Plasmonic hierarchical nanostructures with cascaded field enhancement and their SERS applications

Plasmonic nanostructures with strong near field “hot spots” are highly demanded in many applications such as surface enhanced Raman spectroscopy (SERS). Here, we present some specially designed plasmonic hierarchical nanostructures that combine geometric features of micro- and nanoscales. Owing to the mode coupling and hybridization in these multiscale systems that can produce the cascaded field enhancement (CFE) effect, extremely strong and highly confined field hot spots can be readily generated in nanoscale volumes. Two typical hierarchical nanostructures are presented: an Mshaped grating with 30 nm narrow V-shaped grooves and a nanoparticle-in-cavity (PIC) plasmonic nanoantenna array. A cost-effective, efficient and reliable fabrication technique based on room-temperature nanoimprinting and anisotropic reactive ion etching is developed to fabricate these plasmonic hierarchical nanostructures in large area, during which the nano-features can be finely controlled and tuned. The field distributions and enhancement in the proposed structures are experimentally characterized, which agree very well with the numerical simulations. SERS experiments show the SERS enhancement factor as high as 5×10 8 by employing these hierarchical nanostructures as SERS substrates, which verify the strong light-matter interaction and show the great potential of these devices as low-cost and highly-active substrates for SERS applications.