Terahertz metamaterial
Likewise, the terahertz gap also borders optical or photonic wavelengths; the infrared, visible, and ultraviolet ranges (or spectrums), where well developed lens technologies also exist.
However, the terahertz wavelength, or frequency range, appears to be useful for security screening, medical imaging, wireless communications systems, non-destructive evaluation, and chemical identification, as well as submillimeter astronomy.
[11][12][13] Along with the ability to now interact at terahertz frequencies is the desire to build, deploy, and integrate THz metamaterial applications universally into society.
Moreover, since, many materials do not respond to THz radiation naturally, it is necessary then to build the electromagnetic devices which enable the construction of useful applied technologies operating within this range.
This void also includes phase-shifting and beam-steering devices[note 4] Real world applications in the THz band are still in infancy[8][11][13][14] Moderate progress has been achieved.
In areas where other wavelengths are limited, THz frequencies appear to fill the near future gap for advancements in security, public health, biomedicine, defense, communication, and quality control in manufacturing.
At the same time this frequency band demonstrates capabilities such as passing through and imaging the contents of a plastic container, penetrating a few millimeters of human skin tissue without ill effects, passing through clothing to detect hidden objects on personnel, and the detection of chemical and biological agents as novel approaches for counter-terrorism.
Some THz metamaterial devices are compact cavities, adaptive optics and lenses, tunable mirrors, isolators, and converters.
[16] Additionally, this lack of appropriate sources hinders opportunities in spectroscopy, remote sensing, free space communications, and medical imaging.
[6][14][15][17] As briefly mentioned above, naturally occurring materials such as conventional lenses and glass prisms are unable to significantly interact with the magnetic field of light.
Compared to interaction with the electric field, the magnetic component is imperceptible when in terahertz, infrared, and visible light.
This demonstrated the occurrence of an artificial magnetism,[note 6] and was later applied to terahertz and infrared electromagnetic wave (or light).
[12][18][19] Moreover, because the metamaterial is artificially fabricated during each step and phase of construction, this gives ability to choose how light, or the terahertz electromagnetic wave, will travel through the material and be transmitted.
The control is also derived from electrical-magnetic coupling and response of rudimentary elements that are smaller than the length of the electromagnetic wave travelling through the assembled metamaterial.
It was noted that the realization of magnetism at THz and higher frequencies will substantially affect terahertz optics and their applications.
[12][21] The first terahertz metamaterials able to achieve a desired magnetic response, which included negative values for permeability, were passive materials.
The results were believed to also show that the effect can be tuned throughout the terahertz frequency regime by scaling the dimensions of the structure.
In this case, the now combined and arrayed elements, along with attention to spacing, comprise a flat, rectangular, (planar) structured metamaterial.
In other words, a light source in free space, emits a polarized beam of radiation which is then reflected off the sample (see images to theright).
[12] With increasing frequency that approaches resonance over time the induced currents in the looped wire can no longer keep up with the applied field and the local response begins to lag.
[25] Answering this need, there are proposals for "active metamaterials" which can proactively control the proportion of transmission and reflection components of the source (EM) radiation.
[26][27] By combining metamaterial elements – specifically, split ring resonators – with Microelectromechanical systems technology – has enabled the creation of non-planar flexible composites and micromechanically active structures where the orientation of the electromagnetically resonant elements can be precisely controlled with respect to the incident field.
[29] The theory, simulation, and demonstration of a dynamic response of metamaterial parameters were shown for the first time with a planar array of split ring resonators (SRRs).
Research work has involved investigating, creating, and designing light-weight slow-wave vacuum electronics devices based on traveling wave tube amplifiers.
These are designs that involve folded waveguide, slow-wave circuits, in which the terahertz wave meanders through a serpentine path while interacting with a linear electron beam.
In order to ameliorate the power limitations due to small dimensions and high attenuation, novel planar circuit designs are also being investigated.
[2] In-house work at the NASA Glenn Research Center has investigated the use of metamaterials—engineered materials with unique electromagnetic properties to increase the power and efficiency of terahertz amplification in two types of vacuum electronics slow wave circuits.
The first type of circuit has a folded waveguide geometry in which anisotropic dielectrics and holey metamaterials are which consist of arrays of subwavelength holes (see image to the right).
[33] The second type of circuit has a planar geometry with a meander transmission line to carry the electromagnetic wave and a metamaterial structure embedded in the substrate.
[33][34] The possibility of controlling radiations in the terahertz regime is leading to analysis of designs for sensing devices, and phase modulators.
