Resonance-Tuned Plasmon–Phonon Hybridization in Continuous Bilayer Graphene

Bilayer graphene hosts an infrared-active antisymmetric (Eu) optical phonon whose spectral response is strongly influenced by electronic excitations. While carrier-density control of this phonon has been widely studied, its interaction with collective electronic excitations, graphene plasmons, at nominally fixed carrier density remains less explored. Here we show that resonant plasmonic excitation in chemically doped AB-stacked BLG reshapes the optical phonon response through plasmon–phonon hybridization. A grating-coupled resonator enables efficient far-field excitation of plasmons in a continuous BLG sheet, while a built-in photonic cavity modulates the plasmonic resonance strength. We observe systematic changes in Fano asymmetry, apparent resonance position, spectral weight, and effective line width near the plasmon-strength maximum. Correlation analysis and numerical simulations support plasmon–phonon hybridization as the dominant origin of the observed spectral evolution, with possible intrinsic phonon–electronic coupling. Our findings identify resonance tuning as a distinct route to control plasmon–phonon hybridization and engineer nanoscale mid-infrared light–matter interactions.