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Terahertz and Millimeter Wave Sensing and Applications

A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Physical Sensors".

Deadline for manuscript submissions: closed (30 June 2022) | Viewed by 41142
Please contact the Guest Editor or the Section Managing Editor at ([email protected]) for any queries.

Special Issue Editors


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Guest Editor
Fraunhofer Institute for Industrial Mathematics ITWM, Kaiserslautern, Germany
Interests: terahertz and millimeter wave physics and applications; non-destructive testing; terahertz detectors
Special Issues, Collections and Topics in MDPI journals

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Guest Editor
Fraunhofer Institute for Industrial Mathematics ITWM, Kaiserslautern, Germany
Interests: millimeter wave and terahertz imaging; system development; measurement techniques
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Terahertz and millimeter wave technologies are on the rise in many fields of industrial and scientific applications. Ongoing development of increasingly efficient terahertz and millimeter wave sources, detectors, sensors, and measurement techniques have brought terahertz technologies to a level of industrial maturity, which allow for an integration of the technology in modern production lines and for materials science and characterization. Many of the industrial applications can be summarized under the general term “non-destructive testing” (NDT), where terahertz and millimeter wave technologies can reveal their unique potential. These contact-free, safe, and easy-to-use technologies can address many of today’s challenges in industrial production, maintenance, quality inspection, and materials research, replacing or complementing well-established NDT technologies in these areas. The field of terahertz and millimeter wave sensing, on the other hand, addresses, e.g., gas spectroscopy, biomedical applications, monitoring of ultrafast dynamics, and many more.

Dr. Maris Bauer
Dr. Fabian Friederich
Guest Editors

Manuscript Submission Information

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Keywords

In this Special Issue, we encourage authors from all fields of research and development related to terahertz and millimeter wave sensing and applications with both industrial and scientific backgrounds to submit original manuscripts with a focus on but not limited to the fields of Practical industrial and/or scientific applications
  • New fields of terahertz applications and sensing
  • Non-destructive testing (NDT)
  • Terahertz and millimeter wave imaging
  • Terahertz measurements and system design
  • Terahertz sources, detectors, and sensors

Published Papers (17 papers)

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Editorial

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3 pages, 174 KiB  
Editorial
Terahertz and Millimeter Wave Sensing and Applications
by Maris Bauer and Fabian Friederich
Sensors 2022, 22(24), 9693; https://doi.org/10.3390/s22249693 - 10 Dec 2022
Cited by 4 | Viewed by 1532
Abstract
The field of terahertz and millimeter wave science and technology has evolved in recent years into an area attracting a lot of attention from all sides of science, industry, and the public [...] Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)

