Complex plasmon-exciton dynamics revealed through quantum dot light emission in a nanocavity.

The strong coupling of quantum emitters to plasmonic cavities has emerged as an exciting frontier in quantum plasmonics and optics. Here, we report an extensive set of measurements of plasmonic cavities hosting one to a few semiconductor quantum dots (QDs). Scattering spectra show Rabi splitting, demonstrating that these devices are close to the strong coupling regime. Using Hanbury Brown and Twiss (HBT) interferometry, we observe non-classical emission from the QDs, allowing us to directly determine their number in each device. However, Photoluminescence (PL) spectra measured from QDs coupled to the plasmonic devices are narrower than scattering spectra and show smaller values of the apparent peak splitting, indicating the existence of complex dynamics involving multiple excited states. Using model simulations based on an extended Jaynes Cummings Hamiltonian, we find that the involvement of a dark state of the QDs explains the experimental findings. The coupling of the dark state to the plasmonic cavity makes its emission bright enough to appear as a strong separate peak in the PL spectrum. Interestingly, the intensity of this peak is determined by the interaction with the polaritonic states formed by the coupling of the bright state of the QD to the cavity. Further, a slow decay component in the HBT correlation curves can be attributed to the relaxation of the dark state. The coupling of quantum emitters to plasmonic cavities thus exposes complex relaxation pathways and emerges as an unconventional means to control dynamics of quantum states.