BACKGROUND

Field of the Invention

[0001] The invention relates to photorefractive devices having one or more carbon nanotube-doped polymer layers. The photorefractive devices exhibit improved performance, such an increased diffraction efficiency at low optical power. Also disclosed are methods of making the photorefractive devices and methods of improving the performance of photorefractive devices.

Description of the Related Art

[0002] 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 may be achieved by, for example, steps including: (1) charge generation by laser irradiation; (2) charge transport, resulting in the separation of positive and negative charges; (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, materials that combine good charge generation, good charge transport or photoconductivity, and good electro-optical activity can exhibit good photorefractive properties.

[0003] 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 LiNb0 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. Usually inorganic electro-optical (EO) crystals do not require biased voltage for the photorefractive behavior.

[0004] 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. Patent No. 5,064,264, to Ducharme et al, the contents of which are hereby incorporated by reference. Organic photorefractive materials offer many advantages over the original inorganic photorefractive crystals, such as large optical non-linearities, low dielectric constants, low cost, light weight, 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.

[0005] In recent years, researchers have attempted 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. Some researchers have investigated how various components affect the properties of photorefractive materials. As an example, materials containing carbazole can impart photoconductivity, while phenyl amine groups can improve charge transport properties.

[0006] Most notably, several new organic photorefractive compositions were developed having improved photorefractive properties, such as high diffraction efficiency, fast response time, and long phase stabilities. For example, see U.S. Patent Nos. 6,809,156, 6,653,421, 6,646,107, 6,610,809 and U.S. Patent Application Publication No. 2004/0077794 (Nitto Denko Technical), all of which are hereby incorporated by reference. These references disclose materials and processes for making triphenyl diamine (TPD)-type photorefractive compositions that show very fast response times and good gain coefficients.

[0007] Typically, applying a high biased voltage to photorefractive materials can obtain good photorefractive behavior. Recent efforts have been made to improve the grating holding persistency. For example, PCT Publication No. WO 2008/091716 and U.S. Patent Publication No. 2009/0547336, both of which are hereby incorporated by reference in their entirety, disclose methodologies to utilize approximately half the biased voltage, advantageously resulting in a longer device lifetime by incorporating a polymer layer into the device. The incorporation of the polymer layer in those references improved devices for applications with long grating requirements because the polymer layer reduced the biased voltage, hold grating persistency and protected the devices from voltage breakdown.

[0008] With the development of improved laser writing techniques, there remains a need for improved photorefractive devices that exhibit superior diffraction efficiencies— especially at low optical writing powers. Also, there remains a need to improve grating response and grating decay times in photorefractive materials while, at the same time, inhibiting or preventing voltage breakdown. SUMMARY

[0009] Given the need for improved photorefractive devices that exhibit superior diffraction efficiencies, certain embodiments of the present invention provide a photorefractive device comprising a first electrode layer and a second electrode layer, a photorefractive layer disposed between the first electrode layer and the second electrode layer, and a carbon nanotube-doped polymer layer disposed between the first electrode layer and the photorefractive layer.

[0010] Some embodiments provide a method for fabricating a photorefractive device. In some embodiments, the method for fabricating a photorefractive device, comprises providing a first electrode layer, forming a first carbon nanotube-doped polymer layer by coating one side of the first electrode layer with a mixture comprising carbon nanotubes and a polymer dispersed in a solvent, and placing a photorefractive layer on the carbon nanotube- doped polymer layer, such that the carbon nanotube-doped polymer layer is disposed between the photorefractive layer and the electrode layer.

[0011] Some embodiments provide a method for improving the performance of a photorefractive device by interposing a carbon nanotube-doped polymer layer between an electrode layer and a photorefractive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIGURE 1A illustrates an embodiment (not to scale) in which one carbon nanotube-doped polymer layer is interposed between an electrode layer and a photorefractive material.

[0013] FIGURE IB illustrates an embodiment (not to scale) in which two carbon nanotube-doped polymer layers are interposed between an electrode layer and a photorefractive material on both sides of the photorefractive material.

[0014] FIGURE 2A illustrates an embodiment (not to scale) in which one carbon nanotube-doped polymer layer is interposed between an electrode layer and a photorefractive material on one side of the photorefractive material.

[0015] FIGURE 2B illustrates an embodiment (not to scale) in which two carbon nanotube-doped polymer layers are interposed between an electrode layer and a photorefractive material on both sides of the photorefractive material.

[0016] FIGURE 3 A illustrates embodiments of chromophores that can be used in the photorefractive layer. [0017] FIGURE 3B illustrates embodiments of chromophores that can be used in the photorefractive layer.

[0018] FIGURE 4 illustrates embodiments of chromophores that can be used in the photorefractive layer.

DETAILED DESCRIPTION

[0019] The present disclosure relates to systems and methods for improving the performance of photorefractive devices comprising at least one electrode layer and a photorefractive layer. One or more polymer layers doped with carbon nanotubes are disposed between one or more electrode layers and a photorefractive layer. In some embodiments, the one or more carbon nanotube-doped polymer layers are, by themselves, non-photorefractive. For example, the photorefractive behavior of the device may be derived from solely or in part from the photorefractive layer. Advantageously, as discussed in greater detail below, doping polymer layers that are disposed between an electrode layer and a photorefractive layer with carbon nanotubes increases the diffraction efficiency at low optical powers. In some embodiments, doping polymer layers with carbon nanotubes and interposing them between an electrode layer and a photorefractive layer decreases the biased voltage requirements for photorefractive device applications. Photorefractive devices based upon this design may be used for a variety of purposes including, but not limited to, holographic image recording materials and devices.

[0020] Some embodiments provide a photorefractive device comprising two electrode layers, a photorefractive layer between the two electrode layers, and at least one carbon nanotube-doped polymer layer between one of the electrode layers and the photorefractive layer. In some embodiments, at least one carbon nanotube-doped polymer layer may be interposed between the other electrode layer and the photorefractive layer. In some embodiments, the photorefractive device may optionally further comprise a substrate layer on one of the electrode layers. In other embodiments, the photorefractive device may optionally comprise two substrate layers, one on each of the two electrode layers. FIGURES 1A-2B show several exemplary embodiments of the photorefractive devices.

[0021] FIGURE 1A illustrates one embodiment of the photorefractive devices. The photorefractive device 100 has two electrode layers 104A and 104B, and a photorefractive layer 106 is disposed between the electrode layers 104 A and 104B. A carbon nanotube-doped polymer layer 11 OA is disposed between the top electrode layer 104A and the photorefractive layer 106. [0022] FIGURE IB illustrates another embodiment of the photorefractive device 100. The photorefractive device 100 has two electrode layers 104A and 104B, and a photorefractive layer 106 is disposed between the electrode layers 104A and 104B. Carbon nanotube-doped polymer layer 11 OA is disposed between the top electrode layer 104A and the photorefractive layer. Carbon nanotube-doped polymer layer HOB is disposed between the bottom electrode layer 104B and the photorefractive layer 106.

[0023] FIGURE 2A illustrates another embodiment of the photorefractive device 100. In this embodiment, the photorefractive device 100 has a photorefractive layer 106, two substrate layers 102 A and 102B, two electrode layers 104 A and 104B, and a carbon nanotube- doped polymer layer 110A. The top electrode layer 104A is disposed between the top substrate layer 102 A and the photorefractive layer 106. The carbon nanotube-doped polymer layer 110A is disposed between the top electrode layer 104 A and the photorefractive layer 106. The bottom electrode layer 104B is disposed between the bottom substrate layer 102B and the photorefractive layer 106.

