Two-beam coupling (TBC) in a photorefractive polymer using transmission and reflection geometries is
investigated. With drift (due to an applied electric field) and diffusion, a linearized analysis suggests a phase shift
between the intensity grating and the induced refractive index grating different from the ideal value of 90 degrees,
which is supported by experimental results using a transmission grating geometry. In a self-pumped reflection
grating geometry, which is also experimentally studied, the phase shift can be closer to 90 degrees due to a shorter
grating period. Absorption and absorption gratings during TBC is also experimentally investigated.
We have studied the tunability of the reflection notch of a cholesteric filter containing a negative dielectric anisotropy
LC in a planar alignment. For this purpose, we studied physical, optical and electro-optical characteristics of mixtures
containing chiral dopant S811 and the negative dielectric anisotropy liquid crystal ZLI-2806. Interestingly, smectic A
phases were seen at room temperature for S811 loadings >20% by weight of ZLI-2806. Polarized optical microscopy
(POM) and differential scanning calorimetry (DSC) studies confirmed the formation of a cholesteric phase above room
temperature. A phase diagram was constructed by varying S811from 9-50% by weight in the mixture. Reflection notchs
were not seen at room temperature for compositions of S811 >20%. On heating, the selective reflection notch of the
cholesteric phase appeared and blue shifted with temperature. Various methods of tuning the reflection notch were
examined. Thermal tuning from 2200 nm to 450 nm was observed over the temperature range 23 to 55° C. Application
of a DC field led to electrical tuning (~50 nm) of the notch. The notch was also tuned (>500 nm) optically by exposing a
dye doped cholesteric cell to laser lines at either 532 or 647 nm.
Bragg gratings yield a single diffracted order when irradiated by a coherent beam at the appropriate Bragg angle. In
many cases, nearly all of the energy of the incident beam can be coupled to the diffracted beam. Hence these gratings
can form many useful optical elements, and this has been realized in 1-D, 2-D, and 3-D photonic crystals. Bragg gratings
made with liquid crystals offer the added dimension of dynamic properties through the large electro-optical effect
in liquid crystals. Applications for spatial light modulators are numerous, including optical switches, modulators, active
optical elements (e.g., lenses), laser sources, and tunable filters. We have been exploring a number of approaches for
making liquid crystal Bragg gratings, including holographic polymer-dispersed liquid crystals, cholesteric liquid crystals,
and homogenous nematic liquid crystals in hybrid devices. We have studied the dynamic properties of these Bragg
gratings by electrical, thermal, and optical stimulation. Modification and control of optical and dynamic properties have
been obtained through combinations of liquid crystals with polymers, combinations of various dopant materials, and
interactions of liquid crystals with organic and inorganic interfaces. We discuss the materials, fabrication, characterization,
and physics of liquid crystal Bragg gratings and present the results of various devices we have studied in our lab.
We will also discuss potential applications.
We report on the initial development of a visible initiator for thiol-ene photopolymerization using the 647 nm radiation from a Krypton ion laser. The photoinitiator system consists of the dye oxazine 170 perchlorate and the co-initiator benzoyl peroxide. Electron transfer occurs between the singlet excited state of the oxazine dye and benzoyl peroxide with subsequent decomposition of the peroxide yielding benzoyl oxy radicals capable of free radical initiation. We demonstrate that this photoinitiation system enables holographic patterning of HPDLC gratings as initial Bragg transmission gratings with a periodicity less than one micron using 647 nm radiation. These gratings were electrically switchable between a diffractive and transmissive state. Morphology studies using bright field transmission electron microscopy (BFTEM) indicate the phase separation of nearly spherical shaped nematic liquid crystal droplets of several hundred nanometers in diameter. This demonstration suggests that reflection gratings can be written using this photoinitiator system and 647 nm radiation which have switchable notch wavelengths approaching 2 microns.
Multifunctional acrylate formulations containing nematic liquid crystals have been shown to form holographic polymer dispersed liquid crystal gratings (H-PDLCs) easily using ultra-violet AND/OR visible photoinitiators. Laser wavelengths of 364, 476, 488, 514, 532 and 647 nm have been used for the fabrication of the gratings. Recently, the use of a thiol-ene based monomer system has been shown to overcome some of the adverse effects like post polymerization, voltage creep, and non-uniform shrinkage incurred when using highly functional acrylate monomers. However, Bragg reflection gratings have only been demonstrated utilizing ultra-violet (UV) (363.8 nm Argon ion) photopolymerization. Using UV irradiation and single prism geometry limits the upper end of the reflection notch wavelength. In this work, we report on new visible photoinitiator systems developed for the formation of reflective H-PDLCs using thiol-ene monomers. Using these new photoinitiator systems, reflection notches have been routinely written from the visible to the near infrared (IR) regions. The visible photoinitiator systems included the photoinitiator and radical generator titanocene organo-metallic complex (commercially known as Irgacure 784 (Ciba-Geigy), Rhodamine 6G, Pyrromethene, and a radical generating organic peroxide as coinitiator. Reflection gratings were written using laser wavelengths 442, 488, and 532 nm with diffraction efficiencies (DEs) above 70%. Angle tuning allowed for gratings with reflection notches in the near IR (900-1500 nm) to be written using these initiator systems. Rhodamine 6G was found to be more efficient than the other two initiators. We discuss here this new chemistry, the morphology, and electro-optical properties of the reflection gratings.