Lone-pair-delayed persistent excitonic UV photochromism and charge transfer in Bi-doped and (Bi,Mg)-codoped stoichiometric lithium niobate single crystals

For a deeper understanding of the outstanding real-time photorefractive medium LiNbO${}_{3}$:(Bi,Mg) and excitonic processes in general, stoichiometric LiNbO${}_{3}$ (SLN) crystals are investigated using low-intensity near-ultraviolet (UV) irradiation and infrared-ultraviolet absorption spectroscopy. Absorption bands separated in SLN:Bi and SLN:(Bi,Mg) are assigned to specific Bi defect types and their persistent association with self-trapped excitons (pinned STEs, PSTEs) generated by near-UV irradiation. Processes on widely differing timescales are predicted for one- and two-photon excitations involving different excited states, relevant for further optimization of the system.