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Towards engineering of the selective optical excitations in the topological insulator Bi₂Se₃ Ahamed, Shadab


Topological insulators are widely studied class of materials with fascinating electronic properties and a great potential for application to spintronics. The property that makes these materials interesting is the existence of linearly dispersing electronic states on the surface, occurring due to strong spin-orbit coupling and time-reversal symmetry. The electrons in these topological surface states (TSSs) exhibit helical spin-texture and are forbidden from backscattering due to time-reversal symmetry. Bi₂Se₃ is one such topological insulator that hosts two TSSs: one near the Fermi level and another in the unoccupied states. The optical excitation of the electrons from the occupied TSS to the unoccupied TSS has been an interesting avenue for exciting spin-polarized surface currents in Bi₂Se₃. Several transport experiments have demonstrated the current generation by optical means, confirming the idea, and ARPES experiments have helped in understanding the mechanism of optical excitation. However, several questions remain unanswered including the involvement of the bulk states in the generation scheme, the symmetry of the optical excitation, and therefore the directionality of the injected currents and possibilities to improve the efficiency and selectivity of these excitations. Identifying and controlling these optical transitions, understanding their nature, and isolating the TSS-TSS transition to injected current while suppressing the contribution from the bulk is the essence of the presented work. In particular, we perform a pump-probe ARPES experiment on Bi₂Se₃ single crystals using a 1.55 eV circularly-polarized pump and 6.2eV linearly-polarized probe pulse with two different in-plane orientations of the sample. The pump-induced circular dichroism maps were analyzed at different binding energies to deduce the direction of injected currents and the type of underlying optical transitions associated with them: bulk-bulk, bulk-TSS, or TSS-TSS. We classify the optical transitions into three categories based on the symmetry of the bulk and the TSS alone. As an attempt to explain our findings and simulate our experimental results, we also build a tight-binding model using the Chinook software, starting from the density functional theory calculation on Bi₂Se₃. Finally, we give a prescription to calculate the pump-probe intensity and the pump-induced circular dichroism pattern using the resultant tight-binding model.

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