The time evolution of the G-phonon can be fit with two exponential functions −exp[−(t − t0)/τgrowth] + exp[−(t − t0)/τdecay] convolved with the instrument response. The time constant τgrowth is 220 ± 20 fs for the quick buildup of G-phonons and τdecay is 2400 ± 83 fs for the slow decay (Fig. 2(e) inset). The fitting parameter t0 = 2 ± 15 fs. The G-phonon temperature is determined from the G peak intensity ratio on the Stokes and anti-Stokes sides IS/IAS = exp(ħΩ/kBTG), where ħΩ is the G-phonon energy, kB is the Boltzmann’s constant, and TG is the phonon temperature (Fig. 1(a)). TG is over 3000 K (over 1650 K for pump fluence 0.15 mJ/cm2 as shown in Fig. 1(a)) around t = 400 fs. It decreases to room temperature with a time constant of 2.4 ps, which agrees with previous results2021. The slow decay has been attributed to the energy transfer from G-phonons to other phonon modes via anharmonic coupling2533. The apparent oscillation around t = 600 fs in Fig. 3d might be due to imperfect laser power stability. We did not observe this oscillation in other datasets.