  Pauli blockade occurs when the excited electrons fill up the states near the
conduction bands and block subsequent absorption in semiconductors, and has
been widely applied in mode-locking for passively-pulsed-laser systems. In this
letter, we report the first direct observation that the Pauli blockade is
broken by ultrafast cooling of hot electrons in optically-pumped graphene. With
femtosecond spectroscopy, we demonstrate that the time scale to excite an
electron (~100 fs) is of the same order as that of the electron decay via
electron-electron scattering, which allows the electron excitation interplays
strongly with the cooling of hot electrons. Consequently, Pauli blockade is
dismissed, leading to an unconventionally enhanced optical absorption. We
suggest that this effect is a universal feature of two-dimensional layered
materials, which sheds the light of ultrafast carrier dynamics in nonlinear
physics and inspires the designing of new-generation of ultrafast
optoelectronic devices.
