  Deterministic fractal antennas are employed to realize multimodal plasmonic
devices. Such structures show strongly enhanced localized electromagnetic
fields typically in the infrared range with a hierarchical spatial
distribution. Realization of engineered fractal antennas operating in the
optical regime would enable nanoplasmonic platforms for applications, such as
energy harvesting, light sensing, and bio/chemical detection. Here, we
introduce a novel plasmonic multiband metamaterial based on the Sierpinski
carpet (SC) space-filling fractal, having a tunable and
polarization-independent optical response, which exhibits multiple resonances
from the visible to mid-infrared range. We investigate gold SCs fabricated by
electron-beam lithography on CaF$_{2}$ and Si/SiO$_{2}$ substrates.
Furthermore, we demonstrate that such resonances originate from
diffraction-mediated localized surface plasmons, which can be tailored in
deterministic fashion by tuning the shape, size, and position of the fractal
elements. Moreover, our findings illustrate that SCs with high order of
complexity present a strong and hierarchically distributed electromagnetic
near-field of the plasmonic modes. Therefore, engineered plasmonic SCs provide
an efficient strategy for the realization of compact active devices with a
strong and broadband spectral response in the visible/mid-infrared range. We
take advantage of such a technology by carrying out surface enhanced Raman
spectroscopy (SERS) on Brilliant Cresyl Blue molecules deposited onto plasmonic
SCs. We achieve a broadband SERS enhancement factor up to $10^{4}$, thereby
providing a proof-of-concept application for chemical diagnostics.
