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13 pages, 2066 KiB

Open AccessArticle

Application of Terahertz Time-Domain Spectroscopy to Study the Microheterogeneities of Solutions: A Case Study of Aqueous Sugar Solutions

by Nikita V. Penkov

Photonics 2023, 10(8), 887; https://doi.org/10.3390/photonics10080887 - 1 Aug 2023

Cited by 5 | Viewed by 1295

Abstract

The phenomenon of the formation of microheterogeneities (MHs) in solutions, which, according to chemical handbooks, are considered true solutions, has been known for a long time. MHs have been found in more than 100 binary solutions, many of which are used both in [...] Read more.

The phenomenon of the formation of microheterogeneities (MHs) in solutions, which, according to chemical handbooks, are considered true solutions, has been known for a long time. MHs have been found in more than 100 binary solutions, many of which are used both in various scientific studies and in life. However, the nature of this phenomenon is largely unclear. It is only well-known that MHs are stable areas of increased concentration of one of the components of the solution. The main reason for the poor knowledge of MHs is the use of very few experimental methods, mainly light scattering methods. In this paper, the terahertz time-domain spectroscopy method was used for the first time to study MHs using the example of aqueous solutions of three sugars: glucose, fructose, and sucrose. This method gives the spectra of complex permittivity in the terahertz range, which are very informative when studying the hydrate shells of molecules in solutions. The idea of this study was that structuring sugar molecules with the formation of MHs changes their hydration. The characteristics of sugar hydration in solutions before and after filtration through a 20 nm filter, leading to the destruction of MHs, were compared. It has been shown that the water binding in the MHs of all three solutions is increased compared with the hydrate shells of individual sugar molecules. Also, for MHs’ fructose solution, a decrease in the number of hydrogen bonds between water molecules and an increase in the number of free water molecules was shown, which is not observed in MH glucose and sucrose solutions. This is explained by mutarotations of fructose molecules, leading to permanent significant rearrangements of the water structure in MHs. Thus, terahertz time-domain spectroscopy provides fundamentally new information about the MHs of aqueous solutions at the level of their hydration characteristics. The presence of MHs in solutions is a significant factor that has never been taken into account when studying the hydrate shells of various molecules in solutions using THz spectroscopy. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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9 pages, 1961 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

A Large Area Wide Bandwidth THz Phase Shifter Plate for High Intensity Field Applications

by Can Koral, Zahra Mazaheri and Antonello Andreone

Photonics 2023, 10(7), 825; https://doi.org/10.3390/photonics10070825 - 15 Jul 2023

Cited by 2 | Viewed by 1467

Abstract

We present the design, fabrication, and experimental test of a THz all-dielectric phase shifter plate. The design consists of two wave plate zones coupled in a perpendicular orientation with respect to each other. A large surface area device is realized by an additive [...] Read more.

We present the design, fabrication, and experimental test of a THz all-dielectric phase shifter plate. The design consists of two wave plate zones coupled in a perpendicular orientation with respect to each other. A large surface area device is realized by an additive manufacturing technique using Acrylonitrile Butadiene Styrene (ABS). Its characteristics are analytically evaluated and experimentally measured in the THz band using time domain spectroscopy and imaging routines. The proposed design enables the creation of quasi-ideal phase retardation in between the two planes with good uniformity on a large surface area. We also achieve the flexibility to select the plane of symmetry around the chosen central axes of choice with a sensitive control over the electromagnetic field polarization direction without inducing any temporal shifts in between the wave front components of the traversed beam. Due to its inherent simplicity and robustness, the phase shifter can be easily scaled at higher frequencies and potentially used in several advanced applications, including free-electron laser (FEL) systems where an accurate polarization control of high intensity beams is required. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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13 pages, 7244 KiB

Open AccessEditor’s ChoiceArticle

First Experimental Demonstration of the Wide-Field Amplitude Surface Plasmon Resonance Microscopy in the Terahertz Range

by Vasiliy Valerievich Gerasimov, Oleg Eduardovich Kameshkov, Alexey Konstantinovich Nikitin, Ildus Shevketovich Khasanov, Alexey Georgievich Lemzyakov, Irina Veniaminovna Antonova, Artem Ilyich Ivanov, Nghiem Thi Ha Lien, Nguyen Trong Nghia, Le Tu Anh, Nguyen Quoc Hung and Ta Thu Trang

Photonics 2023, 10(7), 723; https://doi.org/10.3390/photonics10070723 - 24 Jun 2023

Cited by 1 | Viewed by 1644

Abstract

We have demonstrated the wide-field amplitude surface plasmon resonance (SPR) microscopy technique in the terahertz (THz) range. A Zeonex polymer prism was utilized to excite surface plasmon polaritons (SPPs) through attenuated total reflection (ATR) in an Otto configuration. Coherent quasimonochromatic radiation with a [...] Read more.

