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

Abstract

Strong light−matter interaction enables significant modification of fundamental properties of coupled matter, such as chemical reaction rate, conductivity, and energy transfer, which is crucial for designing efficient light-emitting devices and lowthreshold lasing. Strong photon−plasmon coupling in metallic photonic crystal slabs has been extensively exploited for band gap engineering, third-harmonic generation, and enhanced Faraday rotation. However, current research focuses on coupling the waveguide mode to the dipolar plasmon that is inherently lossy and has a small quality factor. This paper reports a metallic photonic crystal slab constructed by placing split nanoring dimer arrays onto an indium−tin−oxide (ITO) waveguide to excite dipolar and highorder waveguide−plasmon polaritons with large Rabi splitting. The dimer unit comprises two identical split nanorings and supports dipolar and high-order hybrid plasmons derived from structural symmetry breaking and plasmon hybridization. Arrangement of the dimer into arrays on the ITO slab creates a strong coupling channel for the hybrid plasmons and the waveguide mode to generate dipolar and high-order waveguide−plasmon polaritons with distinct anticrossing dispersions. The strong coupling effect remarkably modifies the plasmon oscillations by altering the directions and strength of their dipole moment. We used the strongly coupled plasmons with narrow spectral line shapes as a refractive index sensor and obtained the largest sensitivity of 250 nm•RIU −1 and a figure of merit of 11.62 RIU −1 .