Bifunctional metamirrors for simultaneous polarization splitting and focusing

Abstract

Metasurfaces – artificial 2D sheet structures with sub-wavelength periodicity and dimensions of elements – are paving the way to improve traditional optical components by integrating multiple functionalities into one optically flat device. With the progress in nano-fabrication methods, different applications of metasurfaces were demonstrated experimentally, ranging from artificial plasmonic colouring to flat optical components. In this work, we demonstrate implementation of a bifunctional gap-surface-plasmon-based metasurface which, in reflection mode, splits orthogonal linear light polarizations and focuses into different focal spots. The fabricated configuration consists of 50 nm thick gold nanobricks with different lateral dimensions, organized in an array of 240 nm x 240 nm unit cells on the top of a 50 nm thick silicon dioxide layer, which is deposited on an optically thick reflecting gold substrate. Structure is fabricated using standard electron beam lithography and lift-off techniques. Characterization is performed using scanning electron microscopy and optical measurements, including investigation of wavelength dependence of efficiency, focal length and polarization extinction ratio. Our device features high efficiency (up to ~65%) and polarization extinction ratio (up to ~30 dB), exhibiting broadband response in the near-infrared band (750 950 nm wavelength) with the focal length and numerical aperture dependent on the wavelength of incident light. The proposed optical component can be straightforwardly integrated into photonic circuits or fiber optic devices which employ polarization multiplexing.