Metasurface-Enhanced Mid-Infrared Imaging Spectroscopy with Broadband Quantum Cascade Lasers

TL;DR

This paper introduces a compact, broadband metasurface-enhanced mid-infrared imaging system using a radiofrequency-modulated quantum cascade laser, enabling rapid, high-sensitivity molecular detection without complex instrumentation.

Contribution

It presents a novel, miniaturized SEIRA platform combining broadband gradient metasurfaces with a broad-spectrum QCL for efficient, targeted molecular sensing in a simplified, high-throughput setup.

Findings

Achieved up to 3 orders of magnitude faster acquisition time.

Enabled single-frame detection of molecular signatures with a room-temperature IR camera.

Reduced system complexity by eliminating tunable sources and bulky spectrometers.

Abstract

Mid-infrared (mid-IR) spectroscopy offers unparalleled opportunities in sensing through chemically specific detection of molecular absorption fingerprints. Yet, its practical applications are limited by the weak light-matter interaction in the mid-IR range and low brightness of mid-IR light sources. Surface-enhanced infrared absorption (SEIRA) spectroscopy addresses the sensitivity limitations by leveraging resonant photonic structures, in particular, plasmonic and frequency-selective dielectric metasurfaces. However, current implementations of SEIRA approach mainly rely on complex instruments and scanning components such as Fourier-transform infrared spectroscopy and tunable external cavity quantum cascade lasers (EC QCLs). Here, we present a compact and high-throughput imaging-based SEIRA platform that combines broadband gradient metasurfaces with a radiofrequency-modulated QCL that generates remarkably broad instantaneous emission spectrum (250 cm$^{-1}$) covering absorption bands of multiple distinct molecular vibrational modes. By matching the resonance spectrum of the compact (1 mm$^2$) broadband gradient metasurface with the laser emission projected on its surface through a dispersive element, we ensure that every QCL spectral component is uniquely addressed for an efficient targeted enhancement of the electromagnetic field. This enables us to use a low-cost and room-temperature mid-IR camera, acquiring in a single frame the enhanced absorption signatures of analytes deposited on the metasurface as a barcode image, thus reducing the acquisition time by up to 3 orders of magnitude compared to the FTIR and EC QCL based measurements. Eliminating the need for tunable light sources, bulky spectrometers, and expensive low-temperature detectors, our approach enables high-throughput, miniaturized, and highly specific molecular diagnostics for diverse chemical and biological applications.