Terahertz Reflection Spectra Research Articles

Classification of non-referenced continuous-wave terahertz reflection spectra for remote material identification

Commonly, terahertz spectra (both continuous-wave and pulsed) are deconvoluted by reference spectra to remove the water vapor absorption lines and other system related responses. However, in real-life applications obtaining reference spectra can be problematic and adds to the complexity of the system. Thus, a reference-free method for classification of terahertz spectra could be a welcomed advance for remote sensing applications. In this paper, we study how simple machine learning algorithms perform as a reference-free method for terahertz stand-off identification of materials. The algorithms are trained using spectra measured under controlled humidity conditions and tested by a completely independent data set measured under ambient conditions. We apply three different classification algorithms; namely a Gaussian Bayes model, the k nearest neighbors, and a support vector machine. We found that, if the terahertz spectra are processed using a supervised algorithm (Regularized Linear Discriminant Analysis), very high classification scores (¿98.6%) can be retained for the non-referenced spectra. Moreover, the high accuracy is obtained meanwhile the dimensionality is reduced by a factor larger than 160, which further reduces the computational requirements. Hence, we have demonstrated that simple supervised machine learning algorithms can serve as a highly accurate reference-free method for THz material identification. This could be of great importance for real-world remote sensing applications based on terahertz spectroscopy.

Light-induced coherent interlayer transport in stripe-ordered La1.6−xNd0.4SrxCuO4

We have investigated the photoexcited transient responses of stripe-ordered phase in a cuprate superconductor, ${\rm La}_{1.6-x}{\rm Nd}_{0.4}{\rm Sr}_{x}{\rm CuO}_{4}~(x = 0.12)$ using optical-pump terahertz (THz)-probe spectroscopy. Upon the near-infrared photoexcitation with the electric field polarized along the $c$-axis, a clear plasma edge appears in the THz reflection spectrum along the $c$-axis with its position nearly coinciding with the Josephson plasma resonance of similarly doped ${\rm La}_{2-x}{\rm Sr}_{x}{\rm CuO}_{4}~(x = 0.125)$ in the low-temperature superconducting phase. The appearance of light-induced plasma edge sustains up to the onset temperature of the charge-stripe order, indicating the inherent interplay between the light-induced phase and the charge-stripe order. The optical conductivity spectrum of the light-induced state is mostly reproduced by the Drude model with a scattering rate as small as a few meV, and its imaginary part does not exhibit $1/{\omega}$-divergence behavior in any temporal region after the photoexcitation. We discuss the possible origin of the observed coherent interlayer transport behavior as manifested by the narrow Drude response in the THz reflectivity along the $c$-axis.

Aging Analysis of Cable Insulation Material by Terahertz Time-domain Spectroscopy

Insulation materials with high sealing, high insulation, high elasticity, and other characteristics are widely used in the power industry. In this paper, the terahertz time-domain reflection spectrum of cable insulation materials is studied, the optical parameters of cable insulation materials are extracted to characterize the aging state of cable insulation materials, the terahertz time-domain reflection signal of insulation materials with different degrees of aging is tested, and the terahertz reflection spectrum of insulation sheaths is analyzed. The relationship between the aging degree and absorption coefficient in terahertz time-domain spectral reflection mode is obtained. There are obvious differences in the absorption characteristics of cable insulation materials with different degrees of aging in the terahertz band. The longer the aging time is, the greater the absorption coefficient is and the lower the refractive index is. Terahertz time-domain reflectometry plays a role in cable protection with high detection accuracy in the characterization of cable insulation materials.

Study of terahertz reflection spectra of optically thin RDX samples by terahertz imaging with spectral resolution

The THz reflection spectra of optically thin hexogen (RDX) samples were studied by terahertz imaging with spectral resolution. A photoconductive antenna excited by femtosecond laser radiation was used as a source of broadband THz radiation. The presence of a Fourier spectrometer (as well as band-pass THz filters) and a microbolometric THz video camera in the experimental setup made it possible to use the terahertz imaging method to study reflection spectra taking into account scattering in the range of 0.6 to 1 THz. The influence of the optical characteristics of the substrate on the THz reflection spectra was studied. In particular, the conditions for observing the effect of anomalous dispersion for RDX samples with different dispersion in the frequency region of the RDX absorption band 0.8 THz were studied. The obtained results demonstrate the application of the method of terahertz imaging with spectral resolution based on THz video camera for the identification of explosives with concentrations 0.75 to 50 mg / cm2 on various surfaces.