[0024] FIGURE 2B illustrates another embodiment of the photorefractive device 100. In this embodiment, the photorefractive device 100 has a photorefractive layer 106, two substrate layers 102A and 102B, two electrode layers 104A and 104B, and two carbon nanotube-doped polymer layers 11 OA and HOB. The top electrode layer 104A is disposed between the top substrate layer 102 A and the photorefractive layer 106. The top carbon nanotube-doped polymer layer 11 OA is disposed between the top electrode layer 104A and the photorefractive layer 106. The bottom electrode layer 104B is disposed between the bottom substrate layer and the photorefractive layer 106. The bottom carbon nanotube-doped polymer layer HOB is disposed between the bottom electrode layer 104B and the photorefractive layer 106.

[0025] Each of the carbon nanotube-doped polymer layers 11 OA and HOB may include any single polymer, a mixture of two or more polymers, multiple layers that each comprise a different polymer, or combinations thereof. The same is true for polymer layers not doped with carbon nanotubes. The polymer component of the carbon nanotube-doped polymer layer may be selected from a variety of polymers. For example, a polymer component may be formed from polymethyl methacrylate (PMMA), polyimide, amorphous polycarbonate (APC), siloxane sol-gel, or mixtures thereof.

[0026] In some embodiments, the polymer components of the carbon nanotube- doped polymer layers exhibit a low dielectric constant. In some embodiments, the relative dielectric constant of each of the polymer layers ranges from about 2 to about 15, or from about 2 to about 4.5. In some embodiments, the refractive index of the polymer layers ranges from about 1.5 to about 1.7.

[0027] The carbon nanotubes employed in the carbon nanotube-doped polymer layers may be multi-walled carbon nanotubes (MWNT), single-walled carbon nanotubes (SWNT), or combinations thereof. The carbon nanotubes used in the carbon nanotube-doped polymer layer may be functionalized or non-functionalized. In some embodiments, carbon nanotubes that are functionalized may be used. These functionalities may allow better solubility and dispersibility than non-functionalized carbon nanotubes. In some embodiments, the carbon nanotubes are functionalized with hydroxyl groups and/or carboxyl groups. In some embodiments, the carbon nanotubes are short multi-walled carbon nanotubes. In some embodiments the lengths of the carbon nanotubes are in the range of about 0.5 μιη to about 2 μιη. In some embodiments, the lengths of the carbon nanotubes are in the range of about 0.5 μιη to about 30 μιη.

[0028] In addition, the type of nanotubes incorporated into each individual carbon nanotube-doped polymer layer within a given photorefractive device can be the same or different. For instance, the carbon nanotube composition of 11 OA can be different from HOB within one device 100. Carbon nanotubes have electrical conductivity. The electrical conductivity from nanotubes affect the electrical property of polymer layer, further affect the photorefractive performance such as diffraction efficiency or biased voltage peak.

[0029] The weight percent of carbon nanotubes dispersed within the each polymer layer relative to the amount of polymer may vary. In some embodiments, the amount of carbon nanotubes dispersed within the polymer layer is in the range of about 0.001% to about 1% by weight of the polymer. In some embodiments, the amount of carbon nanotubes dispersed within the polymer layer is in the range of about 0.001% to about 0.1% by weight of the polymer. In some embodiments, the amount of carbon nanotubes dispersed within the polymer layer is in the range of about 0.001% to about 0.01% by weight of the polymer.

[0030] In some embodiments, polymer layers that are not doped with carbon nanotubes may be disposed between electrode layers and the photorefractive layer. The polymer make-up of polymer layers not doped with carbon-nanotubes may vary as do the carbon nanotube doped polymer layers and as described above. These polymer layers may also have the same polymer components as the carbon nanotube-doped polymer layers.

[0031] The thicknesses of each of the carbon nanotube-doped polymer layers present in a single photorefractive device may be independently selected. In some embodiments, the thickness the carbon nanotube-doped layer ranges from about 40 μιη to about 30 μιη, from about 30 μηι to about 20 μηι, from about 20 μιη to about 15 μιη, from about 15μιη to about 10 μιη, from about 10 μιη to about 5 μιη, from about 5 μιη to about 2 μm, or less. In some embodiments, the thickness of the carbon nanotube-doped polymer layer is about 40 μιη, about 30 μιη, about 20 μιη, about 15 μιη, about 10 μιη, about 5 μιη, or about 2 μιη.

[0032] In some embodiments, the total combined thickness of the carbon nanotube- doped layers within a given photorefractive device may range from about 2 μιη to about 40 μιη. In some embodiments, the combined thickness of the carbon nanotube-doped layers ranges from about 2 μιη to about 30 μιη, from about 2 μιη to about 20 μιη, from about 10 μιη to about 20 μιη, or from about 20 μιη to about 40 μιη.

[0033] The carbon nanotube-doped polymer layers can be prepared by various techniques known in the art and is not particularly limited. In some embodiments, a carbon nanotube-doped polymer layer is disposed between an electrode layer and a photorefractive layer. In some embodiments, the carbon nanotube-doped polymer is deposited on the electrode layer before mounting to the photorefractive layer. One embodiment of the methods involves applying a mixture to the electrode layer, where the mixture includes carbon nanotubes and at least one polymer dispersed in a solvent. The solvent can then be removed from the applied mixture to form a solid polymer layer. In some embodiments, the mixture is applied to the electrode layers by spin coating or solvent casting. The solvent can then be evaporated under ambient conditions, elevated temperatures or vacuum until a solid polymer layer forms. The photorefractive layer is subsequently mounted to the carbon nanotube-doped polymer layer modified electrode layer.

[0034] The same techniques can be used for photorefractive devices having two electrode layers. The carbon nanotube-doped polymer layer is deposited on each of the electrode layers first to form carbon nanotube-doped polymer layer modified electrode layers, which then can be applied to both sides of the photorefractive layer. In some embodiments, the carbon nanotube-doped polymers may be applied directly to the substrate layer, then the electrode layers may be mounted to the carbon nanotube-doped polymer layers.

[0035] In some embodiments, the mixture applied to the electrode layer includes about 10% to about 45% polymer by weight relative to the solvent. In some embodiments, the mixture includes about 20% to about 45% polymer by weight relative to the solvent. In some embodiments, the mixture includes about 30% to about 40% polymer by weight relative to the solvent. In some embodiments, the mixture can include about 0.001 parts to about 1 parts carbon nanotube by weight relative to 100 parts polymer. In some embodiments, the range of carbon nanotubes in the mixture comprises about 0.001 parts to about 0.1 parts or about 0.001 to about 0.01 parts by weight carbon nanotube relative to 100 parts by weight polymer.

[0036] The solvent in the mixture is also not particularly limited. Generally, any solvent that accommodates adequate mixing of the polymer and carbon nanotubes is suitable. In some embodiments, the solvent can be easily removed (e.g., evaporated) after applying the mixture. In some embodiments, the solvent is an organic solvent. In some embodiments, the organic solvent is selected from the group consisting of dimethylformamide, dichloromethane, and combinations thereof.

[0037] Generally, the mixture can be obtained by dispersing the polymer and carbon nanotubes into the solvent. For example, the components can be dispersed in solvents using a high shear mixer, ultrasonic treatment, and the like. In some embodiments, the polymer is substantially dissolved in a first solvent to obtain a polymer solution, and the carbon nanotube is dispersed in a second solvent optionally using ultrasonic treatment to obtain a carbon nanotube dispersion. The polymer solution and the carbon nanotube dispersion can then be intermixed to obtain the mixture.

