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11 pages, 2828 KiB

Open AccessArticle

Measuring Open Porosity of Porous Materials Using THz-TDS and an Index-Matching Medium

by Mira Naftaly, Iliya Tikhomirov, Peter Hou and Daniel Markl

Sensors 2020, 20(11), 3120; https://doi.org/10.3390/s20113120 - 31 May 2020

Cited by 23 | Viewed by 6102

Abstract

The porosity of porous materials is a critical quality attribute of many products ranging from catalysis and separation technologies to porous paper and pharmaceutical tablets. The open porosity in particular, which reflects the pore space accessible from the surface, is crucial for applications [...] Read more.

The porosity of porous materials is a critical quality attribute of many products ranging from catalysis and separation technologies to porous paper and pharmaceutical tablets. The open porosity in particular, which reflects the pore space accessible from the surface, is crucial for applications where a fluid needs to access the pores in order to fulfil the functionality of the product. This study presents a methodology that uses terahertz time-domain spectroscopy (THz-TDS) coupled with an index-matching medium to measure the open porosity and analyze scattering losses of powder compacts. The open porosity can be evaluated without the knowledge of the refractive index of the fully dense material. This method is demonstrated for pellets compressed of pharmaceutical-grade lactose powder. Powder was compressed at four different pressures and measured by THz-TDS before and after they were soaked in an index-matching medium, i.e., paraffin. Determining the change in refractive index of the dry and soaked samples enabled the calculation of the open porosity. The results reveal that the open porosity is consistently lower than the total porosity and it decreases with increasing compression pressure. The scattering losses reduce significantly for the soaked samples and the scattering centers (particle and/or pore sizes) are of the order of or somewhat smaller than the terahertz wavelength. This new method facilitates the development of a better understanding of the links between material properties (particles size), pellet properties (open porosity) and performance-related properties, e.g., disintegration and dissolution performance of pharmaceutical tablets. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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12 pages, 3828 KiB

Open AccessArticle

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

by Robert B. Kohlhaas, Lars Liebermeister, Steffen Breuer, Marcel Amberg, David de Felipe, Simon Nellen, Martin Schell and Björn Globisch

Sensors 2020, 20(9), 2616; https://doi.org/10.3390/s20092616 - 4 May 2020

Cited by 11 | Viewed by 3507

Abstract

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 In_0.53Ga_0.47As (InGaAs:Fe) grown by molecular beam epitaxy. Due to its ultrashort electron lifetime and high [...] Read more.

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 In_0.53Ga_0.47As (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. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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36 pages, 11339 KiB

Open AccessArticle

Spurious Absorption Frequency Appearance Due to Frequency Conversion Processes in Pulsed THz TDS Problems

by Vyacheslav A. Trofimov, Nan-Nan Wang, Jing-Hui Qiu and Svetlana A. Varentsova

Sensors 2020, 20(7), 1859; https://doi.org/10.3390/s20071859 - 27 Mar 2020

Cited by 2 | Viewed by 2904

Abstract

The appearance of the spurious absorption frequencies caused by the frequency conversion process at the broadband THz pulse propagation in a medium is theoretically and experimentally discussed. The spurious absorption frequencies appear due to both the frequency doubling and generation of waves with [...] Read more.

The appearance of the spurious absorption frequencies caused by the frequency conversion process at the broadband THz pulse propagation in a medium is theoretically and experimentally discussed. The spurious absorption frequencies appear due to both the frequency doubling and generation of waves with sum or difference frequency. Such generation might occur because of the nonlinear response of a medium or its non-instantaneous response. This phenomenon is confirmed by the results of a few physical experiments provided with the THz CW signals and broadband THz pulses that are transmitted through the ordinary or dangerous substances. A high correlation between the time-dependent spectral intensities for the basic frequency and generated frequencies is demonstrated while using the computer simulation results. This feature of the frequency conversion might be used for the detection and identification of a substance. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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11 pages, 4291 KiB

Open AccessArticle

THz Water Transmittance and Leaf Surface Area: An Effective Nondestructive Method for Determining Leaf Water Content

by Mario Pagano, Lorenzo Baldacci, Andrea Ottomaniello, Giovanbattista de Dato, Francesco Chianucci, Luca Masini, Giorgio Carelli, Alessandra Toncelli, Paolo Storchi, Alessandro Tredicucci and Piermaria Corona

Sensors 2019, 19(22), 4838; https://doi.org/10.3390/s19224838 - 6 Nov 2019

Cited by 20 | Viewed by 3986

Abstract

Water availability is a major limiting factor in plant productivity and plays a key role in plant species distribution over a given area. New technologies, such as terahertz quantum cascade lasers (THz-QCLs) have proven to be non-invasive, effective, and accurate tools for measuring [...] Read more.

