  Coherent broadband excitation of plasmons brings ultrafast photonics to the
nanoscale. However, to fully leverage this potential for ultrafast nanophotonic
applications, the capacity to engineer and control the ultrafast response of a
plasmonic system at will is crucial. Here, we develop a framework for
systematic control and measurement of ultrafast dynamics of near-field
hotspots. We show deterministic design of the coherent response of plasmonic
antennas at femtosecond timescales. Exploiting the emerging properties of
coupled antenna configurations, we use the calibrated antennas to engineer two
sought-after applications of ultrafast plasmonics: a subwavelength resolution
phase shaper, and an ultrafast hotspot switch. Moreover, we demonstrate that
mixing localized resonances of lossy plasmonic particles is the mechanism
behind nanoscale coherent control. This simple, reproducible and scalable
approach promises to transform ultrafast plasmonics into a straightforward tool
for use in fields as diverse as room temperature quantum optics, nanoscale
solid state physics and quantum biology.
