Photorefractive composition

Details Photorefractive composition Photorefractive composition

2002-03-29

2003-11-25

Pezzuto, Helen L. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S257000, C526S260000, C526S263000, C526S270000, C526S280000, C526S288000, C526S298000, C526S328500

Reexamination Certificate

active

06653421

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to photorefractive compositions. More particularly, the invention relates to copolymers that include functional groups which provide photorefractive capabilities.
BACKGROUND OF THE INVENTION
Photorefractivity is a phenomenon in which the refractive index of a material can be altered by changing the electric field within the material, such as by laser beam irradiation. The change of the refractive index is achieved by a series of steps, including: (1) charge generation by laser irradiation, (2) charge transport, resulting in the separation of positive and negative charges, and (3) trapping of one type of charge (charge delocalization), (4) formation of a non-uniform internal electric field (space-charge field) as a result of charge delocalization, and (5) refractive index change induced by the non-uniform electric field.
Therefore, good photorefractive properties can be seen only for materials that combine good charge generation, good charge transport or photoconductivity, and good electro-optical activity.
Photorefractive materials have many promising applications, such as high-density optical data storage, dynamic holography, optical image processing, phase conjugated mirrors, optical computing, parallel optical logic, and pattern recognition.
Originally, the photorefractive effect was found in a variety of inorganic electro-optical (EO) crystals, such as LiNbO
3
. In these materials, the mechanism of the refractive index modulation by the internal space-charge field is based on a linear electro-optical effect.
In 1990 and 1991, the first organic photorefractive crystal and polymeric photorefractive materials were discovered and reported. Such materials are disclosed, for example, in U.S. Pat. No. 5,064,264, to Ducharme et al. Organic photorefractive materials offer many advantages over the original inorganic photorefractive crystals, such as large optical nonlinearities, low dielectric constants, low cost, lightweight, structural flexibility, and ease of device fabrication. Other important characteristics that may be desirable depending on the application include sufficiently long shelf life, optical quality, and thermal stability. These kinds of active organic polymers are emerging as key materials for advanced information and telecommunication technology.
In recent years, efforts have been made to optimize the properties of organic, and particularly polymeric, photorefractive materials. As mentioned above, good photorefractive properties depend upon good charge generation, good charge transport, also known as photoconductivity, and good electro-optical activity. Various studies that examine the selection and combination of the components that give rise to each of these features have been done. The photoconductive capability is frequently provided by incorporating materials containing carbazole groups. Phenyl amine groups can also be used for the charge transport part of the material.
Non-linear optical ability is generally provided by including chromophore compounds, such as an azo-type dye, which can absorb photon radiation. The chromophore may also provide adequate charge generation. Alternatively, a material known as a sensitizer may be added to provide or boost the mobile charge required for photorefractivity to occur. Many materials, including a wide range of dyes and pigments, can serve as sensitizers.
The photorefractive composition may be made simply by mixing the molecular components that provide the individual properties required into a host polymer matrix. However, most compositions prepared in this way are not stable over time, because phase separation tends to occur as the components crystallize.
Efforts have been made, therefore, to make polymers that include one or more of the active components in the polymer structure.
An example of a polymer matrix that includes transport components is poly(n-vinylcarbazole) (PVK). With such a matrix, polymers with: high performance could be fabricated as reported by N. Peyghambarian et al. (
Nature
, 1994, 371, 497).
In this case, a, photorefractive composition was made by adding an a{dot over (z)}o dye (DMNPAA; 2,5-dimethyl-4-(p-nitrophenylazo) anisole) as chromophore, and trinitrofluorenone (TNF) as sensitizer. The resulting compositions showed almost 100% diffraction efficiency at laser intensity of 1W/cm
2
and 90 V/&mgr;m biased voltage. However, the response time was slow at over 100 msec.
To achieve good photorefractivity, however, such materials must be doped with large concentrations of chromophore, such as 25 wt % or more. Thus, crystallization and phase separation of the strongly dipolar chromophore remain a major problem.
To completely eliminate the instability caused by phase separation, it has been recognized that it would be desirable to prepare fully functionalised photorefractive polymers, that is, polymers in which both the photoconductivity and the non-minear optical capability reside within the polymer itself.
Building on the original University of Arizona work, efforts have been made to develop fully functional photorefractive polymers, as well as to speed up the response time. For example, PVK polymers in which some of the carbazole groups are tricyanovinylated have been made (N. Peyghambarian et al.,
Applied Phys. Lett
., 1992, 60, 1803). However, the photoconductivity of this polymer was reported as only 0.98 pS/cm and the diffraction efficiency was less than 1%, too low to show good photorefractivity. Subsequently, the same group has reported PVK-based materials with an amazing response time of 4 msec. (N. Peyghambarian et al.,
Applied Physics Leiters
, 1999, 16, 2253).
A number of efforts at materials improvement have used methacrylate-based polymers and copolymers that include photoconductive and chromophore side groups. A paper by T. Kawakami and N. Sonoda, (
Applied Phys. Lett
., 1993, 62, 2167.) discloses acrylate-based polymers containing dicyanovinylideneyl phenylamines as charge transport groups. The diffraction efficiency was reported at around 0.01%.
Japanese Patent Application Laid-open JP-A-1995-318992, to Hitachi Ltd. discloses acrylate-based polymers and copolymers made by conventional polymerization techniques and containing charge transport and non-linear-optical groups, but gives no photorefractive performance data.
A report by H. Sato et al., (Technical report of IEICE., 1995, OME-95-53, OPE95-94, 43) describes the preparation of several copolymers having both charge transport components and non-linear optical components in the side groups of the copolymer. However, the charge transport speeds seem to be too slow for good photorefractive materials.
Japanese Patent Application Laid-open JP-A 1998-333195, to Showa Denko, discloses acrylate-based polymers incorporating triphenylamine groups as charge transport agents. Fast response times (50 msec. at 70 V/&mgr;m biased voltage) is reported, although there is no description or data regarding diffraction efficiency.
A paper by Van Steenwickel et al. (
Macromolecules
, 2000, 33, 4074) describes acrylate-based polymers that include carbazole-based side chains and several stilbene-type side chains. The paper cites a high diffraction efficiency of 60% at 58 V/&mgr;m, but a slow-response time of the sub-second order.
A paper by Y.Chen et al. (
Modern Optics
, 1999, 46, 1003) discusses a methacrylate polymer that has both carbazole-type side chains to provide charge transport capability and nitrophenyl azo-type side chains to provide non-linear optical capability. The materials again show slow response times of over 20 sec.
None of the materials described above achieves the optimum combination of a high diffraction efficiency with a fast response time, long-term stability and easy processability. Thus, there remains a need for photorefractive compositions that combine these attributes.
In recent years, a new type of polymerization, termed living radical polymerization, has been developed for polymerization of functional monomers, including methacrylate and styrene derivatives. Living radical p

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