Special Issue "Advances of THz Spectroscopy"

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Optics and Lasers".

Deadline for manuscript submissions: 28 February 2022.

Special Issue Editors

Dr. Fabio Novelli
E-Mail Website
Guest Editor
Physical Chemistry II and ZEMOS, Ruhr University Bochum, 44801 Bochum, Germany
Interests: terahertz; nonlinear optics; condensed matter; superconductivity; photosynthesis; solvation
Dr. Andrea Perucchi
E-Mail Website
Guest Editor
Elettra Sincrotrone Trieste S.C.p.A., 34139 Trieste, Italy
Interests: THz and IR spectroscopy; synchrotron radiation; strongly correlated electron systems; superconductivity

Special Issue Information

Dear Colleagues,

Important macroscopic properties, from insulation to superconductivity, magnetic properties, and biological functions, originate from collective microscopic excitations. These microscopic excitations are typically found at low energy and can be probed with—and sometimes driven by—terahertz (THz) radiation between ~0.1 and 30 THz.  While the THz frequency range was called the “THz gap” because of the few intense sources available, fast-paced technical developments are now filling this important scientific gap. This is not limited to the generation and detection of THz light, but also includes astonishing developments in its manipulation at the nanoscale, by exploiting the sensing and light-confining properties of plasmons. This Special Issue is dedicated to the developments of THz spectroscopy and to the characterization of the electronic, nuclear, and dielectric properties of matter. This includes, but is not limited to, bulk- or surface-sensitive spectroscopy or microscopy techniques, and the investigation of gases, liquids, soft, and solid-state matter. We kindly invite you to contribute to this Special Issue. 

Dr. Fabio Novelli
Dr. Andrea Perucchi
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. Applied Sciences 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 2300 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

  • THz spectroscopy
  • Far-infrared spectroscopy
  • T-waves
  • Collective excitations
  • Nonlinear optics
  • Antenna
  • Intense THz
  • THz laser
  • Time-resolved (TR) spectroscopy
  • THz plasmons
  • THz nanoscopy

Published Papers (11 papers)

