Combined interface plasmon polariton and x-ray reflectivity determination of the dielectric tensor in ultrathin liquid crystal films

Abstract

We have carried out interface plasmon polariton (IPP) and specular x-ray reflectivity studies of the same multilayer structure containing a thin metallic silver film and a thin (100–600 Å) smectic C* ferroelectric liquid crystal (FLC) film on a glass substrate. An additional thin nylon layer sandwiched between these two films is essential to improve the smectic FLC alignment and its stability. The specular x-ray reflectivity after each stage of layer deposition provides information on the thickness and the electron density of the individual layer. Particularly, this technique allows for the determination of the amplitude and the phase of the electronic density modulation (i.e., the smectic order parameter) of the FLC film. We demonstrate that the x-ray results are essential for the determination of the complex dielectric functions of the FLC film from the shape and the angular position of the IPP resonance. Generally, we have found that for samples with a relatively large smectic order parameter the IPP resonance can be best fitted assuming an anisotropic dielectric tensor and the presence of domains in the FLC film. For well ordered smectic C* films at room temperature, the anisotropy of the dielectric tensor is slightly higher compared to the bulk value and shows a tendency to increase with the decrease of the film thickness in ultrathin films.