Femtosecond dynamics of spin-polarized electrons in topological insulators

A faster control of spins is a major request for the new generation of computing and spintronic systems. In this framework, since many years, 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 (TIs) have received attention in the field of spintronics due to their spectacular features, in particular, the existence, within the insulating gap of bulk states, of spin-polarized surface states (Dirac-cone) that are protected from backscattering by time-reversal symmetry. We have studied the sub-picosecond dynamics in the spin-polarized unoccupied electronic structure of Bi<formula><tex>$_2$</tex></formula>Te<formula><tex>$_3$</tex></formula>, employing circular-polarized light in time and angle resolved photoemission spectroscopy (trARPES). Exploiting the noncollinear optical parametric amplification (NOPA) besides several nonlinear optical processes resulted in tunable ultrashort visible pump pulses with 30 fs length and 1.8 eV energy and ultraviolet probe pulses with about 6 eV energy and 60 fs duration. The stable optical setup and the high repetition rate of an Yb-laser source grants a high signal-to-noise ratio in our photoemission process. The obtained 65 fs time resolution, along with 30 meV energy resolution of the time-of-flight (TOF) energy analyzer, provides us with an exciting possibility to explore the ultrafast electronic dynamics in the unoccupied band structures. Furthermore, circular dichroism (CD) allows access to the spin state of the photoemitted electrons. We found a signature of femtosecond unpolarized bulk bands dynamics in the presence of spin-polarized electrons of the surface states. This observation aided to distinguish the bulk and surface contributions in the spin-electronic current.