  Many proposed experiments involving topological insulators (TIs) require
spatial control over time-reversal symmetry and chemical potential. We
demonstrate reconfigurable micron-scale optical control of both magnetization
(which breaks time-reversal symmetry) and chemical potential in ferromagnetic
thin films of Cr-(Bi,Sb)$_2$Te$_3$ grown on SrTiO$_3$. By optically modulating
the coercivity of the films, we write and erase arbitrary patterns in their
remanent magnetization, which we then image with Kerr microscopy. Additionally,
by optically manipulating a space charge layer in the underlying SrTiO$_3$
substrates, we control the local chemical potential of the films. This optical
gating effect allows us to write and erase p-n junctions in the films, which we
study with photocurrent microscopy. Both effects are persistent and may be
patterned and imaged independently on a few-micron scale. Dynamic optical
control over both magnetization and chemical potential of a TI may be useful in
efforts to understand and control the edge states predicted at magnetic domain
walls in quantum anomalous Hall insulators.
