A terahertz metamaterial is a class of composite
metamaterial
A metamaterial (from the Greek word μετά ''meta'', meaning "beyond" or "after", and the Latin word ''materia'', meaning "matter" or "material") is any material engineered to have a property that is not found in naturally occurring materials. ...
s designed to interact at
terahertz (THz) frequencies. The terahertz
frequency range
A frequency band is an interval in the frequency domain, delimited by a lower frequency and an upper frequency. The term may refer to a radio band or an interval of some other spectrum.
The frequency range of a system is the range over which i ...
used in
materials research is usually defined as 0.1 to 10
THz.
[This corresponds to ]wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s below the millimeter
330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter.
The millimetre (American and British English spelling differences#-re, -er, ...
range, specifically between 3 millimeters ( EHF band) and .03 millimeters; the long-wavelength edge of far-infrared light.
This
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
is also known as the
terahertz gap
Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency
(THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of freq ...
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 wave
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
s with
frequencies
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
higher than
microwaves
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency rang ...
but lower than
infrared radiation
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
and
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
. These characteristics mean that it is difficult to influence terahertz radiation with conventional
electronic
Electronic may refer to:
*Electronics, the science of how to control electric energy in semiconductor
* ''Electronics'' (magazine), a defunct American trade journal
*Electronic storage, the storage of data using an electronic device
*Electronic co ...
components and devices. Electronics technology controls the flow of
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s, and is well developed for
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
s and
radio frequencies
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upper ...
. Likewise, the terahertz gap also borders optical or
photonic wavelengths; the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
,
visible
Visibility, in meteorology, is a measure of the distance at which an object or light can be seen.
Visibility may also refer to:
* A measure of turbidity in water quality control
* Interferometric visibility, which quantifies interference contrast ...
, and
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
ranges (or
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
s), where well developed
lens technologies also exist. However, the terahertz
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
, or
frequency range
A frequency band is an interval in the frequency domain, delimited by a lower frequency and an upper frequency. The term may refer to a radio band or an interval of some other spectrum.
The frequency range of a system is the range over which i ...
, appears to be useful for security screening,
medical imaging
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to rev ...
,
wireless communication
Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...
s systems,
non-destructive evaluation
Nondestructive testing (NDT) is any of a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage.
The terms nondestructive examination (NDE), n ...
, and chemical identification, as well as
submillimeter astronomy
Submillimetre astronomy or submillimeter astronomy (see spelling differences) is the branch of observational astronomy that is conducted at submillimetre wavelengths (i.e., terahertz radiation) of the electromagnetic spectrum. Astronomers plac ...
. Finally, as a
non-ionizing radiation
Non-ionizing (or non-ionising) radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum (photon energy) to ionize atoms or molecules—that is, to completely remove an electron from an atom or molec ...
it does not have the risks inherent in
X-ray screening.
[ Also see index fo]
Presented Projects for Applied Optics 2009
an
Portland State University.
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
metamaterial
A metamaterial (from the Greek word μετά ''meta'', meaning "beyond" or "after", and the Latin word ''materia'', meaning "matter" or "material") is any material engineered to have a property that is not found in naturally occurring materials. ...
s. The metamaterials are based on a lattice structure which mimics
crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...
s. 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
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
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
A phenomenon ( : phenomena) is an observable event. The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be directly observed. Kant was heavily influenced by Gottfried W ...
that cannot be obtained with natural material A natural material is any product or physical matter stop changing it. Minerals and the metals that can be extracted from them (without further modification) are also considered to belong into this category. Natural materials are used as building ma ...
s. This is observed, for example, with a natural glass lens, which interacts with light (the electromagnetic wave
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
) 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
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
and magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
. The interaction of a conventional lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
, 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
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
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
Scientific laws or laws of science are statements, based on repeated experiments or observations, that describe or predict a range of natural phenomena. The term ''law'' has diverse usage in many cases (approximate, accurate, broad, or narrow) a ...
