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.
Layer-thickness measurement is one of the most promising and attractive fields of application for terahertz measurement systems, as they really provide benefits in comparison to competing techniques. In contrast to ultrasound systems, terahertz measurements can be carried out without a coupling medium and is therefore a truly contactless measurement. The possibility to measure individual layers in a multilayer stack is highly advantageous in contrast to established eddy current measurement devices. Unlike X-ray devices, terahertz radiation of common measurement systems is not harmful to biological tissue. Terahertz measurement systems have undergone a remarkable development in terms of the performance as well as in the evaluation algorithms. Increase of speed and enhancement of measurement robustness make these optically complex systems ready for industrial employment. In our contribution, we will cover the development of photonic terahertz measurement systems with a focus on terahertz layer thickness determination.
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