Identify the physical origin of burned-in residual index profiles

Identify the physical origin of the reversible burned-in residual index profile that develops in the programmable lithium-niobate waveguide after prolonged exposure to a fixed non-uniform illumination pattern under bias.

Background

The device exhibits weak residual memory after prolonged exposure to a fixed non-uniform illumination pattern: a residual refractive-index pattern persists for hours after the programming illumination is removed. The effect is reversible under uniform illumination and has been observed only in the current prototype, but its material mechanism remains unresolved. The paper tentatively associates it with persistent photoconductivity, while noting that further material study is needed.