Tunable metamaterial

Since natural materials exhibit very weak coupling through the magnetic component of the electromagnetic wave, artificial materials that exhibit a strong magnetic coupling are being researched and fabricated. These artificial materials are known as metamaterials. The first of these were fabricated (in the lab) with an inherent, limited, response to only a narrow frequency band at any given time. Its main purpose was to practically demonstrate metamaterials. The resonant nature of metamaterials results in frequency dispersion and narrow bandwidth operation where the center frequency is fixed by the geometry and dimensions of the rudimentary elements comprising the metamaterial composite. These were followed by demonstrations of metamaterials 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 along with strategies for varying the frequencies of a single medium (metamaterial). This is in contrast to the fixed frequency metamaterial, which is determined by the imbued parameters during fabrication. ==Tuning strategies for split ring resonators==

Metamaterial-based devices could come to include filters, modulators, amplifiers, transistors, and resonators, among others. The usefulness of such a device could be extended tremendously if the metamaterial’s response characteristics can be dynamically tuned. Control of the effective electromagnetic parameters of a metamaterial is possible through externally tunable components. Single element control Studies have examined the ability to control the response of individual particles using tunable devices such as varactor diodes, semiconductor materials, and barium strontium titanate (BST) thin films. For example, H. T. Chen, in 2008, were able to fabricate a repeating split-ring resonator (SRR) cell with semiconductor material aligning the gaps. This initial step in metamaterial research expanded the spectral range of operation for a given, specific, metamaterial device. Also this opened the door for implementing new device concepts. The importance of incorporating the semiconductor material this way is noted because of the higher frequency ranges at which this metamaterial operates. It is suitable at terahertz (THz) and higher frequencies, where the entire metamaterial composite may have more than 104 unit cells, along with bulk-vertical integration of the tuning elements. Strategies employed for tuning at lower frequencies would not be possible because of the number of unit cells involved. The semiconductor material, such as silicon, is controlled by photoexcitation. This in turn controls, or alters, the effective size of the capacitor and tunes the capacitance. The whole structure is not just semiconductor material. This was termed a 'hybrid', because the semiconductor material was fused with dielectric material; a silicon-on-sapphire (SOS) wafer. Wafers were then stacked - fabricating a whole structure. Multi-element control A multielement tunable magnetic medium was reported by Zhao et al. This structure immersed SRRs in liquid crystals, and achieved a 2% tunable range. BST-loaded SRRs comprising tunable metamaterial, encapsulates all of the tunability within the SRR circuit. In a section below, a research team reported a tunable negative index medium using copper wires and ferrite sheets. The negative permeability behavior appears to be dependent on the location and bandwidth of the ferrimagnetic resonance, a break from wholly non-magnetic materials, which produces a notable negative index band. A coil or permanent magnetic is needed to supply the magnetic field bias for tuning. Electrical tuning Electrical tuning for tunable metamaterials. Magnetostatic control Magnetostatic control for tunable metamaterials. Optical pumping Optical pumping for tunable metamaterials. ==Tunable NIMs using ferrite material==

Yttrium iron garnet (YIG) films allow for a continuously tunable negative permeability, which results 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 multi-layered 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. Pairs 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. ==Liquid crystal tuning for metamaterials==

Tuning plasmonic resonances in metamaterials with liquid crystals Metamaterials containing metallic elements can support a wide range of plasmonic resonances. Introducing a liquid crystal layer into such structures enables control over the wavelength and amplitude of resonance peaks observed in transmittance or reflectance spectra. This tuning arises from reorienting the liquid crystal director, which modifies its dielectric permittivity tensor. Two main mechanisms are responsible: first, the sensitivity of plasmonic resonances to changes in the dielectric environment; and second, the influence of the liquid crystal on the polarization state of incident light before it interacts with the metallic components of the metastructure. Liquid crystals are therefore widely investigated for the active tuning of various plasmonic resonances, including localized surface plasmon resonances and surface lattice resonances in periodic arrays of nanoparticles, Tamm plasmon resonances, and others. Liquid crystal metamaterial tunable in the near-infrared Tuning in the near infrared range is accomplished by adjusting the permittivity of an attached nematic liquid crystal. The liquid crystal material appears to be used as both a substrate and a jacket for a negative index metamaterial. The metamaterial can be tuned from negative index values, to zero index, to positive index values. In addition, negative index values can be increased or decreased by this method. 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 intrinsically 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. Tuning NIMs with liquid crystals Areas of active research in optical materials are metamaterials that are capable of negative values for index of refraction (NIMs), and metamaterials that are capable of zero index of refraction (ZIMs). Complicated steps required to fabricate these nano-scale metamaterials have led to the desire for fabricated, tunable structures capable of the prescribed spectral ranges or resonances. The most commonly applied scheme to achieve these effects is electro-optical tuning. Here the change in refractive index is proportional to either the applied electric field, or is proportional to the square modulus of the electric field. These are the Pockels effect and Kerr effect, respectively. However, to achieve these effects electrodes must be built-in during the fabrication process. This introduces problematic complexity into material formation techniques. Another alternative is to employ a nonlinear optical material as one of the constituents of this system, and depend on the optical field intensity to modify the refractive index, or magnetic parameters. 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 is confirmed experimentally in the microwave range. ==Hybrid metamaterial composites==

