Light-induced instabilities driven by competing helical patterns in long-pitch cholesterics
Laboratoire de Physique UMR 5672, École Normale Supérieure de Lyon, 46 Allée d’Italie, 69364, Lyon Cedex 07, France
2 Physikalisches Institut der Universität Bayreuth, D-95440, Bayreuth, Germany
Accepted: 11 May 2005
Published online: 26 July 2005
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.
PACS: 42.70.Df Liquid crystals – / 42.65.Sf Dynamics of nonlinear optical systems; optical instabilities, optical chaos and complexity, and optical spatio-temporal dynamics –
© EDP Sciences, Società Italiana di Fisica and Springer-Verlag, 2005