Nanometer-scale confinement of whispering gallery plasmonic modes upon slit excitation.

Squeezing optical fields into resonant modes with deep-subwavelength volumes is crucial for advancing quantum optics and nanoscale sensing. A promising approach is to harness surface plasmon modes in resonators composed of stacked metal-insulator-metal cylindrical layers, where optical fields in the visible and infrared ranges are transformed into strongly confined gap modes. In a structure with cylindrical symmetry, whispering gallery modes form a family of eigenmodes with a high azimuthal order, which are typically inaccessible in plasmonic sandwich configurations under plane wave illumination, and their excitation usually requires carefully positioned focused electron probes. Here, we propose an efficient method for exciting whispering gallery modes in gap-surface plasmon resonators using plane wave excitation. The introduction of a nanoscale through-split in the top metal layer enables the formation of a rich set of WGMs in the near-infrared spectrum, resulting in 2-3 orders of magnitude enhancement in total electromagnetic energy accumulated in the spacer. This strategy offers a practical pathway to unveil hidden modal families in plasmonic sandwich resonators to control light-matter interactions at the nanoscale and to drive the further miniaturization of plasmonic devices.