  Perovskite titanates such as SrTiO$_{3}$ (STO) exhibit a wide range of
important functional properties, including high electron mobility,
ferroelectricity, and excellent photocatalytic performance. The wide optical
band gap of titanates limits their use in these applications, however, making
them ill-suited for integration into solar energy harvesting technologies. Our
recent work has shown that by doping STO with equal concentrations of La and Cr
we can enhance visible light absorption in epitaxial thin films while avoiding
any compensating defects. In this work, we explore the optical properties of
photoexcited carriers in these films. Using spectroscopic ellipsometry, we show
that the Cr$^{3+}$ dopants, which produce electronic states immediately above
the top of the O 2p valence band in STO reduce the direct band gap of the
material from 3.75 eV to between 2.4 and 2.7 eV depending on doping levels.
Transient reflectance spectroscopy measurements are in agreement with the
observations from ellipsometry and confirm that optically generated carriers
are present for longer than 2 ns. Finally, through photoelectrochemical
methylene blue degradation measurements, we show that these co-doped films
exhibit enhanced visible light photocatalysis when compared to pure STO.
