Going beyond perfect absorption: Superdirective absorbers (2024)

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  Figure 1

  (a) Front view of an infinite periodic strip array placed over an infinite ground plane and illuminated by a plane wave traveling in the direction of ${\theta }_{\text{i}}$. The distance between the two adjacent strips is $d$. (b) Top view of the array. The strips are loaded with bulk impedances inserted periodically with period $l$. The width of the strips is $w$. Both $l$ and $w$ are much smaller than the wavelength in free space. The periodically loaded strips can be modeled as hom*ogeneous-impedance strips with impedances per unit length $Z_{\text{L}}$. (c) Equivalent circuit of the system.

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  Figure 2

  (a) Finite-width strip array above an infinite PEC ground plane under illumination by a TE-polarized plane wave at ${\theta }_{\text{i}}$. (b) Top view of the structure. The first strip is at the position $y=0$, $z=-h$.

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  Figure 3

  Absorptance as a function of the incident angle ${\theta }_{\text{i}}$ for a designed incident angle of (a) $0^{\circ }$ and (b) ${80}^{\circ }$. For finite-sized absorbers, the solid red curve indicates the reference case (Ref.; all the load impedances are the same as for the designed periodic infinite array), while the dashed blue curve shows the optimized case (Opt.; the optimized loads). The dotted black curve shows the absorptance of the designed infinite absorber (Inf.).

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  Figure 4

  Induced-current distribution for two designed incident angles ${\theta }_{\text{i}}$ of (a) $0^{\circ }$ and (b) ${80}^{\circ }$. The dotted black curve corresponds to infinite absorbers (Inf.). For finite-sized absorbers, the solid red curve and the solid blue curve represent the reference case (Inf.; connected with identical loads) and the optimized case (Opt.; connected with optimized loads), respectively. The dot-dashed purple curve in (b) represents the induced-current distribution when the incident plane wave is traveling from ${77}^{\circ }$.

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  Figure 5

  Absorptance as a function of incident angle ${\theta }_{\text{i}}$ for different-sized strip arrays, where the designed angle of incidence is ${80}^{\circ }$.

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  Figure 6

  Load-impedance distributions for two designed incident angles: (a) ${\theta }_{\text{i}}=0^{\circ }$ and (b) ${\theta }_{\text{i}}={80}^{\circ }$. The red curves show the load impedances of the optimized loads (Opt.), while the blue curves represent the values of the reference (Ref.) load impedance. The solid and dashed curves represent the real and imaginary parts of the load impedance, respectively. The solid green line shows the symmetry axis of the strip array.

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  Figure 7

  (a) Schematic diagram for the comsol multiphysics simulation. The simulation domain is surrounded by a perfectly matched layer (PML). The array is positioned in the center of the simulation domain. The incident angle of the plane wave is ${80}^{\circ }$. (b) Real part of the scattered electric field (with the unit of volt per meter) distribution for the reference uniform array. Numerical results (upper panel) and analytical results (lower panel). (c) Same as (b) for an array with the optimized loads. Numerical results (upper panel) and analytical results (lower panel).

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  Figure 8

  Absorptance as a function of the normalized frequency for arrays of different sizes (indicated in the legend), when the incident angle is ${80}^{\circ }$. The results for the reference uniform absorbers (Ref.) are represented in red, while the curves for the optimized absorbers (Opt.) are shown in blue.

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  Figure 9

  Optimized results for absorptance as a function of incident angle for different periods and expected incident angles.

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  Figure 10

  Absorptance as a function of incident angle for different array sizes.

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