We have demonstrated the wide-field amplitude surface plasmon resonance (SPR) microscopy technique in the terahertz (THz) range. A Zeonex polymer prism was utilized to excite surface plasmon polaritons (SPPs) through attenuated total reflection (ATR) in an Otto configuration. Coherent quasimonochromatic radiation with a wavelength of approximately 197 μm, generated by the Novosibirsk free electron laser, was employed. Our results indicate that the SPR microscopy method is applicable for investigating the planar surfaces of semiconductors at THz frequencies, provided that the SPPs’ cutoff frequency is close to the probing radiation frequency. This condition ensures that the propagation length of the SPPs is comparable to the radiation wavelength. By varying the air gap between the prism and the surface under examination, we acquired images of a polypropylene coating 20 µm thick and a graphene coating 35 nm thick on a flat indium antimonide substrate. The boundary between the coated and uncoated regions can be precisely localized through determination of the kink in the reflection coefficient of the THz radiation beam that illuminates the boundary between the regions if the optimal conditions for the generation of the SPPs in the uncoated region are met. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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17 pages, 19209 KiB

Open AccessArticle

Terahertz Bessel Beams Formed by Binary and Holographic Axicons

by Boris Knyazev, Natalya Osintseva, Maxim Komlenok, Vladimir Pavelyev, Vasily Gerasimov, Oleg Kameshkov, Yulia Choporova and Konstantin Tukmakov

Photonics 2023, 10(6), 700; https://doi.org/10.3390/photonics10060700 - 20 Jun 2023

Viewed by 1590

Abstract

The characteristics of high-power vortex Bessel beams in the terahertz range (

λ = 141

μm) obtained with the use of diffractive axicons (DAs) illuminated by a Gaussian beam of the Novosibirsk free-electron laser were studied. Two of the three possible types of [...] Read more.

The characteristics of high-power vortex Bessel beams in the terahertz range (

λ = 141

μm) obtained with the use of diffractive axicons (DAs) illuminated by a Gaussian beam of the Novosibirsk free-electron laser were studied. Two of the three possible types of DA recently described in our previous paper, namely, binary spiral silicon axicons (BAs), forming beams with a topological charge l equal to 0–4 and 9, and a diamond “holographic” axicon (HA), forming a beam with

l = 9

, were used in the experiments. These axicons formed beams whose cross sections in the region of inner Bessel rings were close to those of ideal Bessel beams, but their intensities varied in azimuth with a frequency of l and

2 l

for the BAs and HA, respectively. However, in the case of the BAs, the beams had a pronounced helical structure at the periphery, whereas for the HA, the beam was axisymmetric. By focusing these beams with a lens, we studied the structure of the so-called “perfect” beams (PBs). While an ideal Bessel beam exhibits a PB as a thin ring, in the case of the BAs, we observed a broadened ring structure consisting of

2 l

short spirals, and for the HA, we observed a narrow ring with

2 l

maxima in azimuth. A comparison of the numerical calculations and experiments showed that the observed azimuthal intensity variations can be attributed to inaccuracies in the preparation of the axicon relief and/or discrepancies between the calculated and actual wavelengths, within a few percent. The results of this work enable the establishment of quality requirements for axicon manufacture and the appropriate selection of the axicon type in accordance with the requirements for the beam. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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13 pages, 6835 KiB

Open AccessArticle

A Novel Triple-Band Terahertz Metamaterial Absorber Using a Stacked Structure of MoS₂ and Graphene

by Fei Cai and Zhifei Kou

Photonics 2023, 10(6), 643; https://doi.org/10.3390/photonics10060643 - 2 Jun 2023

Cited by 9 | Viewed by 1668

Abstract

A MoS₂ and graphene stacked structure is proposed as metamaterials for a triple-band terahertz absorber in this work. The complementary frequency-selective surface of the absorber, consisting of two crossed linear slots and four pairs of concentric circular slots, has three absorptions at [...] Read more.

