Strong Photon-Plasmon Coupling for High-Order Waveguide-Plasmon Polaritons

Strong light-matter interactionenables significant modificationof fundamental properties of coupled matter, such as chemical reactionrate, conductivity, and energy transfer, which is crucial for designingefficient light-emitting devices and low-threshold lasing. Strongphoton-plasmon coupling in metallic photonic crystal slabshas been extensively exploited for band gap engineering, third-harmonicgeneration, and enhanced Faraday rotation. However, current researchfocuses on coupling the waveguide mode to the dipolar plasmon thatis inherently lossy and has a small quality factor. This paper reportsa metallic photonic crystal slab constructed by placing split nanoringdimer arrays onto an indium-tin-oxide (ITO) waveguideto excite dipolar and high-order waveguide-plasmon polaritonswith large Rabi splitting. The dimer unit comprises two identicalsplit nanorings and supports dipolar and high-order hybrid plasmonsderived from structural symmetry breaking and plasmon hybridization.Arrangement of the dimer into arrays on the ITO slab creates a strongcoupling channel for the hybrid plasmons and the waveguide mode togenerate dipolar and high-order waveguide-plasmon polaritonswith distinct anticrossing dispersions. The strong coupling effectremarkably modifies the plasmon oscillations by altering the directionsand strength of their dipole moment. We used the strongly coupledplasmons with narrow spectral line shapes as a refractive index sensorand obtained the largest sensitivity of 250 nm & BULL;RIU-1 and a figure of merit of 11.62 RIU-1.