Research

Jump to: Editorial, Other

17 pages, 4397 KiB  
Article
Dual-Biometric Human Identification Using Radar Deep Transfer Learning
by Ahmad Alkasimi, Tyler Shepard, Samuel Wagner, Stephen Pancrazio, Anh-Vu Pham, Christopher Gardner and Brad Funsten
Sensors 2022, 22(15), 5782; https://doi.org/10.3390/s22155782 - 02 Aug 2022
Cited by 5 | Viewed by 2474
Abstract
Accurate human identification using radar has a variety of potential applications, such as surveillance, access control and security checkpoints. Nevertheless, radar-based human identification has been limited to a few motion-based biometrics that are solely reliant on micro-Doppler signatures. This paper proposes for the [...] Read more.
Accurate human identification using radar has a variety of potential applications, such as surveillance, access control and security checkpoints. Nevertheless, radar-based human identification has been limited to a few motion-based biometrics that are solely reliant on micro-Doppler signatures. This paper proposes for the first time the use of combined radar-based heart sound and gait signals as biometrics for human identification. The proposed methodology starts by converting the extracted biometric signatures collected from 18 subjects to images, and then an image augmentation technique is applied and the deep transfer learning is used to classify each subject. A validation accuracy of 58.7% and 96% is reported for the heart sound and gait biometrics, respectively. Next, the identification results of the two biometrics are combined using the joint probability mass function (PMF) method to report a 98% identification accuracy. To the best of our knowledge, this is the highest reported in the literature to date. Lastly, the trained networks are tested in an actual scenario while being used in an office access control platform to identify different human subjects. We report an accuracy of 76.25%. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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17 pages, 12970 KiB  
Article
Object Recognition in High-Resolution Indoor THz SAR Mapped Environment
by Aman Batra, Fawad Sheikh, Maher Khaliel, Michael Wiemeler, Diana Göhringer and Thomas Kaiser
Sensors 2022, 22(10), 3762; https://doi.org/10.3390/s22103762 - 15 May 2022
Cited by 6 | Viewed by 2067
Abstract
Synthetic aperture radar (SAR) at the terahertz (THz) spectrum has emerging short-range applications. In comparison to the microwave spectrum, the THz spectrum is limited in propagation range but benefits from high spatial resolution. The THz SAR is of significant interest for several applications [...] Read more.
Synthetic aperture radar (SAR) at the terahertz (THz) spectrum has emerging short-range applications. In comparison to the microwave spectrum, the THz spectrum is limited in propagation range but benefits from high spatial resolution. The THz SAR is of significant interest for several applications which necessitate the mapping of indoor environments to support various endeavors such as rescue missions, map-assisted wireless communications, and household robotics. This paper addresses the augmentation of the high-resolution indoor mapped environment for object recognition, which includes detection, localization, and classification. Indoor object recognition is currently dominated by the usage of optical and infrared (IR) systems. However, it is not widely explored by radar technologies due to the limited spatial resolution at the most commonly used microwave frequencies. However, the THz spectrum provides a new paradigm of possible adaptation of object recognition in the radar domain by providing image quality in good compliance to optical/IR systems. In this paper, a multi-object indoor environment is foremost mapped at the THz spectrum ranging from 325 to 500 GHz in order to investigate the imaging in highly scattered environments and accordingly create a foundation for detection, localization, and classification. Furthermore, the extraction and clustering of features of the mapped environment are conducted for object detection and localization. Finally, the classification of detected objects is addressed with a supervised machine learning-based support vector machine (SVM) model. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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10 pages, 4513 KiB  
Communication
Calibration Alignment Sensitivity in Corneal Terahertz Imaging
by Faezeh Zarrinkhat, Mariangela Baggio, Joel Lamberg, Aleksi Tamminen, Irina Nefedova, Juha Ala-Laurinaho, Elsayed E. M. Khaled, Juan M. Rius, Jordi Romeu and Zachary Taylor
Sensors 2022, 22(9), 3237; https://doi.org/10.3390/s22093237 - 22 Apr 2022
Cited by 4 | Viewed by 1516
Abstract
Improving the longitudinal modes coupling in layered spherical structure contributes significantly to corneal terahertz sensing, which plays a crucial role in the early diagnosis of cornea dystrophies. Using a steel sphere to calibrate reflection from the cornea sample assists in enhancing the resolution [...] Read more.
