Fig. 1

SHG principles. a Energy level diagram for SHG. SHG is a parametric process involving virtual states (indicated as dashed horizontal lines), corresponding to the destruction of two fundamental photons followed by the generation of one SH photon. b Classical description of SHG. The fundamental near field driven by the incoming wave induces a nonlinear surface polarization P_surf,n(r,2ω) due to the nonlinear surface susceptibility χ_surf (step 1). The second step is the electromagnetic radiation of the nonlinear sources. Reprinted with permission from The Journal of Physical Chemistry C 2016, 120, 17699–17710. Copyright 2016 American Chemical Society