Metasurfaces: From microwaves to visible

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

The ability to bend, stretch, and roll metamaterial devices on flexible substrates adds a new dimension to aspects of manipulating electromagnetic waves and promises a new wave of device designs and functionalities. This work reviews terahertz and optical metamaterials realized on flexible and elastomeric substrates, along with techniques and approaches to lend tunability to the devices. Substrate electromagnetic and mechanical characteristics suitable for flexible metamaterials are summarized for readers, followed by fabrication and processing techniques, and finally novel approaches used to-date to attain tunability. Future directions and emerging areas of interests are identified with these promising to transform metamaterial design and translate metamaterials into practical devices.

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Flexible metasurfaces and metamaterials: A review of materials and fabrication processes at micro- and nano-scales

The changing balance of forces at the nanoscale offers the opportunity to develop a new generation of spatially reconfigurable nanomembrane metamaterials in which electromagnetic Coulomb, Lorentz and Ampere forces, as well as thermal stimulation and optical signals, can be engaged to dynamically change their optical properties. Individual building blocks of such metamaterials, the metamolecules, and their arrays fabricated on elastic dielectric membranes can be reconfigured to achieve optical modulation at high frequencies, potentially reaching the gigahertz range. Mechanical and optical resonances enhance the magnitude of actuation and optical response within these nanostructures, which can be driven by electric signals of only a few volts or optical signals with power of only a few milliwatts. We envisage switchable, electro-optical, magneto-optical and nonlinear metamaterials that are compact and silicon-nanofabrication-technology compatible with functionalities surpassing those of natural media by orders of magnitude in some key design parameters.

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Reconfigurable nanomechanical photonic metamaterials

Programmable kirigami metamaterials with controllable local tilting orientations on demand through prescribed notches are constructed through a new approach of kiri-kirgami, and their actuation of pore opening via both mechanical stretching and temperature, along with their potential application as skins for energy-saving buildings, is discussed.

Programmable Kiri-Kirigami Metamaterials.

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

Recent developments in transformation optics have led to burgeoning research on gradient index lenses for novel optical systems. Such lenses hold great potential for the advancement of complex optics for a wide range of applications. Despite the plethora of literature on gradient index lenses, previous works have not yet considered the application of anti-reflective coatings to these systems. Reducing system reflections is crucial to the development of this technology for highly sensitive optical applications. Here, we present effective anti-reflective-coating designs for gradient index lens systems. Conventional anti-reflective-design methodologies are leveraged in conjunction with transformation optics to develop coatings that significantly reduce reflections of a flat gradient index lens. Finally, the resulting gradient-index anti-reflective coatings are compared and contrasted with conventional homogeneous anti-reflective coatings.

Transformation-optics-inspired anti-reflective coating design for gradient index lenses.

Broadband performance of lenses designed with quasi-conformal transformation optics

We present the use of quasi-conformal transformation optics techniques to design integrated photonic components on graphene sheets. First, we discuss the required design equations necessary for associating the permittivity tensors achieved from transformation optics techniques to the electronic transport properties of graphene. We then present the design of several novel integrated photonic components using the proposed process, including a quad-beam collimator, waveguide coupler, and waveguide crosser. The theoretical operation of each device will be presented and possible fabrication considerations will be discussed.

Quasi-conformal transformation optics techniques for graphene-based integrated photonic components

There are often discrepancies between the behavior of a metamaterial reflector unit cell in an infinitely periodic environment and in a non-ideal environment in which the reflection phase varies across a metamaterial reflector. This work considers the performance of several capacitively loaded metamaterial unit cells in order to improve the realized gain of a reflector antenna. Each structure exploits capacitive elements to achieve a desired reflection phase gradient across the surface of the metamaterial reflector. The influence of interconnectedness and surface currents on the reflector antenna performance are analyzed and compared with the antenna performance from an ideal impedance surface reflector.

Kenneth L. Morgan

Papers

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

Transformation-optics-inspired anti-reflective coating design for gradient index lenses.

Broadband performance of lenses designed with quasi-conformal transformation optics

Quasi-conformal transformation optics techniques for graphene-based integrated photonic components

Design techniques for loss mitigation in metamaterial reflector antennas