Improving the longitudinal modes coupling in layered spherical structure contributes significantly to corneal terahertz sensing, which plays a crucial role in the early diagnosis of cornea dystrophies. Using a steel sphere to calibrate reflection from the cornea sample assists in enhancing the resolution of longitudinal modes. The requirement and challenges toward applying the calibration sphere are introduced and addressed. Six corneas with different properties are spotted to study the effect of perturbations in the calibration sphere in a frequency range from 100 GHz to 600 GHz. A particle-swarm optimization algorithm is employed to quantify corneal characteristics considering cases of accurately calibrated and perturbed calibrated scenarios. For the first case, the study is carried out with signal-to-noise values of 40 dB, 50 dB and 60 dB at waveguide bands WR-5.1, WR-3.4, and WR-2.2. As expected, better estimation is achieved in high-SNR cases. Furthermore, the lower waveguide band is revealed as the most proper band for the assessment of corneal features. For perturbed cases, the analysis is continued for the noise level of 60 dB in the three waveguide bands. Consequently, the error in the estimation of corneal properties rises significantly (around 30%). Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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16 pages, 8373 KiB  
Article
Detecting the Presence of Intrusive Drilling in Secure Transport Containers Using Non-Contact Millimeter-Wave Radar
by Samuel Wagner, Ahmad Alkasimi and Anh-Vu Pham
Sensors 2022, 22(7), 2718; https://doi.org/10.3390/s22072718 - 01 Apr 2022
Cited by 3 | Viewed by 3120
Abstract
We employ a 77–81 GHz frequency-modulated continuous-wave (FMCW) millimeter-wave radar to sense anomalous vibrations during vehicle transport at highway speeds for the first time. Secure metallic containers can be breached during transport by means of drilling into their sidewalls but detecting a drilling [...] Read more.
We employ a 77–81 GHz frequency-modulated continuous-wave (FMCW) millimeter-wave radar to sense anomalous vibrations during vehicle transport at highway speeds for the first time. Secure metallic containers can be breached during transport by means of drilling into their sidewalls but detecting a drilling signature is difficult because the large vibrations of transport drown out the small vibrations of drilling. For the first time, we demonstrate that it is possible to use a non-contact millimeter-wave radar sensor to detect this micron-scale intrusive drilling while highway-speed vehicle movement shakes the container. With the millimeter-wave radar monitoring the microdoppler signature of the container’s vibrating walls, we create a novel signal-processing pipeline consisting of range–angle tracking, time–frequency analysis, horizontal stripe image convolution, and principal component analysis to create a robust and powerful detection statistic to alarm if drilling is present. To support this pipeline, we develop a statistical model combining the vibrating container and the random vibrations induced by vehicle movement to explore the robustness of the sensor’s detection capabilities. The presented results strongly support the inclusion of a millimeter-wave radar vibration sensor into a transport security system. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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12 pages, 1819 KiB  
Article
Translation-Invariant Zero-Phase Wavelet Methods for Feature Extraction in Terahertz Time-Domain Spectroscopy
by Mahmoud E. Khani and Mohammad Hassan Arbab
Sensors 2022, 22(6), 2305; https://doi.org/10.3390/s22062305 - 16 Mar 2022
Cited by 10 | Viewed by 1949
Abstract
Wavelet transform is an important tool in the computational signal processing of terahertz time-domain spectroscopy (THz-TDS) measurements. Despite its prevalence, the effects of using different forms of wavelet transforms in THz-TDS studies have not been investigated. In this paper, we explore the implications [...] Read more.
Wavelet transform is an important tool in the computational signal processing of terahertz time-domain spectroscopy (THz-TDS) measurements. Despite its prevalence, the effects of using different forms of wavelet transforms in THz-TDS studies have not been investigated. In this paper, we explore the implications of using the maximal overlap discrete wavelet transform (MODWT) versus the well-known discrete wavelet transform (DWT). We demonstrate that the spectroscopic features extracted using DWT can vary over different overlapping frequency ranges. On the contrary, MODWT is translation-invariant and results in identical features, regardless of the spectral range used for its implementation.We also demonstrate that the details coefficients obtained by the multiresolution analysis (MRA) using MODWT are associated with zero-phase filters. In contrast, DWT details coefficients suffer from misalignments originated from the down- and upsampling operations in DWT pyramid algorithm. Such misalignments have adverse effects when it is critical to retain the exact location of the absorption lines. We study the differences of DWT and MODWT both analytically and experimentally, using reflection THz-TDS measurements of α-lactose monohydrate. This manuscript can guide the researchers to select the right wavelet analysis tool for their specific application of the THz spectroscopy. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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10 pages, 5244 KiB  
Article
Temperature and Gate-Length Dependence of Subthreshold RF Detection in GaN HEMTs
by Gaudencio Paz-Martínez, Ignacio Íñiguez-de-la-Torre, Héctor Sánchez-Martín, José Antonio Novoa-López, Virginie Hoel, Yvon Cordier, Javier Mateos and Tomás González
Sensors 2022, 22(4), 1515; https://doi.org/10.3390/s22041515 - 15 Feb 2022
Cited by 6 | Viewed by 2492
Abstract
The responsivity of AlGaN/GaN high-electron mobility transistors (HEMTs) when operating as zero-bias RF detectors in the subthreshold regime exhibits different behaviors depending on the operating temperature and gate length of the transistors. We have characterized in temperature (8–400 K) the detection performance of [...] Read more.
