2020 journal article
Critical Test of the Interaction of Surface Plasmon Resonances with Molecular Vibrational Transitions
JOURNAL OF PHYSICAL CHEMISTRY A, 124(9), 1744–1753.
TL;DR:
The results show that interactions of infrared SPPs with molecular vibrations are in the weak coupling limit, and that enhancements comparable those reported for noble metals are achievable.
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(Web of Science)
Source: Web Of Science
Added: March 30, 2020
We determine the absorption spectra of a gas due to evanescent plasmonic electromagnetic fields in a system where surface interactions (physisorption and chemisorption) are demonstrably negligible. The plasmonic host material, degenerate semiconductor CdO:Dy, has high mobility (366 - 450 cm2/Vs) and carrier density (0.6 - 3.5 x 1020 cm-3) and therefore supports low-loss surface plasmon resonances in the mid-IR. This high mobility layer gives the highest resolution observed in a plasmonic conducting layer in the infrared, higher than gold and rivaling silver at optical frequencies in the resolution of spectral features relative to the plasmon energy. This high resolution permits new understanding of the nature of the interaction of emerging fields with molecular transitions. Using different carrier concentrations, the resonance condition of the surface plasmon polariton (SPP) frequency (ω_SPP) and N2O vibrational absorption spectral frequency (ω_(N_2 O)) can be controlled, thereby allowing a critical test of field-molecule interactions. Experiment and theory both indicate a dispersive N2O line shape for ω_SPPω_(N_2 O) and an abrupt change between the two when the resonance condition ω_SPP=ω_(N_2 O) is reached. A first-order expansion of the Airy equation describes this behavior analytically. The SPP surface enhancement is 6.8±0.5 on resonance, less than enhancements observed in other systems, but in agreement with recent quantitative reports of surface enhanced infrared reflection absorption spectroscopy (SEIRA). Our results show that interactions of infrared SPPs with molecular vibrations are in the weak coupling limit, and that enhancements comparable those reported for noble metals are achievable.