  The dielectric function of heteroepitaxial YBiO$_3$ grown on $a$-Al$_2$O$_3$
single crystals via pulsed laser deposition is determined in the spectral range
from 0.03 eV to 4.5 eV by simultaneous modeling of spectroscopic ellipsometry
and optical transmission data of YBiO$_3$ films of different thickness. The
(111)-oriented YBiO$_3$ films are nominally unstrained and crystallize in a
defective fluorite-type structure with $Fm\bar{3}m$ space group. From the
calculated absorption spectrum, a direct electronic bandgap energy of 3.6(1) eV
and the signature of an indirect electronic transition around 0.5 eV are
obtained. These values provide necessary experimental feedback to previous
conflicting electronic band structure calculations predicting either a
topologically trivial or non-trivial insulating ground state in YBiO$_3$.

Materials Science