Fano resonance boosted cascaded optical field enhancement in a plasmonic nanoparticle-in-cavity nanoantenna array and its SERS application

Cascaded optical field enhancement (CFE) can be realized in some specially designed multiscale plasmonic nanostructures, in which the generation of extremely strong fields at nanoscale volume is crucial for many applications, for example, surface-enhanced Raman spectroscopy (SERS). In this paper, we propose a strategy for realizing a high-quality plasmonic nanoparticle-in-cavity (PIC) nanoantenna array, in which strong coupling between a nanoparticle (NP) dark mode with a high-order nanocavity bright mode can produce strong Fano resonance at the target wavelength. The Fano resonance can effectively boost the CFE in a PIC. A cost-effective and reliable nanofabrication method is developed using room temperature nanoimprinting lithography to manufacture high-quality PIC arrays. This technique guarantees the generation of only one gold NP at the bottom of each nanocavity, which is crucial for the generation of the expected CFE. To demonstrate the performance and application of the PIC array, the PIC array is employed as an active SERS substrate for detecting 4-aminothiophenol molecules. An experimental SERS enhancement factor of 2 x 10(7) is obtained, which verifies the field enhancement and the potential of this device.