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13 pages, 1902 KiB

Open AccessArticle

Design of an Optimized Terahertz Time-Domain Spectroscopy System Pumped by a 30 W Yb:KGW Source at a 100 kHz Repetition Rate with 245 fs Pulse Duration

by Lennart Hirsch, Dionysis Adamou, Daniele Faccio, Marco Peccianti and Matteo Clerici

Appl. Sci. 2024, 14(15), 6688; https://doi.org/10.3390/app14156688 - 31 Jul 2024

Viewed by 977

Abstract

Ytterbium laser sources are state-of-the-art systems that are increasingly replacing Ti:Sapphire lasers in most applications requiring high repetition rate pulse trains. However, extending these laser sources to THz Time-Domain Spectroscopy (THz-TDS) poses several challenges not encountered in conventional, lower-power systems. These challenges include [...] Read more.

Ytterbium laser sources are state-of-the-art systems that are increasingly replacing Ti:Sapphire lasers in most applications requiring high repetition rate pulse trains. However, extending these laser sources to THz Time-Domain Spectroscopy (THz-TDS) poses several challenges not encountered in conventional, lower-power systems. These challenges include pump rejection, thermal lensing in nonlinear media, and pulse durations exceeding 100 fs, which consequently limit the detection bandwidth in TDS applications. In this article, we describe our design of a THz-TDS beamline that seeks to address these issues. We report on the effectiveness of temperature controlling the Gallium Phosphide (GaP) used to generate the THz radiation and its impact on increasing the generation efficiency and aiding pump rejection while avoiding thermal distortions of the residual pump laser beam. We detail our approach to pump rejection, which can be implemented with off-the-shelf products and minimal customization. Finally, we describe our solution based on a commercial optical parametric amplifier to obtain a temporally compressed probe pulse of 55 fs duration. Our study will prove useful to the increasing number of laboratories seeking to move from the high-energy, low-power THz time-domain spectroscopy systems based on Ti:Sapphire lasers, to medium-energy, high-power systems driven by Yb-doped lasers. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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11 pages, 2224 KiB

Open AccessArticle

Characterization of Different Types of Crystallization from Cocoa Butter by Using Terahertz Spectroscopy

by Chao-Hui Feng, Chiko Otani and Hiromichi Hoshina

Appl. Sci. 2024, 14(1), 35; https://doi.org/10.3390/app14010035 - 20 Dec 2023

Cited by 3 | Viewed by 1353

Abstract

Three different cocoa butter polymorphs have been determined by X-ray diffraction (XRD) and terahertz spectroscopy in the range of 0–11 THz. Specific procedures to produce different types of crystallization were detailed. The results from XRD analysis showed that the three polymorphic forms of [...] Read more.

Three different cocoa butter polymorphs have been determined by X-ray diffraction (XRD) and terahertz spectroscopy in the range of 0–11 THz. Specific procedures to produce different types of crystallization were detailed. The results from XRD analysis showed that the three polymorphic forms of cocoa butter were α, β′ (III), and β (V) forms. Terahertz spectroscopy showed different features according to different types of crystallization forms. An observable sharp peak at 6.80 THz can be detected from β (V) type and original samples, whilst α type presented the broad peak at this frequency, respectively. Peaks at 4.25 THz and 5.21 THz were detected in β type (V) form, whilst no noticeable peaks were observed from other samples at those frequencies. This study innovatively showed a great potential to apply terahertz spectroscopy to control the tempering during chocolate manufacturing. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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16 pages, 5725 KiB

Open AccessArticle

CubeSat-Based Observations of Lunar Ice Water Using a 183 GHz Horn Antenna: Design and Optimization

by Vahid Rastinasab, Weidong Hu, Waseem Shahzad and Syed Muzahir Abbas

Appl. Sci. 2023, 13(16), 9364; https://doi.org/10.3390/app13169364 - 18 Aug 2023

Cited by 4 | Viewed by 1453

Abstract

Technological advancements have revolutionized the space industry, facilitating deep space exploration using CubeSats. One objective is to locate potential life-support elements, such as water, on extraterrestrial planets. Water possesses a distinct spectral signature at 183 GHz, useful in remote sensing and environmental monitoring [...] Read more.

