Radiation imagery chemistry: process – composition – or product th – Holographic process – composition – or product
Reexamination Certificate
1999-06-01
2002-10-22
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Holographic process, composition, or product
C430S002000, C359S003000, C359S007000
Reexamination Certificate
active
06468699
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to photorefractive materials that are used as holographic storage materials and, particularly, to methods and materials for optical fixation of information in the photorefractive materials for long term storage where the information can be read out nondestructively with a uniform beam, archived for long term storage, and erased if desired.
2. Description of Prior Art
In volume data storage, information is stored in the form of holograms throughout the three dimensional volume of a light sensitive material. Volume data storage offers great potential for extremely dense mass storage and fast information processing. For example, it is possible to store a terabyte or more of data on a one-centimeter cube.
Photorefractive materials such as ferroelectric materials, are most suitable for read and write or read-only volume memories. The term “photorefractive effect” refers to changes in the index of refraction of an electro-optic material when the material is illuminated with non-uniform light. The index of refraction is defined as the ratio of the speed of light in a vacuum to that in a material.
Photorefractive materials possess suitable traps that are partly occupied by photosensitive electric charges. The traps originate in the material as a result of natural defects during crystal growth. These defects can occur whether the crystal is doped or undoped. Traps can be increased or enhanced by doping. These defects are mechanical flaws in the lattice structure, e.g., through the lattice structure missing atoms at some lattice sites.
A great variety of photorefractive materials is known to exist including organic and inorganic materials. For example, Rakuklujic and Yarviv, Photorefractive materials for optical computing and image processing, SPIE Vol. 881 Optical Computing and Nonlinear Materials (1988) describes a number of well known materials including BaTiO
3
, SBN, BSKNN, LiNbO
3
, KNbO
3
, BSO, GaAs, and InP, which all demonstrate photorefractive effects that may also be enhanced by doping. The Rakuklujic et al. article mathematically defines terms including steady-state change in the refractive index, response time, and photbrefractive sensitivity. The article provides a plurality of material parameters for each material, in both doped and undoped form. These parameters include data corresponding to the wavelength of light that induces a photorefractive effect in each material. The refractive index and photorefractive response time are listed for many materials, e.g., the materials can be selected to vary the photorefractive response time from one second to 100 picoseconds.
The family of tungsten bronze structures has been studies for its photorefractive effect, as reported in Neurgasonkaur et al, Development and modification of photorefractive properties in the tungsten bronze family crystals, 26 Optical Engineering No. 5 pp. 392-405 (May 1987), as well as in R. R. Neurogaonkar et al., Photorefractive tungsten bronze materials and applications, SPIE Vol. 1148 Nonlinear Optical Properties of Materials (1989).
As shown by the aforementioned articles, dopants may be added to the crystal melt to grow crystals having relatively more or less defects than would form during the growth of an undoped crystal. A dopant is typically an atom having a different oxidation state, ionic radius, or affinity for surrounding materials, than the other atoms that are normally found in the lattice. In practice, dopants can insert themselves into the lattice structure to either create or compensate defects, or dopants can decorate the surface of polycrystalline grains. Photorefractive materials are usually doped with a single or multiple dopant species to improve their refractive properties. For instance, lithium niobate is usually doped with iron, of various oxidation states to increase the sensitivity of the material to light. Other dopants that have been used to enhance the photorefractive effect include Ce, Cr, and Mn in various valence states, though an oxidation state of +3 is often preferred. Other dopants of similar oxidation states and which are similarly situated on the periodic table can also be used.
The Electro-Optic Effect—Trap-Charge Theory of Refractance
Holograms are written into photorefractive materials by the action of light on these materials according to the well-known photorefractive effect. The action of light of certain wavelengths and/or activation energy over time produces local field distortions that are associated with an increase or decrease in refraction. The localized differences in refraction are capable of storing data in the form of a trapped image. Two significant problems in the art include fading of the trapped image with time by dissipation of the image during storage or fading of the image during readout. The nonlimiting discussion below provides a generalized theory of why these two image fading phenomenon occur. Other image fading mechanisms may also play a role in the fading phenomenon. Only one technique, namely, that of thermal fixation, has been developed to overcome the problem of image fading during readout. No techniques have been developed to overcome the problem of image dissipation over time in storage conditions. Failure to overcome the problem of image dissipation during storage conditions precludes the use of holograms for long term archival storage.
In a photorefractive material, photosensitive charges move within the material under the influence of light.
FIG. 1
illustrates a photorefractive material
100
having a plurality of trapped photosensitive electric charges
102
and
104
, as well as the motion of these charges when they are excited by light
106
. When the photorefractive material
100
resides in darkness, the photosensitive electric charges
102
and
104
remain where they are originally located. Illumination of the photorefractive material
100
by light
106
excites the photosensitive electric charges
102
and
104
to a mobile state
108
causing the charges to leave their original locations and migrate through the crystal by one or more charge transfer mechanisms
110
and
112
.
The charge transfer mechanisms
110
and
112
are often referred to in the art as drift
110
and diffusion
112
. Drift
110
occurs when photosensitive charges
102
and
104
move under the action of a static electric field that is applied to the material. Diffusion occurs because the photosensitive charges
102
and
104
tend to move from regions of high light intensity to regions of low light intensity. The photosensitive electric charges
102
and
104
migrate through the crystal structure by these mechanisms until they are eventually retrapped, e.g., as at site
114
following a relaxation of energy along pathway
116
.
Excitation of trapped charges
102
and
104
by light
106
causes the charges to move away from the light
106
by drift
110
and diffusion
112
until they are retrapped at other sites, e.g. site
114
. When a photosensitive electric charge, e.g., charge
102
or
104
, migrates, it leaves behind an immobile ionized trap
118
, e.g., trap
118
. This trap creates a space charge electric field, which distorts the material lattice and, consequently, modulates the index of refraction of the material via the electro-optic effect, as explained in more detail below.
FIG. 2
shows two coherent light beams
200
and
202
, which intersect across a three dimensional region
204
within a photorefractive material to create a spatially periodic light interference pattern, as shown in interference pattern
300
of FIG.
3
. The periodic light interference pattern
300
excites photosensitive charges in the material, which migrate away from the light by diffusion and drift in the manner described with respect to FIG.
1
. The motion of the charges disturbs the charge equilibrium that was present in the material before illumination. This disturbance sets up a corresponding electronic charge distribution
302
, which, in turn, creates a corre
Angebranndt Martin
Tegreene Clarence T.
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