[0038] In some embodiments, the electrode layer comprises a transparent electrode. The transparent electrode is further configured as a conducting film. The electrode layer may contain a material selected from the group consisting of metal oxides, metals, and organic films with an optical density less than about 0.2. In some embodiments, the electrode layers may be independently selected from the group consisting of indium tin oxide (ITO), tin oxide, zinc oxide, polythiophene, gold, aluminum, polyaniline, and combinations thereof. In some embodiments, the transparent electrodes are independently indium tin oxide or zinc oxide. When more than one electrode layer is present, just as the individual electrode compositions may vary, the electrode layer thicknesses may vary from one electrode layer to another. In some embodiments, the electrode layers may be of the same thickness. In some embodiments, the electrode layers may be of different thicknesses.

[0039] Interposing a carbon nanotube-doped polymer layers between the photorefractive layer and the electrode layer can provide photorefractive devices that exhibit increased diffraction efficiency and decreased biased voltage relative to devices lacking carbon nanotubes-doped polymer layers. While not wishing to be bound by theory, it is believed that the electrical conductivity of the carbon nanotubes can affect the electrical property of the polymer layer, which further affects the photorefractive performance of the device, such as diffraction efficiency or biased voltage peak. In some embodiments, more than one carbon nanotube-doped polymer layer may be disposed on one side of the photorefractive layer, between the electrode layer and the photorefractive layer. In some embodiments, more than one carbon nanotube-doped polymer layer and/or polymer layer may be disposed on one side of the photorefractive layer, between the electrode layer and the photorefractive layer.

[0040] In some embodiments, the photorefractive device exhibits increased diffraction efficiency relative to a second photorefractive device having polymer layers that are not doped with carbon nanotubes. In some embodiments, the diffraction efficiency relative to the photorefractive device is at least about 10% higher than the second photorefractive device. In some embodiments, the diffraction efficiency of the photorefractive device is at least about 20% higher than the second photorefractive device. In some embodiments, the diffraction efficiency of the photorefractive device is at least about 50% higher than the second photorefractive device. In some embodiments, the diffraction efficiency of the photorefractive device is at least about 100% higher than the second photorefractive device. In some embodiments, the diffraction efficiency of the photorefractive device is at least about 200% higher than the second photorefractive device. In some embodiments, the diffraction efficiency of the photorefractive device is at least about 300% higher than the second photorefractive device. As detailed further below, the diffraction efficiency can be determined, for example, using an approximately 532 nm laser beam with a total optical writing power of about 1.5 mW, about 0.15 mW, or about 0.015 mW.

[0041] The photorefractive devices disclosed herein also exhibit a reduced biased voltage. The low biased voltage advantageously results in a longer device lifetime. This longer life results because higher biased voltage typically cause a device to breakdown easily. In some embodiments, the photorefractive device disclosed herein exhibits a reduced biased voltage compared to a second photorefractive device having polymer layers that are not doped with carbon nanotubes. In some embodiments, the biased voltage exhibited by the disclosed photorefractive device is at least about 10% lower than the second photorefractive device. In some embodiments, the biased voltage exhibited by the disclosed photorefractive device is at least about 20%> lower than the second photorefractive device. As detailed further below, the biased voltage can be determined, for example, using an approximately 532 nm laser beam with a total optical writing power of about 1.5 mW, about 0.15 mW, or about 0.015 mW.

[0042] In some embodiments, the photorefractive layer comprises an organic or inorganic polymer exhibiting photorefractive behavior. In some embodiments, the photorefractive layer possesses a refractive index of approximately 1.7. In some embodiments, the photorefractive layer may comprise a polymer matrix with at least one of a repeat unit including a moiety having photoconductive or charge transport ability and a repeat unit including a moiety having non-linear optical ability, as discussed in greater detail below. Optionally, the photorefractive layer may further comprise other components, such as repeat units including another moiety having non-linear optical ability, as well as sensitizers, chromophores, and plasticizers, as described in U.S. Patent 6,610,809 to Nitto Denko Corporation and hereby incorporated by reference. One or both of the photoconductive and non-linear optical components are incorporated as functional groups into the polymer structure, typically as side groups.

[0043] The thickness of the photorefractive layer can vary. In some embodiments, the photorefractive layer is about 10 μιη to about 200 μιη thick. In some embodiments, the photorefractive layer is about 25 μιη to about 100 μιη thick. Such ranges of thickness allow for the photorefractive layer to provide good grating behavior. The composition of the photorefractive layer is discussed in further detail below.

[0044] The group that provides the charge transport functionality may be any group known in the art to provide such capability. If this group is to be attached to the polymer matrix as a side chain, then the group should be capable of incorporation into a monomer that can be polymerized to form the polymer matrix of the photorefractive composition.

[0045] Non-limiting examples of the substrate layers include soda lime glass, silica glass, borosilicate glass, gallium nitride, gallium arsenide, sapphire, quartz glass, polyethylene terephthalate, and polycarbonate. In some embodiments, the substrate exhibits a refractive index of about 1.5 or less. If more than one substrate layer is used, the substrate layers may be the same or different. In some embodiments, the substrate layer thicknesses may be independently selected.

[0046] Non-limiting examples of the photoconductive, or charge transport, groups are illustrated below. In one embodiment, the photoconductive groups comprise phenyl amine derivatives, such as carbazoles and di- and tri-phenyl diamines. In someembodiments, the moiety that provides the photoconductive functionality is chosen from the group of phenyl amine derivates consisting of the following side chain Structures (i), (ii) and (iii):

Structure (i) wherein Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom and Rai-Rag are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons;

Structure (ii)

wherein Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom and Rbi-Rb ₂₇ are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons;

Structure (iii)

wherein Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom and Rci-Rci ₄ are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.

[0047] The chromophore, or group that provides the non-linear optical functionality may be any group known in the art to provide such capability. If this group is to be attached to the polymer matrix as a side chain, then the group, or a precursor of the group, should be capable of incorporation into a monomer that can be polymerized to form the polymer matrix of the composition.

[0048] The chromophore of the present disclosure is represented by Structure (0):

Structure (0)

wherein Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom such as oxygen or sulfur, and in some embodiments, Q is an alkylene group represented by (CH ₂ ) _P where p is between 2 and 6. Ri is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons, and in some embodiments, Ri is an alkyl group selected from methyl, ethyl, propyl, butyl, pentyl and hexyl. G is a group having a bridge of π-conjugated bond. Eacpt is an electron acceptor group. In some embodiments, Q is selected from the group consisting of ethylene, propylene, butylene, pentylene, hexylene, and heptylene.

[0049] In this context, the term "a bridge of π-conjugated bond" refers to a molecular fragment that connects two or more chemical groups by π-conjugated bond. A π-conjugated bond contains covalent bonds between atoms that have σ bonds and π bonds formed between two atoms by overlap of their atomic orbits (s + p hybrid atomic orbits for σ bonds; p atomic orbits for π bonds).

[0050] The term "electron acceptor" refers to a group of atoms with a high electron affinity that can be bonded to a π-conjugated bridge. Exemplary acceptors, in order of increasing strength, are: C(0)NR ² < C(0)NHR < C(0)NH ₂ < C(0)OR < C(0)OH < C(0)R < C(0)H < CN < S(0) ₂ R < N0 ₂ , wherein R and R ₂ are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons

[0051] As typical exemplary electron acceptor groups, functional groups which are described in U.S. Patent Number 6,267,913, hereby incorporated by reference, can be used. At least a portion of these electron acceptor groups are shown in the structures below. The symbol "t" in the chemical structures below specifies an atom of attachment to another chemical group and indicates that the structure is missing a hydrogen that would normally be implied by the structure in the absence of the

wherein R is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons. [0052] In some embodiments, the moiety that provides the non-linear optical functionality is such a case that G in Structure (0) is represented by a structure selected from the group con isting of the Structures (iv) and (v):

Structure (iv) Structure (v) wherein, in both structures (iv) and (v), Rdi-Rd ₄ are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons, and, in some embodiments, Rdi-Rd ₄ are all hydrogen. R ₂ is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.

