Ultrafast Surface State Spin-Carrier Dynamics in the Topological Insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$

Topological insulators are promising candidates for optically driven spintronic devices, because photoexcitation of spin polarized surface states is governed by angular momentum selection rules. We carry out femtosecond midinfrared spectroscopy on thin films of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$, which has a higher surface state conductivity compared to conventionally studied ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Both charge and spin dynamics are probed utilizing circularly polarized light. With a sub-band-gap excitation, clear helicity-dependent dynamics is observed only in thin ($<20\mathrm{nm}$) flakes. On the other hand, such dependence is observed for both thin and thick flakes with above-band-gap excitation. The helicity dependence is attributed to asymmetric excitation of the Dirac-like surface states. The observed long-lasting asymmetry over 10 ps even at room temperature indicates low backscattering of surface state carriers which can be exploited for spintronic devices.

Figure 1

Helicity-dependent dynamics for a (a) 14, (b) 18, (c) 45, (d) 75 nm flake with a $7\mu \mathrm{m}$ pump and a $7\mu \mathrm{m}$ probe. The difference, i.e., $|\mathrm{RL}|\text{−}|\mathrm{LL}|$ vs time, can be fit with an exponential decay time $\tau =8.2\mathrm{ps}$ for (a) and $\tau =12\mathrm{ps}$ for (b).

Figure 2

Transition diagram with a $7\mu \mathrm{m}$ pump and a $7\mu \mathrm{m}$ probe for (a) thin samples with a relatively low Fermi level. The left figure illustrates the $\sigma +$ pump and $\sigma -$ probe (RL), and the right figure the $\sigma -$ pump and $\sigma -$ probe (LL). LL produces a stronger probe response than RL. (b) Thick samples with a high Fermi level for the $\sigma +$ pump and $\sigma -$ probe (RL), which does not produce a helicity-dependent signal (see the text). The black arrow represents the pump, and the orange arrow represents the probe.

Figure 3

Helicity dependence of a (a) 14, (b) 18, (c) 45, and (d) 75 nm sample with the 800 nm pump and the $7\mu \mathrm{m}$ probe. The difference $|\mathrm{RL}|\text{−}|\mathrm{LL}|$ is fit with an exponential, $\tau =2.8\mathrm{ps}$ for (a), $\tau =12.5\mathrm{ps}$ for the positive part in (b), and $\tau =5.3\mathrm{ps}$ for (c). The fitting is poor for (d) and hence not shown.

Figure 4

Transition diagram for the 800 nm pump and the $7\mu \mathrm{m}$ probe for (a) thin samples with a low Fermi level for the $\sigma -$ pump and $\sigma -$ probe (LL), resulting in a stronger probe response for LL, and (b) thick samples with a high Fermi level, where RL has a stronger probe response (see the text). The black arrow is the pump, and the orange arrow is the probe.