The alignment of nematic liquid crystals (NLCs) on solid substrates is one of the key factors in electro-optic devices. The control of surface orientation of NLCs with the help of polarized light is not only an alternative, contactless method to ensure proper alignment at the cell boundaries, but – because of its reversible character – also opens up new possibilities of applications (rewritable displays, dynamic holography etc.), as well as may lead to unexpected phenomena, such as light-induced dynamic instabilities .
The proposed work targets basic research related to the photoalignment at the liquid crystal-polymer interface. Photoalignment in NLCs is known for decades, and for a given system, the basic mechanisms are well determined in two dimensions (either in-plane, or out-of-plane, depending on the given system, mainly optimized to the desired type of photoalignment). However, the knowledge about the general, three-dimensional mechanism of the process is extremely scarce. The proposed work intends to make significant contribution in this particular field, based on certain recent observations, which indicate that the present description of photoalignment is incomplete .
The principal aspects of the research are the study of the in-plane versus out-of-plane photoinduced reorientation of the nematic director, and the influence of the nematic liquid crystal (NLC) on the process. The laser-induced reorientation in LC cells with a pump-probe technique are planned for different parameters of the system, such as chemical composition of the photosensitive layer, clearing temperature of the NLC, and the glass transition temperature of the azo-dye-assisted polymer. In addition, the properties of the orienting polymer film, like thickness or smoothness, and their influence on photoalignment will also be examined. These investigations are to be completed with the measurement of the azimuthal and zenithal anchoring energies using external electric or magnetic fields.
The work is expected to deliver results that can be further used in investigations beyond the PhD programme, e.g., to extend the two-dimensional model (worked out by our group earlier to explain the in-plane reorientation) to three-dimensions that includes the out-of-plane reorientation, or in exploitation of the observed effects in microfluidics through the control of NLC-flow with polarized light by opto-convection, or through the light-assisted flow steering.
 I. Jánossy, K. Fodor-Csorba, A. Vajda, and T. Tóth Katona,
Laser-induced instabilities in liquid crystal cells with a photosensitive substrate.
Phys. Rev. E89, 012504/1-6 (2014).
 I. Jánossy, T. Tóth-Katona, T. Kósa, and L. Sukhomlinova,
Super-twist generation and instabilities in photosensitive liquid crystal cells.
J. Mol. Liq. AiP (2018). DOI: 10.1016/j.molliq.2017.12.071
MSc degree in physics, or related fields, general interests in soft matter physics and/or optics, good communication skills in English, affinity for experimental work.