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Physical Review Applied

High-spectral-resolution quantum Fourier-transform infrared spectroscopy with pulsed laser excitation

Jasleen Kaur, Yu Mukai, Ryo Okamoto, Shigeki Takeuchi

Published Oct 7, 2024• 5 citations
Publisher

Abstract

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.

DOI
10.1103/physrevapplied.22.044015

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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.
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.
Nonlinear quantum interferometry is widely used in photonic quantum sensing applications to perform measurements with undetected photons. While continuous wave (cw) lasers are typically used to generate correlated photon pairs, pulsed laser pumping can enhance the generation efficiency for photon pairs and enable time-resolved measurements. However, the finite spectral linewidth of pulsed lasers leads to imperfect frequency correlation between generated photon pairs, which can affect the sensitivity of nonlinear interferometry. In this work, we investigate the effect of the finite linewidth of a pump laser on the visibility of spectral domain interference in a nonlinear quantum interferometer where signal photons are detected using a dispersive spectrometer. The proposed theory quantitatively predicts the visibility as a function of optical path difference for a pump source with arbitrary spectrum. As a demonstration, we construct an experimental setup to obtain quantum interference using both cw and pulsed laser excitations under the same experimental conditions. At large optical path differences, a fine interference fringe pattern with high contrast is observed with cw laser excitation, whereas with pulsed laser pumping, the visibility decreases as the optical path delay increases, and the interference fringes appear almost washed out. Our quantitative analysis is useful for determining appropriate pulsed laser parameters for designing nonlinear interferometers for quantum sensing applications using a dispersive spectrometer.
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.