Third order non-linear optically active composites, method of making same and photonic media comprising same

A composite having third order non-linear optical activity comprising, an organic polymer and an inorganic material. The organic polymer has third order nonlinear optical activity and the inorganic material is a sol-gel glass. The organic polymer or a precursor of the organic polymer is mutually soluble with a precursor of the sol-gel glass. The invention also includes photonic media having the above composite and a method of making the composite.

BACKGROUND OF THE INVENTION 
Photonics is an emerging technology viewed by many as the future direction 
of optical signal processing and ultrafast optical computing. Photonics is 
generally a technology where photons rather than electrons are used to 
carry informational signals. 
Nonlinear optical effects of photonic media play an important role in 
photonics. Nonlinear optics refers to the nonlinear relationship between 
the response of the material and the intensity of infinite light. An 
important manifestation of the effect of nonlinear optics is the 
dependence of the refractive index on the intensity of light. Nonlinear 
optical effects are important in photonics because they provide means to 
produce optical switching (optically induced switching of a device from a 
low optical transmission state to a high optical transmission state) and 
optical bistability (behavior of a device whereby the device exhibits two 
optical absorptions within a given range of input values). Optical 
switching and optical bistability are functions needed for optical logic 
and optical memory operation. 
Conjugated organic polymers are considered an important class of optical 
material because they have demonstrated large, non-resonant 
(non-absorptive), optical nonlinearity with ultra fast response time in 
the sub-pico seconds regime, ("Nonlinear Optical and Electroactive 
Polymers" Edr's, P. N. Prasad and D. R. Ulrich, Plenum Press, N.Y. 1988). 
These polymers in their pure state, however, have generally not been found 
to form good photonic media because they typically exhibit high optical 
losses. For example, optical film made of pure poly-p-phenylene vinylene, 
a conjugated organic polymer, exhibits refractive index inhomogeneities 
(i.e. the refractive index varies from domain to domain) and degraded 
optical quality. 
Many glasses, including silica glass, are also considered an important 
class of optical material in that they form excellent photonic media 
because of extremely low optical losses. A major problem with many glasses 
is that their optical nonlinear coefficient (the quantitative 
representation of the strength of the nonlinear optical effects; X.sup.( 
3) -- third order nonlinear coefficient) is extremely low. Optical 
switching in silica wave guides (devices which channel optical waves 
formed in silica) has been observed; however, long interaction lengths are 
necessary to compensate for the extremely low optical nonlinearity. 
Therefore, currently there exists a need for a material having high, 
nonlinear optical coefficients, high optical quality (scattering and 
absorption losses within the material is small). 
Accordingly, an overall object of the invention is to provide a nonlinear, 
high optical quality material. 
A more particular object of this invention to provide a composite material 
capable of being formed into nonlinear, high optical quality wave guides 
with low optical losses. 
A further object of the invention is to provide a material which exhibits 
good mechanical strength, and possess environmental and thermal stability. 
Still another object is to provide a material having the above qualities, 
which can be incorporated into integrated optical device structures, in 
the form of optical fibers and optical wave guides with high optical 
through-puts; and into channel wave guides (structures having a plurality 
of channels that propagate light). 
Another object of the invention is to provide a method of making the 
composite material, having the above qualities, which will provide 
enhanced protection for the active element in the environment. 
SUMMARY OF THE INVENTION 
The invention is a composite having third order non-linear optical activity 
comprising, an organic polymer and an inorganic material, said organic 
polymer having third order non-linear optical activity and said inorganic 
material being a sol-gel glass; wherein said organic polymer or a 
precursor of said organic polymer is mutually soluble with a precursor of 
the sol-gel glass. 
The invention further comprises photonic media comprising the above 
composite and a method of making said composite. 
The method of making a third order nonlinear optically active composite 
comprises the steps of a) mixing a precursor of a sol-gel glass, a 
solvated precursor of an organic polymer having third order nonlinear 
optical activity and a mutual solvent with a sufficient amount of drying 
control chemical additive, to form a sol; b) adding a predetermined amount 
of additional solvent and solvated organic polymer precursor; c) 
maintaining the mixture at about ambient temperature for a sufficient time 
until just prior to gel formation; d) forming a desired structure; and e) 
heat treating to consolidate the sol-gel and convert the organic polymer 
precursor to the desired organic polymer.

DETAILED DESCRIPTION OF THE INVENTION 
Third order non-linear optical activity as used herein refers to physical 
properties, one of whose manifestations is an intensity dependent 
refractive index of a material. 
