Terahertz Time-domain Spectroscopy Research Articles

Multispectral investigation of natural resins.

Resins have been used as remedies since ancient times and various embalming resins have been identified in recent years. In Europe, Mumia vera aegyptiaca, a resinous substance from ancient Egyptian mummies, was even sold in pharmacies as a tonic until the early 20th century. It is difficult to examine the composition of these archeological samples in detail as the well-established analytical techniques, that is, gas chromatography-mass spectrometry or liquid chromatography coupled with tandem mass spectrometry, are destructive and therefore do not allow the analysis of valuable archeological samples. Hence, there is an urgent need for alternative, nondestructive methods for the identification of resin residues. This study aims to explore and compare the use of five spectroscopic methods as an alternative to established analytical procedures. For that, 15 resin samples of known origin and three samples from an Egyptian market were studied. While laser induced-breakdown spectroscopy and terahertz time-domain spectroscopy provide only limited information for resin classification, nuclear magnetic resonance spectroscopy and Fourier-transform infrared spectroscopy can be used to classify the resin samples more accurately. Furthermore, photoluminescence/photoluminescence excitation spectroscopy shows a promising potential in combination with its general advantages, such as cost-efficiency, nondestructive nature, and fast data acquisition.

Terahertz tomography for testing wrapped scintillating crystals

Terahertz time-domain spectroscopy (THz-TDS) is demonstrated to be effective in novel application, for 3D characterization of wrapped scintillator crystals in search for defects affecting the light-yield of these crystals. This is of special importance for fast scintillators currently in demand to be exploited in high-energy physics experiments and medical imaging devices. Typical inorganic (GAGG:Ce) and organic (BC-408) scintillators wrapped in polytetrafluoroethylene (PTFE) and enhanced specular reflector (ESR) tapes have been studied. Time-of-flight information extracted from THz-TDS data reveals positions of the interfaces between the materials, wrapping thickness variations, and allows for the wrapping inspection with resolution better than a single layer, typically ∼200 μm-thick, of PTFE, the currently most common wrapping material. The results are supported by the study of the properties of the wrapping materials in the THz range and evidence of the prospectiveness of THz-TDS technique as a novel tool for nondestructive inspection of wrapped scintillators.

Advanced Data Processing of THz-Time Domain Spectroscopy Data with Sinusoidally Moving Delay Lines

AbstractWe provide a comprehensive technical analysis of the data acquisition process with oscillating delay lines for Terahertz-time domain spectroscopy. The utilization of these rapid stages, particularly in high-repetition-rate systems, is known to enable an effective reduction of noise content through averaging. However, caution must be exercised to optimize the data averaging process, with the goal of significantly optimizing the dynamic range (DR) and signal-to-noise ratio (SNR). Here we discuss some pitfalls to avoid and the effect of improper data handling on the dynamic range obtainable. A free and open-source program, called parrot (Processing All Rapidly & Reliably Obtained THz-traces), is provided alongside this publication to overcome the discussed pitfalls and facilitate the acceleration of experimental setups and data analysis, thereby enhancing signal fidelity and reproducibility.

THz-TDS transflection measurements as a process analyser for tablet mass

Tablet content and content uniformity are essential for the market release of the drug product. For tablets, content and uniformity are determined by the weight ratio of active pharmaceutical ingredient in the tablet and the tablets’ total mass. Novel process analytical technology tools for the control of the ratio of the active pharmaceutical ingredient have been proposed and implemented, but more robust, sensitive, and fast sensors for the control of tablet mass are desirable. In the presented study terahertz time-domain spectroscopy (THz-TDS) is proposed as a potential process analyser for tablet mass. THz-TDS is based on pulsed terahertz signals, which are mapped in the time-domain. Thus, the signal amplitude and arrival time are recorded. THz-TDS measurements of a tablet with a reflection setup result in two signals – a frontside reflection and a backside transflection. The presented study demonstrates that an increase in the tablet mass results in an increase in the time delay of the backside transflection. This is a result of the high refractive index of the solid fraction compared to air. It is suggested that the transflection can be used as an indirect measure of tablet mass for which root mean squared errors of around 1 mg were found. The potential to measure tablets at high acquisition rates (50 Hz) is explored and considered feasible. Additionally, it has been demonstrated in previous work that the time delay of the frontside reflection allows a simultaneous assessment of the tablet height. The presented methodology opens the possibility of in-line monitoring of tablet mass as part of a content and content uniformity strategy at high sampling rates in the production environment. Further, as tablet height and mass can be assessed simultaneously, monitoring and control of the compaction process based on a comprehensive assessment of physical tablet attributes can also be envisioned.

