Special Issue "Advances in THZ Spectroscopy"

A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Structure Analysis and Characterization".

Deadline for manuscript submissions: closed (29 February 2020).

Special Issue Editor

Prof. Dr. Renato Torre
Website
Guest Editor
European Laboratory for Non-Linear Spectroscopy and Department of Physics and Astronomy, University of Firenze Via Carrara 1, Sesto Fiorentino 50019, Firenze, Italy
Interests: non-linear spectroscopy; time-resolved spectroscopy; ultrafast laser spectroscopy; transient grating; optical kerr effect; glass structure and dynamics; glass transition; acoustic propagation; phononic phenomena; liquid crystal; birefringence; order-disorder transitions; water physics

Special Issue Information

Dear colleagues,

With the Terahertz (THz) radiation, the portion of the electromagnetic spectrum covering the frequency range of 0.1-10 THz, corresponding to the radiation wavelengths from 3000 to 30 μm, is usually indicated. In recent years, a series of coherent sources and detection technologies have been developed covering the THz spectrum; hence far-infrared spectroscopy meets a renewed and great interest. This interest ranges from fundamental research that can now investigate the low frequency dynamics with appropriate tools, to technological applications that take advantage of the non-ionizing, non-invasive and penetrable characteristics of THz radiation.

However, THz spectroscopy is still in its infancy and a rapid increase in the efficiency and quality of spectroscopy techniques will develop in the coming years. In particular, for fundamental research the interaction of high-intensity THz pulses with matter is a hot topic, which requires an extension of the theoretical models of non-linear phenomena from optical frequencies to THz ones. Furthermore, among the numerous technological applications, a rapid and efficient THz imaging method remains to be implemented; this is not a trivial task that requires to overcome a series of technical limitations present today in THz technology.

In this special issue, we are planning to collect a series of research studies related to THz spectroscopy. They may include experimental investigations, theoretical models and methods of data analysis for signal extraction; as well as new sources of THz radiation, detection apparatus including methods of imaging and/or characterization of THz radiation.

Prof. Dr. Renato Torre
Guest Editor

Manuscript Submission Information

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Keywords

  • THz laser
  • THz radiation
  • far-infrared spectroscopy
  • optical rectification
  • THz time-domain spectroscopy
  • quantum cascade laser
  • THz Spectroscopy
  • THz imaging
  • T-waves

Published Papers (6 papers)

