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Plasmonic nanostructures, with their unique ability to localize electromagnetic fields into nanoscale volumes to create the so-called hot spots, have been widely studied for the enhancement of nonlinear conversion. Various nonlinear optical processes such second-harmonic generation (SHG), third-harmonic generation, or four-wave mixing have been observed in different designed configurations. The SHG process is known to be forbidden in centrosymmetric nanostructures. Thanks to the broken centro-symmetry at the metal surface as well as to the high degree of the asymmetric spatial variation of the inducing electromagnetic fields, strong SHG in noble metals is experimentally observed via properly design.
In this work, we studied the SHG of vertical and planar split ring resonator (SRR) arrays. Via a unique nanofabrication technique, we are able to accurately control the alignment of nano-structures on top of each other and experimentally realize vertical split ring resonators (VSRRs). As VSRRs allow the coupling of both the incident electric and magnetic field to the excitation of magnetic dipole resonance, the induced strong fields confined within two vertical prongs are beneficial for the SHG enhancement. In addition, a better field confinement is achieved for vertical configurations since the localized fields in the planar SRR gaps are inevitably leaked to the underlying substrate. The nonlinear optical measurements showed a 2.6-fold enhancement of SHG nonlinearity for VSRR metasurface compared to their planar counterparts. Through the analysis of multipole decomposition, we found that except for electric dipole, the dominant mode for VSRRs is electric quadrupolar resonance, while that for planar SRRs is magnetic dipole. This work paves the way in increasing the nonlinear transition quantum efficiency and provides a new insight in designing novel nonlinear sources.
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