Identification of millet origin using terahertz spectroscopy combined with ensemble learning

It’s crucial for both producers and consumers to accurately trace the origin of millet, given the significant differences in price and taste that exist between millets from various origins. The traditional method of identifying the origin of millet is time-consuming, laborious, complex, and destructive. In this study, a new method for fast and non-destructive differentiation of millet origins is developed by combining terahertz time domain spectroscopy with ensemble learning. Firstly, three machine learning algorithms, namely support vector machine (SVM), random forest (RF), and kernel extreme learning machine (KELM), were used to build different discriminative models, and then the impact of six different preprocessing methods on the models’ classification performance was compared. It was observed that models employing Savitzky-Golay preprocessing exhibited pronounced superiority in accurately determining the millet’s geographical origins. Building upon these findings, the research introduces an innovative ensemble learning strategy, leveraging both topsis and stacking techniques, to harness the collective strengths of the three algorithms. The outcomes of this approach reveal its remarkable capacity to distinguish millets originating from five distinct locations without the necessity for any parameter fine-tuning. The accuracy, F1 score, and Kappa on the prediction set are all 100 %, which significantly outperforms the single model, traditional voting method, and stacking method. The culmination of this study suggests that the integration of terahertz time-domain spectroscopy and TOPSIS-Stacking ensemble learning emerges as a promising method for the swift and non-intrusive discrimination of millet geographical origins with remarkable precision.

A New Metric for the Comparison of Permittivity Models in Terahertz Time-Domain Spectroscopy

A New Metric for the Comparison of Permittivity Models in Terahertz Time-Domain Spectroscopy

Research on rubber classification and recognition based on terahertz time-domain spectroscopy and improved honey badger algorithm

The identification of different rubber materials is crucial to ensuring the quality of rubber products. In order to quickly and effectively identify the types of rubber, reduce the impact of counterfeit rubber on the market. This study proposes a rubber identification method based on terahertz time-domain spectroscopy (THz-TDS), Chemometry, and Improved Honey Badger Algorithm (IHBA). Initially, the absorption spectra of eight types of rubber within the 0.2–1.6 THz range are obtained and calculated using THz-TDS. This is followed by data preprocessing using Savitzky-Golay and Principal component analysis(PCA). The optimization effects of genetic algorithm (GA), grid optimization algorithm (GRID), particle swarm optimization algorithm (PSO) and honey badger algorithm (HBA) on support vector machine (SVM) model parameters were compared respectively. The HBA-SVM model achieves 96.88 % recognition accuracy on the prediction set, which is higher than other models and shows excellent parameter optimization ability.To further improve accuracy, Bernoulli chaotic mapping, cosine density factor, and Cauchy mutation are introduced for improvement. Compared with the original model, the IHBA-SVM model improves the accuracy of rubber recognition from 96.88 % to 98.96 %. Furthermore, compared with other models, the IHBA-SVM model achieved the highest classification accuracy. In summary, this study provides technical support and reference for the rapid identification of rubber, which is of great significance for ensuring the quality of rubber products.

Influence of SU-8 curing parameters on the terahertz absorption characteristics

A plethora of polymers are excellent candidates that can be utilized to build various terahertz (THz) based systems for sensing and microfluidics. SU-8 is a versatile epoxy-based polymer with excellent properties, such as biocompatibility and good mechanical properties. Nevertheless, the impact of curing parameters on the THz absorption characteristics of SU-8 remains uncertain. This study explores the impact of various curing conditions on the THz absorption properties of SU-8. The aim is to establish a correlation between THz absorption and the polymer's cross-linking characteristics. Therefore, three key curing parameters have been examined: time, temperature, and ultraviolet (UV) exposure dose. The SU-8 samples that are cured under different conditions are examined using THz Time-Domain Spectroscopy (THz-TDS) to estimate the THz absorption coefficient. Next, the swelling experiment is conducted to evaluate the polymer cross-linking of the cured samples. The results show that curing conditions and, thus, cross-linking routines significantly influence THz wave propagation and attenuation within SU-8 samples. The findings recommended using an optimal curing dose for SU-8 spanning 1240 mJ/cm2 to 1860 mJ/cm2 which results in a relatively lower absorption coefficient while still maintaining a higher state of cross-linking. This research establishes a crucial link between SU-8 processing and its THz response. It can lay the foundation for tailored SU-8 polymer with optimized THz transmission for novel and customized biosensing and microfluidic devices.

