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Bulk optics have been used for various optical instruments such as microscope or camera, but it has limitations in size and thickness because of accumulation of various lenses. Metalenses are an emerging field as one of the next-generation optical technology for miniaturization of optical system. Subwavelength thick nanostructures on metalens control the amplitude, phase, absorption, emission, and transmittance of incident light. Focal length can be control by adjusting phase shift from 0 to 2π according to the change of shape, size, and period of nanostructures. Many kinds of metalens such as GaN nanopillar, Si nanodisk, or TiO2 nanofin have been studied recently, but chromatic aberration correction problem remains as the limitation in metalenses. Some researches such as group delay dispersion have been conducted to correct chromatic aberration, low focusing efficiency ( ~ 10 %) is unresolved limitation. In this work, we suggested a new method to design achromatic metalens for 400, 600 and 800 nm wavelengths focusing. Spatial multiplexing method can correct chromatic aberration very simply. First, three metalenses are designed for each wavelengths. In the next step, meta-atoms of each metalenses are interleaved in one metalens. The coupling between three kinds of meta-atoms is small, therefore we can expect three wavelength focusing in one metalens. We used finite-difference time-domain (FDTD) simulation to investigate field distribution and focusing efficiency of each metalenses. By using this method, we demonstrated achromatic metalens with numerical aperture of 0.89, which focus light at 400, 600, and 800 nm to same focal spot. Focusing efficiency of achromatic metalens for 400, 600 and 800 nm are 38, 44 and 54%, respectively. This design method can attend significant role to demonstrate desired achromatic metalens.
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