Abstract-System-theoretical modeling of terahertz time-domain spectroscopy with ultra-high repetition rate mode-locked lasers

Kevin Kolpatzeck, Xuan Liu, Kai-Henning Tybussek, Lars Häring, Marlene Zander, Wolfgang Rehbein, Martin Moehrle, Andreas Czylwik, and Jan C. Balzer

 Block diagram of a fiber-coupled THz-TDS setup. The optical output signal of the mode-locked laser source is distributed to a terahertz emitter (THz Tx) and, through a variable delay line, to a terahertz detector (THz Rx). The terahertz radiation generated by the emitter is transmitted through a sample and focused into the detector

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-11-16935

Terahertz time-domain spectroscopy (THz-TDS) systems based on ultra-high repetition rate mode-locked laser diodes (MLLDs) and semiconductor photomixers show great potential in terms of a wide bandwidth, fast acquisition speed, compactness, and robustness. They come at a much lower total cost than systems using femtosecond fiber lasers. However, to date, there is no adequate mathematical description of THz-TDS using a MLLD. In this paper, we provide a simple formula based on a system-theoretical model that accurately describes the detected terahertz spectrum as a function of the optical amplitude and phase spectrum of the MLLD and the transfer function of the terahertz system. Furthermore, we give a simple yet exact relationship between the optical intensity autocorrelation and the detected terahertz spectrum. We theoretically analyze these results for typical optical spectra of MLLDs to quantify the effect of pulse chirp on the terahertz spectrum. Finally, we confirm the validity of the model with comprehensive experimental results using a single-section and a two-section MLLD in a conventional THz-TDS system.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-In-situ evaluation of porosity in thermal barrier coatings based on the broadening of terahertz time-domain pulses: simulation and experimental investigations

Dongdong Ye, Weize Wang, Haiting Zhou, Jibo Huang, Wenchao Wu, Hanhong Gong, and Zhen Li

Morphologies of the 8YSZ powders. (a) Particle sizes in the range of 15–55 µm; (b) Particle sizes in the range of 40–96 µm.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-20-28150

Porosity is one of the most important indicators for the characterization of the comprehensive performance of thermal barrier coatings (TBCs). Herein, we explored a fast, nondestructive porosity evaluation method based on the terahertz time-domain broadening effect. Different preparation process parameters were used to deposit the ceramic coatings, and the porosity ranged from 9.09% to 21.68%. Monte Carlo simulations were conducted to reveal the transitive relation between porosity and the terahertz time-domain broadening at different extinction coefficients and transmission distances. A transmission mode with an incidence angle of 0° was used to estimate the terahertz dielectric properties of ceramic coatings and the relative broadening ratio of terahertz pulses at different porosities. As a result, the Monte Carlo simulations showed that the time-domain broadening effect was enhanced when the extinction coefficient and transmission distances increased. As the porosity increased, the refractive index decreased and the extinction coefficient increased. The latter was more sensitive to minor porosity changes as demonstrated by linear fitting comparisons. Meanwhile, the relative broadening ratio increased when the porosity increased, and reserved the sensitivity of the extinction coefficient to porosity changes. The effect was more obvious on the relative broadening ratio which experienced multiple transmissions and reflections. Moreover, the relative broadening ratio could be obtained faster and in an easier manner compared to the dielectric parameters in both the transmission and reflection modes, based on single-step tests with the use of actual terahertz wave inspection. Finally, this study proposed a novel, convenient, online, nondestructive, and noncontact porosity evaluation method that could be potentially utilized to evaluate the integrity of TBCs in gas turbine blades.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Terahertz waves reveal hidden processes in ultrafast artificial photosynthesis

Setup for time-resolved terahertz attenuated total reflection (TR-ATR) spectroscopy. Credit: Osaka University

https://phys.org/news/2019-09-terahertz-reveal-hidden-ultrafast-artificial.html

A team of researchers from Osaka University, in cooperation with Tokyo Institute of Technology, directly observed charge transfer and intermolecular interactions in artificial photosynthesis that occurs on a picosecond (ps) scale (10-12). With time-resolved attenuated total reflection (TR-ATR) spectroscopy in the terahertz (THz) region, they revealed the process of artificial photosynthesis material [Re(CO)2(bpy) {P(OEt)3}2](PF6) in Triethanolamine (TEOA) solvent as a reductant. Their research results were published in Scientific Reports.

