Mid-Infrared Nano-Tomography of Topological Insulator Surfaces

We retrieve the local dielectric function of a fewnanometer-thick surface layer on the three-dimensional topological insulator (Bi 0.5 Sb 0.5 ) 2 Te 3 using mid-infrared nanotomography. Thereby, we identify the contributions of two types of surface states: Band bending leads to an intersubband transition within a massive two-dimensional electron gas, which gives rise to a sharp resonance. Conversely, an additional broadband absorption background may be caused by the topologically protected surface states. Tracing the dielectric response across a nanostructure reveals local changes to the resonance frequency of the intersubband transition, pointing towards nanoscale fluctuations of the doping or the Bi-to-Sb-ratio.