Photorefractivity in polymer-stabilized nematic liquid crystals Page: 4 of 10
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highest gain polymers reported to date is due to orientational responses.7 In light of these results, several groups have
recently explored nematic liquid crystals for photorefractive effects.10-15,17.18 Nematic liquid crystals have long been
known to reorient in much lower electric fields, even reorienting in optical fields. Thus, the space charge fields that are
required to produce comparable changes in the refractive index in nematic liquid crystals are orders of magnitude lower than
for polymeric materials. Space charge fields that induce large photorefractive effects in nematic liquid crystals are on the
order of IOV/cm, compared to the 500,000 V/cm or higher cited for polymeric materials.
The disadvantages of low molar mass liquid crystals as photorefractive media are I)the large fringe spacings
generally required to induce the effect do not permit for Bragg regime diffraction and 2) the general lack of long-lived
gratings. Given these limitations, several groups have recently reported efforts using either polymer-dispersed liquid crystals
or polymer-stabilized liquid crystals (PSLCs).16,19,20 Polymer-dispersed liquid crystals have a high concentration of
polymer, so that phase separation produces droplets within the material. Alignment of the liquid crystal droplet directors with
an applied electric field produces a transparent sample. Polymer-stabilized liquid crystals consist of only 1-2% of a
polymeric material that creates an anisotropic, transparent, gel-like material. Both of these materials alter the charge transport
characteristics of the liquid crystals from purely ion diffusion to include migration of charge through conductive polymers
and/or charge trapping in the polymer. These experiments have proven successful for creating both longer lived gratings and
Bragg regime diffraction. We report here our efforts for creating polymer-stabilized liquid crystals for photorefractive
2. EXPERIMENTAL METHODS
In order for PSLCs to exhibit photorefractivity, they must be photoconductive so that a space-charge field can be
generated. Therefore, easily oxidized and reduced dopants must be added for efficient photoinduced charge generation and
charge migration over bulk distances. With this in mind, the building blocks that make up our PSLCs are shown in Figure 1.
The liquid crystal itself is a eutectic liquid crystal mixture of 35%
(weight %) 4'-(n-octyloxy)-4-cyanobiphenyl (80CB) and 65% 4'- 0
(n-pentyl)-4-cyanobiphenyl (5CB). The liquid crystal is then doped
with the chromophore perylene (2x10'3M), which has a broad
absorption band that peaks at 443 nm and permits the use of the 514 * PER
nm line of an Ar' beam. Perylene is also easily oxidized, with a one-
electron oxidation potential of 0.8 eV vs. a saturated calomel O
electrode. The sample is then doped with 2% (mol %) of NIAC, an 0
acrylate monomer containing an easily reduced naphthalene diimide
moiety (-0.5 eV vs. the saturated calomel electrode). The synthesis EME
of NIAC is described elsewhere.20 Finally, 0.5 % (mol %) of
benzoin methyl ether (BME) is added to function as a photoinitiator. NC
The samples were prepared as follows. Indium-tin-oxide Ks
(ITO) coated glass slides were treated with the surfactant
octadecyltrichlorosilane to induce LC director alignment
perpendicular to the plane of the glass slides (homeotropic
alignment).21 Teflon spacers were used to create 26 Am thick
optical cells. The LC was drawn into the cell through capillary NIAC Polymer SoCs
action, and alignment occurred in approximately 30 mm.
Photopolymerization of the acrylate monomers was performed Figure 1. Components of the photorefractive
within the aligned LC samples with 365 nm light at an intensity of 2 polymer-stabilized liquid cyrstal.
mW/cm2. Polymerization times of 1, 2, 4, and 6 minutes were
utilized. The polymerized samples appeared slightly more hazy and pressure applied to the cell did not misalign the material
as with the unpolymerized samples.
The geometry of the experiment is illustrated in Figure 2. Two coherent, 2.5 mW, 514-nm beams from a continuous
wave Ar' laser were overlapped in the sample. The beams were unfocused and had a le diameter at the sample of 2.5 pm.
Voltages of up to 2 volts were applied to the polymerized samples, producing applied electric fields up to 800 V/cm. For
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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/m1/4/: accessed September 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.