Controllable and Programmable Nonreciprocity Based on Detachable Digital Coding Metasurface

Abstract

electromagnetic (EM) waves. Numerous interesting physical functions are realized by metamaterials [1][2][3][4][5] such as negative refraction, [1] invisibility cloaks, [2,3] and superlens. [4] Metasurfaces are planer versions of bulk metamaterials to control the EM waves on their phases, [6] amplitudes, [7] and polarizations. [8] Evolved from traditional metamaterials described by effective medium parameters, digital coding metamaterials have been proposed [9] to design metasurfaces from digital perspective and realize programmable functionalities using a single metasurface. In the digital coding metasurfaces, two distinct EM responses can be encoded as digits "1" and "0," [10,11] Then the related metasurface design is transformed into digital process. As a result, some groundbreaking researches like convolution operations [12] and information entropy [13] for metasurfaces have been presented, suggesting potential relations between the digital world and the physical world. Within this perspective, we can not only encode the phase responses, but also other EM characteristics like polarizations and orbital angular momentum (OAM) modes. [14,15] Consequently, the digitally programmable methods bring plentiful functions, such as space-time coding, [16,17] and new-architecture communication system. [18][19][20] As an important concept in microwave and optical frequencies, nonreciprocity has raised in myriad physical branchesthermodynamics, mechanics, electromagnetism, optics, and so on. [21] The classic nonreciprocal devices like circulators are indispensable components in radar [22] and communication systems. [23] For traditional electromagnetics, nonreciprocal devices are predominantly based on magnetized materials, [24,25] like ferrites, usually composed of iron oxides and other elements (Al, Ni, Co). [26] However, these kinds of materials are usually large, bulky, uneconomical, and difficult to integrate into the metasurfaces. To overcome these drawbacks, magnetless nonreciprocial metasurfaces (MNMs) have been proposed. [27][28][29][30][31] By interconnecting transistor-like amplifier [27][28][29] and isolator [30,31] into metasurface structure, MNMs can easily achieve smaller size and better integrability. In addition, nonreciprocial metamaterials based on time modulation have also been presented, [32] but their control system complexity is greatly increased.

Information Metamaterial Systems

“…Besides developing the cognitive metamaterial, there are many other directions that are worth to investigate on the information metamaterial. In the future, the suggested research topics include but are not limited to New approaches to control the digital states of meta-atoms, such as using MEMS, microfluid, and amplifier ( Chen et al., 2019 ; Ma et al., 2019 ), besides the mainly used PIN diodes in the current stage. New degree of freedom to define the digital information, such as anisotropic coding ( Liu et al., 2016a , Liu et al, 2016 , 2018 ), polarization coding ( Ma et al., 2017 , 2020 ), frequency coding ( Wu et al., 2018 Wu et al., 2017 ), and amplitude-phase coding ( Bao et al., 2018 Luo et al., 2019 ), besides the mainly used space-domain phase coding and space-time-domain phase coding in the current stage.…”

“…New approaches to control the digital states of meta-atoms, such as using MEMS, microfluid, and amplifier ( Chen et al., 2019 ; Ma et al., 2019 ), besides the mainly used PIN diodes in the current stage.…”

Free-Space Nonreciprocal Transmission Based on Nonlinear Coupled Fano Metasurfaces

“…In addition, time modulation becomes harder as the operating frequency increases because the modulation frequency should scale accordingly [15]. Breaking the passivity of the system through the use of unidirectional gain elements (amplifiers) has also been explored to break reciprocity, leading to small size, ease of fabrication, and integrable CMOS technology for nonreciprocal responses [17][18][19][20]. However, the main disadvantage of such approaches is the large power consumption and noise, because amplifiers are active elements that need to be properly biased in order to operate [20].…”

“…Breaking the passivity of the system through the use of unidirectional gain elements (amplifiers) has also been explored to break reciprocity, leading to small size, ease of fabrication, and integrable CMOS technology for nonreciprocal responses [17][18][19][20]. However, the main disadvantage of such approaches is the large power consumption and noise, because amplifiers are active elements that need to be properly biased in order to operate [20]. Additionally, the use of a CMOS amplifier limits the dynamic range of operation, so these metasurfaces are limited to work in the low power regime not exceeding a few dBm in order to avoid triggering nonlinearities in the amplifiers.…”

Circular Polarization Hologram Realized by Pancharatnam-Berry Phase in Microwave Frequency

“…Metasurfaces, within ultrathin and sub-wavelength features, are applied to realize plenty of functions such as abnormal refraction, beam forming, polarization conversion, and hologram [9]- [15]. Phase-only metasurfaces, which are powerful tools to control the beam properties, have the ability to perform different functions [2] [7].…”