Giant nonlinear processes in plasmonic metasurfaces

Nonlinear metasurfaces based on coupling locally-enhanced plasmonic response to intersubband transitions of multi-quantum-wells have recently provided second-order susceptibilities several orders of magnitude larger than any other non-linear flat structure measured so far. Here, we demonstrate that a proper design of the plasmonic metasurface inclusions can dramatically enhance the overall non-linear response of the structure. The optimized metasurfaces are then applied to achieve efficient second-harmonic generation (SHG) and differential frequency generation (DFG) in the infrared and terahertz frequency bands, respectively. In case of low-power impinging beams (≈1W), our simulations predict large conversion efficiencies of around 0.8% and 0.01% for SHG and DFG, outperforming previously reported efficiencies in 1-2 orders magnitude and confirming the suitability of this type of plasmonic metasurfaces as a highly-efficient flat platform for non-linear photonics.