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Physical Review Applied

Quantum infrared attenuated total reflection spectroscopy

Torataro Kurita, Yu Mukai, Ryo Okamoto, Masaya Arahata, Toshiyuki Tashima, Hiroshi Ota, Katsuhiko Tokuda, Shigeki Takeuchi

Published Jan 28, 2025• 5 citations
Publisher

Abstract

Quantum infrared spectroscopy (QIRS), which harnesses the quantum interference between entangled visible and infrared photon-pair-generation processes in a nonlinear quantum interferometer, has attracted considerable attention for realizing infrared spectroscopy with high-performance visible detectors and light sources. However, a shortcoming of QIRS is its vulnerability to wave-front distortion caused by the sample structure, which can degrade the quantum interference. To address this problem, we propose employing the attenuated total-reflection (ATR) method, which utilizes close contact between the sample and a prism. The ATR method is one of the most utilized methods in infrared spectroscopy since it can be used to measure various types of materials, such as liquid, film, and powder samples, and is therefore useful for real-world applications. Here, we report wavelength-tunable ATR-QIRS measurements. We constructed a Michelson-type nonlinear quantum interferometer with a ${\mathrm{Ca}\mathrm{F}}_{2}$ prism, and measured the absorption of water OH-stretching (approximately $3\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$). The transmission spectrum obtained by our ATR-QIRS system shows good agreement with a numerically calculated transmission spectrum using the Fresnel equation.

DOI
10.1103/physrevapplied.23.014061

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•Jan 1, 2018•0 citations
Quantum infrared spectroscopy (QIRS), which harnesses the quantum interference between entangled visible and infrared photon-pair-generation processes in a nonlinear quantum interferometer, has attracted considerable attention for realizing infrared spectroscopy with high-performance visible detectors and light sources. However, a shortcoming of QIRS is its vulnerability to wave-front distortion caused by the sample structure, which can degrade the quantum interference. To address this problem, we propose employing the attenuated total-reflection (ATR) method, which utilizes close contact between the sample and a prism. The ATR method is one of the most utilized methods in infrared spectroscopy since it can be used to measure various types of materials, such as liquid, film, and powder samples, and is therefore useful for real-world applications. Here, we report wavelength-tunable ATR-QIRS measurements. We constructed a Michelson-type nonlinear quantum interferometer with a CaF2{\mathrm{Ca}\mathrm{F}}_{2}CaF2​ prism, and measured the absorption of water OH-stretching (approximately 3\ensuremath\mum3\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}3\ensuremath\mum). The transmission spectrum obtained by our ATR-QIRS system shows good agreement with a numerically calculated transmission spectrum using the Fresnel equation.
In recent years, nonlinear quantum interferometry has been widely explored for applications like spectroscopy, optical coherence tomography, and imaging [1]. Quantum infrared spectroscopy (QIRS) uses quantum interference of generation processes of frequency-entangled visible-IR photon pairs to perform infrared spectroscopy using visible light sources and silicon-based detectors [2]. This technique utilizes a nonlinear quantum interferometer configuration where the generated IR photons pass through the sample, and the correlated visible photons are detected. The change in the detected visible photon flux with and without the sample helps extract the complex transmittance of the sample in the IR region.
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.
Harnessing the quantum interference of the pair generation processes, infrared quantum spectroscopy, based on nonlinear interferometers with visible-infrared photon-pair sources, enables the extraction of the infrared optical properties of a sample through visible photon detection without the need for an infrared optical source or detector. We develop a theoretical framework for quantum Fourier-transform infrared (QFTIR) spectroscopy. The proposed Fourier analysis method, which fully utilizes the phase information in the interferogram, allows us to determine the complex transmittance and optical constants for a sample in a simple setup without the use of any dispersive optics for spectral selection. In the experimental demonstrations, the transmittance spectrum of a bandpass filter and the refractive index of silica glass are measured in the near-infrared region using QFTIR operated in a low gain regime; these results agree well with the independently measured spectrum using a conventional spectrometer and an value estimated from references. These demonstrations prove the validity and great potential of QFTIR spectroscopy.
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.
Infrared quantum absorption spectroscopy is one of the quantum sensing techniques, by which the infrared optical properties of a sample can be estimated through visible or near infrared photon detection without need for infrared optical source or detector, which has been an obstacle for higher sensitivity and spectrometer miniaturization. However, experimental demonstrations have been limited to wavelengths shorter than 5 µm or in the terahertz region, and have not been realized in the so-called fingerprint region of 1500–500 cm−1 (6.6 to 20 µm), which is commonly used to identify chemical compounds or molecules. Here we report the experimental demonstration of quantum Fourier-transform infrared (QFTIR) spectroscopy in the fingerprint region, by which both absorption and phase spectra (complex spectra) can be obtained from Fourier transformed quantum interferograms obtained with a single pixel visible-light detector. As demonstrations, we obtained the transmittance spectrum of a silicon wafer at around 10 µm (1000 cm−1) and complex transmittance spectrum of a synthetic fluoropolymer sheet, polytetrafluoroethylene, in the wavelength range of 8 to 10.5 µm (1250 to 950 cm−1), where absorption due to stretching modes of C-F bonds is clearly observed. These results open the way for new forms of spectroscopic devices based on quantum technologies.
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.