Bottom-up design of hybrid polymer nanoassemblies elucidates plasmon-enhanced second harmonic generation from nonlinear optical dyes.

Flexible design of hybrid polymer nanoassemblies consisting of nonlinear optical (NLO) polymer nanosheets and gold nanoparticle alignment was done to elucidate near-field effects of localized surface plasmon (LSP) coupling, which was generated from coupled gold nanoparticles, on enhanced second harmonic generation (SHG) from nonlinear optical (NLO) dyes in hybrid nanoassemblies. Structurally well-defined hybrid polymer nanoassemblies comprising NLO polymer nanosheets and aligned gold nanoparticles were fabricated using bottom-up approaches: Langmuir-Blodgett (LB) technique and nanoparticle adsorption. Two hybrid polymer nanoassembled structures were particularly examined: a single-layer NLO polymer nanosheet and gold nanoparticle monolayer (single-layer structure) exhibiting intralayer LSP coupling, and a single-layer NLO polymer nanosheet sandwiched between two-layer gold nanoparticle monolayers (sandwich structure). The latter enables interlayer LSP coupling between the two gold nanoparticle monolayers. Dependence of SHG intensity on the distance between the NLO layer and nanoparticle layer was examined according to the LB layer structure and gold nanoparticle size variation. The SH light intensity from the NLO polymer nanosheet decreased almost exponentially with increasing spacer distance between the NLO polymer nanosheet and gold nanoparticle monolayer in both single-layer and sandwich structures. The decay length depends strongly on the gold nanoparticle size, indicating effective spatial distance for enhanced SHG from NLO polymer nanosheets. Theoretical calculations were used to study the enhancement mechanism. Finite difference time domain (FDTD) calculations reproduced the exponential behavior of SH light intensity as a function of separation distance, which confirmed the importance of coupled gold nanoparticle formation and parallel geometry of near-field coupling of the coupled gold nanoparticles with NLO polymer nanosheets for efficient SHG enhancement. Dipole-type LSP coupling along the long axis of adjacent gold nanoparticles at the fundamental frequency dominates enhancement of SHG from NLO dyes oriented parallel to the long axis of LSP coupling, which occurs at the center of the Au NPs.