[0053] In some embodiments, Eacpt in Structure (0) is =0 an electron acceptor group represented a structure selected from the group consisting of the structures:

wherein R ₅ , R ₆ , R ₇ and Rg are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.

[0054] In some embodiments, chromophore groups are aniline-type groups or dehydronaphtyl amine groups.

[0055] Other types of chromophores may be used. In some embodiments, the chromophore is represented by formula (lib): D - PiC - A (lib)

wherein D is an electron donor group; PiC is a π-conjugated group; and A is an electron acceptor group.

[0056] The term "electron donor" is defined as a group with low electron affinity when compared to the electron affinity of A. Non-limiting examples of electron donor include amino (NRziRz ₂ ), methyl (CH ₃ ), oxy (ORzi), phosphino (PRziRz ₂ ), silicate (SiRzi), and thio (SRzi), and Rzi and Rz ₂ are organic substituents independently selected from alkenyls, alkyls, alkynyls, aryls, cycloalkenyls, cycloalkyls, and heteroaryls. In some embodiments, a heteroaryl has at least one heteroatom selected from O and S.

[0057] The term "π-conjugated group," "PiC" in formula (lib) is independent of the selection of "G" in Structure (0). In some embodiments, suitable π-conjugated groups for PiC include at least one of the following groups: aromatics and condensed aromatics, polyenes, polyynes, quinomethides, and corresponding heteroatom substitutions thereof (e.g. furan, pyridine, pyrrole, and thiophene). In some embodiments, suitable π-conjugated groups for PiC include at least one heteroatom replacement of a carbon in a C=C or C≡C bond and combinations thereof, with or without substitutions. In some embodiments, the suitable π- conjugated groups include no more than two of the preceding groups described in this paragraph. Further, said group or groups may be substituted with a carbocyclic or heterocyclic ring, condensed or appended to the π-conjugated group. Non-limiting examples of π-conjugated groups for PiC in formula lib) include:

wherein m and n are each independently integers of 2 or less.

[0058] The term "electron acceptor" is defined above in formula (lib) is independent of the selection of "Eacpt" in Structure (0). Additionally, "A" is further defined in this instance as an electron acceptor group with high electron affinity when compared to the electron affinity of D. In some embodiments, A is selected from, but not limited to the following: amide; cyano; ester; formyl; ketone; nitro; nitroso; sulphone; sulphoxide; sulphonate ester; sulphonamide; phosphine oxide; phosphonate; N-pyridinium; hetero- substitutions in B; variants thereof; and other positively charged quaternary salts. In some embodiments, A is selected from the group consisting of: N0 ₂ , CN, C=C(CN) ₂ , CF ₃ , F, CI, Br, I,

wherein n is an integer from 1 to 10.

[0059] In some embodiments, the chromophore of formula (lib) configures the composition to be sensitive to multiple light wavelengths in the visible spectrum. In some embodiments the chromophore is selected from one or more of the following compounds:

wherein each Rg-Rig in the above chromophoric compounds is independently selected from the group consisting of hydrogen, C1-C10 alkyl, C1-C10 alkoxy, and C4-C10 aryl, wherein the alkyl and alkoxy groups may be branched or linear. In some embodiments, each Rfi-Rfs ₂ in the above chromophoric compounds is independently selected from H, F, CH ₃ , CF ₃ CN, N0 ₂ , phenyl, CHO, and COCH ₃ . In some embodiments, each Rgi-Rg ₆ in the above chromophoric compounds is independently selected from H, F, CH ₃ , CF ₃ , CN, CH ₂ , phenyl, COCH ₃ .

[0060] In some embodiments, the chromophore of formula (lib) is selected from one or more of l-(4-nitrophenyl)azepane, 4-(azepan-l-yl)benzonitrile, 4-(azepan-l-yl)-2- fluorobenzonitrile, 5 -(azepan- 1 -yl)pyrimidine-2-carbonitrile, 5 -(azepan- 1 -yl)-2-nitrophenol, 1 - (4-nitro-3 -(trifluoromethyl)phenyl)azepane, 1 -(4-(perfluorohexylsulfonyl)phenyl)azepane, 1 -(4- (S-perfluorohexyl-N-perfluoromethylsulfonyl-sulfinimidoyl)ph enyl)azepane, 3-(4- butoxybenzylidene)pentane-2,4-dione, 3-(4-(azepan-l -yl)benzylidene)pentane-2,4-dione, 3-(4- phenoxybenzylidene)pentane-2,4-dione, methyl 3-(4-butoxyphenyl)-2-cyanoacrylate, methyl 3- (4-(azepan-l-yl)phenyl)acrylate, methyl 3-(4-butoxyphenyl)acrylate, ethyl 3-(4-(azepan-l- yl)phenyl)-2-methylacrylate, (Z)-ethyl 2-fluoro-3-(4-phenoxyphenyl)acrylate, ethyl 3-methyl-6- phenoxy-lH-indene-2-carboxylate, ethyl 3-(4-(azepan-l-yl)phenyl)-2-phenylacrylate, 4-((4-(2- butoxyethoxy)phenyl)ethynyl)-2,6-difluorobenzonitrile, 4-((4-(2-butoxyethoxy)phenyl)ethynyl) benzonitrile, 4-((4-(2-butoxyethoxy)phenyl)ethynyl)-2,6-difluorobenzonitri le, 4-((4-(2- ethylhexyloxy)phenyl)ethynyl)-2,6-difluorobenzonitrile, 4-((4-(2-butoxyethoxy)-2,6- difluorophenyl)ethynyl)-2,6-difluorobenzonitrile, 4'-(2-butoxyethoxy)-3,5-difluorobiphenyl-4- carbonitrile, 3,5-difluoro-4'-(2-(2-methoxyethoxy)ethoxy)biphenyl-4-carbon itrile, 2,6-difluoro- 4-((4-(2-(2-methoxyethoxy)ethoxy)-2,6-dimethylphenyl)ethynyl ) benzonitrile, and 4-((2,6- difluoro-4-(2-(2-methoxyethoxy)ethoxy)phenyl)ethynyl)-2,6-di fluorobenzonitrile. In some embodiments, the chromophore can be selected from the following compounds:

[0061] In some embodiments, material backbones, including, but not limited to, polyurethane, epoxy polymers, polystyrene, polyether, polyester, polyamide, polyimide, polysiloxane, and polyacrylate with the appropriate side chains attached, may be used to make the material matrices of the present disclosure.

[0062] In some embodiments, the backbone units are those based on acrylates or styrene. In some embodiments, the backbone units are acrylate-based monomers or methacrylate monomers. The first polymeric materials to include photoconductive functionality in the polymer itself were the polyvinyl carbazole materials developed at the University of Arizona. However, these polyvinyl carbazole polymers tend to become viscous and sticky when subjected to the heat-processing methods typically used to form the polymer into films or other shapes for use in photorefractive devices.

