Optical spin-symmetry breaking for high-efficiency directional helicity-multiplexed metaholograms

Helicity-multiplexed metasurfaces based on symmetric spin–orbit interactions (SOIs) have practical limits because they cannot provide central-symmetric holographic imaging. Asymmetric SOIs can effectively address such limitations, with several exciting applications in various fields ranging from asymmetric data inscription in communications to dual side displays in smart mobile devices. Low-loss dielectric materials provide an excellent platform for realizing such exotic phenomena efficiently. In this paper, we demonstrate an asymmetric SOI-dependent transmission-type metasurface in the visible domain using hydrogenated amorphous silicon (a-Si:H) nanoresonators. The proposed design approach is equipped with an additional degree of freedom in designing bi-directional helicity-multiplexed metasurfaces by breaking the conventional limit imposed by the symmetric SOI in half employment of metasurfaces for one circular handedness. Two on-axis, distinct wavefronts are produced with high transmission efficiencies, demonstrating the concept of asymmetric wavefront generation in two antiparallel directions. Additionally, the CMOS compatibility of a-Si:H makes it a cost-effective alternative to gallium nitride (GaN) and titanium dioxide (TiO2) for visible light. The cost-effective fabrication and simplicity of the proposed design technique provide an excellent candidate for high-efficiency, multifunctional, and chip-integrated demonstration of various phenomena. Metasurfaces are thin films composed of different elements that provide an efficient, miniaturized platform for nano-optics, and a metasurface based on asymmetrical SOIs has been developed that overcomes the limitations with metasurfaces using symmetrical SOIs. Owing to their ability to accommodate the wavefronts of light at the sub-wavelength scale, metasurfaces have found wide application in displays, communications, and data storage. However, metasurfaces based on symmetrical SOIs have limitations with holographic imaging. A team headed by Junsuk Rho at Pohang University of Science and Technology (POSTECH), South Korea has succeeded in designing a metasurface based on asymmetrical SOIs that operates in the visible light domain using low-loss dielectric materials. The team employed a simple, cost-effective fabrication method, and the design offers considerable potential for applications in such areas as asymmetric data inscription and smartphone displays.