Artificial photosynthesis, or a photocatalytic reaction to produce chemical energy from carbon oxide (CO2) and light, is, as with a solar battery, a promising next-generation clean energy. In particular, the photocatalytic reaction using rhenium (Re) complexes is highly efficient. In order to create efficient photocatalytic molecules, it is necessary to examine how the photocatalytic reaction occurs on a picosecond timescale. However, it was impossible to directly observe various phenomena in the photocatalytic reaction.

The researchers tried to obtain information about changes in relative positions of molecules and charge transfer by using time-resolved attenuated total reflection (TR-ATR) spectroscopy. Photocatalytic molecules which absorb light facilitate CO2 reduction to CO, bringing it to a higher energy level. They examined how charge transfer from the reductant TEOA to Re occurred in a photocatalytic reaction.

Because the use of THz waves, whose frequency is lower than those of visible light and infrared light, reveals changes in intermolecular vibrations (that is, binding energies between two neighboring molecules) in the THz (low-frequency) region, this allows one to observe how TEOA molecules around the Re complex move and how electron transfer occurs.

The schematic figures of the temporal evolution from I to III of the interaction between photocatalyst [Re(CO)₂(bpy){P(OEt)₃}₂]+ as a photocatalyst and TEOA molecules as reductants. Credit: Osaka University

However, since most solvents used in photocatalytic studies have a high absorption intensity in the THz region, it is difficult to observe Re in TEOA solvent. Thus, the team combined attenuated total reflection spectroscopy and THz time-domain spectroscopy to carry out TR-ATR in the THz region.

In addition, in order to conduct the ultrafast time-resolved measurements, they combined pump probe spectroscopy with TR-ATR, observing how TEOA molecules moved and how electron transfer occurred on a picosecond-scale timescale during a photocatalytic reaction, a world first. In pump probe spectroscopy, a pump pulse with the wavelength of 400 nm excited a sample and then a probe pulse (THz pulse) was used for probing the sample after an adjustable delay time. As a result, they were able to reveal the intermolecular vibrational mode with a three-step relaxation process on a picosecond timescale after photo-excitation:

• Until 9 ps, the temperature of the Re complex sharply increased due to light absorption, inducing heat transfer from Re ions to TEOA molecules, and the excited state was cooled down.
• From 10 to 14 ps, the distance between TEOA molecules and Re ions was reduced by the rotation of TEOA molecules.
• After 14 ps, charge transfer from the TEOA to Re occurred. The distance between these positively charged molecules grew by the repulsive Coulomb force, separating them.

Professor Kimura from Osaka University says, "The use of THz light allows us to observe the role of the reductant in photocatalytic reaction. TR-ATR spectroscopy in the THz region will help to develop highly efficient photocatalytic reactions. The observation of the relative motion between two molecules and charge dynamics by spectroscopy will assist research on various reaction processes in the fields of biology and chemistry."

Abstract-Effects of thermal aging on cellulose pressboard using terahertz time-domain spectroscopy

In-Sung Lee, Joong Wook Lee,



https://www.sciencedirect.com/science/article/pii/S1567173919301889

We demonstrate a method for examining the effects of thermal aging of cellulose pressboard using terahertz time-domain spectroscopy. The change of refractive indexes at the frequencies of pulsed terahertz waves while applying heat reveals that the degree of thermal aging and the condition of thermal breakdown can be clearly analyzed by performing terahertz measurements. The visual examination method only allows evaluation of the degree of blackness on the external surfaces of cellulose pressboard. However, using terahertz techniques for non-destructive inspection enables determination of the dielectric integrity to check the strength of the electrical insulation inside the cellulose pressboard. The results indicate that the terahertz techniques could be very useful, especially in the electric power industry, which requires a variety of methods for internal non-destructive testing to ensure the safety and reliability of insulation materials in electric power facilities and components.

