Abstract:In this review paper, we report recent progress on Pancharatnam-Berry (PB) phase optical elements, such as lens, grating, and de ector. PB lenses exhibit a fast switching time between two or more focal lengths with large diopter change and aperture size, which is particularly attractive for addressing the accommodation mismatch in head-mounted display devices. On the other hand, PB gratings and de ectors o er a large-angle beam de ection with wide acceptance cone and high e ciency, as compared to conventional volume gratings. Such merits provide great advantages for waveguide-coupling augmented reality headsets. Moreover, the thickness of PB optical elements is only a few micrometers, thus they can be conveniently integrated into modern wearable display systems.

We propose a polarization volume grating (PVG), which exhibits nearly 100% diffraction efficiency and large diffraction angle. Both reflective and transmissive PVGs can be configured depending on application preference. Such a PVG is polarization-sensitive so that it can split an incident unpolarized beam into two well-separated yet polarized beams. These outstanding features make PVG a strong candidate for photonic and display applications. To investigate and optimize the diffraction properties, we build a rigorous simulation model based on finite element method. To illustrate its potential applications, we propose a simple 2D/3D wearable display using a planar waveguide comprising of two reflective PVGs.

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We proposed a switchable beam steering device with cycloidal diffractive waveplate (CDW) for eye tracking in a virtual reality (VR) or augmented reality (AR) display system. Such a CDW diffracts the incident circularly polarized light to the first order with over 95% efficiency. To convert the input linearly polarized light to right-handed or left-handed circular polarization, we developed a broadband polarization switch consisting of a twisted nematic liquid crystal cell and an achromatic quarter-wave retardation film. By cascading 2-3 CDWs together, multiple diffraction angles can be achieved. To suppress the color dispersion, we proposed two approaches to obtain the same diffraction angle for red, green, and blue LEDs-based full color displays. Our device exhibits several advantages, such as high diffraction efficiency, fast response time, low power consumption, and low cost. It holds promise for the emerging VR/AR displays.

We propose a polarization multiplexing structure based on multilayer reflective polarized volume holographic gratings(PVGs) to improve the field of view and brightness of the augmented reality waveguide display. The multilayer structure forms the splicing of different response bandwidths by stacking PVGs with different periodic components, and realizes the expansion of the wavelength (angle) bandwidth. The polarization multiplexing structure controls the polarization of the diffracted light by controlling the rotation direction of the liquid crystal pitch in the liquid crystal material, so that both left-hand and right-hand circularly polarized light are diffracted to enhance efficiency. Based on these two structures, the wavelength bandwidth of PVG is increased by 40 nm, the angular bandwidth is increased by 10°and the diffraction efficiency is nearly doubled. In order to verify the feasibility of these two structures, we use the holographic waveguide display with OLED as the image source. The demonstrated waveguide prototype shows a complete display with a diagonal field of view of 55°. The brightness of virtual image was measured as high as 1100 cd/m 2 with a transparency of 72% for ambient light.