[0063] In contrast, (meth)acrylate -based, and more specifically acrylate-based, polymers, have much better thermal and mechanical properties. That is, they provide better workability during processing by injection-molding or extrusion, for example. This is particularly true when the polymers are prepared by radical polymerization. [0064] The photorefractive polymer composition, in some embodiments, is synthesized from a monomer incorporating at least one of the above photoconductive groups or one of the above chromophore groups. It is recognized that a number of physical and chemical properties are also desirable in the polymer matrix. The polymer may be synthesized in a manner such that it contains both charge transport groups and a chromophore groups. In some embodiments, this copolymer is synthesized using monomers containing charge transport groups that polymerize with monomers containing chromophore groups. Physical properties of the formed copolymer that are of importance include, but are not limited to, the molecular weight and the glass transition temperature, T _g . Also, it is valuable and desirable, although optional, that the composition should be capable of being formed into films, coatings and shaped bodies of various kinds by standard polymer processing techniques, such as solvent coating, injection molding, and extrusion.

[0065] In some embodiments, the polymer generally has a weight average molecular weight, M _w , ranging from about 3,000 to about 500,000, or from about 5,000 to about 100,000 g/mol. The term "weight average molecular weight" as used herein means the value determined by the GPC (gel permeation chromatography) method in polystyrene standards, as is well known in the art.

[0066] In a non-limiting example, the polymer composition used in the photorefractive layer comprises a repeating unit selected from the group consisting of the Structures (i)", (ii)", and (iii)" which provides charge transport functionality:

Structure (i)"

wherein Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom and Rai-Rag are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons;

Structure (ii)"

wherein Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom and Rbi-Rb ₂ 7 are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons;

Structure (iii)"

wherein Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom and Rci-Rci ₄ are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.

[0067] In a non-limiting example, the polymer composition used in the photorefractive layer comprises a repeating unit represented by the Structure (0)" which provides non-linear optical functionality:

Structure (0)

wherein Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom such as oxygen or sulfur, and, in some embodiments, Q is an alkylene group represented by (CH ₂ ) _P where p is between about 2 and about 6. Ri is selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons, and, in some embodiments, Ri is an alkyl group selected from methyl, ethyl, propyl, butyl, pentyl and hexyl. G is a group having a bridge of π-conjugated bond. Eacpt is an electron acceptor group. In some embodiments, Q is selected from the group consisting of ethylene, propylene, butylene, pentylene, hexylene, and heptylene. G and Eacpt are as described above with respect to Structure (0).

[0068] Further non-limiting examples of monomers including a phenyl amine derivative group as the charge transport component include carbazolylpropyl (meth)acrylate monomer; 4-(N,N-diphenylamino)-phenylpropyl (meth)acrylate; N-

[(meth)acroyloxypropylphenyl]-N, N', N'-triphenyl-(l, -biphenyl)-4,4'-diamine; N- [(meth)acroyloxypropylphenyl]-N'-phenyl-N, N'-di(4-methylphenyl)- (1,1 '-biphenyl)-4,4'- diamine; and N-[(meth)acroyloxypropylphenyl]- N'-phenyl- N, N'-di(4-buthoxyphenyl)- (1,1 '- biphenyl)-4,4'-diamine. Such monomers can be used singly or in mixtures of two or more monomers.

[0069] Further non-limiting examples of monomers including a chromophore group as the non-linear optical component include N-ethyl, N-4-dicyanomethylidenyl acrylate and N- ethyl, N-4-dicyanomethylidenyl-3, 4, 5, 6, 10-pentahydronaphtylpentyl acrylate.

[0070] Diverse polymerization techniques are known in the art to manufacture polymers from the above discussed monomers. One such conventional technique is radical polymerization, which is typically carried out by using an azo-type initiator, such as AIBN (azoisobutyl nitrile). In this radical polymerization method, the polymerization catalysis is generally used in an amount of from about 0.01 to about 5 mol%, or from about 0.1 to about 1 mol%, per mole of the sum of the polymerizable monomers.

[0071] In one embodiment of the present disclosure, conventional radical polymerization can be carried out in the presence of a solvent, such as ethyl acetate, tetrahydrofuran, butyl acetate, toluene or xylene. The solvent is generally used in an amount of from about 100 to about 10000 wt%, and, in some embodiments,from about 1000 to about 5000 wt%, per weight of the sum of the polymerizable monomers.

[0072] In an alternative embodiment, conventional radical polymerization is carried out without a solvent in the presence of an inert gas. In one embodiment, the inactive gas comprises one of nitrogen, argon, and helium. The gas pressure during polymerization ranges from about 1 to about 50 atm, or from about 1 to about 5 arm.

[0073] The conventional radical polymerization is, in some embodiments, carried out at a temperature of from about 50° C to about 100° C and is allowed to continue for about 1 to about 100 hours, depending on the desired final molecular weight and polymerization temperature and taking into account the polymerization rate.

[0074] By carrying out the radical polymerization technique based on the teachings and embodiments given above, it is possible to prepare polymers having charge transport groups, polymers having non-linear optical groups, and random or block copolymers carrying both charge transport and non-linear optical groups. Polymer systems may further be prepared from combinations of these polymers. Additionally, by following the techniques described herein, it is possible to prepare such materials with exceptionally good properties, such as photoconductivity, response time, and diffraction efficiency.

[0075] If the polymer is made from monomers that provide only charge transport ability, the photorefractive composition of the invention can be made by dispersing a component that possesses non-linear optical properties through the polymer matrix, as is described in U.S. Patent 5,064,264 to IBM, which is incorporated herein by reference. Suitable materials are known in the art and are well described in the literature, such as D.S. Chemla & J. Zyss, "Nonlinear Optical Properties of Organic Molecules and Crystals" (Academic Press, 1987), incorporated herein by reference. Also, as described in U.S. Patent 6,090,332 to Seth R. Marder et. al., hereby incorporated by reference, fused ring bridge, ring locked chromophores that form thermally stable photorefractive compositions can be used. For typical, non-limiting examples of chromophore additives, the following chemical structure compounds can be used:

[0076] The chosen compound or compounds are may be mixed in the matrix copolymer in a concentration of about up to 80 wt%, or up to about 40 wt%.

[0077] On the other hand, if the polymer is made from monomers that provide only the non-linear optical ability, the photorefractive composition can be made by mixing a component that possesses charge transport properties into the polymer matrix, again as is described in U.S. Patent Number 5,064,264 to IBM. In some embodiments, charge transport compounds are selected because they are good hole transfer compounds, for example, N-alkyl carbazole or triphenylamine derivatives.

[0078] As an alternative, or in addition to, adding the charge transport component in the form of a dispersion of entities comprising individual molecules with charge transport capability, a polymer blend can be made of individual polymers with charge transport and nonlinear optical abilities. For the charge transport polymer, the polymers already described above, such as those containing phenyl-amine derivative side chains, can be used. Since polymers containing only charge transport groups are comparatively easy to prepare by conventional techniques, the charge transport polymer may be made by radical polymerization or by any other convenient method.

[0079] To prepare the non-linear optical containing copolymer, monomers that have side-chain groups possessing non-linear-optical ability may be used. Non-limiting examples of monomers that may be used are those containing the following chemical structures:

wherein Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom such as oxygen or sulfur, and, in some embodiments, Q is an alkylene group represented by (CH ₂ ) _P where p is between about 2 and about 6; R ₀ is a hydrogen atom or methyl group. R is a linear or branched alkyl group with up to 10 carbons. In some embodiments, R is an alkyl group which is selected from methyl, ethyl, or propyl.

