Equalization of RGB coupling efficiencies of metasurface waveguide coupler by adjusting imaginary part of refractive index

Optical metasurfaces offer high-efficiency and flexible wavefront shaping for near-eye displays, especially in wideband waveguide couplers accommodating RGB primary colors. By leveraging the resonance characteristics of sub-wavelength periodic nanostructures, metasurfaces surpass the limitations of traditional optics that rely on multiple components and mediums. In this study, we propose adjustment of the imaginary parts of the material refractive indices as a new method to achieve balanced first-order diffraction efficiencies among RGB colors over a wide field of view (FOV) in an in-coupling metasurface waveguide coupler. Physical mechanism is investigated deeply and systematically in theory. It is found that nanostructure resonances deflect the wavefront and Poynting vector, significantly enhancing first-order diffraction efficiency, while resonance-enhanced absorption plays a crucial role in balancing the diffraction efficiency of RGB primary colors. First experimental demonstration well confirms the practical feasibility of this method and a uniform first-order diffraction efficiency of approximately 20% is achieved among RGB colors across a FOV as large as similar to 30 degrees over a single-piece glass substrate. This research provides insights into the design and mechanisms of metasurface waveguide couplers, advancing our understanding of metasurface-based RGB displays and facilitating further advancements in this field.