Abstract-Quantum-inspired terahertz spectroscopy with visible photons

 

Mirco Kutas, Björn Haase, Jens Klier, Daniel Molter, and Georg von Freymann

Experimental setup. The 1 mm long PPLN crystals are pumped by a continuous-wave laser with a wavelength of 660 nm, generating correlated pairs of signal and terahertz photons. After the crystal the terahertz radiation is separated by an OAP with a through hole and afterwards reflected at a moveable mirror Mi. Pump and generated signal photons are reflected at Ms directly back into the crystal. After the second pass the pump radiation is filtered from the signal radiation by three volume Bragg gratings (VBGs). To obtain a frequency-angular spectrum on the sCMOS camera, the signal radiation is focused through a transmission grating (TG).

https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-8-4-438&id=449480

Terahertz technology offers solutions in nondestructive testing and spectroscopy for many scientific and industrial applications. While direct detection of photons in this frequency range is difficult to achieve, quantum optics provides a highly attractive alternative: it enables the characterization of materials in hardly accessible spectral ranges by measuring easily detectable photons of a different spectral range. Here we report on the application of this principle to terahertz spectroscopy, measuring absorption features of chemicals at sub-terahertz frequencies by detecting visible photons. To generate the needed correlated signal-idler photon pairs, a periodically poled lithium niobate crystal and a 660 nm continuous-wave pump source are used. After propagating through a single-crystal nonlinear interferometer, the pump photons are filtered by narrowband volume Bragg gratings. An uncooled scientific CMOS camera detects the frequency-angular spectra of the remaining visible signal and reveals terahertz-spectral information. Neither cooled detectors nor expensive pulsed lasers for coherent detection are required.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz thickness determination with interferometric vibration correction for industrial applications

Tobias Pfeiffer, Stefan Weber, Jens Klier, Sebastian Bachtler, Daniel Molter, Joachim Jonuscheit, and Georg Von Freymann

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-10-12558

In many industrial fields, like automotive and painting industry, the thickness of thin layers is a crucial parameter for quality control. Hence, the demand for thickness measurement techniques continuously grows. In particular, non-destructive and contact-free terahertz techniques access a wide range of thickness determination applications. However, terahertz time-domain spectroscopy based systems perform the measurement in a sampling manner, requiring fixed distances between measurement head and sample. In harsh industrial environments vibrations of sample and measurement head distort the time-base and decrease measurement accuracy. We present an interferometer-based vibration correction for terahertz time-domain measurements, able to reduce thickness distortion by one order of magnitude for vibrations with frequencies up to 100 Hz and amplitudes up to 100 µm. We further verify the experimental results by numerical calculations and find very good agreement.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement