Doubling the geometric phase of reflective Pancharatnam–Berry diffractive waveplates

Abstract

Cholesteric liquid crystals, a liquid crystal phase with a helical modulation of the anisotropic optic axis, function as holographic optical elements, whereby Bragg reflected light is phase modulated by the Pancharatnam–Berry phase acquired in proportion to twice the spatial phase of helical structure. One of the limitations of such holographic optical elements is that a π rotation in the helix phase is required to modulate the optical phase by 2π. Here, we propose a double-layered reflective structure comprising uniaxial and helical anisotropic materials that can modulate the reflected optical phase twice as much compared to conventional elements. We present results of numerical simulations and confirm them through the fabrication of two devices: (i) a reflective deflector and (ii) an optical vortex generator, in which a two-fold increase is achieved in diffraction angle and topological charge of the generated optical vortex beam, respectively. The proposed device structure should be useful for use in compact catoptric optical systems.