Broadband, Multiband, and Multifunctional All-Dielectric Metasurfaces

All-dielectric bianisotropic metasurfaces with tailored polarization and spectral responses are studied, and a systematic approach to their design is developed. It is shown that a wide range of polarization transformations over broad bandwidths, as well as multiple bands, can be implemented by cascading subwavelength dielectric gratings. The subwavelength gratings, modeled as anisotropic layers, are cascaded to form metasurfaces that control the polarization of impinging waves. These metasurfaces are a low-loss alternative to their lossy, plasmonic counterparts at optical frequencies, and can replace conventional, bulky optical components. The multifunctional performance and compactness (wavelength-sized thickness) of the proposed devices will also find use at millimeter-wave frequencies. Example metasurfaces are presented and their performance is verified through full-wave simulation. This work demonstrates the flexibility with which cascaded subwavelength gratings can realize unprecedented polarization control with varied spectral responses.

Figure 1

Exploded view of a generic, all-dielectric metasurface composed of a stack of subwavelength dielectric gratings with varied axis orientations separated by dielectric spacers. The multilayer metasurface can control the polarization of the impinging waves in a multiband, multifunctional manner.

Figure 2

A subwavelength grating. Under TM ($x$-polarized) and TE ($y$-polarized) incidence, it exhibits different effective permittivities, and can be approximated as a uniaxial slab.

Figure 3

Design variables that need to be determined in the $i\mathrm{th}$ layer of the cascaded metasurface: grating thickness, filling ratio, rotation angle, and thickness of the isotropic spacer.

Figure 4

Cascaded all-dielectric metasurfaces realizing (a) a symmetric linear polarizer, (b) a half-wave plate, (c) an asymmetric linear polarizer, and (d) a symmetric circular polarizer. The thicknesses of the layers are given to the left of each structure. The grating filling ratio and rotation angle (with respect to the $x$ axis) are shown to the right. The grating period is identical in all the layers. $T_{yx}$ denotes the transmission coefficient from a linear $x$-polarized wave to a $y$-polarized wave, and $T_{LR}$ denotes the transmission coefficient from a right-handed to a left-handed circularly polarized wave.

Figure 5

A reflective metasurface that acts as a half-wave plate from the top side and a reflective quarter-wave plate from the bottom side. The plots show that linear $x$- and $y$-polarized incident waves are highly reflected ($R_{xx}^{(i)}=R_{yy}^{(i)}\approx 1$) with ${180}^{\circ }$ and ${90}^{\circ }$ phase difference ($\mathrm{\angle }{R}_{yy}^{(i)}-\mathrm{\angle }{R}_{xx}^{(i)}\approx {180}^{\circ },{90}^{\circ }$) between orthogonal components of reflected fields when excited from each side.

Figure 6

Cascaded all-dielectric metasurfaces realizing (a) a narrowband symmetric circular polarizer, (b) a narrowband asymmetric circular polarizer, (c) a broadband symmetric circular polarizer, and (d) a broadband asymmetric circular polarizer. The plots show that the bandwidth is increased with additional layers. The total thicknesses of the structures are $1.15{\lambda }_0$ (a), $0.96{\lambda }_0$ (b), $1.62{\lambda }_0$ (c), and $2.02{\lambda }_0$ (d), where ${\lambda }_0$ is the wavelength at the center frequency of 75 GHz.

Figure 7

Cascaded subwavelength grating metasurfaces realizing (a) a dual-band (70- and 80-GHz) symmetric circular polarizer, (b) a dual-band asymmetric circular polarizer, (c) a dual-band symmetric linear polarizer, and (d) a dual-band asymmetric linear polarizer. The gratings and spacers are assumed to be made of silicon and the trenches are assumed to be filled with air.

Figure 8

Cascaded all-dielectric metasurfaces realizing (a) a dual-function symmetric and asymmetric left-handed circular polarizer, (b) a dual-function right-handed and left-handed symmetric circular polarizers, (c) a dual-function reflective half-wave plate and right-handed symmetric circular polarizer, and (d) a triple-function asymmetric $y$ linear polarizer (plot in a linear basis), a right-handed symmetric circular polarizer, and a left-handed asymmetric circular polarizer (plot in a circular basis).

Figure 9

Comparison between full-wave simulation and analytical computation. The quad-functional metasurface is designed to perform symmetric and asymmetric $x$ and $y$ linear polarizers at 60, 70, 80, and 90 GHz.

Figure 10

Dual-function metasurfaces realizing (a) $x$ and $y$ symmetric linear polarizers and (b) symmetric and asymmetric $x$ linear polarizers at lower and upper bands. The cascaded metasurfaces show a robust performance under different illumination angles. The wave impinges the metasurfaces in the $x$-$z$ ($\varphi =0^{\circ }$) and $y$-$z$ ($\varphi ={90}^{\circ }$) planes at $\theta ={45}^{\circ }$.

Figure 11

(a) Dual-function responses at arbitrary frequency bands with different bandwidths. The performance of the dual-function left-handed and right-handed symmetric circular polarizer is changed from that of a right-handed symmetric circular polarizer to that of a left-handed symmetric circular polarizer over a 2% change in frequency. (b) Quad-functional metasurface that realizes symmetric and asymmetric right-handed and left-handed circular polarizers in different orders.