Fractal-shaped nanoantennas
have a large potential to enable multiband
devices for surface-enhanced spectroscopy due to their scale-invariant
geometry that gives rise to strongly enhanced local fields across
different spectral ranges with multiscale spatial distributions. In
particular, fractal nanoantennas based on plasmonic metals are promising
for biodetection applications that extend from the near-infrared across
the mid-infrared spectrum. In this context, we introduce novel multiscale
resonant structures based on the inverse Cesaro space-filling fractal
curve with the remarkable property that the number of resonant bands
does not depend on the overall size of the structures. We systematically
study their scattering and near-field resonant properties by resorting
to full-field finite difference time domain simulations in combination
with experimental Fourier transform infrared microspectroscopy. In
particular, by investigating a number of gold antennas fabricated
by electron-beam lithography on CaF2 substrates, we demonstrate
controllable multiband plasmonic resonances from near-infrared to
the mid-infrared spectral regions. Moreover, our findings demonstrate
that large values of near-field enhancement with hierarchical fractal
distributions can be achieved in Cesaro-type nanoantennas across multiple
bands that are ideally suited for chemical detection on a small footprint
area. In order to demonstrate the full potential of Cesaro fractal
nanoantennas for infrared sensing spectroscopy, we show triple band
reliable detection of thin poly(methyl methacrylate) layers with nanoscale
thickness. The engineering of Cesaro-type plasmonic nanoantennas provides
a novel strategy for the realization of active devices with a large
spectral density and reduced footprints that can be conveniently integrated
in future plasmonic–photonic active platforms for energy harvesting
and optical biosensing.
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