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 5×108 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.
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