Brasselet E, Krimer D O, Kramer L
Laboratoire de Physique UMR 5672, Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon Cedex 07, France.
Eur Phys J E Soft Matter. 2005 Aug;17(4):403-11. doi: 10.1140/epje/i2005-10017-7. Epub 2005 Jul 26.
We study theoretically the dynamical reorientation phenomena when a long-pitch cholesteric liquid-crystal film with homeotropic alignment is illuminated by a circularly polarized lightwave. In the present case, the natural cholesteric pitch is of the order of (or larger than) the film thickness. The helical cholesteric structure is thus frustrated by the boundary conditions without illumination. However, above a light intensity threshold reorientation occurs and the bifurcation scenario depends strongly on the natural cholesteric pitch. Recalling that a long-pitch cholesteric is achieved in practice by adding a small amount of chiral agents in a nematic liquid crystal, the observed dynamics can be viewed as the result of the competition between intrinsic and extrinsic unidimensional 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. The all-optical analog in the case of a pure nematic (without chiral dopant), is also discussed.
我们从理论上研究了向列取向的长螺距胆甾相液晶薄膜被圆偏振光波照射时的动态重取向现象。在当前情况下，天然胆甾相螺距与薄膜厚度相当（或大于薄膜厚度）。因此，在无光照时，螺旋状胆甾相结构会受到边界条件的限制。然而，当光强度超过阈值时会发生重取向，并且分岔情况强烈依赖于天然胆甾相螺距。回想一下，在实践中，通过向向列相液晶中添加少量手性剂可实现长螺距胆甾相，观察到的动力学可以看作是内在和外在一维螺旋模式之间竞争的结果。内在部分由掺杂剂的手性诱导的螺旋形变组成，而外在部分与光场通过光向向列相的非均匀角动量转移所诱导的手性有关。我们还讨论了纯向列相（无手性掺杂剂）情况下的全光模拟。
