Fig. 2: Ultrastrong exciton-plasmon coupling in the WS2 monolayer.
Dark-field scattering spectra under (a) 0 and (b) −2% uniaxial strain on the metasurface over different gold film thicknesses (from bottom to top, the gold film thickness (tAu) increases from 17 nm to 24 nm). The scattering splits into lower and upper polariton branches, exhibiting level anticrossing. The vertical dashed lines refer to the WS2 exciton energy (\({\omega }_{{ex}}=2.019\) eV). The right panels are calculated scattering spectra using the model of two coupled oscillators. c Dispersion plots of the measured dark-field scattering spectra. The lower (\({\omega }_{-}\)) and upper (\({\omega }_{+}\)) polariton branches are extracted from the scattering spectra in a,b and fitted (solid lines) with a coupling strength of 165.9 meV at 0 strain (blue lines) and 240.4 meV at −2% strain (purple lines) in the full Hopfield Hamiltonian. The experimental spectral peaks are shown as triangles (0 strain) and circles (−2% strain). The exciton energy (\({\omega }_{{ex}}\)) is shown as the horizontal dashed black line. The plasmonic mode energy (\({\omega }_{{pl}}\)) is shown as the diagonal dashed blue (SC) and purple (USC) lines. The plasmonic response shifts to lower energy when compressive strain is applied, and the local field enhancement is largely enhanced due to the reduced gap size, and both effects lead to significant changes in the polaritonic dispersion. d Normalized second harmonic generation (SHG) intensity compared to scattering in the SC (upper, strain = 0) and USC (lower, strain = −2%) regime, respectively. The simultaneous emergence of energy splitting in dark-field scattering, photoluminescence (Supplementary Fig. 15) and SHG spectra precludes Fano interference phenomena from being responsible for the observed anticrossing. Note that the deviation between SHG and dark-field scattering splittings most likely arises from measurement errors in the strength of the SHG signal at the spectral edge of the photon detector.
