Abstract
Surface-enhanced infrared absorption spectroscopy (SEIRA) is attractive for molecular sensing due to its high sensitivity and access to molecular fingerprint absorptions. In this paper, we report on refractive index sensing of monolayer molecules in a spectral band outside the molecular fingerprint region. In a metagrating composed of a three-layer metal-insulator-metal structure, both propagating surface plasmon resonances (PSPs) and local surface plasmon resonances (LSPRs) are exited from free-space in a broad band of 3 to 9 µm, and their sensing properties are characterized. In response to a self-assembled monolayer of octadecanethiol (ODT) molecules, both PSPs and LSPRs exhibit redshifts in wavelength. The shifts of LSPRs are larger than those of PSPs, as originated from their stronger spatial confinement and larger field enhancement. Our proposed mid-infrared molecular sensor is immune to frequency variations of plasmon resonance and more tolerant to sample feature size variation.
Highlights
Surface plasmons (SPs) are collective oscillations of electrons at the boundaries between materials, which feature strongly confined and intensified fields well suitable for exploring fundamental electromagnetic phenomena [1,2,3,4,5] and for addressing challenges in optical sensing [6,7,8,9,10], and many other applications, such as photovoltaics and lasers [11,12,13]
We demonstrate refractive index sensing of molecules outside their fingerprint region using both local surface plasmon resonances (LSPRs) and propagating surface plasmon resonances (PSPs) in a mid-infrared metagrating
We demonstrated refractive index sensing of monolayer molecules outside their fingerprint region
Summary
Surface plasmons (SPs) are collective oscillations of electrons at the boundaries between materials, which feature strongly confined and intensified fields well suitable for exploring fundamental electromagnetic phenomena [1,2,3,4,5] and for addressing challenges in optical sensing [6,7,8,9,10], and many other applications, such as photovoltaics and lasers [11,12,13]. As resonant wavelength in a plasmonic structure is generally scaled with its feature size [25], by moving resonance to a longer wavelength in the mid-infrared, the sample feature size can be made larger, which reduces difficulties in fabrication. Using structures such as metallic nano-rods [26], nano-slits [27], split ring resonators [28], and patterned graphene [29,30] in surface-enhanced infrared absorption spectroscopy (SEIRA), a number of molecular sensors have been
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