A terahertz metamaterial is a class of composite metamaterials designed to interact at terahertz (THz) frequencies. The terahertz frequency range used in materials research is usually defined as 0.1 to 10 THz.This corresponds to wavelengths below the millimeter range, specifically between 3 millimeters (EHF band) and .03 millimeters; the long-wavelength edge of far-infrared light.
This bandwidth is also known as the terahertz gap because it is noticeably underutilized.The terahertz gap is the set of frequencies in the terahertz region (bandwidth) where unavailable materials have hindered construction of components and systems that might otherwise be universally available. This is because terahertz waves are electromagnetic waves with frequencies higher than microwaves but lower than infrared radiation and visible light. These characteristics mean that it is difficult to influence terahertz radiation with conventional electronic components and devices. Electronics technology controls the flow of electrons, and is well developed for microwaves and radio frequencies. 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. Finally, as a non-ionizing radiation it does not have the risks inherent in X-ray screening. Also see index for Presented Projects for Applied Optics 2009 and NEAR-Lab Thz measurement facility , Portland State University.What is Submillimeter Astronomy? . Arizona Radio Observatory. 2013
About metamaterials
Currently, a fundamental lack in naturally occurring materials that allow for the desired electromagnetic response has led to constructing new artificial composite materials, termed metamaterials. The metamaterials are based on a lattice structure which mimics crystal structures. However, the lattice structure of this new material consists of rudimentary elements much larger than atoms or single molecules, but is an artificial, rather than a naturally occurring structure. Yet, the interaction achieved is below the dimensions of the terahertz radiation wave. In addition, the desired results are based on the resonant frequency of fabricated fundamental elements. The appeal and usefulness is derived from a resonant response that can be tailored for specific applications, and can be controlled electrically or optically. Or the response can be as a passive material.
The development of electromagnetic, artificial-lattice structured materials, termed metamaterials, has led to the realization of phenomena that cannot be obtained with natural materials. This is observed, for example, with a natural glass lens, which interacts with light (the electromagnetic wave) in a way that appears to be one-handed, while light is delivered in a two-handed manner. In other words, light consists of an electric field and magnetic field. The interaction of a conventional lens, or other natural materials, with light is heavily dominated by the interaction with the electric field (one-handed). The magnetic interaction in lens material is essentially nil. This results in common optical limitations such as a diffraction barrier. Moreover, there is a fundamental lack of natural materials that strongly interact with light's magnetic field. Metamaterials, a synthetic composite structure, overcomes this limitation. In addition, the choice of interactions can be invented and re-invented during fabrication, within the laws of physics. Hence, the capabilities of interaction with the electromagnetic spectrum, which is light, are broadened.
Terahertz technology
Terahertz frequencies, or submillimeter wavelengths, which exist between microwave frequencies and infrared wavelengths, are virtually unused in the commercial sector, primarily due are limits to propagating the terahertz band through the atmosphere. However, terahertz devices have been useful in scientific applications, such as remote sensing and spectroscopy.
Terahertz metamaterial devices
Development of metamaterials has traversed the electromagnetic spectrum up to terahertz and infrared frequencies, but does not yet include the visible light spectrum. This is because, for example, it is easier to build a structure with larger fundamental elements that can control microwaves. The fundamental elements for terahertz and infrared frequencies have been progressively scaled to smaller sizes. In the future, visible light will require elements to be scaled even smaller, for capable control by metamaterials.
Along with the ability to now interact at terahertz frequencies is the desire to build, deploy, and integrate THz metamaterial applications universally into society. This is because, as explained above, components and systems with terahertz capabilities will fill a technologically relevant void. Because no known natural materials are available that can accomplish this, artificially constructed materials must now take their place.