. Hence, the capabilities of interaction with the electromagnetic spectrum
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...
, which is light, are broadened.[
]
Terahertz technology
Terahertz frequencies, or submillimeter
Submillimetre astronomy or submillimeter astronomy (see spelling differences) is the branch of observational astronomy that is conducted at submillimetre wavelengths (i.e., terahertz radiation) of the electromagnetic spectrum. Astronomers plac ...
wavelengths, which exist between microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
frequencies and infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
wavelengths can be metaphorically termed "unclaimed territory" where almost no devices exist. Because there are limits to propagating the terahertz band through the atmosphere, the commercial sector has remained uninvolved with such technological development. However, terahertz devices have been useful in the remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
and spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
areas. Moreover, a rich vein of knowledge has been amassed via submillimeter observation techniques. In particular, interdisciplinary researchers involved with astronomy, chemistry, earth science
Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four spheres ...
, planetary science
Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their f ...
, and space science
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consider ...
, have studied thermal emission lines
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
for a diverse and large assortment of gas molecules
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
. The amount of information obtained is specifically amenable to this particular band of electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
. Indeed, the cosmos is suffused in terahertz energy, and meanwhile, almost all of it appears to be overlooked, disregarded, or simply unidentified.[
]
Terahertz metamaterial devices
Development of metamaterials has traversed the electromagnetic spectrum up to terahertz and infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
frequencies, but does not yet include the visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
spectrum. This is because, for example, it is easier to build a structure with larger fundamental elements that can control microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
s. 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
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
, switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
es,[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 modulator
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
s and sensor
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s. This void also includes phase-shifting and beam-steering devices[Beam steering:]
is changing the direction of the main lobe
In a radio antenna's radiation pattern, the main lobe, or main beam, is the lobe containing the higher power. This is the lobe that exhibits the greater field strength.
The radiation pattern of most antennas shows a pattern of "''lobes''" at v ...
of a radiation pattern
In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: “A ...
. Note: In radio systems, beam steering may be accomplished by switching antenna elements or by changing the relative phases of the radio frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
radiation driving the elements. In optical systems, beam steering may be accomplished by changing the refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
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
Far infrared (FIR) is a region in the infrared spectrum of electromagnetic radiation. Far infrared is often defined as any radiation with a wavelength of 15 micrometers (μm) to 1 mm (corresponding to a range of about 20 THz to ...
filters, optical switching modulators, and metamaterial absorber
A metamaterial absorber is a type of metamaterial intended to efficiently absorb electromagnetic radiation such as light. Furthermore, metamaterials are an advance in materials science. Hence, those metamaterials that are designed to be absorbers ...
s. The recent existence of a terahertz radiating source in general are THz quantum cascade laser Quantum-cascade lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jérôme Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, a ...
s, 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
Research is "creativity, creative and systematic work undertaken to increase the stock of knowledge". It involves the collection, organization and analysis of evidence to increase understanding of a topic, characterized by a particular att ...
into technologies which utilize THz frequencies show the capabilities for advanced sensing techniques. In areas where other wavelengths
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
are limited, THz frequencies appear to fill the near future gap for advancements in security, public health
Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the det ...
, biomedicine
Biomedicine (also referred to as Western medicine, mainstream medicine or conventional medicine) , defense
Defense or defence may refer to:
Tactical, martial, and political acts or groups
* Defense (military), forces primarily intended for warfare
* Civil defense, the organizing of civilians to deal with emergencies or enemy attacks
* Defense industr ...
, communication
Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...
, and quality control
Quality control (QC) is a process by which entities review the quality of all factors involved in production. ISO 9000 defines quality control as "a part of quality management focused on fulfilling quality requirements".
This approach places ...
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
Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image).
Imaging technology is the application of materials and methods to create, preserve, or duplicate images.
...
the contents of a plastic container
Plastic containers are containers made exclusively or partially of plastic. Plastic containers are ubiquitous either as single-use or reuseable/durable plastic cups, plastic bottles, plastic bags, foam food containers, Tupperware, plastic tubes ...
