Rotation-tuned single hexagonal air cavity assisting in third-harmonic generation via hybrid modes

A fillable air cavity with large-scale electric field enhancement and a high quality (Q) factor is highly desired. However, it remains challenging in optical and near-infrared regimes. Here, we present a rotated hexagonal air cavity embedded in a Ag–air–Ag waveguide. Under near-infrared excitation, evanescent waves tunnel into the cavity. In addition to the whispering gallery mode and surface plasmon polaritons, the cavity also induces Fabry-Pérot (FP) resonance, whose orientation is tunable via cavity rotation. Thus, the cavity possesses much stronger field enhancement and higher Q than a traditional circular cavity lacking FP resonance. The waveguide exhibits filtering with backward reflection suppression. Then, by integrating a silicon cylinder into the cavity, we demonstrate linear tuning of Mie resonances via radius adjustment. When the electric dipole (ED) resonance is excited, energy is predominantly confined within the cylinder. Different Mie modes will change the orientation of the FP resonance. Importantly, the hybrid modes with ED resonance induce the third-harmonic wave of green light. Compared to a circular cavity, the rotated hexagonal cavity exhibits superior performance. These findings offer a promising strategy for designing efficient air cavities for next-generation multifunctional electro-optical devices.