Abstract
We experimentally demonstrate nearly ideal liquid crystal (LC) polymer Bragg polarization gratings (PGs) operating at a visible wavelength of 450 nm and with a sub-wavelength period of 335 nm. Bragg PGs employ the geometric (Pancharatnam-Berry) phase, and have many properties fundamentally different than their isotropic analog. However, until now Bragg PGs with nanoscale periods (e.g., < 800 nm) have not been realized. Using photo-alignment polymers and high-birefringence LC materials, we employ multiple thin sublayers to overcome the critical thickness threshold, and use chiral dopants to induce a helical twist that effectively generates a slanted grating. These LC polymer Bragg PGs manifest 85-99% first-order efficiency, 19-29° field-of-view, Q ≈ 17, 200 nm spectral bandwidth, 84° deflection angle in air (in one case), and efficient waveguide-coupling (in another case). Compared to surface-relief and volume-holographic gratings, they show high efficiency with larger angular/spectral bandwidths and potentially simpler fabrication. These nanoscale Bragg PGs manifest a 6π rad/μm phase gradient, the largest reported for a geometric-phase hologram while maintaining a first-order efficiency near 100%.
Highlights
IntroductionPolarization Gratings (PGs) are diffractive optical elements [1,2,3] comprising an in-plane anisotropy orientation that varies linearly along a surface, with a fixed anisotropy magnitude
Polarization Gratings (PGs) are diffractive optical elements [1,2,3] comprising an in-plane anisotropy orientation that varies linearly along a surface, with a fixed anisotropy magnitude. They can be formed by azobenzene-containing polymers [1, 4], "form-birefringence" in isotropic materials via high frequency surface profiles [5, 6], plasmonic metasurfaces [7], and photo-aligned bulk liquid crystals (LCs) [8,9,10,11], and operate via the geometric phase [12,13,14,15]
The earliest report of a Bragg PG was an experimental study [4] employing a azo-polymer with low birefringence Δn (∼ 0.01), achieving ≥ 90% diffraction efficiency, with Λ = 2 μm, λ = 633 nm, d = 100 μm, θB = 9.3◦, and Q ≈ 64
Summary
Polarization Gratings (PGs) are diffractive optical elements [1,2,3] comprising an in-plane anisotropy orientation that varies linearly along a surface, with a fixed anisotropy magnitude They can be formed by azobenzene-containing polymers [1, 4], "form-birefringence" in isotropic materials via high frequency surface profiles [5, 6], plasmonic metasurfaces [7], and photo-aligned bulk liquid crystals (LCs) [8,9,10,11], and operate via the geometric phase [12,13,14,15]. Subsequent prior art [24, 25] used numerical simulation and theoretical analysis to predict more generally that PG diffraction efficiency as high as 100% is possible in the Bragg regime for circularly polarized input and oblique incidence
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