Transmission Terahertz Time-domain Spectroscopy Research Articles

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.

Characteristic fingerprint spectrum of α-synuclein mutants on terahertz time-domain spectroscopy

α-Synuclein, a presynaptic neuronal protein encoded by the SNCA gene, is involved in the pathogenesis of Parkinson’s disease. Point mutations and multiplications of α-synuclein (A30P and A53T) are correlated with early-onset Parkinson’s disease characterized by rapid progression and poor prognosis. Currently, the clinical identification of SNCA variants, especially disease-related A30P and A53T mutants, remains challenging and also time consuming. This study aimed to develop a novel label-free detection method for distinguishing the SNCA mutants using transmission terahertz (THz) time-domain spectroscopy. The protein was spin-coated onto the quartz to form a thin film, which was measured using THz time-domain spectroscopy. The spectral characteristics of THz broadband pulse waves of α-synuclein protein variants (SNCA wild type, A30P, and A53T) at different frequencies were analyzed via Fourier transform. The amplitude A intensity (AWT, AA30P, and AA53T) and peak occurrence time in THz time-domain spectroscopy sensitively distinguished the three protein variants. The phase φ difference in THz frequency domain followed the trend of φWT > φA30P > φA53T. There was a significant difference in THz frequency amplitude A′ corresponding to the frequency ranging from 0.4 to 0.66 THz (A′A53T > A′A30P > A′WT). At a frequency of 0.4–0.6 THz, the transmission T of THz waves distinguished three variants (TA53T > TA30P > TWT), whereas there was no difference in the transmission T at 0.66 THz. The SNCA wild-type protein and two mutant variants (A30P and A53T) had distinct characteristic fingerprint spectra on THz time-domain spectroscopy. This novel label-free detection method has great potential for the differential diagnosis of Parkinson’s disease subtypes.

Transformer Oil Temperature Variation Based on Terahertz Time-domain Spectroscopy

The transmission terahertz time-domain spectroscopy system is used to measure the transformer oil terahertz spectrum at different temperatures, the time and frequency domain, time delay, and refractive information of transformer oil with different temperatures are tested. The terahertz wave of the time-domain delay of transformer oil at different temperatures is obtained, the variation of time-domain delay and refractive index with different temperatures in the terahertz band were calculated and the linear variation trend was fitted.

Nondestructive testing of corrosion thickness in coated steel structures with THz-TDS

The corrosion of steel plate structures usually occurs under the coating materials, and it is difficult to be accurately detected by the existing non-destructive testing (NDT) technologies. A novel non-contact terahertz time-domain spectroscopy (THz-TDS) is therefore proposed to measure the corrosion thickness under different coatings, which provides a method to quantify the corrosion degree of steel structures. In this study, the optical parameters of the corrosion products and the common coating materials (epoxy resin, rubber, and cement paste) are investigated by transmission THz-TDS, and the method of extracting the delay time difference in the reflection THz-TDS is proposed to measure the thickness of corrosion layer. The experiments show that the detection of the steel plate corrosion can be achieved from the reflected THz signal based on the peak number change and a significant attenuation of the signal peaks. Moreover, the reflected THz signal can measure the corrosion thickness with an accuracy of more than 90% and the minimum identification thickness is 40 μm. Besides, the thickness of the coating layer is simultaneously measured irrespective of the coating material types. It proves the applicability and accuracy of THz-TDS for NDT corrosion thickness of coated steel structures.

Conductivity in sulfur doped gallium selenide crystals measured by terahertz time-domain spectroscopy

