Terahertz Spectroscopy Research Articles

Low Temperature Exciton Lifetime Enhancement of MAPbBr3 Studied by Broadband Terahertz Spectroscopy

Low Temperature Exciton Lifetime Enhancement of MAPbBr3 Studied by Broadband Terahertz Spectroscopy

Observation of terahertz nonlinear absorption activity in Al2O3

We investigate the terahertz (THz) nonlinear absorption characteristics in a thin Al2O3 crystal using nonlinear THz spectroscopy. A THz saturable absorption performance with a high modulation depth (27.5%) and a relatively low saturable intensity (6.99 µJ/cm2) of Al2O3 is revealed, covering the THz spectrum from 0.1 to 8 THz. The inevitable presence of oxygen vacancies and impurities in Al2O3 single crystal results in the existence of free electrons within the conduction band. The transmission enhancement can be described by the change of the mobility of electrons due to intervalley scattering induced by THz pulses, based on first-principles calculations. Therefore, Al2O3 crystals have promising application potential in mode-locked devices for directly delivering broadband ultrashort THz pulses.

InAs Terahertz Metalens Emitter for Focused Terahertz Beam Generation

Metasurfaces have opened doors to combining multiple photonic functionalities in a single compact device. In particular, the ability to generate short terahertz (THz) pulses with precise wavefront engineering in a single THz metasurface redefined the role metasurfaces can play in THz systems. Here, an InAs metalens emitter which generates and focuses a THz pulse beam is demonstrated using a 130 nm thick InAs metasurface designed as a binary‐phase Fresnel zone plate. The THz beam is focused to a spot of ≈430 μm at 1 THz with a short focal length of 5 mm and large numerical aperture of 0.5. Nanoscale InAs Mie resonators comprising the metasurface enable THz generation with an amplitude as high as 20 times compared to plasmonic THz emitters and several times compared to a 1 mm thick ZnTe crystal. This InAs metasurface emitter provides a new paradigm for designing THz imaging, spectroscopy, and communication systems, where THz beam generation and shaping are performed with a single device without compromising the generation efficiency, while eliminating losses and avoiding limitations of phase matching of conventional nonlinear optics approaches.

High-Resolution Terahertz Spectroscopy for MedicalDiagnostics.

The metabolomics-based approach to diagnostics and therapy monitoring is a fast-emerging trend in modern medicine. Terahertz nonstationary spectroscopy based on the induction and disintegration of freely decaying polarization in the gas mixture during the interaction of radiation with molecules at resonance frequencies is a high-sensitivity method for studying multicomponent gas mixtures, which is promising for identifying metabolites in the thermal decomposition products of biological samples. The paper presents the results of the application of high-resolution terahertz spectroscopy to the study of biological samples taken from patients with certain diseases (pathologically changed tissues of ear-nose-throat organs and similar pathologic tissues formed in other life support systems) to search for characteristic sets of metabolites characterizing the pathology. The world's first measurements of the spectra of pathologic samples of cysts, formed in different life support systems, were carried out, which made it possible to identify similar substances in tissues having the same pathology.

Non-destructive detection of fructose solutions via Terahertz spectroscopy enhanced by Frequency Selective Surface (FSS)

Terahertz (THz) spectroscopy, combined with Frequency Selective Surfaces (FSS), is emerging as a powerful non-destructive technique for detecting and analysing the properties of biological samples. In this study, we explored the use of THz spectroscopy for detecting fructose solutions with concentrations ranging from 0.2 % to 10 %. Using femtosecond laser light sources, the THz transmission and absorption characteristics of fructose were investigated. The integration of FSS significantly enhanced the sensitivity of THz detection, yielding a 16 % improvement at 1.29 THz and 28 % at 1.67 THz. UV–Vis and Fourier Transform Infrared (FTIR) spectroscopy were employed to compare results with traditional methods, highlighting the superior sensitivity of THz spectroscopy, especially at low concentrations. The findings suggest that THz spectroscopy, enhanced by FSS, has strong potential for advancing non-invasive testing in biological and environmental applications.

Correction of systematic low-frequency time delay errors in THz systems by absorption line shape optimization

Time-domain terahertz systems can face challenges due to systematic delay errors introduced by the employed delay mechanism, potentially leading to poor data quality. This article introduces a procedure to address these challenges by correcting low-frequency systematic errors that distort the acquired spectra and incorrectly diminish narrow absorption features. Our procedure solves an optimization problem aiming to find the corrected time-signal pairs that maximize the depth of narrow absorption features, and we highlight how the flexibility of the procedure, in principle, allows for correcting error profiles of arbitrary shape. We demonstrate the effectiveness of this approach through experiments using both simulated and experimentally recorded THz data, and the results show significant improvements in spectral accuracy. We believe this can pave the way for more reliable and precise terahertz spectroscopy measurements, enhancing its applications in various scientific and industrial fields.

