrate that IRT and THz-TDS has excellent detection capability for surface and subsurface defects, respectively. By employing YOLOv7 network with simulation datasets, defects can be effectively identified.

Quantitative assessment whole-rock iron content in magnetite protolith based on terahertz time-domain spectroscopy: publisher’s note

This publisher’s note serves to correct errors in Appl. Opt. 63, 2528 (2024)APOPAI0003-693510.1364/AO.517400.

Flexible terahertz phase shifter for optically controlled polydimethylsiloxane-vanadium dioxide composite film

In the terahertz (THz) band, modulation research has become a focal point, with precise control of the phase shift of THz waves playing a pivotal role. In this study, we investigate the optical control of THz phase shift modulation in a polydimethylsiloxane (PDMS)-vanadium dioxide (VO2) flexible material using THz time-domain spectroscopy. Under the influence of an 808-nm continuous wave (CW) laser with power densities ranging from 0 to 2.74 W/cm2, the PDMS-VO2 flexible material exhibits significant phase shift modulation in the frequency range of 0.2 to 1.0 THz. The maximum optical-pumping phase shift reaches 0.27π rad at 1.0 THz in a composite material with a VO2 mass fraction of 5% and a thickness of 360 µm, and the amplitude transmittance from 0.2 THz to 1.0 THz exceeds 70%. Furthermore, the composite material exhibits good stability under at least 640 switching cycle times, as confirmed through repeatability tests. The proposed composite devices offer a new approach for more flexible phase shift modulation owing to the flexibility of the composite material and the non-contact and precise modulation of light control. Additionally, the stress-adjustable characteristics of flexible materials make them highly suitable for use in wearable THz modulators, highlighting their significant application potential.

Qualitative and Quantitative Detection of Typical Reproductive Hormones in Dairy Cows Based on Terahertz Spectroscopy and Metamaterial Technology.

Progesterone (PROG) and estrone (E1) are typical reproductive hormones in dairy cows. Assessing the levels of these hormones in vivo can aid in estrus identification. In the present work, the feasibility of the qualitative and quantitative detection of PROG and E1 using terahertz time-domain spectroscopy (THz-TDS) and metamaterial technology was preliminarily investigated. First, the time domain spectra, frequency domain spectra, and absorption coefficients of PROG and E1 samples were collected and analyzed. A vibration analysis was conducted using density functional theory (DFT). Subsequently, a double-ring (DR) metamaterial structure was designed and simulated using the frequency domain solution algorithm in CST Studio Suite (CST) software. This aimed to ensure that the double resonance peaks of DR were similar to the absorption peaks of PROG and E1. Finally, the response of DR to different concentrations of PROG/E1 was analyzed and quantitatively modeled. The results show that a qualitative analysis can be conducted by comparing the corresponding DR resonance peak changes in PROG and E1 samples at various concentrations. The best R2 for the PROG quantitative model was 0.9872, while for E1, it was 0.9828. This indicates that terahertz spectral-metamaterial technology for the qualitative and quantitative detection of the typical reproductive hormones PROG and E1 in dairy cows is feasible and worthy of in-depth exploration. This study provides a reference for the identification of dairy cow estrus.

THz-TDS with gigahertz Yb-based dual-comb lasers: noise analysis and mitigation strategies

We investigate terahertz time-domain spectroscopy using a low-noise dual-frequency-comb laser based on a single spatially multiplexed laser cavity. The laser cavity includes a reflective biprism, which enables generation of a pair of modelocked output pulse trains with slightly different repetition rates and highly correlated noise characteristics. These two pulse trains are used to generate the THz waves and detect them by equivalent time sampling. The laser is based on Yb:CALGO, operates at a nominal repetition rate of 1.18 GHz, and produces 110 mW per comb with 77 fs pulses around 1057 nm. We perform THz measurements with Fe-doped photoconductive antennas, operating these devices with gigahertz 1 µm lasers for the first time, to our knowledge, and obtain THz signal currents approximately as strong as those from reference measurements at 1.55 µm and 80 MHz. We investigate the influence of the laser’s timing noise properties on THz measurements, showing that the laser’s timing jitter is quantitatively explained by power-dependent shifts in center wavelength. We demonstrate reduction in noise by simple stabilization of the pump power and show up to 20 dB suppression in noise by the combination of shared pumping and shared cavity architecture. The laser’s ultra-low-noise properties enable averaging of the THz waveform for repetition rate differences from 1 kHz to 22 kHz, resulting in a dynamic range of 55 dB when operating at 1 kHz and averaging for 2 s. We show that the obtained dynamic range is competitive and can be well explained by accounting for the measured optical delay range, integration time, as well as the measurement bandwidth dependence of the noise from transimpedance amplification. These results will help enable a new approach to high-resolution THz-TDS enabled by low-noise gigahertz dual-comb lasers.

