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Undoped and Mg-doped stoichiometric LiTaO 3 with concentrations of 0.5, 0.7, and 1.0 mol % were grown by the double-crucible Czochralski method, and their photorefractive effect was investigated by measuring light-induced birefringence changes. Periodically poled Mg-doped stoichiometric lithium tantalate (PPMgSLT) were fabricated for QPM optical parametric oscillator, whose properties, including slope efficiency and threshold, were measured to investigate influence of photorefractive effect on optical parametric oscillator. The results showed that a concentration of 1 mol % MgO in the melt was enough to suppress photorefractive effect at tens of MW/cm 2 , and the OPO performance of PPMgSLT doped with 0.5 mol % MgO is only 6 % lower than that of PPMgSLT doped with 1 mol % MgO at room temperature, even though light-induced birefringence existed for the crystal doped with 0.5 mol % MgO.

Interband photorefractive gratings induced by deep ultraviolet light at λUV = 257 nm are investigated in pure and Mg-doped near-stoichiometric LiTaO3. By controlling the stoichiometry, the response time and dynamic range of the interband photorefractive effect can be very much improved. Measured time constants are in the order of a few tens of milliseconds for UV light intensities of about 100 mW/cm2. Further, we report that Mg doping of near-stoichiometric LiTaO3 increases the grating response rate by a factor of 50 compared with the one in undoped crystals. In Mg-doped near-stoichiometric LiTaO3, quasi-fixing of holographic gratings with UV light and nondestructive readout in the visible has been observed. These quasi-fixed gratings are written in the deep traps of the crystal.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have investigated the dark decay mechanism by measuring dark decay time constants of two-color holograms recorded in undoped and slightly doped near-stoichiometric LiNbO 3 and LiTaO 3 crystals in the temperature range from 50 to 200 °C. All the samples in this work obey an Arrhenius-type dependence of time constant on absolute temperature, and yield nearly same activation energies of 1.08 eV. The results show that proton compensation mechanism dominated the dark decay process. Lifetimes of holograms at room temperature in LiTaO 3 are one order of magnitude longer than that in LiNbO 3 if having the same proton concentration.

Near-stoichiometric LiNbO 3 crystals slightly doped with Mn from different Li concentrations in the melt were grown by the top seeded solution growth method. The light-induced (small polarons) absorption under UV illumination was probed using near-IR of 780 nm. The dark decay process shows a stretched-exponential behavior. The dependences of the light-induced absorption, the decay time constant, and the stretching factor on the pumping intensity were presented. The photovoltaic current intensity and the photoconductivity under UV illumination were measured by applying a DC field along the z-axis of the sample. The photoconductivity intensity showed superlinear dependence of intensity. The experimental results can be explained quantitatively by developing the two-center charge transport model with direct excitation and recombination between Mn and small polarons.

Two-color holography is an effective solution to the volatile readout problem in volume holographic data storage based on photorefractive materials. Popular materials for two-color holography are reduced doped and nondoped near-stoichiometric lithium niobate crystals. However, the lifetime at room temperature is from several weeks to several months depending on the reduction state of the material. Moreover, reductive treatment will degrade the nonvolatility of two-color holograms. The important issue for two-color holography is how to increase the lifetime. In this contribution, lifetimes of two-color nonvolatile holograms recorded in as-grown near-stoichiometric lithium niobate and tantalate crystals were compared by extrapolating the high-temperature data. The dark-decay time constants obey an Arrhenius dependence on absolute temperature and yield activity energy of 1.06 eV around in all measured crystals. Lifetimes of holograms in nondoped and slightly doped crystals depend on the proton concentration. Lifetimes of hologram in lithium tantalate are one order of magnitude longer than those in lithium niobate at the same proton concentration. The lifetime of two-color holograms in lithium tantalite is longer than 20 years.

Microscale to nanoscale ferroelectric domain engineering of a near-stoichiometric LiNbO 3 crystal was investigated by using a scanning force microscope. The single crystal LiNbO 3 was grown by the double-crucible Czochralski method with automatic powder supply system. The electric field required to inverse the polarization direction of the domain was about one-ninth of the filed for a conventional congruent LiNbO 3 . The near-stoichiometric LiNbO 3 crystal fixed on metal substrate was polished to a thickness of approximately 5 μm. Polarization directions of the domains were locally inverted by applying voltages with a conductive cantilever of the scanning force microscope. Furthermore, the domain structure was patterned in the LiNbO 3 samples, where the domains were inverted by scanning with the cantilever on the sample while applying voltages.

Holograms are recorded in near-stoichiometric LiNbO3 doped with 8 ppm of Mn by use of two-color holographic recording method, where an IR laser of 778 nm is used for writing and an UV laser of 350 nm is used for gating. Dependences of two-color sensitivity and M/# on gating and writing intensities are presented. Significant improvements of sensitivity, M/# and dark decay are obtained compared with reduced near-stoichiometric LiNbO3. To our knowledge, the measured two-color sensitivity of 0.21 cm/J with a gating intensity of 1.5 W/cm2 is the highest among the values reported so far, which is attributed to large light-induced absorption of small polarons. 50 plane-wave holograms are recorded by the conventional angle-multiplexing method.

Two-color recording is achieved in a Tb-doped near-stoichiometric LiNbO 3 crystal of 3-mm thickness by use of extraordinary 778 nm information-carrying beams and ordinary 350 nm gating light. The dependences of key parameters, the two-color recording sensitivity and the dynamic range expressed by the saturated diffraction efficiency, on intensities of recording and gating beams are presented. The saturated diffraction efficiency increases with total writing intensity, and then reaches a saturated value of 56% at an intensity of 20W/cm 2 . The sensitivity increases linearly with gating intensity at low intensity, and then saturates with a value of 0.08 cm/J at a higher intensity than 1.6 W/cm 2 . The saturated sensitivity is much higher than the reported value in literature. This material shows very significant improvement on two-color holography performance. 50 plane-wave holograms are recorded by the angle-multiplexing method and an M/# of 1.13 is obtained.

We observed that newly developed near-stoichiometric LiNbO3 crystals doubly doped with Tb and Fe have three different types of energy levels: UV absorption centers just above the valence band, metastable shallow electron traps slightly below the conduction band, the deep traps located about 1.9 eV below the conduction band. Irradiation with UV light induced a stable absorption band extending from (lambda) equals 650 nm to the absorption edge, which is caused by the photoinduced charge transfer from UV-sensitive absorption centers to deep traps 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.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.