Nonlinear Optical Resonances from Ballistic Electron Funnelling

We introduce a new mechanism for second-harmonic generation through geometrically rectifying─funnelling─ballistic electrons in THz optical resonators. Our resonant rectifiers inherently act as second-order harmonic generators, rectifying currents without the presence of a potential barrier. Particle-in-cell simulations reveal that femtosecond electron-surface scattering plays a critical role in this process. We differentiate electron funnelling from nonlocal plasmonic drag and bulk Dirac anharmonicity, showing that funnelling can reduce the required field intensity for second-harmonic generation by 3-4 orders of magnitude. We provide design guidelines for generating funnelling-induced second-harmonic generation, including resonance mode matching and materials selection. This approach offers a practical pathway for low-field, geometrically tunable THz upconversion and rectification, operating from sub-10 THz to multiple tens of THz in graphene.