A MoS₂ and graphene stacked structure is proposed as metamaterials for a triple-band terahertz absorber in this work. The complementary frequency-selective surface of the absorber, consisting of two crossed linear slots and four pairs of concentric circular slots, has three absorptions at 0.6 THz (99.7%), 1.5 THz (95.4%), and 2.5 THz (99.5%). The polarization of the THz absorber is less sensitive to the incident angle within a certain range. By controlling the material properties of MoS₂ and graphene, the peak absorption frequency can be tuned within a certain range. The stacked structure of different 2D materials provides new ideas for the design of the THz absorber, which is important for THz in detection, communication, and imaging applications. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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11 pages, 4383 KiB

Open AccessArticle

Prism Couplers with Convex Output Surfaces for Nonlinear Cherenkov Terahertz Generation

by Galiya Kh. Kitaeva, Dmitrii A. Markov, Daniil A. Safronenkov and Natalia V. Starkova

Photonics 2023, 10(4), 450; https://doi.org/10.3390/photonics10040450 - 13 Apr 2023

Cited by 2 | Viewed by 1401

Abstract

Sideways THz generation in Mg:LiNbO₃ crystal is studied considering Si-prism-lens couplers with different output surface curvatures. A theoretical approach is developed for modeling the angular distributions of THz radiation power inside the crystal, inside the Si coupler and outside in free space. [...] Read more.

Sideways THz generation in Mg:LiNbO₃ crystal is studied considering Si-prism-lens couplers with different output surface curvatures. A theoretical approach is developed for modeling the angular distributions of THz radiation power inside the crystal, inside the Si coupler and outside in free space. Our calculations show how the imposition of a plano-convex lens on the standard flat prism can substantially improve the THz generation efficiency. The ratio between the lens curvature radius and the distance from the curvature center to the point of generation on the lens axis is found to be one of the most important parameters. The developed general approach can be used for the further design of the optimal THz extraction elements of a different configuration. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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12 pages, 5361 KiB

Open AccessCommunication

Prospects for the Implementation of an Intense Source of Ultraviolet Radiation Based on a Gas-Discharge Plasma in a Quasi-Optical Cavity Excited by a Pulse of Terahertz Radiation

by Galina Kalynova, Yuriy Kalynov and Andrei Savilov

Photonics 2023, 10(4), 440; https://doi.org/10.3390/photonics10040440 - 12 Apr 2023

Cited by 1 | Viewed by 1194

Abstract

An electrodynamic system is described that provides the creation of an electromagnetic wave field of high intensity at a frequency of 1 THz due to a combination of accumulation in time and compression in space of a wave pulse coming from an electron [...] Read more.

An electrodynamic system is described that provides the creation of an electromagnetic wave field of high intensity at a frequency of 1 THz due to a combination of accumulation in time and compression in space of a wave pulse coming from an electron cyclotron maser (gyrotron). This system is based on the use of a three-mirror cavity consisting of two focusing mirrors and one flat corrugated Bragg-type photonic structure providing coupling between the gyrotron wave pulse and the operating wave of the cavity. The aim of this work is to use a “spot” of the intense terahertz field inside the cavity to provide a point-like plasma discharge in a gas stream injected into this spot; such a discharge can be a source of extreme ultraviolet radiation. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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11 pages, 16353 KiB

Open AccessArticle

Optical Properties of LiGdF₄ Single Crystal in the Terahertz and Infrared Ranges

by Gennady A. Komandin, Sergey P. Lebedev, Stella L. Korableva, Oleg A. Morozov, Vladimir M. Kyashkin, Vadim V. Semashko and Pavel P. Fedorov

Photonics 2023, 10(1), 84; https://doi.org/10.3390/photonics10010084 - 11 Jan 2023

Cited by 2 | Viewed by 1859

Abstract

The basic mechanisms of absorption of THz range radiation in optically perfect LiGdF₄ single crystals were studied using the broadband experimental data and the dielectric response function analysis within the harmonic oscillator model. The polarized IR reflection spectra have allowed one to [...] Read more.

