  The nonlinear processes of frequency conversion like second harmonic
generation (SHG) usually obey certain selection rules, resulting from the
preservation of different kind of physical quantities, e.g., the angular
momentum. For SHG created by monolayer of transition-metal dichalcogenides
(TMDCs) like WS2, the valley-exciton locked selection rule predicts an SHG
signal in the cross-polarization state. By combining plasmonic nanostructures
with a monolayer of TMDC, a hybrid metasurface is realized which affects this
nonlinear process due to an additional polarization conversion process. Here,
we observe that the plasmonic metasurface modifies the light-matter interaction
with the TMDC resulting in an SHG signal that is co-polarized with respect to
the incident field, which is usually forbidden for solely monolayers of TMDC.
We fabricate such hybrid metasurfaces by placing plasmonic nanorods on top of a
monolayer WS2 and study the valley-exciton locked SHG emission from such system
for different parameters, such as wavelength and polarization. Furthermore, we
show the potential of the hybrid metasurface for tailoring nonlinear processes
by adding additional phase information to the SHG signal using the
Pancharatnam-Berry phase effect. This allows direct tailoring of the SHG
emission to the far-field.
