Plasmon gap mode-assisted third-harmonic generation from metal film-coupled nanowires

A numerical investigation on the third-order nonlinear optical properties of a plasmonic system composed by metal film-coupled nanowires is reported. The linear and nonlinear optical characteristics are studied by finite-difference time-domain (FDTD) method. To substantially improve the nonlinear effect, the geometric parameters of the system are carefully engineered to excite strong plasmon gap resonance with dramatically enhanced electric field intensity at the gap between the nanowires and the film. The third-harmonic generation (THG) property is examined by nonlinear FDTD simulation. It shows that the THG efficiency estimated from the nonlinear optical absorption can be ∼1 × 10−5 under an incident power density of 5.2 GW/cm2. Plasmonic resonance is necessary to achieve highly efficient THG since the system on resonance shows the THG intensity 4 orders of magnitude higher than that of an off-resonance system.