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Research

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Open AccessArticle
Towards Intense THz Spectroscopy on Water: Characterization of Optical Rectification by GaP, OH1, and DSTMS at OPA Wavelengths
Materials 2020, 13(6), 1311; https://doi.org/10.3390/ma13061311 - 13 Mar 2020
Abstract
Water is the most prominent solvent. The unique properties of water are rooted in the dynamical hydrogen-bonded network. While TeraHertz (THz) radiation can probe directly the collective molecular network, several open issues remain about the interpretation of these highly anharmonic, coupled bands. In [...] Read more.
Water is the most prominent solvent. The unique properties of water are rooted in the dynamical hydrogen-bonded network. While TeraHertz (THz) radiation can probe directly the collective molecular network, several open issues remain about the interpretation of these highly anharmonic, coupled bands. In order to address this problem, we need intense THz radiation able to drive the liquid into the nonlinear response regime. Firstly, in this study, we summarize the available brilliant THz sources and compare their emission properties. Secondly, we characterize the THz emission by Gallium Phosphide (GaP), 2–{3–(4–hydroxystyryl)–5,5–dimethylcyclohex–2–enylidene}malononitrile (OH1), and 4–N,N–dimethylamino–4′–N′–methyl–stilbazolium 2,4,6–trimethylbenzenesulfonate (DSTMS) crystals pumped by an amplified near-infrared (NIR) laser with tunable wavelength. We found that both OH1 as well as DSTMS could convert NIR laser radiation between 1200 and 2500 nm into THz radiation with high efficiency (> 2 × 10−4), resulting in THz peak fields exceeding 0.1 MV/cm for modest pump excitation (~ mJ/cm2). DSTMS emits the broadest spectrum, covering the entire bandwidth of our detector from ca. 0.5 to ~7 THz, also at a laser wavelength of 2100 nm. Future improvements will require handling the photothermal damage of these delicate organic crystals, and increasing the THz frequency. Full article
(This article belongs to the Special Issue Advances in THZ Spectroscopy)
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Open AccessArticle
Investigation of Fresh Gastric Normal and Cancer Tissues Using Terahertz Time-Domain Spectroscopy
Materials 2020, 13(1), 85; https://doi.org/10.3390/ma13010085 - 23 Dec 2019
Cited by 1
Abstract
In recent times, terahertz (THz) technologies have been actively applied in many biomedical research work, including gastric cancer diagnosis. In order to provide an effective removal of tumor during surgery, it is necessary to clearly distinguish it from different membranes of the stomach. [...] Read more.
In recent times, terahertz (THz) technologies have been actively applied in many biomedical research work, including gastric cancer diagnosis. In order to provide an effective removal of tumor during surgery, it is necessary to clearly distinguish it from different membranes of the stomach. In this work, we reported an investigation of various normal and cancer fresh gastric tissues using terahertz time-domain spectroscopy in the reflection mode. Refractive index and absorption coefficient of moderately differentiated and poorly differentiated gastric adenocarcinomas, as well as both serosa and mucosa were obtained in the frequency range from 0.2 to 1 THz. All cancer tissues were distinguishable from normal ones. The influence of the morphology of the investigated tissues on the obtained optical properties is discussed. The obtained results demonstrated a potential of THz time-domain spectroscopy to discriminate a tumor from normal serous and mucous gastric membranes. Thus, this method might be applied to gastric cancer diagnosis. Full article
(This article belongs to the Special Issue Advances in THZ Spectroscopy)
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Open AccessFeature PaperArticle
Terahertz Time-Domain Reconstruction of Coating Microstratigraphy on Gilded Surfaces
Materials 2019, 12(23), 3822; https://doi.org/10.3390/ma12233822 - 21 Nov 2019
Abstract
Here, a systematic study in order to assess the potential of THz time domain reflectometry for measuring the thicknesses of overpaint layers applied on original gilded surfaces was carried out. The work is part of a thorough characterization campaign, which is going on [...] Read more.
Here, a systematic study in order to assess the potential of THz time domain reflectometry for measuring the thicknesses of overpaint layers applied on original gilded surfaces was carried out. The work is part of a thorough characterization campaign, which is going on at the Rijksmuseum for addressing the conservation problems of a set of 19th century gilded picture frames on which heavy coatings were applied in previous undocumented restoration interventions. To perform such non-invasive thickness measurements, an analytical protocol based on Gaussian fits of the THz pulse-echo temporal profiles was optimized through the preparation of suitable technical samples and the comparison with direct thickness measurements. Finally, the methodology was validated by characterizing the microstratigraphy of an original sculptural element from a gilded picture frame in the Rijksmuseum collection. The results achieved show the effectiveness of the present approach in revealing multi-layered dielectric microstructures with a spatial resolution of about 30 µm when using a spectral range up to 1.5 THz. Full article
(This article belongs to the Special Issue Advances in THZ Spectroscopy)
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Open AccessArticle
Electronic Modulation of THz Radiation at NovoFEL: Technical Aspects and Possible Applications
Materials 2019, 12(19), 3063; https://doi.org/10.3390/ma12193063 - 20 Sep 2019
Cited by 2
Abstract
The Novosibirsk Free Electron Laser (NovoFEL) facility is able to produce high-power tunable terahertz (THz) laser radiation in quasi-continuous mode. The ability to control/shape this THz radiation is required in a number of user experiments. In this work we propose a modulation approach [...] Read more.
The Novosibirsk Free Electron Laser (NovoFEL) facility is able to produce high-power tunable terahertz (THz) laser radiation in quasi-continuous mode. The ability to control/shape this THz radiation is required in a number of user experiments. In this work we propose a modulation approach suitable for free electron lasers based on recuperation design. It allows for generating THz macropulses of a desirable length, down to several microseconds (limited by a quality factor of FEL optical resonator). Using this approach, macropulses in the time window from several microseconds to several hundred microseconds have been shown for three possible frequency ranges: mid-infrared (~1100 cm−1), far-infrared (~200 cm−1) and THz (~40 cm−1). In each case, the observed rise and decay of the macropulse have been measured and interpreted. The advantage of using short macropulses at the maximum peak power available has been demonstrated with the time-resolved Electron Paramagnetic Resonance (EPR) spectroscopy. Full article
(This article belongs to the Special Issue Advances in THZ Spectroscopy)
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Review

