  Strong spin-orbit coupling, resulting in the formation of
spin-momentum-locked surface states, endows topological insulators with
superior spin-to-charge conversion characteristics, though the dynamics that
govern it have remained elusive. Here, we present an all-optical method that
enables unprecedented tracking of the ultrafast dynamics of spin-to-charge
conversion in a prototypical topological insulator Bi$_2$Se$_3$/ferromagnetic
Co heterostructure, down to the sub-picosecond timescale. Compared to pure
Bi$_2$Se$_3$ or Co, we observe a giant terahertz emission in the
heterostructure than originates from spin-to-charge conversion, in which the
topological surface states play a crucial role. We identify a 0.12-picosecond
timescale that sets a technological speed limit of spin-to-charge conversion
processes in topological insulators. In addition, we show that the
spin-to-charge conversion efficiency is temperature independent in Bi$_2$Se$_3$
as expected from the nature of the surface states, paving the way for designing
next-generation high-speed opto-spintronic devices based on topological
insulators at room temperature.