Performance simulation of terahertz waveguide resonance biochemical sensor based on nanoporous gold films

A highly sensitive terahertz (THz) waveguide resonance biochemical sensor is designed and simulated. The sensor consists of a silicon prism, a dielectric layer and a nanoporous gold film. The nanoporous gold film acts as both a THz waveguiding layer and a biochemical molecular enrichment layer, which can enhance the interaction between the THz waveguide mode and the adsorbed biochemical molecules, consequently improving the sensor’s sensitivity. When the THz transverse electric (TE) or transverse magnetic (TM) modes are excited by the prism-coupling method, the THz absorption of the nanoporous gold film results in the sharp resonance dips in the THz reflection spectrum. The resonance frequencies of the THz waveguide modes and the sensitivity to either liquid refractive index (RI) or adsorbed molecules can be determined with the measured reflection spectra, and the sensor’s sensitivity and its figure of merit (FOM) can be improved by adjusting the thickness and RI of the dielectric layer. The simulation results at 45º incidence angle indicate that the resonance frequencies of the TE and TM modes of the sensor linearly change with increasing either liquid RI or the amount of adsorbed molecules, and the RI sensitivity and the FOM with the TM mode are 13.42 THz/RIU and 167.70/RIU, respectively. Compared with the TM mode, the TE mode has a lower sensitivity to lqiuid RI but a high sensitivity to adsorbed molecules. The reason for these differences is that with the TE mode the evanescent field penentrating out of the nanopous gold film is weaker than that with the TM mode.

Terahertz imaging with metamaterials for biological applications

Terahertz (THz) technology has become more widespread due to its diverse range of potential applications, particularly when combined with various functional metamaterials using cutting-edge nanotechnology techniques. In this report, we introduce a highly improved THz imaging technology by comparing complementary metamaterials intuitively based on Babinet’s principle. The THz reflectance spectra for the complementary metamaterials exhibit a significant and distinct association with the polarization angle. Four different polarization angles and metamaterial pattern geometries were tested for the reflectance imaging of a target image pattern. Field enhancement on the metamaterial surface was also investigated using finite element simulations to support the experimental results. Optimizing the metamaterial based on the experimental and calculation results led to high image contrast and quality. The proposed label-free imaging platform was then employed to produce clear contrast images for mouse brain tissue and HEK-293 cells, thus highlighting the potential application of this system to real biological samples.

Phase Function Effects on Identification of Terahertz Spectral Signatures Using the Discrete Wavelet Transform

We describe the application of the Discrete Wavelet Transform (DWT) in extracting the characteristic absorption signatures of materials from terahertz reflection spectra. We compare the performance of different mother wavelets, including Daubechies, Least Asymmetric (LA), and Coiflet, based on their phase and gain functions and filter lengths. We show that the phase functions of the wavelet and scaling filters result in spectral shifts to the absorption lines in the wavelet domain. We provide a solution by calculating advancement coefficients necessary to achieve effective zero-phase-function DWT. We demonstrate the utility of this signal processing technique using α-lactose monohydrate/polyethylene samples with different levels of rough surface scattering. In all cases, the DWT-based algorithm successfully extracts resonant signatures at 0.53 and 1.38 THz, even when they are obscured by the rough surface scattering effects. The DWT analysis with accompanying phase corrections can be utilized as a robust technique for material identification in non-destructive evaluation (NDE) using terahertz spectroscopy.

Tunable terahertz reflection spectrum based on zinc-blende structure crystals excited by ultrasonic

The tunable terahertz reflection spectrum of zinc-blende structure crystals (ZnSe, CuCl and InP), formed in the first bandgap, excited by ultrasonic is investigated in the present paper. Numerical simulation results show that, under the pressure of 0 GPa–100 GPa, the volume change of the crystal indicates that the zinc-blende structure crystals are suitable acousto-optic crystal. According to the optical phonon resonance dispersion, the frequency selective bandwidth of ZnSe, CuCl and InP crystal are 22.5 meV, 21.4 meV and 32.4 meV, respectively, indicating that the frequency selection range of InP crystal is wide. The transverse optical polariton is dis-propagating in the first bandgap and reflects. With the increase of ultrasonic frequency, the reflection peaks moves towards the restrahlen band, and the amplitude of the reflection spectrum decreases until it disappears. More transverse optical phonon combinations make the width of the reflection peak wider. The tunable terahertz reflection spectrum can be achieved by adjusting the ultrasonic frequency, a satisfactory condition for tunable selection and filtering in terahertz range.

