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Special Issue "Optical Spectroscopy, Sensing, and Imaging from UV to THz Range"

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

Deadline for manuscript submissions: closed (30 June 2020).

Special Issue Editors

Prof. Dr. Vincenzo Spagnolo
Website SciProfiles
Guest Editor
PolySense Lab—Department of Physics, Politecnico di Bari, Via Amendola 173, 70126 Bari, Italy
Interests: optoacoustic gas sensing; quantum cascade lasers; spectroscopic techniques for real-time device monitoring; thermal modeling of optoelectronic devices
Special Issues and Collections in MDPI journals
Prof. Dr. Weidong Chen
Website
Guest Editor
Laboratoire de Physico-Chimie de l'Atmosphère, Université du Littoral Côte d'Opale, 189A, Avenue Maurice Schumann 59140 Dunkerque, France
Interests: developments of photonic instrumentation involving high-sensitivity spectroscopic techniques for applied spectroscopy; optical metrology of key atmospheric species (trace gases and aerosols) for applications in atmospheric and environmental sciences; optical parametric laser source generation by frequency conversion and its applications to applied spectroscopy
Special Issues and Collections in MDPI journals
Prof. Dr. Lei Dong
Website
Guest Editor
Institute of Laser Spectroscopy, Shanxi University, 92 Wucheng Road, Taiyuan, Shanxi 030006, China
Interests: development in laser spectroscopy applied to sensitive and selective trace gas detection and laser applications in industrial process control; chemical analysis; medical diagnostics and environmental monitoring
Prof. Dr. Pietro Patimisco
Website
Guest Editor
PolySense Lab—Department of Physics, University of Bari, Via Amendola 173, 70126 Bari, Italy
Interests: photoacoustic spectroscopy; terahertz sources; photoluminescence; cavity-enhanced absorption spectroscopy; quartz tuning fork; optical waveguide; Raman spectroscopy

Special Issue Information

Dear Colleagues,

The recent advancements in optical sources and detectors has opened up new opportunities for sensing and imaging techniques and applications. This 2019 Special Issue of the journal Sensors entitled “Optical Spectroscopy, Sensing, and Imaging from UV to THz Range” will focus on all aspects of the research and development related to these areas. Original research papers that focus on the development of optical sources for sensing, the design and experimental verification of new sensors and imaging systems, as well as papers that focus on their field testing and campaign measurement are welcome. Both reviews and original research articles will be published. Reviews should provide an up-to-date well-balanced overview of the current state-of-the-art in a particular field and include main results from other groups. We look forward to and welcome your participation in this Special Issue.

Prof. Dr. Vincenzo Spagnolo
Prof. Dr. Weidong Chen
Prof. Dr. Lei Dong
Dr. Pietro Patimisco
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Sensors is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • optical sources for sensing
  • sensing mechanisms
  • gas and liquid sensors
  • fiber optic sensors
  • chemical sensors
  • bio-medical sensors
  • physical sensors
  • imaging
  • spectral imaging
  • multispectral imaging
  • imaging spectroscopy

Published Papers (13 papers)

