Uniform pupil expansion of full-color AR waveguides enabled by transfer-based PVGs on sapphire substrates

We demonstrate a full-color augmented reality waveguide system that integrates polarization volume gratings (PVGs) onto sapphire substrates, which function both as optical waveguides and as a retarder to improve pupil expansion uniformity. Unlike conventional systems requiring thickness modulation or additional retarders, our approach leverages the birefringence of sapphire to induce in-plane polarization transformation after total internal reflection, enabling uniform out-coupling across the eyebox. To mitigate disruption of circular polarization during photoalignment, which is caused by the strong birefringence of sapphire substrates, we employ a water-assisted lift-off method combined with a PDMS-assisted delamination process to relocate PVG structures from glass onto sapphire with high fidelity. The resulting PVG-integrated sapphire substrates are used to construct a full-color architecture, with two sapphire substrates designed for red and cyan spectral bands. Experimental results demonstrate consistent image brightness across the eyebox with a 30° field of view, confirming improved spatial uniformity in out-coupling efficiency enabled by the sapphire waveguides.