  Nanoparticle-on-mirror plasmonic nanocavities, capable of extreme optical
confinement and enhancement, have triggered state-of-the-art progress in
nanophotonics and development of applications in enhanced spectroscopies and
molecular detection. However, the optical quality factor and thus performance
of these nanoconstructs are undermined by the granular polycrystalline metal
films used as a mirror. Here, we report an atomically smooth single-crystalline
platform for low-loss nanocavities using chemically-synthesized gold
microflakes as a mirror. Nanocavities constructed using gold nanorods on such
microflakes exhibit a rich structure of plasmonic modes, which are highly
sensitive to the thickness of optically-thin (down to ~15 nm) microflakes. The
atomically smooth single-crystalline microflakes endow nanocavities with
significantly improved quality factor (~2 times) and scattering intensity (~3
times) compared with their counterparts based on deposited films. The developed
low-loss nanocavities further allow for the integration with a mature platform
of fiber optics, opening opportunities for realizing nanocavity-based
miniaturized photonic devices with high performance.
