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arXiv:1611.01283 · Scientific Reports

Nonlinear infrared spectroscopy free from spectral selection

Anna V. Paterova, Shaun Lung, Dmitry A. Kalashnikov, Leonid A. Krivitsky

Published Feb 20, 2017• 52 citations
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Abstract

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.

DOI
10.1038/srep42608

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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 describe a proof-of-concept of a method for measurement of both real (refraction) and imaginary (absorption) part of the refractive index in the infrared (IR) range by measuring an interference pattern in the visible range without the need for any spectral and spatial selection. The concept is based on 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 of interest, its properties can be found from the observations of the visible photon.
Infrared (IR) spectroscopy and imaging are important tools in metrology, including material analysis, sensing and characterization. Although conventional methods of IR metrology are available, they face challenges related to the high cost and low efficiency of IR light sources and photodetectors. These challenges can be addressed by circumventing the need for IR-range components using tools and techniques of quantum optics. In this thesis, we describe a technique for IR measurements, which requires using only accessible light sources and detectors operating in the visible range. Our technique is based on the nonlinear interference of correlated photon pairs generated via spontaneous parametric down conversion (SPDC). In SPDC, a photon from the pump laser generates a pair of correlated photons within a nonlinear crystal. One photon of the pair can be generated in the visible range, and the other in the IR range. Two identical nonlinear crystals pumped by the same laser form the nonlinear interferometer, where interference of the visible SPDC photons can be observed. The interference pattern of the visible photons depends on amplitudes and phases of the IR photons, which are used to probe the properties of the medium under study. Hence, by observing the interference of the visible photons we infer the IR properties of the media, without direct measurements of IR light. We applied the nonlinear interference method to realize the IR spectroscopy, IR optical coherence tomography (OCT), IR imaging and IR polarimetry. In the IR spectroscopy experiment, we simultaneously and independently measured both refractive index and absorption coefficient of various samples in the IR range, while only detecting photons in the visible range. In IR OCT and IR imaging schemes, we measured the reflectivity of the interfaces of the sample, performed 3D raster and wide field imaging through opaque (in the visible range) materials. In the IR polarimetry, we measured the birefringence of the samples. We studied glass, organic polymer, Silicon and CO2 gas, but our method can be applied to a broad variety of samples. Our method is widely tunable and has high precision and accuracy over mid-IR wavelengths. This work paves the way for further practical adoption of the method in the field of IR metrology, without the need for IR grade detectors and sources.
We demonstrate a new scheme for infrared spectroscopy with visible light sources and detectors. The technique relies on the nonlinear interference of correlated photons, produced via spontaneous parametric down conversion in a nonlinear crystal. Visible and infrared photons are split into two paths and the infrared photons interact with the sample under study. The photons are reflected back to the crystal, resembling a conventional Michelson interferometer. Interference of the visible photons is observed and it is dependent on the phases of all three interacting photons: pump, visible and infrared. The transmission coefficient and the refractive index of the sample in the infrared range can be inferred from the interference pattern of visible photons. The method does not require the use of potentially expensive and inefficient infrared detectors and sources, it can be applied to a broad variety of samples, and it does not require a priori knowledge of sample properties in the visible range.
Spectroscopy in the mid-infrared region is an indispensable tool for identifying molecular types in various fields, including physics, chemistry, and medical sciences. However, conventional infrared light sources, detectors, and noise from blackbody radiation have been the obstacles to miniaturization and higher sensitivity of infrared spectrometers. Quantum infrared spectroscopy, which uses visible and infrared photon pairs in a quantum entangled state, has attracted attention as a new sensing technology that enables infrared spectroscopy with detectors in the visible range. However, the bandwidth of conventional quantum entangled light sources is at most 1 µm or less, which hinders broadband measurements, which are important in spectroscopic applications. Here we have realized an ultra-broadband entangled state of visible–infrared photons with wavelengths from 2 to 5 µm, harnessing a specially designed nonlinear crystal with chirped poling structure inside. Furthermore, we constructed a nonlinear quantum interferometer using the ultra-broadband quantum entangled photons and realized broadband infrared spectroscopy of inorganic and organic materials using a visible detector made of silicon. Our results show that quantum infrared spectroscopy can achieve ultra-broadband spectroscopic measurements and pave the way for the highly sensitive, ultra-compact infrared spectrometers using quantum entangled photons.
Recent years have seen the development of quantum sensing concepts utilizing nonlinear interferometers based on correlated photon pairs generated by spontaneous parametric down-conversion (SPDC). Using SPDC far from frequency degeneracy allows a “division of labor” between the mid-infrared photon for the strongest sample interaction and the correlated near-infrared photon for low-noise detection. The small number of photons provided by SPDC and the resulting inferior signal-to-noise ratio are, however, a limiting factor preventing the wide applicability of the novel sensing concept. Here, we demonstrate a nonlinear interferometer based on pump-enhanced SPDC with strongly improved emission rates while maintaining broadband spontaneous emission. For validation of the concept, we demonstrate high-resolution mid-infrared spectroscopy with near-infrared detection, showcasing improved accuracy. Although the number of mid-infrared photons is about five orders of magnitude smaller than in classical spectrometers, the sensitivity of the quantum spectrometer becomes comparable, marking an essential step toward real-world applications.