A bi-directional photovoltaic shift on the surface of topological insulators

Topological insulators (TIs) possess spin-polarized Dirac states on the surface as a result of the non-trivial topology of the bulk band structure. The surface of TIs is considered as a promising spin-electronic application. Recently, it was suggested that a generation of spin-polarized photocurrent would be possible with bulk-insulating TIs by utilizing surface photovoltage (SPV) effect. To date, SPV effect was observed for the bulk-insulating TI, Bi₂Te₂Se. However, due to the limited controllability of bulk carrier, only a uni-directional shift of the surface potential has been demonstrated. A bi-directional SPV is important for taking control of the ambipolar surface current by light.
Here, we have performed time- and angle- resolved photoemission spectroscopy on bulk-insulating TIs of n- and p-type Bi₂Te₃. The experiment was carried out with linearly polarized pump (hν=1.48 eV) and probe (5.92 eV) pulses generated by Ti: sapphire laser system operating at a repetition rate of 250 kHz.
We report the bi-directional SPV realized on the TI Bi₂Te₃. The topological surface states showed downward and upward photovoltaic shifts for the n- and p-type samples, respectively. Our finding opens a way to ambipolar optical control of spin-polarized current generation on the surface of TIs.