Low‐Profile Electromagnetic Holography by Using Coding Fabry–Perot Type Metasurface with In‐Plane Feeding

Abstract

this technology, the spatial resolution and imaging efficiency of the metasurface holograms are significantly improved, compared with the conventional geometrical optical structures suffering high-noise and low-reliability properties. Accordingly, some new achievements such as the polarization-independent, multi-color, non-linear, and high-efficiency holograms have been implemented with much more convenience under the metasurface-based technologies. [10][11][12][13][14][15][16] Digital coding metasurfaces have attracted much attention in recent years, which build up a bridge for the physical world and the digital information world. [17,18] The phase or amplitude response of the digital units are always encoded as N-bit with adjustment of some meta-unit parameters. For example, for a coding unit of transmission-type metaunit, the transmission phases of 0° and 180° refer to be digital bits "0" and "1", which bring great innovation for traditional designs. We can directly realize the transmission, reception, and processing of information in the microwave fields without considering the base-band digital modulation and processing, which greatly simplifies the design of the conventional transmitters and receivers. In addition, new wireless communication systems and advanced imaging systems have also been developed using this technology with good performance. [19][20][21] On this basis, adding active elements such as PIN diodes, varactors, and micro-electro-mechanical systems (MEMS) into the digital meta-unit, [22][23][24][25] the programmable metasurfaces can be realized through a field programmable gate array (FPGA). Electromagnetic reprogrammable coding metasurface holograms have been completed by using 1-bit reflection-type metasurface. [26] However, in the actual implementations of the conventional metasurface holography and the programmable digital-coding metasurface holography, the position of the feeding source is always placed far from the metasurfaces, which greatly increases the profile of metasurfaces and emerges unwanted blockage under the reflection-type designs. [27] Nowadays, the folded reflectarray, as an improved form of Cassegrain reflectarray, has also received much attention. [28][29][30] Although this structure reduces the profile and overcomes the shielding effect of the reflection-type metasurfaces, it brings some challenges in designing the feed and the rotation of polarization. [28][29][30] The introduction of Fabry-Perot (FP) cavity will pave a way to solve these problems. Compared with the folded reflectarray, Metasurface holography has been well studied due to its features that are superior to the traditional holographic technologies. However, high profiles of the reflection-and transmission-type configurations in the metasurface holography restrict its applications. On the other hand, digital coding metasurfaces have received great attention recently because they build a link between the information theory and surface electromagnetic fields, which also bring new viewpoint and convenience fo...

Metasurface Holography in the Microwave Regime

“…Considering the influence of the transmission phase of the partially reflective surface, the traditional optical ray model was modified in consequence. More importantly, this design overcomes the shortcoming of the bulky profile, and it constitutes the first work to achieve 2D functions from metasurfaces in a leaky-mode mechanism [102]. In 2020, Cui et al proposed to synthesize the amplitude and phase in the region of interest simultaneously and independently utilizing high-efficiency phase-only metasurfaces [103].…”

Dual‐Polarized Tri‐Channel Encrypted Holography Based on Geometric Phase Metasurface

“…However, imaging measurement in the far field at microwave frequencies is a difficult task, and hence it is necessary to replace the Fraunhofer's diffraction by the Green's function to calculate the transformation of the near field. [12,63] Second, the GSW algorithm is applied to holographic imaging with continuous phase distribution. To apply it to the proposed 1-bit phase metasurface holography concept, the calculated phase needs to be discretized at equal intervals, and the phase discretization process also needs to be included in each iteration process.…”

“…Moreover, the restricted dimensions of the 80 Â 80 unit cells metasurface also contributes to the limited quality of the hologram. [16,63,66] However, it should be noted that at higher frequencies with smaller of unit cells, more elements can be arranged on the metasurface to greatly improve the performance of the hologram.…”

“…Some improvements have be adopted here to make it suitable for the 1‐bit phase metasurface‐based holography in the microwave region. First, it should be noted that the original GSW algorithm uses Fraunhofer diffraction as a function to describe the propagation of electromagnetic waves in space, [ 61–63 ] which is often utilized in the far field region with the imaging distance . However, imaging measurement in the far field at microwave frequencies is a difficult task, and hence it is necessary to replace the Fraunhofer's diffraction by the Green's function to calculate the transformation of the near field.…”

Phase and Polarization Modulations Using Radiation‐Type Metasurfaces

“…As a solution, metasurfaces inte grated with feeds are studied, and these metasurfaces mainly focusing on narrowing the distance between external feeds. For example, foldedreflection type metasurfaces [24] and Fabry-Perot type metasurfaces [25] have been realized and contributed to the profile reduction. It is noted that these metasurfaces are risky and challenging schemes because of obvious mutual coupling between metasurfaces themselves and external feeds; thus, optimization procedure is a must.…”