Figure 3. (a) Representative PL spectra under different excitation energies at 15K base temperature. The PL spectra are renormalized and shifted with respect to the spectral peak for comparison. The upper halves of the PL lineshape (normalized PL> 0.5) are nearly the same, and the lower parts of PL lineshape expand at the higher excitation energy. The expansion weighs heavily at the higher energy tails as magnified in the inset. (b) Representative PL spectra at various cryostat temperature. The linewidth broadening owing to the lattice temperature displays different pattern against that of exciton temperature. The inset shows the linewidth from Lorentz fitting as a function of the lattice temperature (c) Simulated PL spectra with the mechanism of acoustic phonon assisted exciton emission, where the lattice temperature is kept constant (\(T_{\text{lattice}}=15K\)) and the exciton temperature is the sole variable. The high energy side tail expands obviously accompanying with the linewidth broadening. The inset shows the linewidth broadening as a function of the exciton temperature. Simulation result of PL spectra with excitation energy of 2.31eV (d) and 2.75eV (e), the exciton temperature is estimated to be 39K and 55K (~24K and 40K higher than the lattice temperature) under the excitation energies of 2.31eV and 2.75eV, respectively.
Figure 3(a) shows the representative PL spectra under different excitation energies. These PL spectra are renormalized and shifted with respect to the PL energy peak for better comparison. Note that the top halves of the PL spectra where the normalized intensity > 0.5 are nearly the same across the excitation energy range. Contrarily the tail at the high energy side expands with the elevating excitation energy as illustrated in the inset. This linewidth broadening has a contrasting manner to the lattice temperature induced line shape broadening (Figure 3(b)) which displays a whole line shape broadening other than just an expansion in the tail. To simulate the phonon-assisted PL we set the exciton temperature as the single variable and keep lattice temperature as a constant (~15K). Figure 3(c) shows the simulated PL spectrum of A1s exciton under the acoustic phonon assisted photoluminescence mechanism with all defined parameters from M. M. Glazov and B. Urbaszek’s work28 (detailed in SI). The expansion at the higher energy edge leads to the effective linewidth broadening (inset of Fig.3(c)), remarkably reproducing the experimental features in Fig.2(d). The simulation perfectly describes our experimental results and it clearly indicates that the acoustic phonon assisted photoluminescence process makes significant contribution to the whole PL spectrum in high quality samples. Comparing our experimental (Figure 3(a)) with simulation results (Fig.3(c)), we conclude that the higher excitation energy leads to the higher exciton temperature and finally raises non-Lorentz high energy tail. As demonstrated in the Fig.3(d) and (e), the effective exciton temperature (\(T_{\text{exciton}}\)) is 24Khigher than the lattice temperature (\(T_{\text{lattice}}\)) when the excitation is at 2.31eV and 40K higher at 2.75eV , respectively.