  The versatility of quantum gas experiments greatly benefits from the ability
to apply variable potentials. Here we describe a method which allows the
preparation of potential structures for microcavity photons via spatially
selective deformation of optical resonator geometries with a heat induced
mirror surface microstructuring technique. We investigate the thermalization of
a two-dimensional photon gas in a dye-filled microcavity composed of the custom
surface-structured mirrors at wavelength-scale separation. Specifically, we
describe measurements of the spatial redistribution of thermal photons in a
coupled double-ridge structure, where photons form a Bose-Einstein condensate
in a spatially split ground state, as a function of different pumping
geometries.
