Tunable optical diffractive structures from liquid crystalline materialsBošnjaković, Dejan (Avtor) Drevenšek Olenik, Irena (Mentor) nematic liquid crystalsferromagnetic materialsphotoresist polymeric materialslaser-based micro structuringoptical diffractive structurestransmission gratingssurface anchoring energyThis thesis is linked to parallel experimental research conducted by researchers in the Physics of complex matter group. It is focused on the theoretical modelling of electric and magnetic tuning of optical diffractive properties of grating structures fabricated as periodic configurations of a liquid crystal (LC) and a polymer. Due to the surface relief present on the sidewalls of the polymer ribbons, which is generated during their fabrication by the direct laser writing (DLW) process, the LC-director is oriented along the grooves in the relief. With the help of an external electric or magnetic field, the orientation of the LC medium between the ribbons can be changed, and, as a result, the optical properties of the structure are modified. For a theoretical description of optical properties, it is necessary to solve Maxwell's equations in a medium with periodic spatial modulation of optical dielectric permittivity tensor. We performed a numerical procedure based on the rigorous coupled-wave analysis (RCWA). The calculations began by modelling "empty" gratings, in which the channels between the polymer ribbons are filled with air. With this, we determined the parameters of the polymer scaffolds that best reproduced the experimental data. Then, we modelled structures in which the magnetic field caused a reorientation of a ferromagnetic LC in the plane of the grating. And also structures in which the electric field caused a reorientation of the LC in the direction perpendicular to it. Initially, numerical calculations were performed with the assumption that the reorientation of the LC medium between two polymer ribbons takes place homogeneously, thus neglecting the details related to surface anchoring. We then upgraded the simulations by calculating the spatially dependent orientational structure of the LC as a function of the applied electric or magnetic field by minimizing the Landau-de Gennes free energy, including the surface anchoring energy. These calculations were performed numerically and also analytically. Analytical results were obtained using the one-constant approximation and the approximations of the high and the low applied fields. The obtained theoretical results for electrically and magnetically tunable gratings agree very well with the experimental results. Therefore, the developed methodology provides an efficient tool for designing diffractive optical elements (DOEs) based on LC and polymers. It also enables simulations of their operation controlled by external fields. At the end of the thesis, we also show that the developed methodology can be utilized to analyse the operation of gratings with a hybrid control, in which active regulation takes place with an electric and a magnetic field. This type of configuration is very interesting for usage in practical devices, as it provides much shorter switching times than standard LC-based optical gratings that typically rely on applying only one active switching process.20232023-04-15 08:15:02Doktorsko delo/naloga145304VisID: 131197COBISS_ID: 149366275sl