Breaking Reciprocity with Space‐Time‐Coding Digital Metasurfaces

Abstract

which can manipulate electromagnetic (EM) waves in unconventional ways, and have enabled many exotic physical phenomena and effects, also inspiring novel devices and engineering applications. [1] Their 2D versions, commonly referred to as metasurfaces, are experiencing a strong surge of interest owing to a number of attractive features, including ultrathin thickness, low loss, easy fabrication, and potential conformability. In the wake of the pioneering work by Yu et al. [2] on generalized Snell's laws enabled by imparting an abrupt phase shift, metasurfaces have demonstrated unprecedented capabilities in wavefront engineering, amplitude modulation, and polarization conversion, just to mention a few. [2][3][4][5][6] However, metasurfaces that only impart space-gradient phase discontinuities are inherently constrained by Lorentz reciprocity. This implies, for instance, that the time-reversed version of a reflected wave propagates along the same direction as the original incident wave at the same frequency.The quest for breaking reciprocity is of longstanding interest in EM engineering, and is currently eliciting renewed attention in view of its pivotal role in lifting some fundamental limitations in communication systems as well as energy harvesting and heat management. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] For instance, in wireless communication systems, a nonreciprocal antenna could radiate a very directive beam without being bound to receiving its reflected echo. [20] A common way to attain nonreciprocal effects, especially at microwave frequencies, is to break the time-reversal symmetry by means of biased magnetic materials (e.g., ferrites). These, however, are typically bulky, costly, and difficult to integrate and scale up to optical wavelengths, [10] which motivates the strong interest in magnetless approaches. Some of these are based on nonlinear materials, which are not bound by the reciprocity theorem, but are power-dependent and require sufficiently high signal intensity. [11,12] Other magnetless approaches rely on transistor-based devices [13] and moving media, [14,15] but are limited in terms of operating frequency, and are difficult to extend to the optical regime. Timevarying approaches have emerged as attractive alternatives based on time-modulated devices, [16][17][18][19][20][21][22][23][24] which have smaller size, lower cost, and better integrability. In 2015, Shaltout et al. [19] Metasurfaces are artificially engineered ultrathin structures that can finely tailor and control electromagnetic wavefronts. There is currently a strong interest in exploring their capability to lift some fundamental limitations dictated by Lorentz reciprocity, which have strong implications in communication, heat management, and energy harvesting. Time-varying approaches have emerged as attractive alternatives to conventional schemes relying on magnetic or nonlinear materials, but experimental evidence is currently limited to devices such as circulators and antennas. Here, the recently...

Optomechanics of Chiral Dielectric Metasurfaces

“…On the other side, nanoscale artificial materials have been the major object of research in the field of electromagnetic structured media for more than two decades . Wavelength‐scale patterns of scatterers have shown the capability to produce bright colors and extravagant surface effects, often mocking what elegantly realized in biological systems .…”

Programmable anisotropic digital metasurface for independent manipulation of dual-polarized THz waves based on a voltage-controlled phase transition of VO2microwires

“…The versatility and reliability of wavefront engineering by metasurfaces furnish an inspiring platform for realizing novel physics phenomena including perfect imaging [3], invisibility cloaking [4], vortex beam [5] and numerous innovative functional metadevices [6,7]. Beyond the scope of analog metasurfaces, digital metasurfaces have recently been reported as an alternative approach to engineer the scattering patterns by multiple discrete digital states ("0" or "1" state in 1-bit coding case) while they remarkably provide a broader range of wave-matter functionalities [8,9]. Despite the rapid growth, all of the aforementioned coding schemes are isotropic coding metamaterials; hence, due to their isotropic geometry of the metaparticles, these devices only support unique responses for excitation with a certain single polarization.…”

Tunable, reconfigurable, and programmable metamaterials

“…The programmable metamaterials are composed of particles in which the pin‐diodes are loaded. By controlling the bias voltages, these diodes show distinct circuit parameters at “ON” and “OFF,” corresponding to the digital codes “0” and “1.” The programmable metamaterials essentially work as the wave‐based information system with programmable coding sequences . When the bias voltages are connected to a field‐programmable gate array (FPGA), the wave based information process on physical levels of metamaterials can be realized in real time.…”

“…The programmable metamaterials essentially work as the wave-based information system with programmable coding sequences. [89][90][91][92][93][94] When the bias voltages are connected to a field-programmable gate array (FPGA), the wave based information process on physical levels of metamaterials can be realized in real time. Different functions can be acquired with the different coding patterns determined by FPGA on programmable metamaterials.…”

“…In recent years, many functions and devices based on programmable metamaterials have been reported including holograms, 95 imager systems, 89,[96][97][98][99] and communication systems. 90,[100][101][102][103] In this article, we will summarize the recent progress on metamaterials from two aspects: tunable and reconfigurable metamaterials in terahertz and microwave frequencies; the functional devices and novel systems by the programmable metamaterials.…”