  In an ideal 3D topological insulator (TI), the bulk is insulating and the
surface conducting due to the existence of metallic states that are localized
on the surface; these are the topological surface states. Quaternary Bi-based
compounds of Bi$_{2-x}$Sb$_{x}$Te$_{3-y}$Se$_{y}$ with finely-tuned bulk
stoichiometries are good candidates for realizing ideal 3D TI behavior due to
their bulk insulating character. However, despite its insulating bulk in
transport experiments, the surface region of
Bi$_{2-x}$Sb$_{x}$Te$_{3-y}$Se$_{y}$ crystals cleaved in ultrahigh vacuum also
exhibits occupied states originating from the bulk conduction band. This is due
to adsorbate-induced downward band-bending, a phenomenon known from other
Bi-based 3D TIs. Here we show, using angle-resolved photoemission, how an EUV
light beam of moderate flux can be used to exclude these topologically trivial
states from the Fermi level of Bi$_{1.46}$Sb$_{0.54}$Te$_{1.7}$Se$_{1.3}$
single crystals, thereby re-establishing the purely topological character of
the low lying electronic states of the system. We furthermore prove that this
process is highly local in nature in this bulk-insulating TI, and are thus able
to imprint structures in the spatial energy landscape at the surface. We
illustrate this by `writing' micron-sized letters in the Dirac point energy of
the system.

Materials Science