Abstract-Optoelectronic frequency-modulated continuous-wave terahertz spectroscopy with 4 THz bandwidth

 Lars Liebermeister, Simon Nellen, Robert B. Kohlhaas, Sebastian Lauck, Milan Deumer, Steffen Breuer, Martin Schell, Björn Globisch 

https://www.nature.com/articles/s41467-021-21260-x

Broadband terahertz spectroscopy enables many promising applications in science and industry alike. However, the complexity of existing terahertz systems has as yet prevented the breakthrough of this technology. In particular, established terahertz time-domain spectroscopy (TDS) schemes rely on complex femtosecond lasers and optical delay lines. Here, we present a method for optoelectronic, frequency-modulated continuous-wave (FMCW) terahertz sensing, which is a powerful tool for broadband spectroscopy and industrial non-destructive testing. In our method, a frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing, employing a time-delayed copy of the same beat signal. Consequently, the receiver current is inherently phase-modulated without additional modulator. Owing to this technique, our broadband terahertz spectrometer performs (200 Hz measurement rate, or 4 THz bandwidth and 117 dB peak dynamic range with averaging) comparably to state-of-the-art terahertz-TDS systems, yet with significantly reduced complexity. Thickness measurements of multilayer dielectric samples with layer-thicknesses down to 23 µm show its potential for real-world applications. Within only 0.2 s measurement time, an uncertainty of less than 2 % is achieved, the highest accuracy reported with continuous-wave terahertz spectroscopy. Hence, the optoelectronic FMCW approach paves the way towards broadband and compact terahertz spectrometers that combine fiber optics and photonic integration technologies.

Abstract-Broadband terahertz spectroscopy at high magnetic field using the 25 Tesla Split Florida-Helix magnet

David J. Hilton,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10983/109830S/Broadband-terahertz-spectroscopy-at-high-magnetic-field-using-the-25/10.1117/12.2519796.short?SSO=1

We describe the development of a novel high magnetic field ultrafast spectrometer that uses the 25 Tesla Split Florida-Helix magnet. This system has a large numerical aperture that permits the free space propagation of ultrafast pulses to a sample in this large external magnetic field. The system has an operating bandwidth that spans 0.3 to 10 THz. We discuss the utility of this novel instrument for nonequilibrium dynamical measurements and stabilization of alternate orders in magnetic active materials.

© (2019) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Broadband Terahertz Spectroscopy of Imidazolium-Based Ionic Liquids

Sen Mou, Andrea Rubano, Domenico Paparo

https://cdn-pubs.acs.org/doi/abs/10.1021/acs.jpcb.7b10886

Ionic liquids are liquid salts at ambient temperature composed of organic cations and organic/inorganic anions. Outstanding physical and chemical properties of ionic liquids lead to increasing application in scientific and industrial field. Ionic liquids have been already investigated by different spectroscopic techniques, including terahertz (THz) time-domain spectroscopy. The usual THz frequency range extends up to 2-3 THz, a relatively narrow band, which can only show the intermolecular vibrational modes. Here we report about broadband THz spectroscopy of ILs up to 13 THz. Bandwidth of intermolecular absorption band presents unexpected behavior and strong sharp intramolecular absorptions are shown. In addition, we found violation of the approximation of harmonic oscillator used to predict the peak shift of intermolecular absorption band.

Abstract-Coexistence of Kosmotropic and Chaotropic Impacts of Urea on Water as Revealed by Terahertz Spectroscopy

Keiichiro Shiraga, Koichiro Tanaka, Yuichi Ogawa, Takuro Tajima

https://www.researchgate.net/publication/322148885_Coexistence_of_Kosmotropic_and_Chaotropic_Impacts_of_Urea_on_Water_as_Revealed_by_Terahertz_Spectroscopy

Whether urea can serve as a kosmotrope or chaotrope has long been a topic of debate. In this study, broadband THz spectroscopy (from 0.2 to 12 THz) of aqueous solutions of urea was used to characterize the hydration state and the hydrogen bond structure of water around urea. Three low-frequency vibration modes of urea were found around 2 THz, 4 THz, and above 12 THz. After eliminating the contribution of these modes, the “urea-vibration-free” complex dielectric constant was decomposed into the relaxation modes of bulk water and the oscillation modes of water. When hydration water is defined to be reorientationally retarded relative to bulk, our analysis revealed that the hydration number is 1.9 independent of the urea concentrations up to 5 M and this number is close agreement with that of water constrained by the strong acceptor hydrogen bonds of urea oxygen. Regarding the hydrogen bond structure, it was found that the tetrahedral-like water structure is mostly preserved (though the hydrogen-bond lifetime is significantly shortened) but the population of non-hydrogen-bonded water molecules fragmented from the network is markedly increased, presumably due to the urea’s NH2 inversion. These experimental results point to coexistence of apparently two contradictory aspects of urea: dynamical retardation (the kosmotropic aspect) by the –CO group and slight structural disturbance (the chaotropic aspect) by the –NH2 group.

Abstract-Broadband terahertz spectroscopy of the insulator-metal transition driven by coherent lattice deformation at the SmNiO3/LaAlO3 interface

W. Hu, S. Catalano, M. Gibert, J.-M. Triscone, and A. Cavalleri

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.161107

We investigate the nonequilibrium insulator-metal transition driven in a SmNiO3 thin film by coherent optical excitation of the LaAlO3 substrate lattice. By probing the transient optical properties over a broad frequency range (100−800cm−1), we analyze both the time-dependent metallic plasma and the infrared optical phonon line shapes. We show that the light-induced metallic phase in SmNiO3has the same carrier density as the equilibrium metallic phase. We also report that the LaAlO3substrate acts as a transducer only at the earlier time delays, as the vibrations are driven coherently. No long-lived structural rearrangement takes place in the substrate. Finally, we show that the transient insulator-metal transition occurs both below and above the Néel temperature. We conclude that the supersonic melting of magnetic order measured with ultrafast x rays is not the driving force of the formation of the metallic phase. We posit that the insulator-metal transition may origin from the rearrangement of ordered charges at the interface propagating into the film.

• 
• 
•