Effect of high-temperature annealing on terahertz optoelectronic properties of nitrogen-doped polycrystalline diamond

Nitrogen-doped polycrystalline diamond (N-PCD) is one of the most important carbon-based electronic materials. Here we present a systmatic investigation of the effect of high-temperature annealing (HTA) on basic physical properties of N-PCD wafer grown by microwave plasma-enhanced chemical vapor deposition (MPECVD). The metallographic and optical microscopes, X-ray diffraction,Raman spectroscopy and X-ray photoelectron spectroscopy are applied for the characterization of N-PCD before and after HTA. Particularly, we study the optoelectrnic properties of N-PCD before and after HTA by using terahertz (THz) time-domain spectroscopy. THz transmittance, dynamical and static dielectric constants and optical conductivity for N-PCD are measured. For N-PCD before HTA, we can observe a THz absorption peak induced by weak H-bonds in N-PCD, which was predicted theoretically in 1999. For N-PCD after HTA, we find that the corresponding optical conductivity can be rightly descriped by the Drude-Lorentz formula. Thus, via fitting the experimental results with the theoretical formula, we can determine optically the key electronic parameters of N-PCD after HTA, such as the electron density, the electronic relaxation time and the Lorentz frequency. The temperature dependence of these parameters is examined in the range of 80–300 K. This work demondtrates that HTA is a practical and efficient way in improving the electronic and optoelectronic properties of N-PCD. The results obtained from this research can benefit an in-depth understanding of the effect of HTA on physical properties of N-PCD from a viewpoint of semiconductor physics.

AirTac: A Contactless Digital Tactile Receptor for Detecting Material and Roughness via Terahertz Sensing

Tactile sensing is an indispensable capability for humans or intelligent devices to engage in complex physical interactions. Mainstream tactile sensors are contact-based, which show limitation in measuring deformable objects and demanding high maintenance effort. As a complementary solution, a new paradigm of contactless tactile sensing is attracting much interest. While promising, they can only identify coarse-grained single tactile perception properties, either material type or surface roughness. In this paper, we propose airTac, which includes a novel contactless digital tactile receptor model, capable of simultaneously extracting these two basic tactile properties in a fine-grained resolution, by harnessing the massive bandwidth of terahertz(THz) frequency band. airTac is designed based on our key finding: while the impact of material type and surface roughness on THz signal intertwine with each other, they are actually separable, i.e., roughness manifests a certain pattern in distorting relative high-frequency components of the whole THz spectrum. Therefore, we custom-design a bio-inspired deep neural network model to decouple the intertwined THz signal, and distill the tactile perception properties embedded underlying the signal. We prototype airTac using a THz time domain spectroscopy, and perform extensive evaluation over 9 different daily material types with 39 different surface roughness. airTac achieves a material identification accuracy of 97.43% and a roughness classification accuracy of 91.46%, which demonstrates that airTac can identify common materials in daily life and exceed a fingertip spatial resolution of 1mm.

Quantitative analysis of multicomponent mixtures of rubber and additives based on terahertz time-domain spectroscopy and Krawtchouk moments

This paper presents a quantitative analytical method based on terahertz time-domain spectroscopy and chemometrics, which was used to detect the content of diphenyl-p-phenylenediamine in a five-component mixture comprising tire rubber and additives. We separately extracted the features of the spectra by using principal component analysis and Krawtchouk moments, and then used partial least squares regression and support vector regression to develop quantitative analysis models. The experimental results show that support vector regression outperforms partial least squares regression in the quantitative analysis of multicomponent mixtures, and the Krawtchouk moments method effectively enhances the prediction accuracy of the model. The Krawtchouk moment combined with support vector regression model shows excellent predictive ability. The correlation coefficient and root mean square error of Krawtchouk moment combined with support vector regression model for the calibration set were 0.9749 and 1.7256%, respectively, and for the prediction set were 0.9669 and 1.9777%, respectively. The results demonstrate that terahertz time-domain spectroscopy combined with Krawtchouk moments and support vector regression is an effective method for determining the antioxidant content in rubber and additive compounds.

