Graded metasurfaces exploit the local momentum imparted by an impedance gradient to mold the impinging wave front. This approach suffers from fundamental limits on the overall conversion efficiency, and it is challenged by fabrication limitations on the spatial resolution. Here, we introduce the concept of metagratings, formed by periodic arrays of carefully tailored bianisotropic inclusions and show that they enable wave front engineering with unitary efficiency and significantly lower fabrication demands. We employ this concept to design reflective metasurfaces for wave front steering without limitations on efficiency. A similar approach can be extended to transmitted beams and arbitrary wave front transformation, opening opportunities for highly efficient metasurfaces for extreme wave manipulation.

Metagratings: Beyond the Limits of Graded Metasurfaces for Wave Front Control.

Achieving multiple diversified functionalities in a single flat device is crucial for electromagnetic (EM) integration, but available efforts suffer the issues of device thickness, low efficiency, and restricted functionalities. Here, a general strategy to design high-efficiency bifunctional devices based on metasurfaces composed by anisotropic meta-atoms with polarization-dependent phase responses is described. Based on the derived general criterions, two bifunctional metadevices, working in reflection and transmission modes, respectively, that can realize two distinct functionalities with very high efficiencies (≈90% in reflection geometry and ≈72% in transmission one) are designed and fabricated. Microwave experiments, including both far-field and near-field characterizations, are performed to demonstrate the predicted effects, which are in excellent agreement with numerical simulations. The findings in this study can motivate the realizations of high-performance bifunctional metadevices in other frequency domains and with different functionalities, which are of crucial importance in EM integration.

High‐Performance Bifunctional Metasurfaces in Transmission and Reflection Geometries

A study of optical aberrations for flat lenses based on phase discontinuities is reported. The wave aberration function and the analytical expression of the aberrations up to the 4th order are derived to describe the performance of both ideal and practical flat lenses. We find that aberration-free focusing is possible under axial illumination but off-axis aberrations appear when the excitation is not normal to the interface. An alternative design for an aplanatic metasurface on a curved substrate is proposed to focus light without coma and spherical aberrations.

Aberrations of flat lenses and aplanatic metasurfaces

Metasurfaces offer great opportunities to control light, but so far most ``metadevices'' work either in pure reflection or pure transmission mode, leaving half of electromagnetic (EM) space untapped. Thus the authors design meta-atoms with polarization-dependent transmission and reflection properties, to efficiently manipulate EM waves in either mode. They fabricate three ultrathin devices with multiple polarization-dependent functionalities and very high efficiencies on both transmission and reflection sides. These findings significantly expand the capabilities of metasurfaces for more demanding and diverse applications.

High-Efficiency and Full-Space Manipulation of Electromagnetic Wave Fronts with Metasurfaces

We propose a spoof surface plasmon polariton (SPP) emitter which is composed of ultrathin corrugated metallic strips, exhibiting the directional radiation property. The spoof SPP emitter provides a way to quickly convert the SPP mode to a radiated mode. By controlling phase modulations produced by the phase-gradient metasurface on the ultrathin metallic strips, we demonstrate theoretically and experimentally that spoof SPP waves are converted into spatial propagating waves with high efficiency, which are further radiated with flexible beam steering. The proposed method sets up a link between SPP waves and radiation waves in a highly controllable way, which would possibly open an avenue in designing new kinds of microwave and optical elements in engineering.

Efficient conversion of surface-plasmon-like modes to spatial radiated modes

We propose an ultra-thin planar metasurface with phase discontinuities for highly efficient beam steering. The effect benefits from the broadband transparency and flexible phase modulation of stacked metal/dielectric multi-layers that is perforated with coaxial annular apertures. Proof-of-principle experiments verify that an efficiency of 65% and a deflection angle of 18° at 10 GHz are achieved for the transmitted beam, which are also in good agreement with the finite-difference-method-in-time-domain (FDTD) simulations. The scheme shall be general for the design of beam-steering transmitters in all frequencies.

Highly efficient beam steering with a transparent metasurface.

We propose a scheme of metal/graphene plasmonic gratings for negative reflection. The existence of graphene ribbons, introducing abrupt discontinuity on tangential components of magnetic fields for scattering waves across a graphene interface, substantially alters the dispersion of surface states on plasmonic gratings such that negative reflection that is robust against the incidence angle and can be tuned in a wide frequency range as a function of Fermi energy of graphene. Circularly polarized incidence waves are reflected and split along specular and negative directions, with respective to transverse magnetic and electric polarization. Our findings are potentially helpful for light steering in integrated optical circuits.

Negative reflection from metal/graphene plasmonic gratings.

Broadband and wide angle microwave absorption with optically transparent metamaterial

Electromagnetic wave beam regulating and controlling device

We theoretically and experimentally investigate subwavelength imaging with a flat lens constructed with fishnet metamaterial. By adopting a ring antenna with a diameter of 5 mm, a high-quality image, with the full width at half maximum no more than half of the operational wavelength at 10 GHz, is measured in free space at a distance of 19 mm away from the flat lens. The spatial resolution is about one third of the operational wavelength considering the electrical size of the ring antenna, which is also verified by finite-difference time-domain simulations. The subwavelength imaging arises from the negative coupling between the adjacent spoof plasmonic waveguides of fishnet metamaterial and is proved valid in a wide frequency range.

Xiaopeng Su

Papers

Highly efficient beam steering with a transparent metasurface.

Negative reflection from metal/graphene plasmonic gratings.

Broadband and wide angle microwave absorption with optically transparent metamaterial

Electromagnetic wave beam regulating and controlling device

Subwavelength imaging with a fishnet flat lens