Terahertz Time-domain Spectroscopy Research Articles

The Birefringence and Extinction Coefficient of Ferroelectric Liquid Crystals in the Terahertz Range

In this paper, the refractive index and extinction coefficient of ferroelectric liquid crystals have been examined by the terahertz time-domain spectroscopy system. Two modes of ferroelectric liquid crystal materials, deformed helix ferroelectric liquid crystal (DHFLC), and electric suppressed helix ferroelectric liquid crystal (ESHFLC) are tested as experimental samples. Nematic liquid crystal (NLC) was also investigated for comparison. The birefringence of DHFLC 587 slowly increases with the growth of frequency, and it averages at 0.115. Its extinction coefficients gradually incline to their stable states at 0.06 for o-wave and 0.04 for e-wave. The birefringence of ESHFLC FD4004N remains between around 0.165 and 0.175, and both of its e-wave and o-wave extinction coefficients are under 0.1, ranging from 0.05 to 0.09. These results of FLC will facilitate the examination and improve the response performance of THz devices using fast liquid crystal materials.

Hydrogen bonding and Hirshfeld surface analysis of gallic acid and its monohydrate based on terahertz spectroscopy

The terahertz (THz) spectroscopy of gallic acid (GA) and gallic acid monohydrate (GAMD) in the frequency range of 0.2–3.0 THz were investigated using terahertz time-domain spectroscopy (THz-TDS) system. The THz absorption spectrum of GAMD exhibited a red-shift compared to that of GA. To resolve the absorption spectra of the two substances, density functional theory (DFT) was employed to calculate and simulate the theoretical spectra of the two substances in this frequency range, while the vibration mode automatic relevance determination (VMARD) method combined with GaussView and VMD software was utilized to determine the origin of each absorption peak. Additionally, Hirshfeld surface analysis and the Atoms-in-Molecules (AIM) topological analysis were conducted to explore the intermolecular interactions between GA and GAMD. The results revealed that the characteristic absorption peaks of both GA and GAMD are attributed to overall atomic and molecular motions, whereas those of GAMD are accompanied by water molecule translation. Furthermore, H…O/O…H hydrogen bonding was identified as the main contributor to the interaction between GA and GAMD. The addition of water molecules led to differentiation in the position and strength of intermolecular interactions; specifically, hydrogen bonding in GA primarily involved the carboxyl group with the hydroxyl group, while hydrogen bonding in GAMD occurred between the water molecule and the hydroxyl group of the central molecule.

Structure characteristics and microwave/terahertz dielectric response of low-permittivity (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr3(MoO4)9 high-entropy ceramics

Microwave dielectric ceramics play a vital role in wireless communication systems. However, it is a great challenge to obtain ideal performances of single-phase ceramics. In this work, (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr3(MoO4)9 (abbreviated as L5ZMO) high-entropy ceramics were successfully prepared by the solid-state reaction method, the pure trigonal L5ZMO structure can be synthesized in the mixture calcined at 700 °C. Rietveld refinement based on XRD data reveals that lattice parameters of L5ZMO ceramics are affected by the radius of the five rare earth elements, which indicates that the rare earth elements have been solubilized and randomly distributed in the Ln-site. The relative density illustrates a strong dependence on sintering temperature, which gradually increases with increasing sintering temperature. Based on the analysis of bond characteristics, the Mo–O bond exhibits a vital effect on the dielectric properties of L5ZMO ceramics. Besides, the infrared reflection spectrum and terahertz time-domain spectroscopy analysis show that ionic polarization is the dominant polarization in the microwave/terahertz bands. Meanwhile, the tan δ can be further reduced by optimizing process parameters. Notably, the L5ZMO high-entropy ceramics sintered at 750 °C exhibit outstanding dielectric properties of εr = 10.46, Q·f = 59,713 GHz, and τf = −21.94 ppm/°C, indicating that the high-entropy design optimizes τf (−38.8 ppm/°C) of the simple La2Zr3(MoO4)9 ceramics. The attempt provides theoretical guidance for the application of high-entropy design concepts in microwave/terahertz bands.

A reconstructing approach to reduce distortion in detection of THz pulses via electro-optic sampling

Terahertz (THz) band is an important range in photonics, and provides numerous advantages in various applications. One of the most popular detection methods for THz pulses is electro-optic sampling (EOS). EOS provides many benefits; however, in this method distortion damages the output signal, and limits the bandwidth of this technique. In this article, a calculation-based approach is proposed to remove the effect of distortion in EOS detection process. This manner is based on definition of a base-level spectrum which modifies the output of EOS to eliminate unwanted disorders. The introduced method is computational, inexpensive, feasible, fast, adaptable, and effective. Moreover, a detailed comprehensive step-by-step model of THz time-domain spectroscopy (THz-TDS) with a simple and obvious perspective for ZnTe and GaP crystals is provided.

