Voltage Creep in Holographic PDLC Gratings
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gained much attention over the last two decades. The ability to employ electro-optic based phenomena without the use of polarizers is attractive for a number of potential applications including switchable architectural windows, light shutters, and direct-view displays. Contrast arises from the ability to switch the nematic director between two states. In a conventional cell, the directors are aligned when the field is on causing the cell to become transparent while a reverse mode cell employs an electric field to induce scattering. One major variable in these conventional films is the magnitude of the voltage needed to modulate the director. This variable is also very important in dictating the performance of holographic elements formed using polymer-dispersed liquid crystals. By spatially polymerizing a starting syrup, the location of the phase-separated LC domains can be controlled thereby forming complex spatial patterns of LC domains. Our group has been successful in forming Bragg gratings both in transmission 2 and reflection 3 geometries using the principles of photo polymerization-induced phase separation of LC's. Considerable interest in this technology is based on the ability to fabricate switchable diffractive optical elements. These have applications in a number of applications including switchable diffractive devices, optical interconnects, programmable waveguide structures, and switchable colored films 4 . The switching voltage of these films is dictated by a number of factors. It is well known that the switching voltage is inversely proportional to the LC droplet size such that smaller
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Mat. Res. Soc. Symp. Proc. Vol. 559 01999 Materials Research Society
domains necessitate larger switching fields. Droplet size issues are paramount in the determination of switching voltage for diffractive elements because the LC domains are typically one to two orders of magnitude smaller than domains in conventional PDLC structures. In a simplistic model first developed by Wu5 and expanded by Sutherland 6 to apply to grating structures, the critical switching field necessary to induce complete switching is defined below.
E =- ,,• , -,C+2J[k (l
1), 112
In this equation CTLc and ap are the low frequency conductivities, a is the average diameter of the LC droplet, k is the bend elastic constant, As is the dielectric anisotropy of the LC, and I is the droplet shape anisotropy (major axis/minor axis diameter).
The work presented here looks at the origins of an increase in the switching voltage of transmission gratings formed holographically over -7 days time. This creep of the voltage is detrimental to long-term usage of this material system. Gratings were formed and then the switching voltage over time was obtained. Dielectric and IR spectroscopy measurements are used to probe the causes of this voltage creep. EXPERIMENTAL PROCEDURES Photosensitive PDLC formulations consisted of a multifunctional acrylate monomer, photoinitiator, coinitiator, and liquid crystal. Syrups contained approximately 35% LC (E7Merck)
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