  We investigate the ultrafast spin dynamics of the prototypical topological
insulator $\mathrm{Bi_{2}Se_{3}}$ using time-resolved Kerr-rotation
(polarization-change) measurements across near-infrared wavelengths. The
Kerr-rotation angle $\Delta \theta_{K}$ of $\mathrm{Bi_{2}Se_{3}}$ was found to
significantly depend on photon energy around a resonance transition ($\sim 1.0\
\mathrm{eV}$) of bulk states, as well as the ellipticity of the pump light, in
the presence of spin excitation. The observed photon-energy dependence of
$\Delta \theta_{K}$ can be well simulated by assuming spin-dependent
refractive-index changes in the presence of band-gap renormalization and
state-filling effect upon photoexcitation. Our study delivers comprehensive
insights into the opto-spintronic properties of bulk $\mathrm{Bi_{2}Se_{3}}$
and the fundamental physical processes underlying polarization changes. These
findings are expected to be crucial in developing ultrafast magneto-optical
memory devices, which can perform read-and-write operations in the Terahertz
regime.