Water availability is a major limiting factor in plant productivity and plays a key role in plant species distribution over a given area. New technologies, such as terahertz quantum cascade lasers (THz-QCLs) have proven to be non-invasive, effective, and accurate tools for measuring and monitoring leaf water content. This study explores the feasibility of using an advanced THz-QCL device for measuring the absolute leaf water content in Corylus avellana L., Laurus nobilis L., Ostrya carpinifolia Scop., Quercus ilex L., Quercus suber L., and Vitis vinifera L. (cv. Sangiovese). A recently proposed, simple spectroscopic technique was used, consisting in determining the transmission of the THz light beam through the leaf combined with a photographic measurement of the leaf area. A significant correlation was found between the product of the leaf optical depth (τ) and the leaf surface area (L_A) with the leaf water mass (M_w) for all the studied species (Pearson’s r test, p ≤ 0.05). In all cases, the best fit regression line, in the graphs of τL_A as a function of M_w, displayed R² values always greater than 0.85. The method proposed can be combined with water stress indices of plants in order to gain a better understanding of the leaf water management processes or to indirectly monitor the kinetics of leaf invasion by pathogenic bacteria, possibly leading to the development of specific models to study and fight them. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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12 pages, 3099 KiB

Open AccessArticle

A Sensitive and Versatile Thickness Determination Method Based on Non-Inflection Terahertz Property Fitting

by Xuequan Chen and Emma Pickwell-MacPherson

Sensors 2019, 19(19), 4118; https://doi.org/10.3390/s19194118 - 23 Sep 2019

Cited by 15 | Viewed by 3083

Abstract

The accuracy of thin-film characterization in terahertz spectroscopy is mainly set by the thickness uncertainty. Physical thickness measurement has limited accuracy for thin-film samples thinner than a few hundreds of micrometers and is sometimes even impossible. The temporal resolution of time-domain terahertz spectrometers [...] Read more.

The accuracy of thin-film characterization in terahertz spectroscopy is mainly set by the thickness uncertainty. Physical thickness measurement has limited accuracy for thin-film samples thinner than a few hundreds of micrometers and is sometimes even impossible. The temporal resolution of time-domain terahertz spectrometers is not sufficient to resolve such thin films. Previously reported numerical methods mainly only work for materials with low dispersion and absorption. Here, we propose a novel method for thickness determination by fitting a non-inflection offset exponential function to the material optical properties. Theoretical analysis predicts the best fitting to only be achieved when the correct thickness is given. Transmission measurements on a thin-film polymer, water, and a lactose pallet verify the theory and show the accurate thickness determination and property characterization on materials which are either achromatic or dispersive, transparent or absorptive, featureless or resonant. The measurements demonstrate the best versatility and sensitivity compared to the state-of-art. The method could be widely adapted to various types of research and industrial applications. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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Review

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16 pages, 2988 KiB

Open AccessReview

Review of Terahertz Pulsed Imaging for Pharmaceutical Film Coating Analysis

by Décio Alves-Lima, Jun Song, Xiaoran Li, Alessia Portieri, Yaochun Shen, J. Axel Zeitler and Hungyen Lin

Sensors 2020, 20(5), 1441; https://doi.org/10.3390/s20051441 - 6 Mar 2020

Cited by 50 | Viewed by 7960

Abstract

Terahertz pulsed imaging (TPI) was introduced approximately fifteen years ago and has attracted a lot of interest in the pharmaceutical industry as a fast, non-destructive modality for quantifying film coatings on pharmaceutical dosage forms. In this topical review, we look back at the [...] Read more.

Terahertz pulsed imaging (TPI) was introduced approximately fifteen years ago and has attracted a lot of interest in the pharmaceutical industry as a fast, non-destructive modality for quantifying film coatings on pharmaceutical dosage forms. In this topical review, we look back at the use of TPI for analysing pharmaceutical film coatings, highlighting the main contributions made and outlining the key challenges ahead. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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21 pages, 3739 KiB

Open AccessReview

Non-Contact, Non-Destructive Testing in Various Industrial Sectors with Terahertz Technology

by Yu Heng Tao, Anthony J. Fitzgerald and Vincent P. Wallace

Sensors 2020, 20(3), 712; https://doi.org/10.3390/s20030712 - 28 Jan 2020

Cited by 168 | Viewed by 13671

Abstract

In this article, we survey various non-contact, non-destructive testing methods by way of terahertz (THz) spectroscopy and imaging designed for use in various industrial sectors. A brief overview of the working principles of THz spectroscopy and imaging is provided, followed by a survey [...] Read more.

