Decoupling nonlinear programmability from resonance tuning in a spectrally stable graphene–GaN terahertz nanocavity

Electrically tunable graphene metasurfaces commonly rely on Fermi-level modulation to reshape plasmonic resonances, a process that inherently couples amplitude control to undesirable resonance-frequency shifts. This work introduces a mechanism to bypass this trade-off: field-programmed second-harmonic generation (SHG) within a spectrally stable graphene–GaN terahertz nanocavity. By applying a vertical DC field, we activate an effective second-order nonlinear response at the graphene–GaN interface through electric-field-induced second-harmonic generation. Crucially, the cavity-backed hybrid mode remains nearly invariant under bias, effectively decoupling the nonlinear source modulation from resonance-frequency tuning. Full-wave nonlinear simulations confirm electrically programmable SHG emission with minimal resonance pulling, stable near-field profiles, and multi-resonant SH spectra. This platform, leveraging multi-resonant enhancement from hybrid cavity-plasmon modes, provides a robust architecture for spectrally stable, active nonlinear terahertz metasurfaces.