In-line porosity and hardness monitoring of tablets by means of optical coherence tomography

In-line monitoring of critical quality attributes (CQAs) during a tableting process is an essential step toward a real-time release strategy. Such CQAs can be the tablet mass, the API content, dissolution, hardness and tensile strength. Since dissolution testing is laborious and time-consuming and cannot be performed in-line, it is desirable to replace dissolution testing with predictive models based on other CQAs that affect the dissolution characteristics, such as the tablet porosity and hardness. Traditionally, porosity is determined offline via gas adsorption methods or other techniques, such as Terahertz spectroscopy or gas in scattering media absorption spectroscopy. Tablet hardness is typically established using a hardness tester. While these destructive tests can readily be performed at-line, they have limited applicability in in-line settings for a high-percentage inspection. Optical coherence tomography (OCT) has recently been proposed as a possible tool for determining quality attributes. This work describes the first application of OCT for the prediction of tablet porosity and hardness. OCT measurements of tablets produced in a ConsiGma 25™ tableting line and a Stylcam 200R compaction simulator in several compaction force settings were performed and correlated with the porosity and hardness. It was demonstrated that OCT can easily be installed in-line and provide real-time information about critical material attributes. These insights confirm the applicability of OCT as a real-time quality control tool and its potential to replace time-consuming and destructive offline measurements.

Roller compaction: Measuring ribbon porosity by terahertz spectroscopy and machine learning

Roller compaction is a crucial unit operation in pharmaceutical manufacturing, with its ribbon porosity widely recognised as a critical quality attribute. Terahertz spectroscopy has emerged as a fast and non-destructive technique to measure porosity in pharmaceutical products. From a sensing perspective, the irregular shape and uneven surface of fragmented ribbon pieces can affect the accuracy and precision of the measurements, particularly for techniques that probe only a small sampling volume. It is known that the porosity is not uniform within the ribbon structure, with variations expected across the width of the ribbon and in the microstructure corresponding to its surface texture. However, typical pharmaceutical analysis methods, such as envelope density, only report an average bulk porosity, are slow to operate and limited in accuracy. To address this challenge, we developed and trained convolutional neural network models using THz spectra as input to classify four types of topography typically encountered in ribbons: ridge, valley, flat plane and edge points. The classifiers achieved 91% validation accuracy in both identifying outliers and differentiating between ribbons of smooth and knurled surfaces. For the more challenging task of distinguishing between the ridges and valleys of knurled surfaces, an 81% testing accuracy was achieved. Once each measurement is paired with its topography, resolving the density distribution within the sample is possible. This data can be combined to arrive at an average bulk porosity value compatible with conventional pharmaceutical analysis.

High-efficiency Yb:YAG thin-disk chirped pulse amplifier delivering 884-femtosecond laser with tunable repetition rates and high stability

We experimentally demonstrated a compact, highly efficient, stable, chirped pulse amplification (CPA) thin-disk regenerative system with a tunable repetition rate. The laser head consists of a 9 at. % Yb:YAG thin-disk with a diameter of 8.8 mm and a thickness of 150 μm, is designed for a 48-pump pass configuration. In CPA-based regenerative amplifier configuration, a maximal output power of 85 W at 500 kHz is achieved with a conversion efficiency of 53.1 %. This corresponds to a pulse energy of 170 μJ. To the best of our knowledge, this is the highest conversion efficiency in CPA thin-disk regenerative amplifiers. The pulse width is compressed to 884 fs with a pair of grating, resulting in a peak power of 192 MW. At 85 W, a near diffraction limit beam quality factor M2 of 1.40 is measured, which is essential in precision applications. In addition, remarkable long-term power stability is confirmed with a root-mean-square (RMS) fluctuation of 0.12 % over a 24-hour duration. The reported femtosecond amplifier is believed to be a promising tool for various applications such as extreme manufacturing, terahertz spectroscopy, etc.

Multimodal Non-Destructive In Situ Observation of Crystallinity Changes in High-Density Polyethylene Samples with Relation to Optical Parameters during Tensile Deformation

Abstract: Many non-destructive optical testing methods are currently used for material research, providing various information about material parameters. At RECENDT, a multimodal experimental setup has been designed that combines terahertz (THz) spectroscopy, optical coherence tomography (OCT), infrared (IR), and Raman spectroscopy with a tensile test stage. This setup aims to gather material information such as crystallinity and optical parameters of high-density polyethylene (HDPE) during a tensile test. The setup compares common IR and Raman spectroscopy and the less common optical methods THz and OCT. Complementarity is achieved through different frequency ranges and measurement approaches, resulting in different measured optical material parameters and depths. During tensile testing, HDPE samples with varying crystallinity were analysed, and the determined optical parameters such as refractive index, birefringence, scattering coefficient of decay, and penetration depth can be correlated with the change in crystallinity. These findings demonstrate that the optical methods and their outcomes can be interconnected. With further optimization of the experimental setup, it would be possible to observe the alignment of fibres in fibre composite panels and the stress distribution of polymers effectively. This opens interesting possibilities for polymer characterization in the future, including quality control during moulding processes and material testing.

