Special Issue "THz Spectroscopy: Basic Science and Application"

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

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

Special Issue Editor

Prof. Jerzy Łusakowski
E-Mail
Guest Editor
Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warsaw, Poland
Interests: THz spectroscopy; semiconductor plasma; magnetotransport

Special Issue Information

Dear Colleagues,

The last two decades have witnessed a rapid growth of THz-related achievements both in basic science and applications. Different paths of THz research based on time–domain and Fourier spectroscopy systems, monochromatic excitations, applications of strong magnetic fields or intense radiation lead to more and more advanced experiments, revealing basic properties of matter at the quantum level and development of new THz techniques. THz devices and THz-oriented technologies and materials have become indispensable tools in advanced laboratories.

This Special Issue of Applied Sciences on “THz Spectroscopy: Basic Science and Applications” aims at presenting a collection of papers which will show the current status of research referring to quantum phenomena occurring at THz frequencies, THz properties of advanced materials, and performance of THz devices. We encourage authors to submit papers describing results of their studies on various aspects of THz science, ranging from sources and detectors, through THz optical components and plasmonic devices to metamaterials or topological insulators.

We express our hope that this Special Issue will serve as a useful reference both for specialists looking for new ideas and newcomers wishing to find out the beauty of the world observed in the THz light.

Prof. Jerzy Łusakowski
Guest Editor

Manuscript Submission Information

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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

  • THz sources and detectors
  • THz spectroscopy
  • THz metamaterials
  • semiconductor plasma and plasmonic devices
  • THz metamaterials
  • THz experimental techniques
  • narrow-gap semiconductors
  • topological insulators

Published Papers (6 papers)