Research has begun with first, demonstrating the practical terahertz metamaterial. 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. These are devices such as directed light sources, lenses, switches,Switching: The controlling or routing of signals in circuits to execute logical or arithmetic operations or to transmit data between specific points in a network. Note: Switching may be performed by electronic, optical, or electromechanical devices. Source: from Federal Standard 1037C modulators and sensors. This void also includes phase-shifting and beam-steering devicesBeam steering: is changing the direction of the main lobe of a radiation pattern. Note: In radio systems, beam steering may be accomplished by switching antenna elements or by changing the relative phases of the radio frequency radiation driving the elements. In optical systems, beam steering may be accomplished by changing the refractive index of the medium through which the beam is transmitted or by the use of mirrors or lenses. Source: from Federal Standard 1037C Real world applications in the THz band are still in infancy
Moderate progress has been achieved. Terahertz metamaterial devices have been demonstrated in the laboratory as tunable far-infrared filters, optical switching modulators, and metamaterial absorbers. The recent existence of a terahertz radiating source in general are THz quantum cascade lasers, optically pumped THz lasers, backward wave oscillators (BWO) and frequency multiplied sources. However, technologies to control and manipulate THz waves are lagging behind other frequency domains of the spectrum of light.
Furthermore, research into technologies which utilize THz frequencies show the capabilities for advanced sensing techniques. 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. This terahertz band has the distinction of being non-invasive and will therefore not disrupt or perturb the structure of the object being radiated. 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. Terahertz metamaterials, because they interact at the appropriate THz frequencies, seem to be one answer in developing materials which use THz radiation.
Researchers believe that artificial magnetic (paramagnetic) structures, or hybrid structures that combine natural and artificial magnetic materials, can play a key role in terahertz devices. Some THz metamaterial devices are compact cavities, adaptive optics and lenses, tunable mirrors, isolators, and converters.
Challenges in this field
Generating THz electromagnetic radiation
Without available terahertz sources, other applications are held back. In contrast, semiconductor devices have become integrated into everyday living. This means that commercial and scientific applications for generating the appropriate frequency bands of light commensurate with the semiconductor application or device are in wide use. Visible and infrared lasers are at the core of information technology. Moreover, at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications.
However, applications for the terahertz regime, previously defined as the terahertz gap of 0.1 to 10 THz, is an impoverished regime by comparison. Sources for generating the required THz frequencies (or wavelength) exist, but other challenges hinder their usefulness. Terahertz laser devices are not compact and therefore lack portability and are not easily integrated into systems. In addition, low-power-consumption, solid state terahertz sources are lacking. Furthermore, the current devices also have one or more shortcomings of low power output, poor tuning abilities, and may require cryogenic liquids for operation (liquid helium). Additionally, this lack of appropriate sources hinders opportunities in spectroscopy, remote sensing, free space communications, and medical imaging.
Meanwhile, potential terahertz frequency applications are being researched globally. Two recently developed technologies, Terahertz time-domain spectroscopy and quantum cascade lasers could possibly be part of a multitude of development platforms worldwide. However, the devices and components necessary to effectively manipulate terahertz radiation require much more development beyond what has been accomplished to date (2012).
Magnetic field interaction 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. The significant interaction (permittivity) occurs with the electric field. In natural materials, any useful magnetic interaction will taper off in the gigahertz range of frequencies. Compared to interaction with the electric field, the magnetic component is imperceptible when in terahertz, infrared, and visible light. So, a notable step occurred with the invention of a practical metamaterial at microwave frequencies,It was essentially a proof of principle demonstration, which was later commonly applied to the higher-frequency domain of terahertz and infrared. See negative index metamaterials. because the rudimentary elements of metamaterials have demonstrated a coupling and inductive response to the magnetic component commensurate with the electric coupling and response. This demonstrated the occurrence of an artificial magnetism,See main article: Paramagnetism and was later applied to terahertz and infrared electromagnetic wave (or light). In the terahertz and infrared domain, it is a response that has not been discovered in nature.
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. This degree of choice is not possible with conventional materials. 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.