Electro-optics of antiferroelectric orthoconic reflective displays

The contrast of antiferroelectric liquid crystal (AFLC) displays working on transmissive mode is degraded by the presence of pretransitional effect. Contrast is significantly enhanced by the use of orthoconic materials, i.e., AFLC mixtures with 45° smectic cone half angle. However, current orthoconic materials usually show short helical pitch (<1 μm), what hinders the surface stabilization of the material with standard alignments. Indeed, cell thickness should be small compared to helical pitch, in order to surface-stabilize the material. Cell thickness nonetheless cannot be arbitrarily chosen, since AFLCs behave as linear retardation plates whose performance is a function of optical path. In the case of transmissive cells, thickness is fixed about 1.5 - 2 μm, i.e., wider than helical pitch. As an alternative, the use of reflective cells has been proposed. In these cells, the optical path is doubled; therefore, the same optical performance can be obtained, in principle, with 0.8 - 1 μm reflective cells than with transmissive cells twice as thick, whereas surface-stabilization is improved. In this work, the electro-optical behavior of orthoconic reflective cells is studied. Multiplexed seven-level driving schemes have been employed for dynamic analysis of orthoconic displays, allowing video-rate multiplexed analogue grayscale.