Charge Carriers Localization Effect Revealed through Terahertz Spectroscopy of MXene: Ti3C2Tx.

The transport properties of charge carriers in MXene, a promising material, have been studied using terahertz time-domain spectroscopy (THz-TDS) to examine its potential applications in optical and electronic devices. However, previous studies have been limited by narrow frequency ranges, which have hindered the understanding of the intrinsic mechanisms of carrier transport in MXenes. To address this issue, ultrabroadband THz-TDS with frequencies of up to 15 THz to investigate the complex photoconductances of MXene (Ti3C2Tx) films with different thicknesses are employed. The findings indicate that the electronic localization is substrate-dependent, and this effect decreases with an increase in the number of layers. This is attributed to the screening effect of the high carrier density in Ti3C2Tx. Additionally, the layer-independent photocarrier relaxations revealed by optical pump THz probe spectroscopy (OPTP) provide evidence of the carrier heating-induced screening effect. These results are significant for practical applications in both scientific research and various industries.

The dotTHz Project: A Standard Data Format for Terahertz Time-Domain Data

From investigating molecular vibrations to observing galaxies, terahertz technology has found extensive applications in research and development over the past three decades. Terahertz time-domain spectroscopy and imaging have experienced significant growth and now dominate spectral observations ranging from 0.1 to 10 THz. However, the lack of standardised protocols for data processing, dissemination, and archiving poses challenges in collaborating and sharing terahertz data between research groups. To tackle these challenges, we present the dotTHz project, which introduces a standardised terahertz data format and the associated open-source tools for processing of dotTHz files. The dotTHz project aims to facilitate seamless data processing and analysis by providing a common framework. All software components are released under the MIT licence through GitHub repositories to encourage widespread adoption, modification, and collaboration. We invite the terahertz community to actively contribute to the dotTHz project, fostering the development of additional tools that encompass a greater breadth and depth of functionality. By working together, we can establish a comprehensive suite of resources that benefit the entire terahertz community.

Analyzing the effects of nonthermal pretreatments on the quality of microwave vacuum dehydrated beef using terahertz time-domain spectroscopy and near-infrared hyperspectral imaging.

Both nonthermal pretreatment and nondestructive analysis are effective technologies in improving drying processes. This study evaluated the effects of different pretreatment methods on the quality of beef dehydrated by microwave vacuum drying (MVD) and compared the MVD process performance comprising real-time moisture content (MC), MC loss, colour content, and shrinkage rate using different optical sensing methods including terahertz time-domain spectroscopy (THz-TDS) and near-infrared hyperspectral imaging (NIR-HSI). Results indicated that osmotic pretreatment improved the drying rate of MVD beef with lower changes in colour and shrinkage rate. Both THz-TDS-based and NIR-HSI-based on-site direct scanning and in-situ in-direct sensing showed accurate prediction results, with best R2p of 0.9646 and 0.9463 for MC and R2p of 0.9817 and 0.9563 for MC loss prediction, respectively. NIR-HSI visualisation of MC results showed that ultrasound pretreatment curbed but osmotic pretreatment promoted nonuniform distribution during MVD. This research should guide improving the industrial MVD drying process.

Design and fabrication of a microcoil metamaterial absorber for the sub-terahertz region.

The development of electromagnetic wave absorbers operating in the sub-terahertz (sub-THz) region is necessary in 6G communications. We designed and fabricated a sub-THz metamaterial absorber based on metal microcoils embedded and periodically arranged in a dielectric substrate. The microcoil parameters were optimized by calculating the electromagnetic response of the metamaterial using finite element analysis. An actual metamaterial was then fabricated based on the optimized parameters and characterized using THz time-domain spectroscopy. Our microcoil absorber exhibits an absorptance of >80% and a high shielding performance at about 250 GHz. The resonance frequency can be precisely adjusted by modifying the microcoil array dimensions.