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Research

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Open AccessArticle
High-Throughput Screening of Free Proline Content in Rice Leaf under Cadmium Stress Using Hyperspectral Imaging with Chemometrics
Sensors 2020, 20(11), 3229; https://doi.org/10.3390/s20113229 - 05 Jun 2020
Abstract
Tracking of free proline (FP)—an indicative substance of heavy metal stress in rice leaf—is conducive to improve plant phenotype detection, which has important guiding significance for precise management of rice production. Hyperspectral imaging was used for high-throughput screening FP in rice leaves under [...] Read more.
Tracking of free proline (FP)—an indicative substance of heavy metal stress in rice leaf—is conducive to improve plant phenotype detection, which has important guiding significance for precise management of rice production. Hyperspectral imaging was used for high-throughput screening FP in rice leaves under cadmium (Cd) stress with five concentrations and four periods. The average spectral of rice leaves were used to show differences in optical properties. Partial least squares (PLS), least-squares support vector machine (LS-SVM) and extreme learning machine (ELM) models based on full spectra and effective wavelengths were established to detect FP content. Genetic algorithm (GA), competitive adaptive weighted sampling (CARS) and PLS weighting regression coefficient (Bw) were compared to screen the most effective wavelengths. Distribution map of the FP content in rice leaves were obtained to display the changes in the FP of leaves visually. The results illustrated that spectral differences increased with Cd stress time and FP content increased with Cd stress concentration. The best result for FP detection is the ELM model based on 27 wavelengths selected by CARS and Rp is 0.9426. Undoubtedly, hyperspectral imaging combined with chemometrics was a rapid, cost effective and non-destructive technique to excavate changes of FP in rice leaves under Cd stress. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
Environmental Monitoring of Methane with Quartz-Enhanced Photoacoustic Spectroscopy Exploiting an Electronic Hygrometer to Compensate the H2O Influence on the Sensor Signal
Sensors 2020, 20(10), 2935; https://doi.org/10.3390/s20102935 - 22 May 2020
Cited by 2
Abstract
A dual-gas sensor based on the combination of a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor and an electronic hygrometer was realized for the simultaneous detection of methane (CH4) and water vapor (H2O) in air. The QEPAS sensor employed an interband [...] Read more.
A dual-gas sensor based on the combination of a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor and an electronic hygrometer was realized for the simultaneous detection of methane (CH4) and water vapor (H2O) in air. The QEPAS sensor employed an interband cascade laser operating at 3.34 μm capable of targeting a CH4 absorption line at 2988.8 cm−1 and a water line at 2988.6 cm−1. Water vapor was measured with both the electronic hygrometer and the QEPAS sensor for comparison. The measurement accuracy provided by the hygrometer enabled the adjustment of methane QEPAS signal with respect to the water vapor concentration to retrieve the actual CH4 concentration. The sensor was tested by performing prolonged measurements of CH4 and H2O over 60 h to demonstrate the effectiveness of this approach for environmental monitoring applications. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
Free-Standing Complementary Asymmetric Metasurface for Terahertz Sensing Applications
Sensors 2020, 20(8), 2265; https://doi.org/10.3390/s20082265 - 16 Apr 2020
Cited by 1
Abstract
We designed and tested a highly sensitive metasurface device based on free-standing complementary asymmetric split-ring resonators at terahertz frequencies. It is utilized for sensing a galactose film. We characterized the device using the induced red shift of a Fano resonance observed in the [...] Read more.
We designed and tested a highly sensitive metasurface device based on free-standing complementary asymmetric split-ring resonators at terahertz frequencies. It is utilized for sensing a galactose film. We characterized the device using the induced red shift of a Fano resonance observed in the THz transmission. The sensor has a high sensitivity of 91.7 GHz/RIU due to a significant interaction between the galactose overlayer and the metasurface. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
Inspection of Trivalent Chromium Conversion Coatings Using Laser Light: The Unexpected Role of Interference on Cold-Rolled Aluminium
Sensors 2020, 20(8), 2164; https://doi.org/10.3390/s20082164 - 11 Apr 2020
Abstract
Laser-based inspection of trivalent chromium conversion coatings on rough, cold-rolled aluminium substrates is studied from a basic physics perspective by means of angle and wavelength dependent measurements. As a result, we show that the correlation between the scattered laser light and the coating [...] Read more.
Laser-based inspection of trivalent chromium conversion coatings on rough, cold-rolled aluminium substrates is studied from a basic physics perspective by means of angle and wavelength dependent measurements. As a result, we show that the correlation between the scattered laser light and the coating weight of the conversion layer is dominated by the phenomenon of interference. The combined experimental and numerical approach of our study is based on an appropriate layer model which was developed from a set of reference measurements of confocal microscopy, electron microscopy and X-ray photoelectron spectroscopy. The aluminium alloy AA3003 with a trivalent chromium conversion coating serves as an example. Our derived model is capable to reconstruct the reflectance of a laser beam at grazing incidence even for a pronounced surface roughness of R q 300   n m , for different coating thicknesses less than 70 n m corresponding to coating weights between zero and 0.5 g / m 2 and for laser wavelengths from 405 n m to 785 n m . In our discussion we outline the possibility to transfer the results to other aluminium alloys and/or other metallic substrates, as well as their potential for industrial applications such as 100% inline-capability, costs, velocity and ruggedness. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
