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Recent Advances in THz Sensing and Imaging
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Recent Advances in THz Sensing and Imaging

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

Deadline for manuscript submissions: 25 June 2025 | Viewed by 2398

Special Issue Editors

Dr. Maxime Bernier

E-Mail Website
Guest Editor
Center for Radiofrequencies, Optics and Microelectronics in the Alpes, Chambery, France
Interests: THz spectroscopy and imaging; modelization of THz signatures; THz system metrology
Dr. Gwenael Gaborit

E-Mail Website
Guest Editor
Center for Radiofrequencies, Optics and Microelectronics in the Alpes, Chambery, France
Interests: THz generation; THz detection; E-field measurement; UWB; non-linear optics
Dr. Pierre-Baptiste Vigneron

E-Mail Website
Guest Editor
Center for Radiofrequencies, Optics and Microelectronics in the Alpes, Chambery, France
Interests: THz generation; THz detection; non-linear optics

Special Issue Information

Dear Colleagues,

At present, THz science is sufficiently mature to address issues in an increasing number of application domains. Moreover, THz technologies (emitters, detectors, imagers, all-in-one…) are becoming sophisticated and performant enough to propose miscellaneous technical solutions for academics and industries (and soon, for everyone). Nevertheless, there are so many fundamental and technological advances and discoveries to come that we are undoubtedly entering an extraordinary era for THz. This Special Issue deals with THz (i) device developments, (ii) signal processing, and (iii) application.

Colleagues, we know that you have been working hard to gain deeper knowledge of THz–matter interactions in sensing applications or nondestructive testing. Some of you may focus more specifically on modelling methods for these interactions in order to either improve existing measurement techniques/devices or to develop innovative ones. This Special Issue is a great opportunity for all of us to disseminate the recent advances in THz science and technologies dedicated to sensing and imaging applications.

Dr. Maxime Bernier
Dr. Gwenael Gaborit
Dr. Pierre-Baptiste Vigneron
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 submissions that pass pre-check are 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 2600 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

  • gas sensing
  • nondestructive testing
  • THz prospection
  • subwavelength imaging
  • THz image processing
  • sensing and modeling of heterogeneous samples
  • QCL THz
  • THz-QWIP
  • THz lab-on-chip
  • THz detectors

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Further information on MDPI's Special Issue policies can be found here.

Published Papers (3 papers)

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Research

16 pages, 24312 KiB  
Article
Fast Terahertz Reflection Imaging for In-Line Detection of Delaminations in Glass Fiber-Reinforced Polymers
by Peter Fosodeder, Michael Pfleger, Kausar Rahman, Tom Dutton, Sophie Cozien-Cazuc, Sandrine van Frank and Christian Rankl
Sensors 2025, 25(3), 851; https://doi.org/10.3390/s25030851 - 30 Jan 2025
Viewed by 512
Abstract
Terahertz (THz) is an emerging technology particularly well suited for the non-destructive investigation of inner structures in polymers. To realize its full potential, THz imaging systems adapted to industrial constraints as well as more application studies in areas of interest are needed. In [...] Read more.
Terahertz (THz) is an emerging technology particularly well suited for the non-destructive investigation of inner structures in polymers. To realize its full potential, THz imaging systems adapted to industrial constraints as well as more application studies in areas of interest are needed. In this work, we present a fast and flexible THz imaging system comprising hardware and software and demonstrate its capabilities for the investigation of defects in glass fiber-reinforced polymers (GFRPs), particularly for the detection of drilling-induced delaminations. Measurement data obtained by raster scanning of GFRP samples are gathered in 3D volumetric images. THz images of the drilled holes are then compared to reference images of the same holes obtained from X-ray computed tomography measurements. We show that THz imaging is capable of identifying not only artificial defects in the form of aluminum and Teflon inlays, but also real defects such as delaminations generated by drilling operations, and is suitable for non-destructive testing in industrial conditions. Full article
(This article belongs to the Special Issue Recent Advances in THz Sensing and Imaging)
►▼ Show Figures

