  Volkov-Pankratov surface bands arise in smooth topological interfaces, i.e.
interfaces between a topological and a trivial insulator, in addition to the
chiral surface state imposed by the bulk-surface correspondence of topological
materials. These two-dimensional bands become Landau-quantized if a magnetic
field is applied perpendicular to the interface. I show that the energy scales,
which are typically in the 10-100 meV range, can be controlled both by the
perpendicular magnetic field and the interface width. The latter can still be
varied with the help of a magnetic-field component in the interface. The Landau
levels of the different Volkov-Pankratov bands are optically coupled, and their
arrangement may allow one to obtain population inversion by resonant optical
pumping. This could serve as the elementary brick of a multi-level laser based
on Landau levels. Moreover, the photons are absorbed and emitted either
parallel or perpendicular to the magnetic field, respectively in the Voigt and
Faraday geometry, depending on the Volkov-Pankratov bands and Landau levels
involved in the optical transitions.
