Fig. 3: Tunable ultrastrong exciton-plasmon coupling in the WS2 monolayer.
a Mechanically tunable plasmonic resonance (energy at \({\omega }_{{pl}}\)) and plasmonic enhancement in photoluminescence peak intensity of the strongly coupled plasmonic system. b Normalized dark-field scattering intensity as a function of excitation energy and strain. The horizontal purple line marks the onset of USC. The black solid lines are the extracted polariton energies of the lower (LPB) and upper (UPB) polariton branches. The white dashed line is the WS2 exciton energy (\({\omega }_{{ex}}=2.019\) eV). The plasmonic response shifts to lower/higher energy when compressive/tensile strain is applied, and the local field enhancement is significantly enhanced/reduced owing to the reduced/enlarged gap size, and both effects change the coupling strength. c Normalized coupling strength as a function of strain. The onset of USC is marked by the horizontal dashed line. d Ground-state energy modification as a function of strain. The black dashed line is the calculation result from the coupling strength (\(g\)) and the polariton energies (\({\omega }_{+},{\omega }_{-}\)) from the fitting to the full Hopfield Hamiltonian. The absolute change in ground-state energy reaches 13.4 meV at −2% compressive strain. Error bars in (c) and (d) result from the variation in dark-field scattering from multiple batches of samples and are extracted from the standard error of the fit.