Highly Sensitive Photoacoustic Microcavity Gas Sensor for Leak Detection
Sensors 2020, 20(4), 1164; https://doi.org/10.3390/s20041164 - 20 Feb 2020
Cited by 3
Abstract
A highly sensitive photoacoustic (PA) microcavity gas sensor for leak detection is proposed. The miniature and low-cost gas sensor mainly consisted of a micro-electro-mechanical system (MEMS) microphone and a stainless-steel capillary with two small holes opened on the side wall. Different from traditional [...] Read more.
A highly sensitive photoacoustic (PA) microcavity gas sensor for leak detection is proposed. The miniature and low-cost gas sensor mainly consisted of a micro-electro-mechanical system (MEMS) microphone and a stainless-steel capillary with two small holes opened on the side wall. Different from traditional PA sensors, the designed low-power sensor had no gas valves and pumps. Gas could diffuse into the stainless-steel PA microcavity from two holes. The volume of the cavity in the sensor was only 7.9 μL. We use a 1650.96 nm distributed feedback (DFB) laser and the second-harmonic wavelength modulation spectroscopy (2f-WMS) method to measure PA signals. The measurement result of diffused methane (CH4) gas shows a response time of 5.8 s and a recovery time of 5.2 s. The detection limit was achieved at 1.7 ppm with a 1-s lock-in integral time. In addition, the calculated normalized noise equivalent absorption (NNEA) coefficient was 1.2 × 10−8 W·cm−1·Hz−1/2. The designed PA microcavity sensor can be used for the early warning of gas leakage. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
Application of Micro Quartz Tuning Fork in Trace Gas Sensing by Use of Quartz-Enhanced Photoacoustic Spectroscopy
Sensors 2019, 19(23), 5240; https://doi.org/10.3390/s19235240 - 28 Nov 2019
Cited by 2
Abstract
A novel quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a micro quartz tuning fork (QTF) is reported. As a photoacoustic transducer, a novel micro QTF was 3.7 times smaller than the usually used standard QTF, resulting in a gas sampling volume of ~0.1 [...] Read more.
A novel quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a micro quartz tuning fork (QTF) is reported. As a photoacoustic transducer, a novel micro QTF was 3.7 times smaller than the usually used standard QTF, resulting in a gas sampling volume of ~0.1 mm3. As a proof of concept, water vapor in the air was detected by using 1.39 μm distributed feedback (DFB) laser. A detailed analysis of the performance of a QEPAS sensor based on the micro QTF was performed by detecting atmosphere H2O. The laser focus position and the laser modulation depth were optimized to improve the QEPAS excitation efficiency. A pair of acoustic micro resonators (AmRs) was assembled with the micro QTF in an on-beam configuration to enhance the photoacoustic signal. The AmRs geometry was optimized to amplify the acoustic resonance. With a 1 s integration time, a normalized noise equivalent absorption coefficient (NNEA) of 1.97 × 10−8 W·cm−1·Hz−1/2 was achieved when detecting H2O at less than 1 atm. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
A Sensitive Carbon Dioxide Sensor Based on Photoacoustic Spectroscopy with a Fixed Wavelength Quantum Cascade Laser
Sensors 2019, 19(19), 4187; https://doi.org/10.3390/s19194187 - 26 Sep 2019
Cited by 3
Abstract
A photoacoustic spectroscopy (PAS) based carbon dioxide (CO2) sensor with a fixed wavelength quantum cascade laser (FW-QCL) was demonstrated. The emission wavelength of the FW-QCL at 4.42 μm in the mid-infrared spectral region matched a fundamental CO2 absorption line. Amplitude [...] Read more.
A photoacoustic spectroscopy (PAS) based carbon dioxide (CO2) sensor with a fixed wavelength quantum cascade laser (FW-QCL) was demonstrated. The emission wavelength of the FW-QCL at 4.42 μm in the mid-infrared spectral region matched a fundamental CO2 absorption line. Amplitude modulation of the laser intensity was used to match the resonant photoacoustic (PA) cell. The noise from the background was reduced with the correlation demodulation technique. The experimental results showed that the sensor had excellent signal stability and a concentration linear response. When the integration time was 1 s, a 1σ minimum detection limit (MDL) of 2.84 parts per million (ppm) for CO2 detection was achieved. The long-term stability of the sensor was evaluated by means of an Allan deviation analysis. With an integration time of ~100 s, the MDL was improved to 1 ppm. This sensor was also used to measure the CO2 concentration from some common emission sources, such as cigarette smoking, automobile exhaust, and the combustion of some carbon-containing materials, which confirmed the stability and robustness of the reported FW-QCL based CO2-PAS sensor system. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
Acoustic Coupling between Resonator Tubes in Quartz-Enhanced Photoacoustic Spectrophones Employing a Large Prong Spacing Tuning Fork
Sensors 2019, 19(19), 4109; https://doi.org/10.3390/s19194109 - 23 Sep 2019
Cited by 7
Abstract
