Tunable metamaterial

A tunable metamaterial is a metamaterial which has the capability to arbitrarily adjust changes in the magnetic response of the medium, or the response of the refractive index. It encompasses the developments beyond the bandwidth limitations in left-handed materials by constructing various types of metamaterials.

Tunable metamaterials are a segment of the continuing research into metamaterials that produce a negative refractive index. Since conventional materials exhibit very weak coupling through the magnetic component of the electromagnetic wave, artificial materials that have this abiltity are being researched and fabricated. These artificial materials are known as metamaterials. The first of these have been fabricated (in the lab) to respond to only one frequency or one frequency band. These were followed by demonstrations of metamtaerials that were tunable only by changing the geometry and/or position of their components. These have been followed by metamaterials that are tunable in wider frequency ranges.

Frequency selective surface based metamaterials

Frequency selective surface (FSS) has become an alternative to the fixed frequency metamaterial, where static geometries and spacings of unit cells determine the frequency response of a given metamaterial. For each different frequency response a new set of geometrical shapes and spacings would have to be embedded in a newly fabricated material. This is discussed below in a section "Tunable NIMs using yttrium iron garnet."

FSS based metamaterials allow for optional changes of frequencies in a single medium (metamaterial) rather than a restricition to a fixed frequency response. An example of where this altenative is highly advantageous is in deep space or even a craft in orbit. The expense of regular missions to access a single piece of equipment for tuning and maintenance would prohibitive. Remote tuning, in this case, is advantageous.

FSS was first developed to control the transmission and reflection characteristics of an incident wave. This has resulted in smaller cell size along with increases in bandwidth and the capability to shift frequencies in real time for artificial materials.

This type of structure can be used to create a metamaterial surface with the intended application of artificial magnetic conductors or applications for boundary conditions. Another application is as stopband device for surface wave propagation along the interface. This is because surface waves are a created as a consequence of an interface between two media having dissimilar refractive indexes. Depending on the application of the system that includes the two media, there may be a need to attenuate surface waves or utilize them.

An FSS based metamaterial employs a (miniature) model of equivalent LC circuitry. At low frequencies the physics of the interactions is essentially defined by the static LC model. At higher frequencies the static LC concepts become unavailable. This is due to dependence on phasing and wave polarization. When the FSS is engineered for electromagnetic band gap (EBG) characteristics, the FSS is designed to enlarge its stop band properties in relation to dispersive, surface wave (SW) frequencies. Furthermore, as an EBG it is designed to reduce its dependence on the propagating direction of the surface wave travelling across the surface (interface).

Tunable NIMs using yttrium iron garnet

A tunable negative index metamaterial was demonstrated in 2006, and published in 2007. Negative index metamaterials (NIMs) have become fascinating because of notable capabilities such as backward wave propagation and subwavelength resolution imaging. These have novel applications such as super lenses, leaky wave antennas, and miniature delay lines. Before the fabrication of a "tunable material", most NIMs were resonant metamaterials (SRR), photonic crystals or planar periodic arrays of passive lumped circuit elements, (metamaterial antennas and waveguide elements). These operated in a narrow bandwidth and were not tunable – different parameters would have to be embedded in a new fabrication of the same material. In other words, to operate at a different frequency the periodicity and size of the elements would have to be changed. Furthermore, scaling the materials to terahertz and optical frequencies has proved challenging. Instead, by using a thin yttrium-iron-garnet (YIG) slab, or in other instances - an anisotropic liquid crystal, continuous frequency tuning of the negative index is possible.

The YIG film allowed for a continuously tunable negative permeability, which resulted in a tunable frequency range over the higher frequency side of the ferromagnetic resonance of the (YIG). Complementary negative permittivity is achieved using a single periodic array of copper wires. Eight wires were spaced 1 mm apart and a ferromagnetic film of a multilayered YIG at 400 mm thickness was placed in a K band waveguide. The YIG film was applied to both sides of a gadolinium gallium garnet substrate of 0.5 mm thickness. Ferromagnetic resonance was induced when the external H magnetic field was applied along the X axis.

The external magnetic field was generated with an electromagnet. A pair of E–H tuners were connected before and after the waveguide containing the NIM composite. The tunability was demonstrated from 18 to 23 GHz. Theoretical analysis, which followed, closely matched the experimental results.

An air gap was built into the structure between the array of copper wires and the YIG. This reduces coupling with the ferrite, YIG material. When negative permeability is achieved across a range of frequencies, the interaction of the ferrite with the wires in close proximity, reduces the net current flow in the wires. This is the same as moving toward positive permittivity. This would be an undesired result as the material would no longer be a NIM. The separation also reduces the effective loss of the dielectric, induced by the interaction of the wire's self-field with permeability. Furthermore, there are two sources of conduction in the copper wire. First, the electric field in a (microwave) waveguide creates a current in the wire. Second, any arbitrary magnetic field created by the ferrite when it moves into a perpendicular configuration induces a current. Additionally, at frequencies where µ is negative, the induced microwave magnetic field is opposite to the field excited in a TE10 mode of propagation in a waveguide. Hence, the induced current is opposite to the current resulting from the electric field in a waveguide.

