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JSAP-OSA Joint Symposia

Spectral resolution of quantum Fourier transform infrared spectroscopy using pulsed laser excitation

Jasleen Kaur, Yu Mukai, Ryo Okamoto, Shigeki Takeuchi

Published Jan 1, 2024• 0 citations
Publisher

Abstract

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.

DOI
10.1364/jsapo.2024.20a_c43_3

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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.
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.
Quantum infrared spectroscopy (QIRS), which uses quantum interference of the generation processes of photon pairs, enables infrared spectroscopy using visible light sources and detectors. Until recently, continuous wave (cw) lasers with a narrow linewidth have been used for excitation in most QIRS experiments. Although the use of pulsed laser pumping is attractive, it has been expected that the inherent finite linewidth of pulsed lasers may degrade the spectral resolution of QIRS. Here, we show that this is not always true. The theoretical analysis shows that the resolution of quantum Fourier transform infrared (QFTIR) spectroscopy is not affected by the linewidth of the pump laser. To verify this, we construct an experimental setup for QFTIR spectroscopy using both cw and pulsed laser excitation under the same experimental conditions. High-resolution spectra are successfully obtained for a silicon parallel plate and an acetylene gas cell as samples using a pulsed pump laser with a finite linewidth, and no resolution degradation is observed compared to the case of cw pumping. These results will open up the use of QFTIR spectroscopy using visible light sources and detectors in novel applications including high-speed time-resolved infrared spectroscopy, which will be beneficial for various fields including physics, chemistry, and environmental science.
Nonlinear interferometers based on correlated photon pairs allow mid-infrared spectroscopy by measuring only near-infrared photons with a silicon-based detector, which offers higher detectivities than mid-infrared detectors. Here, we use the nonlinear interferometer not only to determine the transmission of a sample but also for spectral analysis - analogue to classical Fourier transform infrared (FTIR) spectrometers. The Quantum FTIR analyzes the absorption of gas mixtures with high spectral resolution over a broad mid-infrared wavelength range. Improved emission rates of the photon pair source yield sensitivities similar to a classical FTIR despite an extremely low light exposure on the sample.
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 $3\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$). The transmission spectrum obtained by our ATR-QIRS system shows good agreement with a numerically calculated transmission spectrum using the Fresnel equation.