Resonance pinning via coupling between Ferrell-Berreman and plasmon modes in hyperbolic metamaterials.

We propose a plasmonic platform consisting of a gold nanoantenna array integrated with a multilayer hyperbolic metamaterial (HMM) substrate and demonstrate resonance wavelength pinning enabled by the out-of-plane epsilon-near-zero (ENZ) response of the HMM. The pinning effect originates from strong coupling between the localized surface plasmon resonance (LSPR) of the nanoantennas and the Ferrell-Berreman (FB) mode supported by the multilayer structure. Numerical results reveal a pronounced anticrossing behavior with a maximum Rabi splitting of 411 meV, confirming the formation of hybrid plasmonic modes. Compared with a conventional SiO2 substrate, the proposed structure reduces resonance shifts induced by antenna-length perturbations by approximately a factor of three, significantly relaxing fabrication tolerances. In addition, the hybrid modes exhibit excellent robustness against variations in incident angle, antenna width, and antenna height, maintaining stable spectral responses over a broad parameter range. Furthermore, replacing ITO with Al:ZnO in the HMM enables effective tuning of the operating spectral range of the pinning effect, demonstrating the flexibility of engineering the FB mode through the choice of conducting oxide. These findings establish a new mechanism for resonance wavelength pinning based on the out-of-plane ENZ response of multilayer HMMs and provide a versatile route toward robust plasmonic and nanophotonic devices.