  Manipulating solids using the time-periodic drive of a laser pulse is a
promising route to generate new phases of matter. Whether such `Floquet-Bloch'
manipulation can be achieved in topological magnetic systems with disorder has
so far been unclear. In this work, we realize Floquet-Bloch manipulation of the
Dirac surface-state mass of the topological antiferromagnet (AFM)
MnBi$_2$Te$_4$. Using time- and angle-resolved photoemission spectroscopy
(tr-ARPES), we show that opposite helicities of mid-infrared circularly
polarized light result in substantially different Dirac mass gaps in the AFM
phase, despite the equilibrium Dirac cone being massless. We explain our
findings in terms of a Dirac fermion with a random mass. Our results underscore
Floquet-Bloch manipulation as a powerful tool for controlling topology even in
the presence of disorder, and for uncovering properties of materials that may
elude conventional probes.