The responsivity of AlGaN/GaN high-electron mobility transistors (HEMTs) when operating as zero-bias RF detectors in the subthreshold regime exhibits different behaviors depending on the operating temperature and gate length of the transistors. We have characterized in temperature (8–400 K) the detection performance of HEMTs with different gate lengths (75–250 nm). The detection results at 1 GHz can be reproduced by a quasi-static model, which allows us to interpret them by inspection of the output ID  VDS curves of the transistors. We explain the different behaviors observed in terms of the presence or absence of a shift in the zero-current operating point originating from the existence of the gate-leakage current jointly with temperature effects related to the ionization of bulk traps. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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11 pages, 613 KiB  
Article
Crossing Frequency Method Applicable to Intermediate Pressure Plasma Diagnostics Using the Cutoff Probe
by Si-jun Kim, Jang-jae Lee, Young-seok Lee, Chul-hee Cho and Shin-jae You
Sensors 2022, 22(3), 1291; https://doi.org/10.3390/s22031291 - 08 Feb 2022
Cited by 13 | Viewed by 2215
Abstract
Although the recently developed cutoff probe is a promising tool to precisely infer plasma electron density by measuring the cutoff frequency (fcutoff) in the S21 spectrum, it is currently only applicable to low-pressure plasma diagnostics below several torr. To [...] Read more.
Although the recently developed cutoff probe is a promising tool to precisely infer plasma electron density by measuring the cutoff frequency (fcutoff) in the S21 spectrum, it is currently only applicable to low-pressure plasma diagnostics below several torr. To improve the cutoff probe, this paper proposes a novel method to measure the crossing frequency (fcross), which is applicable to high-pressure plasma diagnostics where the conventional fcutoff method does not operate. Here, fcross is the frequency where the S21 spectra in vacuum and plasma conditions cross each other. This paper demonstrates the fcross method through three-dimensional electromagnetic wave simulation as well as experiments in a capacitively coupled plasma source. Results demonstrate that the method operates well at high pressure (several tens of torr) as well as low pressure. In addition, through circuit model analysis, a method to estimate electron density from fcross is discussed. It is believed that the proposed method expands the operating range of the cutoff probe and thus contributes to its further development. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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15 pages, 3167 KiB  
Article
W-Band Photonic Receiver for Compact Cloud Radars
by Dmitry Strekalov, Ninoslav Majurec, Andrey Matsko, Vladimir Ilchenko, Simone Tanelli and Razi Ahmed
Sensors 2022, 22(3), 804; https://doi.org/10.3390/s22030804 - 21 Jan 2022
Cited by 3 | Viewed by 2038
Abstract
We introduce an RF-photonics receiver concept enabling the next generation of ultra-compact millimeter wave radars suitable for cloud and precipitation profiling, planetary boundary layer observations, altimetry and surface scattering measurements. The RF-photonics receiver architecture offers some compelling advantages over traditional electronic implementations, including [...] Read more.
We introduce an RF-photonics receiver concept enabling the next generation of ultra-compact millimeter wave radars suitable for cloud and precipitation profiling, planetary boundary layer observations, altimetry and surface scattering measurements. The RF-photonics receiver architecture offers some compelling advantages over traditional electronic implementations, including a reduced number of components and interfaces, leading to reduced size, weight and power (SWaP), as well as lower system noise, leading to improved sensitivity. Low instrument SWaP with increased sensitivity makes this approach particularly attractive for compact space-borne radars. We study the photonic receiver front-end both analytically and numerically and predict the feasibility of the greater than unity photonic gain and lower than ambient effective noise temperature of the device. The receiver design is optimized for W-band (94 GHz) radars, which are generally assessed to be the primary means for observing clouds in the free troposphere as well as planetary boundary layer from space. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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12 pages, 3305 KiB  
Article
Investigations on Practical Issues in Solid Immersion Lens Based Sub-Wavelength Terahertz Imaging Technique: System Stability Verification and Interference Pattern Removal
by Da-Hye Choi, Jun-Hwan Shin, Il-Min Lee and Kyung Hyun Park
Sensors 2021, 21(21), 6990; https://doi.org/10.3390/s21216990 - 21 Oct 2021
Cited by 6 | Viewed by 1781
Abstract
Terahertz (THz) imaging techniques are attractive for a wide range of applications, such as non-destructive testing, biological sensing, and security imaging. We investigate practical issues in THz imaging systems based on a solid immersion lens (SIL). The system stability in terms of longitudinal [...] Read more.
