Coupled surface plasmons and optical guided wave exploration of near-surface director profile

For a liquid crystal (LC) cell with thin silver claddings it is possible, using a high index coupling prism, to excite both surface plasmon modes and ordinary optical guided modes. In a situation where the tilt of the director varies from homogeneous to homeotropic through the cell, then for p-polarized incident radiation the p-polarized surface plasmon mode and the ordinary guided waves may couple to each other. When the plane containing the director is normal to the incident plane, there is also polarization conversion leading to strong coupling between the p-polarized surface plasmon and s-like guided modes. From theoretical analyses together with numerical modelling it is shown how this coupling between the surface plasmon mode and guided waves gives a high sensitivity to the surface director tilt profile near the walls, higher than that of the surface plasmon mode alone. Experimental confirmation of this has been realized using a hybrid aligned nematic (HAN) LC cell with the director in a plane normal to the incident plane. The results fully confirm the model predictions showing that this coupling of surface plasmons to guided waves provides a powerful tool for near-surface director studies.