  The applications of plasmonics to energy transfer from free-space radiation
to molecules are currently limited to the visible region of the electromagnetic
spectrum due to the intrinsic optical properties of bulk noble metals that
support strong electromagnetic field confinement only close to their plasma
frequency in the visible/ultraviolet range. In this work, we show that
nanoporous gold can be exploited as plasmonic material for the mid-infrared
region to obtain strong electromagnetic field confinement, co-localized with
target molecules into the nanopores and resonant with their vibrational
frequency. The effective optical response of the nanoporous metal enables the
penetration of optical fields deep into the nanopores, where molecules can be
loaded thus achieving a more efficient light-matter coupling if compared to
bulk gold. In order to realize plasmonic resonators made of nanoporous gold, we
develop a nanofabrication method based on polymeric templates for metal
deposition and we obtain antenna arrays resonating at mid-infrared wavelengths
selected by design. We then coat the antennas with a thin (3 nm) silica layer
acting as target dielectric layer for optical energy transfer. We study the
strength of the light-matter coupling at the vibrational absorption frequency
of silica at 1250 cm-1 through the analysis of the experimental Fano lineshape
that is benchmarked against identical structures made of bulk gold. The boost
of optical energy transfer from free-space mid-infrared radiation to molecular
vibrations in nanoporous 3D nanoantenna arrays can open new application routes
for plasmon-enhanced physical-chemical reactions.