, penetrating a few millimeters
330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter.
The millimetre (American and British English spelling differences#-re, -er, ...
of human skin tissue without ill effects, passing through clothing to detect hidden objects on personnel, and the detection of chemical
A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...
and biological agent
A biological agent (also called bio-agent, biological threat agent, biological warfare agent, biological weapon, or bioweapon) is a bacterium, virus, protozoan, parasite, fungus, or toxin that can be used purposefully as a weapon in bioterroris ...
s as novel approaches for counter-terrorism
Counterterrorism (also spelled counter-terrorism), also known as anti-terrorism, incorporates the practices, military tactics, techniques, and strategies that governments, law enforcement, business, and intelligence agencies use to combat or el ...
.[
] 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
Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
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 device
A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity li ...
s have become integrated into everyday living. This means that commercial and scientific
Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe.
Science may be as old as the human species, and some of the earliest archeological evidence for ...
applications for generating the appropriate frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
bands of light commensurate with the semiconductor application or device are in wide use. Visible and infrared lasers
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
are at the core of information technology
Information technology (IT) is the use of computers to create, process, store, retrieve, and exchange all kinds of data . and information. IT forms part of information and communications technology (ICT). An information technology system (I ...
. 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
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
) exist, but other challenges hinder their usefulness. Terahertz laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
devices are not compact and therefore lack portability and are not easily integrated into systems
A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and express ...
. 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
In physics, cryogenics is the production and behaviour of materials at very low temperatures.
The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...
liquids for operation (liquid helium
Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity.
At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temp ...
).[ Additionally, this lack of appropriate sources hinders opportunities in ]spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
, remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
, free space communications, and medical imaging
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to rev ...
.[
]
Meanwhile, potential terahertz frequency applications are being researched globally. Two recently developed technologies, Terahertz time-domain spectroscopy
In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique in which the properties of matter are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample's effect on ...
and quantum cascade lasers Quantum-cascade lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jérôme Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, ...
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 prism
Prism usually refers to:
* Prism (optics), a transparent optical component with flat surfaces that refract light
* Prism (geometry), a kind of polyhedron
Prism may also refer to:
Science and mathematics
* Prism (geology), a type of sedimentary ...
s are unable to significantly interact with the magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
of light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
. The significant interaction (permittivity
In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ''ε'' ( epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in ...
) occurs with the electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
. In natural material A natural material is any product or physical matter stop changing it. Minerals and the metals that can be extracted from them (without further modification) are also considered to belong into this category. Natural materials are used as building ma ...
s, any useful magnetic interaction will taper off in the gigahertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one h ...
range of frequencies
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
. Compared to interaction with the electric field, the magnetic component is imperceptible when in terahertz, infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
, and visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
. 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
Negative-index metamaterial or negative-index material (NIM) is a metamaterial whose refractive index for an electromagnetic wave has a negative value over some frequency range.
NIMs are constructed of periodic basic parts called unit cells, whi ...
. 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
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
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
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
, 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.[
]
Electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
, which includes light, carries energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
and momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
that may be imparted to matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...
with which it interacts. The radiation and matter have a symbiotic relationship. Radiation does not simply act on a material, nor is it simply acted on upon by a given material; radiation interacts with matter.
The magnetic interaction, or induced coupling, of any material can be translated into permeability. The permeability of naturally occurring materials is a positive value. A unique ability of metamaterials is to achieve permeability values less than zero (or negative values), which are not accessible in nature. Negative permeability was first achieved at microwave frequencies with the first metamaterials. A few years later, negative permeability was demonstrated in the terahertz regime.
Materials which can couple magnetically are particularly rare at terahertz or optical frequencies.
