  Vibrational strong coupling of molecules to optical cavities based on
plasmonic resonances has been explored recently, because plasmonic near-fields
can provide strong coupling in sub-diffraction limited volumes. Such field
localization maximizes coupling strength, which is crucial for modifying the
vibrational response of molecules and, thereby, manipulating chemical
reactions. Here, we demonstrate an angle-independent plasmonic nanodisk
substrate that overcomes limitations of traditional Fabry-Perot optical
cavities, because the design can strongly couple with all molecules on the
surface of the substrate regardless of molecular orientation. We demonstrate
that the plasmonic substrate provides strong coupling with the C=O vibrational
stretch of deposited films of PMMA. We also show that the large linewidths of
the plasmon resonance allows for simultaneous strong coupling to two,
orthogonal water symmetric and asymmetric vibrational modes in a thin film of
copper sulfate monohydrate deposited on the substrate surface. A
three-coupled-oscillator model is developed to analyze the coupling strength of
the plasmon resonance with these two water modes. With precise control over the
nanodisk diameter, the plasmon resonance is tuned systematically through the
modes, with the coupling strength to both modes varying as a function of the
plasmon frequency, and with strong coupling to both modes achieved
simultaneously for a range of diameters. This work may aid further studies into
manipulation of the ground-state chemical landscape of molecules by perturbing
multiple vibrational modes simultaneously and increasing the coupling strength
in sub-diffraction limited volumes.
