Slot Waveguide Liquid Crystal Phase Shifters with Planar and Homeotropic Alignment

The use of liquid crystals (LC) as cladding in optical switching applications enables large phase shifts with low power consumption, due to the possibility of changing the orientation of LC’s molecules by applying an external electric field. As a result, variations of such a driving electric field, leads to changes in the effective refractive index of the LC and, consequently, in the optical phase at the output of such waveguides. Establishing an appropriate molecular organization inside the cells is thus fundamental for the function and optimization of such devices. In this work we present a detailed study based on Monte Carlo simulations of the distortions and spatial variations of the director of the LC obtained as a consequence of variations of a driving external field. A simple slot waveguide model was used for the scope, focusing our investigations on the consequences of using devices based on Silicon-Organic Hybrid (SOH) and Polydimethylsiloxane (PDMS). These two materials induce planar (SOH) and homeotropic (PDMS) alignment of the LC molecule at the waveguide surfaces. Different sizes of both the rail of the slot waveguide and of the whole system were also considered in order to study their effect on the optical behaviour of the device. To evaluate the ordering of the LC molecules, the Lebwohl-Lasher (LL) lattice model, which describes in a simple but efficient way the molecular interaction of a nematic system, was used. Moreover the external field was modelled by adding a term to the Hamiltonian which describes its coupling with the mesogenic molecules. The results are presented in terms of resulting optical patterns, along with relevant quantitative information like the transmitted light intensity variation on the applied electrical field strength.