A theoretical model describing the acoustic coupling between two resonator tubes in spectrophones exploiting custom-made quartz tuning forks (QTFs) is proposed. The model is based on an open-end correction to predict the optimal tube length. A calculation of the sound field distribution from [...] Read more.
A theoretical model describing the acoustic coupling between two resonator tubes in spectrophones exploiting custom-made quartz tuning forks (QTFs) is proposed. The model is based on an open-end correction to predict the optimal tube length. A calculation of the sound field distribution from one tube exit allowed for the estimation of the optimal radius as a function of the QTF prong spacing and the sound wavelength. The theoretical predictions have been confirmed using experimental studies employing a custom QTF with a fundamental flexural mode resonance frequency of 15.8 kHz and a quality factor of 15,000 at atmospheric pressure. The spacing between the two prongs was 1.5 mm. Spectrophones mounting this QTF were implemented for the quartz-enhanced photoacoustic detection of water vapor in air in the mid-infrared spectral range. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
Identification of Soybean Varieties Using Hyperspectral Imaging Coupled with Convolutional Neural Network
Sensors 2019, 19(19), 4065; https://doi.org/10.3390/s19194065 - 20 Sep 2019
Cited by 3
Abstract
Soybean variety is connected to stress resistance ability, as well as nutritional and commercial value. Near-infrared hyperspectral imaging was applied to classify three varieties of soybeans (Zhonghuang37, Zhonghuang41, and Zhonghuang55). Pixel-wise spectra were extracted and preprocessed, and average spectra were also obtained. Convolutional [...] Read more.
Soybean variety is connected to stress resistance ability, as well as nutritional and commercial value. Near-infrared hyperspectral imaging was applied to classify three varieties of soybeans (Zhonghuang37, Zhonghuang41, and Zhonghuang55). Pixel-wise spectra were extracted and preprocessed, and average spectra were also obtained. Convolutional neural networks (CNN) using the average spectra and pixel-wise spectra of different numbers of soybeans were built. Pixel-wise CNN models obtained good performance predicting pixel-wise spectra and average spectra. With the increase of soybean numbers, performances were improved, with the classification accuracy of each variety over 90%. Traditionally, the number of samples used for modeling is large. It is time-consuming and requires labor to obtain hyperspectral data from large batches of samples. To explore the possibility of achieving decent identification results with few samples, a majority vote was also applied to the pixel-wise CNN models to identify a single soybean variety. Prediction maps were obtained to present the classification results intuitively. Models using pixel-wise spectra of 60 soybeans showed equivalent performance to those using the average spectra of 810 soybeans, illustrating the possibility of discriminating soybean varieties using few samples by acquiring pixel-wise spectra. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
Geometric Aberration Theory of Offner Imaging Spectrometers
Sensors 2019, 19(18), 4046; https://doi.org/10.3390/s19184046 - 19 Sep 2019
Cited by 1
Abstract
A third-order aberration theory has been developed for the Offner imaging spectrometer comprising an extended source; two concave mirrors; a convex diffraction grating; and an image plane. Analytic formulas of the spot diagram are derived for tracing rays through the system based on [...] Read more.
A third-order aberration theory has been developed for the Offner imaging spectrometer comprising an extended source; two concave mirrors; a convex diffraction grating; and an image plane. Analytic formulas of the spot diagram are derived for tracing rays through the system based on Fermat’s principle. The proposed theory can be used to discuss in detail individual aberrations of the system such as coma, spherical aberration and astigmatism, and distortion together with the focal conditions. It has been critically evaluated as well in a comparison with exact ray tracing constructed using the commercial software ZEMAX. In regard to the analytic formulas, the results show a high degree of practicality. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Open AccessArticle
Influence of Tuning Fork Resonance Properties on Quartz-Enhanced Photoacoustic Spectroscopy Performance
Sensors 2019, 19(18), 3825; https://doi.org/10.3390/s19183825 - 04 Sep 2019
Cited by 1
Abstract
A detailed investigation of the influence of quartz tuning forks (QTFs) resonance properties on the performance of quartz-enhanced photoacoustic spectroscopy (QEPAS) exploiting QTFs as acousto-electric transducers is reported. The performance of two commercial QTFs with the same resonance frequency (32.7 KHz) but different [...] Read more.
A detailed investigation of the influence of quartz tuning forks (QTFs) resonance properties on the performance of quartz-enhanced photoacoustic spectroscopy (QEPAS) exploiting QTFs as acousto-electric transducers is reported. The performance of two commercial QTFs with the same resonance frequency (32.7 KHz) but different geometries and two custom QTFs with lower resonance frequencies (2.9 KHz and 7.2 KHz) were compared and discussed. The results demonstrated that the fundamental resonance frequency as well as the quality factor and the electrical resistance were strongly inter-dependent on the QTF prongs geometry. Even if the resonance frequency was reduced, the quality factor must be kept as high as possible and the electrical resistance as low as possible in order to guarantee high QEPAS performance. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Review