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Research

Article
Intermolecular Hydrogen-Bond Interactions in DPPE and DMPC Phospholipid Membranes Revealed by Far-Infrared Spectroscopy
Appl. Sci. 2021, 11(21), 10038; https://doi.org/10.3390/app112110038 - 27 Oct 2021
Viewed by 393
Abstract
The vibrational signature in the far-infrared region of two different phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), was investigated as a function of relative humidity from 0 to 75% in order to evaluate the effect of headgroup composition on the formation of intermolecular interactions. [...] Read more.
The vibrational signature in the far-infrared region of two different phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), was investigated as a function of relative humidity from 0 to 75% in order to evaluate the effect of headgroup composition on the formation of intermolecular interactions. The substructures of the frequency region between 50 and 300 cm−1 were identified, and changes in the frequency and intensity of the related vibrations with hydration were analyzed. Interestingly, in PE, two additional vibrational bands with respect to PC were found at 162 and 236 cm−1 and assigned to intermolecular hydrogen bonds between the hydrogen-bond-donating groups, -NH3+, and hydrogen-bond-accepting groups, —P—O− and —COO, of adjacent molecules. The presence of these interactions also affected the penetration of water, severely reducing the hydration capability of PE lipids. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Electric Field and Temperature Effects on the Ab Initio Spectroscopy of Liquid Methanol
Appl. Sci. 2021, 11(12), 5457; https://doi.org/10.3390/app11125457 - 12 Jun 2021
Viewed by 563
Abstract
Although many H-bonded systems have been extensively investigated by means of infrared (IR) spectroscopy, the vibrational response to externally applied electric fields of polar liquids remains poorly investigated. However, local electric fields along with quantum-mechanical interactions rule the behavior of H-bonded samples at [...] Read more.
Although many H-bonded systems have been extensively investigated by means of infrared (IR) spectroscopy, the vibrational response to externally applied electric fields of polar liquids remains poorly investigated. However, local electric fields along with quantum-mechanical interactions rule the behavior of H-bonded samples at the molecular level. Among the many H-bonded systems, liquid methanol holds a key place in that it exhibits a very simple H-bond network where, on average, each molecule acts as a single H-bond donor and, at the same time, as a single H-bond acceptor. Here we report on the IR spectra emerging from a series of state-of-the-art ab initio molecular dynamics simulations of bulk liquid methanol under the action of static and homogeneous electric fields. In addition, the same analysis is here conducted in the absence of the external field and for different temperatures. Although some electric-field-induced effects resemble the response of other polar liquids (such as the global contraction of the IR spectrum upon field exposure), it turns out that, distinctly from water, the “electrofreezing” phenomenon is unlikely to happen in liquid methanol. Finally, we provide atomistic analyses magnifying the completely different nature of electric-field- and temperature-induced effects on bulk liquid methanol and on its vibrational response. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Metal-Organic Hybrid Metamaterials for Spectral-Band Selective Active Terahertz Modulators
Appl. Sci. 2021, 11(6), 2765; https://doi.org/10.3390/app11062765 - 19 Mar 2021
Cited by 1 | Viewed by 519
Abstract
Optically controlled spectral-band selective terahertz (THz) modulators based on metal-organic hybrid metamaterials were investigated. An artificially structured material, which consists of two single split-ring resonators put together on the split gap side, was patterned on a silicon substrate to generate frequency-selective properties. An [...] Read more.
Optically controlled spectral-band selective terahertz (THz) modulators based on metal-organic hybrid metamaterials were investigated. An artificially structured material, which consists of two single split-ring resonators put together on the split gap side, was patterned on a silicon substrate to generate frequency-selective properties. An active layer of an organic thin film (fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester, also called PCBM) was deposited on the metamaterial-silicon structure for modulating the transmission of incident THz radiation. The metal-organic hybrid metamaterials enabled active control of spectral bands present in the transmission spectra of THz waves. In addition, the changes in the photo-excited carrier density due to the transfer of charges between the layers were quantitatively analyzed by simulation results. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Mail Inspection Based on Terahertz Time-Domain Spectroscopy
Appl. Sci. 2021, 11(3), 950; https://doi.org/10.3390/app11030950 - 21 Jan 2021
Cited by 2 | Viewed by 735
Abstract
One of the most prominent applications of terahertz time-domain spectroscopy is the spectral investigation of materials covered by visibly opaque objects. Therefore, terahertz waves are well suited to inspect the content of mail. We report on our work on mail inspection in this [...] Read more.