Terahertz channel modeling based on surface sensing characteristics

The dielectric properties of environmental surfaces, including walls, floors and the ground, etc., play a crucial role in shaping the accuracy of terahertz (THz) channel modeling, thereby directly impacting the effectiveness of communication systems. Traditionally, acquiring these properties has relied on methods such as terahertz time-domain spectroscopy (THz-TDS) or vector network analyzers (VNA), demanding rigorous sample preparation and entailing a significant expenditure of time. However, such measurements are not always feasible, particularly in novel and uncharacterized scenarios. In this work, we propose a new approach for channel modeling that leverages the inherent sensing capabilities of THz channels, specifically by obtaining channel measurement data through the analysis of refractive indices. By comparing the results obtained through channel sensing with that derived from THz-TDS measurements, we demonstrate the its ability to yield dependable surface property information. Integrating it into a ray-tracing algorithm for channel modeling in both a miniaturized cityscape scenario and an indoor environment, the results show consistency with experimental measurements, thereby validating its effectiveness in real-world settings.

Contrasting Ultra-Low Frequency Raman and Infrared Modes in Emerging Metal Halides for Photovoltaics.

Lattice dynamics are critical to photovoltaic material performance, governing dynamic disorder, hot-carrier cooling, charge-carrier recombination, and transport. Soft metal-halide perovskites exhibit particularly intriguing dynamics, with Raman spectra exhibiting an unusually broad low-frequency response whose origin is still much debated. Here, we utilize ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination of the vibrational response for a wide range of metal-halide semiconductors: FAPbI3, MAPbI x Br3-x , CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. We rule out extrinsic defects, octahedral tilting, cation lone pairs, and "liquid-like" Boson peaks as causes of the debated central Raman peak. Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose-Einstein statistics toward low frequency. These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.

Broadband terahertz absorption and Q-switching behavior of 5-chloro-2-nitroaniline (5C2NA) crystals

Abstract Growth of transparent 5-chloro-2-nitroaniline (5C2NA) crystals was achieved through the slow evaporation technique. Analysis via X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy confirmed the crystalline nature of the organic crystals and revealing distinct molecular fingerprints of 5C2NA respectively. UV-VIS and Photoluminescence (PL) spectroscopy were employed to study the material's band gap and ground state absorption respectively. Thermogravimetric analysis (TGA) analysis indicated the stability of the grown crystals up to 211ºC. Dielectric measurements and Urbach plot suggested the presence of fewer defects in 5C2NA crystals. Time domain terahertz spectroscopy was utilized to observe the variation of absorption coefficient and refractive index in the terahertz frequency regime. Nonlinear optical effects such as saturable absorption (SA) and reverse saturable absorption (RSA), are pivotal in the development of all-optical logic gates. The transition between SA and RSA possesses a significant importance in optoelectronic applications. In this study, we investigate the 5C2NA crystal, revealing its ability to exhibit both SA and RSA under Z scan technique with varying pump intensities. The switching properties observed in 5C2NA can be harnessed for applications such as all-optical logic gates, rapid optical switching, optical limiting, mode storage, and more.

Terahertz Transmission through a Gold Mirror or Electrode.

Hundreds of nanometer-thick metal layers are used as electrical conductors in various technologies and research fields. The intensity of the radiation transmitted by such devices is a small fraction and is often neglected. Here, it is shown that intense terahertz time-domain spectroscopy can probe the absolute electro-optical properties of a 100 nm thick gold sample in transmission geometry without the need to apply electrical contacts or handle wires. The terahertz conductivity of the metal film agrees with that obtained from standard contact measurements of the static component within the error bars. This experimental approach can help to quantify the electrical properties of opaque and conductive materials such as the composite electrodes used in photovoltaic or electrochemical applications, and in the quality control of metal films.

Unveiling enamel demineralization mechanisms by sensitive dielectric differentiation based on terahertz nanospectroscopy.

The early stage of dental caries, i.e. demineralization, has always been a topic of concern to dentists. Understanding the essential mechanism of its occurrence is of great significance for the prevention and treatment of dental caries. However, owing to limitations in resolution and the detection capabilities of diagnostic tools, the study of enamel demineralization has always been a challenge. Terahertz (THz) technology, especially the combination of scanning near-field optical microscopy (s-SNOM) and THz time-domain spectroscopy (TDS), due to its nanoscale resolution, has shown great advantages in the field of biological imaging. Here, a THz s-SNOM system is used to perform near-field imaging of enamel before and after demineralization at the nanoscale. It can be found that near-field signals decrease significantly after demineralization. This is due to the changes of the crystal lattice and the transfer of mineral ions during demineralization, which leads to a decrease in the permittivity of the enamel. The novel approach in this study reveals the essence of demineralization and lays the groundwork for additional research and potential interventions.