In this work, the conductivity of intrinsic GaSe, S doped 2.5 mass% GaSe (GaSe: S(2.5%)), and S doped 7 mass% GaSe (GaSe: S(7%)) crystals, in a frequency range of 0.3–2.5 THz, is measured by transmission terahertz time-domain spectroscopy, and fitted with Drude-Smith-Lorentz model which is introduced by lattice vibration effect. It is found that the real part of conductivity decreases with the augment of S doping, which is caused by the gradual shift of the Fermi energy level of GaSe crystals to the charge neutrality level due to the generation of substitution impurities and gap impurities by S doping, resulting in the reduction of carrier concentration. The intrinsic GaSe and GaSe: S(2.5%) have a clear lattice vibration peak at about 0.56 THz, while GaSe: S(7%) has no lattice vibration peak near 0.56 THz, which is mainly due to the S doping increasing the structural hardness of the crystal and reducing the interlayer rigidity vibration of the crystal. All three samples have the obvious narrow lattice vibration peaks at about 1.81 THz, and the intensities that first decrease and then increase with the augment of S doping, which is mainly due to the fact that a small amount of S doping can reduce the local structural defects of GaSe and weaken the intensity of the narrow lattice vibration peak, while excessive S doping can generate the β-type GaS crystal, increase the local structural defects of the crystals and the intensity of the narrow lattice vibration peak. With the increase of S doping, the intensity of the broad lattice vibration peak of GaSe crystal weakens or even disappears at about 1.07 THz and 2.28 THz, mainly due to the S doping resulting in the substitution of S for impurities and GaS gap impurities, which reduces the fundamental frequency phonon vibration intensity, thereby weakening the lattice vibration caused by the second-order phonon difference mode of the crystal. The results show that the appropriate concentration of S doping can effectively suppress the lattice vibration of GaSe crystal and reduce the conductivity and power loss in the THz band. This study provides important data support and theoretical basis for the design and fabrication of low loss THz devices.

Measurement of Stress Optical Coefficient for Silicone Adhesive Based on Terahertz Time Domain Spectroscopy

The bonding structure is affected by environmental loads during use, causing internal stress in the adhesive layer, which leads to the debonding and expansion of the bonding layer. Therefore, it is important to accurately measure the stress distribution of the bonding layer to assess the life of the bonding structure. In this study, based on the transmission and reflection terahertz time-domain spectroscopy (THz-TDS) technique, the stress optical coefficients of a silicone adhesive were measured, and the calculation models of the transmission and reflection stress optical coefficients were derived. In the reflection calculation model, the caliper THz thickness measurement method was proposed to compensate for the thickness change of the silicone adhesive, under tensile stress. Under the transmission THz-TDS stress optical coefficient calculation model, the stress optical coefficient C of the silicone adhesive is 0.1142 ± 0.0057 MPa−1, and the stress optical coefficient C of the reflective system is 0.1135 ± 0.0051 MPa−1. The test results show that the reflective THz-TDS can also be used to measure the optical stress coefficient of the material, which compensates for the shortcomings of the traditional transmission measurement method, and lays a foundation for the characterization of the internal stress of the adhesive layer of the adhesive structure.

Study on the laser-induced damage of thin films by terahertz time-domain spectroscopy

The undamaged and damaged areas of the ITO thin film deposited on a silicon substrate induced by laser with different energies were tested with the transmission terahertz time-domain spectroscopy system. Their time-domain spectra and frequency-domain spectra of 0.4–1.0 THz were obtained, and the differences were analyzed between the undamaged and damaged areas in these two spectra. The results show that the peak-to-peak value in the time domain and the amplitude in the frequency domain of the damaged area evidently increased compared with those of the undamaged area, and with the increase in laser-induced energy, resulting in the larger damaged area and heavier surface roughness, these two parameters gradually decrease. For the damaged area of the ITO thin film with the undamaged substrate, the refractive index and absorption coefficient are lower than those of the undamaged area. Therefore, the variation in the terahertz time-domain and frequency-domain spectra could be utilized to distinguish the damage of the optical thin film irradiated by laser, which provides a new approach for the laser-induced damage identification and the technical support in effect of the laser-induced damage on the properties of the optical thin film element in the terahertz band.

Observation of magnetoconductivity with terahertz probes for ferromagnetic Fe films

Ultrafast characterization of spin-dependent transport behavior in ferromagnetic systems without contact probes has been of strong demand, recently. We have experimentally investigated spin-dependent complex conductivity for ferromagnetic Fe films of various thickness by means of terahertz time-domain transmission spectroscopy. Comparison of the transmitted terahertz wave amplitude and the spin-dependent conductivity reveals that the magnetization state of films effectively determines the complex conductivity. Non-invasive observation of spin-dependent conductivity by contact-free terahertz probe method is proven to be promising in further investigating spintronic materials, particularly on an ultrafast timescale.

