Vibrational Strong Coupling in Direct Laser Printed Plasmonic MIM Nanopatch Antennas

Strong Coupling of mid-infrared (IR) vibrational transitions to optical cavities provides a way to modify and control a material’s chemical reactivity and potentially facilitate novel chemical detection technology. Vibrational transitions strongly coupled to single plasmonic cavities have proven difficult to achieve due to the relatively weak oscillator strength of vibrational transitions and high losses of plasmonic structures, requiring open plasmonic systems such as surface plasmon polaritons with subwavelength field confinement only in one or two dimensions and can therefore accommodate large populations of coupled oscillators to enable strong coupling. On the other hand, Metal-Insulator-Metal (MIM) patch-antennae are a closed plasmonic resonator with subwavelength field confinement in all three dimensions that has proven capable of strong light-matter interactions with minimal interaction volume. There, the electromagnetic fields are highly confined to deep subwavelength scales upon which a large field enhancement can overcome the inherently weak oscillator strength of vibrational transitions, lowered oscillator populations, and plasmonic losses to create a strongly coupled system. However, such patch antennae are fabricated via costly and time-consuming electron beam lithography techniques.<br/>Here, we demonstrate and characterize vibrational strong coupling in nanoscale direct laser printed mid-IR MIM plasmonic nanopatch antennae. Such a fabrication scheme represents a cost-effective, highly accessible means of fabricating and rapidly evaluating mid-IR nanometer-sized plasmonic resonators capable of producing strong light-matter interactions. Our L-shaped nanopatch antennae system exhibits strong coupling between localized dipole modes and the C=O vibrational transition at 1731 cm^-1 of the polymer insulator. Upon analysis of the measured electromagnetic response via analytical and numerical FEM simulations, we find that the fundamental dipole modes are a superposition of responses from each arm. This leads to an effective resonance length of the MIM resonator that further reduces the mode volume by an order of magnitude compared to other similarly reported MIM systems. Furthermore, we find from full-wave electromagnetic simulations that an asymmetry of the arm lengths in the L-antenna leads to a chiral splitting of higher order modes when illuminated with circularly polarized light. Thus enabling the potential detection of chiral molecules via circularly dichroic vibrational strong coupling.