  We report temporal changes in Raman-scattering spectra of detwinned
YBa$_2$Cu$_3$O$_{6+x}$ single crystals under exposure to red laser light
polarized along the crystallographic $a$-axis. A recent publication by Bakr et
al. (Phys. Rev. B 88 214517 (2013)) identified new Raman-active modes that
appear upon cooling below $T \sim 200$ K, and attributed these modes to charge
ordering phenomena observed in x-ray scattering and nuclear magnetic resonance
experiments on the same materials. Here we report that the intensities of these
modes depend not only on temperature and oxygen content, but also on the
cumulative photon dose absorbed by the YBa$_2$Cu$_3$O$_{6+x}$ samples. The
light-induced changes in the Raman spectra exhibit a stretched-exponential time
dependence, with a characteristic time that increases strongly upon cooling.
They also depend strongly on doping and are most pronounced for $x \sim 0.6$.
In fully illuminated samples, the mode intensities exhibit a monotonic
temperature dependence indicative of a second-order phase transition. The
findings indicate a metastable state generated by light-induced rearrangement
of the oxygen dopants. We hypothesize that the new Raman phonons are associated
with a three-dimensional charge-ordered state induced by light illumination,
analogous to a state that was recently observed by resonant x-ray scattering in
oxygen-disordered YBa$_2$Cu$_3$O$_{6+x}$ films.
