Critical Test of the Interaction of Surface Plasmon Resonances with Molecular Vibrational Transitions

We determine the infrared 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, the degenerate semiconductor CdO:Dy, has high mobility (366-450 cm²/(V s)) and carrier density ((0.6-3.5) × 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 material in the infrared, higher than that of gold and rivaling that of silver. The high resolution permits a 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) can be controlled, thereby allowing a critical test of field-molecule interactions. Experiment and theory both indicate a dispersive N₂O line shape for ω_SPP < ω_N, an absorptive line shape for ω_SPP < ω_N, and an abrupt change between the two when the resonance condition ω_SPP < ω_N 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, lower 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 can be achieved.