Enhancing Third-Harmonic Generation with Spatial Nonlocality

As the geometrical feature of a nanostructure approaches the Thomas-Fermi screening length, the electron-level interaction induced nonlocality leads to the longitudinal resonance modes above bulk plasmon resonance frequency. In this work, we investigate the contribution of longitudinal modes to the enhancement of third harmonic generation process. Specifically, we study a single Ag nanowire and a Ag nanowire dimer coated with third-order nonlinear dielectrics. By implementing hydrodynamic and nonlinear models together, we find that the spectral overlap of the longitudinal resonance modes with third order harmonics enables the improvement of the nonlinear conversion efficiency. The optimized results show that despite of reduced field enhancement for the fundamental resonance, the third-harmonic absorption intensities in nonlocal case can surpass the local calculation results by hundreds of times. Maximum third-harmonic scattering intensities can also be realized though appropriate design of both structures. In contrast to previous studies that mainly focus on the negative effects of nonlocality, our study indicates that the nonlocal effects may benefit our system with proper designs, opening a new door for quantum plasmonic research.