We report on time-resolved ultrafast optical spectroscopy study of the topological insulator (TI) Bi$_2$Se$_3$. We unravel that a net spin polarization can not only be generated using circularly polarized light via interband transitions between topological surface states (SSs), but also via transitions between SSs and bulk states. Our experiment demonstrates that tuning photon energy or temperature can essentially allow for photoexcitation of spin-polarized electrons to unoccupied topological SSs with two distinct spin relaxation times ($sim$25 fs and $sim$300 fs), depending on the coupling between SSs and bulk states. The intrinsic mechanism leading to such distinctive spin dynamics is the scattering in SSs and bulk states which is dominated by $E_g^2$ and $A_{1g}^1$ phonon modes, respectively. These findings are suggestive of novel ways to manipulate the photoinduced coherent spins in TIs.
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