  Weyl and Dirac relativistic fermions are ubiquitous in topological matter.
Their relativistic character enables high energy physics phenomena like the
chiral anomaly to occur in solid state, which allows to experimentally probe
and explore fundamental relativistic theories. Here we show that on smooth
interfaces between a trivial and a topological material, massless Weyl and
massive Dirac fermions intrinsically coexist. The emergence of the latter,
known as Volkov-Pankratov states, is directly revealed by magneto-optical
spectroscopy, evidencing that their energy spectrum is perfectly controlled by
the smoothness of topological interface. Simultaneously, we reveal the optical
absorption of the zero-energy chiral Weyl state, whose wavefunction is
drastically transformed when the topological interface is smooth. Artificial
engineering of the topology profile thus provides a novel textbook system to
explore the rich relativistic energy spectra in condensed matter
heterostructures.
