Terahertz Time-domain Reflection Research Articles

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.

Towards simultaneous determination of tablet porosity and height by terahertz time-domain reflection spectroscopy

The quality control of pharmaceutical tablets is still based on testing small sample numbers using at- and off-line testing methods. Traditional in-process controls, such as tablet mass, height, mechanical strength, and disintegration time are time- and resource-consuming and poorly suited to support an effective transition towards continuous manufacturing. Another suitable parameter to monitor during production would be tablet porosity. Porosity can be linked to mechanical strength and disintegration but typically requires knowledge of tablet dimensions and mass. Tablet porosity measurements based on terahertz time-domain spectroscopy (THz-TDS) offer a fast and non-destructive approach to in-process control testing for physical tablet properties. This study presents THz-TDS reflection measurements as an alternative to the previously reported transmission setup. It is shown that the proposed method can determine porosity based on the reflected amplitude from the tablet surface, but also allows for precise determination of tablet height in the same measurement. The tablet mass can be estimated by combining the height and porosity measurements. This opens up for the opportunity to determine the tablet's mechanical strength by using the possible correlation to the determined porosity.

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.

Self-Referenced Method for Geometrical Distortion Removal in THz Time-Domain Reflection Imaging

In this article, we develop a method for removing the phase drift induced by physically distorted object in terahertz time-domain reflection imaging (THz-TDRI). The proposed approach is defined as self-referenced, as it does not rely on any numerical parameter optimization nor extra instrumental components, and it is based on the unique manipulation of time-domain imaging data. In fact, we demonstrate that the problem can be solved assuming a linear contribution of the temporal shift induced by surface curvature. We illustrate how the self-referenced method is modeled and implemented, and we report the results obtained on two objects with different characteristics: a tilted and highly reflective surface, and a warped and heterogeneous surface. The proposed method demonstrates how to successfully remove the phase alterations induced on the reflected electric field, and how to repair the heavily corrupted images in the frequency-domain.

Characterizing the Thermally Grown Oxide in Thermal Barrier Coating by Terahertz Time Domain Spectroscopy

This paper presents a method to simultaneously measure the thickness and refractive index of the thermally grown oxide (TGO) in thermal barrier coating (TBC) by using a reflective terahertz time-domain spectroscopy (THz-TDS) system. First, an optical transmission model of THz radiation in the multilayer structure of TBC is established. Owing to the different structures of TBC before and after forming the TGO layer, two different transmission models are established, respectively. Then, the experimental signals from the samples after different thermal cycles are obtained by the THz-TDS system. By fitting the experimentally measured reflected THz signals from the TBC samples to the proposed optical model using an optimization algorithm, the thickness and refractive index of the TGO are determined. In this work, four samples with different thicknesses of TGO layers are analyzed. The results of thickness of TGO layer are verified by SEM observation, and a reasonable agreement is obtained.

Mechanism of THz dielectric constant enhancement in multi-component oxide glasses investigated by infrared and THz spectroscopies

Dielectric properties of multi-component silicate oxide glasses are investigated with a focus on high dielectric constant oxyfluorosilicate (OFS) glasses by using the infrared reflection spectroscopy and terahertz (THz) time-domain spectroscopy. On the basis of multiple Lorentz oscillator model, vibrational parameters and most responsible ionic pairs are identified for major modes in OFS glasses. The lowest frequency mode is found to dominate the total THz dielectric strength. Among various glasses, the high frequency (optical) dielectric constant shows a superlinear dependence on the electronic polarizability of oxygens. In contrast, the low frequency (THz) mode contribution to the dielectric constant is enhanced by the ionicity (expressed by the polarization ionicity parameter) and exhibits an even steeper dependence on the electronic polarizability. This feature well explains the mechanism of attaining the highest THz dielectric constant in an OFS glass (e.g. ZNbKLSNd glass). Also, the oxygen ion effective charges bearing the lowest frequency modes of OFS glasses are evaluated and found to behave consistently with the polarization ionicity parameter, confirming the relevance of both of these parameters as good ionicity indicators.

Degree of inversion of A/B lattice sites and microwave/millimeter wave/terahertz dielectric properties of MgAl2-x(Zn0.5Mn0.5)xO4 ceramics

In this work, spinel-structured MgAl2-x(Zn0.5Mn0.5)xO4 (0 ≤ x ≤ 0.08) single-phase ceramics were prepared through a solid-state reaction route. The substitution of (Zn0.5Mn0.5)3+ for Al3+ at the octahedral site affected the degree of inversion of A/B lattice sites, bond length/strength/valence, and covalency of metal-oxygen bond in the tetrahedron and hence microwave dielectric properties of MgAl2O4. The variation in εr and tanδ of ceramics is investigated in the millimeter wave-terahertz frequency band by combining infrared reflection spectrum and terahertz time-domain spectroscopy. A high Q×f value of 111,010 GHz @ 12.01 GHz, low εr = 8.3, and slightly lower τf = −60 ppm/°C is obtained for MgAl1.98Zn0.01Mn0.01O4 ceramics, which is tuned by adding a small amount of SrTiO3. The composite ceramics exhibited a near-zero τf (2.8 ppm/°C), high Q×f (55,400 GHz @ 11.15 GHz), and low εr (= 8.5), showing a great potential application prospect for 5G/6G wireless communication.

