Polymer identification via undetected photons using a low footprint nonlinear interferometer

TL;DR

This paper introduces a compact nonlinear interferometer that uses undetected photons to rapidly and accurately identify plastics by their vibrational spectra, enabling portable environmental monitoring without mid-infrared detectors.

Contribution

The authors develop and demonstrate a micro-integrated, thermally-stabilized nonlinear interferometer capable of rapid, high-resolution polymer identification using only near-infrared detection.

Findings

Achieved a signal-to-noise ratio of 34 at 100 Hz measurement rate

Successfully identified common polymers like polypropylene, polyethylene, and polystyrene

Operates effectively at room temperature without mid-infrared technology

Abstract

Plastic pollution has become a critical global challenge, with microplastics pervading ecosystems and entering human food chains. Effectively monitoring this widespread contamination demands rapid, reliable, and portable material identification techniques that often elude conventional Raman and FTIR spectroscopy. Undetected photon spectroscopy within a nonlinear interferometer (NLI) offers a solution, allowing the retrieval of mid-infrared absorption spectra by detecting only near-infrared signal photons using standard silicon-based technology. Here, we demonstrate a highly compact, micro-integrated, thermally-stabilised NLI with a Michelson-like geometry designed for the rapid spectroscopy of plastics. We benchmarked its room-temperature performance, demonstrating a signal-to-noise ratio of 34 with a measurement rate of 100 Hz and a spectral resolution of 6 cm$^{-1}$. We show that we can accurately and rapidly retrieve the characteristic vibrational absorption spectra of common polymers such as polypropylene, polyethene, and polystyrene, without using mid-infrared technology. These results establish our compact module as a promising field-deployable platform for robust, real-time environmental monitoring systems and other mid-infrared spectroscopy applications.