Technological advancements have revolutionized the space industry, facilitating deep space exploration using CubeSats. One objective is to locate potential life-support elements, such as water, on extraterrestrial planets. Water possesses a distinct spectral signature at 183 GHz, useful in remote sensing and environmental monitoring applications. Detecting this signature provides crucial information about water and ice presence and distribution on celestial bodies, aiding future exploration and colonization efforts. Mostly in space remote sensing uses corrugated horn antennae due to high gain and radiation patterns but fabrication of corrugated antenna is very challenging or even impossible in some cases. To ease this challenge, in our research we propose ideas to transform a corrugated horn antenna into a smooth-walled design by using MATLAB Cubic smoothing Splines algorithms. We compare simulation results between smooth-walled and corrugated antennas, and we can see some improvements in insertion losses, Voltage Standing Wave ratio (VSWR), and gain. We also manufactured this 183 GHz antenna using a commercially available 3D printer by utilizing Acrylonitrile Butadiene Styrene (ABS) material. The antenna surface was then coated with a thin layer of copper using conductive paint. In the end, we practically evaluate smooth-walled antenna functionality and compare it with the theriacal results. Validating the antenna’s functionality proposes a cost-effective and accessible production method to be used in a CubeSat engineering model or university students’ project. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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18 pages, 9024 KiB

Open AccessArticle

Frequency Range Optimization for Continuous Wave Terahertz Imaging

by Blaž Pongrac, Andrej Sarjaš and Dušan Gleich

Appl. Sci. 2023, 13(2), 974; https://doi.org/10.3390/app13020974 - 11 Jan 2023

Cited by 2 | Viewed by 1786

Abstract

With shorter wavelengths than microwaves and greater penetration depth than infrared light, waves in the terahertz spectrum offer unique material testing opportunities. Terahertz technology offers non-invasive and non-destructive testing in the form of spectroscopy and imaging. The most used systems for terahertz imaging [...] Read more.

With shorter wavelengths than microwaves and greater penetration depth than infrared light, waves in the terahertz spectrum offer unique material testing opportunities. Terahertz technology offers non-invasive and non-destructive testing in the form of spectroscopy and imaging. The most used systems for terahertz imaging are time-domain spectroscopy systems. However, frequency domain spectroscopy systems could offer excellent frequency resolution and be more suitable for biomedical applications. Terahertz imaging based on frequency domain spectroscopy systems is slow, and suffers from frequency tuning errors. A novel one-dimensional imaging principle is presented in this paper. In addition, frequency range optimization based on convolutional neural networks and occlusion sensitivity is utilized for frequency range optimization. Frequency range optimization is used to determine the optimal frequency range for data acquisition. The optimal frequency range or bandwidth should be wide enough for effective phase detection, and should be at the intersection of several spectral footprints in the observed medium. The intersection of spectral footprints is estimated using the proposed frequency range optimization algorithm based on a convolutional neural network and occlusion sensitivity algorithm. The proposed algorithm selects the most sensitive frequency band of THz spectrum automatically, and enables very fast acquisitions for object inspection and classification. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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12 pages, 29389 KiB

Open AccessArticle

Measuring Maritime Paint Thickness under Water Using THz Cross-Correlation Spectroscopy

by Johan Østergaard Knarreborg, Jonathan Hjortshøj-Nielsen, Bjørn Hübschmann Mølvig, Thorsten Bæk, Peter Uhd Jepsen and Simon Jappe Lange

Appl. Sci. 2022, 12(23), 12397; https://doi.org/10.3390/app122312397 - 4 Dec 2022

Cited by 2 | Viewed by 1896

Abstract

The shipping industry is a major contributor to global greenhouse gas (GHG) emissions, which is why it is important to optimize every aspect of the efficiency of ocean-going vessels. This includes the antifouling paint that ensures hydrodynamic efficiency. Measuring the thickness of the [...] Read more.

