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Planar achromatic metalenses with a thickness of the order of wavelength have attracted much attention for their potential applications in ultra-compact optical devices. However, realizing single-layer achromatic metalenses across a wide bandwidth requires that the corresponding meta-atoms have complex cross-sections for correct phase profile and dispersion compensation. Herein, we introduce an effective Abbe number and use lens maker equations to design a dual-layer achromatic metalens in which we compensate the dispersion by using a plano-convex liked metalens combined with a plano-concave liked metalens. The stacked metalens are designed based on simple high refractive index dielectric cylindrical meta-atoms with different radii, which simplify the design and fabrication processes. We demonstrate that a dual-layer achromatic metalens has a small focal length difference across the visible wavelength range and an average focusing efficiency above 50%, which proves that the design method is promising for many potential applications in multi-functional flat optical devices.

narrowband high-reflectivity Bragg gratings in a novel multimode fiber. Opt Commun 233 (2003), 91-95. 6. J. Lim, Q. Yang, B.E. Jones, and P.R. Jackson, Strain and temperature sensors using multimode optical fiber Bragg gratings and correlation signal processing. ABSTRACT: Several Krylov subspace iterative algorithms are compared as the solvers for the discrete dipole approximation method to analyze the electromagnetic scattering problem. Fast Fourier transform technique is exploited to accelerate the computation of matrix-vector product. Numerical examples for homogeneous spheres indicate that the results of radar cross section agree well with the exact solutions. Some misleading concepts in literatures are clarified. We give a new comparison of the computational complexities of these methods. Faster solvers are recommended according to the size of scatterer.

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