Effect of 3D Printing Parameters on the Refractive Index, Attenuation Coefficient, and Birefringence of Plastics in Terahertz Range

The refractive indices, attenuation coefficients, and level of birefringence of various 3D printing plastics may change depending on the printing parameters. Transmission terahertz time‐domain spectroscopy was used to look for such effects in Copolyester (CPE), Nylon, Polycarbonate (PC), Polylactic acid, and Polypropylene. The thickness of each sample was measured using an external reference structure and time‐of‐flight measurements. The parameters varied were printer nozzle size, print layer height, and print orientation. Comparison of these parameters showed that a printer’s nozzle size and print layer height caused no change in real refractive index or attenuation coefficient. A change in printing orientation from vertical to horizontal caused an increase both in real refractive index and in attenuation coefficient. In vertically printed samples, the increase in birefringence was proportional to the increase in layer height and inversely proportional to nozzle size. There was no measurable intrinsic birefringence in the horizontally printed samples. These effects should be taken into account in the design of FDM 3D printed structures that demand tailored refractive indices and attenuation coefficients, while also providing a foundation for nondestructive evaluation of FDM 3D printed objects and structures.

Metal-insulator transition in (Pr,Y)0.7Ca0.3CoO3 in the far-infrared spectral region

Bulk ceramics and thick nanogranular films of mixed-valence cobaltite ${({\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{Y}}_{y})}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3}$ were studied using time-domain terahertz transmission spectroscopy and infrared reflection spectroscopy. Thanks to the high probing frequency, the overall increase in the conductivity during the insulator to metal transition was observed both in the ceramics and in the nanogranular films. The terahertz conductivity spectrum shows that the transition is not a simple cross-over between two states. As the charges become delocalized during the insulator to metal transition, a broad resonant mode corresponding to the eigenfrequency of the binding potential appears in the terahertz conductivity spectra in the ceramics, and it further softens as the metallic state prevails. This is interpreted as a consequence of the complexity of charge motion in a temperature-dependent energy landscape, changing dramatically from the tightly bound charges on atomic distance in the insulating phase to almost free charges in the metallic phase. This behavior is not directly observed in the nanogranular films as their nature introduces spectral signatures of an additional localization due to grain boundaries which obscure the pronounced effects observed in the bulk. Nevertheless fine spectral signatures accompanying the phase transition are still resolved. The insulator to metal transition is also accompanied by a weak softening of infrared-active phonon modes, which is related to the expansion of lattice parameters upon phase transition.

Physical Evidence of Stress-Induced Conformational Changes in Polymers

Polymer mechanics and characterization is an active area of research where a keen effort is directed towards gaining a predictive and correlative relationship between the applied loads and the specific conformational motions of the macromolecule chains. Therefore, the objective of this research is to introduce the preliminary results based on a novel technique to in situ probe the mechanical properties of polymers using non-invasive, non-destructive, and non-contact terahertz spectroscopy. A dielectric elastomer actuator (DEA) structure is used as the loading mechanism to avoid obscuring the beam path of transmission terahertz time-domain spectroscopy. In DEAs, the applied voltage results in mechanical stresses under the active electrode area with far-reaching stretching in the passive area. Finite element analysis is used to model and simulate the DEA to quantify the induced stresses at the observation site over a voltage range spanning from 0 V to 3000 V. Additionally, a novel analysis technique is introduced based on the Hilbert-Huang transform to exploit the time-domain signals of the ultrathin elastomeric film and to defy the limits set forth by the current state-of-the-art analysis techniques. The computational result shows a nonlinear relationship between the effective stresses and the applied voltage. Analysis of the terahertz time-domain signals shows a shift in the delay times and a decrease in signal peak amplitudes, whereas these characteristics are implicitly related to the change in the index of refraction. In all, the results evidentially signify the interrelationship between the conformational changes and applied mechanical stress.

