Terahertz metamaterials
Encyclopedia
Terahertz metamaterials are a new class of composite, artificial materials
which 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 wavelength
s 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.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.
Terahertz waves are electromagnetic waves with frequencies higher than microwaves but lower than infrared radiation
and visible light. They possess many advantages for applications in radio astronomy
spectroscopy, non-destructive testing
of spacecraft
, non-ionizing medical imaging and tumor detection, high resolution close range radar, and security detection of chemicals, biological agents, and weapons
. However, this frequency region is largely under-utilized, and is referred to as the “terahertz gap” of the electromagnetic spectrum
. Applications of frequencies
in the terahertz radiation
range hold the promise of efficient advancement in notably important technologies.
has led to constructing new artificial composite materials, termed metamaterial
s. The metamaterials are based on a lattice structure which mimics crystal structure
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 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
.
and out to 30 micrometer (10 terahertz) wavelengths has been metaphorically termed unclaimed territory where few devices, and perhaps none, exist. The submillimeter, or terahertz band, exists between technologies in traditional microwave radiation and optical domains
. Because atmospheric propagation is limited, the commercial sector has passed over this frequency band. However, terahertz technology has been instrumental for high-resolution spectroscopy
. Moreover, a rich vein of knowledge has been amassed via submillimeter remote sensing techniques. In particular, interdisciplinary researchers in astrophysics
, and the earth sciences have mapped thermal emission lines for a wide variety of lightweight molecules. The amount of information obtained is specifically amenable to this particular band of electromagnetic radiation
. In fact, the universe is bathed in terahertz energy; most of it going unnoticed and undetected.
s, has led to the realization of phenomena that cannot be obtained with natural material
s. 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. Metamaterial
s, 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.
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 microwave
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
, switch
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 modulators and sensor
s. 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 absorbers
. The recent existence of a terahertz radiating source in general are THz quantum cascade laser
s. 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, penetrate a few millimeters of human skin tissue without ill effects, clothing to detect hidden objects on personnel, the detection of chemical
and biological agent
s 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, isolator
s, and converters
.
Semiconductor device
s have become integrated into everyday living. Commercial and scientific applications for generating the appropriate frequency
bands of light, or the electromagnetic spectrum
, commensurate with the semiconductor application or device are in wide use. Visible and infrared lasers are at the core of information technology
, and 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 .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. These laser
devices are not compact and therefore lack portability and are not easily integrated into systems. In addition, low-consumption, solid state
terahertz sources are lacking. 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
).
This lack of appropriate sources hinders opportunities in spectroscopy
, remote sensing
, free space communications, and medical imaging
.
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 (December 2009).
s are unable to significantly interact with the magnetic field
of light. The significant interaction (permittivity
) occurs with the electric field
. In natural material
s 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
. This is because the rudimentary elements of metamaterials have demonstrated a coupling and inductive response
to the magnetic component commensurate to 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.
Electromagnetic radiation
, which includes light, carries energy
and momentum
that may be imparted to matter
with which it interacts. The radiation and matter have a symbiotic relationship. Radiation does not simply act on a material, nor does 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 natural occurring materials is a positive value. A unique ability of metamaterials is to achieve permeability values less than zero (or negative values). These values 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
state 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 poasible 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
at the atom
ic level. This drawback can be overcome by using metamaterials that mirror atomic magnetic coupling
, on a scale magnitudes larger than the atom.
. 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.
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 metmaterials 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 metamterial. Since it was designed to operate at terahertz frequencies, photolithography is used to etch the elements onto a substrate.
The Split-Ring Resonator (SRR) is a common metamaterial in use for a variety of experiments. Magnetic responses (permeability
) at terahertz frequencies can be achieved with a structure composed of non-magnetic elements, such as SRRs, which demonstrate different responses at resonant frequencies and near resonant frequencies. The desired, artificially fabricated, magnetic response is realized over a relatively large bandwidth, and can be tuned throughout the terahertz frequency spectrum. The periodic array
allows the material to behave as a medium with an effective magnetic permeability µeff(ω), where ω is frequency. In other words, at resonance µeff is achieved.
