  Plasmonic metasurfaces provide an effective way to increase the efficiency of
several nonlinear processes while maintaining nanoscale dimensions. In this
work, nonlinear metasurfaces based on film-coupled silver nanostripes loaded
with Kerr nonlinear material are proposed to achieve efficient four-wave mixing
(FWM). Highly localized plasmon resonances are formed in the nanogap between
the metallic film and nanostripes. The local electric field is dramatically
enhanced in this subwavelength nanoregion. These properties combined with the
relaxed phase matching condition due to the ultrathin area lead to a giant FWM
efficiency, which is enhanced by nineteen orders of magnitude compared to a
bare silver screen. In addition, efficient visible and low-THz sources can be
constructed based on the proposed nonlinear metasurfaces. The FWM generated
coherent wave has a directional radiation pattern and its output power is
relatively insensitive to the incident angles of the excitation sources. This
radiated power can be further enhanced by increasing the excitation power. The
dielectric nonlinear material placed in the nanogap is mainly responsible for
the ultrastrong FWM response. Compact and efficient wave mixers and optical
sources spanning different frequency ranges are envisioned to be designed based
on the proposed nonlinear metasurface designs.