Published research pertaining to some natural magnetic materials states that these materials do respond to frequencies above the microwave range, but the response is usually weak, and limited to a narrow band of frequencies. This reduces the possible useful terahertz devices. It was noted that the realization of magnetism at THz and higher frequencies will substantially affect terahertz optics and their applications.[
This has to do with magnetic ]coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...
at the atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, and ...
ic level. This drawback can be overcome by using metamaterials that mirror atomic magnetic coupling A magnetic coupling is a coupling that transfers torque from one shaft to another, but using a magnetic field rather than a physical mechanical connection.
Magnetic shaft couplings are most often used for liquid pumps and propeller systems, since ...
, on a scale of magnitudes larger than the atom.[
]
The first THz metamaterials
The first terahertz metamaterials able to achieve a desired magnetic response, which included negative values for permeability, were passive materials. Because of this, "tuning" was achieved by fabricating a new material, with slightly altered dimensions to create a new response. However, the notable advance, or practical achievement, is actually demonstrating the manipulation of terahertz radiation with metamaterials
A metamaterial (from the Ancient Greek, Greek word Meta, μετά ''meta'', meaning "beyond" or "after", and the Latin word ''materia'', meaning "matter" or "material") is any material engineered to have a property that is not found in naturally ...
.
For the first demonstration, more than one metamaterial structure was fabricated. However, the demonstration showed a range of 0.6 to 1.8 terahertz. 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. This was followed by a demonstrations at 6 THz, and 100 THz.
With the first demonstration, scaling of elements, and spacing, allowed for success with the terahertz range of frequencies. As with metamaterials in lower frequency ranges, these elements were non-magnetic materials, but were conducting elements. The design allows a resonance that occurs with the electric and magnetic components simultaneously. And notable is the strong magnetic response of these artificially constructed materials.
For the elements to respond at resonance, at specified frequencies, this is arranged by specifically designing the element. The elements are then placed in a repeating pattern, as is common for metamaterials. In this case, the now combined and arrayed elements, along with attention to spacing, comprise a flat, rectangular, (planar) structured metamaterial. Since it was designed to operate at terahertz frequencies, photolithography is used to etch the elements onto a substrate.[
]
Magnetic responses and refractive index
The split-ring resonator
A split-ring resonator (SRR) is an artificially produced structure common to metamaterials. Its purpose is to produce the desired magnetic susceptibility (magnetic response) in various types of metamaterials up to 200 terahertz.
These media cre ...
(SRR) is a common metamaterial in use for a variety of experiments.[ Magnetic responses ( permeability) at terahertz ]frequencies
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
can be achieved with a structure composed of non-magnetic
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...
elements, such as copper-wire SRR, which demonstrate different responses centered around a resonant frequency. Split ring resonators show a capability for tuning across the terahertz regime. Furthermore, the repeating structure made up the constituent materials follows the same strategy of averaging the electromagnetic field as it manipulates and transmits the terahertz radiation This averaging technique is called an effective medium response.
Effective permeability ''µ-eff'' is boosted from the inductance
Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of the ...
of the rings and the capacitance
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
occurs at the gaps of the split rings. In this terahertz experiment ''ellipsometry
Ellipsometry is an optical technique for investigating the dielectric properties (complex refractive index or dielectric function) of thin films. Ellipsometry measures the change of Polarization (waves), polarization upon reflection or transmissi ...
'' is applied, rather than waveguides. 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). The emitted polarization is intended, and angle of polarization is known. A polarization change is reflected (off the sample material) is then measured. Information on the phase difference (if any) and the reflected polarization is considered.[
The local magnetic field of the cell material can be understood as a magnetic response. Below resonance the local magnetic field increases This magnetic response stays in ]phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
with the electric field. Because the SRR cell is actually a non-magnetic material, this local magnetic response is temporary and will retain magnetic characteristics only so long as there is an externally applied magnetic field. Thus the total magnetization will drop to zero when the applied field is removed. In addition, the local magnetic response is actually a fraction of the total magnetic field. This fraction is proportional to the field strength and this explains the linear dependency. Likewise there is an aggregate linear response over the whole material. This tends to mimic alignments and spins at the atomic level.[
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. Then as the frequency increases further the induced local field response lags further until it is completely out of phase with the excitation field. This results in a magnetic permeability that is falling below unity and includes values less than zero. The linear ]coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...