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Open AccessReview
Coherent THz Hyper-Raman: Spectroscopy and Application in THz Detection
Materials 2019, 12(23), 3870; https://doi.org/10.3390/ma12233870 - 23 Nov 2019
Abstract
Recently we have demonstrated a new nonlinear optical effect in the THz interval of frequencies. The latter is based on the use of femtosecond optical pulses and intense, sub-ps, broadband terahertz (THz) pulses to generate a THz-optical four- and five-wave mixing in the [...] Read more.
Recently we have demonstrated a new nonlinear optical effect in the THz interval of frequencies. The latter is based on the use of femtosecond optical pulses and intense, sub-ps, broadband terahertz (THz) pulses to generate a THz-optical four- and five-wave mixing in the investigated material. The spectrum of the generated signal is resolved in time and wavelength and displays two pronounced frequency sidebands, Stokes and anti-Stokes, close to the optical second harmonic central frequency 2 ω L , where ω L is the optical central frequency of the fundamental beam, thus resembling the spectrum of standard hyper-Raman scattering, and hence we named this effect ‘THz hyper-Raman’—THYR. We applied this technique to several crystalline materials, including α-quartz and gallium selenide. In the first material, we find that the THYR technique brings spectroscopic information on a large variety of low-energy excitations that include polaritons and phonons far from the Γ-point, which are difficult to study with standard optical techniques. In the second example, we show that this new tool offers some advantages in detecting ultra-broadband THz pulses. In this paper we review these two recent results, showing the potentialities of this new THz technique. Full article
(This article belongs to the Special Issue Advances in THZ Spectroscopy)
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Open AccessReview
Interpreting the Terahertz Spectrum of Complex Materials: The Unique Contribution of the Bayesian Analysis
Materials 2019, 12(18), 2914; https://doi.org/10.3390/ma12182914 - 09 Sep 2019
Cited by 2
Abstract
In the last few decades, experimental studies of the terahertz spectrum of density fluctuations have considerably improved our knowledge of the mesoscopic dynamics of disordered materials, which also have imposed new demands on the data modelling and interpretation. Indeed, lineshape analyses are no [...] Read more.
In the last few decades, experimental studies of the terahertz spectrum of density fluctuations have considerably improved our knowledge of the mesoscopic dynamics of disordered materials, which also have imposed new demands on the data modelling and interpretation. Indeed, lineshape analyses are no longer limited to the phenomenological observation of inelastic features, as in the pioneering stage of Neutron or X-ray spectroscopy, rather aiming at the extraction from their shape of physically relevant quantities, as sound velocity and damping, relaxation times, or other transport coefficients. In this effort, researchers need to face both inherent and practical obstacles, respectively stemming from the highly damped nature of terahertz modes and the limited energy resolution, accessible kinematic region and statistical accuracy of the typical experimental outcome. To properly address these challenges, a global reconsideration of the lineshape modelling and the enforcement of evidence-based probabilistic inference is becoming crucial. Particularly compelling is the possibility of implementing Bayesian inference methods, which we illustrated here through an in-depth discussion of some results recently obtained in the analysis of Neutron and X-ray scattering results. Full article
(This article belongs to the Special Issue Advances in THZ Spectroscopy)
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