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We introduce and experimentally demonstrate an achromatic polarization grating (PG), which manifests high diffraction efficiency (> 99.5%) over a broad range of spectrum. Unlike conventional phase gratings, this family of PGs has unique diffraction properties including three non-zero diffraction orders (m = 0,±1) with up to 100% efficiency and strongly polarization sensitive first-order diffraction. It has long been recognized that these diffractive optical elements are useful for beamsplitting, polarimetry, displays, and more. A conventional (Circular-type) PG implemented with a spiraling, in-plane, linear birefringence has a modest spectral range (Δλ/λ0 congruent to 6.8%) over which it possesses > 99.5% efficiency. We have identified a two-layer twisted PG structure that achieves achromatic diffraction that achieves a five-fold improvement of the high efficiency bandwidth (Δλ/λ0 congruent to 34.3%). We have successfully implemented this structure with reactive mesogens (polymerizable liquid crystals) with a small amount of left- and right-hand chiral agents, and here report on its operation over nearly the entire range of visible light. We also investigated the behavior of the achromatic PG with the finite-difference time-domain method using an Open Source software package WOLFSIM, developed at NC State University, in order to evaluate the angular selectivity and the paraxial limit.
Oh et al. (Fri,) studied this question.