The shipping industry is a major contributor to global greenhouse gas (GHG) emissions, which is why it is important to optimize every aspect of the efficiency of ocean-going vessels. This includes the antifouling paint that ensures hydrodynamic efficiency. Measuring the thickness of the antifouling on top of all other paint layers using THz cross-correlation spectroscopy (THz-CCS) underwater could enable vessel operators to monitor the state of the paint on ship hulls and plan any vessel’s sailing route and maintenance optimally. However, due to the high absorption of water in the THz domain, measuring through any significant amount of water is impossible, making a water removal method necessary. This study shows how a THz-CCS system can be packaged for underwater measurements using a molded silicone contact seal. In combination with a spectroscopic model for data treatment, the thickness of a single paint layer is retrieved underwater. This paves the way for a more advanced system capable of measuring multilayer maritime paint underwater, which will enable shipping companies to continuously monitor the paint layers’ thickness. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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14 pages, 1281 KiB

Open AccessArticle

Cubic Nonlinearity of Tellurite and Chalcogenide Glasses: Terahertz-Field-Induced Second Harmonic Generation vs. Optical Kerr Effect

by Sergey Bodrov, Yuriy Sergeev, Ekaterina Burova, Aleksey Korytin, Aleksey Murzanev, Alexander Romashkin and Andrey Stepanov

Appl. Sci. 2022, 12(22), 11608; https://doi.org/10.3390/app122211608 - 15 Nov 2022

Cited by 2 | Viewed by 1556

Abstract

Third-order nonlinear susceptibilities

χ^{(3)} (- 2 ω; ω, ω, 0)

and

χ^{(3)} (- ω; ω, - ω, ω)

responsible for electric-field-induced second harmonic generation and the optical Kerr effect were measured and directly compared for [...] Read more.

Third-order nonlinear susceptibilities

χ^{(3)} (- 2 ω; ω, ω, 0)

and

χ^{(3)} (- ω; ω, - ω, ω)

responsible for electric-field-induced second harmonic generation and the optical Kerr effect were measured and directly compared for tellurite and chalcogenide glasses. The nonlinear coefficients were found by measuring the second harmonic radiation from samples under the action of an external field of terahertz pulses and by the classical z-scan technique, respectively. The influence of ambient air and helium gas on second harmonic generation was analyzed. It was demonstrated that both susceptibilities

χ^{(3)} (- 2 ω; ω, ω, 0)

and

χ^{(3)} (- ω; ω, - ω, ω)

have close absolute values, which shows the dominant role of nonresonant electronic nonlinearity in the studied glasses. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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19 pages, 7276 KiB

Open AccessArticle

VOC-Net: A Deep Learning Model for the Automated Classification of Rotational THz Spectra of Volatile Organic Compounds

by M. Arshad Zahangir Chowdhury, Timothy E. Rice and Matthew A. Oehlschlaeger

Appl. Sci. 2022, 12(17), 8447; https://doi.org/10.3390/app12178447 - 24 Aug 2022

Cited by 9 | Viewed by 2404

Abstract

Conventional black box machine learning (ML) algorithms for gas-phase species identification from THz frequency region absorption spectra have been reported in the literature. While the robust classification performance of such ML models is promising, the black box nature of these ML tools limits [...] Read more.

Conventional black box machine learning (ML) algorithms for gas-phase species identification from THz frequency region absorption spectra have been reported in the literature. While the robust classification performance of such ML models is promising, the black box nature of these ML tools limits their interpretability and acceptance in application. Here, a one-dimensional convolutional neural network (CNN), VOC-Net, is developed and demonstrated for the classification of absorption spectra for volatile organic compounds (VOCs) in the THz frequency range, specifically from 220 to 330 GHz where prior experimental data is available. VOC-Net is trained and validated against simulated spectra, and also demonstrated and tested against experimental spectra. The performance of VOC-Net is examined by the consideration of confusion matrices and receiver-operator-characteristic (ROC) curves. The model is shown to be 99+% accurate for the classification of simulated spectra and 97% accurate for the classification of noisy experimental spectra. The model’s internal logic is examined using the Gradient-weighted Class Activation Mapping (Grad-CAM) method, which provides a visual and interpretable explanation of the model’s decision making process with respect to the important distinguishing spectral features. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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13 pages, 8814 KiB

Open AccessArticle

Non-Imaging Fall Detection Based on Spectral Signatures Obtained Using a Micro-Doppler Millimeter-Wave Radar

by Yael Balal, Afik Yarimi and Nezah Balal

Appl. Sci. 2022, 12(16), 8178; https://doi.org/10.3390/app12168178 - 16 Aug 2022

Cited by 2 | Viewed by 1885

Abstract

Falls are the leading cause of accidents among the elderly population. In recent years, radar has been employed in fall detection due to its superior sensing capabilities, small dimensions, low cost and primarily non-intrusive sensing capabilities in addition to its robustness under a [...] Read more.

