Photorefractivity in polymer-stabilized nematic liquid crystals Page: 1 of 10
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Photorefractivity in polymer-stabilized nematic liqui crystals
Gary P. Wiederrecht' and Michael R. Wasielewskiab J
JUL 3 019
'Chemistry Division, Argonne National Laboratory, Argonne, IL 60439-
bDepartment of Chemistry, Northwestern University, Evanston, IL 60208-
Polymer-stabilized liquid crystals, consisting of low concentrations of a polymeric electron acceptor, are shown to
exhibit significantly enhanced photorefractive properties. The charge generation and transport properties of these composite
systems are strongly modified from nematic liquid crystals doped with electron donors and acceptors. The new composites
are produced by polymerizing a small quantity of a 1,4:5,8-naphthalenediimide electron acceptor functionalized with an
acrylate group in an aligned nematic liquid crystal. Photopolymerization creates an anisotropic gel-like medium in which the
liquid crystal is free to reorient in the presence of a space charge field, while maintaining charge trapping sites in the
polymerized regions of the material. The presence of these trapping sites results in the observation of longer lived, higher
resolution holographic gratings in the polymer-stabilized liquid crystals than observed in nematic liquid crystals alone.
These gratings display Bragg regime diffraction. Asymmetric beam coupling, photo-conductivity, and four-wave mixing
experiments are performed to characterize the photophysics of these novel materials.
Keywords: Photorefractive Materials; Polymer-Stabilized Liquid Crystals; Non-Linear Optics; Photoconductivity
Photorefractive materials hold great promise for optical device applications in the areas of reversible optical
holography, noise-free optical image amplification, phase conjugate mirrors, and other optical signal processing
techniques.1-3 The photorefractive effect is a light-induced change in the refractive index of a nonlinear optical material. It
results from the creation of an electric field induced by directional charge transport over macroscopic distances. If the
material is electro-optic, the electric (or space charge) field can then modulate the refractive index of the material. When the
effect is properly optimized, an image can be stored and retrieved with no loss of optical fidelity using phase conjugation
Research on the photorefractive effect has blossomed over the past several years. As a consequence of the
outstanding optical quality and commercial availability of inorganic ferroelectric materials such as barium titanate and
lithium niobate, photorefractive holographic data storage systems have been designed and developed to the point that
commercialization of these systems is now being attempted.4 Although these materials perform well, the availability of
materials that are useful over a wide range of wavelengths and have significantly lower cost would dramatically enhance the
versatility of the photorefractive effect. In this pursuit, the 1990's have seen the advent of photorefractive polymers and
liquid crystals.5-17 These materials have many attractive properties, such as low dielectric constants, which minimize
dielectric shielding of the space charge field, low cost, and relatively simple syntheses. However, the development of useful
organic materials is challenging because the photorefractive effect requires the simultaneous optimization of electro-optic
properties, charge generation efficiency, charge transport over macroscopic distances, and charge trapping. Nonetheless,
organic materials have been developed to the point that their photorefractive gains are now larger than those exhibited by
their inorganic counterparts.
One of the reasons for the large photorefractive gains in organic materials is the development of low glass transition
temperature polymers that permit orientational motion of the nonlinear optical chromophores within the space-charge field.
This provides a birefringence contribution to the index of refraction change of the photorefractive grating over and above
that of the electro-optic effect. In fact, recent studies have shown that a large fraction of the photorefractive gain in the
The submitted manuscript has been created ing on its behalf, a paid-up, nonexclusive,
by the University of Chicago as Operator of irrevocable worldwide license in said article
Argonne National Laboratory ('Argonne') to reproduce, prepare derivative works, dis-
under Contract No. W-31-109-ENG-38 with tribute copies to the public, and perform pub-
the U.S. Department of Energy. The U.S. licy and display publicly, by or on behalf of
""e JKMm OF - S D UmpT W tI MTED Government retains for itself, and others act- the Government
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Wiederrecht, G.P. & Wasielewski, M.R. Photorefractivity in polymer-stabilized nematic liquid crystals, report, July 1, 1998; Illinois. (digital.library.unt.edu/ark:/67531/metadc704637/m1/1/: accessed August 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.