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Editorial

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1 pages, 145 KiB

Open AccessEditorial

Foreword to the Special Issue on Terahertz Nondestructive Testing

by Robert A. Smith

Appl. Sci. 2021, 11(24), 11724; https://doi.org/10.3390/app112411724 - 10 Dec 2021

Cited by 2 | Viewed by 2110

Abstract

It is my pleasure to introduce this Special Issue on “Terahertz Nondestructive Testing”, one of the most promising new technologies to be applied to non-destructive inspection problems [...] Full article

(This article belongs to the Special Issue Terahertz Applications for Nondestructive Testing)

Research

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11 pages, 1883 KiB

Open AccessArticle

A Terahertz Fast-Sweep Optoelectronic Frequency-Domain Spectrometer: Calibration, Performance Tests, and Comparison with TDS and FDS

by Janis Kutz, Lars Liebermeister, Nico Vieweg, Konstantin Wenzel, Robert Kohlhaas and Mira Naftaly

Appl. Sci. 2022, 12(16), 8257; https://doi.org/10.3390/app12168257 - 18 Aug 2022

Cited by 10 | Viewed by 1874

Abstract

We report calibration and performance tests of a terahertz fast-sweep optoelectronic frequency-domain spectrometer designed for industrial applications, aimed at quantifying its performance specifications and demonstrating its suitability for envisaged usage. The frequency scale is calibrated using atmospheric water vapour lines and a silicon [...] Read more.

We report calibration and performance tests of a terahertz fast-sweep optoelectronic frequency-domain spectrometer designed for industrial applications, aimed at quantifying its performance specifications and demonstrating its suitability for envisaged usage. The frequency scale is calibrated using atmospheric water vapour lines and a silicon wafer etalon; the amplitude linearity is verified using a set of silicon plates. Instrument performance is tested by measuring transmission properties of a variety of representative samples and comparing with a time-domain spectrometer and a frequency-domain spectrometer. Full article

(This article belongs to the Special Issue Terahertz Applications for Nondestructive Testing)

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9 pages, 12853 KiB

Open AccessArticle

Kilohertz Pixel-Rate Multilayer Terahertz Imaging of Subwavelength Coatings

by Daniel Molter, Kim-Sophie Ellenberger, Jens Klier, Stefan Duran, Joachim Jonuscheit, Georg von Freymann, Nico Vieweg and Anselm Deninger

Appl. Sci. 2022, 12(10), 4964; https://doi.org/10.3390/app12104964 - 13 May 2022

Cited by 12 | Viewed by 2348

Abstract

The acquisition speed of terahertz time-domain spectroscopy systems has undergone a significant improvement in recent years. With the development of dual-laser-based sampling techniques such as ASOPS or ECOPS, waveform acquisition rates in the kilohertz range have become feasible. Here, we present measurements of [...] Read more.

The acquisition speed of terahertz time-domain spectroscopy systems has undergone a significant improvement in recent years. With the development of dual-laser-based sampling techniques such as ASOPS or ECOPS, waveform acquisition rates in the kilohertz range have become feasible. Here, we present measurements of sub-wavelength layers in multilayer systems at a rate of 1.6 kHz, where the individual layer thicknesses are analyzed in real time. For demonstration, we image layers on metallic and non-metallic disks with a 300 mm diameter, acquiring 240,000 pixels in only 2.5 min. By combining a rotation of the samples with a linear translation of the measurement head, we maximize the measurement yield. Owing to a “single-shot” (625 µs) dynamic range of 40 dB and a bandwidth above 3 THz of the ECOPS system, we achieve sub-micron repeatability for each layer. Full article

(This article belongs to the Special Issue Terahertz Applications for Nondestructive Testing)

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18 pages, 3347 KiB

Open AccessEditor’s ChoiceArticle

THz Time-Domain Ellipsometer for Material Characterization and Paint Quality Control with More Than 5 THz Bandwidth

by Helge Ketelsen, Rüdiger Mästle, Lars Liebermeister, Robert Kohlhaas and Björn Globisch

Appl. Sci. 2022, 12(8), 3744; https://doi.org/10.3390/app12083744 - 8 Apr 2022

Cited by 10 | Viewed by 3055

Abstract

Quality control of car body paint in the automotive industry is a promising industrial application of terahertz technology. Terahertz time-domain spectroscopy in reflection geometry enables accurate, fast, and nondestructive measurement of individual layer thicknesses of multi-layer coatings. For high precision thickness measurements, the [...] Read more.

