Theorie photoinduzierter bzw. -angeregter dynamischer Phänomene in farbstoffdotierten nematischen Flüssigkristallen
Final Report Abstract
We have investigated the impact of flow on the dynamical phenomena induced by light in pure and dye-doped nematic liquid crystals. The electric field component of light wave reorients the molecules of nematic liquid crystals if it is sufficiently intense. This causes a change in the optical properties of the medium (i.e. an re-orientation of the optical axes), so a great variety of nonlinear dynamical phenomena can be observed. At the same time, the rotatory motion of the molecules gives rise to fluid flow, which was not taken into account in previous theoretical studies of these phenomena. In the case of circularly polarized light incident perpendicularly on the nematic, we have found that flow does not alter the general bifurcation scenario, though it leads to a substantial quantitative shift of the bifurcation thresholds. In the case of linearly polarized light incident at a slightly oblique angle on the nematic, we have found that the inclusion of flow does change qualitatively the bifurcation scenario at moderate to high intensities. The predictions of our new theory agree very well with experimental observations with respect to the various bifurcations and dynamical regimes. In particular, our theory predicts a very uncommon route to chaos via a sequence of homoclinic gluing bifurcations in a certain parameter region wich are expected to be confirmed soon in corresponding experiments. We have also considered the generalization of light-induced nonlinear reorientation dynamics of nematics to the case of a dynamical forcing. As an example, we chose a situation of a homeotropic nematic liquid crystal film driven by circular polarized light beam at normal incidence with a periodically modulated intensity near a Hopf bifurcation. Our results are discussed in the general context of dynamical systems: Our system offers in fact an attractive way to investigate various general concepts of nonlinear dynamics. Furthermore we have studied theoretically the dynamical reorientation phenomena when a long-pitch cholesteric liquid crystal film with homeotropic alignment is illuminated by a circularly polarized light. The natural cholesteric pitch is chosen to be of the order of (or larger than) the film thickness. We have found much more complex dynamical phenomena than for pure nematics, which can be traced back to the competition between intrinsic and extrinsic one-dimensional helical patterns. The intrinsic part consists of the helical deformations induced by the chirality of the dopant, whereas the extrinsic part is related to the chirality induced by the optical field through the non-uniform angular momentum transfer of light to a nematic. We have also shown that thermomechanical effects are responsible for the lowering of the LIFT threshold in dye-doped nematic liquid crystals.
Publications
- D. O. Krimer, G. Demeter and L. Kramer, Phys. Rev. E 71, 051711 (2005)
- D. O. Krimer, G. Demeter, and L. Kramer, NATO Science Series "Self-assembly, pattern formation and growth phenomena in nano-systems" Eds: A.A.Golovin, A.A.Nepomnyashchy, Springer 2006, pp.83-122
- D. O. Krimer, L. Kramer, E. Brasselet, T. V. Galstian and L. J. Dube, J. Opt. Soc. Am. B, 22, 1681 (2005)
- E. Brasselet, D. O. Krimer, and L. Kramer, Europ. Phys. J. E , 17, 403 (2005)
- E. Brasselet, T. V. Galstian, L. J. Dube, D. O. Krimer and L. Kramer, J. Opt. Soc. Am. B, 22, 1671 (2005)
- G. Demeter, D. O. Krimer and L. Kramer, Phys. Rev. E 72, 051712 (2005)