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.

Dielectric property measurement of human sweat using attenuated total reflection terahertz time domain spectroscopy.

Sweat is one of the essential biofluids produced by the human body, and it contains various physiological biomarkers. These biomarkers can indicate human health conditions such as disease and illness. In particular, imbalances in the concentration of electrolytes can indicate the onset of disease. These same imbalances affect the dielectric properties of sweat. In this study, we used attenuated total reflection terahertz time domain spectroscopy to obtain the frequency-dependent dielectric properties of human sweat in a frequency range from 200 GHz to 2.5 THz. We have investigated the variation of dielectric properties of sweat collected from different regions of the human body, and we have observed that the real and imaginary part of dielectric permittivity decreases with the increase in frequency. A combination of left-hand Jonscher and Havriliak-Negami processes is used to model the results and reveal the presence of relaxation processes related to sodium and calcium ions concentrations. This information may help design novel biosensors to understand the human health condition and provide a hydration assessment.

Non-Destructive Identification of Drugs in Plastic Packaging Using Attenuated Total Reflection Terahertz Time Domain Spectroscopy

In this study, we demonstrate that drugs in plastic packaging can be identified without being opened using attenuated total reflection terahertz time domain spectroscopy. In this system, the terahertz wave undergoes total internal reflection at the interface between prism and sample, producing an evanescent wave at the interface. The penetration depth of the evanescent waves is larger than the thickness of typical plastic packaging in the sub-terahertz frequency region; therefore, it becomes possible to detect the sample without opening the package. Here, we show that some saccharides samples such as lactose in plastic packaging can be identified using its spectral fingerprint by placing the packaged lactose on the prism. This method has the potential to be used in the non-destructive testing and analysis of a wide variety of samples, such as medicine sachets, to reduce medication dispensing errors in pharmacies.

Terahertz circular polarization sensing for protein denaturation based on a twisted dual-layer metasurface.

Protein denaturation has very important research value in nutrition, biomedicine, and the food industry, which is caused by the changes in the molecular structure of the protein. Since the collective vibrational and torsional modes of protein molecules are within the terahertz (THz) frequency range, THz spectroscopy can characterize the protein denaturation with several advantages of non-contact, label-free, real-time, and non-destructive. Therefore, we proposed a reflective THz time-domain polarization spectroscopy sensing method, and use a flexible twisted dual-layer metasurface film as a sensor to realize the thermal denaturation sensing, concentration sensing, and types identification of protein aqueous solutions. The experiment tested three proteins (bovine serum albumin, whey protein, and ovalbumin), and the results show that: for the thermal denaturation sensing, its detection sensitivity can reach 6.30 dB/% and the detection accuracy is 0.77%; for the concentration sensing, the detection sensitivity and detection accuracy reach 52.9 dB·mL/g and 3.6·10-5 g/mL, respectively; in addition, different protein types can be distinguished by the difference of the circular polarization spectra.

Reflection terahertz time-domain spectroscopy for imaging and identifying concealed interfaces in insulated systems.

Polarization-resolved reflection spectroscopy enabled by a custom ultrahigh molecular weight polymer dove prism is used to identify spectral characteristics that manifest in the time domain at terahertz frequencies, which shows promise in combination with terahertz time-domain reference-free reflection imaging at a concealed interface. The method is used to produce 100×100 pixel images of an interface concealed by an ultrahigh molecular weight polymer using TE and TM polarized terahertz fields. The construction of material-specific image filters is guided by a theoretical reflection model by identifying reflection pulse characteristics unique to an interface. Application of these filters to the collected terahertz images distinguishes materials at the interface including the concealing plastic, water, air, glass, and metal that are correctly identified and imaged.

Terahertz circular dichroism sensing of living cancer cells based on microstructure sensor

Terahertz (THz) waves have the advantages of being noninvasive and nonionizing because of their low radiation energy, so they have potential applications in the biomedical field, but thus far, those have been limited by the strong absorption in water and low detection sensitivity. Herein, we propose a reflective THz time-domain circular dichroism (CD) sensing system and a silicon subwavelength grating as the microstructure sensor to generate and detect the THz chiral polarization states, to realize quantitative detection of living cell numbers and qualitative identification of cell kinds in a liquid environment. Three kinds of hepatoma cell proliferation and inhibition with different concentrations of aspirin were measured by this sensing method, and the experimental results show that the sensitivities for CD resonance intensity and frequency shift can reach 3.44 dB mL/106 cells and 5.88 GHz mL/106 cells, respectively, and the minimum detection concentration is in the order of 104 cells/mL for THz detection in a liquid environment for the first time. This new THz sensing system and sensing method are expected to become a broadband, label-free, noncontact, real-time detection technology that can be used for quantitative detection and qualitative identification of cells or other active biochemical materials.