By high optical quality is meant materials having optical attenuation of 
less than about 10 dB/cm for a light of wavelength of 1.06.mu.. 
Organic polymers having third order, non-linear optical activity, are 
suitable for the organic polymers, in accordance with this invention. 
Typically these polymers are conjugated polymer systems. Preferred members 
of conjugated polymer systems include those which can be synthesized via a 
sulfonium salt precursor route. Examples of such conjugated systems are 
described in U.S. Pat. Nos. 3,401,152 and 3,706,677. Poly-p-phenylene 
vinylene (PPV), homopolymeric and copolymeric derivatives of PPV and 
heteroatomic analogs of PPV and their copolymers with PPV or derivatives 
of PPV, are the most preferred organic polymers having third order, 
non-linear optical activity. An example of a homopolymeric derivative 
would be 2,5 - dimethoxy derivatives. An example of a heteroatomic analog 
of the PPV is poly-thienylene vinylene. 
By organic polymer precursor is meant a precursor to the organic polymers 
having the above properties, which may be converted by chemical and/or 
thermal treatment into the desired organic polymer of the composite. The 
organic polymer precursor is itself an organic polymer. 
There are water-woluble sulfonium polyelectrolyte polymers which are 
precursors of PPV. The PPV is then formed by a thermal elimination 
reaction of the precursor. Other polymers and their polymeric precursors 
are described in "High Molecular Weight Polyphenylene Vinylene", F. E. 
Karasz, J. D. Capistran, D. R. Gagnon and R. W. Lenz, Mol. Cryst. Liq. 
Cryst. 118, 327-332 (1985), and in "Preparation of Poly (phenylene 
vinylene) from cycloalkylene sulfonium Salt Monomers and Polymers" Robert 
W. Lenz, Chien-Chung Han, John Stenger-Smith and F. E. Karasz, Journal of 
Polymer Science: Part A: Polymer Chemistry, Vol. 26, 3241-3248 (1988). 
The inorganic material is a sol-gel derived glass substance. By sol-gel 
glass is meant a glass formed by a sol-gel processing technique. Suitable 
sol-gel processed glasses include but are not limited to the Group IV 
oxide glasses. A preferred sol-gel glass is silica sol-gel glass. 
By sol-gel precursor is meant a low molecular weight (nonpolymeric, for 
example a monomer) soluble, inorganic material which can react to form 
ultimately a three dimensional cross-linked polymeric material. The 
sol-gel precursor is used to form, by the sol-gel processing technique, 
the sol-gel processed glass. Examples of suitable sol-gel precursors which 
may be used in accordance with this invention are Group IV tetraalkoxy 
compounds which can be processed by the sol-gel method to yield a Group IV 
oxide glass. An example of a preferred sol-gel precursor is tetramethyl 
orthosilicate, (TMOS, also known as silicon tetramethoxide), which is 
commercially available. TMOS is the precursor for the preferred silica 
sol-gel glass. 
The sol-gel processing technique is well known to those skilled in the art 
and can generally be described as a three step process whereby a 
multivalent inorganic alkoxide is used as a precursor to produce an 
inorganic glass. In the first step, the solvated precursor is hydrolyzed 
by addition of water, undergoes partial reaction and thereby forms very 
small colloidal particles dispersed in the solution. This dispersion is 
called a sol. As the reaction proceeds, a three dimensional polymeric 
network (gel) is formed. In the final step, the gel is heated to 
consolidate the material by reducing the void content and expelling the 
solvent and the volatile, low molecular weight reaction product, to 
produce a glass. A more detailed description of the sol-gel processing 
technique may be found in "SiO.sub.2 gel Glasses"L. L. Hench, S. H. Wang, 
and S. C. Park, SPIE Proceedings of the Symposium on "Advances in Optical 
Materials" Vol. 505, p. 90 (1984). 
Mutually soluble as used herein means that the organic polymer or a 
precursor of the organic polymer is soluble in the same solvent with the 
precursor of the sol-gel processed glass. Typically any solvent which will 
mutually solubilize the components can be used. Suitable solvents include 
water and alcohols. Preferred solvents are water, methanol and ethanol. 
The composites of the present invention may be formed by mixing a solvent, 
a sol-gel precursor and a solvated organic polymer precursor with a 
sufficient amount of drying control chemical additive, to form a sol. 
