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 detection by upconversion to the near-infrared using picosecond pulses

 

 Schematic of the experimental setup. A pulsed laser at about 1550 nm (orange) is used to pump the terahertz source and a frequency-doubling crystal to generate the NIR pump beam (red). Reflective bandpass filters are used to narrow the spectrum before the pump enters the nonlinear medium together with the terahertz radiation (green). SHG: second harmonics generation, PPLN: periodically poled lithium niobate crystal, RBP: reflective bandpass, M: mirror, ITO: indium-tin-oxide coated glass, PCA: photoconductive antenna, f1,f2${\mathrm{f}}_1,{\mathrm{f}}_2$: lenses (focal lengths of f1${\mathrm{f}}_1$: 125 mm, f2${\mathrm{f}}_2$: 400 mm).

Tobias Pfeiffer, Daniel Molter, and Georg von Freymann

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-20-29419

The detection of terahertz photons by using silicon-based devices enabled by visible photons is one of the fundamental ideas of quantum optics. Here, we present a classical detection principle using optical upconversion of terahertz photons to the near-infrared spectral range in the picosecond pulse regime, which finally enables the detection with a conventional sCMOS camera. By superimposing terahertz and optical pump pulses in a periodically poled lithium-niobate crystal, terahertz photons at 0.87 THz are converted to optical photons with wavelengths close to the central pump wavelength of 776 nm. A tunable delay between the pulses helps overlap the pulses and enables time-of-flight measurements. Using a sCMOS camera, we achieve a dynamic range of 47.8 dB with a signal to noise ratio of 23.5 dB at a measurement time of one second, in our current setup.

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

Abstract-Terahertz quantum sensing

 Björn Haase, Daniel Molter, Felix Riexinger, Mirco Kutas, Patricia Bickert,

https://advances.sciencemag.org/content/6/11/eaaz8065

Quantum sensing is highly attractive for accessing spectral regions in which the detection of photons is technically challenging: Sample information is gained in the spectral region of interest and transferred via biphoton correlations into another spectral range, for which highly sensitive detectors are available. This is especially beneficial for terahertz radiation, where no semiconductor detectors are available and coherent detection schemes or cryogenically cooled bolometers have to be used. Here, we report on the first demonstration of quantum sensing in the terahertz frequency range in which the terahertz photons interact with a sample in free space and information about the sample thickness is obtained by the detection of visible photons. As a first demonstration, we show layer thickness measurements with terahertz photons based on biphoton interference. As nondestructive layer thickness measurements are of high industrial relevance, our experiments might be seen as a first step toward industrial quantum sensing applications.

Abstract-Terahertz Quantum Sensing

Mirco Kutas, Björn Haase, Patricia Bickert, Felix Riexinger, Daniel Molter, Georg von Freymann

https://arxiv.org/abs/1909.06855

Quantum sensing is highly attractive for accessing spectral regions in which the detection of photons is technically challenging: sample information is gained in the spectral region of interest and transferred via entanglement into another spectral range, for which highly sensitive detectors are available. This is especially beneficial for terahertz radiation, as the corresponding photon energy lies in the range of a few meV - an energy where no semiconductor detectors are available and coherent detection schemes or cryogenically cooled bolometers have to be employed. Here, we report on the first demonstration of quantum sensing in the terahertz frequency range in which the terahertz photons interact with a sample in free space and information about the sample thickness is obtained by the detection of visible photons. A nonlinear single-crystal interferometer setup with a periodically poled lithium niobate crystal (PPLN) and a 660 nm pump source is used, generating visible (signal) photons and associated (idler) photons in the terahertz frequency range. Separation from the pump photons and detection of the visible signal photons is achieved by using highly efficient and narrowband volume Bragg gratings and an uncooled scientific complementary metal-oxide-semiconductor (sCMOS) camera. The acquired frequency-angular spectra show quantum interference in the Stokes as well as the Anti-Stokes part of collinear forward generation caused by spontaneous parametric down-conversion (SPDC) and down-conversion as well as up-conversion of thermal photons. The information encoded in the quantum interference can be used to determine the thickness of coatings or functional layers that are mainly transparent in the terahertz spectral range. As a first demonstration, we show layer thickness measurements with terahertz photons based on induced coherence without induced emission.

Abstract-Terahertz cross-correlation spectroscopy driven by incoherent light from a superluminescent diode

Daniel Molter, Michael Kolano, and Georg von Freymann

Fig. 1 Experimental setup: The output of the superluminescent diode (SLD) is optionally bandpass filtered (BP) and amplified by an EDFA. Afterwards, the light is sent to a standard TDS-like setup consisting of a beam splitter (50:50), delay section, terahertz receiver (Rx) and emitter (Tx) as well as reflective terahertz optics. Photomixers for CW terahertz generation and detection are used.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-9-12659

We present a novel terahertz spectroscopy principle by using incoherent light from a super luminescent diode for terahertz cross-correlation spectroscopy. The combless nature of this light source leads to a truly continuous terahertz spectrum. We demonstrate the possibility to influence the terahertz spectral bandwidth of the system by changing the bandwidth of different bandpass filters in the system. Depending on the employed bandpass filter we achieve peak dynamic ranges of 60 dB or a terahertz spectral width of about 1.7 THz. The applicability of the measurement system to spectroscopic terahertz measurement tasks is demonstrated.

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

Abstract-Spontaneous parametric down-conversion of photons at 660 nm to the terahertz and sub-terahertz frequency range

Björn Haase, Mirco Kutas, Felix Riexinger, Patricia Bickert, Andreas Keil, Daniel Molter, Michael Bortz, and Georg von Freymannark

Fig. 1 Phase-matching schemes. The periodic poling of the nonlinear crystal leads to a k-vector component kΛ, which can have two directions. This yields phase-matched terahertz generation in forward and backward direction. Pump (p), signal (s) and idler (THz) wave vectors have to be considered as non-collinear in down- (sd) and up-conversion (su).

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-5-7458

We report on spontaneous parametric down-conversion (SPDC) in periodically poled lithium niobate (PPLN) using 660 nm pump wavelength and the type 0 phase-matching condition to the terahertz and even sub-terahertz frequency range. Detection of the frequency-shifted signal photons is achieved by using highly efficient and narrowband volume Bragg gratings and an uncooled sCMOS camera. The acquired frequency-angular spectrum shows backward and forward generation of terahertz and sub-terahertz photons by SPDC, as well as up-conversion and higher order quasi phase-matching (QPM). The frequency-angular spectrum is theoretically calculated using a Monte-Carlo integration scheme showing a high agreement with the measurement. This work is one important step toward quantum sensing and imaging in the terahertz and sub-terahertz frequency range.

© 2019 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

Abstract-Interferometry-aided terahertz time-domain spectroscopy

Daniel Molter, Manuel Trierweiler, Frank Ellrich, Joachim Jonuscheit, and Georg Von Freymann

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-7-7547

Terahertz time-domain spectroscopy as well as all optical pump-probe techniques with ultrashort pulses relies on the exact knowledge of an optical delay between related laser pulses. Classical realizations of the measurement principle vary the optical path length for one of the pulses by mechanical translation of optical components. Most commonly, only an indirect measurement of the translation is carried out, which introduces inaccuracies due to imprecise mechanics or harsh environment. We present a comprehensive study on the effect of delay inaccuracies on the quality of terahertz spectra acquired with time-domain spectroscopy systems and present an interferometric technique to directly acquire the optical delay simultaneously to the terahertz measurement data. This measurement principle enables high-precision terahertz spectroscopy even in harsh environment with non-systematic disruptions.

© 2017 Optical Society of America