Figure 1

Figure 1
<p>Drilled GFRP plates: (<b>a</b>) 3 mm thick plate, (<b>b</b>) 6 mm thick plate. Dashed black boxes: holes drilled with the same drill bit sequentially from the top left corner to bottom right corner. Dashed red boxes: sections cut out afterwards to perform XCT measurements.</p> Full article ">Figure 2
<p>THz reflection imaging scheme with 2 PCAs (emitter and detector) and 4 off-axis parabolic mirror guiding and focusing the THz beam onto the sample.</p> Full article ">Figure 3
<p>Interaction of THz radiation with a delamination: snapshots of a simulation of a THz pulse propagating towards a GFRP plate (dark area) with a delamination (small slit). (<b>a</b>) THz pulse traveling from the emitter (Tx) towards the GFRP plate (dark area). (<b>b</b>) Incoming THz pulse partially reflected at the top surface of the GFRP plate and the delamination. The image shows both partial reflections propagating towards the detector (Rx) after coupling out of the sample again.</p> Full article ">Figure 4
<p>Scanning trajectory and real-time visualization. (<b>a</b>) Schematic of the GFRP plate’s drilling process and scanning trajectory in the <math display="inline"><semantics> <mrow> <mi>x</mi> <mo>−</mo> <mi>y</mi> </mrow> </semantics></math> plane. Scan performed continuously. (<b>b</b>) Screenshot of the imaging software interface during scanning, showing the live visualization of the measurement data. The left side of the interface allows to set the scanning parameters and control the axis movement. On the right side, the current B-scan and the progress of two C-scans showing the pulse amplitude and time delay.</p> Full article ">Figure 5
<p>Optical detection of a delamination by reflection imaging. (<b>a</b>) Schematic image of a drilled hole in a GFRP plate including a delamination (dark area). The THz radiation propagates from the emitter (Tx) into the sample (black dashed arrow) and generates partial reflections (blue arrows) at interfaces that propagate back towards the detector (Rx). (<b>b</b>) Raw time-domain THz measurement signal (blue) and its Hilbert transform (orange).</p> Full article ">Figure 6
<p>Signal processing algorithm. The raw data produced by the imaging system (i.e., measurement positions and THz time-domain signals) are processed by a low-pass and moving average filter. The 3D volumetric image is calculated by applying the Hilbert transform to the time-domain signal, interpolating the measurement data on a 3D spatial grid. Additionally, the dataset is rotated in space such that the sample surface is aligned in the x-y plane.</p> Full article ">Figure 7
<p>The 3D THz imaging of artificial inlays in a GFRP plate. (<b>a</b>) Sketch of the sample showing the lateral location and depth of the aluminum inlays (dark areas) and PTFE inlays (gray areas). Location and orientation of the slices shown in (<b>c</b>,<b>d</b>) are indicated by the dashed lines/boxes. (<b>b</b>) Top and bottom surfaces extracted from the measured 3D THz dataset. (<b>c</b>) B-scan along the blue dashed line in (<b>a</b>), THz reflections from top and bottom surfaces (green boxes) and reflections from the individual aluminum inlays (red boxes). (<b>d</b>) C-scans extracted at a depth of 0.75 mm and 1.13 mm from dashed regions in (<b>a</b>), location of the individual detected inlays (red boxes).</p> Full article ">Figure 8
