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Metasurfaces, the two-dimensional (2D) sub-wavelength artificial structures, where light is not required to have a deep penetration, have shown the ability to tailor the amplitude, phase and polarization of light. The functionalities of various optical components can be realized by metasurface-based design, such as beam splitters, filters, waveplates, deflector, lens and holograms. Here, we propose a new type of metasurface based on the concept of ultra-thin film interference and experimentally demonstrate its feasibilities in beam deflector, light focusing and broadband meta-hologram in visible spectrum. Considering an ultra-thin thin film interference system, a sandwich structure, composed of air, a lossy material layer and a metallic mirror, the reflection of this system can be regarded as the linear superposition of the partial reflections from first interface and from the cavity after several roundtrips. First, we calculate the phases and reflections of various thicknesses of amorphous silicon (a-Si) on top of aluminum layer under normal illumination of an unpolarized light in the wavelength region from 400 to 850nm. 2 π phase coverage can be achieved by changing the film thickness of a-Si within 50 nanometers. We select two thicknesses (2-level phase modulation) for the demonstration of meta-devices. The ultra-flat grating metasurface for beam steering are designed. The reflection angles of grating metasurface can be modulated by changing its period, while the specular reflection is inhibited. We further demonstrate computer-generated holograms (CGH) based on ultra-thin interference metasurface. The holographic images are reconstructed by the combinations of phase- and amplitude- modulation. These devices show the great potential and CMOS-compatibility in the application of optics, display, security printing, and metasurface-based optical storage system.
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