Metasurfaces: From microwaves to visible

Applying metamaterial concepts to dielectric systems offers low losses compared with metallic structures. Here, silicon-based metamaterial and nanophotonic advances are reviewed. The prospect of creating metamaterials with optical properties greatly exceeding the parameter space accessible with natural materials has been inspiring intense research efforts in nanophotonics for more than a decade. Following an era of plasmonic metamaterials, low-loss dielectric nanostructures have recently moved into the focus of metamaterial-related research. This development was mainly triggered by the experimental observation of electric and magnetic multipolar Mie-type resonances in high-refractive-index dielectric nanoparticles. Silicon in particular has emerged as a popular material choice, due to not only its high refractive index and very low absorption losses in the telecom spectral range, but also its paramount technological relevance. This Review overviews recent progress on metamaterial-inspired silicon nanostructures, including Mie-resonant and off-resonant regimes.

Metamaterial-inspired silicon nanophotonics

Advances in reflectarrays and array lenses with electronic beam-forming capabilities are enabling a host of new possibilities for these high-performance, low-cost antenna architectures. This paper reviews enabling technologies and topologies of reconfigurable reflectarray and array lens designs, and surveys a range of experimental implementations and achievements that have been made in this area in recent years. The paper describes the fundamental design approaches employed in realizing reconfigurable designs, and explores advanced capabilities of these nascent architectures, such as multi-band operation, polarization manipulation, frequency agility, and amplification. Finally, the paper concludes by discussing future challenges and possibilities for these antennas.

Reconfigurable Reflectarrays and Array Lenses for Dynamic Antenna Beam Control: A Review

In recent years we have learned to fabricate structures smaller than electromagnetic wavelengths, and to assemble them into metamaterials with exotic optical properties for previously unimaginable applications. One such property is perfect absorption of incident light, with no reflection or transmission, across many wavelengths. The authors review the physics, design principles, and classification of thin perfect absorbers, and outline avenues for progress.

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Thin perfect absorbers for electromagnetic waves: Theory, design, and realizations

Metamaterials are being applied to the development and construction of many new devices throughout the electromagnetic spectrum. Limitations posed by the metamaterial operational bandwidth and losses can be effectively mitigated through the incorporation of tunable elements into the metamaterial devices. There are a wide range of approaches that have been advanced in the literature for adding reconfiguration to metamaterial devices all the way from the RF through the optical regimes, but some techniques are useful only for certain wavelength bands. A range of tuning techniques span from active circuit elements introduced into the resonant conductive metamaterial geometries to constituent materials that change electromagnetic properties under specific environmental stimuli. This paper presents a survey of the development of reconfigurable and tunable metamaterial technology as well as of the applications where such capabilities are valuable.

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Reconfigurable and Tunable Metamaterials: A Review of the Theory and Applications

The transformation optics technique for designing novel electromagnetic and optical devices offers great control over wave behavior, but is difficult to implement primarily due to limitations in current metamaterial design and fabrication techniques. This paper demonstrates that restricting the spatial transformation to a conformal mapping can lead to much simpler material parameters for more practical implementation. As an example, a flat cylindrical-to-plane-wave conversion lens is presented and its performance validated through numerical simulations. It is shown that the lens dimensions and embedded source location can be adjusted to produce one, two, or four highly directive planar beams. Two metamaterial designs for this lens that implement the required effective medium parameters are proposed and their behavior analyzed.

Conformal mappings to achieve simple material parameters for transformation optics devices

Integrated photonics is expected to play an increasingly important role in optical communications, imaging, computing and sensing with the promise for significant reduction in the cost and weight of these systems. Future advancement of this technology is critically dependent on an ability to develop compact and reliable optical components and facilitate their integration on a common substrate. Here we reveal, with the utility of the emerging transformation optics technique, that functional components composed of planar gradient index materials can be designed and readily integrated into photonic circuits. The unprecedented design flexibility of transformation optics allows for the creation of a number of novel devices, such as a light source collimator, waveguide adapters and a waveguide crossing, which have broad applications in integrated photonic chips and are compatible with current fabrication technology. Using the finite-difference time-domain method, we perform full-wave numerical simulations to demonstrate their superior optical performance and efficient integration with other components in an on-chip photonic system. These components only require spatially-varying dielectric materials with no magnetic properties, facilitating low-loss, broadband operation in an integrated photonic environment. Using transformation optics, researchers have built miniature gradient-index components for collimating, coupling and splitting light beams. Transformation optics is a popular theoretical approach for designing novel metamaterial-based photonic devices such as invisibility cloaks and perfect light absorbers. Qi Wu, Jeremiah Turpin and Douglas Werner from The Pennsylvania State University in the USA have now used transformation optics to design miniature gradient-index components. The small size and all-dielectric nature of these components means that they can be easily implemented by standard nanofabrication techniques such as silicon-on-insulator patterning, and are well-suited for integration with planar on-chip photonic systems. The researchers suggest that the approach could also yield nanoscale circuitry for controlling surface plasmons in applications such as optical communications, computing and sensing.

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Integrated photonic systems based on transformation optics enabled gradient index devices

Recent advancements in transformation optics (TO) and metamaterials have inspired tremendous interest in the electromagnetic community, creating a variety of novel antennas with enhanced performance, such as broad bandwidth, large gain, and high polarization efficiency. Although there could be infinitely many transformations for designing a given device, most of them result in rather complicated material compositions. This paper compares two recently introduced TO techniques, both of which lead to much simpler material requirements. In particular, a linear geometrical transformation or a quasi-conformal mapping was employed to design multi-beam collimating lenses, which possess either homogeneous or isotropic constituent materials. A systematic comparison is made for the first time between these two TO design approaches for a specific example of a quad-beam focusing lens, where the advantages and disadvantages of each method are clearly identified. Full-wave numerical simulations were performed to demonstrate the well-collimated beams produced by the TO lenses designed by either transformation. The characteristics of the two lens antennas, such as radiation pattern and bandwidth, were contrasted, providing valuable guidance on design tradeoffs for a specific application.

Transformation Optics Inspired Multibeam Lens Antennas for Broadband Directive Radiation

A reconfigurable metasurface made of Ge2Sb2Te5 phase-change material was experimentally demonstrated in the 1.55 μm wavelength range. A nanostructured Ge2Sb2Te5 film on fused silica substrate was optimized to switch from highly transmissive (80%) to highly absorptive (76%) modes with a 7:1 contrast ratio in transmission independent of polarization, when thermally transformed from the amorphous to crystalline state. The metasurface was designed using a genetic algorithm optimizer linked with an efficient full-wave electromagnetic solver.

Jeremiah P. Turpin

Papers

Reconfigurable and Tunable Metamaterials: A Review of the Theory and Applications

Conformal mappings to achieve simple material parameters for transformation optics devices

Integrated photonic systems based on transformation optics enabled gradient index devices

Transformation Optics Inspired Multibeam Lens Antennas for Broadband Directive Radiation

Reconfigurable near-IR metasurface based on Ge 2 Sb 2 Te 5 phase-change material