Optical parameters extraction of soil and its microplastics contamination using terahertz spectroscopy

The primary focus of this study investigates the potential of continuous-wave terahertz frequency-domain spectroscopy(CW-THz-FDS) as a rapid, non-destructive technique for detecting microplastic contamination in soil. Focusing on low-density polyethylene (LDPE) as a model microplastic, we prepared soil mixtures with varying LDPE concentrations (1 %,5 %,10 %,15 % by weight). We used a THz-FDS system operating between 0.1 and 0.7 THz to analyze the optical properties (transmission coefficient, attenuation and refractive index) of these mixtures and LDPE samples separately. Notably both soil and LDPE particles were of micrometer scale. Our results revealed distinct, concentration-dependent variations in these properties. The refractive index for the higher concentration (15 %) ranged from 2.2 to 2.3, while the attenuation coefficient varies between 10 and 14 cm−1. Crucially, we identified polylactic acid [PLA] as a suitable sample holder material due to its high transmission in the THz range. Consequently, this study suggests that terahertz spectroscopy can be a promising technique for detecting microplastic contamination in soil, offering a non-destructive and sensitive method for environmental monitoring.

Large Dynamic Range Spectral Measurement in Terahertz Region Based on Frequency Up-Conversion Detection via OH1 Crystal.

Terahertz spectroscopy systems, which integrate terahertz sources and detectors, have important applications in many fields such as materials science and security checking. Based on highly sensitive frequency up-conversion detection, large dynamic range spectral measurements in a terahertz region are reported. Our system realized the detection sensitivity at a 10 aJ level with a 2-(3-(4-hydroxystyryl)-5,5-dime-thylcyclohex-2-enylidene) malononitrile (OH1) crystal and a dynamic range up to seven orders. Based on this system, we verified the validity of the spectral measurement with tests which were conducted on monohydrate glucose, anhydrous glucose and mixed tablet samples with a thickness of 0.8 mm in 1~3 THz, respectively. Also, a mini coppery elbow tube with an inner diameter of 1 mm was used for the transmission of a terahertz wave to simulate some strip biological tissue samples. By allowing terahertz to transmit through this tube filled with 0.5 g glucose powder, we successfully obtained the absorption spectrum with a minimum transmittance at the absorption peak in the order of 10-4.

Enhancement of broadband terahertz trace absorption spectrum based on angular multiplexing of trapezoidal dielectric grating

Terahertz spectroscopy provide a fast, label-free, and non-destructive method for detecting bio-medical molecules, with wide applications in national security, pharmacy, and healthcare. However, detecting terahertz absorption spectra of trace analytes still faces significant challenges. In recent years, metamaterial parameter multiplexing technology has been used to detect the terahertz fingerprint spectra of trace substances. This paper proposes a new structure based on the angle-multiplexing enhancement principle of a trapezoidal dielectric grating to enhance the terahertz absorption spectra of thin-film analytes. By scanning the incident wave angle, a series of resonance absorption peaks can be obtained. The enhanced absorption spectrum formed by the envelope of absorption spectrum peaks increases by 141 times compared to directly measuring the absorption spectrum of the same film. This structure provides a new and effective means for terahertz trace detection using metamaterial parameter multiplexing technology.

Identification and quantification of adulteration in collagen powder by terahertz spectroscopy − the effect of spectral characteristics on performance is considered

Terahertz spectroscopy is an emerging rapid detection method that can be used to detect and analyze food quality issues. However, models developed based on various spectral characteristics of terahertz have shown different performances in food identification. Therefore, we preliminarily analyzed the effect of terahertz spectral characteristics on the identification and quantification of collagen powder adulterated with food powders (plant protein powder, corn starch, wheat flour) with the use of random forest (RF), linear discriminant analysis (LDA), and partial least squares regression (PLSR), and determined the spectral characteristics suitable for identification and quantitative analysis. Then, the selected spectral characteristics data were preprocessed using baseline correction (BC), gaussian filter (GF), moving average (MA), and savitzky-golay (SG). Feature variables were extracted from preprocessed spectral characteristics data using genetic algorithm (GA), random forest (RF), and least angle regression (LAR). The study indicated that the BC-GA-LDA classification model based on the absorption coefficient spectra achieved an accuracy of 96.96% in identifying adulterated collagen powder. Additionally, the GA-PLSR model developed based on the power spectra demonstrated excellent performance in predicting adulteration levels, with the coefficient of determination (Rp2) values ranging from 0.93 to 0.99. The results showed that the rational selection of terahertz spectral characteristics is highly feasible for the accurate detection of collagen powder adulteration.