Falls are the leading cause of accidents among the elderly population. In recent years, radar has been employed in fall detection due to its superior sensing capabilities, small dimensions, low cost and primarily non-intrusive sensing capabilities in addition to its robustness under a range of heat and lighting conditions. In this paper, we present a technique for identifying when a person is falling using a low-power millimeter-wave radar operating in the W-band. This detection, conducted in real time, is based on the transmission of a continuous wave and heterodyning of the received signal reflected from the person to obtain micro-Doppler shifts associated with the person’s motion. These results make it possible to obtain a high-quality time-frequency distribution and spectrogram, from which the person’s unique fall movement characteristics can be determined. In this paper, we present experimental results based on 94 GHz real radar data obtained from a falling person. This carrier frequency is higher than that of current systems, allowing higher frequency resolution and more accurate results. Compared to other tracking systems, this sensor does not simulate or violate privacy. However, the high-frequency system enables high-resolution realizations with high reliability. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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8 pages, 1472 KiB

Open AccessArticle

Detection of Polystyrene Microplastic Particles in Water Using Surface-Functionalized Terahertz Microfluidic Metamaterials

by Sae June Park and Yeong Hwan Ahn

Appl. Sci. 2022, 12(14), 7102; https://doi.org/10.3390/app12147102 - 14 Jul 2022

Cited by 7 | Viewed by 2449

Abstract

We propose a novel method for detecting microplastic particles in water using terahertz metamaterials. Fluidic channels are employed to flow the water, containing polystyrene spheres, on the surface of the metamaterials. Polystyrene spheres are captured only near the gap structure of the metamaterials [...] Read more.

We propose a novel method for detecting microplastic particles in water using terahertz metamaterials. Fluidic channels are employed to flow the water, containing polystyrene spheres, on the surface of the metamaterials. Polystyrene spheres are captured only near the gap structure of the metamaterials as the gap areas are functionalized. The resonant frequency of terahertz metamaterials increased while we circulated the microplastic solution, as polystyrene spheres in the solution are attached to the metamaterial gap areas, which saturates at a specific frequency as the gap areas are filled by the polystyrene spheres. Experimental results were revisited and supported by finite-difference time-domain simulations. We investigated how this method can be used for the detection of microplastics with various solution densities. The saturation time of the resonant frequency shift was found to decrease, while the saturated resonant frequency shift increased as the solution density increased. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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8 pages, 6807 KiB

Open AccessBrief Report

A Bunch of YBCO Josephson Generators for the Analysis of Resonant Cold-Electron Bolometers

by Leonid S. Revin, Dmitry V. Masterov, Alexey E. Parafin, Sergey A. Pavlov, Dmitry A. Pimanov, Alexander V. Chiginev, Anton V. Blagodatkin, Igor V. Rakut’, Evgenii V. Skorokhodov, Anna V. Gordeeva and Andrey L. Pankratov

Appl. Sci. 2022, 12(23), 11960; https://doi.org/10.3390/app122311960 - 23 Nov 2022

Cited by 3 | Viewed by 1285

Abstract

The resonant properties of Cold-Electron Bolometers (CEBs) located at a 0.3 K cryostat plate are measured using a 50 μm long high-temperature YBa₂Cu₃O

_{7 - δ}

(YBCO) Josephson junction oscillator, placed on a 2.7 K plate of the same [...] Read more.

The resonant properties of Cold-Electron Bolometers (CEBs) located at a 0.3 K cryostat plate are measured using a 50 μm long high-temperature YBa₂Cu₃O

_{7 - δ}

(YBCO) Josephson junction oscillator, placed on a 2.7 K plate of the same cryostat. For these purposes, a bunch of YBCO Josephson oscillators with various lengths of dipole antennas and overlapping generation bands has been developed and investigated in 50–500 GHz frequency range. Two setups of Josephson junction placement were compared, and as a result, various narrow-band receiving systems with CEBs have been measured, demonstrating the feasibility of the presented approach. Full article

(This article belongs to the Special Issue Applications of Terahertz Sensing and Imaging)

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