Metasurfaces are composed of periodic pattern of subwavelength structure. They have been of great interest to the scientific community due to their interaction with light in ways that cannot be found in nature. Among various types of metasurfaces, metalenses are ones with promising applications because they can bend and focus light in a confined space. In contrast to conventional refractive lenses, the metalenses are ultra-thin slab with only a few nanometers in thickness suitable for limited space system. However, designing a very thin metalens with requirement of high efficiency is a huge challenge. We designed a high NA metalens with a diameter of less than 25 μm to focus an 830 nm laser beam onto the waveguide situated only a few microns from the laser. The metalens were designed with FDTD simulation and fabricated by distributing Si nanofin structures in a particular pattern using standard electron beam lithography. Determining the efficiency of the metalens itself is a challenge due to small focal spot and background light. Thus, grating of similar physical shape of nano-bricks was used instead for finding efficiency of the metalens. By inspecting diffracted light from metasurface grating, the efficiency of metalens was determined. Here we present the process of designing the metalens, fabrication and testing its efficiency to provide the best solution for limited space optical system.
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