Optical analysis of aligned Ni nanowire arrays with different degree of oxidation for terahertz polarizer application

The optical properties of aligned nickel nanowire arrays (NiNWAs) with different degrees of oxidation for terahertz (THz) polarizer applications have been investigated by using THz time-domain spectroscopy. In frequency-domain spectra, the full width at half maxima of transmitted peaks was broadened and the peak positions have a blue shift with increasing oxidation levels, besides the enhancement in peak intensity. It is indicated that the oxidation of Ni nanowires (NWs) has a significant influence on the interaction between Ni NWs and THz wave. The transmittance of the aligned NiNWAs increases with annealing temperature increasing. Conversely, the degree of polarization and extinction ratio (ER) decreases. A corresponding relationship between the change of ER and degree of oxidation is summarized by means of thermogravimetric analysis. The change of ER for the annealing sample with the degree of oxidation of 0.507% is 27.32%, which induced the polarization properties of aligned NiNWAs to be sensitive to the oxidation of Ni NWs. These findings can provide new positive features in the development of future polarization-based device applications for THz electronics and photonics.

Defect detection of composite materials for cable insulation layer based on terahertz

Hidden defects can seriously affect the insulation performance of cables and pose safety hazards. In this article, a novel Non-Destructive evaluation technique for insulation materials based on terahertz time-domain spectroscopy technology is proposed. Firstly, terahertz time-domain spectroscopy technology was used to perform reflective scanning on insulation materials. Various imaging algorithms were used to image terahertz defects. The results showed that the maximum imaging and peak to peak time imaging were effective. At the same time, bilateral filtering was used to process terahertz defect images. The results showed that bilateral filtering not only filtered out noise in terahertz images, but also retained edge details. Terahertz time-domain spectroscopy technology offers a fresh approach for detecting cable insulation material defects non-destructively, and finds application in various electrical environment inspections.

Cherenkov-type terahertz generation by optical rectification in KD2PO4 (DKDP) crystal

The potential of KD2PO4 (or DKDP) crystal as a terahertz generator is demonstrated, despite its extremely strong terahertz absorption. By combining the Cherenkov radiation scheme and surface-emitting geometry, femtosecond Ti:sapphire laser pulses of ∼200−300 μJ energy were converted to broadband (∼4 THz bandwidth) terahertz transients with the efficiency of ∼8×10−6. The transients produced electro-optic signals of a ∼0.15 modulation depth and ∼103 dynamic range in a 1-mm thick ZnTe detector crystal. This suggests DKDP as a potential generator crystal for terahertz time-domain spectroscopy. From our measurements, we estimated the nonlinear coefficient d36≈3 pm/V of DKDP for optical rectification, which is an order of magnitude larger than the value known from second-harmonic-generation experiments. At the same time, the obtained value of d36 is an order of magnitude smaller than the value calculated from the clamped electro-optic coefficient r63 for the MHz frequency range. At high pump intensities, we observed multiple filamentation of the pump beam and white light generation, spatially separated from terahertz generation.

Automatic histogram-based defect detection in glass fibre reinforced polymer composites using terahertz time-domain spectroscopy reflection imaging

We developed a method for automatic detection of defects in glass fibre reinforced polymer (GFRP) based on a terahertz time-domain spectroscopy (TDS) system. Two GFRP specimens with round defects of different sizes (inclusions and delaminations) were measured by means of a high-speed TDS reflective scanner with an electronically controlled optical sampling scheme. The algorithm provided a series of C-scans, which were then pre-selected to identify only those with features potentially associated with defects. A thresholding method based on Gaussian fitting of the histogram was developed. The results showed that 87 % of defects were detected for the specimen with inclusions and 100 % for the specimen with delaminations. The algorithm also provided information about the depth of defects, as each C-scan was assigned to a time slice. The results demonstrate that the TDS-based system, along with the proposed histogram-based method, can automatically detect defects in GFRP. © 2017 Elsevier Inc. All rights reserved.

Quantitative analysis of millet mixtures based on terahertz time-domain spectroscopy and improved coati optimization algorithm

The significant difference in economic and nutritional value between new and old cereals has led to a plethora of grain mix-ups on the market, and it is necessary to distinguish the old and new grains accurately and quickly. In this study, millet, which has high economic and nutritional value in cereals, was used as the research object. Terahertz time domain spectroscopy combined with chemometrics was used to detect its mixture. The experimental findings show that, compared to the partial least squares regression model, and the support vector regression models optimized by particle swarm and grid algorithms, the support vector regression model optimized by the coati optimization algorithm (COA) exhibits superior performance. In order to further improve the prediction accuracy of the support vector regression model, an improved COA was proposed. The quality and diversity of the initial population are improved by introducing chaotic mapping, tent mapping, and opposition based learning mechanisms, followed by the introduction of adaptive weights and Wright flight mechanisms, which enhance the flexibility and convergence speed of the algorithm, and finally, the introduction of the sine-cosine algorithm to improve the search range. The experiments demonstrate that a support vector regression model optimized by an enhanced coati’s algorithm provides superior quantitative analysis of millet mixtures as compared to other models tested. The optimized model achieves high prediction accuracy, with correlation coefficients reaching 0.9872 and 0.9742 for millet mixtures with gradient ratios of 10% and 2%, respectively. This research provides technical support and a reference for rapid, nondestructive quantitative detection of millet mixtures while opening up new possibilities for subsequent quantitative detection of other cereals.

