Fig. 3.

Temperature dependence of the WSe₂-PC system. a The temperature dependence of the exciton, cavity, and polariton energies. The exciton energy E_exc(T) is measured from the weakly coupled TM excitons (red diamonds) and is extracted by fitting the PL spectrum using Gaussian line shape. Temperature dependence of the exciton is fitted by Eq. (6) (dashed blue line). The cavity energy E_cav(T) is found to be approximately constant with temperature and indicated by the dashed blue line. The polariton energies E_LP,UP(T) (black circles) are extracted by fitting the PL spectra at k_x = 3.1 μm⁻¹, k_y = 0 μm⁻¹ using Gaussian line shape. They anti-cross and split from the exciton and PC mode energies at 100 K and below, showing strong coupling in this range. All the error bars are extracted by least square data fitting. b The strong coupling to weak-coupling transition measured by the temperature dependence of g (black stars), $\sqrt{\left({\gamma }_{\mathrm{exc}}^2+{\gamma }_{\mathrm{cav}}^2\right)\mathrm{∕}2}$ (red circles), and (γ_exc − γ_cav)/2 (blue triangles). The red and blue lines are fits by Eq. (7) for γ_exc while γ_cav is approximately constant with temperature. g drops to below $\sqrt{\left({\gamma }_{\mathrm{exc}}^2+{\gamma }_{\mathrm{cav}}^2\right)\mathrm{∕}2}$ at about 115 K, showing the transition to the weak-coupling regime as indicated by the dashed line. Rabi splitting persists up to about 185 K, till g becomes smaller than (γ_exc − γ_cav)/2. All the data points are extracted by fitting the PL spectrum using Gaussian line shape, and the error bars are extracted by least square data fitting. c Temperature dependence of the integrated PL intensity of the PC-WSe₂ polariton, showing only mild change in intensity. The inset shows the spectrum at 10 K integrated over θ_x = −30° to 30° and over the spectral range shown. The higher energy side shoulder corresponds to exciton-like LP emission at large θ_x where the density of states is high