One of the most prominent applications of terahertz time-domain spectroscopy is the spectral investigation of materials covered by visibly opaque objects. Therefore, terahertz waves are well suited to inspect the content of mail. We report on our work on mail inspection in this spectral range including machine design, optical layouts, data analysis, and implementations. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Performance Evaluation of a THz Pulsed Imaging System: Point Spread Function, Broadband THz Beam Visualization and Image Reconstruction
Appl. Sci. 2021, 11(2), 562; https://doi.org/10.3390/app11020562 - 08 Jan 2021
Cited by 5 | Viewed by 752
Abstract
Terahertz (THz) technology is a promising research field for various applications in basic science and technology. In particular, THz imaging is a new field in imaging science, where theories, mathematical models and techniques for describing and assessing THz images have not completely matured [...] Read more.
Terahertz (THz) technology is a promising research field for various applications in basic science and technology. In particular, THz imaging is a new field in imaging science, where theories, mathematical models and techniques for describing and assessing THz images have not completely matured yet. In this work, we investigate the performances of a broadband pulsed THz imaging system (0.2–2.5 THz). We characterize our broadband THz beam, emitted from a photoconductive antenna (PCA), and estimate its point spread function (PSF) and the corresponding spatial resolution. We provide the first, to our knowledge, 3D beam profile of THz radiation emitted from a PCA, along its propagation axis, without the using of THz cameras or profilers, showing the beam spatial intensity distribution. Finally, we evaluate the THz image formation on a test-sample composed by a regular linen natural pattern. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Refractive Indices of Ge and Si at Temperatures between 4–296 K in the 4–8 THz Region
Appl. Sci. 2021, 11(2), 487; https://doi.org/10.3390/app11020487 - 06 Jan 2021
Viewed by 603
Abstract
Refractive indices of high resistivity Si and Ge were measured at temperatures between 4–296 K and at frequencies between 4.2–7.7 THz using a Fourier-transform spectrometer (FTS) in transmission mode. A phenomenological model of the temperature dependence of the refractive index is proposed. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Characterisation of Biological Materials at THz Frequencies by Attenuated Total Reflection: Lard
Appl. Sci. 2020, 10(23), 8692; https://doi.org/10.3390/app10238692 - 04 Dec 2020
Cited by 3 | Viewed by 750
Abstract
The penetration depth of an evanescent wave in Attenuated Total Reflection (ATR) is dependent on the wavelength of the radiation utilised. At THz frequencies, the penetration depth into biological tissues is in the order of 0.1 to 0.5 mm; rendered pig lard was [...] Read more.
The penetration depth of an evanescent wave in Attenuated Total Reflection (ATR) is dependent on the wavelength of the radiation utilised. At THz frequencies, the penetration depth into biological tissues is in the order of 0.1 to 0.5 mm; rendered pig lard was used as a model sample in this study. A method for the direct measurement of the evanescent wave penetration depth is presented which allows for the estimation of the dispersion of the complex refractive index by using the reflection of the evanescent wave from varying sample depths. The method employs frustrated total internal reflection, and has been demonstrated by using the THz/Far-IR beamline at the Australian synchrotron, and modelled using finite difference time domain (FDTD) simulations. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Tunable Dual Broadband Terahertz Metamaterial Absorber Based on Vanadium Dioxide
Appl. Sci. 2020, 10(20), 7259; https://doi.org/10.3390/app10207259 - 17 Oct 2020
Cited by 5 | Viewed by 748
Abstract
With the rapid development of terahertz technology, tunable high-efficiency broadband functional devices have become a research trend. In this research, a dynamically tunable dual broadband terahertz absorber based on the metamaterial structure of vanadium dioxide (VO2) is proposed and analyzed. The [...] Read more.
With the rapid development of terahertz technology, tunable high-efficiency broadband functional devices have become a research trend. In this research, a dynamically tunable dual broadband terahertz absorber based on the metamaterial structure of vanadium dioxide (VO2) is proposed and analyzed. The metamaterial is composed of patterned VO2 on the top layer, gold on the bottom layer and silicon dioxide (SiO2) as the middle dielectric layer. Simulation results show that two bandwidths of 90% absorption reach as wide as 2.32 THz from 1.87 to 4.19 THz and 2.03 THz from 8.70 to 10.73 THz under normal incidence. By changing the conductivity of VO2, the absorptance dynamically tuned from 2% to 94%. Moreover, it is verified that absorptance is insensitive to the polarization angle. The physical origin of this absorber is revealed through interference theory and matching impedance theory. We further investigate the physical mechanism of dual broadband absorption through electric field analysis. This design has potential applications in imaging, modulation and stealth technology. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Nonlinear TeraHertz Transmission by Liquid Water at 1 THz
Appl. Sci. 2020, 10(15), 5290; https://doi.org/10.3390/app10155290 - 30 Jul 2020
Cited by 4 | Viewed by 1330
