Fig. 1. Exciton-polaron interactions and optical properties in monolayer WSe₂.

a Schematic illustration of Fermi-polaron dynamics before the below-gap pump excitation (left) and after the excitation (right). The attractive interaction between excitons and holes is weakened whereas the repulsive interaction is less affected under the off-resonant pump. b An optical microscopic image of hBN-encapsulated WSe₂ device (Inset: a schematic of the device structure). c Reflectance contrast spectra as a function of the probe photon energy (bottom axis) and Fermi energy $E_{\mathrm{F},\mathrm{h}}$ (left axis). Here, RC$=\left(R-R^{\prime }\right)/R^{\prime }$, where $R$ is reflection signal from the WSe₂ monolayer sample and $R^{\prime }$ is reference spectrum from the background (hBN) nearby the sample. The dashed line ($E_{\mathrm{F},\mathrm{h}}=$ 0 meV) is the regime where an excitonic Stark effect is measured (Fig. 2a, c). The dotted line is for the hole density ($E_{\mathrm{F},\mathrm{h}}=$ −12.4 meV). Figure 2b, d are the measured data at this hole-doped density. d The oscillator strength parameters of attractive and repulsive polarons calculated from (c). Vertical error bars are obtained from the fits.