Localization of laser-induced breakdown in aggregates of silver nanoshells

Nanoshells are useful in near-infrared (NIR) applications because of the red shift of the Mie resonance into NIR. Silica/gold/silver nanoshells were prepared via gold anchoring and characterized by Mie-theoretical calculations and transmission electron microscopy, which indicated a broad and enhanced NIR absorption and a partially coated, mottled Ag/Au structure on the silica core. The mottled nanoshells were found to spontaneously aggregate in solution to form irregular islands. Their surface roughness was examined by the fractal dimension analysis. When the aggregates were irradiated with 1064-nm laser pulses, highly localized emission due to laser-induced breakdown was observed, which is attributed to the non-uniform metal distribution over the aggregate surface and the localization of the optical field induced by the disordered arrangement of the nanoshells in the aggregate.