Femtosecond dynamics of spin-polarized electrons in topological insulators

A fast control of spins is a major quest in spintronic systems. Ultrashort light pulses have been utilized to trigger and detect the spin dynamics of electrons in magnetic materials and multilayers. Recently, three-dimensional topological insulators have received attention due to the existence, within the insulating band gap of bulk states, of spin-polarized surface states that are protected from backscattering by time-reversal symmetry. We studied sub-picosecond dynamics in the spin-polarized unoccupied electronic structure of BiTe, employing circular-polarized light in time- and angle-resolved photoemission spectroscopy. Noncollinear optical parametric amplification and several nonlinear optical processes result in tunable, ultrashort visible pump pulses with a duration of 30 fs and 1.8 eV energy and ultraviolet probe pulses with about 60 fs duration and about 6 eV energy. The stable optical setup and the high repetition rate of an Yb-laser source gives a high signal-to-noise ratio in our photoemission process. The 65 fs time resolution, along with 30 meV energy resolution of the time-of-flight energy analyzer, provides the opportunity to explore the ultrafast electronic dynamics in the unoccupied band structures. Furthermore, circular dichroism allows access to the spin state of the photoemitted electrons. A signature of femtosecond unpolarized bulk bands dynamics appears in the presence of spin-polarized electrons of the surface states. This helps distinguish the bulk and surface contributions in the spin-electronic current.