Metamaterials were originally researched as a passive response material. The passive response was and still is determined by the patterning of the metamaterial elements. In other words, the majority of research has focused on the passive properties of the novel transmission, e.g., the size and shape of the inclusions, the effects of metal film thickness, hole geometry, periodicity, with passive responses such as a negative electric response, negative index or gradient index etc. In addition, the resonant response can be significantly affected by depositing a dielectric layer on metal hole arrays and by doping a semiconductor substrate. The result is significant shifting of the resonance frequency. However, even these last two methods are part of the passive material research. Electromagnetic metamaterials can be viewed as structured composites with patterned metallic subwavelength inclusions. As mesoscopic physical systems, these are built starting from the unit cell level. These unit cells are designed to yield prescribed electromagnetic properties. A characteristic of this type of metamaterial is that the individual components have a resonant (coupling) response to the electric, magnetic or both components of the electromagnetic radiation of the source. The EM metamaterial as an artificially designed transmission medium, has so far delivered desired responses at frequencies from the microwave through to the near visible. The introduction of a natural semiconductor material within or as part of each metamaterial cell results in a new design flexibility. The incorporation, application, and location of semiconductor material is strategically planned so as to be strongly coupled at the resonance frequency of the metamaterial elements. The hybrid metamaterial composite is still a passive material. However, the coupling with the semiconductor material then allows for external stimulus and control of the hybrid system as a whole, which produces alterations in the passive metamaterial response. External excitation is produced in the form, for example, photoconductivity, nonlinearity, or gain in the semiconductor material.{{Cite journal| last1 =Chen| first1 =Hou-Tong| last2 =O'Hara| first2 =John F.| last3 =Azad| first3 =Abul K.| last4 =Taylor| first4 =Antoinette J.|author4-link=Antoinette Taylor| last5 =Averitt| first5 =Richard D.| last6 =Shrekenhamer| first6 =David B.| last7 =Padilla| first7 =Willie J.|title =Experimental demonstration of frequency-agile terahertz metamaterials|journal =Nature Photonics|volume =2|page =295 |date =May 2008| url =http://www2.bc.edu/~padillaw/PDF/Nature_Photonics_2_295_2008.pdf ==Tunable spectral range via electric field control==

Terahertz (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. ==Frequency selective surface based metamaterials==

Frequency selective surfaces (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. Because arrayed unit cells maintain static positions throughout operation, a new set of geometrical shapes and spacings would have to be embedded in a newly fabricated material for each different radiated frequency and response. Instead, FSS based metamaterials allow for optional changes of frequencies in a single medium (metamaterial) rather than a restriction to a fixed frequency response. An example of where this alternative is highly advantageous is in deep space or with a satellite or telescope in orbit. The expense of regular space missions to access a single piece of equipment for tuning and maintenance would be prohibitive. Remote tuning, in this case, is advantageous. 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 LC model analysis and numerical simulation. This is also known as the static LC model. At higher frequencies the static LC concepts become unavailable. This is due to dependence on phasing. When the FSS is engineered for electromagnetic band-gap (EBG) characteristics, the FSS is designed to enlarge its stopband properties in relation to dispersive, surface wave (SW) frequencies (microwave and radio frequencies). Furthermore, as an EBG it is designed to reduce its dependence on the propagating direction of the surface wave traveling across the surface (interface). Description An HIS, or AMC, can be described as a type of electromagnetic band gap (EBG) material or a type of synthetic composite that is intentionally structured with a magnetic conductor surface for an allotted, but defined range of frequencies. AMC, or HIS structures often emerge from an engineered periodic dielectric base along with metallization patterns designed for microwave and radio frequencies. The metalization pattern is usually determined by the intended application of the AMC or HIS structure. Furthermore, two inherent notable properties, which cannot be found in natural materials, have led to a significant number of microwave circuit applications. The phase of the reflected electric field has normal incidence the same phase of the electric field impinging at the interface of the reflecting surface. The variation of the reflection phase is continuous between +180◦ and −180◦ relative to the frequency. Zero is crossed at one frequency, where resonance occurs. A notable characteristic is that the useful bandwidth of an AMC is generally defined as +90◦ to −90◦ on either side of the central frequency. AMC as an FSS band gap s. :Bottom image - Looking down on top of the high-impedance surface, showing a triangular lattice of hexagonal metal plates. The configuration creates a capacitive and inductive surface. It can be utilized as band gap material at prescribed frequencies. It is also designed to enhance antenna operation as a novel periodic material. At radio frequencies, the fields associated with surface waves can extend thousands of wavelengths into the surrounding space, and they are often best described as surface currents. They can be modeled from the viewpoint of an effective dielectric constant, or an effective surface impedance. The surface impedance is derived from the ratio of the electric field at the surface to the magnetic field at the surface, which extends far into the metal beyond the skin depth. When a texture is applied to the metal surface, the surface impedance is altered, and its surface wave properties are changed. At low frequencies, it is inductive, and supports transverse-magnetic (TM) waves. At high frequencies, it is capacitive, and supports transverse electric (TE) waves. Near the LC resonance frequency, the surface impedance is very high. In this region, waves are not bound to the surface. Instead, they radiate into the surrounding space. A high-impedance surface was fabricated as a printed circuit board. The structure consists of a triangular lattice of hexagonal metal plates, connected to a solid metal sheet by vertical conducting vias. ==Uniplanar compact photonic-bandgap==

The uniplanar compact photonic-bandgap (UC-PBG) is proposed, simulated, and then constructed in the lab to overcome elucidated limitations of planar circuit technology. Like photonic bandgap structures it is etched into the ground plane of the microstrip line. The geometry is square metal pads. Each metal pad has four connecting branches forming a distributed LC circuit.{{Cite book ==See also==