Effective permeability µ-eff is boosted from the inductance
of the rings and the capacitance
occurs at the gaps of the split rings. In prior microwave
frequency experiments bulk metamaterial is used, such as waveguides to transmit the source of radiation. In this terahertz experiment ellipsometry
is applied. 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. The change in polarization of the radiation reflected off the sample material is then measured. This is used to obtain phase
information and the polarization state of the emitted and reflected radiation
. This information is then is used to demonstrate the boost in effective magnetic permeability at terahertz frequencies.
An external magnetic field
is applied with the THz radiation. Then the radiation induces a current
in the looped wire of the SRR cell. This current then induces a local magnetic field
(a vector quantity). The local magnetic field can be understood as a magnetic response
. Well below the resonance frequency ω0 the local magnetic field increases over time
corresponding to increasing frequency. This magnetic response stays in phase
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. All this has to do with alignments and spins at the atomic level.
As the frequency continues to increase, approaching resonance, 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 above ω0, 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, over time - including values less than zero. The linear coupling
between the induced local field and the fluctuating applied field is in contrast to the non-linear characteristics of ferromagnetism
, hence no permanent magnetic effect is achieved.
Three different SRR samples were compared. The wavelength of the resonant excited field is λ
and the material is able to scale 1/7 λ. These are the necessary conditions for the metamaterial to become a medium with µeff. The sample was placed inside a vacuum
produced inside a compartment. A mercury arc lamp was used as the electromagnetic source, and shined onto the sample, at an angle
of 30°. The SRRs are expected to respond magnetically when the magnetic field penetrates the rings (S-polarization
) and to exhibit no magnetic response when the magnetic field is parallel to the plane of the SRR (P-polarization
). The frequency range of 0.6 THz to 1.8 THz was used for the measurements. The reflectance ratio of S- and P- polarizations was matched with strong magnetic responses of SRRs when the magnetic field penetrates the rings (S-polarization). Three different artificial magnetic structures are designated D1, D2, and D3. See the graphical representation of the magnetic responses here. D1 strong magnetic response at 1.25 THz, with a ratio of just below 1.5. To show that it is the material that is used to vary the effective permeability, two other samples are used to show that this resonance should scale with dimensions in accordance to Maxwell's equations
. Therefore D2 has a strong magnetic response at peaks at 0.95 THz, and the D3 sample peaks at 0.8 THz. This demonstrates the scalability of these magnetic metamaterials throughout the THz range and potentially into optical frequencies. To further demonstrate verification of the results, a mathematical simulation was performed which repeated the demonstration. The results of the simulation were in good agreement with the actual results for materials D1, D2, and D3.
and magnetic permeability - the parameters of normal materials, are artificially expanded. In normal materials, resonance
s fade away above gigahertz frequencies
. Instead, resonances at terahertz frequencies have been effectively demonstrated for metamaterial
s. This now allows for interesting new effects in linear optics as well as in nonlinear optics
. Furthermore, a negative magnetic permeability would allow for negative-index materials at optical frequencies, which seemed totally out of reach just a few years ago.
To fulfill a need to achieve localized magnetic resonant responses for terahertz optical frequencies
, an array of single nonmagnetic metallic split rings can be used to implement a magnetic resonance at 100 THz. The split-ring resonator mimicked an LC oscillator which generated waves with frequency ωLC = (LC)−1/2.
See the image to the right:
An LC circuit
is a resonant circuit or tuned circuit that consists of an inductor
, and a capacitor
. When connected together, an electric current
can alternate between them at the circuit's resonant frequency. LC circuits are used either for generating signals at a particular frequency
, or picking out a signal at a particular frequency from a more complex signal. They are key components in many applications such as oscillators, filter
s, tuners
and frequency mixer
s.
To couple an incident light beam to the LC resonance one of two conditions must be met. The first condition is that electric field vector E of the incident light source has a component that is normal to the plates of the capacitor. The second condition is the magnetic field vector H of the incident light has a component normal to the plane of the coil
. When the second condition is met, a localized magnetic field is created which counteracts the magnetic field of the light source and can result in a negative permeability. Such metamaterials were first realized at frequencies around 10 GHz (3-cm wavelengths) - and could be fabricated on stacked electronic circuit boards. In this case another two orders of magnitude, to 100 THz, had been achieved. This puts visible frequencies for negative refraction index much closer.