Single-cycle, 643 mW average power terahertz source based on tilted pulse front in lithium niobate.

We present the highest, to the best of our knowledge, average power from a laser-driven single-cycle THz source demonstrated so far, using optical rectification in the tilted pulse front geometry in cryogenically cooled lithium niobate, pumped by a commercially available 500 W ultrafast thin-disk ytterbium (Yb) amplifier. We study repetition rate-dependent effects in our setup at 100 and 40 kHz at this high average power, revealing different optimal fluence conditions for efficient conversion. The demonstrated sources with multi-100 mW average power at these high repetition rates combine high THz pulse energies and high repetition rate and are thus ideally suited for nonlinear THz spectroscopy experiments with significantly reduced measurement times. The presented result is a first benchmark for high average power THz time-domain spectroscopy systems for nonlinear spectroscopy, driven by very high average power ultrafast Yb lasers.

Design of an Optimized Terahertz Time-Domain Spectroscopy System Pumped by a 30 W Yb:KGW Source at a 100 kHz Repetition Rate with 245 fs Pulse Duration

Ytterbium laser sources are state-of-the-art systems that are increasingly replacing Ti:Sapphire lasers in most applications requiring high repetition rate pulse trains. However, extending these laser sources to THz Time-Domain Spectroscopy (THz-TDS) poses several challenges not encountered in conventional, lower-power systems. These challenges include pump rejection, thermal lensing in nonlinear media, and pulse durations exceeding 100 fs, which consequently limit the detection bandwidth in TDS applications. In this article, we describe our design of a THz-TDS beamline that seeks to address these issues. We report on the effectiveness of temperature controlling the Gallium Phosphide (GaP) used to generate the THz radiation and its impact on increasing the generation efficiency and aiding pump rejection while avoiding thermal distortions of the residual pump laser beam. We detail our approach to pump rejection, which can be implemented with off-the-shelf products and minimal customization. Finally, we describe our solution based on a commercial optical parametric amplifier to obtain a temporally compressed probe pulse of 55 fs duration. Our study will prove useful to the increasing number of laboratories seeking to move from the high-energy, low-power THz time-domain spectroscopy systems based on Ti:Sapphire lasers, to medium-energy, high-power systems driven by Yb-doped lasers.

Electron density measurement of Ar, N2, O2, and Ar mixtures (with N2 and O2) gas in inductively coupled plasma (ICP) using terahertz time domain spectroscopy

In this study, an inspection method using terahertz waves was proposed for the diagnosis of electron density using inductively coupled plasma (ICP). Plasma with low and high electron density for Ar, N2, O2, and Ar mixed (with N2 and O2) gases was generated by varying the RF power of the ICP and mass flow rate. Based on the theoretical analysis about propagation characteristics of the THz waves in Ar, N2, O2, and Ar mixed (with N2 and O2) gas plasma, the electron density of the ICP was measured according to RF power and mass flow rate. These results were verified using a Langmuir probe that can measure the electron density distribution.

Label-free detection of inclusion body formation in E. coli with application of terahertz

Extensive production of recombinant protein is essential for use in various fields. Bacterial cells, especially the Escherichia coli (E. coli) expression system, are widely used for the production of recombinant proteins due to their advantages in time and cost. However, the formation of inclusion bodies (IBs) caused by the production of recombinant protein is a significant issue. Therefore, a precise tool for the detection of IBs is in high demand. In this study, we demonstrated a label-free and rapid sensing platform for the detection of IBs formation in E. coli using terahertz-time domain spectroscopy (THz-TDS) system and terahertz metamaterials. The detection was based on the local polarity difference between E. coli samples with induced (positive) and non-induced (negative) protein overexpression. The formation of IBs in positive samples resulted in a decrease in local polarity compared to negative samples. This local polarity difference, strongly related to intermolecular interactions, influenced THz absorption as the THz regime involves weak intermolecular vibrational modes. Experimental results showed that the THz transmittance (ΔT) and resonance frequency shift (Δf) were significantly different between positive and negative samples, confirming the effectiveness of our method. Specifically, positive samples exhibited a ΔT value of 26.57% and a Δf value of 0.12 THz, while negative samples showed a ΔT value of 31.83% and a Δf value of 0.14 THz. These results highlight the capability of our platform to detect IBs rapidly and accurately.