Terahertz (THz) imaging techniques are attractive for a wide range of applications, such as non-destructive testing, biological sensing, and security imaging. We investigate practical issues in THz imaging systems based on a solid immersion lens (SIL). The system stability in terms of longitudinal misalignment of the SIL is experimentally verified by showing that the diffraction-limited sub-wavelength beam size (0.7 λ) is maintained as long as the SIL is axially located within the depth-of-focus (~13 λ) of the objective lens. The origin of the fringe patterns, which are undesirable but inevitable in THz imaging systems that use continuous waves, is analytically studied, and a method for minimizing the interference patterns is proposed. By combining two THz images obtained at different axial positions of the object and separated by λ/4, the interference patterns are significantly reduced, and the information hidden under the interference patterns is unveiled. The broad applicability of the proposed method is demonstrated by imaging objects with different surface profiles. Our work proves that the resolution of conventional THz imaging systems can easily be enhanced by simply inserting a SIL in front of the object with high tolerance in the longitudinal misalignment and provides a method enabling THz imaging for objects with different surface profiles. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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11 pages, 986 KiB  
Article
Terahertz Imaging for Paper Handling of Legacy Documents
by Min Zhai, Alexandre Locquet and David S. Citrin
Sensors 2021, 21(20), 6756; https://doi.org/10.3390/s21206756 - 12 Oct 2021
Cited by 3 | Viewed by 1631
Abstract
Despite predictions of the paperless office, global demand for printing and writing paper remains strong, and paper appears to be here to stay for some time. Not only firms, but also governments, libraries, and archives are in possession of large collections of legacy [...] Read more.
Despite predictions of the paperless office, global demand for printing and writing paper remains strong, and paper appears to be here to stay for some time. Not only firms, but also governments, libraries, and archives are in possession of large collections of legacy documents that still must be sorted and scanned. In this study, terahertz-based techniques are demonstrated to address several routine tasks related to the automated paper handling of unsorted legacy documents. Specifically, we demonstrate terahertz-based counting of the number of sheets in unconsolidated paper stacks, as well as locating stapled documents buried in paper stacks. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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19 pages, 9776 KiB  
Article
All-Electronic Emitter-Detector Pairs for 250 GHz in Silicon
by Kęstutis Ikamas, Dmytro B. But, Albert Cesiul, Cezary Kołaciński, Tautvydas Lisauskas, Wojciech Knap and Alvydas Lisauskas
Sensors 2021, 21(17), 5795; https://doi.org/10.3390/s21175795 - 28 Aug 2021
Cited by 14 | Viewed by 2338
Abstract
The spread of practical terahertz (THz) systems dedicated to the telecommunication, pharmacy, civil security, or medical markets requires the use of mainstream semiconductor technologies, such as complementary metal-oxide-semiconductor (CMOS) lines. In this paper, we discuss the operation of a CMOS-based free space all-electronic [...] Read more.
The spread of practical terahertz (THz) systems dedicated to the telecommunication, pharmacy, civil security, or medical markets requires the use of mainstream semiconductor technologies, such as complementary metal-oxide-semiconductor (CMOS) lines. In this paper, we discuss the operation of a CMOS-based free space all-electronic system operating near 250 GHz, exhibiting signal-to-noise ratio (SNR) with 62 dB in the direct detection regime for one Hz equivalent noise bandwidth. It combines the state-of-the-art detector based on CMOS field-effect-transistors (FET) and a harmonic voltage-controlled oscillator (VCO). Three generations of the oscillator circuit are presented, and the performance characterization techniques and their improvement are explained in detail. The manuscript presents different emitter–detector pair operation modalities, including spectroscopy and imaging. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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13 pages, 2119 KiB  
Article
Ultra-High Repetition Rate Terahertz Time-Domain Spectroscopy for Micrometer Layer Thickness Measurement
by Kevin Kolpatzeck, Xuan Liu, Lars Häring, Jan C. Balzer and Andreas Czylwik
Sensors 2021, 21(16), 5389; https://doi.org/10.3390/s21165389 - 10 Aug 2021
Cited by 13 | Viewed by 2284
Abstract
Terahertz time-domain spectroscopy systems driven by monolithic mode-locked laser diodes (MLLDs) exhibit bandwidths exceeding 1 THz and a peak dynamic range that can compete with other state-of-the-art systems. Their main difference compared to fiber-laser-driven systems is their ultra-high repetition rate of typically dozens [...] Read more.
Terahertz time-domain spectroscopy systems driven by monolithic mode-locked laser diodes (MLLDs) exhibit bandwidths exceeding 1 THz and a peak dynamic range that can compete with other state-of-the-art systems. Their main difference compared to fiber-laser-driven systems is their ultra-high repetition rate of typically dozens of GHz. This makes them interesting for applications where the length of the terahertz path may not be precisely known and it enables the use of a very short and potentially fast optical delay unit. However, the phase accuracy of the system is limited by the accuracy with which the delay axes of subsequent measurements are synchronized. In this work, we utilize an all-fiber approach that uses the optical signal from the MLLD in a Mach–Zehnder interferometer to generate a reference signal that we use to synchronize the detected terahertz signals. We demonstrate transmission-mode thickness measurements of stacked layers of 17μm thick low-density polyethylene (LDPE) films. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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14 pages, 1893 KiB  