A miniaturized dual band terahertz metamaterial based absorber as a biosensor for non-melanoma skin cancer diognostic

Early diagnosis of Non-melanoma skin cancers (NMSCs) is most important for successful treatment of the disease.The detection of NMSCs involves visual inspection or invasive method of detection by skin biopsy. Recently, terahertz spectroscopy has come into limelight for detection of biomarkers in low terahertz (THz) region in between 0.1 – 10 THz which will be in resonance with the biomolecules. This work entails in designing a microscale THz metamaterial absorber for discriminating the attributes between Non-melanoma skin cancerous skin and normal skin. The proposed absorber is composed of space filling curved aluminum layer over a polymide substrate. The absorber culminates its absorbance peak of 99.8 % at 0.503 THz and 99.5 % at 1.076 THz, revealing its dual band trait. Moreover, variation of refractive index of the medium from 1.3 to 1.4 shows shift in absorption peak with a sensitivity of 95.76 GHz/RIU and 100 GHz/RIU for first and second band respectively.

Probing Majorana Wave Functions in Kitaev Honeycomb Spin Liquids with Second-Order Two-Dimensional Spectroscopy.

Two-dimensional coherent terahertz spectroscopy (2DCS) emerges as a valuable tool to probe the nature, couplings, and lifetimes of excitations in quantum materials. It thus promises to identify unique signatures of spin liquid states in quantum magnets by directly probing properties of their exotic fractionalized excitations. Here, we calculate the second-order 2DCS of the Kitaev honeycomb model and demonstrate that distinct spin liquid fingerprints appear already in this lowest-order nonlinear response χ_{yzx}^{(2)}(ω_{1},ω_{2}) when using crossed light polarizations. We further relate the off-diagonal 2DCS peaks to the localized nature of the matter Majorana excitations trapped by Z_{2} flux excitations and show that 2DCS thus directly probes the inverse participation ratio of Majorana wave functions. By providing experimentally observable features of spin liquid states in the 2D spectrum, our Letter can guide future 2DCS experiments on Kitaev magnets.

Temperature‐Dependent Recombination Dynamics of Photocarriers in CsPbBr3 Microcrystals Revealed by Ultrafast Terahertz Spectroscopy

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

Probing inhomogeneous cuprate superconductivity by terahertz Josephson echo spectroscopy

AbstractInhomogeneities crucially influence the properties of quantum materials, yet methods that can measure them remain limited and can access only a fraction of relevant observables. For example, local probes such as scanning tunnelling microscopy have documented that the electronic properties of cuprate superconductors are inhomogeneous over nanometre length scales. However, complementary techniques that can resolve higher-order correlations are needed to elucidate the nature of these inhomogeneities. Furthermore, local tunnelling probes are often effective only far below the critical temperature. Here we develop a two-dimensional terahertz spectroscopy method to measure Josephson plasmon echoes from an interlayer superconducting tunnelling resonance in a near-optimally doped cuprate. The technique allows us to study the multidimensional optical response of the interlayer Josephson coupling in the material and disentangle intrinsic lifetime broadening from extrinsic inhomogeneous broadening for interlayer superconducting tunnelling. We find that inhomogeneous broadening persists up to a substantial fraction of the critical temperature, above which this is overcome by the thermally increased lifetime broadening.

Balanced air-biased detection of terahertz waveforms.

A novel, to the best of our knowledge, balanced air-biased coherent detection scheme for capturing ultrabroadband terahertz (THz) waveforms is implemented. The balanced detection scheme allows for coherent detection at the full repetition rate of the laser system without requiring bias modulation, signal generators, or lock-in amplifiers while doubling the dynamic range and quadrupling the signal-to-noise ratio compared to conventional air-biased coherent detection. These advantages are achieved by rotating the bias electrodes by 90° relative to the conventional scheme. With a 1 kHz driving laser, the scheme enables sub-second, high-fidelity waveform acquisition with a continuously moving delay stage, demonstrated by collecting 200 waveforms in 100 s. The balanced detection scheme paves the way for much faster and higher quality 2D ultrabroadband terahertz spectroscopy.

All-Dielectric Metasurface-Based Terahertz Molecular Fingerprint Sensor for Trace Cinnamoylglycine Detection.

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