Abstract In recent years, active metasurfaces have emerged as a reconfigurable nanophotonic platform for the manipulation of light. Here, application of an external stimulus to resonant subwavelength scatterers enables dynamic control over the wavefront of reflected or transmitted light. In principle, active metasurfaces are capable of controlling key characteristic properties of an electromagnetic wave, such as its amplitude, phase, polarization, spectrum, and momentum. A ‘universal’ active metasurface should be able to provide independent and continuous control over all characteristic properties of light for deterministic wavefront shaping. In this article, we discuss strategies for the realization of this goal. Specifically, we describe approaches for high performance active metasurfaces, examine pathways for achieving two-dimensional control architectures, and discuss operating configurations for optical imaging, communication, and computation applications based on a universal active metasurface.

/pdf/toward-a-universal-metasurface-for-optical-imaging-1cef33dt.pdf

Toward a universal metasurface for optical imaging, communication, and computation

Exploring the interaction of light with materials periodically structured in space and time is intellectually rewarding and, simultaneously, a computational challenge. Appropriate computational tools are urgently needed to explore how such upcoming photonic materials can control light on demand. Here, we introduce a semi-analytical approach based on the transition matrix (also known as T-matrix) to analyze the optical response of a spatiotemporal metasurface. The metasurface consists of a periodic arrangement of time-varying scattering particles. In our approach, we depart from an individual scatterer's T-matrix to construct the effective T-matrix of the metasurface. From that effective T-matrix, all observable properties can reliably be predicted. We verify our semi-analytical approach with full-wave numerical simulations. We demonstrate a speed-up with our approach by a factor of more than 500 compared to a finite-element simulation. Finally, we exemplify our approach by studying the effect of time modulation on a Huygens' metasurface and discuss some emerging observable features.

Modeling four-dimensional metamaterials: a T-matrix approach to describe time-varying metasurfaces.

: Exploring the interaction of light with materials periodically structured in space and 17 time is intellectually rewarding and, simultaneously, a computational challenge. Appropriate 18 computational tools are urgently needed to explore how such upcoming photonic materials can 19 control light on demand. Here, we introduce a semi-analytical approach based on the transition 20 matrix (also known as T-matrix) to analyze the optical response of a spatiotemporal metasurface. 21 The metasurface consists of a periodic arrangement of time-varying scattering particles. In our 22 approach, we depart from an individual scatterer’s T-matrix to construct the effective T-matrix of 23 the metasurface. From that effective T-matrix, all observable properties can reliably be predicted. 24 We verify our semi-analytical approach with full-wave numerical simulations. We demonstrate a 25 speed-up with our approach by a factor of more than 500 compared to a finite-element simulation. 26 Finally, we exemplify our approach by studying the effect of time modulation on a Huygens’ 27 metasurface and discuss some emerging observable features.

Modeling four-dimensional metamaterials: A T-matrix approach to describe time-varying metasurfaces

Abstract The current commercially viable light detection and ranging systems demand continuous, full-scene, and dynamic two-dimensional point scanning, while featuring large aperture size to ensure long distance operation. However, the biasing architecture of large-area arrays with numerous individually controlled tunable elements is substantially complicated. Herein, inverse design of a perimeter-controlled active metasurface for two-dimensional dynamic beam steering at mid-infrared regime is theoretically presented. The perimeter-control approach simplifies biasing architecture by allowing column-row addressing of the elements. The metasurface consists of a periodic array of plasmonic patch nanoantennas in a metal-insulator-metal configuration, wherein two active layers of indium arsenide are incorporated into its building block. The metasurface profile facilitates wide phase modulation of ≈355°$\approx 355^{\circ} $ on the reflected light at the individual element level through applying independent voltages to its respective columns and rows. The multi-objective genetic algorithm (GA) for optimizing user-defined metrics toward shaping desired far-zone radiation pattern is implemented. It is demonstrated that multi-objective GA yields better results for directivity and spatial resolution of perimeter-controlled metasurface by identifying the design tradeoffs inherent to the system, compared to the single-objective optimizer. A high directivity and continuous beam scanning with full and wide field-of-view along the azimuth and elevation angles are respectively maintained.

/pdf/inverse-design-of-perimeter-controlled-inas-assisted-21r6vvmz.pdf

Inverse design of perimeter-controlled InAs-assisted metasurface for two-dimensional dynamic beam steering

Optical metasurfaces are becoming ubiquitous optical components to control light properties. However, most of these devices are passive and cannot be arbitrarily reconfigured according to the change in the surrounding environment. Here the authors propose an innovative design strategy relying on the position of topological singularities to address full phase modulation of light with almost unity efficiency. The active metasurface unit cells consist of asymmetric Gires–Tournois resonators filled with either silicon or hetero‐structured materials to leverage on the thermo‐optical or electro‐optical effects, respectively. In both cases, a full phase modulation associated with 100% reflection amplitude is observed even when dealing with extremely low refractive index change, on the order of 0.01. Improving the deflection efficiencies for each deflection angle is performed by optimizing the refractive index modulation profile in the extended unit cell using an advanced statistical learning optimization methodology. Consequently, active beam steering designs for active thermo‐optical effect with ultimate performance exceeding 90% have been optimized. Furthermore, active wavefront splitting using electro‐optics materials is optimized to reach ultimate modulation performances with nearly 92% efficiency. The realization of highly efficient active beam‐forming operating at high frequencies would open important applications in imaging microscopy, and 3D light detection and ranging (LiDAR).

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/lpor.202200880

Universal Active Metasurfaces for Ultimate Wavefront Molding by Manipulating the Reflection Singularities

Chip-scale beam steering units, which would replace currently used mechanical gimbals, could revolutionize the field of free space optical communications. We review chip-scale technologies, which enable electronic beam reconfigurations and steering without mechanically moving parts. We assess the feasibility of using different electrically steerable apertures such as active metasurfaces and optical phased arrays for laser communications. Our optical link budget analysis shows that, for metasurface apertures of 1 cm in diameter and input powers of 5 W, the free space link range can approach ~ 10,000 km. We also provide an outlook how the link range can be increased further. 

Electrically controlled reconfigurable apertures for free space optical communications

Laser communications hold the potential to bring internet-like speeds (exceeding Gb/s) to data transmission in space. One of the major challenges of the laser communications is the necessity to accurately direct narrow beams. In the present talk, we discuss how electrically reconfigurable active metasurfaces can address this challenge. We perform an optical link budget analysis and discuss how the link range can be extended by increasing the aperture diameter or the input optical power. To assess maximal available output powers from realistic metasurface-based apertures, we experimentally probe the gate-tunable performance of indium tin oxide (ITO)-based active metasurfaces upon high-power illumination. 

Jared Sisler

Papers

Toward a universal metasurface for optical imaging, communication, and computation

Electrically controlled reconfigurable apertures for free space optical communications

Active metasurfaces for laser communications