Abstract
Surface-enhanced infrared absorption spectroscopy is attractive for molecular sensing due to its access to chemical bonds with high detection sensitivity. Such a spectroscopic method typically operates on localized resonances in subwavelength structured antennas and metamaterials. In this paper, we demonstrate monolayer octadecanethiol detection by using the leaky guided mode in a metal–insulator–metal waveguide, whose angle-tunable dispersion enables coupling to molecular vibrations with a frequency-variable optical resonance. Our results show that, by changing the incident angle from 15° to 75°, the resonance frequency of the leaky guided mode is scanned around the CH2 vibration modes with frequency detuning from −200 cm−1 to 350 cm−1 in wavenumber. As the frequency detuning increases, the vibration signal of both the CH2 symmetric and asymmetric modes increases first and then decreases. The maximum vibration signal of 1%–1.5% is reached at positive and negative frequency detuning values of ±100 cm−1. These sensing properties are explained with a coupled-oscillator model, which suggests that both enhanced near-field and coupling strength between the optical resonance and molecular vibration play an important role for the optimal sensing performance.
Highlights
By varying the incident angle from 15○ to 75○, the resonance frequency of the leaky guided mode is scanned around the CH2 vibrations of monolayer octadecanethiol (ODT) molecules, and a maximum vibration signal of 1%–1.5% is measured intermediately at both positive and negative frequency detuning values of ±100 cm−1
By varying the incident angle, the TM01 waveguide mode was excited and scanned around CH2 vibration lines, which enabled a straightforward way in adjusting optical resonance for optimal sensing performance
Our results revealed that maximum vibration signal was obtained at both positive and negative frequency detuning values of ±100 cm−1
Summary
Surface-enhanced infrared absorption spectroscopy (SEIRAS) has attracted considerable interest in the fields of spectroscopy, material science, and sensor technologies due to its ability to measure molecular bonds with high sensitivity in a non-invasive way. Based on the enhanced interaction between analyte molecules and intensified near-fields in resonant plasmonic structures such as metallic nanorods, nanoslits, arrayed disks, and split-ring resonators, a variety of important molecule species have been measured in SEIRAS with the sensitivity up to monolayer or even sub-monolayer molecules. Besides metallic antennas, patterned graphene recently has been applied in SEIRAS for molecular sensing by taking advantage of strong field confinement of the graphene plasmon in the mid-infrared region. A wealth of signature vibration bonds in proteins, polymer films, and gas molecules has been revealed in the structures of graphene nanoribbons and hybrid graphene–metal nanostructures. These reported metamaterial antennas used in SEIRAS are mostly based on localized plasmon resonances, whose frequencies are determined by the patterned antenna geometry. A wealth of signature vibration bonds in proteins, polymer films, and gas molecules has been revealed in the structures of graphene nanoribbons and hybrid graphene–metal nanostructures.. A wealth of signature vibration bonds in proteins, polymer films, and gas molecules has been revealed in the structures of graphene nanoribbons and hybrid graphene–metal nanostructures.15–18 These reported metamaterial antennas used in SEIRAS are mostly based on localized plasmon resonances, whose frequencies are determined by the patterned antenna geometry. Propagating surface plasmons and Rayleigh anomaly diffraction modes have been proposed as alternatives for molecular sensing in SEIRAS.19–21 Despite their less near-field enhancement as compared with the localized plasmonic mode, their angle-tunability offers a flexibility in controlling the optical resonance frequency for coinciding with that of molecular vibration modes without adjusting the antenna physical geometry Propagating surface plasmons and Rayleigh anomaly diffraction modes have been proposed as alternatives for molecular sensing in SEIRAS. Despite their less near-field enhancement as compared with the localized plasmonic mode, their angle-tunability offers a flexibility in controlling the optical resonance frequency for coinciding with that of molecular vibration modes without adjusting the antenna physical geometry
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