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Optics Continuum

Background-free absorption spectroscopy with strongly absorbing media

Neil Irvin Cabello, Shinta Ozawa, Takao Fuji, Tetsuhiro Kudo

Published Jun 19, 2026• 0 citations
Publisher

Abstract

We demonstrate that mid-infrared background-free absorption spectroscopy is an effective method for characterizing high-concentration gas samples, while measurements of large absorbance are normally restricted by the dynamic range of FTIR spectrometers. In this study, the MIR spectra of high-concentration carbon dioxide gas were measured using background-free absorption spectroscopy. We found that the separation of the upconverted free-induction decay (FID) bands corresponding to the P - and the R -branches about 2350 cm −1 increases with gas concentration. By measuring the separation between the two FID bands, the concentration of carbon dioxide gas can be extrapolated even at high concentrations. We also discuss the origin of this increasing band separation observed in the upconverted FID signal, which arises not only from the imaginary part of the susceptibility but also from its real part. Background-free spectroscopy circumvents Fourier transform infrared spectroscopy’s detector dynamic range limitation, while the present technique itself does not require high dynamic range.

DOI
10.1364/optcon.604925

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We demonstrate that mid-infrared background-free absorption spectroscopy is an effective method for characterizing high-concentration gas samples, while measurements of large absorbance are normally restricted by the dynamic range of FTIR spectrometers. In this study, the MIR spectra of high-concentration carbon dioxide gas were measured using background-free absorption spectroscopy. We found that the separation of the upconverted free-induction decay (FID) bands corresponding to the P - and the R -branches about 2350 cm −1 increases with gas concentration. By measuring the separation between the two FID bands, the concentration of carbon dioxide gas can be extrapolated even at high concentrations. We also discuss the origin of this increasing band separation observed in the upconverted FID signal, which arises not only from the imaginary part of the susceptibility but also from its real part. Background-free spectroscopy circumvents Fourier transform infrared spectroscopy’s detector dynamic range limitation, while the present technique itself does not require high dynamic range.
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.
Abstract Fourier-transform infrared spectroscopy (FTIR) is an indispensable analytical method that allows label-free identification of substances via fundamental molecular vibrations. However, traditional FTIR spectrometers require mid-infrared (MIR) elements, including low-efficiency MIR photodetectors. SU(1,1) interferometry has previously enabled FTIR with undetected MIR photons via spontaneous parametric down-conversion in the low-parametric-gain regime, where the number of photons per mode is much less than one and sensitive photodetectors are needed. In this work, we develop a high-parametric-gain SU(1,1) interferometer for MIR-range FTIR with undetected photons. Using our method, we demonstrate three major advantages: a high photon number at the interferometer output, a considerably lower photon number at the sample, and improved interference contrast. In addition, we broaden the spectral range of the interferometer by aperiodic poling in the gain medium. Exploiting the broadband SU(1,1) interferometer, we measure and evaluate the MIR absorption spectra of polymers in the 3-μm region.
We propose and demonstrate a new method for single-shot multiplex absorption spectroscopy that permits enhanced sensitivity in the simultaneous measurement of multiple spectral lines in rapidly changing gas-phase media, such as turbulent flames. It uses an ultrashort laser pulse that propagates through the absorbing medium, for which the relevant absorption information resides in the free-induction decay that is trailing behind the transmitted pulse. Time gating out most of the transmitted pulse, but not the free-induction decay, enhances the relative fraction of light that contains absorption information when the spectrum is measured. This procedure reduces the background associated with the input light, thus enhancing detection sensitivity.
In recent years, quantum Fourier transform infrared (QFTIR) spectroscopy has emerged as an alternative to conventional absorption spectroscopy in the mid-infrared region of the spectrum. By harnessing induced coherence and spectral correlations of photon pairs in a nonlinear Michelson interferometer, this technique offers promising potential for the practical detection of organic gases. However, little research was conducted to bring QFTIR spectrometers closer to domestic or in-field usage. In this work, we present the first use of a QFTIR spectrometer for open-path detection of multiple interfering organic gases in ambient air. We built a nonlinear Michelson interferometer with 1.7 m-long arms to increase the absorption length, coupled with analysis techniques from classical differential absorption spectroscopy used for gas-traces detection. We thus characterize our spectrometer’s sensitivity to acetone, methanol, and ethanol vapors and demonstrate the accurate identification of mixtures of these gases released in ambient air. We show this characteristic is preserved over time by performing a measurement overnight and tracking the evolution of different gases’ average concentrations. These results constitute the first use-case of a QFTIR spectrometer as a detector of organic gases and, thus, represent an important milestone toward the development of such detectors in practical situations.
Traditional absorption spectroscopy has a fundamental difficulty in resolving small absorbance from a strong background due to the instability of laser sources. Existing background-free methods in broadband vibrational spectroscopy help to alleviate this problem but face challenges in realizing either low extinction ratios or time-resolved field measurements. Here, we introduce optical-parametric-amplification-enhanced background-free spectroscopy, in which the excitation background is first suppressed by an interferometer, and then the free-induction decay that carries molecular signatures is selectively amplified. We show that this method can improve the limit of detection in linear interferometry by order(s) of magnitude without requiring lower extinction ratios or a time-resolved measurement, which can benefit sensing applications in detecting trace species.