Abstract-Insights into the water status in hydrous minerals using terahertz time-domain spectroscopy

Yuanyuan Ma, Haochong Huang, Sibo Hao, Kunfeng Qiu, Hua Gao, Lu Gao, Weichong Tang, Zili Zhang, Zhiyuan Zheng


https://www.nature.com/articles/s41598-019-45739-2

The determinations of water status incorporated in hydrous minerals are of considerable significances in geoscience fields. Coincidentally, the aqueous sensitivity of terahertz radiation has motivated numerous explorations in several cross-domain applications. Terahertz time-domain spectroscopy is employed as a major probing technique coupling of traditional detecting methods to uncover the mask of water status in copper sulfate pentahydrate as well as mineral quartz in this article. Based on the quantitative identification of water status in copper sulfate pentahydrate, the water incorporated in mineral quartz is verified qualitatively. Notable differences of optical constants originating from the water content are obtained for copper sulfate pentahydrate and mineral quartz. These present works indicate that terahertz technology can be considered as a promising method to satisfy the ever-increasing requirements in hydrous mineral analyses.

Abstract-Nondestructive imaging of hidden defects in aircraft sandwich composites using terahertz time-domain spectroscopy

Qiang Wang, Xinyi Li, Tianying Chang, Jin Zhang, Lingyu Liu, Hongbin Zhou, Jinpeng Bai,



https://www.sciencedirect.com/science/article/pii/S1350449518306091

Aircraft sandwich composites are superior to ordinary composites and have been largely used in the manufacture of military aircraft. This study attempts to investigate the efficacy of terahertz (THz) time-domain spectroscopy (TDS) imaging technology in detecting hidden defects in aircraft glass fiber (GF) sandwich composites. The defects of various sizes, such as debonding, delamination, and multi-delamination, at different depths in GF A-sandwich structure composites with foam core, C-sandwich structure composites with honeycomb core, and sheet-to-sheet cementing structure composites were evaluated. The THz frequency-domain amplitude and time-domain amplitude imaging algorithms were used to visualize the defects simulated by ultrathin double-layer Teflon inserts. And the suitable image processing methods which include wavelet-based fusion and multiscale edge representation were employed. With a combination of high-resolution THz C-scan and B-scan imaging, both the horizontal size and location, and the vertical depth and thickness of the defects were obtained in three dimensions. This study experimentally demonstrated that THz imaging technology can clearly detect various hidden defects in aircraft GF sandwich composites through reflection or transmission imaging mode.

Abstract-A Terahertz Time Domain Spectroscopy-based Network Analyzer

Ramer, Jan-Martin ; Jan-Martin Ramer is with the department of material characterization of Fraunhofer institute for physical measurement techniques IPM, Fraunhofer- Platz 1, 67663 Kaiserslautern and the department of physics of the University of Kaiserslautern as well as the state research center OPTIMAS, Erwin-Schrodinger-Straße 56, 67663 Kaiserslautern (e-mail: Jan- Martin.Raemer@ipm.fraunhofer.de). ; von Freymann, G.

http://ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&arnumber=7001556&sortType%3Dasc_p_Sequence%26filter%3DAND%28p_IS_Number%3A4357488%29%26rowsPerPage%3D50


We present a terahertz time domain spectroscopybased setup based on photoconductive antennas and a femtosecond fiber laser for measuring the transmission S parameters of electronic devices. To this end, radiation is focused into horn antennas attached to the waveguide-coupled devices under test and coupled out of the device using a similar setup. The terahertz emitter is fiber-coupled in order to allow for flexible selection the measurement geometry. As test cases, the S21 parameters of several H band (220-330 GHz) amplifiers are measured and show promising results. Also, the response of a W band amplifier is measured, marking the lower frequency boundary for this setup. In both cases, the measurements are compared to measurements retrieved using a commercially available vector network analyzer with frequency extenders, showing rather good agreement. Transmission measurements of “thru” waveguides used as a reference imply our system’s ability to measure up to at least 2.5 THz, widely exceeding the bandwidth of currently available network analyzers.