[0080] One technique for preparing a copolymer involves the use of a precursor monomer containing a precursor functional group for non-linear optical ability. Typically, this precursor is represented by the following general Structure (1):

Ro

H ₂ C=C

COO

V

Structure (1)

wherein Ro is a hydrogen atom or methyl group and V is selected from the group consisting of the following structures (vi) and (vii):

Structure (vi) Structure (vii)

wherein, in both structures (vi) and (vii), Q represents an alkylene group comprising 1 to 10 carbon atoms with or without a hetero atom such as oxygen or sulfur, and, in some embodiments, Q is an alkylene group represented by (CH ₂ ) _P where p is between about 2 and about 6. Rdi-Rd ₄ are independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons, and in some embodiments, Rdi-Rd ₄ are hydrogen; and wherein Ri represents a linear or branched alkyl group with up to 10 carbons, and in some embodiments, Ri is an alkyl group selected from methyl, ethyl, propyl, butyl, pentyl or hexyl. [0081] To prepare copolymers, both the non- linear optical monomer and the charge transport monomer, each of which can be selected from the types mentioned above, may be used. The procedure for performing the radical polymerization in this case involves the use of the same polymerization methods and operating conditions, with the same preferences, as described above.

[0082] After the precursor copolymer has been formed, it can be converted into the corresponding copolymer having non-linear optical groups and capabilities by a condensation reaction. Typically, the condensation reagent may be selected from the group consisting of:

wherein R ₅ , R ₆ , R ₇ and Rg are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.

[0083] The condensation reaction can be done at room temperature for about 1 to about 100 hrs, in the presence of a pyridine derivative catalyst. A solvent, such as butyl acetate, chloroform, dichloromethylene, toluene or xylene can be used. Optionally, the reaction may be carried out without the catalyst at a solvent reflux temperature of about 30°C or above for about 1 to about 100 hrs.

[0084] It has been discovered that use of a monomer containing a precursor group for non-linear-optical ability, and conversion of that group after polymerization tends to result in a polymer product of lower polydispersity than the case if a monomer containing the non-linear- optical group is used. This is, therefore, one technique for formation of the photorefractive composition.

[0085] There are no restrictions on the ratio of monomer units for the copolymers comprising a repeating unit including the first moiety having charge transport ability, a repeating unit including the second moiety having non-linear-optical ability, and, optionally, a repeating unit including the third moiety having plasticizing ability. However, as a typical representative example, the ratio per 100 weight parts of a (meth)acrylic monomer having charge transport ability relative to a (meth)acrylate monomer having non-linear optical ability ranges between about 1 and about 200 weight parts and in some embodiments, ranges between about 10 and about 100 weight parts. If this ratio is less than about 1 weight part, the charge transport ability of copolymer itself is weak and the response time tends to be too slow to give good photorefractivity. However, even in this case, the addition of already described low molecular weight components having non-linear-optical ability can enhance photorefractivity. On the other hand, if this ratio is more than about 200 weight parts, the non-linear-optical ability of copolymer itself is weak, and the diffraction efficiency tends to be too low to give good photorefractivity. However, even in this case, the addition of already described low molecular weight components having charge transport ability can enhance photorefractivity.

[0086] Optionally, other components may be added to the polymer matrix to provide or improve the desired physical properties mentioned earlier in this section. For good photorefractive capability, a photosensitizer may be added to serve as a charge generator. A wide choice of such photosensitizers is known in the art. One suitable sensitizer includes a fullerene. "Fullerenes" are carbon molecules in the form of a hollow sphere, ellipsoid, tube, or plane, and derivatives thereof. One example of a spherical fullerene is C ₆ o. While fullerenes are typically comprised entirely of carbon molecules, fullerenes may also be fullerene derivatives that contain other atoms, e.g., one or more substituents attached to the fullerene. In some embodiments, the sensitizer is a fullerene selected from C ₆ o, C ₇₀ , C ₈₄ , each of which may optionally be substituted. In an embodiment, the fullerene is selected from soluble C ₆ o derivative [6,6]-phenyl-C61-butyricacid-methylester, soluble C ₇₀ derivative [6,6]-phenyl-C ₇ r butyricacid-methylester, or soluble Cg ₄ derivative [6,6]-phenyl-C85-butyricacid-methylester. Fullerenes can also be in the form of carbon nanotubes, either single-wall or multi-wall. The single -wall or multi-wall carbon nanotubes can be optionally substituted with one or more substituents. Another suitable sensitizer includes a nitro-substituted fluorenone. Non-limiting examples of nitro-substituted fluorenones include nitrofluorenone, 2,4-dinitrofluorenone, 2,4,7- trinitrofluorenone, and (2,4,7-trinitro-9-fluorenylidene)malonitrile. Fullerene and fluorenone are non-limiting examples of photosensitizers that may be used. The amount of photosensitizer required is usually less than about 3 wt%.

[0087] The compositions can also be mixed with one or more components that possess plasticizer properties into the polymer matrix to form the photorefractive composition. Any commercial plasticizer compound can be used, such as phthalate derivatives or low molecular weight hole transfer compounds, for example N-alkyl carbazole or triphenylamine derivatives or acetyl carbazole or triphenylamine derivatives. N-alkyl carbazole or triphenylamine derivatives containing electron acceptor group, depicted in the following structures 4, 5, or 6, can help the photorefractive composition more stable, since the plasticizer contains both N-alkyl carbazole or triphenylamine moiety and non-liner optics moiety in one compound.

[0088] Non-limiting examples of the plasticizer include ethyl carbazole; 4-(N,N- diphenylamino)-phenylpropyl acetate; 4-(N,N-diphenylamino)-phenylmethyloxy acetate; N- (acetoxypropylphenyl)-N, N', N'-triphenyl-(l, -biphenyl)-4,4'-diamine; N-

(acetoxypropylphenyl)-N'-phenyl-N, N'-di(4-methylphenyl)- (l , l '-biphenyl)-4,4'-diamine; and N-(acetoxypropylphenyl)- N'-phenyl- N, N'-di(4-buthoxyphenyl)- (l ,l '-biphenyl)-4,4'-diamine. Such compounds can be used singly or in mixtures of two or more monomers. Also, un- polymerized monomers can be low molecular weight hole transfer compounds, for example 4- (N,N-diphenylamino)-phenylpropyl (meth)acrylate; N-[(meth)acroyloxypropylphenyl]-N, N', N'-triphenyl-(l ,l '-biphenyl)-4,4' -diamine; N-[(meth)acroyloxypropylphenyl]-N'-phenyl-N, N'- di(4-methylphenyl)- (l , l '-biphenyl)-4,4'-diamine; and N-[(meth)acroyloxypropylphenyl]- N'- phenyl- N, N'-di(4-buthoxyphenyl)- (1 , 1 '-biphenyl)-4,4'-diamine. Such monomers can be used singly or in mixtures of two or more monomers.

[0089] In some embodiments, as another type of plasticizer, N-alkyl carbazole or triphenylamine derivatives, which contains electron acceptor group, as depicted in the following Structures 4, 5, or 6, can be used:

Structure 4

wherein Rai is independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; p is 0 or 1 ;

Structure 5

wherein Rbi-Rb ₄ are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; p is 0 or 1;

Structure 6

wherein Rci-Rc ₃ are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons; p is 0 or 1; wherein Eacpt is =0 or an electron acceptor group and represent a structure selected from the group consisting of the structures:

wherein R ₅ , R ₆ , R ₇ and Rg are each independently selected from the group consisting of a hydrogen atom, a linear alkyl group with up to 10 carbons, a branched alkyl group with up to 10 carbons, and an aromatic group with up to 10 carbons.

[0090] In some embodiments of the invention, polymers of comparatively low T _g when compared with similar polymers prepared in accordance with conventional methods are provided. The inventors have recognized that this provides a benefit in terms of lower dependence on plasticizers. By selecting copolymers of intrinsically moderate T _g and by using methods that tend to depress the average T _g , it is possible to limit the amount of plasticizer required for the composition to no more than about 30% or about 25%, and in some embodiments lower, such as no more than about 20%.