Continuous-wave terahertz in-line holographic diffraction tomography with the scattering fields reconstructed by a physics-enhanced deep neural network

Diffraction tomography is a promising, quantitative, and nondestructive three-dimensional (3D) imaging method that enables us to obtain the complex refractive index distribution of a sample. The acquisition of the scattered fields under the different illumination angles is a key issue, where the complex scattered fields need to be retrieved. Presently, in order to develop terahertz (THz) diffraction tomography, the advanced acquisition of the scattered fields is desired. In this paper, a THz in-line digital holographic diffraction tomography (THz-IDHDT) is proposed with an extremely compact optical configuration and implemented for the first time, to the best of our knowledge. A learning-based phase retrieval algorithm by combining the physical model and the convolution neural networks, named the physics-enhanced deep neural network (PhysenNet), is applied to reconstruct the THz in-line digital hologram, and obtain the complex amplitude distribution of the sample with high fidelity. The advantages of the PhysenNet are that there is no need for pretraining by using a large set of labeled data, and it can also work for thick samples. Experimentally with a continuous-wave THz laser, the PhysenNet is first demonstrated by using the thin samples and exhibits superiority in terms of imaging quality. More importantly, with regard to the thick samples, PhysenNet still works well, and can offer 2D complex scattered fields for diffraction tomography. Furthermore, the 3D refractive index maps of two types of foam sphere samples are successfully reconstructed by the proposed method. For a single foam sphere, the relative error of the average refractive index value is only 0.17%, compared to the commercial THz time-domain spectroscopy system. This demonstrates the feasibility and high accuracy of the THz-IDHDT, and the idea can be applied to other wavebands as well.

Ultrafast spin-to-charge conversion in antiferromagnetic (111)-oriented L12-Mn3Ir

Antiferromagnetic L12-Mn3Ir combines outstanding spin-transport properties with magnons in the terahertz (THz) frequency range. However, the THz radiation emitted by ultrafast spin-to-charge conversion via the inverse spin Hall effect remains unexplored. In this study, we measured the THz emission and transmission of a Permalloy/(111)-oriented L12-Mn3Ir multilayer by THz time-domain spectroscopy. The spin Hall angle was determined to be approximately constant at 0.035 within a frequency range of 0.3–2.2 THz, in comparison with the THz spectroscopy of a Permalloy/Pt multilayer. Our results not only demonstrate the potential of L12-Mn3Ir as a spintronic THz emitter but also provide insights into the THz spin transport properties of L12-Mn3Ir.

Electromagnetically induced transparency in a strongly coupled orthogonal polarization-insensitive planar terahertz metamaterial

We demonstrate experimentally and numerically the electromagnetically induced transparency (EIT) effect in a strongly coupled planar terahertz (THz) metamaterial. The circular-shaped four-arc geometry combined with cross resonators ensures the polarization-insensitive response of the EIT for the orthogonal polarization incident THz. The EIT response can be varied by changing the gap between the arc and cross-shaped resonators. The field profiles indicate a strong coupling between the resonators leading to the EIT effect. In order to understand the underlying physical mechanism, we employed a coupled harmonic oscillator model, which suggests an increase in coupling when the distance between resonators is reduced. THz time-domain spectroscopy of the fabricated samples with the same shape and size of the simulated structures was used to verify the numerical findings. Our study uses a symmetric and easy-to-fabricate planar metasurface that can pave the way for the design and construction of THz photonic components, such as optical switches and slow light devices.