The first responses are shown with a lattice constant of a = 450 nm. Additionally, this corresponds to a total number of 56 × 56 = 3136 SRR microstructures. Coupling is controlled through the polarization of the incident light - the interaction of the electric field components with the capacitor and the interaction of the magnetic field components with the inductor. Other lattice constants shown will have a different total number of SRR microstructures.
The LC resonance occurs at 3 µm. Resonant responses occur at lattice constants of 450 nm, 650 nm, and 900 nm. Two distinct resonant responses occur for all three of these lattice constants. Additionally, all three lattice constants are notably smaller than the LC resonant frequency. Coupling to the LC resonance can only occur if there is a component normal from the polarized electric field to the plates of the capacitance. If the electric field is rotated 90° then resonance around the 3-µm wavelength completely disappears.
Next, closed rings rather than split rings are radiated to compare results. Linear polarization does not occur for either position of the metamaterial. Hence, unlike the split ring resonators, no resonance occurs at 3-µm. Finally, measurements are performed under an angle of up to 40° with respect to the surface normal, such that the magnetic field vector of the incident light acquires a component normal to the coils. As expected, the 3-µm resonance persists and does not shift.
Later, in 2005, resonant magnetic nanostructures were fabricated that experimentally exhibited a negative permeability in the mid-infrared range. This was the first practical demonstration to do so. This was seen as an important step toward achieving negative refractive index in the IR range.
at terahertz frequencies, but not a negative index of refraction. These two studies are nevertheless important because a negative magnetic permeability is necessary to achieve negative refraction. In addition, these experiments demonstrated that optical negative index metamaterials
are possible because of the acquired magnetic response (permeability). In 2005 experimental observation of a negative refractive index for the optical range, specifically, for the wavelengths close to 1.5 μm (200 THz frequency) was accomplished.
This accomplishment was in agreement with prior theoretical predictions that a layer of pairs of parallel metal nanorods can produce a negative refractive index.
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
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.
Semiconductor-SRR metamaterial-based terahertz electrical modulators will be useful for real-time terahertz imaging, fast sensing and identification, and even in short range secure terahertz communications.
The metamaterial was composed of an array of gold crosses fabricated on top of an n-doped
semiconductor (GaAs
) layer.
The crossbars were effectively electric dipole
s. In the vicinity of the resonance
frequency the crossbars create a negative effective permittivity for this metamaterial. Upon reaching negative permittivity
, a major fraction of the electromagnetic wave is reflected from the metamaterial surface. The other part is of course transmitted, hence a stop band occurs around the dipole resonance frequency. Here is where the n-doped GaAs layer comes into play. The conductivity of the semiconductor layer is the tuning device for the transmitted part of the EM wave. And the semiconductor layer can be purposely tuned.
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.
elements into which cuts are periodically introduced. At frequencies above the resonant frequency and below plasma frequency, the permittivity is negative and, because the resonant
frequency can be set to virtually any value in a metamaterial, phenomena usually associated with optical frequencies including negative ε can be reproduced at
low frequencies.
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. 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 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.
responses in metamaterials has progressed far enough to where application demonstrations are surfacing.
A process is demonstrated for tuning the magnetic resonance frequency of a fixed split-ring resonator array, by way of adding material near the split-ring elements. The sensitivity of the fine tuning suggests possible applications as a sensor device. The resonant frequency responds to silicon nanospheres.
Applying drops of a silicon-nanospheres/ethanol solution to the surface of the sample decreases the magnetic resonance frequency of the split-ring array in incremental steps of 0.03 THz. This fine tuning is done post fabrication and is demonstrated to be reversible. The exhibited sensitivity of the split-ring resonance frequency to the presence of silicon nanospheres also suggests further application possibilities as a sensor device.