Additional solvent and solvated organic polymer precursor, may be added to 
dilute the sol dispersion, to slow down the gelation process and produce 
better quality composite films, (predetermined amount). Such a mixture may 
be maintained at an appropriate temperature until just prior to incipient 
gel formation. Films may then be cast, the sol-gel can be consolidated and 
the organic polymer precursor may be converted into the desired organic 
polymer. 
The solvated organic polymer precursor may be an organic polymer precursor 
in a solvent. An example of a preferred solvated organic polymer precursor 
is an organic polymer precursor in about a 3% to about 5% aqueous 
solution. Most preferably the solvated organic polymer precursor is a 4% 
aqueous solution. 
Examples of drying control chemical additives include formic acid, 
formamide, glycerol and oxalic acid. A detailed discussion of these drying 
control chemical additives is found in The Science of Ceramic Chemical 
Processing, eds Larry L. Hench and Donald R. Ulrich, Wiley Interscience 
Publishing Co., 1986, pp.52-64. For example, formic acid may be utilized 
in an amount equivalent to between about 0.1% to about 1% of the weight of 
the sol-gel precursor. 
Films may be cast on a suitable substrate using methods known to those 
skilled in the art. Examples of suitable methods include but are not 
limited to Spin Coating and Doctor Blading. 
The Spin Coating method produces sub-micrometer thick films. The Doctor 
Blading method may be preferred when films of between 1-3.mu.m in 
thickness are needed. Films made by these known techniques are kept at 
ambient temperature in a dark and clean environment for a sufficient time 
to allow some of the solvent and the volatiles to slowly evaporate. 
Generally, about 3-5 hours is sufficient for the solvent to evaporate. 
The PPV precursor is converted into PPV and the sol is consolidated by heat 
treating the materials or by other methods known to those skilled in the 
art. More specifically, the product may be placed in an oven for heat 
treatment, preferably a vacuum oven. The sol may be consolidated by 
placing the film into a vacuum oven at between about 60 to 80.degree. C. 
for about 3 to 8 hours; then at about 100 to 130.degree. C. for about 10 
to 15 hours, to age the sol-gel, maintain the sol-gel quality and limit 
cracking. Then the organic polymer precursor is converted into the desired 
organic polymer by heat treating in the vacuum oven at about 200 to 
280.degree. C. for about 10 hours. 
This method produces composites of high optical quality. The composites of 
the invention retain the third order non-linear optical activity of the 
organic polymer and exhibit high optical quality. These materials may then 
be utilized to form photonic media of high optical quality, with nonlinear 
properties. Examples of photonic media include optical fibers, optical 
wave guides and channel wave guides having high optical through-puts. 
The following examples and preparations describe the manner and process of 
making and using the invention and set forth the best mode contemplated by 
the inventor of carrying out the invention, but are not to be construed as 
a limitation thereof. 
EXAMPLES 
To form a composite in accordance with this invention which comprises PPV 
and silica sol-gel, equal volumes of TMOS, about 4% aqueous solution of 
PPV precursor (filtered) and methanol were mixed with an amount of formic 
acid, equivalent to about 1% of TMOS by weight. The above components were 
mixed with constant agitation at about 60.degree. C. for about 30 minutes 
until a sol was formed. 
At about 24-27.degree. C. (ambient temperature) about 8 additional volumes 
of methanol and a sufficient amount of aqueous PPV precursor solution was 
added to the above mixture until one of the preferred ratios below was 
reached. 
Examples of preferred ratios of PPV and TMOS (by weight) are: 
______________________________________ 
# PPV precursor 
TMOS 
______________________________________ 
1 1 1 
2 1 1.5 
3 1 2 
4 1 4 
5 1 6 
______________________________________ 
The mixture was placed in an ultrasonic bath to clarify the solution. The 
solution was then kept at about ambient temperature until the gel was 
about to form. Generally, the more TMOS in the mixture, the shorter the 
time required for gelation to begin. For the preferred ratios of 1:1 to 
1:2 of PPV precursor to TMOS, about 8 hours was required. 
Although any of the known methods may be utilized, the films were cast 
using the Spin Coating method. The sol-gel was consolidated and the PPV 
precursor was converted into PPV by placing the films into a vacuum oven 
at about 70.degree. C. for about 4 hours, then at 120.degree. C. for about 
10 hours, and finally 200.degree. C. for about 10 hours. 
Other embodiments of the invention will be apparent to the skilled in the 
art from a consideration of this specification or practice of the 
invention disclosed herein. It should be understood that there may be 
other embodiments which fall within the spirit and scope of the invention 
as defined by the following claims.