Bioinspired self-similar all-dielectric antennas: probing the effect of secondary scattering centres by Raman spectroscopy

All-dielectric optical antennas offer unique advantages in fabrication of highly reproducible Raman
probes and light management at the meso- and nano-scale. Hot-spot engineering is a key step for
enhancing the local field and promoting light channelling from far- to near-field. However, unlike the
case of plasmonic antennas, hot-spot design and implementation in all-dielectric systems is still largely
underdeveloped. Inspired by building-up principles that govern absorption, trapping and scattering of
visible light in a variety of biological structures and guided by finite element optical simulations, here we
explored the potential of self-similar dielectric optical antennas in light coupling with Mie-type modes.
The systems investigated were based on the combination of primary and secondary antennas, made of
micron- and nano-sized TiO2 nanoshells. Different types of configurations, including randomly and
hierarchically organized antennas were tested. Raman experiments revealed that hierarchically ordered
structures offer superior SERS performances in terms of field enhancement and reproducibility in
comparison to the randomly arranged counterparts, which are in turn more efficient, yet less
reproducible, than simple micro-sized antennas taken as a reference. These results open the door to
further investigations in view of developing next-generation platforms for SERS and photocatalysis.