Doubly Resonant Plasmonic Hot Spot–Exciton Coupling Enhances Second Harmonic Generation from Au/ZnO Hybrid Porous Nanosponges

Abstract

We introduce zinc oxide (ZnO) functionalized porous gold nanoparticles that exhibit strong second harmonic (SH) emission due to an efficient coupling of localized surface plasmons to ZnO excitons. The nanosponges are perforated with a random network of 10 nm sized ligaments, localizing plasmons in a high density of hot spots. We use a broadband, few-cycle ultrafast laser to probe coherent nonlinear emission from individual bare gold and ZnO-functionalized sponges. While the third harmonic spectrum of the hybrid particles redshifts with respect to that of bare gold sponges, a distinct blueshift is seen in their SH spectra. SH emission around 390 nm, slightly below the ZnO band gap, is enhanced by 10×. We attribute this to doubly resonant plasmon–exciton interactions: the laser drives nanosponge plasmon hot spot resonances, and this locally enhanced field induces two-photon excitation of localized ZnO excitons. This opens a path toward the design of efficient coherent nonlinear optical sources by combining ...