\textbf{Nematicity in FeSe single crystals probed by pump-probe spectroscopy}

The anisotropic quasiparticle dynamics in FeSe single crystals have been studied by polarized pump-probe spectroscopy. Two distinguishable relaxation components were unambiguously observed in transient reflectivity changes ($\Delta R$/$R)$. The orientation-dependent fast component with the timescale of 0.1-1.5 ps associated with the electronic structure clearly shows two-fold symmetry, which further reveals the gap opening along k$_{\mathrm{y}}$ below the temperature of structure phase transition ($T_{\mathrm{s}})$ and the electronic nematicity can persist up to 200 K. For the slow component with the timescale of 8-25 ps, it is assigned to the energy relaxation through spin sub-system and also shows a two-fold symmetry below $T_{\mathrm{s}}$. However, this two-fold symmetry is dramatically weakened above $T_{\mathrm{s}}$ and surprisingly persists up to at least 200 K. Consequently, the high-temperature nematic fluctuations in FeSe may be driven by the order parameters which associated with both charge (orbital) and spin sub-systems.