Tunable terahertz reflection spectrum based on band gaps of GaP materials excited by ultrasonic

Tunable terahertz (THz) reflection spectrum, ranged from 0.2 to 8 THz, in band gaps of gallium phosphide (GaP) materials excited by ultrasonic is investigated in the present paper, in which tunable ultrasonic and terahertz wave collinear transmission in the same direction is postulated. Numerical simulation results show that, under the acousto-optic interaction, band gaps of transverse optical phonon polariton dispersion curves are turned on, this leads to a dis-propagation of polariton in GaP bulk. On the other side, GaP material has less absorption to THz wave according to experimental studies, as indicates that THz wave could be reflected by the band gaps spontaneously. The band gaps width and acousto-optic coupling strength are proportional with ultrasonic frequency and its intensity in ultrasonic frequency range of 0–250 MHz, in which low-frequency branch of transverse optical phonon polariton dispersion curves demonstrate periodicity and folding as well as. With the increase of ultrasonic frequency, frequency of band gap is blue-shifted, and total reflectivity decreased with −1-order and −2-order reflectivity decrease. The band gaps converge to the restrahlen band infinitely with frequency of ultrasonic exceeding over 250 MHz, total reflectivity of which is attenuated. As is show above, reflection of THz wave can be accommodated by regulating the frequency and its intensity of ultrasonic frequency. Relevant technology may be available in tunable THz frequency selection and filtering.

Terahertz Imaging and Electromagnetic Model of Axon Demyelination in Alzheimer's Disease

We investigate the utility of terahertz (THz) spectroscopy in identifying Alzheimer's disease in human brain tissue. Using reflection-mode time-domain THz spectroscopy, two-dimensional images of formalin-fixed and paraffin-embedded tissue samples of the hippocampus area were recorded in the 60 GHz-2 THz band. The THz images were compared with microscopic images of the same samples after hematoxylin and eosin staining. We demonstrate that the THz reflection spectra, particularly from white matter, reveal detectable differences between postmortem brain tissues exhibiting Alzheimer's disease and normal controls. The THz reflectivity of white matter areas was on average 4.2% higher than that of gray matter regions in tissue samples with known Alzheimer's history, whereas for normal control samples the contrast was only 2%. Additional studies further enhance this hypothesis, and the demyelination of white matter in Alzheimer's tissues suggests a possible cause for the differences in the THz reflection spectra. We also compare the THz response of the tissues with conventional Luxol-fast blue staining to demonstrate the correlation between the two methods for predicting demyelination. In addition, we present a simplified electromagnetic model of white matter axons exhibiting various degrees of demyelination to further support this hypothesis through full-wave electromagnetic simulations. This study offers, for the first time, proof of concept for the feasibility of detecting Alzheimer's disease using THz spectroscopy on ex vivo samples.

Determination of the complex refractivity of Au, Cu and Al in terahertz and far-infrared regions from reflection spectra measurements

A scheme to determine the complex refractivity of gold (Au), Copper (Cu) and Aluminum (Al) from measurements of ellipsometer and spectrometer are proposed in this paper. The reflection spectra of the metals from 4THz to 40THz are measured with spectrometer. The determined refractivity by Kramers-Kronig (KK) algorithm coincides with the measured results from ellipsometer in far infrared region. Drude model is invited to make the wing correction on the terahertz reflection spectra, which helps to eliminate the effects of the noises from spectrometer on KK algorithm. The calculated refractive indexes from measured spectra in terahertz region are in consistent with those from corrected reflection spectra. The advantage of the scheme is to obtain terahertz dispersion properties based on limited information in infrared region.

Extracting Complex Refractive Index from Polycrystalline Glucose with Self-Referenced Method for Terahertz Time-Domain Reflection Spectroscopy

A self-referenced method for extracting the complex refractive index of material was proposed. The method utilized signals reflected from the front and rear surfaces of a slice sample as reference and sample signals, respectively. Before using the self-referenced method, a hybrid filtering technique for eliminating systematic and random noises of time-domain terahertz reflection spectroscopy was used. A terahertz reflection spectrum of crystalline glucose was measured and three feature absorption peaks were obtained from 0.2 to 2.0 THz. We suggest that intermolecular vibrational modes may contribute to the observed absorption spectra in the THz frequency range.

Observation of Disk Scattering Slab Modes Transitioning to Disk Scattering Modes on Metal–Dielectric Heterostructures in the Terahertz Waveband

It has been demonstrated that, in the terahertz (THz) waveband, disk scattering slab modes (DSSMs) can exist in a heterostructure composed of a metallic subwavelength disk array coated on one surface of a dielectric slab. By observing the THz reflection spectrum, it is found that DSSMs can exist when the slab is sufficiently thick relative to the THz pulse wavelength. These thick slab modes, namely, disk scattering modes (DSMs), can be seen as degenerated DSSMs. Based on experimental measurements and theoretical analysis via a dipole-scattering model developed here, it is shown that the features of DSMs can be tuned by changing the geometric parameters of the disk array. These results are important for the design of new THz devices, such as THz filters.

Thermally stimulated 3–15 THz emission at plasmon-phonon frequencies in polar semiconductors

The possibilities of distinguishing highly coherent terahertz emission at a specified frequency from the incoherent thermal emission of a hot body are considered. It is experimentally shown that the smooth planar surface (with no diffraction guides) of heated GaAs and AlGaAs wafers emits directed continuous-wave (cw) terahertz radiation at coupled surface plasmon-phonon vibrational frequencies. The recording of terahertz reflectance spectra is demonstrated as a method for the identification of plasmons, optical phonons, and coupled plasmon-phonon vibrations in semiconductors.