between the induced local field and the fluctuating applied field is in contrast to the non-linear characteristics of ferromagnetism
Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
[
Later, a magnetic response in these materials were demonstrated at 100 terahertz, and in the infrared regime. Proving the magnetic response was an important step towards later controlling the ]refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
. Finally, negative index of refraction was achieved for terahertz wavelengths at 200 terahertz using layer pairs metallic nanorods in parallel. This work is also complemented by surface plasmon studies in the terahertz regime.
Work also continues with studies of applying external controls such as electronic switching and semiconductor structures to control transmission and reflection properties.[
][
]
Reconfigurable terahertz metamaterials
Electromagnetic metamaterials
A metamaterial (from the Greek word μετά ''meta'', meaning "beyond" or "after", and the Latin word ''materia'', meaning "matter" or "material") is any material engineered to have a property that is not found in naturally occurring materials. ...
show promise to fill the Terahertz gap (0.1 – 10 THz). The terahertz gap is caused by two general shortfalls. First, almost no naturally occurring materials are available for applications which would utilize terahertz frequency sources. Second is the inability to translate the successes with EM metamaterials in the microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
and optical domain, to the terahertz domain.[
Moreover, the majority of research has focused on the passive properties of artificial periodic THz transmission, as determined by the patterning of the metamaterial elements e.g., the effects of the size and shape of inclusions, metal film thickness, hole geometry, periodicity, etc. It has been shown that the resonance can also be affected by depositing a dielectric layer on the metal hole arrays and by doping a semiconductor substrate, both of which result in significant shifting of the resonance frequency. However, little work has focused on the "active" manipulation of the extraordinary optical transmission though it is essential to realize many applications.][
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. Strategies include illuminating the structure with laser light, varying an external static magnetic field where the current does not vary, and by using an external bias voltage supply (semiconductor controlled). These methods lead to the possibilities of high-sensitive spectroscopy, higher power terahertz generation, short-range secure THz communication, an even more sensitive detection through terahertz capabilities. Furthermore, these include the development of techniques for, more sensitive terahertz detection, and more effective control and manipulation of terahertz waves.][
]
Employing MEM technology
By combining metamaterial elements – specifically, split ring resonators – with Microelectromechanical systems
Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
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.
Dynamic electric and magnetic metamaterial response at THz frequencies
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).
Survey of terahertz metamaterial devices
Terahertz metamaterials are making possible the study of novel devices.[Alternate copy here]
Novel amplifier designs
In the terahertz compact moderate power amplifiers are not available. This results in a region that is underutilized, and the lack of novel amplifiers can be directly attributed as one of the causes.
Research work has involved investigating, creating, and designing light-weight slow-wave vacuum electronics devices based on traveling wave tube amplifier
A traveling-wave tube (TWT, pronounced "twit") or traveling-wave tube amplifier (TWTA, pronounced "tweeta") is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range. The TWT belongs t ...
s. These are designs that involve folded waveguide
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
, slow-wave circuits, in which the terahertz wave
Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency
(THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of fre ...
meanders through a serpentine path while interacting with a linear electron beam. Designs of folded-waveguide traveling-wave tubes are at frequencies of 670, 850, and 1030 GHz. In order to ameliorate the power limitations due to small dimensions and high attenuation, novel
A novel is a relatively long work of narrative fiction, typically written in prose and published as a book. The present English word for a long work of prose fiction derives from the for "new", "news", or "short story of something new", itsel ...