Fiber Coupled Transceiver with 6.5 THz Bandwidth for Terahertz Time-Domain Spectroscopy in Reflection Geometry.

We present a fiber coupled transceiver head for terahertz (THz) time-domain reflection measurements. The monolithically integrated transceiver chip is based on iron (Fe) doped In0.53Ga0.47As (InGaAs:Fe) grown by molecular beam epitaxy. Due to its ultrashort electron lifetime and high mobility, InGaAs:Fe is very well suited as both THz emitter and receiver. A record THz bandwidth of 6.5 THz and a peak dynamic range of up to 75 dB are achieved. In addition, we present THz imaging in reflection geometry with a spatial resolution as good as 130 µm. Hence, this THz transceiver is a promising device for industrial THz sensing applications.

Through-substrate terahertz time-domain reflection spectroscopy for environmental graphene conductivity mapping

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapor deposited graphene films through silicon support. We validate the technique against measurements performed using the established transmission based THz-TDS. Our through-substrate approach allows unhindered access to the graphene top surface and thus, as we discuss, opens up pathways to perform in situ and in-operando THz-TDS using environmental cells.

A Single-Resonant-Structure and Optically Transparent Broadband THz Metamaterial Absorber

Broadband THz metamaterial absorbers have important applications in terahertz modulation, imaging, and communication, but current designs usually involve complex nesting or stacking resonant structures. This paper reports a single-resonant-structure and optically transparent broadband terahertz metamaterial absorber. The absorber demonstrates > 80% broadband absorption in a wide frequency range of 0.4~1.04 THz, or 88.9% of the central frequency. The thickness of the absorber is only 1/15 of the wavelength corresponding to the lowest absorption frequency. The absorption spectrum has been experimentally verified with a reflective terahertz time-domain spectrometer. Further study shows that the absorber is insensitive to the polarization angle, and the absorbance decreases very little for incident angles less than 30°. The absorption spectrum can be finely tuned by varying the structure parameters.

Utilizing multilayer structures to enhance terahertz characterization of thin films ranging from aqueous solutions to histology slides.

We propose a multilayer geometry to characterize thin-film samples in reflection terahertz time domain spectroscopy. Theory indicates that this geometry has higher sensitivity compared to ordinary transmission or reflection geometries when characterizing both low- and high-absorption samples. Pure water and water-ethanol mixtures are measured to verify the characterization accuracy of the proposed geometry and its capability to measure trace liquids. Paraffin-embedded oral cancer tissue is imaged to further show how the proposed geometry enhances the sensitivity for solid low-absorptive films.

Inspecting human colon adenocarcinoma cell lines by using terahertz time-domain reflection spectroscopy.

Techniques to inspect and analyze human colorectal cancer cell lines by using terahertz time-domain attenuated total reflection spectroscopy (THz TD-ATR) were investigated. The characteristics of THz absorption spectra of two colorectal cancer cell lines DLD-1 and HT-29 in aqueous solutions with different concentrations were studied. Different spectral features were observed compared to normal cell line. Identification results based on different parameters including absorption coefficient, refractive index, real and imaginary parts of complex permittivity, dielectric loss tangent were discussed. This research may be promising for quick and instant inspection of liquid samples by using THz time-domain spectroscopy in medical applications.

Method to tune a high-order resonance of metamaterials for broadband

In this paper, we present a method to tune high-order resonances of sectional asymmetric metamaterial structures. By drawing the positions of the resonances nearer to each other, we can broaden the bandwidth to about 7 times that of the traditional mono-ring metamaterial absorber. Furthermore, our method makes a simple modification to the structure without increasing fabrication difficulty. For verification, we have designed and fabricated a sectional asymmetric structure and set up a fiber coupled reflective terahertz time-domain spectroscopy system to measure its spectra. The measured results are in good agreement with the simulation results.

Graphene based terahertz phase modulators

Electrical control of amplitude and phase of terahertz radiation (THz) is the key technological challenge for high resolution and noninvasive THz imaging. The lack of active materials and devices hinders the realization of these imaging systems. Here, we demonstrate an efficient terahertz phase and amplitude modulation using electrically tunable graphene devices. Our device structure consists of electrolyte-gated graphene placed at quarter wavelength distance from a reflecting metallic surface. In this geometry, graphene operates as a tunable impedance surface which yields electrically controlled reflection phase. Terahertz time domain reflection spectroscopy reveals the voltage controlled phase modulation of π and the reflection modulation of 50 dB. To show the promises of our approach, we demonstrate a multipixel phase modulator array which operates as a gradient impedance surface.