Relationship between the characteristic temperature and terahertz permittivity of different rank coals considering FeS2 content

In this study, the characteristic temperatures and terahertz permittivity of lignite with varying pyrite content were determined using a synchronous thermal analyzer and terahertz time-domain spectroscopy. A mathematical relationship between these two parameters was established by combining experimental data. The results indicate that within the range of 0–9% pyrite content, there is a trend where pyrite initially enhances and then weakens the promotion effect on coal spontaneous combustion. A quadratic polynomial can describe this relationship, with the extremum occurring at 6%. Regarding permittivity, as the pyrite content increases from 0% to 9%, the real part of permittivity exhibits irregular variations, while the imaginary part shows a well-defined linear increase. This behavior is attributed to enhanced relaxation polarization, leading to more significant dielectric losses. Furthermore, through the combination of experimental data, a robust quadratic relationship is observed between characteristic temperature and terahertz permittivity. The credibility of this relationship is validated by comparing it with experimental results. These findings contribute to the advancement of the theoretical understanding of coal's permittivity and spontaneous combustion characteristics, and provide a theoretical possibility for the rapid estimation of spontaneous combustion characteristics in pyrite-bearing coal seams using electromagnetic methods.

Non-destructive mapping of electrical properties of semi-insulating compound semiconductor wafers using terahertz time-domain spectroscopy

With an increasing demand for THz electronics applications, it is essential to obtain the electrical properties of semiconductor materials used in their fabrication. This study presents an approach to estimate the electrical properties of semi-insulating compound semiconductors using terahertz time-domain spectroscopy (THz-TDS). By employing THz-TDS in transmission mode, the resistivity and carrier concentration of semi-insulating Silicon Carbide (SiC) and Indium Phosphide (InP) wafers are non-destructively measured. The study utilizes the simplified Drude model and the Nelder-Mead algorithm to estimate the electrical properties from the THz-TDS transmission (S21) measurement data. The resistivity values of the SiC and InP samples, (1.42 ± 0.15) × 105 Ω cm and (3.0 ± 0.1) × 107 Ω cm, respectively, are in accordance with the manufacturer specifications. Moreover, the feasibility of non-destructive mapping of the electrical properties of the wafers is demonstrated, offering a novel tomographic inspection approach for online monitoring. This technique holds significant promise for enhancing production yields in the semiconductor industry.

Ultrafast pump-probe spectroscopy via chirped-pulse up-conversion with dispersion compensation.

In this study, ultrafast transient signals were detected on a single-shot basis using chirped-pulse up-conversion spectroscopy with dispersion compensation. Unlike in the conventional time-encoding technique using chirped pulses, distortion of the ultrafast waveform was reduced by applying dispersion compensation to the chirped probe pulses and using sum-frequency generation with the chirped readout pulses. The method was applied to terahertz time-domain spectroscopy and near-infrared pump-probe spectroscopy, providing ultrafast observations with an improved temporal resolution comparable to the transform-limited pulse durations. Terahertz waveforms, Kerr rotation signals, and phonon-polariton oscillations were measured accurately with no significant waveform distortion, thereby showing the proposed scheme to be promising for single-shot pump-probe spectroscopy in a wide range of spectroscopic applications.

Terahertz Metasurfaces Exploiting the Phase Transition of Vanadium Dioxide.

Artificially designed modulators that enable a wealth of freedom in manipulating the terahertz (THz) waves at will are an essential component in THz sources and their widespread applications. Dynamically controlled metasurfaces, being multifunctional, ultrafast, integrable, broadband, high contrasting, and scalable on the operating wavelength, are critical in developing state-of-the-art THz modulators. Recently, external stimuli-triggered THz metasurfaces integrated with functional media have been extensively explored. The vanadium dioxide (VO2)-based hybrid metasurfaces, as a unique path toward active meta-devices, feature an insulator-metal phase transition under the excitation of heat, electricity, and light, etc. During the phase transition, the optical and electrical properties of the VO2 film undergo a massive modification with either a boosted or dropped conductivity by more than four orders of magnitude. Being benefited from the phase transition effect, the electromagnetic response of the VO2-based metasufaces can be actively controlled by applying external excitation. In this review, we present recent advances in dynamically controlled THz metasurfaces exploiting the VO2 phase transition categorized according to the external stimuli. THz time-domain spectroscopy is introduced as an indispensable platform in the studies of functional VO2 films. In each type of external excitation, four design strategies are employed to realize external stimuli-triggered VO2-based THz metasurfaces, including switching the transreflective operation mode, controlling the dielectric environment of metallic microstructures, tailoring the equivalent resonant microstructures, and modifying the electromagnetic properties of the VO2 unit cells. The microstructures' design and electromagnetic responses of the resulting active metasurfaces have been systematically demonstrated, with a particular focus on the critical role of the VO2 films in the dynamic modulation processes.