planar circuit designs are also being investigated.[
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).][
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. Computational results are more promising with this circuit. Preliminary results suggest that the metamaterial structure is effective in decreasing the electric field magnitude in the substrate and increasing the magnitude in the region above the meander line, where it can interact with an electron sheet beam. In addition, the planar circuit is less difficult to fabricate and can enable a higher current. More work is needed to investigate other planar geometries, optimize the electric-field/electron-beam interaction, and design focusing magnet geometries for the sheet beam.][
][This section uses public domain material from NAS]
Technical Reports service
Novel terahertz sensors and phase modulators
The possibility of controlling radiations in the terahertz regime is leading to analysis of designs for sensing devices, and phase modulators. Devices that can apply this radiation would be particularly useful. Various strategies are analyzed or tested for tuning metamaterials that may function as sensors. Likewise linear phase shift can be accomplished by using control devices. It also necessary to have sensors that can detect certain battlefield hazards
See also
*Metamaterial cloaking
Metamaterial cloaking is the usage of metamaterials in an cloaking device, invisibility cloak. This is accomplished by manipulating the paths traversed by light through a novel optical material. Metamaterials direct and control the Wave propagati ...
*Metamaterial antennas
Metamaterial antennas are a class of antennas which use metamaterials to increase performance of miniaturized ( electrically small) antenna systems. Their purpose, as with any electromagnetic antenna, is to launch energy into free space. However, ...
*Nonlinear metamaterials A nonlinear metamaterial is an artificially constructed material that can exhibit properties not found in nature. Its response to electromagnetic radiation can be characterized by its permittivity and magnetic permeability, material permeability. Th ...
*Photonic crystal
A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of Crystal structure, natural crystals gives rise to X-ray crystallograp ...
*Seismic metamaterials
A seismic metamaterial, is a metamaterial that is designed to counteract the adverse effects of seismic waves on artificial structures, which exist on or near the surface of the earth. Current designs of seismic metamaterials utilize configurations ...
*Acoustic metamaterials
An acoustic metamaterial, sonic crystal, or phononic crystal, is a material designed to control, direct, and manipulate sound waves or phonons in gases, liquids, and solids (crystal lattices). Sound wave control is accomplished through manipulating ...
*Plasmonic metamaterials A plasmonic metamaterial is a metamaterial that uses surface plasmons to achieve optical properties not seen in nature. Plasmons are produced from the interaction of light with metal-dielectric materials. Under specific conditions, the incident ligh ...
* Spoof surface plasmon
*Transformation optics
Transformation optics is a branch of optics which applies metamaterials to produce spatial variations, derived from coordinate transformations, which can direct chosen bandwidths of electromagnetic radiation. This can allow for the construction ...
*Theories of cloaking
Theories of cloaking discusses various theories based on science and research, for producing an electromagnetic cloaking device. Theories presented employ transformation optics, event cloaking, dipolar scattering cancellation, tunneling light tran ...
*Metamaterials (journal)
''Metamaterials'' was a peer-reviewed scientific journal that was established in March 2007. It was published by Elsevier in association with the Metamorphose Network of Excellence. The coordinating editor was Mikhail Lapine. The journal was publ ...
*Metamaterials Handbook
''Metamaterials Handbook'' is a two-volume handbook on metamaterials edited by Filippo Capolino (University of California).
The series is designed to cover all theory and application topics related to electromagnetic metamaterials. Disciplin ...
* Metamaterials: Physics and Engineering Explorations
Notes
References
:*
General references
*
*
*Federal Standard 1037
"b" to "byte"
*Federal Standard 1037C
Glossary of Telecommunication Terms
*Jany, S. J
Glossary of Laser Terminology
(PDF download)
External links
Google scholar
List of Papers by JB Pendry
Imperial College, Department of Physics, Condensed Matter Theory Group
Video: John Pendry lecture: The science of invisibility
April 2009, SlowTV
U.S. Air Force Research Lab
Researchers Combine Terahertz Radiation and Metamaterial Technology to Detect Explosives
Optoelectronic metamaterials for sub-wavelength imaging in the mid infra red regime
(A – Z) – University of North Carolina at Chapel Hill. See ''nanometer'' here.
*
{{DEFAULTSORT:Terahertz Metamaterials
Metamaterials
Terahertz technology