Radiation imagery chemistry: process – composition – or product th – Holographic process – composition – or product
Reexamination Certificate
1999-01-11
2001-04-24
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Holographic process, composition, or product
C430S290000, C430S001000, C359S003000, C534S573000, C534S738000
Reexamination Certificate
active
06221535
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a photorefractive composite.
Photorefractive composites are potentially useful in the production of holograms for optical processing and information storage. For a review of such composites see the article by W. E. Moerner in Nature, Vol 371, pages 475 et seq (October 1994).
The photorefractive effect was first observed in inorganic materials, e.g. barium, titanate and lithium niobate. Since the demonstration of the first organic polymer based photorefractive (PR) system in 1991 [1], this class of materials has been developed to a point where they have now equalled or surpassed [2,3] many of the performance characteristics of both organic and inorganic PR crystals. Together with the low cost and versatility of organic polymer based systems this makes them highly attractive for commercial applications in optical data storage and optical data processing. Recently a PR polymer has been shown to exhibit 86% steady state diffraction efficiency [2], an outstanding increase over earlier systems, moving PR polymers further toward implementation. This composite comprises a PVK:TNF charge-transport network (which has been known for a long time and is used in photocopiers), an electro-optic polar dye (DMNPAA) and a material to lower the resulting composite viscosity at room temperature (a plasticiser) ECZ. The structures of TNF, PVK, ECZ and DMNPAA are shown below. In this prior composition, the ECZ (16% by weight) allows the re-orientation of the dye molecule at room temperature due to the lowering of the glass transition temperature (Tg), but is otherwise inert as far as the photorefractive process is concerned. This inert substance is unwanted if it is desired to achieve the highest concentration of all the active components. Several groups, however, have reported this to be a capricious and unstable system [4-6] which suffers from non-trivial sample preparation, stringent storage requirements (low humidity and dust free environment), and a risk of short device lifetimes. This system has since been reported by many groups to be extremely difficult to synthesise with good optical quality due to the crystallisation of the dye (DMNPAA) from the matrix.
DESCRIPTION OF THE INVENTION
According to a first aspect of the present invention there is provided a photorefractive composition comprising a charge transport matrix and an electrooptic dye having an aliphatic group of four or more carbon atoms.
By using an electrooptic dye containing a aliphatic group of four or more carbon atoms, the dye becomes less polar per unit length and hence more soluble in a non-polar charge transport host. This allows the incorporation of higher concentrations of the dye into the host matrix without crystallisation problems and, subsequently, the composite is very easy to fabricate without specialised conditions. Another advantage is that the dyes reduce the glass transition temperature of the resulting matrix itself and there is no need for an additional plasticiser such as ECZ in order for the re-orientational effect to be present. This allows all of the composite to be active in the photorefractive process.
The dye preferably incorporates an unsubstituted alkyl group. Preferably the group is branched. This alkyl group may have 4 or more carbon atoms and may for example be a sec-butyl or t-butyl group but more preferably has a larger number of carbon atoms. For preference the alkyl group has at least 5 and more preferably at least 8 carbon atoms. A particularly preferred alkyl group is the 2-ethythexyl group. A further example is the 3-methylbutyl group.
Generally the alkyl group will not have more than 30 carbon atoms. A preferred maximum is 20.
The electrooptic compound may be an azo compound or a stilbene compound.
The electrooptic dye may for example by of the formula I
where R
1
is an alkyl group of at least 4 carbon atoms, R
2
and R
3
are the same or different and are alkyl groups having up to 3 carbon atoms, X is carbon or nitrogen.
Such compounds are novel and provide a second aspect of the invention.
Preferably R
1
is branched and preferably has at least 5 carbon atoms. For preference R
1
is 2-ethylhexyl. A further favoured option for R
1
is 3-methylbutyl.
R
2
and R
3
are preferably methyl.
X is preferably nitrogen.
The compound in which R
1
=2-ethylhexyl, R
2
=R
3
=methyl, and X=Y=nitrogen is represented by formula (II) below and is a preferred embodiment of the second aspect of the invention.
The systematic name of (II) is 1-(2′-ethylhexyloxy)-2,5-dimethyl-4-(4″-nitrophenylazo)benzene and it is referred to herein as EHDNPB.
The charge transport matrix for use in the composition of the first aspect of the invention preferably comprises PVK as a conductive matrix and a charge sensitising agent. This charge sensitising agent may for example be TNF. Alternative charge sensitising agents are C
60
, C
70
and 2,4,7,9-tetranitrofluorenone (TeNF) (for use in the presence of oxygen). The presence of charge carrier trap sites can be of importance. Maximum hologram spatial frequency, stability under readout illumination and dark stability are strongly dependent on the properties of charge carrier trap sites. The traps must be deep enough to inhibit thermal liberation processes and under readout conditions of illumination and poling field optical libation from the traps should ideally also be suppressed. The intrinsic trap density can be reduced by preparation of materials and sample fabrication in a nitrogen atmosphere. Deep and complex trap systems may be introduced by the addition of higher doping levels of fullerene C
60
, ultraviolet irradiation, and thermal decomposition.
Alternative charge transfer matrices (photoconductive matrices) which may be used are poly(methylphenyl silane) (PMPSi) and poly(p-phenylenevinylene) (PPV) derivatives such as poly[1,4-phenylene-1,2-diphenoxyphenylvinylene](DPOP-PPV). A further possibility is poly(epoxypropylcarbazole) (PEPC). Such charge transfer matrices may not require the use of charge sensitising agents but they can be added.
The amount of the electrooptic dye incorporated in the composition of the first aspect of the invention may for example be 10 to 70% by weight of the composition, more preferably 40 to 60% by weight on the same basis.
The preferred matrix is PVK:TNF. The PVK:TNF ratio may for example be (40-50:1).
Preferred polymer composites of this invention are based PVK:TNF and which include an electro-optic chromophore. This chromophore incorporates a racemic ethylhexyl group. This non-polar functionality has two important roles. Firstly, it renders the dye a plasticiser and as such activates the orientational enhancement mechanism [7]. Secondly, it increases the solubility of the dye in PVK inhibiting crystallisation. The resulting composite reproducibly forms stable, optically transparent films with good reorientational mobility. No separate plasticising agent such as N-ethylcarbazole (ECZ) [2], is required.
As indicated above, compositions in accordance with the first aspect of the invention are easy to fabricate and do not require the incorporation of ECZ. The devices made from such compositions also have a relatively long device lifetime. Thus, by way of example, we have fabricated devices containing 55% of EHDNPB in a PVK:TNF host and such devices had a lifetime exceeding 8 months without special storage conditions. By comparison, we were only able to introduce 10% DMNPAA into the PVK:TNF host without crystallisation occurring within a matter of days.
We have been able to produce a high optical quality, long-life guest-host photorefractive polymer composite which exhibits a 60% device steady state diffraction efficiency and 120 cm
−1
two-beam coupling gain, well in excess of its absorption of 3.5 cm
−1
, at a wavelength of 676 nm. In contrast to alternative composite materials of comparable high dye content, this performance has been achieved without
Blackburn Richard D.
Cox Alan M.
King Terence A.
Leigh David
Wade Frances A.
Angebranndt Martin
Nixon & Vanderhye
The Victoria University of Manchester
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