  High-lying Rydberg states of Mott-Wannier excitons are receiving considerable
interest due to the possibility of adding long-range interactions to the
physics of exciton-polaritons. Here, we study Rydberg excitation in bulk
synthetic cuprous oxide grown by the optical float zone technique and compare
the result with natural samples. X-ray characterization confirms both materials
are mostly single crystal, and mid-infrared transmission spectroscopy revealed
little difference between synthetic and natural material. The synthetic samples
show principal quantum numbers up to $n=10$, exhibit additional absorption
lines, plus enhanced spatial broadening and spatial inhomogeneity. Room
temperature and cryogenic photoluminescence measurements reveal a significant
excess of copper vacancies in the synthetic material. These measurements
provide a route towards achieving \mbox{high-$n$} excitons in synthetic
crystals, opening a route to scalable quantum devices.
