Over recent years, the detection of organic compounds has arisen as a cornerstone for security, health, environmental and pollution issues. The ability to measure low concentrations of gases such as methane, methanol, or nitrogenous compounds, allows to perform tasks such as practical medical diagnoses, explosives detection, or the prevention of pollution from various sources, including industry and agriculture. The Laser technology has enabled the detection of such compounds thanks in particular to Fourier Transform Infrared (FTIR) spectroscopy. The mid-infrared part of the electromagnetic spectrum is of particular interest, for it features molecular transitions involving vibrational and rotational levels, specific to each molecules. However, in order to span this part of the spectrum, conventional FTIR spectrometers rely on noisy and low-efficiency detectors, which also require cooling to operate properly. Applications of FTIR spectroscopy in the mid-infrared region are therefore limited for the time being, particularly in terms of spectral resolution at low gas concentrations.
More recently, the growing interest for induced coherence in quantum photonic interference has opened perspectives for more practical and sensitive FTIR spectrometers in the mid-infrared region. This phenomenon was first introduced by Zou et al. in 1991 [1], and involves an interference on the idler photon from a pair emitted via spontaneous parametric down-conversion (SPDC). Remarkably, the visibility of this interference depends on the transmissivity of an optical element placed in the signal photon's path, element that never interacted with the idler photon. Applications of this phenomenon to infrared spectroscopy were only proposed during the last decade [2], with first applications to FTIR spectroscopy proposed by Paterova et al. in 2018 [3] and Lindner et al. in 2021 [4], who combined induced coherence in a nonlinear Michelson interferometer with Fourier Transform spectroscopy techniques. This allowed them to measure the absorption spectrum of gases in the mid-infrared region, by detecting a near-infrared photon with less noisy, more efficient, and cheaper detectors. These experiments pioneered the field of quantum FTIR spectroscopy, feeding the hope for more sensitive detection of organic compounds in the mid-infrared spectrum.
In this work, we built a quantum FTIR spectrometer displaying the highest sensitivity ever achieved. In order to reach this sensitivity, we built such a nonlinear Michelson interferometer, with enhanced stability allowing to probe the gas in 1.7m-long interferometer arms. The resulting total absorption consequently increases by two orders of magnitude compared to the most recent experiment of Lindner et al. [4]. In addition, we used new post-processing techniques, by filtering out the noise in the spectrum reconstructed via Fourier transform, in a more selective way than the more usual apodization post-processing. This allowed us to resolve faint absorption lines of gases that would otherwise be imperceptible. The resulting sensitivity was high enough to detect the absorption spectrum of ethanol and methanol vapors in ambient air, as well as absorption lines of methane and water vapour from human breath and earth atmosphere. In addition, thanks to the large spectral band allowed by FTIR spectroscopy, multiple gas were identified from the same sample. This way, we showed that quantum FTIR spectroscopy can indeed be used for practical applications, such as medical breath analysis and on-filed gas detection.
[1] X.Y. Zou, L.J. Wang, and L. Mandel, "Induced Coherence and Indistinguishability in Optical Interference", Physical Review Letters, 67 (3), 318 – 321 (1991).
[2] D.A. Kalashnikov, A.V. Paterova, S.P. Kulik, L.A. Krivitsky, "Infrared spectroscopy with visible light", Nature Photonics 10,98-101 (2016).
[3] A. Paterova, H. Yang, C. An, D. Kalashnikov, and L. Krivitsky, "Measurement of infrared optical constants with visible photons", New Journal of Physics, 20, 043015 (2018).
[4] C. Lindner, J. Kunz, S.J. Herr, S. Wolf, J. Kießling, and F. Kühnemann, "Nonlinear interferometer for Fourier-transform mid-infrared gas spectroscopy using near-infrared detection", Optics Express, 29 (3), 4035 - 4047 (2021).
