Dual‐Physics Manipulation of Electromagnetic Waves by System‐Level Design of Metasurfaces to Reach Extreme Control of Radiation Beams

DOI: 10.1002/admt.201600196 technology[30,31] can also be applied to design new artificial lenses. By periodically and gradually placing surface refractive index textures, the surface waves are transformed to spatial waves based on the Floquet theory, which can be exploited to develop low-profile antennas.[32–34] Furthermore, the inverse problem of transforming spatial waves into surface waves has also been proposed,[35] which can be used for constructing the perfect absorber. Previous work[32–35] can mainly be attributed to surface impedance modulations, as there is a direct relation between the surface refractive index and the surface impedance. In this work, we propose a method whereby a metasurface system is used to control and modulate the surface waves and spatial waves simultaneously by tailoring both the surface refractive index and surface impedance and thus realize a 2D scanning (in both the azimuth and elevation directions) of radiation beams. The basic unit cell of the whole metasurface system is a metallic patch with a grounded dielectric. The unit-cell texture can be considered either as the surface refractive index or the surface impedance. In our design, we tailored the textures by tuning the surface refractive index such as to generate surface Luneburg lenses that can convert point sources clinging to the Luneburg lens to surface plane waves in different directions. Thus, the Luneburg lenses can be regarded as the feeding network. Then the surface plane waves with different wave vectors will be modulated by the one-dimensional (1D) impedance metasurface placed between the Luneburg lenses. Finally, the surface waves depart from the impedance metasurface to radiate into free space, and the radiation beams can cover the 2D spatial scanning region by simply changing the feeding location and the working frequency. The mechanism of the proposed radiation can be interpreted as a leakywave[33] or holographic radiation.[36,37] This article provides insight into the fact that metasurfaces can make a system-level EM design by taking advantage of many physical properties of materials without needing active elements and feeding circuits. The proposed metasurface has a much lower profile and lower cost than a normal phase-array system. Hence we believe that it has great potential in the areas of remote sensing, multi-input multi-output (MIMO) communications, and radar systems. This work can be easily promoted to the terahertz frequencies. The EM theory of the proposed system-level design can be seen in Figure 1, in which the two Luneburg lenses can be considered as the feeding network and the 1D impedance modulator is the radiation aperture. The Luneburg lenses, which can transform the point sources to corresponding plane surface waves, are used to generate many directional surface plane waves to excite the 1D impedance-modulation surface. Then the surface plane waves under different directions are converted into spatial waves with different azimuth angles. In recent years, because of their ability to control electromagnetic (EM) waves, metasurfaces[1–6] have attracted great attention of scientists and engineers. Thanks to their smaller loss, lower cost, and lower profile than bulk metamaterials,[7–10] metasurfaces have proven to be very compatible and promising for engineering applications. There are two kinds of methods for controlling EM waves using metasurfaces, one of which is suitable for spatial waves (propagating waves), and the other for surface waves. The modeling and theoretical analyses for either method are also very different.[11–13] In controlling spatial waves, some new concepts, such as the generalized Snell’s law[14] and Huygens surface,[15] may be considered as milestones in the development of metasurfaces. In a previous report,[14] V-shaped textures were first proposed on a single layer to realize a design that covers 360 degrees of spatial phase. However, the conversion effciency is very low. Ref. [15] introduced the electric and magnetic resonances to construct the Huygens metasurface, which can make the transmission efficiency nearly 100%. By following these outstanding ideas, many articles have been published that show successful controlling of the transmission[16–18] and reflection[19,20] characteristics when the incident waves interact with the metasurfaces. For this kind of method, the metasurfaces can be regarded as spatial filters. In the meantime, the abilities and functionalities of EM devices have been enhanced, and hence massive active and tunable metasurfaces[21–25] have been proposed using PIN diodes, variodes, or transistors governed by the applied voltage or boasting other properties from the microwave to the optical band. Recently, even field-programmable metasurfaces by means of active technology have been presented.[26] Although the active metasurfaces can dynamically control the EM waves, they may suffer from complicated designs and high cost. The surface waves can be considered as two-dimensional (2D) propagating waves with an attenuation in the transverse direction. To control the surface waves by metasurfaces, the traditional permeability and permittivity of the EM medium will be replaced by the effective surface refractive index and surface impedance to represent the EM characteristics of metasurfaces. Surface-wave lenses created by tailoring the surface refractive index have been presented in the literature to shape the surface wave fronts.[27–29] Transformation optics