EXAMPLES

[0091] It has been discovered that photorefractive devices produced using the systems and methods disclosed above can achieve an increase in the sensitivity of that is about 1.2 to about 3 times higher than that of photorefractive devices having polymer layers that are not doped with carbon nanotubes. That is, the photorefractive device exhibits superior diffraction efficiency at lesser optical writing powers.

[0092] These benefits are further described by the following examples, which are intended to be illustrative of the embodiments of the disclosure, but are not intended to limit the scope or underlying principles in any way.

(a) Monomers containing charge transport groups - TPD acrylate monomer:

[0093] Triphenyl diamine type (N-[acroyloxypropylphenyl]-N, N', N'-triphenyl- (1,1 '-biphenyl)-4,4' -diamine) (TPD acrylate) were purchased from Wako Chemical, Japan. The TPD acrylate type monomers have the structure:

(b) Monomers containing non-linear-optical groups

[0094] The non-linear-optical precursor monomer 5-[N-ethyl-N-4- formylphenyl]amino-pentyl acrylate was synthesized according to the following synthesis scheme:

STEP I :

[0095] In a solution of bromopentyl acetate (about 5 mL or 30 mmol) and toluene (about 25 mL), triethylamine (about 4.2 mL or 30 mmol) and N-ethylaniline (about 4mL or 30 mmol) were added at about room temperature. This solution was heated to about 120°C overnight. After cooling down, the reaction mixture was rotary-evaporated. The residue was purified by silica gel chromatography (developing solvent: hexane/acetone = about 9/1). An oily amine compound was obtained. (Yield: about 6.0g (80%))

STEP II:

[0096] Anhydrous DMF (about 6 mL or 77.5 mmol) was cooled in an ice-bath. Then, POCI ₃ (about 2.3 mL or 24.5 mmol) was added dropwise into a roughly 25 mL flask, and the mixture was allowed to come to room temperature. The amine compound (about 5.8 g or 23.3 mmol) was added through a rubber septum by syringe with dichloroethane. After stirring for about 30 min., this reaction mixture was heated to about 90 °C and the reaction was allowed to proceed overnight under an argon atmosphere.

[0097] After the overnight reaction, the reaction mixture was cooled, and poured into brine water and extracted by ether. The ether layer was washed with potassium carbonate solution and dried over anhydrous magnesium sulfate. After removing the magnesium sulfate, the solvent was removed and the residue was purified by silica gel chromatography (developing solvent: hexane/ethyl acetate = about 3/1). An aldehyde compound was obtained. (Yield: about 4.2g (65%))

STEP III:

[0098] The aldehyde compound (about 3.92 g or 14.1 mmol) was dissolved with methanol (about 20 mL). Into this mixture, potassium carbonate (about 400 mg) and water (about lmL) were added at room temperature and the solution was stirred overnight. The next day, the solution was poured into brine water and extracted by ether. The ether layer was dried over anhydrous magnesium sulfate. After removing the magnesium sulfate, the solvent was removed and the residue was purified by silica gel chromatography (developing solvent: hexane/acetone = about 1/1). An aldehyde alcohol compound was obtained. (Yield: about 3.2 g

(96%))

STEP IV:

[0099] The aldehyde alcohol (about 5.8 g or 24.7 mmol) was dissolved with anhydrous THF (about 60 mL). Into the solution, triethylamine (about 3.8 mL or 27.1 mmol) was added and the solution was cooled by ice-bath. Acrolyl chloride (about 2.1 mL or 26.5 mmol) was added and the solution was maintained at 0°C for 20 minutes. Thereafter, the solution was allowed to warm up to room temperature and stirred at room temperature for 1 hour, at which point TLC indicated that all of the alcohol compound had disappeared. The solution was poured into brine water and extracted by ether. The ether layer was dried over anhydrous magnesium sulfate. After removing the magnesium sulfate, the solvent was removed and the residue acrylate compound was purified by silica gel chromatography (developing solvent: hexane/acetone = about 1/1). The compound yield was about 5.38 g (76%), and the compound purity was about 99% (by GC).

c) Synthesis of non-linear-optical chromophore 7-FDCST

[0100] The non-linear-optical precursor 7-FDCST (7 member ring dicyanostyrene, 4-homopiperidino-2-fluorobenzylidene malononitrile) was synthesized according to the following two-step synthesis scheme:

[0101] A mixture of 2,4-difluorobenzaldehyde (about 25 g or 176 mmol), homopiperidine (about 17.4 g or 176 mmol), lithium carbonate (about 65 g or 880 mmol), and DMSO (about 625 mL) was stirred at about 50°C for about 16 hours. Water (about 50mL) was added to the reaction mixture. The products were extracted with ether (about lOOmL). After removal of ether, the crude products were purified by silica gel column chromatography using hexanes-ethyl acetate (about 9: 1) as eluent and crude intermediate was obtained (about 22.6 g,). 4-(dimethylamino)pyridine (about 230 mg) was added to a solution of the 4-homopiperidino-2- fluorobenzaldehyde (about 22.6g or 102 mmol) and malononitrile (about 10.1 g or 153 mmol) in methanol (about 323 mL). The reaction mixture was kept at room temperature and the product was collected by filtration and purified by recrystallization from ethanol. The compound yield was about 18.1g (38%)

(d) Plasticizer

[0102] N-ethylcarbazole is commercially available from Aldrich and was used after recrystallization.

(e) Matrix Polymer

Production Example 1 - Preparation of TPD Acrylate / Chromophore Type 10:1 Copolymer by AIBN Radical Initiated Polymerization

m/n=10/l

[0103] The charge transport monomer N-[(meth)acroyloxypropylphenyl]-N, Ν', N'- triphenyl-(l, -biphenyl)-4,4'-diamine (TPD acrylate) (about 43.34 g), and the non-linear- optical precursor monomer 5-[N-ethyl-N-4-formylphenyl]amino-pentyl acrylate (about 4.35 g), prepared as described in step (b) above, were put into a three-necked flask. After toluene (about 400 mL) was added and purged by argon gas for about 1 hour, azoisobutylnitrile (about 118 mg) was added into this solution. Then, the solution was heated to about 65 °C, while continuing to purge with argon gas.

[0104] After about 18 hours of polymerization, the polymer solution was diluted with toluene. The polymer was precipitated from the solution and added to methanol, and the resulting polymer precipitate was collected and washed in diethyl ether and methanol. The white polymer powder was collected and dried. The yield of polymer was about 66%. [0105] The weight average and number average molecular weights were measured by gel permeation chromatography, using polystyrene standard. The results were M _n = about 10,600, M _w = about 17,100, giving a polydispersity of about 1.61.

(f) Preparation of Polymer Solution

[0106] The polymer solution was prepared by dissolving a polymer (APC, PMMA, Sol-gel or polyimide) powder— generally between about 10% to about 45% by weight of polymer— in dichloromethane, stirring the resultant solution under ambient conditions for at least about 12 hours to substantially dissolve the polymer, and filtering the resultant solution through an about 0.2 μιη PTFE filter.

(g) Preparation of Carbon Nanotube Dispersion

[0107] The carbon nanotube dispersion was prepared by dispersing about 0.5%> by weight of hydroxyl (-OH) or carboxyl (-COOH or -COO ^" ) functionalized MWNTs (short multi-walled nanotubes) powder in DMF or dichloromethane. The functionalized short nanotubes were purchased from Cheap Tubes Inc. and used as received (>95% purity) (-OH functionalized nanotubes parts number SKU03040206; -COOH functionalized nanotubes parts number SKU03040306). The functionalized nanotubes were CCVD grown, acid purified, and functionalized through repeated reductions and extractions in concentrated acids. The outer diameters of the nanotubes were less than 8 nm, with lengths ranging between about 0.5 μιη to about 2.0 μιη. The functional content was about 3.86% by weight.

