Highly Nonlinear Plexcitons Revealed by Single‐Nanoparticle Saturable Spectroscopy

ABSTRACT Strong optical nonlinearities are essential for a wide range of photonic technologies, including optical computing, ultrafast pulse compression, and quantum logic gates. Plexcitons, hybrid quasiparticles formed through strong coupling between plasmons and excitons, can in principle combine the strong nonlinearity of the excitonic component and the deep subwavelength confinement of the plasmonic component. However, the nonlinear modulation depth realized so far in plexcitonic systems remains far below theoretical predictions. Here, we measure the saturable reflectance contrast at the single‐nanoparticle level in a compact plexcitonic system composed of monolayer WS 2 inserted in a nanocube‐on‐mirror nanocavity. Compared with bare monolayer WS 2 , the strongly coupled system exhibits a ∼ 3500‐fold enhancement in the measured nonlinear response and achieves an integrated nonlinear modulation depth of 83%. Similar behavior is also observed in a nanobipyramid‐on‐mirror nanocavity. These compact plexcitonic systems provide a platform for exploring many‐body interaction mechanisms on ultrafast timescales, and establish building blocks for quantum photonic simulators, optical neural‐network computing, and ultrafast polaritonic switches.