Article
Discrete Fourier Transform Radar in the Terahertz-Wave Range Based on a Resonant-Tunneling-Diode Oscillator
by Hiroki Konno, Adrian Dobroiu, Safumi Suzuki, Masahiro Asada and Hiroshi Ito
Sensors 2021, 21(13), 4367; https://doi.org/10.3390/s21134367 - 25 Jun 2021
Cited by 11 | Viewed by 1800
Abstract
We used a resonant-tunneling-diode (RTD) oscillator as the source of a terahertz-wave radar based on the principle of the swept-source optical coherence tomography (SS-OCT). Unlike similar reports in the terahertz range, we apply the stepwise frequency modulation to a subcarrier obtained by amplitude [...] Read more.
We used a resonant-tunneling-diode (RTD) oscillator as the source of a terahertz-wave radar based on the principle of the swept-source optical coherence tomography (SS-OCT). Unlike similar reports in the terahertz range, we apply the stepwise frequency modulation to a subcarrier obtained by amplitude modulation instead of tuning the terahertz carrier frequency. Additionally, we replace the usual optical interference with electrical mixing and, by using a quadrature mixer, we can discriminate between negative and positive optical path differences, which doubles the measurement range without increasing the measurement time. To measure the distance to multiple targets simultaneously, the terahertz wave is modulated in amplitude at a series of frequencies; the signal returning from the target is detected and homodyne mixed with the original modulation signal. A series of voltages is obtained; by Fourier transformation the distance to each target is retrieved. Experimental results on one and two targets are shown. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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17 pages, 20163 KiB  
Article
Spatial Domain Terahertz Image Reconstruction Based on Dual Sparsity Constraints
by Xiaozhen Ren and Yuying Jiang
Sensors 2021, 21(12), 4116; https://doi.org/10.3390/s21124116 - 15 Jun 2021
Cited by 5 | Viewed by 1802
Abstract
Terahertz time domain spectroscopy imaging systems suffer from the problems of long image acquisition time and massive data processing. Reducing the sampling rate will lead to the degradation of the imaging reconstruction quality. To solve this issue, a novel terahertz imaging model, named [...] Read more.
Terahertz time domain spectroscopy imaging systems suffer from the problems of long image acquisition time and massive data processing. Reducing the sampling rate will lead to the degradation of the imaging reconstruction quality. To solve this issue, a novel terahertz imaging model, named the dual sparsity constraints terahertz image reconstruction model (DSC-THz), is proposed in this paper. DSC-THz fuses the sparsity constraints of the terahertz image in wavelet and gradient domains into the terahertz image reconstruction model. Differing from the conventional wavelet transform, we introduce a non-linear exponentiation transform into the shift invariant wavelet coefficients, which can amplify the significant coefficients and suppress the small ones. Simultaneously, the sparsity of the terahertz image in gradient domain is used to enhance the sparsity of the image, which has the advantage of edge preserving property. The split Bregman iteration scheme is utilized to tackle the optimization problem. By using the idea of separation of variables, the optimization problem is decomposed into subproblems to solve. Compared with the conventional single sparsity constraint terahertz image reconstruction model, the experiments verified that the proposed approach can achieve higher terahertz image reconstruction quality at low sampling rates. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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9 pages, 1397 KiB  
Communication
Detection of Microplastic in Salts Using Terahertz Time-Domain Spectroscopy
by Jaeseung Im, Taewon Goo, Jugyoung Kim, Soobong Choi, Sung Ju Hong and Young-Mi Bahk
Sensors 2021, 21(9), 3161; https://doi.org/10.3390/s21093161 - 02 May 2021
Cited by 10 | Viewed by 3362
Abstract
We report on a prototypical study of the detection of microplastic embedded in table salts by using terahertz time-domain spectroscopy. In the experiment, high-density polyethylene (HDPE) of sizes from 150 to 400 μm are used as a representative microplastic and mixed with table [...] Read more.
We report on a prototypical study of the detection of microplastic embedded in table salts by using terahertz time-domain spectroscopy. In the experiment, high-density polyethylene (HDPE) of sizes from 150 to 400 μm are used as a representative microplastic and mixed with table salts. Analyzing terahertz transmittance with an effective medium model, we extract various optical properties such as refractive index, absorption coefficient, and real/imaginary parts of the dielectric constant of the mixture. Consequently, the optical properties exhibit volume-ratio-dependence in 0.1–0.5 THz regimes. Especially, the refractive index and the real part of the dielectric constant possess monotonic frequency dependence, meaning that the quantities can be relevant indicators for the detection of the microplastic in terms of practical applications. Our work proves that terahertz time-domain spectroscopy can pave a way to recognize microplastic mixed with salts and be expanded for detecting various micro-sized particles. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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Other

