SHG behaviors due to coupled plasmon mode in Au nanorod trimer

The intense and confined optical fields due to surface plasmons (SP’s) are useful as ultrasmall light sources for nonlinear optics (NLO) in sub-diffraction spaces. In general, the NLO susceptibilities are closely associated with the geometry of the materials, and the selection rule is applied, depending on the symmetries of them. The odd-ordered susceptibilities are allowed from the structures with any symmetries, while the even-ordered ones are allowed only from the non-centrosymmetric structures. The general rule is true of the plasmonic metal nanoparticles; the even-ordered susceptibilities are forbidden under the electric dipolar approximations from the centro-symmetric spherical and rod-shaped metal nanoparticles, which are commonly used in the studies of SP. The NLO responses occur less efficiently though the terms related to higher-order multipoles in these structures.