Double-Layered Metasurfaces Fabricated by Single-Step Lithography for Efficient Phase Control

A metasurface is an ultrathin-layered artificial material which consists of an array of nanoscaled optical element and is capable of manipulating light by adjusting the phase in subwavelength scale. Such structures are typically fabricated by cascaded CMOS (Complementary Metal-Oxide-Semiconductor) processes including deposition, etching, and lift-off techniques. Recently, as the demand for optical functionality increases, multiple metasurfaces are designed and integrated in a cascaded fashion which, as a trade-off, causes more fabrication complexity and cost. In this article, we present the double-layered metasurface for phase and wavefront control using dielectric materials, which can be easily fabricated by a single lithographic step without the need for additional etching or lift-off process. The proposed metasurface consists of amorphous silicon (a-Si) nanobeams acting as phase retarders based on the Mie-type resonance and an additional polymer layer serving as nanostructured spacers between the constituent a-Si layers. We analyze the effect of actual fabrication techniques on the performance of our proposed metasurface and demonstrate that the double-layered phase gradient metasurface (PGM) deflects light at approximately twice the angle compared to that of single-layered PGM. Furthermore, we extend our study to double-layered metalenses and achieve a high numerical aperture (NA) exceeding 0.9. The proposed design concept demonstrates the potential for enhancing light manipulation and paves the way for the development of advanced optical devices with improved performance and a simplified fabrication process.