K. Buse

Non-volatile holographic storage in doubly doped lithium niobate crystals

Photorefractive materials are being widely investigated for applications in holographic data storage. Inhomogeneous illumination of these materials with an optical interference pattern redistributes charge, builds up internal electric fields and so changes the refractive index. Subsequent homogeneous illumination results in light diffraction and reconstructs the information encoded in the original interference pattern. A range of inorganic and organic photorefractive materials are known, in which thousands of holograms of high fidelity can be efficiently stored, reconstructed and erased. But there remains a problem with volatility: the read-out process usually erases the stored information and amplifies the scattered light. Several techniques for ‘fixing’ holograms have been developed, but they have practical disadvantages and only laboratory demonstrators have been built. Here we describe a resolution to the problem of volatility that should lead to the realization of a more practical system. We use crystals of lithium niobate — available both in large size and with excellent homogeneity — that have been doped with two different deep electron traps (iron and manganese). Illumination of the crystals with incoherent ultraviolet light during the recording process permits the storage of data (a red-light interference pattern) that can be subsequently read, in the absence of ultraviolet light, without erasure. Our crystals show up to 32 per cent diffraction efficiency, rapid optical erasure of the stored data is possible using ultraviolet light, and light scattering is effectively prevented.

Two-center holographic recording

We describe a two-center holographic recording method for the storage of persistent holograms in doubly doped lithium niobate crystals. We use a two-center model, and we show that our experimental observations can be explained by the model. We describe experimental methods for finding the unknown material parameters of LiNbO3:Fe:Mn crystals for the two-center model, and we discuss the optimization of two-center recording.

Lifetime of small polarons in iron-doped lithium–niobate crystals

Pulsed illumination of lithium–niobate crystals with green light excites electrons from deep traps into the intrinsic defect NbLi5+ (Nb on Li site in the valence state 5+) and creates NbLi4+ centers (small polarons). The electrons trapped in this more shallow center increase the light absorption in the red and near infrared. The dark decay of the polaron concentration is observed by monitoring the relaxation of these absorption changes. Iron-doped lithium–niobate crystals with different concentrations of NbLi are investigated for various illumination conditions and temperatures. The relaxation shows a stretched-exponential behavior which is in disagreement with the predictions of the standard rate-equation-based model. The observed lifetimes of the polarons range from tens of nanoseconds to some milliseconds. Computer simulations reveal that all results can be explained considering distance-dependent excitation and recombination rates, i.e., the lifetime of an individual polaron depends on the distance to...

Doubling the Efficiency of Third Harmonic Generation by Positioning ITO Nanocrystals into the Hot-Spot of Plasmonic Gap-Antennas

We incorporate dielectric indium tin oxide nanocrystals into the hot-spot of gold nanogap-antennas and perform third harmonic spectroscopy on these hybrid nanostructure arrays. The combined system shows a 2-fold increase of the radiated third harmonic intensity when compared to bare gold antennas. In order to identify the origin of the enhanced nonlinear response we perform finite element simulations of the nanostructures, which are in excellent agreement with our measurements. We find that the third harmonic signal enhancement is mainly related to changes in the linear optical properties of the plasmonic antenna resonances when the ITO nanocrystals are incorporated. Furthermore, the dominant source of the third harmonic is found to be located in the gold volume of the plasmonic antennas.

Role of cerium in lithium niobate for holographic recording

Cerium-doped lithium niobate crystals are tested for holographic recording. A photochromic effect is observed in crystals doped with cerium and manganese. But two-center recording in the sample is not as effective as in iron and manganese doubly doped crystals. Photocurrent measurements in cerium and iron singly doped crystals indicate that the photovoltaic constant in the cerium-doped crystal is only one third of that of the iron-doped one. This is the main reason accounting for the low sensitivity of cerium-doped lithium niobate crystals. However, in the diffusion dominated case, i.e., for reflection geometry, cerium-doped lithium niobate may give a strong effect.

Highly tunable low-threshold optical parametric oscillation in radially poled whispering gallery resonators.

Whispering-gallery resonators (WGRs), based on total internal reflection, possess high quality factors in a broad spectral range. Thus, nonlinear-optical processes in such cavities are ideally suited for the generation of broadband or tunable electromagnetic radiation. Experimentally and theoretically, we investigate the tunability of optical parametric oscillation in a radially structured WGR made of lithium niobate. With a 1.04  μm pump wave, the signal and idler waves are tuned from 1.78 to 2.5  μm--including the point of degeneracy--by varying the temperature between 20 and 62 °C. A weak off centering of the radial domain structure extends considerably the tuning capabilities. The oscillation threshold lies in the mW-power range.

Sensitivity improvement in two-center holographic recording

Persistent holograms are recorded with green light in LiNbO(3) crystals doped with Mn and Fe. The recording sensitivity is 20 times better than that obtained by recording with red light. Partial loss of persistence is caused by using green light for recording.

Role of iron in lithium-niobate crystals for the dark-storage time of holograms

The dark decay of holograms stored in iron-doped photorefractive lithium-niobate crystals is studied for samples containing up to 0.25 wt% Fe2O3 (iron concentration 71×1018 cm−3). The oxidation/reduction state of the crystals, i.e., the concentration ratio of Fe2+ and Fe3+ ions, is changed in a wide range by thermal annealing. The dark decay is attributed to two effects: An ionic dark conductivity arising from mobile protons and an electronic dark conductivity caused by tunneling of electrons between iron sites. The latter is proportional to the effective trap density, i.e., to the density of charge carriers which can be moved between the iron sites. The proportionality factor is the specific dark conductivity which increases exponentially with the third root of the entire iron concentration.

THREE-VALENCE CHARGE-TRANSPORT MODEL FOR EXPLANATION OF THE PHOTOREFRACTIVE EFFECT

A new charge-transport model assuming one center in three different valence states is discussed for explanation of the photorefractive effect. Quantitative description of experimental results in KNb03:Fe by the model is demonstrated. Many similarities with the so-called “two-center” model are found although the microscopic explanation of the light-induced charge transport is rather different.

Infrared holographic recording in LiNbO3:Fe and LiNbO3:Cu

Abstract Holograms may be recorded in photorefractive LiNbO3:Fe and LiNbO3:Cu with infrared light pulses (wavelength λ = 1064 nm, pulse duration tp = 20 ns), if the crystals are previously or simultaneously illuminated with green light pulses (λ = 532 nm, tp = 15 ns). Refractive index changes and time constants are studied for crystals with different iron and copper concentrations after thermal reduction and oxidation treatments. A model explaining this effect is discussed.