Metasurfaces have been strongly investigated to realize a paradigm shift from classical optics. Unlikely the classical optics dealing with light rays in accordance with geometric-optic principles, metasurfaces allow the control of wavefront in subwavelength thickness on flat surfaces [1,2]. As conventional convex lenses, metalenses on flat surface without any macroscopic surface curvature have tremendous capability in future flat optics. They can also be used to replace the expensive compound lens systems in a number of consumer electronic products such as optical storages, digital cameras, microscopes, etc. Providing a flat, form factor, metalenses have a large degree of freedom in designing miniaturized sensors as well.
Typically, metasurfaces are composed of an array of planar nanostructures that locally provide the change of amplitude or phase of light in reflection or transmission. The phase modulation on each nanostructure leads to the change of wavefront for a new class of flat optical elements [3-5]. For example, high-contrast transmit arrays (HCTAs) have been reported to demonstrate subwavelength-thick lenses with high-numerical aperture and large focal efficiency [6,7]. This is a promising approach to make metasurfaces consisting of an array of subwavelength dielectric nanoposts on flat surface; however, a full coverage of phase from 0 to 2 pi is not readily achievable due to phase defects, when the post diameter is chosen to be varied. The phase defects are originated from resonances at the wavelength of interest, hindering a gradual increase of phase with respect to the variation of post diameters. Such defects deteriorate optical performance compared with conventional curved lenses, particularly in focal spot sizes and focusing efficiencies.
Here we propose a novel method to repair phase defects and achieve a full, 2 pi phase coverage with free of defects, which provides complete phase matches with theoretical calculations. We apply this method to demonstrate the convex-lens-like metalens with high numerical aperture (NA), small focal spot size, and high focusing efficiency in near-infrared region (1.55 microns). Together with the theoretical design and simulation, we prepare metalenses using silicon photolithography and nanofabrication and analyze experimental observations. The measured full width at half maximum (FWHM) of the focal spot and focusing efficiency show a high performance for numerical apertures of 0.3 ~0.7. This achievement offers considerable opportunities for various applications using metasurfaces based on controlled wavefront with free of defects.
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