Mid-infrared spectroscopy allows for the direct characterization of molecular structures with chemical specificity unique to this spectral range. Nanophotonics has extended this approach to nanometer-scale samples and low numbers of surface-bound molecules by exploiting the strong near-field enhancement of subwavelength resonators.
Such surface-enhanced infrared absorption spectroscopy techniques have traditionally relied on plasmonic platforms based on metallic antennas, which are limited by low quality factor (Q-factor) resonances imposed by resistive loss [1]. Nanostructured resonators based on high-index low-loss dielectric materials can overcome this limitation. However, so far, the prospects of combining high-Q dielectric resonators with molecular spectroscopy have not been realized.
Here, we introduce an imaging-based nanophotonic method for detecting mid-infrared molecular fingerprints, and implement it for the chemical identification and compositional analysis of surface-adsorbed analytes. In contrast to previous approaches where high-Q resonances in metasurfaces are generated via the interference of super-radiant and sub-radiant modes, our design exploits the collective behavior of Mie resonances to provide strong near-field enhancements and spectrally clean high-Q resonances, which enables the highly selective enhancement of molecular fingerprint information [2].
We experimentally realize a two dimensional array of high-Q metasurface pixels, where the resonance positions of individual metapixels are tuned to discrete frequencies. This configuration allows us to assign each resonance position to a specific pixel of the metasurface, enabling us to detect molecular absorption signatures at multiple spectral points simultaneously. By comparing the imaging-based readout of this spatially encoded vibrational information before and after the coating of target analyte molecules, we retrieve chemically specific molecular barcodes suitable for chemical identification and compositional analysis.
Specifically, we will present additional results and analysis on how our method can detect the characteristic signatures for biological, polymer, and pesticide molecules, covering application areas such as biosensing, materials science, and environmental monitoring.
Crucially, our method can be combined with broadband light sources and detectors to resolve molecular absorption fingerprints without the need for spectrometry, frequency scanning, or moving mechanical parts, paving the way towards sensitive and versatile miniaturized mid-infrared spectroscopy devices.
[1] Neubrech et al., Chem. Rev. 117, 5110–5145 (2017)
[2] Tittl et al., Science 360, 1105–1109 (2018)
Photonic & plasmonic nanomaterials , Enhanced spectroscopy and sensing , Optical sensing from solid state to bio-medicine
