Achromatic polarization gratings as highly efficient thin-film polarizing beamsplitters for broadband light

Abstract

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.