Anisotropy has played a critical role in many material systems, but its controllable creation and modulation have been a long-lasting challenge for the scientific communities. Polarization-addressed anisotropy appears more attractive among all approaches due to its excellent controllability, simplicity, and accuracy, but only a limited number of material systems are applicable for such a concept, which are largely focused on oriented growth. Here, we establish a polarization-dependent anisotropic etching system made of Au@oligomer core–shell nanoparticles (NPs). As the oligomer coatings can be photochemically degraded via two-photon photolithography, the plasmonic near-field enhancement supported by the Au NP cores renders much faster degradation of the oligomer shells along the polarization, resulting in anisotropic Au@oligomer hybrid NPs. Such shape anisotropy leads to polarization-dependent photoluminescence with embedded dyes of methylene blue, which can be used as single-particle-based polarization detector. The oligomer lobes capped at the sides of the Au NP can also function as a protection agent for anisotropic photochemical growth of Au NPs, which evolve into Au nanorods and mushrooms with controlled irradiation time. Such polarization-directed etching of oligomer shells has unique advantages of high local-selectivity, controllability, and versatility for on-chip nanofabrication, which opens many new opportunities for integrated nanophotonic devices.
Nonlinear optical signals of metallic nanoparticle films usually reach the maximum at the percolation threshold with a semicontinuous nanostructure due to the enhancement of the percolating effect. Here, we deposited Ag nanoparticles on the percolating Au nanoparticle film to construct conductive Ag/Au bilayer films and comparatively explored second harmonic generation (SHG) of single-component (Au or Ag) and two-component Ag/Au nanoparticle films. Intriguingly, we observed a 680% SHG enhancement of the Ag/Au bilayer film compared to that of the percolating Au nanoparticle film, which is caused by a cooperative effect of surface plasmon resonances, interband transitions, and the percolation effect. These observations provide a strategy to design nonlinear photonic nanodevices with optimized nonlinear performance.
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