Nonvolatile and quasi-nonvolatile holographic recording in near-stoichiometric lithium niobate doubly doped with Tb and Fe

We have observed that newly developed near-stoichiometric LiNbO₃ crystals doubly doped with Tb and Fe have three different types of energy levels: ultraviolet (UV) absorption centers just above the valence band, metastable shallow electron traps slightly below the conduction band, and deep traps located about 1.9 eV below the conduction band. Irradiation with UV light induced a stable absorption band extending from λ ∾ 650 nm to the absorption edge, which is caused by the photoinduced charge transfer from UV-sensitive absorption centers to deep traps (Fe³⁺) via the conduction band. The electron lifetimes at shallow and deep traps could be controlled by doping concentrations. Based on these favorable energy states, nonvolatile two-color holographic recording has been demonstrated by use of 852 nm recording beams and UV gating light. Quasi-nonvolatile one-color recording at 532 nm has also been demonstrated in these codoped crystals. Hologram recording from the UV-exposed, colored state revealed a much improved sensitivity in comparison to that from the uncolored state. The obtained M/# was 1.73 for a 3.3 mm-thick crystal.