Nazarene Mocam Frias
Nazarene Mocam Frias
Cm_ΆFuTheoretical and experimental study on photorefractive mesogenic compoistesv
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| Photorefractive (PR) mesogenic composites provide a variety of applications in low power photonic devices, and being an excellent man-machine interface, it is undeniably at the vanguard of the current research. In particular, the synthesis of PR organic materials is the subject of extensive material development to optimize device requirements such as resolution, performance, and response characteristics. Recently, we demonstrated the enhancement of resolution by dissolving a highly functionalized copolymer, and a photoconductive sensitizer in a low-molar mass liquid crystal. The composite showed the liquid crystalline phase even though the copolymer lacks a mesophase at all temperature.Although the basic conditions leading to photorefraction are the same in all PR materials, the mechanisms are quite different. In this work, we proposed a theoretical model of the PR effect exhibited by the above novel mesogenic composite. The irradiation of inhomogeneous light intensity leads to the freeing of photocharges by charge transfer complexes. This induces charge separation by drift and diffusion, and forms a space charge field, if the photoconductivity is larger than the dark current and if the Debye-Huckel screening radius is much smaller than the characteristic scale of intensity variation. While the theoretical approach used in deriving the rate equations, current continuity, and the space charge field is the same as that of the band-conduction model in inorganic PR materials, the differences in photochemical response and crystal symmetry was taken into account. It was found that the space charge field is dependent on the diffusion coefficients and conductivities. Another contribution to the PR effect in the composite is the electrohydrodynamic instability. However, this requires a material to be anisotropically conducting in the dc field regime. This contribution was neglected in our sample. The electrooptic effect in nematic liquid crystal is due to the reorientation of the director which is caused by the perturbing action of the field, the viscoelastic torques, and the surface energy. In examining the dynamics of the reorientation angle, continuum mechanics, conservation laws, and entropy inequality were employed with the assumptions that the medium is incompressible and at constant temperature. The equilibrium state equation was obtained from the variational principle to which the free energy in the established state must have an extremum value. The variational equations was linearized with the aid of perturbation theory. To validate the model, we conducted a two-beam coupling experiment to two composites. Sample A was aligned homeotropically by the action of the copolymer, while B was aligned homogeneously by placing it between unidirectionally rubbed PVA layers. We measured the gain coefficient with different applied dc voltages, and different tilted angles. Results showed that both samples have roughly the same gain coefficient vs. dc field curve, but the dependence of gain coefficient on the tilted angle is different. The difference was found to be a manifestation of the surface energy. The surface energy enters into the dynamical equations only if the anchoring strength is weak such that molecules are perturbed at the cell interface as in the planar case. Finally, we concluded that the molecular reorientation in our samples was induced by drift and diffusion in accordance with electrooptic, viscoelastic, and anchoring properties. The experimental results behave in good agreement with the theoretical model. |
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| 1. | Measurement of
photorefractive phase shift in mesogenic composites. H. Ono, T. Kawamura, N. M. Frias, K. Kitamura, N. Kawatsuki, and H. Norisada Appl. Phys. Lett. 75 (1999) 3632-3634. |
| 2. | Photorefractive mesogenic
materials. H. Ono, T. Kawamura, N. M. Frias, K. Kitamura, N. Kawatsuki, and H. Norisada Advanced Materials 12 (2000) 143-146. |
| 3. | Holographic Bragg
grating in photorefractive polymer-dissolved liquid-crystal
composites. H. Ono, T. Kawamura, N. M. Frias, K. Kitamura, N. Kawatsuki, H. Norisada, and T. Yamamoto Appl. Phys. Lett. 88 (2000) 3853-3858. |
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| 1. | QOOONR@ζSVρp¨wΦWAuο\eWiζRͺϋj1253 Tilted angle dependence of photorefractive gain coefficients in polymer-dissolved liquid crystals Nazarene Mocam Frias, Hiroshi Ono |
| 2. | QOOUNQ The symposium on hybrid nano
materials toward future industries Theoretical investigation of photorefractivity in liquid crystal cells with hybrid layered structure. N. M. Frias and H. Ono |
| 3. | QOOUNU@The 7th International Symposium on the 21th
Century COE Program of Nagaoka University of Technology, Bandung,
Indonesia Photoenhancement nonfaradaic processes in photorefractive nematic liquid crystal N. M. Frias and H. Ono |