Fig. 4: Ultrastrong coupling in multilayer WS2 flakes.
a–d, Dark-field scattering spectra of a multilayer WS2 plasmonic system. Dashed white lines denote the exciton energy (\({\omega }_{{ex}}=2.01\) eV) for a,b \({\omega }_{{ex}}=2.003\) eV for c,d and the diagonal dashed orange and purple lines indicate the plasmon energy (\({\omega }_{{pl}}\)). The open circles represent lower (\({\omega }_{-}\)) and upper (\({\omega }_{+}\)) polariton energies obtained from the dark-field scattering spectra of individual plasmonic systems. The solid lines show the lower and upper polariton dispersions using the full Hopfield Hamiltonian. 3 L and 4 L represent the trilayer and quadrilayer cases. e Normalized coupling strength (\(g/{\omega }_{{ex}}\)) as a function of the WS2 layers under 0 (orange) and −2% compressive strain (purple). The horizontal dashed line marks the onset of USC. Error bars are derived from the variation in dark-field scattering from different batches of samples and are extracted from the standard error of the fit.
