Topological Insulators as Ultrafast Auston Switches in On-Chip THz-Circuits

We demonstrate that the three-dimensional topological insulator Bi-2 Se-3 can be exploited as an ultrafast photoswitch in high-frequency optoelectronic circuits without the need to apply a voltage bias to the topological insulator. Bi-2 Se-3 -films are optically excited with a circularly polarized laser pulse. The circular photogalvanic effect gives rise to an ultrafast surface current. We use coplanar THz-striplines and an on-chip pump-probe scheme to sample the ultrafast transient photocurrent. For such subpicosecond electromagnetic transients, the stripline circuit introduces significant broadening due to dispersion and attenuation. Therefore, we compute the effective refractive index of the striplines and introduce a back-transformation of the distorted signal to explore the original ultrafast currents in the topological insulator. Our results show how to use topological insulators as unipolar THz-sources in on-chip stripline circuits.