Periodic alignment of liquid crystal molecules in silica micro-capillaries

The micro–holes which form structures in photonic crystal fibers may be filled with various substances. The usage of liquid crystals as filling materials allows to take advantage of their ability to reorient and to dynamically change the refractive index, and consequently to modify the propagation characteristics. The main property of the liquid crystal molecules is an ability to the long-range ordering. The direction of their arrangement can be changed by different techniques. One of the methods to make alignment textures is the photo-alignment technique. Photo-alignment method, based on photo-orientation of molecules in LC cells, appears to be potentially the best technique to use in the case of micro-capillaries. It results from the fact that in this technique the direct access to the ordering layer is not required. The procedure of photo–alignment in micro–capillaries consists in preparing thin layers of material inside the micro-holes which acquire orienting properties after irradiation with linearly polarized light beam of appropriate wavelength. Molecules orientation depends on the polarization of the laser beam used during irradiation of the polymer layer. The usage of amplitude masks in the irradiation allows to obtain a periodic alignment of molecules variable within a single sample. In this case, a given orientation is forced on equal areas in constant distances. The limit of resolution of the amplitude mask, for which the molecules of the liquid crystal in micro–capillaries with polymer layer inside order in required direction has been determined experimentally. The possibility to make alignment textures in micro–holes which form structures in photonic crystal fibers will allow for reproducible sample preparation as well as obtaining new, currently unreachable propagation effects. In particular, the production of long period fiber gratings on the basis of fibers filled with liquid crystals will be possible.