<p>Drilled hole #1. (<b>a</b>,<b>b</b>) Pair of C-scans (X-ray left and THz right) at a depth of 0.3 mm. A clearly visible defect in the top area of the hole is highlighted (dashed box) in both the X-ray and THz images. (<b>c</b>,<b>d</b>) Additional pair of C-scans at a depth of 0.6 mm. The X-ray image shows the same but slightly narrower defect, whereas the THz image indicates the presence of a material interface in the right half of the dashed region.</p> Full article ">Figure 9
<p>Drilled holes #2 and #3. (<b>a</b>,<b>b</b>) X-ray CT scan of hole #2 showing a delamination at a depth of 0.3 mm in the top left corner (blue frame). The same feature is also identified as a bright spot in the THz scan at the same depth. (<b>c</b>,<b>d</b>) A delamination in hole #3 is found at a depth of 0.5 mm (blue frame). Similarly, two large reflections are found in the same region in the THz scan.</p> Full article ">Figure 10
<p>Drilled holes #4 and #5. (<b>a</b>) X-ray image of hole #4 0.4 mm below the surface of a 6 mm GFRP plate and (<b>b</b>) corresponding THz image. (<b>c</b>) Hole #5 drilled into a 3 mm GFRP plate and imaged using X-ray CT 0.6 mm below the surface. (<b>d</b>) Corresponding THz image showing identical defect features around the top region of the hole.</p> Full article ">
17 pages, 4616 KiB  
Article
All-Metal Metamaterial-Based Sensor with Novel Geometry and Enhanced Sensing Capability at Terahertz Frequency
by Sagnik Banerjee, Ishani Ghosh, Carlo Santini, Fabio Mangini, Rocco Citroni and Fabrizio Frezza
Sensors 2025, 25(2), 507; https://doi.org/10.3390/s25020507 - 16 Jan 2025
Viewed by 702
Abstract
This research proposes an all-metal metamaterial-based absorber with a novel geometry capable of refractive index sensing in the terahertz (THz) range. The structure consists of four concentric diamond-shaped gold resonators on the top of a gold metal plate; the resonators increase in height [...] Read more.
This research proposes an all-metal metamaterial-based absorber with a novel geometry capable of refractive index sensing in the terahertz (THz) range. The structure consists of four concentric diamond-shaped gold resonators on the top of a gold metal plate; the resonators increase in height by 2 µm moving from the outer to the inner resonators, making the design distinctive. This novel configuration has played a very significant role in achieving multiple ultra-narrow resonant absorption peaks that produce very high sensitivity when employed as a refractive index sensor. Numerical simulations demonstrate that it can achieve six significant ultra-narrow absorption peaks within the frequency range of 5 to 8 THz. The sensor has a maximum absorptivity of 99.98% at 6.97 THz. The proposed absorber also produces very high-quality factors at each resonance. The average sensitivity is 7.57/Refractive Index Unit (THz/RIU), which is significantly high when compared to the current state of the art. This high sensitivity is instrumental in detecting smaller traces of samples that have very correlated refractive indices, like several harmful gases. Hence, the proposed metamaterial-based sensor can be used as a potential gas detector at terahertz frequency. Furthermore, the structure proves to be polarization-insensitive and produces a stable absorption response when the angle of incidence is increased up to 60°. At terahertz wavelength, the proposed design can be used for any value of the aforementioned angles, targeting THz spectroscopy-based biomolecular fingerprint detection and energy harvesting applications. Full article
(This article belongs to the Special Issue Recent Advances in THz Sensing and Imaging)
►▼ Show Figures