(h) Preparation of Carbon Nanotube-doped Polymer Layer

[0108] The carbon nanotube-doped polymer layer was prepared by intermixing the polymer solution and carbon nanotube dispersion to obtain a mixture having about 0.001 to about 0.01 parts by weight of carbon nanotube relative to about 100 parts polymer. The resulting mixture was applied to a transparent electrode layer composed of ITO by spin-coating or solvent casting. Solvent components were removed from the applied mixture by heat treatment up to 100° C at a predetermined heating program for about 6 hours. The applied mixture was further subjected to vacuum heating at about 130° C for about 1 hour to form an about 0.5 μιη to an about 50 μιη thick carbon nanotube-doped polymer layer on the electrode.

(f) Preparation of photorefractive layer

[0109] The photorefractive layer was prepared with following components:

(i) Matrix polymer (described above): 50 wt%

(ii) Prepared chromophore of 7F-DCST 30 wt% (iii) Ethyl carbazole plasticizer 20 wt%

[0110] To prepare the photorefractive composition, the components listed above were dissolved with toluene and stirred overnight at room temperature. After removing the solvent by rotary evaporator and vacuum pump, the residue was scratched and gathered. This powdery residue mixture— which is used to form the photorefractive layer— was put on a slide glass and melted at about 125° C to make an approximately 200-300 μιη thickness film, or pre- cake.

Example 1 - Preparation of Photorefractive Devices

[0111] A photorefractive device was prepared having generally the same structure and components as shown in FIGURE 2b: two ITO-coated glass substrates (electrode and substrate), two carbon nanotube-doped polymer layers, and a photorefractive layer. The photorefractive device was fabricated using the following steps:

[0112] (i) Polymer Solution: About 35% by weight of APC (amorphous polycarbonate) powder was dissolved in dichloromethane.

[0113] (ii) Carbon Nanotube Dispersion: About 0.5% by weight of hydroxyl (-OH) functionalized MWNTs (short multi-walled carbon nanotubes) powder was dispersed in dichloromethane by ultrasonic treatment.

[0114] (iii) Forming Carbon Nanotube-doped Polymer Layer: The polymer solution and the carbon nanotube dispersion were intermixed to obtain a solution having about 0.006 parts by weight of carbon nanotube relative to 100 parts polymer. The resulting mixture was applied by spin coating onto the ITO film and dried at 130° C for about 1 hour. These steps provided an about 5 μιη thick carbon nanotube-doped polymer layer.

[0115] (iv) Assembling the Photorefractive Device: The photorefractive film or pre- cake was transferred from the glass plate and interposed between the two carbon nanotube- doped polymer layers to form a photorefractive device as shown in FIGURE 2B. The total combined thickness for the polymer layers was about 10 μιη (about 5 μιη each) and the photorefractive layer was about 100 μιη thick.

Example 2

[0116] A photorefractive device was obtained in the same manner as in Example 1 except that the MWNTs were carboxyl (-COOH or -COO ^" ) functionalized instead of being hydroxyl (-OH) functionalized. Also, each of the polymer layers was about 10 μιη thick, thus, the total combined thickness of the polymer layers was about 20 μιη. Example 3

[0117] A photorefractive device was obtained in the same manner as in Example 1 except each of the polymer layers was about 10 μιη thick, thus, the total combined thickness of the polymer layers was about 20 μιη.

Example 4

[0118] A photorefractive device was obtained in the same manner as in Example 1 except each of the polymer layers was about 20 μιη thick, thus, the total combined thickness of the polymer layers was about 40 μιη.

Example 5

A photorefractive device was obtained in the same manner as in Example 1 except that the MWNTs were carboxyl (-COOH or -COO-) functionalized instead of being hydroxyl(-OH) functionalized. Also, each of the polymer layers was about 20 μιη thick, thus, the total combined thickness of the polymer layers was about 40 μιη.

Comparative Example 1

[0119] A photorefractive device was obtained in the same manner as in the Example

1 except that it was fabricated without carbon nanotubes in the polymer layers.

Comparative Example 2

[0120] A photorefractive device was obtained in the same manner as in the Example

2 except that it was fabricated without carbon nanotubes in the polymer layers.

Comparative Example 3

[0121] A photorefractive device was obtained in the same manner as in the Example 4 except that it was fabricated without carbon nanotubes in the polymer layers.

Measurement Method: Diffraction Efficiency

[0122] The diffraction efficiency was measured as a function of the applied field, by four-wave mixing experiments at about 532 nm with two s-polarized writing beams and a p- polarized probe beam. The angle between the bisector of the two writing beams and the sample normal was about 60 degrees and the angle between the writing beams was adjusted to provide an approximately 2.5 μιη grating spacing in the material (about 20 degrees). The optical power was the about same for both writing beams. Optical powers of about 0.45 mW/cm ² , about 0.045 mW/cm ² , and about 0.0045 mW/cm ² — which correlates with a total optical power of about 1.5 mW, about 0.15 mW, and about 0.015 mW, respectively, after correction for reflection losses— were applied to consider the diffraction efficiency at low powers. The beams were collimated to a spot size of approximately 500 μιη. The optical power of the probe was about 100 μW.

[0123] The measurement of diffraction efficiency peak bias was done as follows: The electric field (V/um) applied to the photorefractive sample was varied from about 0 V/μιη all the way up to about 100 V/μιη with certain time period (typically about 400 s), and the sample was illuminated with the two writing beams and the probe beam during this time period. Then, the diffracted beam was recorded. According to the theory,

wherein E ₀ is the component of E ₀ along the direction of the grating wave-vector and E _q is the trap limited saturation space-charge field. The diffraction efficiency will show maximum peak value at certain applied bias. The peak diffraction efficiency bias thus is a very useful parameter to determine the device performance.

[0124] The diffraction efficiencies measurements are summarized in Table 1 below.

TABLE 1 - Diffraction Efficiency of Photoelectric device

[0125] As illustrated in Table 1, the sensitivity is greatly increased (about two to about four times) when using carbon nanotube-doped polymer layers in the photorefractive device, particularly at lower optical power levels. That is, the diffraction efficiency is substantially higher when the photorefractive device includes carbon nanotube-doped photorefractive devices. For example, Example 1 exhibits a diffraction efficiency of about 16% at 0.015 mW, while Comparative Example 1 exhibits an efficiency of about 3%. Thus, this comparison shows about a four-fold increase in diffraction efficiency.

[0126] Moreover, when using carbon nanotube-doped polymer layers in the photorefractive device, the biased voltage is decreased. For instance, Example 1 exhibits a biased voltage of about 2.2 kV at 0.015 mW, while Comparative Example 1 exhibits a biased voltage of about 2.7 kV. Thus, this comparison shows about a 20%> decrease in biased voltage.

[0127] Also, when using different functional group modified carbon nanotubes- doped polymer layers in the photorefractive device, the diffraction efficiency and biased voltage did not show clear difference. For example, Example 2 exhibits a biased voltage of about 3.3 kV and a diffraction efficiency of about 20% at 0.015 mW, while Example 3 exhibits a biased voltage of about 3.3 kV and a diffraction efficiency of about 24%. Thus, this comparison shows about similar biased voltage and diffraction efficiency between hydroxyl and carboxyl modified carbon nanotubes.