Control of Plexcitonic Strong Coupling via Substrate‐Mediated Hotspot Nanoengineering

Abstract Plexcitonic strong coupling has ushered in an era of room‐temperature quantum electrodynamics at the nanoscale. Realizing its potential applications from single‐molecule spectroscopy to room‐temperature quantum technologies on an industrial level requires scalable and mass‐producible plasmonic cavities that provide ease of access and control for quantum emitters. Here, a strategy for multidimensional hotspot engineering is proposed via a rational selection of substrates, which facilitates elevation of a gold bowtie nanocavity hotspot to the top of the device and provides a field enhancement of ≈482 (a 1.6‐fold increase compared to a conventional bowtie‐on‐glass cavity at the bottom of the nanogap). The formation mechanism for these antenna modes is discussed from the perspective of charge carrier motion; and their advantages, particularly in view of their dominantly in‐plane polarized near‐fields, are further elaborated in a spatiotemporal study of plexcitonic strong coupling, which reveals ultrafast quantum dynamics and potential for applications related to 2D materials whose excitonic dipoles are typically oriented in‐plane. The conceptual discovery of this substrate‐enabled hotspot nanoengineering could readily be extended to tailor hotspots in other plasmonic platforms, and may inspire a plethora of novel research directions from plasmon‐enhanced spectroscopy and sensing to the design of quantum logic gates and quantum metasurfaces.