Double Bragg Diffraction by Thin Liquid Crystal 2D Grating

The diffraction properties of a 2D grating realized with heliconical cholesteric liquid crystal (ChOH) have been studied by extended computer simulation using the finite element method. In particular, the dependence of the diffraction efficiency and the positions of various reflection orders arising in a 2D ChOH on its geometric parameters—such as the transverse period, sample thickness, and the types of polarization of the incident radiation—has been investigated in detail. The structure is made by a periodic alternation of stripes with different pitches producing the transversal grating, while the longitudinal grating is given by the heliconical order of the material. It is shown that a double Bragg diffraction originates from the 2D grating with maximum efficiency for circularly polarized light with the same handedness as the liquid crystal helix. Unique optical properties of this structure have been highlighted. Spectral and angular symmetry of plus/minus orders of diffraction is broken by a small increase in the angle of incidence, while mutual tunability of the double diffraction is achieved through the applied electric field. We also highlight a peculiar feature of this structure: by varying the applied field, it is possible to tune one diffraction resonance with respect to the other one and to switch off the double diffraction.