Dynamic control of the optical properties of a liquid crystal waveguide by means of an applied electric field

The application of Liquid Crystals to integrated optics is actively investigated because the mechanical and optical properties of these materials can be exploited to realize new schemes of opto-electronic devices. In particular the molecular alignment induced in nematic liquid crystals by external electric fields can be very useful for mode splitting, tunable filtering and optical switching. The aim of this work is to experimentally demonstrate the possibility of dynamically reshaping the refractive index profile of a planar waveguide, and hence the propagation characteristics of the modes, by applying a small external electric field. Graded index planar waveguides have been realized by using a nematic liquid crystal in a distorted hybrid configuration as the guiding layer. The application of external electric fields enables to modify the orientation of the local molecular axis and hence to reshape in an easy and controlled way the refractive index profile. Extensive m-lines spectroscopy measurements were performed in order to determine the anisotropic refractive index profiles for various electric field strengths. The experimental results are in good agreement with what expected from a simple model based on the dielectric, optical and elastic properties of the liquid crystal guiding layer.