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

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.

Figures (5)

• Figure 1: Illustration of rapid imaging-based SEIRA spectroscopy enabled by the synergy of resonance gradient metasurfaces and RF-modulated QCL generating broad instantaneous emission spectrum. Laser emission is projected on the metasurface through a dispersive element, providing spatially resolved SEIRA signal. The gradient of the metasurface resonance is tailored to match the spectral component of the broadband laser addressing each location on the chip. The reflectivity of the resonance gradient metasurface is then captured in a single frame of a room-temperature mid-IR imaging camera, allowing to detect the absorption signature of analytes deposited on the metasurface in a barcode-like format. The insets show the false color SEM images of the gradient metasurface.
• Figure 2: Non-enhanced imaging spectroscopy with broadband quantum cascade laser (a) Schematic of the broadband QCL imaging spectroscopy components. Left: quantum cascade laser is driven into broadband emission regime using radiofrequency drive. Center: broadband laser emission is projected on a sample surface through a dispersive element and then imaged with a mid-IR camera. Right: as each frequency component of BQCL addresses a specific coordinate on the sample surface, the spatial profile of the laser spot represents the sample transmission spectrum. (b) Spatial intensity profile of a projected laser spot (black curve) compared to the spectrum of BQCL emission measured with FTIR (red curve). (c) Image of the BQCL spot on the surface of CaF$_2$ chip recorded with a mid-IR camera. (d,e) Comparison of the absorption spectra of (d) polystyrene and (e) polymetylmetacrylate films on CaF$_2$ measured with FTIR (black curves) and extracted from the spatial intensity profiles of the transmitted BQCL beam (red curves).
• Figure 3: Broadband gradient metasurface for imaging-based SEIRA spectroscopy (a) Scheme of the unit cells of the resonance gradient metasurface consisting of two tilted Ge ellipses on CaF$_2$ substrate scaled along one of the chip coordinates. (b) Map of the reflection spectra of the gradient metasurface showing the evolution of the resonant peak with the chip coordinate. (c) Optical image of the gradient metasurface chip. Top axis shows the physical chip coordinate, bottom axis - the wavenumber of the resonant peak at the corresponding location on the chip. The red overlay shows the BQCL spot imaged with a mid-IR camera with the same scale (also shown in Fig. \ref{['fig:spectroscopy']}c). (d) Reflection images of the gradient metasurface chip measured at different excitation frequencies with a mid-IR microscopy using external cavity tunable QCLs as its light source. The overlay shows the intensity profile of the BQCL spot focused on the sample surface in single-frequency emission regime (no RF drive). (e) Stack of FTIR transmission spectra of the broadband resonance gradient metasurface covered with PMMA measured at different chip coordinates. Polymer absorption has minor contribution to the spectra (indicated with small red arrow). (f) Stack of FTIR reflection spectra of the same chip. The amplitude of the peaks is strongly modulated by PMMA absorption (amplitude indicated with red arrow). Reference PMMA absorption profile is shown with black curves in both panels (inverted in panel f for convenience).
• Figure 4: Calibration of the alignment of gradient metasurfaces with broadband QCL emission for rapid SEIRA-based imaging spectroscopy. (a) Scheme of BQCL imaging spectroscopy integrated with a gradient metasurface. Spatial distribution of the graident metasurface resonance matches the dispersion of the BQCL beam wavelength after the diffraction grating. (b,c) Schematics of calibration configurations for the imaging spectroscopy system. (b) Mismatched configuration: the dispersions of the BQCL spot and the metasurface resonance have the opposite sign. When the gradient metasurface is scanned along the BQCL spot, light is reflected only from the area where the dispersions intersect (marked with white dashed line). (c) Matched configuration: each frequency component of the BQCL addresses the location on the gradient with the corresponding resonance frequency. (d) Calibration data for mismatched configuration. Each vertical section of the map represents the profile of the BQCL spot for a certain shift of the chip from the spot center. (e) Calibration data for matched configuration. Image of the BQCL spot reflected from the gradient chip demonstrates uniformly high reflectance due to matching of the laser frequency components with local metasurface resonance.
• Figure 5: Demonstration of the rapid single-frame imaging-based SEIRA of analytes with broadband QCL and gradient metasurfaces (a-c) FTIR reflection spectra of gradient metasurface covered with layers of different analytes: (a) 300 nm polystyrene film, (b) 80 nm PMMA film, and (c) 45 nm alanine film. The envelope of the spectra stack measured at different locations along the gradient shows the absorption profiles of the analytes. (d-f) Images of the BQCL spots reflected from the gradient metasurface covered with the respective analytes. The images are normalized to reflection image from gold film measured in the same configuration. (g-i) Red curves: enhanced absorption spectra extracted from the intensity profiles of single-frame reflection images of the gradient metasurface. Blue curves: scaled reference absorption spectra extracted from reflection spectra of analytes on gold measured with FTIR.