“…Recently, researchers have developed a variety of plasmonic nanostructures to enhance the nonlinearity of thin films [3]. For example, bow-tie-shaped gold elements [4,5], plasmonic nanogap cavity [6], nanohole array [7], individual nanohole [8], nanoparticles [9], doubly resonant Si nanoresonators [10], MoS 2 on a silicon waveguide [11], film-coupled silver nanostrips [12], graphene-based plasmonic gratings [13,14] and metal gratings [15][16][17][18][19] have been shown to largely enhance the nonlinear optical conversion efficiency. Moreover, heterostructures such as coupled semiconductormetal nanoparticle films [20], polymer/Ag/glass structure [21], and bilayer gold nanopillar-nanoaperture structure [22] have been demonstrated to enhance SHG.…”