Abstract
Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics. Previous metasurfaces have been limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. Here we report a polarization-insensitive, high-contrast transmissive metasurface composed of crystalline titanium dioxide pillars in the form of metalens at the wavelength of 633 nm. The focal spots are as small as 0.54, which is very close to the optical diffraction limit of 0.5. The simulation focusing efficiency is up to 88.5%. A rigorous method for metalens design, the phase realization mechanism and the trade-off between high efficiency and small spot size (or large numerical aperture) are discussed. Besides, the metalenses can work well with an imaging point source up to ±15° off axis. The proposed design is relatively systematic and can be applied to various applications such as visible imaging, ranging and sensing systems.
Highlights
In recent years, manipulating electromagnetic waves has become a vital research topic with the development of integrated optical circuits and optical communication systems
The performance of the metalenses was evaluated by three dimension finite difference time domain (FDTD) simulations
We designed a set of 5.4-μm diameter metalenses with a numerical aperture (NA) as high as 0.8 that focused the x-polarized light from the back side of a substrate (200-mm thick fused silica) to points located at distances ranging from f = 2 μm to f = 14 μm away from the metalenses
Summary
In recent years, manipulating electromagnetic waves has become a vital research topic with the development of integrated optical circuits and optical communication systems. Metasurfaces based on various cells ranging from the V-shaped [1,5,6,7], U-shaped [8], cross-shaped [9] and L-shaped [10] nanoantennas to nanobricks [11,12,13] and Pancharatnam-Berry phase [14] elements [15,16,17,18,19,20] have been proposed to achieve transmissive flat lenses Their efficient operation range has seldom worked at the visible wavelength because of the intrinsic optical loss of the constituent materials (usually consisting of either silicon or plasmonic noble metals), which leads to the reported metalenses demonstrated at visible or near infrared wavelengths [5,21,22] often having ordinary efficiencies of below 60%. The reported metalenses were usually designed for linear [11,12] or circular [19,20] polarizations of incident light, which have limited their potential application in polarization-independent devices
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