Bi-layered stack plasmonic metasurface for high-resolution near-field second-harmonic generation microscopy applications

This work presents a comparative numerical study of three plasmonic metasurface designs for near-field second-harmonic generation (SHG) microscopy applications requiring enhanced forward (FW) second-harmonic (SH) emission and high near-field resolution. We compare a localized surface plasmon resonance (LSPR)-based metasurface, a surface-lattice resonance (SLR)-based metasurface, and a bi-layered stack metasurface. All designs employ V-shaped gold nanoantennae as the nonlinear building blocks, providing high local-field enhancement at the pump wavelength and driving SH emission in the UV–visible range. The LSPR-based metasurface provides acceptable near-field resolution but limited FW emission. On the other hand, the SLR-based design, thanks to higher Q-factor enhancement through collective resonances, boosts the SH emission, while the near-field resolution is inhibited due to the required thick cladding of the SLR-based metasurface design. The proposed bi-layered metasurface integrates a passive, centrosymmetric nanobar array supporting an SLR beneath the nonlinear V-antenna layer, separated by a spacer optimized for vertical interaction. This design results in a 200× increase in FW SH emission intensity and enhanced near-field resolution compared to the LSPR case. The results demonstrate that the stack plasmonic metasurface enables simultaneous optimization of SHG efficiency and resolution, promising for plasmonic-assisted near-field SHG microscopy applications.