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Soft Matter and Biological Physics
Eur. Phys. J. E 3, 21-28 (2000)
DOI: 10.1007/s101890070037

Optical studies in liquid crystals using the high-accuracy universal polarimeter

J. Ortega1, J. Etxebarria2 and C.L. Folcia2

1  Departamento de Física Aplicada II, Facultad de Ciencias, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain
2  Departamento de Física de la Materia Condensada, Facultad de Ciencias, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain

wmporapj@lg.ehu.es
wmpetecj@lg.ehu.es

(Received 13 September 1999 and Received in final form 25 January 2000)

Abstract
The so-called high-accuracy universal polarimeter (HAUP) is introduced as a tool to study optical properties of liquid crystals. It is shown that besides homogeneous crystals, the HAUP method also permits to investigate inhomogeneous materials, provided that the optical parameters vary slowly on a wavelength scale. The technique is especially useful when small effects are sought. Measurements are presented for several twisted nematic samples. Twist angles are measured with high accuracy and non-adiabatic effects for light propagation, such as apparent indicatrix rotations or effective gyrations, are easily detected. The results agree well with the predictions of a first-order perturbation treatment for light propagation through a stratified medium. Finally, the changes of the director profile are analyzed in a planar sample that presents a chiral nematic to smectic-A transition. The experimental data indicate that if the rubbing directions of the cell are not strictly parallel, the molecules at the surfaces undergo small twists in both phases, and the twist parameters are determined.

PACS
61.30.-v - Liquid crystals.
78.20.-e - Optical properties of bulk materials and thin films.
64.70.Md - Transitions in liquid crystals.

© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2000