We present an overview of our research effort on volume holographic digital data storage. Innovations, developments, and new insights gained in the design and operation of working storage platforms, novel optical components and techniques, data coding and signal processing algorithms, systems tradeoffs, materials testing and tradeoffs, and photon-gated storage materials are summarized.

Holographic data storage

An electrophotographic photosensitive member that can not easily cause charging lines even where it is an electrophotographic photosensitive member employing as a conductive layer a layer containing metal oxide particles is disclosed. Also disclosed are a process cartridge and an electrophotographic apparatus which have such an electrophotographic photosensitive member. The electrophotographic photosensitive member has a conductive layer which contains titanium oxide particles coated with tin oxide doped with phosphorus or tungsten.

Electrophotographic photosensitive member, process cartridge and electrophotographic apparatus

Research into and the development of data storage devices is a race to keep up with the continuing demand for more capacity, more density, and faster readout rates. Improvements in conventional memory technologies-magnetic hard disk drives, optical disks, and semiconductor memories-have managed to keep pace with the demand for bigger, faster memories. However, there is strong evidence that these two-dimensional surface storage technologies are approaching fundamental limits. An alternative approach for next-generation memories is to store data in three dimensions. This article describes developments in holographic 3D memories, in which high density is achieved by superimposing many holograms within the same volume of recording material. Holographic storage is a promising candidate for next-generation storage. Research has demonstrated that holographic storage systems with desirable properties can be engineered. The next step is to build these systems at costs competitive with those of existing technologies and to optimize the storage media.

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Photorefractive damage, optical absorption, photoconductivities, and photogalvanic currents of stoichiometric LiNbO(3) single crystals with different Mg doping levels have been investigated. Nominally pure stoichiometric LiNbO(3) shows lower photorefractive damage resistance than congruent crystal; however, stoichiometric crystals doped with MgO of more than 1.8 mol. % exhibit no measurable photorefractive damage at 532 nm to intensities of as much as 8 MW/cm(2) . This remarkable damage resistance can be attributed not only to increased photoconductivity but also to decreased photogalvanic current. Stoichiometric Mg:LiNbO(3) also demonstrates the shortest absorption edge, 302 nm, and a single-domain nature with low scattering losses.

Stoichiometric Mg:LiNbO(3) as an effective material for nonlinear optics.

The physical basis of the domain engineering in ferroelectrics and its application to lithium niobate crystals were reviewed. The unified kinetic approach to the domain structure evolution in electric field was formulated and its validity for understanding the variety of observed domain evolution scenarios was demonstrated. The kinetics and statics of the domain structure in the crystals of lithium niobate family including congruent, stoichiometric, and MgO doped ones have been discussed. The main stages of the periodical poling process and related problems have been pointed out. The basic poling techniques applied for creation of the periodical domain structures in bulk crystals and waveguides were compared. The recent applications of the periodically poled lithium niobate for light frequency conversion using second harmonic generation and optical parametric oscillation, excitation of the surface acoustic waves, and generation of terahertz radiation have been discussed. The special attention has been paid for achievements in fabrication of high-power optical parametric oscillation and integrated optical devices with periodically poled lithium niobate. The future trends in periodical poling and development of the nanodomain engineering which will allow to create the nanoscale domain patterns necessary for utilization of the new nonlinear interactions were reviewed.

Micro- and nano-domain engineering in lithium niobate

Growth and characterization of MgO-doped LiNbO3 for electro-optic devices

We report on the fabrication of a first‐order periodically inverted domain structure in LiTaO3 and LiNbO3 for quasi‐phase‐matched devices using proton exchange with one‐directional heating. First‐order periodically inverted domains with a 3.2 μm period and more than 40 μm depth were formed beyond the proton exchanged region. This domain inversion took place during the proton exchange process, far below the Curie temperatures. Taking advantage of these deep domains, we demonstrated second‐harmonic generation without fabricating a channel waveguide.

Fabrication of periodically inverted domain structures in LiTaO3 and LiNbO3 using proton exchange

Bulk laser damage threshold of 2‐in.‐diam. MgO‐doped LiNbO3 crystals grown using the Czochralski technique has been measured using a Q‐switched Nd3+:LiYF4 laser. The threshold showed dependence on the crystal quality. The highest bulk laser damage threshold (14 J/cm2 at 1.053 μm wavelength with 1‐ns pulse width) was obtained in the LiNbO3 crystal doped with 1 mol % of MgO. The lowering of bulk laser damage threshold was observed in the crystals doped with MgO at more than 3 mol %. These crystals contained the local aggregations of MgO which increased the scattering centers and decreased the transparency.

Investigation of bulk laser damage threshold of lithium niobate single crystals by Q‐switched pulse laser

Growth and characterization of off-congruent LiNbO3 single crystals grown by the double crucible method

There is disclosed a harmonic modulation device of waveguide type including a semiconductor laser, a waveguide for generating a nonlinear optical harmonic of the semiconductor laser by keeping phase matching, means for leading an optical beam of the semiconductor laser to the waveguide, electrodes disposed on the waveguide, and means for modulating a refractive index of the waveguide electro-optically to modulate a phase matching condition and thereby modulate an intensity of the harmonic.

Fumio Nitanda

Papers

Growth and characterization of MgO-doped LiNbO3 for electro-optic devices

Fabrication of periodically inverted domain structures in LiTaO3 and LiNbO3 using proton exchange

Investigation of bulk laser damage threshold of lithium niobate single crystals by Q‐switched pulse laser

Growth and characterization of off-congruent LiNbO3 single crystals grown by the double crucible method

Short-wavelength laser light source