Spatially twisted liquid-crystal devices

Nematic liquid-crystal devices are a powerful tool to structure light in different degrees of freedom, both in classical and in quantum regimes. Most of these devices exploit the possibility of introducing a position-dependent phase retardation either with a homogeneous alignment of the optic axis—e.g., liquid-crystal-based spatial light modulators—or, conversely, with a uniform but tunable retardation and patterned optic axis, e.g., q-plates. The pattern is the same in the latter case on the two alignment layers. Here, a more general case is considered, wherein the front and back alignment layers are patterned differently. This creates a non-symmetric device, which can exhibit different behaviors depending on the direction of beam propagation and effective phase retardation. In particular, we fabricate multi-q-plates by setting different topological charges on the two alignment layers. The devices have been characterized by spatially resolved Stokes polarimetry, with and without applied electric voltage, demonstrating new functionalities.