Meir Razvag

Wavelength-selective photonic switching in paraelectric potassium lithium tantalate niobate

A generic wavelength-selective switching method is presented. The method is based on the voltage-controlled photorefractive effect in paraelectric crystals. The switch consists of a crystal at the paraelectric phase in which a space charge grating was stored by a photorefractive process. Diffraction occurs by the application of a uniform electric field that causes the space charge grating to induce an index grating. Results of the diffraction efficiency as a function of the applied field and the time dependence of the diffracted beam are presented. The performance envelop of a switch based on this effect is briefly discussed.

Electroholographic neurons implemented on potassium lithium tantalate niobate crystals

We describe a new approach for constructing large-scale artificial neural networks. The novelty of our approach is based on the concept of electroholography (EH), which permits interconnecting of electronic neurons by minute-volume holograms, using the voltage-controlled photorefractive effect in paraelectric crystals. Crystals of potassium lithium tantalate niobate (KLTN) in the paraelectric phase are shown to be suitable for implementing this concept. A small network composed of two KLTN crystals on which holographic connections are recorded is presented to demonstrate the EH approach.

Electric-field multiplexing of volume holograms in paraelectric crystals

Electric-field multiplexing (EFM) results from the tuning of the effective wavelength of the light beam inside a photorefractive crystal. This tuning results from the application of an external electric field to the crystal during holographic recording. We demonstrate the high Bragg selectivity of this multiplexing technique in paraelectric crystals and compare it with the selectivity obtained in the ferroelectric phase. The effects of the two major physical parameters of working in the paraelectric phase, the temperature and the external electric field applied during the writing stage, are investigated. Experimental results of the EFM of three image-bearing holograms recorded in reflection geometry are presented along with a qualitative analysis of the Bragg selectivity in paraelectric crystals.

Photorefractive hologram fixing by a 4 K cooldown to the phase transition in K1−xLixTa1−yNbyO3

A fixing process of photorefractive holograms is reported. The process is based on the formation of an ionic space charge which becomes mobile at the phase transition temperature. A fixed grating was formed in paraelectric potassium lithium niobate tantalate by a 4 K cooldown to Tc, and a warmup back to the paraelectric phase. The diffraction efficiency of the fixed grating was 66% of the original photorefractive space charge grating.

Dipolar holographic gratings induced by the photorefractive process in potassium lithium tantalate niobate at the paraelectric phase

We present experimental evidence indicating the formation of dipolar holographic gratings in potassium lithium tantalate niobate at the paraelectric phase slightly above the phase transition. These gratings are formed in the dark, following the writing of space-charge-based photorefractive gratings, under an external electric field. The dipolar gratings create a spatial modulation of the low-frequency dielectric constant, and the latter induces a spatially correlated modulation of the polarization, which is transformed into birefringence gratings through the quadratic electro-optic effect.

Electric Field Multiplexing Of Holograms in paraelectric K/sub 1-y/Li/sub y/Ta/sub 1-x/Nb/sub x/O/sub 3/ Crystals

there is the added dimension of data volatility-the very process that makes this class of material attractive from a storage perspective is also its liability. The illumination used to record additional holograms or read out a previously stored hologram causes partial redistribution of the existing space charge fields and eventual erasure of the recorded information. This erasure manifests itself as a decrease in the BER to a point at which the data can no longer be reliably recovered. Solutions to this problem involve f ~ n g ofthe holograms by thermal or electrical means so that the space charge field is insensitive to the illumination or by gating the recording so that the establishment of a new space charge field requires the simultaneous application of two stimuli, such as two wavelengths, whereas reading can be done with just one wavelength and hence no disruption of the space charge fields. We report here BER studies of various organic and inorganic recording media performed using a precision holographic teststand.’ Multiple data input pages of up to 1.0 Mbit each are recorded and retrieved and the subsequent raw BER determined. In a pixelmatching configuration, a one-to-one imaging ofthe input SLM through the material onto the detector array with no over-sampling, the optical quality of the material is critical to preserve alignment and minimize distortions. The effectiveness of various fixing and gating processes will also be discussed. This research has been supported in part by the Defense Advanced Research Projects Agency through the NSIClARPA Photorefractive Information Storage Materials (PRISM) Consortium (MDA972-94-2-0008). *IBMAlmaden Research Center, San Jose, California 95120