Surface plasmon enhanced second harmonic behavior of a noncentrosymmetric dolmen-type Au nanostructure

This paper presents the second harmonic generation (SHG) behavior of a dolmen-type Au nanostructure under surface plasmon (SP) resonance conditions. The SP-enhanced optical fields of the plasmonic metal nanoparticles are suitable for nonlinear wave mixing at the nanoscale. However, the SP-enhanced fields alone are insufficient for second-order nonlinear optics, including the SHG effect. As in traditional nonlinear optics, it is helpful to prepare the metal nanoparticles in a noncentrosymmetric shape to obtain the large SHG signals in the far fields. The dolmen-type Au nanostructure consists of three centrosymmetric rectangular Au nanorods (AuNRs), two of which are arranged in parallel and the third is perpendicular. The U-shaped arrangement breaks the centrosymmetry in this complex nanostructure. The noncentrosymmetric dolmen-type AuNRs exhibited <^>25 times higher SHG signals than the reference single AuNR. According to the numerically mapped surface charges and near fields, the enhanced SHG signals were related to the hybridized bonding state between the dipolar plasmon of the monomer moiety and the quadrupolar plasmon of the dimer moiety. The far-field SHG signals are attributed to the intense near field, which was not centrosymmetric in the nanogap region between the monomer and dimer moieties. The dependence of the SHG signals was also investigated as a function of the separation between the monomer and dimer moieties. The SHG conversion efficiency shows a considerable increase as the separation narrows down to <^>20% of the AuNR size. Our present research demonstrates that second-order nonlinear plasmonic behavior can be engineered by appropriately arranging the constituent simply shaped metal nanoparticles.

Automated summary

This paper presents the second harmonic generation behavior of a dolmen-type Au nanostructure under surface plasmon resonance conditions. The study demonstrates that arranging metal nanoparticles in a noncentrosymmetric shape can enhance the second-order nonlinear optics effect, resulting in higher SHG signals. The enhanced SHG signals are attributed to the hybridized bonding state between the dipolar plasmon of the monomer moiety and the quadrupolar plasmon of the dimer moiety.