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16 pages, 6070 KiB  
Perspective
Plasmonic Field-Effect Transistors (TeraFETs) for 6G Communications
by Michael Shur, Gregory Aizin, Taiichi Otsuji and Victor Ryzhii
Sensors 2021, 21(23), 7907; https://doi.org/10.3390/s21237907 - 27 Nov 2021
Cited by 23 | Viewed by 3087
Abstract
Ever increasing demands of data traffic makes the transition to 6G communications in the 300 GHz band inevitable. Short-channel field-effect transistors (FETs) have demonstrated excellent potential for detection and generation of terahertz (THz) and sub-THz radiation. Such transistors (often referred to as TeraFETs) [...] Read more.
Ever increasing demands of data traffic makes the transition to 6G communications in the 300 GHz band inevitable. Short-channel field-effect transistors (FETs) have demonstrated excellent potential for detection and generation of terahertz (THz) and sub-THz radiation. Such transistors (often referred to as TeraFETs) include short-channel silicon complementary metal oxide (CMOS). The ballistic and quasi-ballistic electron transport in the TeraFET channels determine the TeraFET response at the sub-THz and THz frequencies. TeraFET arrays could form plasmonic crystals with nanoscale unit cells smaller or comparable to the electron mean free path but with the overall dimensions comparable with the radiation wavelength. Such plasmonic crystals have a potential of supporting the transition to 6G communications. The oscillations of the electron density (plasma waves) in the FET channels determine the phase relations between the unit cells of a FET plasmonic crystal. Excited by the impinging radiation and rectified by the device nonlinearities, the plasma waves could detect both the radiation intensity and the phase enabling the line-of-sight terahertz (THz) detection, spectrometry, amplification, and generation for 6G communication. Full article
(This article belongs to the Special Issue Terahertz and Millimeter Wave Sensing and Applications)
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