Multimode ultra-strong and strong coupling to Mid-IR vibrational transitions in direct laser written plasmonic nano-patch antennas

Plasmonic resonators have become a key system for enhancing light-matter interactions due to its deep subwavelength confinement and enhanced field strengths; with metal-insulator-metal (MIM) plasmonic nano-patch antennas or cavities achieving some of the largest field confinements. Applying these systems to the field of vibrational strong coupling (VSC) is of particular importance as the relatively large volumes of the mid-IR photonic cavities and weak oscillator strengths of vibrational transitions restricts VSC studies and devices to large ensembles of molecules, thus representing a potential limitation of this promising field. In this work, we employ a novel multimode L-shaped MIM cavity to enable multimode strong coupling between two plasmonic modes, resonances associated with the length each cavity arm, and a vibrational transition in the insulator material. Strong and ultra-strong splittings are observed in the three-branched system. In addition to the deeply wavelength mode volumes <<(λ/2n)³ realized, the 3D-printed nature of these attoliter-sized nano-cavities offer the capability to explore intricate plasmonic cavity designs that would be required for VSC applications such as controlling chemical reactions and creating chemical sensors.