Holographic polymer-dispersed liquid crystals for photonics

In the frame of this project we will generate complex structures from photosensitive composites of polymers and liquid crystals (H-PDLC) by holographic methods. The optical properties of these materials are governed by nanodroplets of liquid crystals within the polymer matrix. One thus can, for instance, switch an H-PDLC from an opaque to a completely transparent medium for light by application of an external electric field. The colossal light- induced refractive-index change further makes it possible to create so called photonic crystals, i.e., to periodically modulate the dielectric susceptibility, so that band gaps for photons are generated (stop bands). We are targeting on three major aims in this project: (1) To investigate and understand the kinetics of phase separation between the polymer matrix and the liquid crystal droplets (2) To explore the relation between the structural properties on a nanometre scale and the optical properties in the submicrometre range (3) To optimise H-PDLCs for photonic applications ("all optical networking") and neutron-optic components To reach these goals, we will employ non-destructive in-situ techniques: light optical holographic such as two- wave-mixing, interferometric beam coupling experiments, holographic scattering and polarisation microscopy, as well as neutron diffraction and scattering from light-induced structures. The latter investigations are performed using a unique instrument (HoloNS), which allows simultaneous diffraction of neutrons and light during creation of the structures. The project will be run in cooperation with scientific institutes in Slovenia and Germany. The expected outcome will be: creation of novel nanostructured materials as well as new fundamental knowledge on them. These media will have an outstanding importance for optical technologies, in particular for the fabrication of next-generation photonic elements. Furthermore a new class of materials opens up for the construction of neutron optical components, maybe even for the production of neutronic crystals.

Holographic polymer dispersed liquid crystals (HPDLCs) are composites which adopt the advantages of both its constituents: they can be optically patterned and such structures then are stable, strongly anisotropic and electrically switchable. In the present project we targeted on exploring the relationship between structure and macroscopic properties, the details of the photopolymerization kinetics and phase separation mechanism, on characterizing as well as understanding the peculiar properties of those structures, and to exploit them for holographic optical elements (e.g., photonic crystals) and neutron optical elements. Three major scientific advances were made: we found that (1) the phototriggered kinetics occurs in two steps with a long term polymerization that prevails in the dark, (2) thick anisotropic gratings in HPDLCs are overcoupled with relevant contributions of both, phase and extinction modulation, and (3) such gratings are extremely efficient diffractive elements for cold neutrons. We prepared the composite material according to the formulations given in literature. They were inserted between two glass plates with unusually thick spacers of 50 -100 micrometres and subsequently exposed to a holographic interference pattern. Due to the photosensitivity of HPDLCs we recorded gratings of various spacings (400 - 2000 nm). The formation kinetics were studied by light-optical diffraction, interferometric beam-coupling and electron paramagnetic resonance techniques. Temperature and electric field dependence of the structures were investigated with respect to their change in optical anisotropy and diffraction efficiency. Extremely overcoupled complex gratings were obtained whose significant parameters could be extracted by employing a newly developed model. A particular emphasis was laid on the application of HPDLCs in neutron optics. When starting the project, we anticipated that by choosing proper thickness, grating spacing and exposure, HPDLCs could serve as beam-splitters for cold neutrons. This claim was finally put into practice by using a material in which the liquid crystal component was replaced by nanoparticles. The project`s outcome led to more insight into the mechanisms of grating formation, their peculiarities under various external conditions, and a profound perspective for applying these composite materials in neutron optics. Part of the results were obtained in close collaboration with groups of scientists from the J. Stefan-Institute in Ljubljana, Slovenia.