Detail:

Abstract:
　　Topological insulators (TIs) are characterized by an unusual electronic structure exhibiting both insulating bulk and robust metallic surface states (SSs). This unique electronic structure combining external light excitation leads to TIs a great promise for opto-spintronics and ultrafast spintronics applications. Therefore, understanding the charge and spin dynamics in TIs becomes quite essential. Here, a systematic study of these dynamics in prototypical TI Bi_2Se_3 has been carried out using ultrafast pump-probe optical spectroscopy. 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. For the first time, 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 (~25 fs and ~300 fs). We reveal that 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 new findings are suggestive of novel ways to manipulate the photoinduced coherent spins in TIs, which may have profound implications in future TI-based ultrafast spintronic devices.