Finite elements numerical analysis of the dynamic response of POLICRYPS gratings

Recently an improved technique was introduced in order to realize permanent switchable liquid crystal (LC) volume gratings in polymer/LC composite mixtures. By ensuring that diffusion takes place faster than actual polymerization, researchers succeeded into fabricating grating structures in which well-defined polymer slices alternate with uniformly aligned slices of LC material. The new gratings, which are know as POLICRYPS (Polymer-Liquid-Crystal-Polymer-Slices) or POLIPHEM (Polymer-Liquid-Crystal-Polymer-Holograms-Electrically-Manageable) have the potential of very high diffraction efficiency with lower driving voltages compared to their H-PDLC counterparts [3]. Some initial works on electro-optical assessment of POLICRYPS gratings in the visible and infrared wavelengths have been reported so far, but until now no robust model that accurately describes their switching mechanism has been presented. In this communication, we present for the first time investigations into the switching dynamics of POLICRYPS-like geometries using numerical techniques based on finite elements analysis. We use software tools developed by the UCL modeling group (during European project MonLCD) to build a 2D rectangular model of the LCslice sandwiched between two polymer slices. The liquid crystal 5CB and the polymer NOA61 (Norland) are considered in our calculations. Our model is extended to account for anchoring of the liquid crystal material at the interface with the vertical polymeric walls running normal to the grating vector. Both strong and weak vertical anchoring cases are considered. Starting from the Oseen-Frank expression of the elastic energy, the switching-on and switching-off times of the LC slices are calculated by numerically solving the corresponding energy dissipation equation.