The basic mechanisms of absorption of THz range radiation in optically perfect LiGdF₄ single crystals were studied using the broadband experimental data and the dielectric response function analysis within the harmonic oscillator model. The polarized IR reflection spectra have allowed one to determine the phonon contribution in the absorption coefficient in the THz range, while transmission spectra in the THz range were used to obtain the birefringence value and the effects of various mulitparticle processes. Additionally, we established the optical and electrodynamic parameters of the LiGdF₄ single crystal, which are necessary for the design of nonlinear optical devices. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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9 pages, 515 KiB

Open AccessCommunication

Calculation of the Proportion of Free Water Molecules in Aqueous Solutions Using the Parameters of Their Dielectric Permittivity in the Terahertz Range, Based on the Onsager Theory

by Nikita V. Penkov

Photonics 2023, 10(1), 44; https://doi.org/10.3390/photonics10010044 - 31 Dec 2022

Cited by 8 | Viewed by 2034

Abstract

The question of the structure of aqueous solutions is one of the most fundamental and complex, while it is relevant to all natural science disciplines. An important parameter of the dynamically equilibrium structure of an aqueous solution is the number of free water [...] Read more.

The question of the structure of aqueous solutions is one of the most fundamental and complex, while it is relevant to all natural science disciplines. An important parameter of the dynamically equilibrium structure of an aqueous solution is the number of free water molecules. To date, there are no reliable and fully justified methods for determining this parameter. Recently, the terahertz time-domain spectroscopy (THz-TDS) method has been developing. It makes it possible to record the spectra of the complex permittivity in the THz region, where an orientation relaxation band of free water molecules is detected for aqueous solutions. The purpose of this work is to establish the relationship of the parameters of THz dielectric permittivity with the number of free water molecules. For this purpose, the process of polarization of water in the THz region was theoretically considered using the formalism of electrodynamics of continuous media. The Onsager theory is taken as a basis and its development is carried out for the case of high-frequency fields. As a result, an analytical ratio was obtained for calculating the proportion of free water molecules in solutions. A comparison with other well-known, more simplified and poorly substantiated approaches is presented. Calculations of the fraction of free molecules for a number of aqueous solutions have been carried out. It can be argued that the first theoretically justified approach to calculating the population of free water molecules in a solution, which does not contain internal contradictions, is presented. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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8 pages, 1780 KiB

Open AccessArticle

Terahertz Time-Domain Polarimetry for Principal Optical Axes of Anisotropic Crystals

by Alexander Mamrashev, Fedor Minakov, Nazar Nikolaev and Valery Antsygin

Photonics 2021, 8(6), 213; https://doi.org/10.3390/photonics8060213 - 10 Jun 2021

Cited by 12 | Viewed by 2580

Abstract

We propose a method for measuring the terahertz properties for two principal optical axes of anisotropic crystals without optical activity using terahertz time-domain spectroscopy (THz-TDS). The method put forward in this paper utilizes the inherent polarization sensitivity of the THz-TDS electro-optic detection system. [...] Read more.

We propose a method for measuring the terahertz properties for two principal optical axes of anisotropic crystals without optical activity using terahertz time-domain spectroscopy (THz-TDS). The method put forward in this paper utilizes the inherent polarization sensitivity of the THz-TDS electro-optic detection system. We demonstrate the practical application of the method by measuring the temperature dependence of the refractive index and the absorption coefficient of a lithium triborate crystal for three optical axes. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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Review

Jump to: Research

32 pages, 5244 KiB

Open AccessReview

Review of Bioplastics Characterisation by Terahertz Techniques in the View of Ensuring a Circular Economy

by Andreja Abina, Tjaša Korošec, Uroš Puc and Aleksander Zidanšek

Photonics 2023, 10(8), 883; https://doi.org/10.3390/photonics10080883 - 29 Jul 2023

Cited by 6 | Viewed by 2215

Abstract

The increasing scarcity of natural resources, worsening global climate change, environmental degradation, and rising demand for food are forcing the biotechnology and plastics industries to seek and apply circular economy models that would lead to a sustainable transition in the production and use [...] Read more.