Quality control of car body paint in the automotive industry is a promising industrial application of terahertz technology. Terahertz time-domain spectroscopy in reflection geometry enables accurate, fast, and nondestructive measurement of individual layer thicknesses of multi-layer coatings. For high precision thickness measurements, the frequency dependent complex refractive index of all layers must be calibrated very accurately. THz time-domain ellipsometry is self-referencing and provides reliable, frequency resolved material properties with high signal-to-noise ratio. The method is characterized by a high sensitivity to optical material properties and layer thicknesses. We present characterization results in the frequency range 0.1–6 THz for typical automotive paints and different substrates such as polypropylene (PP), which features a high material anisotropy. We demonstrate that the broadband material properties derived from ellipsometry allow for inline thickness measurements of multi-layer car body paints with high accuracy. Full article

(This article belongs to the Special Issue Terahertz Applications for Nondestructive Testing)

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14 pages, 1153 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Polymer Pellet Fabrication for Accurate THz-TDS Measurements

by Keir N. Murphy, Mira Naftaly, Alison Nordon and Daniel Markl

Appl. Sci. 2022, 12(7), 3475; https://doi.org/10.3390/app12073475 - 29 Mar 2022

Cited by 13 | Viewed by 3238

Abstract

We investigate fabrication of compacts using polytetrafluoroethylene (PTFE) and polyethylene (PE), and the effect of compaction conditions on their terahertz transmission properties. The conditions used to fabricate compressed powder samples for terahertz time-domain spectroscopy (THz-TDS) can impact the accuracy of the measurements and [...] Read more.

We investigate fabrication of compacts using polytetrafluoroethylene (PTFE) and polyethylene (PE), and the effect of compaction conditions on their terahertz transmission properties. The conditions used to fabricate compressed powder samples for terahertz time-domain spectroscopy (THz-TDS) can impact the accuracy of the measurements and hence the interpretation of results. This study investigated the effect of compaction conditions on the accuracy of the THz-TDS analysis. Two polymers that are commonly used as matrix materials in terahertz spectroscopy studies were explored using a compaction simulator and a hydraulic press for sample preparation. THz-TDS was used to determine the refractive index and loss coefficient to compare the powder compacts (pellets) to the values of solid material. Sample porosity, axial relaxation and tensile strength were measured to assess the material’s suitability for terahertz spectroscopy. It was found that PTFE is the preferable material for creating THz-TDS samples due to its low porosity and high tensile strength. PE was found to show significant porosity at all compaction pressures, making it an unsuitable material for the accurate determination of optical parameters from THz-TDS spectroscopy measurements. The larger particle sizes of PE resulted in compacts that exhibited significantly lower tensile strength than those made from PTFE making handling and storage difficult. Full article

(This article belongs to the Special Issue Terahertz Applications for Nondestructive Testing)

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11 pages, 3414 KiB

Open AccessFeature PaperArticle

Non-Destructive Porosity Measurements of 3D Printed Polymer by Terahertz Time-Domain Spectroscopy

by Mira Naftaly, Gian Savvides, Fawwaz Alshareef, Patrick Flanigan, GianLuc Lui, Marian Florescu and Ruth Ann Mullen

Appl. Sci. 2022, 12(2), 927; https://doi.org/10.3390/app12020927 - 17 Jan 2022

Cited by 8 | Viewed by 3056

Abstract

The porosity and inhomogeneity of 3D printed polymer samples were examined using terahertz time-domain spectroscopy, and the effects of 3D printer settings were analysed. A set of PETG samples were 3D printed by systematically varying the printer parameters, including layer thickness, nozzle diameter, [...] Read more.

