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Nonlinear Frequency Generation and Conversion: Materials and Devices

First demonstration of nonlinear interferometry in the terahertz frequency range

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

Published Mar 4, 2021• 0 citations
Publisher

Abstract

Using nonlinear interferometers is highly attractive for accessing spectral regions in which the photon detection is technically challenging. Especially for terahertz radiation this is a long-lasting problem, as their photon energy is in the range of a few meV. Therefore, the properties of terahertz photons after interaction with a sample are transferred to visible photons, whose detection can be realized by widely available imaging sensors. We demonstrated this concept with terahertz photons propagating in free space, determining the thickness of polytetrafluoroethylene pla tes by only detecting visible photons. We utilized a nonlinear interferometer with one periodically poled lithium niobate crystal driven by a 660-nm pump source. In the crystal, pairs of visible signal and correlated terahertz photons are created and separated into different paths of the interferometer afterwards. The terahertz photons pass the sample and gain information that can be transferred to the visible photons. To detect the signal photons with an uncooled sCMOS camera, the pump photons are filtered out by narrowband volume Bragg gratings. As the signal photons are mainly generated by down - and up-conversion of thermal photons besidesspontaneous parametric down-conversion, the frequency-angular spectra show nonlinear interference in the Stokes and anti-Stokes regions. This interference can be used to assess information of coatings that are mainly transparent in the terahertz frequency range. Establishing on this first demonstration of a nonlinear interferomete r with terahertz photons, we improved the visibility by a factor of 3 by modifying the experimental setup, helping this concept on its way towards industrial applications.

DOI
10.1117/12.2578410

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Using nonlinear interferometers is highly attractive for accessing spectral regions in which the photon detection is technically challenging. Especially for terahertz radiation this is a long-lasting problem, as their photon energy is in the range of a few meV. Therefore, the properties of terahertz photons after interaction with a sample are transferred to visible photons, whose detection can be realized by widely available imaging sensors. We demonstrated this concept with terahertz photons propagating in free space, determining the thickness of polytetrafluoroethylene pla tes by only detecting visible photons. We utilized a nonlinear interferometer with one periodically poled lithium niobate crystal driven by a 660-nm pump source. In the crystal, pairs of visible signal and correlated terahertz photons are created and separated into different paths of the interferometer afterwards. The terahertz photons pass the sample and gain information that can be transferred to the visible photons. To detect the signal photons with an uncooled sCMOS camera, the pump photons are filtered out by narrowband volume Bragg gratings. As the signal photons are mainly generated by down - and up-conversion of thermal photons besidesspontaneous parametric down-conversion, the frequency-angular spectra show nonlinear interference in the Stokes and anti-Stokes regions. This interference can be used to assess information of coatings that are mainly transparent in the terahertz frequency range. Establishing on this first demonstration of a nonlinear interferomete r with terahertz photons, we improved the visibility by a factor of 3 by modifying the experimental setup, helping this concept on its way towards industrial applications.
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 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. As a first demonstration, we show layer thickness measurements with terahertz photons based on biphoton interference. As non-destructive layer thickness measurements are of high industrial relevance, our experiments might be seen as a first step towards industrial quantum-sensing applications.
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.
In quantum sensing experiments, photons in the spectral region of interest interact with a sample and their properties are transferred via biphoton-correlation to another spectral region, where widely available and highly developed detectors can be used. Especially spectral regions where the photon detection is quite difficult can benefit from this novel technique. 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 determining the thickness of polytetrafluoroethylene plates by only detecting visible photons [1].
Generation and Detection of terahertz radiation still remain a technological challenge, especially when compared to the visible or near infrared spectral range. For this reason, approaches are continuously pursued to transfer the terahertz radiation to other frequency ranges after interaction with the sample in order to utilize the advantages of the advanced detector technology. Novel quantum-optical concepts offer a further possibility to realize this transfer. This makes them an attractive alternative for the terahertz frequency range, as they only require cost-effective components such as visible continuous wave lasers and silicon-based detectors. We report on the first demonstration of imaging with undetected photons in the terahertz frequency range.
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.
We have demonstrated, for the first time, coherent terahertz (THz)-wave phase detection using nonlinear optical interference based on backward THz-wave parametric oscillation (BW-TPO). Slant-stripe-type periodically poled lithium niobate (PPLN) crystal were used as the BW-TPO gain medium to generate and detect the backward-propagating THz-wave at 0.3 THz. Phases of all pump, idler, and THz-wave were detected from the interference between the generation side idler and the detection signal. This is the first report of a comprehensive study of the phase information transfer by modulating and detecting all phases contributing to coherent THz-wave detection. This interference with suppressed phase noise achieved a THz-wave phase detection with a high visibility of 0.89. This achievement not only has sensing applications that simultaneously measure the real and imaginary parts of the refractive index as a stand-alone system but also reaches the limit of classical detection sensitivity when combined with highly sensitive THz-wave detection. Furthermore, this works paves way to the realization of ultra-sensitive THz-wave detection beyond the standard quantum limit by the non-classical states of squeezed light in our system.