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Research

Open AccessArticle
Homodyne Spectroscopy with Broadband Terahertz Power Detector Based on 90-nm Silicon CMOS Transistor
Appl. Sci. 2021, 11(1), 412; https://doi.org/10.3390/app11010412 - 04 Jan 2021
Viewed by 373
Abstract
Over the last two decades, photomixer-based continuous wave systems developed into versatile and practical tools for terahertz (THz) spectroscopy. The high responsivity to the THz field amplitude of photomixer-based systems is predetermined by the homodyne detection principle that allows the system to have [...] Read more.
Over the last two decades, photomixer-based continuous wave systems developed into versatile and practical tools for terahertz (THz) spectroscopy. The high responsivity to the THz field amplitude of photomixer-based systems is predetermined by the homodyne detection principle that allows the system to have high sensitivity. Here, we show that the advantages of homodyne detection can be exploited with broadband power detectors combined with two photomixer sources. For this, we employ a THz detector based on a complementary metal-oxide-semiconductor field-effect transistor and a broadband bow-tie antenna (TeraFET). At 500 GHz and an effective noise bandwidth of 1 Hz, the response from one photomixer-based THz source resulted in an about 43 dB signal-to-noise ratio (SNR). We demonstrate that by employing a homodyne detection system by overlaying the radiation from two photomixers, the SNR can reach up to 70 dB at the same frequency with an integration time 100 ms. The improvement in SNR and the spectroscopic evidence for water vapor lines demonstrated up to 2.2 THz allow us to conclude that these detectors can be successfully used in practical continuous wave THz spectrometry systems. Full article
(This article belongs to the Special Issue THz Spectroscopy: Basic Science and Application)
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Open AccessArticle
Off-Axis Diffractive Optics for Compact Terahertz Detection Setup
Appl. Sci. 2020, 10(23), 8594; https://doi.org/10.3390/app10238594 - 30 Nov 2020
Cited by 1 | Viewed by 370
Abstract
Medical and many other applications require small-volume setups enabling terahertz imaging. Therefore, we aim to develop a device for the in-reflection examination of the samples. Thus, in this article, we focus on the diffractive elements for efficient redirection and focusing of the THz [...] Read more.
Medical and many other applications require small-volume setups enabling terahertz imaging. Therefore, we aim to develop a device for the in-reflection examination of the samples. Thus, in this article, we focus on the diffractive elements for efficient redirection and focusing of the THz radiation. A terahertz diffractive optical structure has been designed, optimized, manufactured (using extrusion-based 3D printing) and tested. Two manufacturing methods have been used—direct printing of the structures from PA12, and casting of the paraffin structures out of 3D-printed molds. Also, the limitations of the off-axis focusing have been discussed. To increase the efficiency, an iterative algorithm has been proposed that optimizes off-axis structures to focus the radiation into small focal spots located far from the optical axis, at an angle of more than 30 degrees. Moreover, the application of higher-order kinoform structure design allowed the maintaining of the smallest details of the manufactured optical element, using 3D printing techniques. Full article
(This article belongs to the Special Issue THz Spectroscopy: Basic Science and Application)
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Open AccessArticle
THz-TDS for Detecting Glycol Contamination in Engine Oil
Appl. Sci. 2020, 10(11), 3738; https://doi.org/10.3390/app10113738 - 28 May 2020
Cited by 1 | Viewed by 540
Abstract
There continues to be a need for an in-situ sensor system to monitor the engine oil of internal combustion engines. Engine oil needs to be monitored for contaminants and depletion of additives. While various sensor systems have been designed and evaluated, there is [...] Read more.
There continues to be a need for an in-situ sensor system to monitor the engine oil of internal combustion engines. Engine oil needs to be monitored for contaminants and depletion of additives. While various sensor systems have been designed and evaluated, there is still a need to develop and evaluate new sensing technologies. This study evaluated Terahertz time-domain spectroscopy (THz-TDS) for the identification and estimation of the glycol contamination of automotive engine oil. Glycol contamination is a result of a gasket or seal leak allowing coolant to enter an engine and mix with the engine oil. An engine oil intended for use in both diesel and gasoline engines was obtained. Fresh engine oil samples were contaminated with four levels of glycol (0 ppm, 150 ppm, 300 ppm, and 500 ppm). The samples were analyzed with THz-TDS and converted to frequency domain parameters of refractive index and absorption coefficient. While both parameters showed potential, the absorption coefficient had the best potential and was able to statistically discriminate among the four contamination levels. Full article
(This article belongs to the Special Issue THz Spectroscopy: Basic Science and Application)
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Open AccessArticle
Titanium-Based Microbolometers: Control of Spatial Profile of Terahertz Emission in Weak Power Sources
Appl. Sci. 2020, 10(10), 3400; https://doi.org/10.3390/app10103400 - 14 May 2020
Viewed by 498
Abstract
Terahertz (THz) imaging and spectroscopy set-ups require fine optical alignment or precise control of spatial mode profile. We demonstrate universal, convenient and easy-to-use imaging—resonant and broadband antenna coupled ultrasensitive titanium-based—dedicated to accurately adjust and control spatial mode profiles without additional focusing optical components [...] Read more.
Terahertz (THz) imaging and spectroscopy set-ups require fine optical alignment or precise control of spatial mode profile. We demonstrate universal, convenient and easy-to-use imaging—resonant and broadband antenna coupled ultrasensitive titanium-based—dedicated to accurately adjust and control spatial mode profiles without additional focusing optical components of weak power THz sources. Versatile operation of the devices is shown using different kinds of THz—electronic multiplier sources, optical THz mixer-based frequency domain and femtosecond optoelectronic THz time-domain spectrometers as well as optically pumped molecular THz laser. Features of the microbolometers within 0.15–0.6 THz range are exposed and discussed, their ability to detect spatial mode profiles beyond the antennas resonances, up to 2.52 THz, are explored. Polarization-sensitive mode control possibilities are examined in details. The suitability of the resonant antenna-coupled microbolometers to resolve low-absorbing objects at 0.3 THz is revealed via direct, dark field and phase contrast imaging techniques as well. Full article
(This article belongs to the Special Issue THz Spectroscopy: Basic Science and Application)
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Open AccessArticle
Grating Metamaterials Based on CdTe/CdMgTe Quantum Wells as Terahertz Detectors for High Magnetic Field Applications
Appl. Sci. 2020, 10(8), 2807; https://doi.org/10.3390/app10082807 - 18 Apr 2020
Viewed by 574
Abstract
The cyclotron and magnetoplasmon resonances were studied at 2 K in grating metamaterials fabricated on wafers with one or two modulation doped CdTe/CdMgTe quantum wells. The gratings (with the period varied between 2 μm and 8 μm) were prepared with an [...] Read more.
The cyclotron and magnetoplasmon resonances were studied at 2 K in grating metamaterials fabricated on wafers with one or two modulation doped CdTe/CdMgTe quantum wells. The gratings (with the period varied between 2 μ m and 8 μ m) were prepared with an electron beam lithography either by etching or by evaporation of Au. The gratings were studied with an atomic force microscope which revealed a correlation between the depth and width of etched grooves at a constant time of etching. The sharpest resonances observed are due to excitation of magnetoplasmon in the case of Au gratings on a wafer with one quantum well. Etched samples with two quantum wells showed the strongest tuneability of magnetoplasmon resonances with the period of the grating and illumination with white light. We showed that the samples studied can be used as resonant or quasi-resonant terahertz detectors tuneable with magnetic field and white light. Full article
(This article belongs to the Special Issue THz Spectroscopy: Basic Science and Application)
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Open AccessFeature PaperArticle
Terahertz Spectroscopy of Thermal Radiation from AlGaN/GaN Heterostructure on Sapphire at Low Temperatures
Appl. Sci. 2020, 10(3), 851; https://doi.org/10.3390/app10030851 - 25 Jan 2020
Cited by 1 | Viewed by 885
Abstract
Terahertz spectroscopy of thermal radiation from electrically pumped AlGaN/GaN structures on sapphire substrate was investigated in this work. Comparison of experimental THz spectroscopy results to theoretical spectra calculations shows that thermal radiation of the sample lattice is the main mechanism causing the emission [...] Read more.
Terahertz spectroscopy of thermal radiation from electrically pumped AlGaN/GaN structures on sapphire substrate was investigated in this work. Comparison of experimental THz spectroscopy results to theoretical spectra calculations shows that thermal radiation of the sample lattice is the main mechanism causing the emission above T = 155 K, and it is mainly influenced by sapphire substrate. Here, the emission was attributed to the radiative electron transitions in shallow impurities and nitrogen vacancies as well as to radiative decay of longitudinal optical phonons (387 cm - 1 ) in sapphire substrate. We have successfully demonstrated that THz emission spectroscopy can be used to define the temperature at which thermal emission from AlGaN/GaN HEMT structures dominates the emission spectrum. Full article
(This article belongs to the Special Issue THz Spectroscopy: Basic Science and Application)
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