Complex-Amplitude Programmable Versatile Metasurface Platform Driven by Guided Wave.

Metasurfaces have shown unparalleled controllability of electromagnetic (EM) waves. However, most of the metasurfaces need external spatial feeding sources, which renders practical implementation quite challenging. Here, a low-profile programmable metasurface with 0.05λ0 thickness driven by guided waves is proposed to achieve dynamic control of both amplitude and phase simultaneously. The metasurface is fed by a guided wave traveling in a substrate-integrated waveguide, avoiding external spatial sources and complex power divider networks. By manipulating the state of the p-i-n diodes embedded in each meta-atom, the proposed metasurface enables 1-bit amplitude switching between radiating and nonradiating states, as well as a 1-bit phase switching between 0° and 180°. As a proof of concept, two advanced functionalities, namely, low sidelobe-level beam scanning and Airy beam generation, are experimentally demonstrated with a single platform operating in the far- and near-field respectively. Such complex-amplitude, programmable, and low-profile metasurfaces can overcome integration limitations of traditional metasurfaces, and open up new avenues for more accurate and advanced EM wave control within an unprecedented degree of freedom.

Four-channel meta-hologram enabled by a frequency-multiplexed mono-layered geometric phase metasurface.

In recent years, frequency-multiplexed metasurfaces have received extensive attention due to the increasing demand for multifunction integration and communication capacity. However, multi-channel studies achieved with a mono-layered frequency-multiplexed metasurface are limited. Herein, a universal design strategy for a frequency-multiplexed mono-layered geometric phase metasurface is proposed by utilizing Pancharatnam-Berry (PB) phase modulations. The elementary meta-atom is judiciously designed to transmit the cross-polarized component of a circularly polarized incident wave at four distinct frequencies with independent 360° phase shifts and a constant amplitude of 0.48, close to the theoretical limit of 0.5. As a proof-of-concept demonstration, a four-channel meta-hologram is designed to achieve distinct holographic images of "three foci", "five foci", "J" and "X" at 7.2 GHz, 9.1 GHz, 10.9 GHz, and 15.2 GHz respectively. The images are projected in the desired azimuth planes by exploiting the time-shifting properties of the Fourier transform. The experimental and full-wave simulation results are in good agreement, which indicates that the proposed strategy has great potentials in various applications, such as multi-channel imaging and information encryption technology.

Design of a frequency-multiplexed metasurface with asymmetric transmission.

Metasurfaces presenting diversified functionalities have broadened the prospect of manipulating the phase, amplitude, and polarization from the optical to microwave fields. Although the frequency-multiplexing strategy is one of the intuitive and effective approaches to expand the number of channels, demonstrations reporting on the combination between directional asymmetric transmission and frequency-multiplexing via an ultrathin flat device are limited. In this study, a novel, to the best of our knowledge, strategy is proposed to generate four independent holographic images under opposite illumination directions at two operating frequencies, utilizing a single metasurface composed of two types of metallic resonators and one grating layer. Specifically, each scattering channel with independent information makes full use of the whole metasurface. Simulation and experimental results show good agreement, highlighting the attractive capabilities of the multi-functional metasurface platform, which provides more freedom for the manipulation of electromagnetic waves.

Broadband tunable metasurface platform enabled by dynamic phase compensation.

Broadband metasurfaces have attracted significant attention for a variety of applications in imaging and communication systems. Here, a method to alleviate the chromatic aberrations issue is proposed in the microwave region using dynamic phase compensation enabled by a reconfigurable metasurface. The dispersion characteristic of the meta-atom implemented with varactor diodes can be flexibly manipulated electronically, such that the dispersion-induced phase distortions over a wide frequency band can be compensated dynamically to achieve broadband performances. Various aberration-free functionalities can be realized with the proposed active metasurface. Near-field measurements are performed on a fabricated prototype to demonstrate aberration-free beam bending and hologram imaging, showing good agreement with simulation results. Such an active metasurface platform paves the way to efficient devices for wireless power transfer, sensors, and communication and antenna systems at radio or much higher frequencies.

Meta-hologram enabled by a double-face copper-cladded metasurface based on reflection-transmission amplitude coding.

Here, we propose a double-face copper-cladded meta-hologram that can efficiently manipulate the amplitude of electromagnetic waves in both transmission and reflection spaces, depending on the polarization state of the incident electromagnetic wave. The proposed meta-hologram is validated by encoding the transmission-reflection amplitude information of two independent images into a single metasurface. The holographic images obtained from measurements agree qualitatively with simulation results. The proposed metasurface presents a novel, to the best of our knowledge, scheme for electromagnetic wavefront control in the whole space and overcomes the limitations of narrow frequency band operation.

Helicity-switched hologram utilizing a polarization-free multi-bit coding metasurface.

In this work, a polarization-free coding metasurface is proposed to manipulate circularly polarized waves. Compared to a Pancharatnam-Berry phase metasurface, the proposed design not only allows for overcoming anti-symmetrical response characteristics between orthogonal circularly polarized states to enable achieving identical functionality under both right-handed and left-handed circularly polarized wave illuminations and avoiding polarization-conversion losses but also offers additional degree of freedom in the control of handedness. As a proof-of-concept demonstration, a polarization-free multi-bit coding metasurface is designed to realize helicity-switched holograms in the microwave region. Experimental measurements performed on a fabricated prototype reveal outstanding imaging quality with extremely high imaging efficiency above 76% for arbitrary polarizations at 10 GHz. Our proposed method expands the route in manipulating circularly polarized waves and can be applied over the whole electromagnetic spectrum for wavefront manipulation.

Multi-focus hologram utilizing Pancharatnam-Berry phase elements based metamirror.

An ultrathin reflection-type metamirror is proposed for multi-focusing with any desired focusing fashion including focal number and location. The metamirror is composed of reflection-type Pancharatnam-Berry (P-B) phase elements, which are able to provide full reflection phase of 2π, together with near-unity reflection efficiency by judiciously engineering the rotation angle of each latter element. A holographic algorithm is utilized to calculate the phase distribution at the interface of the metamirror to achieve the desired multi-focus spots. Experimental demonstrations performed in microwave region show good imaging quality with high reflection efficiency and imaging efficiency. The proposed metamirror provides a high-performance solution for low-cost and lightweight beam-shaping and beam-focusing devices.

Coding Huygens' metasurface for enhanced quality holographic imaging.

In this paper, coding Huygens' metasurface (CHM) is proposed for holographic imaging with enhanced quality. A weighted holographic algorithm is used to calculate the phase distribution at the interface and to design the CHM. Experimental demonstration performed in the microwave region validates holographic imaging with the ability to modulate energy distribution among focal points and improve image quality. By judiciously engineering both electric and magnetic dipolar resonators, the proposed digital Huygens' meta-atom is able to provide a full transmission-phase covering the whole range of 2π together with a near-unity transmission efficiency. The proof-of-concept experiments show that holographic imaging quality can be indeed improved by using digital meta-atoms with several bits. Furthermore, the modulation of intensity distribution among focal points is experimentally realized by using the 3-bits CHM. The proposed CHM hologram shows great potential in a variety of application fields, such as programmable high-resolution imaging lenses, microscopy, data storage, information processing, and computer-generated holograms.