  Strongly confined surface plasmon-polariton modes can be used for efficiently
delivering the electromagnetic energy to nano-sized volumes by reducing the
cross sections of propagating modes far beyond the diffraction limit, i.e., by
nanofocusing. This process results in significant local-field enhancement that
can advantageously be exploited in modern optical nanotechnologies, including
signal processing, biochemical sensing, imaging and spectroscopy. Here, we
propose, analyze, and experimentally demonstrate on-chip nanofocusing followed
by impedance-matched nanowire antenna excitation in the end-fire geometry at
telecom wavelengths. Numerical and experimental evidences of the efficient
excitation of dipole and quadrupole (dark) antenna modes are provided,
revealing underlying physical mechanisms and analogies with the operation of
plane-wave Fabry-P\'erot interferometers. The unique combination of efficient
nanofocusing and nanoantenna resonant excitation realized in our experiments
offers a major boost to the field intensity enhancement up to $\sim 12000$,
with the enhanced field being evenly distributed over the gap volume of
$30\times 30\times 10\ {\rm nm}^3$, and promises thereby a variety of useful
on-chip functionalities within sensing, nonlinear spectroscopy and signal
processing.
