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.
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