  Resonant optical antennas supporting plasmon polaritons (SPPs) - collective
excitations of electrons coupled to electromagnetic fields in a medium - are
relevant to sensing, photovoltaics, and light-emitting devices, among others.
Due to the SPP dispersion, a conventional antenna of fixed geometry, exhibiting
a narrow SPP resonance, cannot simultaneously operate in two different spectral
bands. In contrast, here it is demonstrated that in metallic disks, separated
by a nanometric spacer, the hybridized antibonding SPP mode stays in the
visible range, while the bonding one can be pushed down to the mid-infrared
range. Such an SPP dimer can sense two materials of nanoscale volumes, whose
fingerprint central frequencies differ by a factor of 5. Additionally, the
mid-infrared SPP resonance can be tuned by employing a phase-change material
(VO2) as a spacer. The dielectric constant of the phase-change material is
controlled by heating the material at the frequency of the antibonding optical
mode. These findings open the door to a new class of optoelectronic devices
able to operate in significantly different frequency ranges in the linear
regime, and with the same polarization of the illuminating wave.