Temperature dependence of terahertz optical characteristics and carrier transport dynamics in p-type transparent conductive CuCr1−xMgxO2 semiconductor films

Terahertz reflectance spectra and electrical transport of CuCr1−xMgxO2 films have been studied in temperature range of 150–300 K. With increasing temperature, the phonon mode near 15 THz shows a redshift trend, and free carrier absorption below 6 THz becomes more prominent. Moreover, hole effective mass increases linearly from 0.028 to 0.48 m0 with the temperature and composition. Hall coefficient shows a turning-point at about 220, 206, and 194 K for the composition of x = 0.02, 0.06, and 0.10, respectively. The phenomena can be attributed to the transition of carrier transport mechanism from a thermal activation behavior to a variable range-hopping one.

Non-destructive Inspection of Copper Corrosion via Coherent Terahertz Light Source

In order to investigate the infrared activities of corrosion products of copper in terahertz (THz) frequency band, THz absorption spectra of copper (I) oxide (Cu2O) and copper (II) oxide (CuO) powders are measured. In view of the practical application for non-destructive inspection of copper wire deterioration, THz reflectance spectra of copper conductive wires with and without oxidization were also measured. THz spectroscopy results have shown some clear specific absorption bands in 1-5THz region, and the THz reflection spectra have revealed clear reflection intensity difference at around 1.7THz band between corroded and not-corroded copper wires. It is also shown that the surrounding polyethylene (PE) of copper electric wire is very transparent for THz wave, thus non-destructive inspection of the corrosion status of copper wire surrounded by the invisible PE insulator can be possible by using THz wave.

Characterization of roughness parameters of metallic surfaces using terahertz reflection spectra

This paper reports on the effect of random Gaussian roughness with rms roughness values of 5-20 microm on the terahertz reflection spectra of metallic aluminum surfaces using Fourier transform IR spectroscopy. By comparing experimental data with a theoretical model based on the Kirchhoff approximation, the rms roughness of a surface is accurately determined. The rms roughness determined by this method is in good agreement with the rms roughness measured using a stylus surface profilometer. In addition, we demonstrate that this method can be used to clearly resolve rough surfaces that differ in rms roughness by approximately 1 microm.

Surface roughness effects on the terahertz reflectance of pure explosive materials

We studied the effect of surface roughness on terahertz reflectance spectra of explosives at both quasinormal and oblique incidence by Fourier-transform spectroscopy. The optical constants of 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane and 1,3,5 trinitro-1,3,5 triazacyclohexane were determined from the reflectance spectra of pure optically flat pellets and clear spectral signatures were found in the 0.5–8THz range. In the case of a realistic surface roughness of several tens of microns, the observation of the spectral signatures at quasinormal incidence is hindered by the specular reflectance roll-off at terahertz frequencies. On increasing the angle of incidence, the roll-off occurred at higher frequency and the surface roughness was optically measured. Oblique incidence geometries can benefit the development of terahertz standoff detection devices.

Far-infrared signature of the superconducting gap in intercalated graphiteCaC6

Terahertz reflectance spectra of the Ca-intercalated graphite ${\text{CaC}}_{6}$ reveal a superconducting gap below 11 K. The gap signature lacks a sharp onset to full reflectivity at $2{\ensuremath{\Delta}}_{0}$ but rather shows a distribution of gap values consistent with an anisotropic gap. The experimental data were successfully fitted to the gap distribution obtained from density-functional calculations of Sanna et al. [Phys. Rev. B 75, 020511(R) (2007)]. The temperature dependence of the superconducting gap is characteristic for a BCS type superconductor.

Comparison of subtractive Kramers-Kronig analysis and maximum entropy model in resolving phase from finite spectral range reflectance data

The maximum entropy model (MEM) and Kramers-Kronig (K-K) analysis were compared with the aim of phase retrieval from reflectance. The object was to test two different phase-retrieval methods when reflectance is known at a finite frequency range and data fitting is not performed beyond the finite frequency band. In addition, it was assumed that the phase is known only at one or two anchor points. As an example, we study the terahertz reflection spectrum related to a semiconductor and an optical spectrum of potassium chloride. It is shown that the MEM resolves the complex refractive index of a medium, in the vicinity of initial and final points of the spectra, better than singly and doubly subtractive K-K relations. Both methods give only satisfactory results in the event of one anchor point, but in the case of two anchor points, the MEM is better than doubly subtractive K-K. It is proposed that the MEM should be used instead of K-K analysis, for a priori information of phase at two anchor points, for the purpose of resolving the complex refractive index of a medium from reflectance with high accuracy.