Fourier-transform infrared spectroscopy with undetected photons from high-gain spontaneous parametric down-conversion

TL;DR

This paper introduces a high-gain SU(1,1) interferometer for Fourier-transform infrared spectroscopy that enhances sensitivity and spectral range by utilizing undetected photons in the mid-infrared region.

Contribution

The work develops a high-parametric-gain SU(1,1) interferometer enabling MIR FTIR with undetected photons, improving photon output, reducing sample photon exposure, and broadening spectral coverage.

Findings

Achieved higher photon number at the interferometer output.

Reduced photon number interacting with the sample.

Demonstrated broadband MIR absorption spectra of polymers.

Abstract

Fourier-transform infrared spectroscopy (FTIR) is an indispensable analytical method that allows label-free identification of substances via fundamental molecular vibrations. However, the sensitivity of FTIR is often limited by the low efficiency of mid-infrared (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 new 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 analyze different methods to broaden the spectral range of the interferometer by aperiodic poling and temperature gradient in the gain medium. Exploiting the broadband SU(1,1) interferometer, we measure and evaluate the MIR absorption spectra of polymers in the 3-{\mu}m region.