Application of terahertz time-domain spectroscopy in atmospheric pressure plasma jet diagnosis

The measurement of plasma density is of great significance in the study of magnetically constrained nuclear fusion and the transmission of electromagnetic waves in plasma. In this paper, the terahertz time-domain spectroscopy (THz-TDS) system is used to diagnose the plasma density of the atmospheric pressure plasma jet. The transmittance properties of the atmospheric pressure plasma jet are measured by a fiber-coupled transmission THz-TDS system. Then the electron density of the plasma jet is calculated based on the measurement results according to the relationship with the complex refractive index. The plasma jets with different electron densities are also investigated by varying the discharge voltages and the ionization gases. The terahertz transmittance spectra in measurement and calculation agree well. This study is of great significance for the applications of THz-TDS in high-accuracy, non-contact plasma diagnosis.

Terahertz spectroscopy and imaging for gastric cancer diagnosis

Terahertz waves are sensitive to differences in biological tissue hydration, we present promising results regarding the feasibility of applying this with terahertz time-domain spectroscopy and imaging for early detection of cancer through the characterisation of human gastrointestinal tissue with cancer-affected regions. To do that, healthy (normal) and carcinoma-affected gastric tissue samples at different stages were measured using transmission terahertz time-domain spectroscopy in the frequency range of 0.15–2.00 THz. Absorption coefficients and refractive index spectra of both normal and carcinoma-affected tissue were extracted and analysed. The results confirm that the techniques may be powerful tools to perform qualitative, early diagnosis of human cancer.

Quality assessment of terahertz time-domain spectroscopy transmission and reflection modes for graphene conductivity mapping.

We present a comparative study of electrical measurements of graphene using terahertz time-domain spectroscopy in transmission and reflection mode, and compare the measured sheet conductivity values to electrical van der Pauw measurements made independently in three different laboratories. Overall median conductivity variations of up to 15% were observed between laboratories, which are attributed mainly to the well-known temperature and humidity dependence of non-encapsulated graphene devices. We conclude that terahertz time-domain spectroscopy performed in either reflection mode or transmission modes are indeed very accurate methods for mapping electrical conductivity of graphene, and that both methods are interchangeable within measurement uncertainties. The conductivity obtained via terahertz time-domain spectroscopy were consistently in agreement with electrical van der Pauw measurements, while offering the additional advantages associated with contactless mapping, such as high throughput, no lithography requirement, and with the spatial mapping directly revealing the presence of any inhomogeneities or isolating defects. The confirmation of the accuracy of reflection-mode removes the requirement of a specialized THz-transparent substrate to accurately measure the conductivity.

Label-free protein detection using terahertz time-domain spectroscopy.

Protein analysis is the foundation to understanding the mechanisms of complex biological processes. As one of the most widely used techniques to determine protein species and contents, protein dot blot aids biology research but needs corresponding antibodies for marking. A label-free detection method based on terahertz time-domain spectroscopy (THz-TDS) is proposed and demonstrated to improve this traditional technology. A membrane loaded with protein samples is directly scanned using a transmission THz-TDS system for spectral imaging. Different kinds of proteins can be distinguished by the refractive index extracted from the THz transmission spectrum. The intensity or shade imaged with the THz transmission spectrum can help detect the protein quantitatively. The feasibility of this new protein assay is demonstrated by the results of systematic testing with actual samples prepared with the dot-blot protocol.

Comparative Study of Material Parameter Extraction Using Terahertz Time-Domain Spectroscopy in Transmission and in Reflection

As no terahertz signal is transmitted through opaque materials, reflection terahertz time-domain spectroscopy is obviously the dedicated setup. On the other hand, optical properties of transparent and thick samples are extracted more accurately with transmission terahertz time-domain spectroscopy. In this paper, we report the intrinsic frontier above or below which it is preferable to use either reflection or transmission setup for probing with the best accuracy possible a given sample.

