The spin-polarized surface states of topological insulators (TIs) are expected to host a number of unusual electromagnetic and quantum-mechanical phenomena. These properties make them attractive for applications in spintronics and quantum computing. However, high quality TI materials are often difficult to synthesize and control. Many proposed experiments involving TIs require spatial control over time-reversal symmetry breaking and electron chemical potential. We have developed a set of microscopy and control techniques that allow us to study and manipulate both the chemical potential and magnetization of TI thin films on a few-micrometer scale.

A central challenge with TI materials is to reliably tune the chemical potential of their electrons with respect to the Dirac point and the bulk bands. We demonstrate persistent, bidirectional optical control of the chemical potential of non-magnetic (Bi,Sb)2Te3 thin films grown on SrTiO3. By optically modulating a space-charge layer in the SrTiO3 substrates, we induce a persistent field effect in the TI films comparable to electrostatic gating techniques but without additional materials or processing. This enables us to optically write and erase p-n junctions in these films, which we study with scanning photocurrent microscopy. The ability to optically write and erase mesoscopic electronic structures in a TI may aid in the investigation of the unique properties of the topological insulating phase. The gating effect also generalizes to other thin-film materials grown on SrTiO3, suggesting that this optical gating effect could provide local optical control of chemical potential in a wide range of ultrathin and 2D electronic systems.

Breaking time-reversal symmetry at the surface of a TI can lead to exotic physical phenomena such as 1D quantized edge states and unique magnetoelectric effects. Ferromagnetic TI materials provide a means of breaking time-reversal symmetry without an externally-applied magnetic field. We use scanning Kerr microscopy to study the behavior of ferromagnetic domains in thin films of Cr-doped (Bi,Sb)2Te3 grown on different substrates in and different regimes of magnetic doping. Moreover, we demonstrate a magneto-optical recording technique that allows us to write and erase micron-scale patterns in the local magnetization of these films. The ability to control time-reversal symmetry breaking at a micron scale may be particularly useful in studying the 1D quantized edge states predicted to exist along magnetic domain walls in these materials. Furthermore, we show that optical gating effect we studied in non-magnetic TI materials also applies to magnetically-doped thin films. We suggest that persistent, bidirectional optical control of chemical potential and magnetization in TI thin films may enable a wide variety of experiments in TI and low-dimensional physics.