Infrared (IR) spectroscopy is an indispensable tool for many practical applications including material analysis and sensing. Existing IR spectroscopy techniques face challenges related to the inferior performance and the high cost of IR-grade components. Here, we develop a new method, which allows studying properties of materials in the IR range using only visible light optics and detectors. It is based on the nonlinear interference of entangled photons, generated via Spontaneous Parametric Down Conversion (SPDC). In our interferometer, the phase of the signal photon in the visible range depends on the phase of an entangled IR photon. When the IR photon is traveling through the media, its properties can be found from observations of the visible photon. We directly acquire the SPDC signal with a visible range CCD camera and use a numerical algorithm to infer the absorption coefficient and the refraction index of the sample in the IR range. Our method does not require the use of a spectrometer and a slit, thus it allows achieving higher signal-to-noise ratio than the earlier developed method.
We propose and experimentally demonstrate a dielectric metasurface that allows monitoring of polarization deviations from an arbitrary elliptical input anchor state simply by tracking in real-time the output ratio between the powers of horizontal and vertical components after the metasurface. Importantly, this ratio can be enhanced corresponding to increased responsivity. Such nontrivial functionality is achieved by designing binary metasurfaces that realize tailored nonunitary and chiral polarization transformation. We experimentally demonstrate the operation at telecommunication wavelengths with enhanced responsivity up to 25 for various anchor states, including the strongly elliptical and circular. We also achieve the uncertainty of deviation measurement that is significantly better than the fundamental limit for nonchiral metasurfaces.
Spectral measurements in the infrared (IR) optical range provide unique fingerprints of materials which are useful for material analysis, environmental sensing, and health diagnostics 1 Entangled photons continue to play a crucial role in advancing many areas of quantum technologies, including cryptography 4,5 , computing 6,7 and metrology 8,9 . They can be obtained using a variety of methods, with SPDC in non-linear optical crystals, being well established 10 .We consider a specific type of interference technique, referred to as a non-linear interferometry, which is analogue to a conventional Mach-Zehnder or Young interferometer but with the two splitting mirrors being substituted with two SPDC crystals 3 . In a non-linear interferometer, two SPDC crystals are pumped by a common laser, so that down-converted photons (signal and idler) from one crystal are injected into the second crystal. Signal and idler photons from the two crystals interfere and produce a distinctive interference pattern in frequency and spatial domains. Depending on experimental configuration one can observe interference either in intensity or in the second-order correlation function [11][12][13] .One remarkable feature of non-linear interferometers is that the interference pattern for signal photons is determined by a total phase, acquired by all three propagating photons: the signal, the idler and the pump 2,3 . This is different than conventional interferometry, where the interference pattern is defined solely by the phase of the signal photon. From the interference pattern of the signal photon, it is possible to infer a relative phase of an idler photon. Actual detection of idler photons is not required. This scheme has found its applications in imaging
We develop and experimentally realize all-dielectric metasurfaces that can transform any input two-photon polarization-entangled state to an arbitrary target state, achieving a tailored control over the degree of entanglement through a theoretically optimal non-Hermitian operation.
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