The increasing scarcity of natural resources, worsening global climate change, environmental degradation, and rising demand for food are forcing the biotechnology and plastics industries to seek and apply circular economy models that would lead to a sustainable transition in the production and use of bioplastics. Circular economy models can improve the economic productivity of bio-based plastics and have a positive impact on the environment by reducing conventional plastic waste and the consumption of petrochemical feedstocks for plastic production. In addition, some agricultural wastes that have the potential to be used as bioplastics can be reused. Terahertz (THz) systems are already used in the plastics and rubber industries for non-destructive testing, detection, imaging, and quality control. Several reports have highlighted the potential applications of THz spectroscopy and imaging in polymer analysis and plastics characterisation. This potential is even greater with chemometric methods and artificial intelligence algorithms. In this review, we focus on applications that support the transformation of the biotechnology sector to the circular economy, particularly via the transition from conventional plastics to bioplastics. In this review, we discuss the potential of THz systems for the characterisation and analysis of bioplastics and biopolymers. The results of previous studies on biopolymers in the THz frequency range are summarised. Furthermore, the potential of using artificial intelligence approaches such as machine learning as advanced analytical methods in THz spectroscopy and imaging, in addition to the conventionally used chemometric methods, is discussed. The results of this review highlight that THz technology can contribute to closed technological circles in important areas of biotechnology and the related plastics and rubber industries. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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27 pages, 6061 KiB

Open AccessReview

Regulating Terahertz Photoconductivity in Two-Dimensional Materials

by Xiao Xing, Zeyu Zhang and Guohong Ma

Photonics 2023, 10(7), 810; https://doi.org/10.3390/photonics10070810 - 12 Jul 2023

Cited by 3 | Viewed by 2516

Abstract

Two-dimensional materials represented by graphene have attracted extensive interest owing to the unique layer-dependent physical properties that are tunable with various external fields. In addition, by stacking two or more 2D materials together, a new material with the desired properties can be tailored [...] Read more.

Two-dimensional materials represented by graphene have attracted extensive interest owing to the unique layer-dependent physical properties that are tunable with various external fields. In addition, by stacking two or more 2D materials together, a new material with the desired properties can be tailored and designed. Fully understanding the dynamical photoconductive response in 2D materials is uttermost important to design and develop the advanced optoelectronic devices. Terahertz (THz) time-domain spectroscopy (TDS) and time-resolved THz spectroscopy are powerful spectroscopic tools with the advantages of being contact-free and noninvasive, which have been widely used to study the photoconductivity (PC) of 2D materials. In this review, firstly, we provide a short introduction of the 2D materials and THz spectroscopy, and then a brief introduction of the experimental setup and experimental data analysis based on time-resolved THz spectroscopy are presented. After that, we overview the latest progress on the regulation of the THz PC that includes: (1) regulating the THz PC of graphene (Gr) and transition metal dichalcogenide (TMD) thin films with oxygen adsorption; (2) regulating the THz PC of Gr and Gr/TMDs heterostructures by electric gating and a built-in field introduced by a substrate; (3) regulating the THz PC of Gr/TMD heterostructures via optical gating; and (4) we overview the latest progress on the observation of elementary excitations in 2D materials with THz PC spectra following optical excitation and THz PC regulation via the photoexcitation of quasi-particles. Finally, we conclude the review and present a short overview of future research directions. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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16 pages, 2857 KiB

Open AccessEditor’s ChoiceReview

Development and Application of THz Gyrotrons for Advanced Spectroscopic Methods

by Svilen Sabchevski and Mikhail Glyavin

Photonics 2023, 10(2), 189; https://doi.org/10.3390/photonics10020189 - 10 Feb 2023

Cited by 10 | Viewed by 2541

Abstract

Nowadays, gyrotrons are used in numerous and diverse fields of research and technology. Their most prominent application is to electron cyclotron resonance plasma heating and current drive-in reactors for controlled thermonuclear fusion. Another matured field is the thermal microwave treatment of materials in [...] Read more.

Nowadays, gyrotrons are used in numerous and diverse fields of research and technology. Their most prominent application is to electron cyclotron resonance plasma heating and current drive-in reactors for controlled thermonuclear fusion. Another matured field is the thermal microwave treatment of materials in industrial-grade gyrotron-based technological systems. The unique spectral properties of gyrotron radiation, frequency tunability, and the possibility for precise control and modulation of both the output power and frequency have made the gyrotrons attractive and appropriate radiation sources for various novel advanced spectroscopic techniques. Among them are ESR (electron spin resonance), NMR-DNP (nuclear magnetic resonance with a signal enhancement through dynamic nuclear polarization), XDMR (X-ray detected magnetic resonance), acoustic molecular spectroscopy, as well as high-precision spectroscopy for measuring the SFS (super-fine splitting of the energy levels of positronium). In this review paper, we present both the current status and the most remarkable recent achievements of these methods implemented in gyrotron-based spectroscopy systems and discuss the main trends in the development of their dedicated radiation sources operating in the THz frequency range. Full article

(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)

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