The porosity and inhomogeneity of 3D printed polymer samples were examined using terahertz time-domain spectroscopy, and the effects of 3D printer settings were analysed. A set of PETG samples were 3D printed by systematically varying the printer parameters, including layer thickness, nozzle diameter, filament (line) thickness, extrusion, and printing pattern. Their effective refractive indices and loss coefficients were measured and compared with those of solid PETG. Porosity was calculated from the refractive index. A diffraction feature was observed in the loss spectrum of all 3D printed samples and was used as an indication of inhomogeneity. A “sweet spot” of printer settings was found, where porosity and inhomogeneity were minimised. Full article

(This article belongs to the Special Issue Terahertz Applications for Nondestructive Testing)

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7 pages, 1982 KiB

Open AccessArticle

Nondestructive Tomographic Imaging of Rust with Rapid THz Time-Domain Spectroscopy

by Hwan Sik Kim, Seung Yeob Baik, Joong Wook Lee, Jangsun Kim and Yeong Hwan Ahn

Appl. Sci. 2021, 11(22), 10594; https://doi.org/10.3390/app112210594 - 11 Nov 2021

Cited by 8 | Viewed by 2311

Abstract

In this study, we developed a rapid nondestructive tool for testing rust spread in a metal covered by a paint layer by using a THz time-domain spectroscopy system at a speed of 100 Hz/pixel. Time-of-flight imaging helps identify rust formation by exclusively obtaining [...] Read more.

In this study, we developed a rapid nondestructive tool for testing rust spread in a metal covered by a paint layer by using a THz time-domain spectroscopy system at a speed of 100 Hz/pixel. Time-of-flight imaging helps identify rust formation by exclusively obtaining the reflection from the steel below the paint surface. The use of frequency-selective imaging allows us to manipulate the contrast in rust imaging. Higher contrast is generally obtained when monitoring using the higher frequency component. In addition, we monitored the spread of rust in a steel plate under the influence of two different chemical solutions: NaCl and acid. We found that in the early stages, the decrease in THz reflection was governed by the high-frequency components due to the formation of lepidocrocite, whereas the low-frequency component develops as the proportion of hematite increases with time. Full article

(This article belongs to the Special Issue Terahertz Applications for Nondestructive Testing)

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Review

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18 pages, 1846 KiB

Open AccessReview

Selected Applications of Terahertz Pulses in Medicine and Industry

by Philip F. Taday, Michael Pepper and Donald D. Arnone

Appl. Sci. 2022, 12(12), 6169; https://doi.org/10.3390/app12126169 - 17 Jun 2022

Cited by 10 | Viewed by 3896

Abstract

This article contains a brief summary of areas where terahertz technology is making an impact in research and industrial applications. We cover some of its uses in the pharmaceutical setting, where both imaging and spectroscopy play important roles. Medical applications are also being [...] Read more.

This article contains a brief summary of areas where terahertz technology is making an impact in research and industrial applications. We cover some of its uses in the pharmaceutical setting, where both imaging and spectroscopy play important roles. Medical applications are also being pursued in many research laboratories, primarily for imaging purposes and following on from the first results just over 20 years ago. The three-dimensional imaging capability of pulsed terahertz allows for the observation of tumours below the surface of tissue, such as basal cell carcinoma of skin. The recent use of the technology in studies of cultural heritage has shown to increase our understanding of the past. The power of terahertz is exemplified by the discussion on its importance in different industries, such as semiconductor circuit manufacturing and automotive assembly. Full article

(This article belongs to the Special Issue Terahertz Applications for Nondestructive Testing)

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11 pages, 2735 KiB

Open AccessEditor’s ChoiceReview

An Overview of Terahertz Imaging with Resonant Tunneling Diodes

by Jue Wang, Mira Naftaly and Edward Wasige

Appl. Sci. 2022, 12(8), 3822; https://doi.org/10.3390/app12083822 - 10 Apr 2022

Cited by 15 | Viewed by 4977

Abstract

Terahertz (THz) imaging is a rapidly growing application motivated by industrial demands including harmless (non-ionizing) security imaging, multilayer paint quality control within the automotive industry, insulating foam non-invasive testing in aerospace, and biomedical diagnostics. One of the key components in the imaging system [...] Read more.

Terahertz (THz) imaging is a rapidly growing application motivated by industrial demands including harmless (non-ionizing) security imaging, multilayer paint quality control within the automotive industry, insulating foam non-invasive testing in aerospace, and biomedical diagnostics. One of the key components in the imaging system is the source and detector. This paper gives a brief overview of room temperature THz transceiver technology for imaging applications based on the emerging resonant tunneling diode (RTD) devices. The reported results demonstrate that RTD technology is a very promising candidate to realize compact, low-cost THz imaging systems. Full article

(This article belongs to the Special Issue Terahertz Applications for Nondestructive Testing)

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