  We demonstrate stimulated emission depletion (STED) microscopy with 20 nm
gold nanospheres coated by fluorescent silica. Compared with previous
demonstrations of STED with a hybrid plasmonic fluorescent label, the current
implementation offers a substantially smaller label and a better resolution
improvement of up to 2.5-fold beyond the diffraction limit of confocal
microscopy. This is achieved at approximately 2 times lower intensity than
conventional STED based on dyes alone, and in an aqueous environment,
demonstrating the relevance to bio-imaging. Finally, we also show, for the
first time in this context, a 3-fold reduction in the rate of photobleaching
compared to standard dye-based STED, thus, enabling brighter images.
