  Nanoantennas offer the ultimate spatial control over light by concentrating
optical energy well below the diffraction limit, whereas their quality factor
(Q) is constrained by large radiative and dissipative losses. Dielectric
microcavities, on the other hand, are capable of generating a high Q-factor
through an extended photon storage time but have a diffraction-limited optical
mode volume. Here we bridge the two worlds, by studying an exemplary hybrid
system integrating plasmonic gold nanorods acting as nanoantennas with an
on-resonance dielectric photonic crystal (PC) slab acting as a low-loss
microcavity and, more importantly, by synergistically combining their
advantages to produce a much stronger local field enhancement than that of the
separate entities. To achieve this synergy between the two polar opposite types
of nanophotonic resonant elements, we show that it is crucial to coordinate
both the dissipative loss of the nanoantenna and the Q-factor of the low-loss
cavity. In comparison to the antenna-cavity coupling approach using a
Fabry-Perot resonator, which has proved successful for resonant amplification
of the antenna's local field intensity, we theoretically and experimentally
show that coupling to a modest-Q PC guided resonance can produce a greater
amplification by at least an order of magnitude. The synergistic
nanoantenna-microcavity hybrid strategy opens new opportunities for further
enhancing nanoscale light-matter interactions to benefit numerous areas such as
nonlinear optics, nanolasers, plasmonic hot carrier technology, and
surface-enhanced Raman and infrared absorption spectroscopies.
