Metasurface-empowered spectral and spatial light modulation for disruptive holographic displays

The holographic display, one of the most realistic ways to reconstruct optical images in three-dimensional (3D) space, has gained a lot of attention as a next-generation display platform for providing deeper immersive experiences to users. So far, diffractive optical elements (DOEs) and spatial light modulators (SLMs) have been used to generate holographic images by modulating electromagnetic waves at each pixel. However, such architectures suffer from limitations in terms of having a resolution of only a few microns and the bulkiness of the entire optical system. In this review, we describe novel metasurfaces-based nanophotonic platforms that have shown exceptional control of electromagnetic waves at the subwavelength scale as promising candidates to overcome existing restrictions, while realizing flat optical devices. After introducing the fundamentals of metasurfaces in terms of spatial and spectral wavefront modulation, we present a variety of multiplexing approaches for high-capacity and full-color metaholograms exploiting the multiple properties of light as an information carrier. We then review tunable metaholograms using active materials modulated by several external stimuli. Afterward, we discuss the integration of metasurfaces with other optical elements required for future 3D display platforms in augmented/virtual reality (AR/VR) displays such as lenses, beam splitters, diffusers, and eye-tracking sensors. Finally, we address the challenges of conventional nanofabrication methods and introduce scalable preparation techniques that can be applied to metasurface-based nanophotonic technologies towards commercially and ergonomically viable future holographic displays.