First optical observation of Kossel diagrams in chiral smectic phases

By means of optical microscopy, Kossel diagrams of chiral smectic phases are observed for the dirst time. Due to Bragg reflection, these diagrams occur in the focal plane of the microscope lens if the sample is illuminated with convergent monochromatic light. The Kossel diagram for the SmC* phase ― observed for the chiral compound CE3 (BDH) ― consists of a central ring corresponding to the periodicity p/2 of the |m|=2 mode of the tensor order parameter e(r) describing the helical structure. The temperature dependence of the pitch p(T) determined by the Kossel method is in agreement with spectroscopic investigations of the Bragg scattering in back reflection. The Kossel diagrams of the unknown chiral smectic structure SmM* consist also of one central ring; no additional reciprocal lattice vectors are observed in the visible wavelength range for this phase. In addition to the Kossel diagrams, we report on the observation of Airy spirals in the SmC* phase Au moyen d'un microscope polarisant en configuration conoscopique on etudie pour la premiere fois les diagrammes de Kossel des phases chirales SmC* et SmM*. Dans le cas de la phase SmC* on observe un seul anneau de Kossel correspondant a la periodicite p/2 de la composante tensorielle |m|=2 de la structure helicoidale du parametre d'ordre e(r). La variation du pas p(T) en fonction de la temperature, obtenue a partir des diagrammes de Kossel, est en accord avec des mesures faites independamment par reflexion de Bragg en incidence normale aux couches smectiques. Le diagramme de Kossel de la phase SmM* comporte aussi un seul anneau central. En plus d'anneau de Kossel, dans le cas de la phase SmC* on observe des spirales d'Airy