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Open AccessReview
Photoacoustic-Based Gas Sensing: A Review
Sensors 2020, 20(9), 2745; https://doi.org/10.3390/s20092745 - 11 May 2020
Cited by 1
Abstract
The use of the photoacoustic effect to gauge the concentration of gases is an attractive alternative in the realm of optical detection methods. Even though the effect has been applied for gas sensing for almost a century, its potential for ultra-sensitive and miniaturized [...] Read more.
The use of the photoacoustic effect to gauge the concentration of gases is an attractive alternative in the realm of optical detection methods. Even though the effect has been applied for gas sensing for almost a century, its potential for ultra-sensitive and miniaturized devices is still not fully explored. This review article revisits two fundamentally different setups commonly used to build photoacoustic-based gas sensors and presents some distinguished results in terms of sensitivity, ultra-low detection limits, and miniaturization. The review contrasts the two setups in terms of the respective possibilities to tune the selectivity, sensitivity, and potential for miniaturization. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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Other

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Open AccessLetter
Reducing Afterpulsing in InGaAs(P) Single-Photon Detectors with Hybrid Quenching
Sensors 2020, 20(16), 4384; https://doi.org/10.3390/s20164384 - 06 Aug 2020
Abstract
High detection efficiency appears to be associated with a high afterpulse probability for InP-based single-photon avalanche diodes. In this paper, we present a new hybrid quenching technique that combines the advantages of both fast active quenching and high-frequency gated-passive quenching, with the aim [...] Read more.
High detection efficiency appears to be associated with a high afterpulse probability for InP-based single-photon avalanche diodes. In this paper, we present a new hybrid quenching technique that combines the advantages of both fast active quenching and high-frequency gated-passive quenching, with the aim of suppressing higher-order afterpulsing effects. Our results showed that the hybrid quenching method contributed to a 10% to 85% reduction of afterpulses with a gate-free detection efficiency of 4% to 10% at 1.06 μm, with 40 ns dead time, compared with the counter-based hold-off method. With the improvement of the afterpulsing performance of high-frequency gated single-photon detectors, especially at relatively high average detection efficiencies with wide gate widths, the proposed method enables their use as high-performance free-running detectors. Full article
(This article belongs to the Special Issue Optical Spectroscopy, Sensing, and Imaging from UV to THz Range)
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