Metamaterial phase shifter

Phase shifters are critical elements in several electronically tuned microwave systems in defense, space and commercial communications applications. The excessive cost and weight of phase shifters has limited deployment of electronically scanned antennas in some aerospace applications. There is a significant demand in the industry for lightweight, high-powered phase shifters in microwave systems. Recent advances in metamaterials possessing negative index of refraction, and strong dispersion characteristics that are tunable in the microwave range, are seen as a new opportunity for novel microwave technologies. The phase shifter is derived, in part from a previously demonstrated tunable negative index metamaterials (NIMs).

The tunability and low loss observed in this type of NIMs make them ideal materials for designing tunable, compact and lightweight phase shifters. Theoretical and experimental research of "field tunable NIM" was a combination of yttrium iron garnet (YIG) and K-band waveguide (copper wires) are the basis for magnetic field tunability in the microwave frequency region. Furthermore, the permeability of the NIM was simultaneously tuned along with refractive index. The change in permeability or refractive index leads to a change in phase velocity of the signal and therefore the phase of the transmission coefficient.

Tunable Josephson metamaterial

The parametric amplifier consists of a metamaterial embedded in a half-wavelength microwave cavity. The metamaterial is a superconducting niobium coplanar waveguide where the center conductor is a series array of 480 Josephson junctions. Each junction is split into two junctions in parallel, forming a SQUID magnetometer and making the phase velocity of the metamaterial tunable with magnetic flux enclosed by the SQUID.

Liquid crystal tuning for metamaterials

Liquid crystal metamaterial tunable in the near-infrared

Near-infrared metamaterials can possess a reconfigurable index of refraction from negative through zero to positive values. Reconfigurability is achieved by cladding thin layers of liquid crystal both as a superstrate and a substrate on an established negative-index metamaterial, and adjusting the permittivity of the liquid crystal.

Tunability of wire-grid metamaterial immersed into nematic liquid crystal

Sub-wavelength metal arrays, essentially another form of metamaterial, usually operate in the microwave and optical frequencies. A liquid crystal is both transparent and anisotropic at those frequencies. In addition, a liquid crystal has the inherent properties to be both intrisincally tunable and provide tuning for the metal arrays. This method of tuning a type of metamaterial can be readily used as electrodes for applying switching voltages.

Tunable optical NIMs employing anisotropic liquid crystals

There are two possible design approaches for tunable optical NIMs that incorporate anisotropic liquid crystals (LCs). The first approach utilizes an external field to change the director orientation of the LC molecules to tune the refractive index of the metamaterial, while the second approach employs temperature. The LC is used to tune the response of the magnetic resonator rather than changing the electric properties of the NIM. Second, the inherent anisotropic properties of the LCs are employed in this method.

Liquid crystal tuning of silicon-on-ring-resonators

Ring resonators are optical devices designed to show resonance for specific wavelengths. In silicon-on-insulator layered structures, they can be very small, exhibit a high Q factor and have low losses that make them efficient wavelength-filters. The goal is to achieve a tunable refractive index over a larger bandwidth.

Structural tunability in metamaterials

A novel approach is proposed for efficient tuning of the transmission characteristics of metamaterials through a continuous adjustment of the lattice structure, and confirm it experimentally in the microwave range.

Tunable teraherz metamaterials

Semiconductor controlled metamaterial

Hybrid metamaterials will still exhibit ‘passive’ properties (such as a negative electric response, negative index or gradient index), as determined by the patterning of the metamaterial elements. However, the aforementioned coupling engenders control of the passive metamaterial response by means of an external stimulus of the natural material response (such as photoconductivity, nonlinearity, gain). For example, amplitude control has been demonstrated through carrier injection13,14 or depletion in a semiconductor substrate.

Terahertz electric field control

Teraherz (THz) metamaterials can show a tunable spectral range, where the magnetic permeability reaches negative values. These values were established both theoretically and experimentally. The demonstrated principle represents a step forward toward a metamaterial with negative refractive index capable of covering continuously a broad range of THz frequencies and opens a path for the active manipulation of millimeter and submillimeter beams.

See also

• Acoustic metamaterials
• Metamaterial
• Negative index metamaterials
• Metamaterial antennas
• Perfect lens
• Photonic metamaterials
• Photonic crystal
• Split-ring resonator
• Terahertz metamaterials

Electromagnetic interactions

• Bloch wave
• EM radiation
• Electron mobility
• Permeability (electromagnetism)*
• Permittivity*
• Wavenumber
• Photo-dember
• Impedance