  We report intracavity Bragg scattering induced by photorefractive (PR) effect
in high-Q lithium niobate (LN) ring resonators at cryogenic temperatures. We
show that, when a cavity mode is strongly excited, the PR effect imprints a
long-lived periodic space-charge field. This residual field in turn creates a
refractive index modulation pattern that dramatically enhances the back
scattering of an incoming probe light, and results in selective and
reconfigurable mode splittings. This PR-induced Bragg scattering effect,
despite being undesired for many applications, could be utilized to enable
optically programmable photonic components.