In this article, we survey various non-contact, non-destructive testing methods by way of terahertz (THz) spectroscopy and imaging designed for use in various industrial sectors. A brief overview of the working principles of THz spectroscopy and imaging is provided, followed by a survey of selected applications from three industries—the building and construction industry, the energy and power industry, and the manufacturing industry. Material characterization, thickness measurement, and defect/corrosion assessment are demonstrated through the examples presented. The article concludes with a discussion of novel spectroscopy and imaging devices and techniques that are expected to accelerate industry adoption of THz systems. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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35 pages, 7281 KiB

Open AccessReview

Industrial Applications of Terahertz Sensing: State of Play

by Mira Naftaly, Nico Vieweg and Anselm Deninger

Sensors 2019, 19(19), 4203; https://doi.org/10.3390/s19194203 - 27 Sep 2019

Cited by 229 | Viewed by 12615

Abstract

This paper is a survey of existing and upcoming industrial applications of terahertz technologies, comprising sections on polymers, paint and coatings, pharmaceuticals, electronics, petrochemicals, gas sensing, and paper and wood industries. Finally, an estimate of the market size and growth rates is given, [...] Read more.

This paper is a survey of existing and upcoming industrial applications of terahertz technologies, comprising sections on polymers, paint and coatings, pharmaceuticals, electronics, petrochemicals, gas sensing, and paper and wood industries. Finally, an estimate of the market size and growth rates is given, as obtained from a comparison of market reports. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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Other

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11 pages, 4510 KiB

Open AccessLetter

Secure Deep Learning for Intelligent Terahertz Metamaterial Identification

by Feifei Liu, Weihao Zhang, Yu Sun, Jianwei Liu, Jungang Miao, Feng He and Xiaojun Wu

Sensors 2020, 20(19), 5673; https://doi.org/10.3390/s20195673 - 5 Oct 2020

Cited by 9 | Viewed by 3515

Abstract

Metamaterials, artificially engineered structures with extraordinary physical properties, offer multifaceted capabilities in interdisciplinary fields. To address the looming threat of stealthy monitoring, the detection and identification of metamaterials is the next research frontier but have not yet been explored. Here, we show that [...] Read more.

Metamaterials, artificially engineered structures with extraordinary physical properties, offer multifaceted capabilities in interdisciplinary fields. To address the looming threat of stealthy monitoring, the detection and identification of metamaterials is the next research frontier but have not yet been explored. Here, we show that the crypto-oriented convolutional neural network (CNN) makes possible the secure intelligent detection of metamaterials in mixtures. Terahertz signals were encrypted by homomorphic encryption and the ciphertext was submitted to the CNN directly for results, which can only be decrypted by the data owner. The experimentally measured terahertz signals were augmented and further divided into training sets and test sets using 5-fold cross-validation. Experimental results illustrated that the model achieved an accuracy of 100% on the test sets, which highly outperformed humans and the traditional machine learning. The CNN took 9.6 s to inference on 92 encrypted test signals with homomorphic encryption backend. The proposed method with accuracy and security provides private preserving paradigm for artificial intelligence-based material identification. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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12 pages, 1541 KiB

Open AccessLetter

Terahertz Time-Domain Polarimetry in Reflection for Film Characterization

by Sandrine van Frank, Elisabeth Leiss-Holzinger, Michael Pfleger and Christian Rankl

Sensors 2020, 20(12), 3352; https://doi.org/10.3390/s20123352 - 12 Jun 2020

Cited by 3 | Viewed by 3311

Abstract

Terahertz time-domain spectroscopy is a useful technique to characterize layered samples and thin films. It gives access to their optical properties and thickness. Such measurements are done in transmission, which requires access to the sample from opposite sides. In reality this is not [...] Read more.

Terahertz time-domain spectroscopy is a useful technique to characterize layered samples and thin films. It gives access to their optical properties and thickness. Such measurements are done in transmission, which requires access to the sample from opposite sides. In reality this is not always possible. In such cases, reflection measurements are the only option, but they are more difficult to implement. Here we propose a method to characterize films in reflection geometry using a polarimetric approach based on the identification of Brewster angle and modeling of the measured signal to extract the refractive index and thickness of the sample. The technique is demonstrated experimentally on an unsupported single layer thin film sample. The extracted optical properties and thickness were in good agreement with established transmission terahertz spectroscopy measurements. The new method has the potential to cover a wide range of applications, both for research and industrial purposes. Full article

(This article belongs to the Special Issue Advanced Industrial Terahertz Sensing Applications)

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