Hideki Hatano

Holographic properties of doped stoichiometric LiNbO3 crystals

Doped stoichiometric LiNbO3 (SLN) crystals show more advantages over traditional congruent LiNbO3 (CLN). This paper is focused on the measurement of the holographic performances of doped SLN crystals, including the sensitivity, dynamic range, and angular selectivity. The principles and methods for the measurements are first described, with more attention paid to the measurement of erasure time constants. The results show that the SLN crystal co-doped with Ce and Mn has the largest dynamic range, owing to the extremely long erasure time constants even by the multiplexing method. The SLN crystal co-doped with Tb and Fe and reduced in rich nitrogen atmosphere has the fastest response time and highest sensitivity. The oxidized and as-grown SLN crystals co-doped with Tb and Fe exhibit photochromic effect under UV illumination. With UV pre-illumination these crystals have sensitivity competitive to the heavily Fe doped CLN. The measured selective angles for most of the SLN samples of proper optical quality agree well with the theoretical calculation.

Investigation on dark decay of two-color holograms in near-stoichiometric LiNbO 3 and LiTaO 3

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 LiNbO3 and LiTaO3 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 LiTaO3 are one order of magnitude longer than that in LiNbO3 if having the same proton concentration.

Intensity dependence of two-color holography performance in Tb-doped near-stoichiometric LiNbO3

Two-color recording is achieved in a Tb-doped near-stoichiometric LiNbO3 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/cm2. 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/cm2. 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.

Light-induced charge transport in Mn-doped near-stoichiometric LiNbO 3 under cw UV illumination

Near-stoichiometric LiNbO3 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.

Long lifetime of two-color nonvolatile holograms in near-stoichiometric lithium tantalate crystals

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.

Two-color photorefractive holography in Mn-doped near-stoichiometric lithium niobate crystals

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.