Fluorescence Enhancement of Crystal Violet in Low-Loss Plasmonic Nanocavity Formed by Gold Nanorod on Single-Crystalline Gold Microflake

Extreme optical confinement and field enhancement induced by nanoparticle-on-film plasmonic nanocavities are an effective strategy to improve the spontaneous emission efficiency of quantum emitters, and the quality of the metal film is critical to their performance in enhanced spectroscopic applications. Here, we construct plasmonic nanocavities by dispersing gold nanorods on single-crystalline gold microflakes (GMFs) and sputter-deposited gold film separated by a tunable thickness poly(methyl methacrylate) (PMMA) layer on the same glass substrate, respectively. The effects of GMF and gold film on the fluorescence of crystal violet molecules doped in the PMMA spacer layer are experimentally studied and compared. Compared with sputter-deposited gold film, the nanocavities formed on GMFs produce a 143-fold fluorescence enhancement and 5.4-fold lifetime reduction for CV molecules at the optimal PMMA thickness of 13 nm, which are about 6 and 3 times the counterparts formed on gold film, respectively. The great improvement in the plasmonic enhancement effect of the nanocavities formed on ultrasmooth single-crystalline GMFs can be attributed to the reduced optical losses caused by rough surface and grain boundaries in gold film. The low-loss high-performance plasmonic nanocavities based on ultrasmooth single-crystalline GMFs provide a platform for applications in high-speed nanoscale light sources, surface-enhanced spectroscopy, and ultrasensitive sensors.