Abstract
Optical films and surfaces using geometric phase are increasingly demonstrating unique and sometimes enhanced performance compared to traditional elements employing propagation phase. Here, we report on a diffraction grating with wider angular bandwidth and significantly higher average first-order efficiency than the nearest prior art of metasurfaces, volume holographic gratings, and surface-relief gratings configured to achieve a steep deflection angle. More specifically, we demonstrate a liquid crystal (LC) polymer Bragg polarization grating (PG) with large angular bandwidth and high efficiency in transmission-mode for 532 nm wavelength and 400 nm period. Angular bandwidth was significantly increased by arranging two slanted grating layers within the same monolithic film. First, we studied the optical properties with simulation and identified a structure with 48° angular bandwidth and 70% average first-order efficiency. Second, we fabricated a sample using a photo-aligned chiral nematic LC, where the two grating slants were controlled by the chiral dopants. We measured 40° angular bandwidth, 76% average efficiency, and 96% peak efficiency. Strong input polarization sensitivity (300:1 contrast) and spectral bandwidth (200 nm) mostly matched prior PGs. This approach is especially advantageous for augmented-reality systems and nonmechanical beam steering.
Highlights
Deflecting light to large angles with a thin transmissive optical element is challenging to do with high efficiency for a wide range of incident angles and wavelengths[1]
We aim to increase the angular response of Bragg polarization grating (PG) by creating two or more slants within the same physical grating, similar to Volume holographic gratings (VHGs)
While we cannot superimpose gratings within the same volume, we can layer them because the last liquid crystal polymer (LCP) surface spontaneously aligns subsequent layers
Summary
Deflecting light to large angles with a thin transmissive optical element is challenging to do with high efficiency for a wide range of incident angles and wavelengths[1]. We demonstrate a Bragg polarization grating with sub-wavelength period and significantly increased angular bandwidth, in both simulation and experiment. This element maintains high first-order efficiency, wide spectral bandwidth, and strong polarization sensitivity. One important distinction amongst the four grating technologies cited above relates to their fundamental principle of diffraction: Bragg PGs function via the geometric phase, while the others (VHGs, SRGs, and nanobeam metasurfaces) function via the propagation phase[18]. General behavior distinct from the latter, including stronger input polarization sensitivity and high single-order efficiency with often lower higher-order leakages
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