Rapid and non-destructive identification of plastic particles through THz technology and machine learning

For fast and accurate non-destructive differentiation of plastic pellets with similar appearance during import and export inspections, as well as for quality control purposes. Terahertz technology offers a non-destructive analysis approach for samples from diverse fields. By integrating deep learning algorithms with terahertz time-domain spectroscopy (THz-TDS) and linear discriminant analysis (LDA), it is possible to establish a highly accurate terahertz spectroscopy qualitative identification model. Additionally, the incorporation of principal component analysis (PCA) enables the application of this model to higher dimensional data. Notable differences in absorption coefficient, phase difference, and refractive index are observed among different plastic particles. The implementation of this rapid, accurate, and non-destructive detection method can greatly facilitate the testing and identification of plastic particles in customs and quality inspection departments.

Measurement of Water Uptake and States in Nafion Membranes Using Humidity-Controlled Terahertz Time-Domain Spectroscopy.

Perfluorinated sulfonic acid ionomers are well known for their unique water uptake properties and chemical/mechanical stability. Understanding their performance-stability trade-offs is key to realizing membranes with optimal properties. Terahertz time-domain spectroscopy has been demonstrated to resolve water states inside industrially relevant membranes, producing qualitatively agreeable results to conventional gravimetric analysis and prior demonstrations. Using the proposed humidity-controlled terahertz time-domain spectroscopy, here we quantify this detailed water information inside commercially available Nafion membranes at various humidities for direct comparison against literature values from dynamic vapor sorption, differential scanning calorimetry, and Fourier transform infrared spectroscopy on selected samples. Using this technique therefore opens up opportunities for rapid future parameter space investigation for membrane optimization.

Terahertz time-domain spectroscopy for the inspection of dry fibre preforms

Liquid moulded polymer matrix composites (LM-PMCs) are increasingly used in aerospace, automotive and other industrial applications. Liquid moulding processes featuring dry fibre preforms provide flexibility and enable cost reductions for secondary load-bearing structures. However, preform variability and manufacturing reproducibility remain major obstacles to wider use in primary structures. The open literature records only marginal use of non-destructive inspection (NDI) methods for dry multilayer preforms due to technological limitations and cost of NDI methods. In this work, terahertz time-domain spectroscopy (THz-TDS) is used for inspecting three dry multilayer glass fibre preforms featuring different defects, for the first time. A novel time-domain enhancement method is compared with classical image processing methods, aiming at improving image contrast and detecting potential defects. Furthermore, THz B-Scan is used for verifying the accuracy of interply defect detection. Finite difference time domain is simulated for analyzing THz magnitude variation in time-domain. Finally, quantitative evaluation is applied to further illustrate the significant potential of THz-TDS for the inspection of dry fibre preforms. The results show that the errors on defects lengths, widths, angles, and diameters for THz-TDS are in the ranges 8.2 %–34.3 %, 18.67 %–75 %, 0.29 %–6.67 %, and 1.33 %–10 % respectively.

Multi-stacked polarization insensitive broadband terahertz metamaterial

In this article, we present a polarization-insensitive terahertz metamaterial designed by stacking resonators capable of providing ultra-wideband terahertz transmissions. Our design includes a square ring resonator situated between two windmill-shaped resonators, separated by a polyimide spacer. We optimized the spacer thickness to achieve a broadband response in transmission. These optimized broadband metamaterial designs were fabricated through multiple steps of the photolithography process. Terahertz time-domain spectroscopy of the fabricated samples indicates broadband terahertz transmission, in agreement with both simulation findings and results calculated from the transmission line model for the multi-layered metamaterial geometry. Our research reveals a strong near-field coupling between resonators, leading to wideband transmission of terahertz waves. The stacking of these metamaterials is crucial in designing broadband bandpass filters and broadband modulators for terahertz photonics while keeping the resonance strength almost intact.

Investigation of dielectric properties and conductivity of polyvinyl chloride composites by terahertz time-domain spectroscopy

Polyvinyl chloride (PVC) composites have been studied using terahertz time-domain spectroscopy (THz-TDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) to highlight the prospects for their applications. The terahertz dielectric properties and conductivity of PVC composites in the frequency window of 0.2–1.6 THz have been investigated. The universal dielectric response model has been applied to fit the real part of THz conductivity. It shows that a super-linear behavior is presented in the conductivity, and the conductivity can be tuned by varying the content of PVC and the type of composite. In addition, relationships are established among the types and concentrations as well as the dielectric properties and conductivity. These results are expected to be applied to the preparation of terahertz devices and to the monitoring of materials in industrial production.

Study on the Terahertz Spectroscopy Properties of Graphene Quantum Dots Based on Microfluidic Chip

Graphene quantum dots are quasi-zero-dimensional nanomaterials with unique physical and chemical properties. This study utilized a terahertz (THz) time-domain spectroscopy system to analyze the absorption characteristics of THz-waves by graphene quantum dots at different concentrations. Additionally, we applied electric fields and magnetic field to explore the THz-wave absorption properties of graphene quantum dots in greater detail. The results indicate that the THz absorbance of graphene quantum dots is positively correlated with sample concentration and applied electric field strength. However, it is negatively correlated with the intensity of the applied magnetic field. This work combines THz technology and microfluidic devices to propose a viable methodology for conducting in-depth study on graphene quantum dots.