Metamaterial research is an extremely important global activity that promises to change our lives in many different ways. These include making objects invisible and the dramatic impact of metamaterials upon the energy and medical sectors of society. Behind all of the applications, however, lies the business of creating metamaterials that are not going to be crippled by the kind of loss that is naturally heralded by use of resonant responses in their construction. This review sets out some solutions to the management of loss and gain, coupled to controlled and nonlinear behavior, and discusses some critical consequences concerning stability. Under the general heading of active and tunable metamaterials, an international spectrum of authors collaborates here to present a set of solutions that addresses these issues in several directions. As will be appreciated, the range of possible solutions is really fascinating, and it is hoped that these discussions will act as a further stimulus to the field.

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Active and tunable 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

We demonstrate a thermally tunable optical metamaterial with negative permeability working in the visible range. By covering coupled metallic nanostrips with aligned nematic liquid crystals (NLCs), the magnetic response wavelength of the metamaterial is effectively tuned through control of the ambient temperature, changing the refractive index of LC via phase transitions. By increasing the ambient temperature from 20 degree to 50 degree, the magnetic response wavelength shifts from 650nm to 632nm. Numerical simulations confirm our tests and match the experimental observations well.

Tunable magnetic response of metamaterials

We demonstrate a thermally tunable optical metamaterial with negative permeability working in the visible range. By covering coupled metallic nanostrips with aligned nematic liquid crystals (NLCs), the magnetic response wavelength of the metamaterial is effectively tuned through control of the ambient temperature, changing the refractive index of LC via phase transitions. By increasing the ambient temperature from 20 to 50 °C, the magnetic response wavelength shifts from 650 to 632 nm. Numerical simulations confirm our tests and match the experimental observations well.

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This paper presents the results of a feasibility study for the design of multi-band tunable metamaterials based on the use of micro-split SRR (MSSRR) structures. In this study, we have designed and constructed a conventional split-ring resonator (SRR) unit cell (type A) and two modified SRR unit cells having the same design parameters except that they contain two (type B) or four (type C) additional micro-splits on the outer square ring, along the arm having the main split. Transmission characteristics of the resulting MSSRR cells are obtained both numerically and experimentally and compared to those of the ordinary SRR unit cell. It is observed that the presence of the additional micro-splits leads to the increase of resonance frequency by substantial amounts due to the series capacitance effect. Next, we have designed and constructed 2×2 homogeneous arrays of magnetic resonators which consist of the same type of cells (either A, or B, or C). Such MSSRR blocks are found to provide only a single frequency band of operation around the magnetic resonance frequency of the related unit cell structure. Finally, we have designed and constructed 2×2 and 3×2 inhomogeneous arrays which contain columns of different types of metamaterial unit cells. We have shown that these inhomogeneous arrays provide two or three different frequency bands of operations due to the use of different magnetic resonators together. The number of additional micro-splits in a given MSSRR cell can be interactively controlled by various switching technologies to modify the overall metamaterial topology for the purpose of activating different sets of multiple resonance frequencies. In this context, use of electrostatically actuated RF MEMS switches is discussed, and their implementation is suggested as a future work, to control the states of micro-splits in large MSSRR arrays to realize tunable multi-band metamaterials.

A tunable multi-band metamaterial design using micro-split SRR structures

Tunable properties of materials are highly desirable in many applications. Metamaterials with negative properties (permittivity, permeability, or both) can have many applications if such properties can be made tunable or can be switched alternatively between positive and negative property behavior. This paper is a numerical study of the effect of substrate properties on the effective properties of a metamaterial slab. We present both simulation results and measurement data for a specific split-ring resonator structure for two different substrate thicknesses and demonstrate very good agreement. Then, using finite element simulation, varying the permittivity of the substrate from 1 to 14 while keeping its physical thickness fixed, we show that the resonance frequency drops from ∼16to∼6GHz. Alternately, when the physical thickness of the substrate is varied from 0.05to2mm, keeping its permittivity fixed, the resonance frequency decreases from ∼13.2to∼9.2GHz. In each case, the effective refractive index is re...

Tuning the effective properties of metamaterials by changing the substrate properties

This paper calculates the S-parameters of the metamaterial structure using finite element simulation. To verify the simulation, a slab composed of arrays of the Omega structures was prepared, and the S-parameters of the metamaterial slab sample were measured using a focused beam free-space microwave measurement setup. The direct comparison of S-parameters shows perfect agreement of the simulation and the measurement. A strong resonance at about 7.5 GHz is observed

Direct Comparison of the Magnitude and Phase of Measured S-parameters of Metamaterials with Finite Element Simulation

Size and efficacy are two of the main challenges of an AR display. A compact form factor and low power consumption are highly desirable in AR glasses. Self-emitting display technologies such as OLED and MicroLED do not need illumination thus enabling simpler and smaller optics, but their efficacy is quite low, especially for pixel sizes of 5um or below. Spatial Light Modulator (SLM) based technologies, such as DMD, LCoS (Liquid Crystal on Silicon), and LBS (Laser Beam Scanning), take advantage of high efficacy light sources and can achieve a high brightness AR display combining low power consumption and high pixel density, which is highly preferable in many applications. The DMD, together with the efficient and ever-improving LED light source, has been highly favored in the battery-powered portable projection display market, due to its high efficacy in a compact form factor as well as its insensitivity to polarization. Here we present a compact optical engine architecture for AR glasses, taking into consideration both size and efficacy. The design simplifies the traditional DMD illumination to a compact size targeting a thin profile similar to a backlit LCD. This compact size is achieved with a small compromise to engine efficacy. Multiplying the pixel density with a glass plate actuator offers a performance improvement that easily justifies the slight increase in size. This architecture offers an excellent optical engine option for AR glasses which is both compact and high performance.

Zhongyan Sheng

Papers

Tuning the effective properties of metamaterials by changing the substrate properties

Direct Comparison of the Magnitude and Phase of Measured S-parameters of Metamaterials with Finite Element Simulation

Compact digital micromirror device (DMD)-based optical architecture for augmented reality (AR) glasses