Figure 1

Figure 1
<p>(<b>a</b>) Front view of the proposed design. (<b>b</b>) Side view of the proposed design. Geometrical dimensions of the structure are u = 86 µm, r = 40 µm, b = 6 µm, r<sub>1</sub> = 30 µm, r<sub>2</sub> = 20 µm, and r<sub>3</sub> = 10 µm, respectively. a = 2 µm, b = 6 µm, and t = 2 µm.</p> Full article ">Figure 2
<p>Absorption spectra of the proposed structure with hexaband configuration.</p> Full article ">Figure 3
<p>Side view of a conventional sensor [<a href="#B31-sensors-25-00507" class="html-bibr">31</a>,<a href="#B32-sensors-25-00507" class="html-bibr">32</a>].</p> Full article ">Figure 4
<p>Comparison between the absorption plot of our proposed design and the conventional design (depicted in <a href="#sensors-25-00507-f003" class="html-fig">Figure 3</a>) [<a href="#B31-sensors-25-00507" class="html-bibr">31</a>,<a href="#B32-sensors-25-00507" class="html-bibr">32</a>].</p> Full article ">Figure 5
<p>Results of parametric analysis plotting values of absorption A as a function of frequency for different values of unit cell dimensions <span class="html-italic">u</span> [µm] (<b>a</b>), ground plate thickness <span class="html-italic">t</span> [µm] (<b>b</b>), and height of the largest ring <span class="html-italic">b</span> [µm] (<b>c</b>).</p> Full article ">Figure 5 Cont.
<p>Results of parametric analysis plotting values of absorption A as a function of frequency for different values of unit cell dimensions <span class="html-italic">u</span> [µm] (<b>a</b>), ground plate thickness <span class="html-italic">t</span> [µm] (<b>b</b>), and height of the largest ring <span class="html-italic">b</span> [µm] (<b>c</b>).</p> Full article ">Figure 6
<p>Plots of simulated absorption spectra for different values of the polarization angle (<span class="html-italic">ϕ</span>) [deg].</p> Full article ">Figure 7
<p>Plots of simulated absorption spectra for different values of incidence angles (<span class="html-italic">θ</span>) [deg].</p> Full article ">Figure 8
<p>Real (blue solid line) and imaginary (red solid line) parts of the simulated impedance of the structure are plotted as a function of frequency.</p> Full article ">Figure 9
<p>Simulated effective permittivity [F/m] (blue) and permeability [A/m] (red) of the structure are plotted as a function of frequency. The real and the imaginary parts are depicted in solid and dashed lines, respectively.</p> Full article ">Figure 10
<p>Simulated surface current distribution 2D map at the resonant frequency of (<b>a</b>) 5.972 THz, (<b>b</b>) 6.272 THz, (<b>c</b>) 6.977 THz, (<b>d</b>) 7.067 THz, (<b>e</b>) 7.715 THz, and (<b>f</b>) 7.934 THz.</p> Full article ">Figure 11
<p>Shift in the absorption peaks in the absorption spectrum of the structure when the refractive index increases from 1 to 1.05.</p> Full article ">Figure 12
<p>Scatter plots of resonance frequency with respect to values of surrounding medium refractive index in the range from 1 to 1.05 with a step width of 1.01 for each absorption peak; (<b>a</b>) 1st Peak = 5.972 THz, (<b>b</b>) 2nd Peak = 6.272 THz, (<b>c</b>) 3rd Peak = 6.977 THz, (<b>d</b>) 4th Peak = 7.067 THz, (<b>e</b>) 5th Peak = 7.715 THz, and (<b>f</b>) 6th Peak = 7.934 THz.</p> Full article ">Figure 12 Cont.
<p>Scatter plots of resonance frequency with respect to values of surrounding medium refractive index in the range from 1 to 1.05 with a step width of 1.01 for each absorption peak; (<b>a</b>) 1st Peak = 5.972 THz, (<b>b</b>) 2nd Peak = 6.272 THz, (<b>c</b>) 3rd Peak = 6.977 THz, (<b>d</b>) 4th Peak = 7.067 THz, (<b>e</b>) 5th Peak = 7.715 THz, and (<b>f</b>) 6th Peak = 7.934 THz.</p> Full article ">
12 pages, 3043 KiB  
Article
Fabry–Perot Effect Suppression in Gas Cells Used in THz Absorption Spectrometers. Experimental Verification
by George K. Raspopin, Alexey V. Borisov, Arnaud Cuisset, Francis Hindle, Semyon V. Yakovlev and Yury V. Kistenev
Sensors 2024, 24(22), 7380; https://doi.org/10.3390/s24227380 - 19 Nov 2024
Viewed by 701
Abstract
A standard measuring gas cell used in absorption spectrometers is a cylinder enclosed by two transparent windows. The Fabry–Perot effects caused by multiple reflections of terahertz waves between these windows produce significant variations in the transmitted radiation intensity. Therefore, the Fabry–Perot effects should [...] Read more.
A standard measuring gas cell used in absorption spectrometers is a cylinder enclosed by two transparent windows. The Fabry–Perot effects caused by multiple reflections of terahertz waves between these windows produce significant variations in the transmitted radiation intensity. Therefore, the Fabry–Perot effects should be taken into account to correctly measure absorption spectra in Bouguer law-based absorption spectroscopy. One approach to reducing the Fabry–Perot effects is based on inserting an additional external movable window with the standard measuring gas cell. This was proposed and numerically analyzed in our previous work. This paper is aimed at the experimental validation of this method when using amplitude modulation (AM) spectroscopy. Also, a comparison of the efficiency of reducing the Fabry–Perot effects using this method is experimentally compared to frequency modulation spectroscopy. The latter was shown to effectively reduce the Fabry–Perot effects compared to AM spectroscopy with the standard measuring gas cell, and the use of the external movable window was shown to further improve the elimination of Fabry–Perot effects. Full article
(This article belongs to the Special Issue Recent Advances in THz Sensing and Imaging)
►▼ Show Figures

