  The nematic order (nematicity) is considered one of the essential ingredients
to understand the mechanism of Fe-based superconductivity. In most Fe-based
superconductors (pnictides), nematic order is reasonably close to the
antiferromagnetic order. In FeSe, in contrast, a nematic order emerges below
the structure phase transition at T_s = 90 K with no magnetic order. The case
of FeSe is of paramount importance to a universal picture of Fe-based
superconductors. The polarized ultrafast spectroscopy provides a tool to probe
simultaneously the electronic structure and the magnetic interactions through
quasiparticle dynamics. Here we show that this approach reveals both the
electronic and magnetic nematicity below and, surprisingly, its fluctuations
far above Ts to at least 200 K. The quantitative pump-probe data clearly
identify a correlation between the topology of the Fermi surface (FS) and the
magnetism in all temperature regimes, thus providing profound insight into the
driving factors of nematicity in FeSe and the origin of its uniqueness.
