Tunable metamaterial
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.
[5] For example, H. T. Chen, in 2008, were able to fabricate a repeating split-ring resonator (SRR) cell with semiconductor material aligning the gaps.
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.
[5] In a section below, a research team reported a tunable negative index medium using copper wires and ferrite sheets.
[3] In aerospace applications (for example) negative index metamaterials are likely candidates for tunable, compact and lightweight phase shifters.
Because the designated metamaterials can handle the appropriate power levels, have strong dispersion characteristics, and are tunable in the microwave range these show potential to be desirable phase shifters.
The liquid crystal material appears to be used as both a substrate and a jacket for a negative index metamaterial.
In addition, a liquid crystal has the inherent properties to be both intrinsically tunable and provide tuning for the metal arrays.
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.
[12] 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.
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 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 incorporation, application, and location of semiconductor material is strategically planned so as to be strongly coupled at the resonance frequency of the metamaterial elements.
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.
[6] Terahertz (THz) metamaterials can show a tunable spectral range, where the magnetic permeability reaches negative values.
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.
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.
The expense of regular space missions to access a single piece of equipment for tuning and maintenance would be prohibitive.
This has resulted in smaller cell size along with increases in bandwidth and the capability to shift frequencies in real time for artificial materials.
At low frequencies the physics of the interactions is essentially defined by the LC model analysis and numerical simulation.
[17] A type of FSS based metamaterial has the interchangeable nomenclature Artificial Magnetic Conductor (AMC) or High Impedance Surface (HIS).
Furthermore, control of the radio frequency or microwave radiation pattern is efficiently increased, and mutual coupling between antennas is also reduced.
AMC, or HIS structures often emerge from an engineered periodic dielectric base along with metallization patterns designed for microwave and radio frequencies.
Furthermore, two inherent notable properties, which cannot be found in natural materials, have led to a significant number of microwave circuit applications.
For example, AMC surfaces as antenna ground planes are able to effectively attenuate undesirable wave fluctuations, or undulations, while producing good radiation patterns.
They behave as a network of parallel resonant LC circuits, which act as a two-dimensional electric filter to block the flow of currents along the sheet.
[19] This structure can then serve as an artificial magnetic conductor (AMC), because of its high surface impedance within a certain frequency range.
In addition, as an artificial magnetic conductor it has a forbidden frequency band, over which surface waves and currents cannot propagate.
[19] The uniplanar compact photonic-bandgap (UC-PBG) is proposed, simulated, and then constructed in the lab to overcome elucidated limitations of planar circuit technology.