Figure 1

Figure 1
<p>Simulation of the measurements of water vapor absorption spectra near 557 GHz for: (<b>a</b>) water vapor concentration ~5000 ppm, <span class="html-italic">p</span> = 0.01 atm, T = 293 K; (<b>b</b>) water vapor concentration ~1000 ppm, <span class="html-italic">p</span> = 0.001 atm, T = 293 K. Red dotted lines—the water vapor absorption coefficient calculated using parameters from the 2020 HITRAN spectral database. Blue lines—water vapor absorption coefficient calculated for standard measuring gas cell with 1 m length, plane-parallel windows of 5 mm thickness, and refractive index <math display="inline"><semantics> <mrow> <mi>n</mi> </mrow> </semantics></math> = 2.1.</p> Full article ">Figure 2
<p>Illustration of FWHM and FSR parameters for a standard measuring gas cell with plane-parallel optically transparent windows.</p> Full article ">Figure 3
<p>Calculated amplitude of the second harmonic signal (normalized to maximum) as a function of the modulation index <math display="inline"><semantics> <mrow> <mi>m</mi> </mrow> </semantics></math>.</p> Full article ">Figure 4
<p>Block scheme of the experimental setup.</p> Full article ">Figure 5
<p>Photo of the measuring gas cell with external moving window placed on the linear transducer.</p> Full article ">Figure 6
<p>Water vapor line absorption coefficient shape for ~5000 ppm concentration, <span class="html-italic">p</span> = 1 atm, T = 293 K. Blue line—water vapor absorption line shape measured in the standard measuring gas cell with additional external movable window, orange line—water vapor absorption line shape measured in the standard gas cell, red dotted line—the water vapor absorption coefficient calculated using the 2020 HITRAN spectral database.</p> Full article ">Figure 7
<p>Water vapor line absorption coefficient shape for ~4320 ppm concentration, <span class="html-italic">p</span> = 0.04 atm, T = 293 K. Blue line—water vapor absorption line shape measured in the standard measuring gas cell with additional external movable window, orange line—water vapor absorption line shape measured in the standard gas cell, red dotted line—the water vapor absorption coefficient calculated using the 2020 HITRAN spectral database.</p> Full article ">Figure 8
<p>Second harmonic signal for the analyzed water vapor absorption line 556.938 GHz. Blue line—the second harmonic signal measured using the standard measuring gas cell normalized to the maximum value, red line—the same signal calculated for this water vapor absorption line using the 2020 HITRAN spectral database.</p> Full article ">
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