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Timestamp: 2019-04-24 06:50:29+00:00

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Beyond the wave manipulation at a single frequency, efficiency bandwidth control and functional dispersion engineering over metasurfaces are key challenges towards practical applications. Here we propose a type of wideband dielectric metasurfaces made of ultra-thin and layered high-index dielectric patches. The inclusions can be considered as effective material with designable effective refractive index and dispersion. Beam-deflection metasurfaces composed of such inclusions are characterized with the bandwidth approaching and surpassing the limit of conventional blazed gratings in transmission and reflection manners. The bandwidths are more than twice of that in popular single-layer dielectric metasurfaces made of pillar and disk building blocks. In addition, the proposed design benefits from operation over wide range of incident angles and with large tolerance to fabrication errors. More complicated beam manipulation can be fulfilled similarly with great potential for wideband planar optics.
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Fig. 1 (a) The layered inclusion studied in this work and the designs using dielectric thick pillar (b) and thin disk (c). The periodicity and the thickness information is marked relative to the center wavelength.
Fig. 2 Transmission spectrum of phase shifters made of layered structures (a), pillars (b), and disks (c). The solid lines are for amplitude and dash lines for phase. Different elements are represented by different colors. (d)–(f) Variation of the effective refractive index of the inclusions with frequency retrieved from the responses in (a)–(c). Only real part is considered.
Fig. 3 (a) Field profile in the layered π phase shifter. (b) Total field profile in the pillar π phase shifter and the two guided modes in it. (c) Total field profile in the disk π element and the two guided modes in it. Magnetic component Hx in the y–z plane is plotted with the excitation of Ey and kz. The dash lines indicate the edges of the Si patterns.
Fig. 4 (a–c) Angular response of the layered, pillar and disk inclusions. The solid lines are for amplitude and dash lines for phase. Different elements are represented by different colors. (d–f) Effective refractive index as a function of the incident angle.
Fig. 5 (a) Schematic illustration of the three metasurfaces by repeating two supercells in each. (b) Efficiency spectrum of the metasurfaces for 15° beam deflection. Solid line is from Eq. (2). Dot line, plus line and cross line are for layered, pillar and disk metasurfaces, respectively. (c) Variation of ΔΦ in the supcercell over frequency with the same legend as in (b). (d) Transmission amplitude and phase of the first (red) and the last elements (blue) in the layered supercell. Solid lines are amplitude and star markers are phase.
Fig. 6 Beam deflection in layered, pillar and disk metasurfaces at the center frequency f0 and 1.2 f0. Real part of Hx is plotted before and after the metasurface supercell.
Fig. 7 (a) Angular bandwidth of the metasurfaces. (b) Near field (real part of Hx) in layered, pillar and disk metasurface supercells with 45° excitation.
Fig. 8 Beam deflection efficiency of the devices when introducing different amount of the width error (a), the thickness error (b), and the registration error (c) at the center frequency with the normal excitation. The results are averaged for 30 times calculations.
Fig. 9 Examples of ideal phase spectrum of elements at the two ends of the supercell for unity-efficiency deflection.
Fig. 10 (a) Geometry of the reflective layered inclusion. (b) Deflection efficiency of the reflective layered design over frequency as the dot line. The solid line is from Eq. (2). (c) Phase spectrum of 8 elements in a supercell. (d) Efficiency for beam deflection over incident angle. Dot line for layered metasurface. Solid line for effective-medium grating with the same thickness. (d) Reflected near field (Hx) by the layered metasurface at different frequencies and with different incident angles. Position of the metasurface is shown as the dash line, and incident field is not shown for clarity.

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