太赫兹时域光谱反射式层析成像技术

The modified polypropylene(PP) material has been widely used in dashboard, bumper and other auto parts because of its characteristics of flame retardant, high impact resistance, etc. It is a necessary means to improve the quality of auto parts by using the nondestructive testing technology to detect the modified PP material. A transmission terahertz time-domain spectroscopy(THz-TDS) imaging system and a reflective THz-TDS imaging system were built. The optical parameters of modified PP materials were extracted by adopting the transmission THz-TDS system. The refractive index, one of the optical parameters in terahertz band can be solved through the mathematics model, its value was 1.53. A kind of modified PP material sample with flat bottom holes was designed. The reflective THz-TDS imaging system was adopted for measuring the sample, the deconvolution method was used to filter the time domain THz signal, which largely improved the signal-to-noise ratio. What's more, THz-TDS tomography imaging technology was proposed based on the time of flight. Combined with the measured refractive index, the 3D structure of the modified PP material was reconstructed with the time of flight tomography imaging technique, the thickness accuracy is 0.01 mm.

Terahertz surface emission of d -band electrons from a layered tungsten disulfide crystal by the surface field

Terahertz (THz) time-domain emission spectroscopy in both transmission and reflection configurations has been employed to understand the THz radiation property and surface properties of tungsten disulfide ($\mathrm{W}{\mathrm{S}}_{2}$). We observed only one polarization of THz radiation under different polarization of pump beam and a saturation effect with the increasing of pump power. The results are different from that of $\mathrm{Mo}{\mathrm{S}}_{2}$ based on optical rectification in spite of similar physical and optoelectronic properties of them. The nonlinear optical coefficient calculation based on first-principle method combined with the azimuthal angle dependence of THz radiation implies that THz radiation is insensitive to the azimuthal angle in $\mathrm{W}{\mathrm{S}}_{2}$. From the pump polarization angle dependence of THz radiation, we find that the contribution due to the nonlinear effect is only 12% approximately. All these suggest the main THz mechanism from $\mathrm{W}{\mathrm{S}}_{2}$ is due to the surface depletion field induced by the surface states. We also analyzed the surface field features of $\mathrm{W}{\mathrm{S}}_{2}$ with the maximum surface depletion field of approximate $1.2\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{cm}$. Fresnel law combined with the dipole radiation model is also used to analyze the angular dependence of THz radiation. The results can not only afford a fundamental THz radiation property of layered materials, but also promote the development of THz devices based on layered materials.

Polarization-resolved terahertz third-harmonic generation in a single-crystal superconductor NbN: Dominance of the Higgs mode beyond the BCS approximation

Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in s-wave superconductors, where charge-density fluctuations (CDF) have also been shown to contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscopy for a single-crystal NbN film. The result shows that the resonant THG in the transmitted light always appears in the polarization parallel to that of the incident light with no appreciable crystal axis dependence. When we compare this with the theoretical calculation here with the BCS approximation and the dynamical mean-field theory for a model of NbN constructed from first principles, the experimental result strongly indicates that the Higgs mode rather than the CDF dominates the THG resonance in NbN. A possible mechanism for this is discussed such as the retardation effect in the phonon-mediated pairing interaction beyond BCS.

Calculation and Study of Graphene Conductivity Based on Terahertz Spectroscopy

Based on terahertz time-domain spectroscopy system and two-dimensional scanning control system, terahertz transmission and reflection intensity mapping images on a graphene film are obtained, respectively. Then, graphene conductivity mapping images in the frequency range 0.5 to 2.5 THz are acquired according to the calculation formula. The conductivity of graphene at some typical regions is fitted by Drude-Smith formula to quantitatively compare the transmission and reflection measurements. The results show that terahertz reflection spectroscopy has a higher signal-to-noise ratio with less interference of impurities on the back of substrates. The effect of a red laser excitation on the graphene conductivity by terahertz time-domain transmission spectroscopy is also studied. The results show that the graphene conductivity in the excitation region is enhanced while that in the adjacent area is weakened which indicates carriers transport in graphene under laser excitation. This paper can make great contribution to the study on graphene electrical and optical properties in the terahertz regime and help design graphene terahertz devices.