Abstract
The solvation properties of liquid water originate from the transient network of hydrogen-bonded molecules. In order to probe the coupling between the different modes of this network, nonlinear terahertz (THz) spectroscopy techniques are required. Ideally, these techniques should use a minimal volume and [...] Read more.
The solvation properties of liquid water originate from the transient network of hydrogen-bonded molecules. In order to probe the coupling between the different modes of this network, nonlinear terahertz (THz) spectroscopy techniques are required. Ideally, these techniques should use a minimal volume and capitalize on sensitive field-resolved detection. Here we performed open aperture z-scan transmission experiments on static liquid cells, and detect the THz fields with electro-optical techniques. We show that it is possible to quantify the nonlinear response of liquid water at ~1 THz even when large signals originate from the sample holder windows. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Reconfigurable Terahertz Metamaterial Using Split-Ring Meta-Atoms with Multifunctional Electromagnetic Characteristics
Appl. Sci. 2020, 10(15), 5267; https://doi.org/10.3390/app10155267 - 30 Jul 2020
Cited by 7 | Viewed by 803
Abstract
We propose a reconfigurable terahertz (THz) metamaterial (RTM) to investigate its multifunctional electromagnetic characteristics by moving the meta-atoms of split-ring resonator (SRR) array. It shows the preferable and capable adjustability in the THz frequency range. The electromagnetic characteristics of the proposed RTM device [...] Read more.
We propose a reconfigurable terahertz (THz) metamaterial (RTM) to investigate its multifunctional electromagnetic characteristics by moving the meta-atoms of split-ring resonator (SRR) array. It shows the preferable and capable adjustability in the THz frequency range. The electromagnetic characteristics of the proposed RTM device are compared and analyzed by moving the meta-atoms in different polarized transverse magnetic (TM) and transverse electric (TE) modes. The symmetrical meta-atoms of RTM device exhibit a resonant tuning range of several tens of GHz and the asymmetrical meta-atoms of RTM device exhibit the better tunability. Therefore, an RTM device with reconfigurable meta-atoms possesses the resonance shifting, polarization switching, electromagnetically induced transparency (EIT) switching and multiband to single-band switching characteristics. This proposed RTM device provides the potential possibilities for the use of THz-wave optoelectronics with tunable resonance, EIT analog and tunable multiresonance characteristics. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Article
Tunable Terahertz Metamaterial Using an Electric Split-Ring Resonator with Polarization-Sensitive Characteristic
Appl. Sci. 2020, 10(13), 4660; https://doi.org/10.3390/app10134660 - 06 Jul 2020
Cited by 11 | Viewed by 1043
Abstract
We present a tunable terahertz (THz) metamaterial using an electric split-ring resonator (eSRR), which exhibits polarization-sensitive characteristics. The proposed eSRR is composed of double symmetrical semicircles and two central metal bars. By changing the lengths of two metal bars, the electromagnetic responses can [...] Read more.
We present a tunable terahertz (THz) metamaterial using an electric split-ring resonator (eSRR), which exhibits polarization-sensitive characteristics. The proposed eSRR is composed of double symmetrical semicircles and two central metal bars. By changing the lengths of two metal bars, the electromagnetic responses can be tuned and switched between dual-band and triple-band resonances in transverse magnetic (TM) mode. Furthermore, by moving the bottom metal bar to change the gap between the two metal bars, the first resonance is stable at 0.39 THz, and the second resonance is gradually blue-shifted from 0.83 to 1.33 THz. The tuning range is 0.50 THz. This means that the free spectrum ranges (FSR) could be broadened by 0.50 THz. This proposed device exhibits a dual-/triple-band switch, tunable filter, tunable FSR and polarization-dependent characteristics. It provides an effective approach to perform tunable polarizer, sensor, switch, filter and other optoelectronics in THz-wave applications. Full article
(This article belongs to the Special Issue Advances of THz Spectroscopy)
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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Title: Nonlinear TeraHertz Transmission by Liquid Water at 1 THz
Authors: Fabio Novelli; Chun Yu Ma; Nidhi Adhlakha; Ellen M Adams; Thorsten Ockelmann; Debasish Das Mahanta; Paola Di Pietro; Andrea Perucchi; Martina Havenith
Affiliation: Department of Physical Chemistry II, Ruhr University Bochum, 44801 Bochum, Germany
Abstract: The solvation properties of liquid water originate from the transient network of hydrogen-bonded molecules. In order to probe the coupling between the different modes of this network, nonlinear THz spectroscopy techniques are required. Ideally, these techniques should use a minimal volume and capitalize on sensitive field-resolved detection. Here we performed open aperture z-scan transmission experiments on static liquid cells, and detect the THz fields with electro-optical techniques. We show that it is possible to quantify the nonlinear response of liquid water at ~1 THz even when large signals originate from the sample holder windows.

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