Moreover, the causal relation between amplitude switching and phase shifting enables broadband modulation.
from nearby military high-power lasers, while still allowing the sensor to conduct necessary battlefield.
modes, terahertz radiation is ideal to excite and probe these modes and to detect DNA by its terahertz properties at a specific binding state. This is a proposal for a rapid processing and reading of up to 100 arrayed gene sensors for diagnostic applications.
Metamaterial
Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior....
which interact at terahertz (THz) frequencies. The terahertz frequency range used in materials research
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...
is usually defined as 0.1 to 10 THz. This corresponds to wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s below the millimeter range, specifically between 3 millimeters (EHF
Extremely high frequency
Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low infrared light, also referred to as terahertz radiation...
band) and .03 millimeters; the long-wavelength edge of far-infrared light
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
.
This bandwidth
Bandwidth
Bandwidth is the difference between the upper and lower frequencies in a contiguous set of frequencies. It is typically measured in hertz, and may sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on context...
is also known as the terahertz gap.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.
Terahertz waves are electromagnetic waves with frequencies higher than microwaves but lower than infrared radiation
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
and visible light. They possess many advantages for applications in radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
spectroscopy, non-destructive testing
Terahertz nondestructive evaluation
Terahertz nondestructive evaluation pertains to devices, and techniques of analysis occurring in the terahertz domain of electromagnetic radiation...
of spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
, non-ionizing medical imaging and tumor detection, high resolution close range radar, and security detection of chemicals, biological agents, and weapons
WMD
WMD may refer to:* Weapon of mass destruction, a term used to describe munitions with the capacity to kill large numbers of human beings indiscriminately* Weighted mean difference, a measure in statistical meta-analysis...
. However, this frequency region is largely under-utilized, and is referred to as the “terahertz gap” of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. Applications of frequencies
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
in the terahertz radiation
Terahertz radiation
In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz...
range hold the promise of efficient advancement in notably important technologies.
About metamaterials
Currently, a fundamental lack in naturally occurring materials that allow for the desired electromagnetic responsePermeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
has led to constructing new artificial composite materials, termed metamaterial
Metamaterial
Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior....
s. The metamaterials are based on a lattice structure which mimics crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
s. However, the lattice structure of this new material consists of rudimentary elements
Negative index metamaterials
Negative index metamaterials or negative index materials are artificial structures where the refractive index has a negative value over some frequency range. This does not occur in any known natural materials, and thus is only achievable with engineered structures known as metamaterials...
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
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
. 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
Passivity (engineering)
Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems...
.
Terahertz technology
More broadly the submillimeter-wave energy can be defined 1000–100 um (300 GHz–3 THz). Beyond 3 THz,and out to 30 micrometer (10 terahertz) wavelengths has been metaphorically termed unclaimed territory where few devices, and perhaps none, exist. The submillimeter, or terahertz band, exists between technologies in traditional microwave radiation and optical domains
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
. Because atmospheric propagation is limited, the commercial sector has passed over this frequency band. However, terahertz technology has been instrumental for high-resolution spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
. Moreover, a rich vein of knowledge has been amassed via submillimeter remote sensing techniques. In particular, interdisciplinary researchers in astrophysics
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
, and the earth sciences have mapped thermal emission lines for a wide variety of lightweight molecules. The amount of information obtained is specifically amenable to this particular band of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
. In fact, the universe is bathed in terahertz energy; most of it going unnoticed and undetected.
Terahertz metamaterial devices
The development of electromagnetic, artificial-lattice structured materials, termed metamaterialMetamaterial
Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior....
s, has led to the realization of phenomena that cannot be obtained with natural material
Natural material
A natural material is any product or physical matter that comes from plants, animals, or the ground. Minerals and the metals that can be extracted from them are also considered to belong into this category.* Biotic materials...
s. This is observed , for example, with a natural glass lens
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
, 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
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
and magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. The interaction of a conventional lens
Microscopy
Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...
, 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
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
limitations such as a diffraction barrier. Moreover, there is a fundamental lack of natural materials that strongly interact with light's magnetic field. Metamaterial
Metamaterial
Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior....
s, 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
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. which is light, are broadened.
Development of metamaterials has traversed the electromagnetic spectrum up to terahertz and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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 microwave
Negative index metamaterials
Negative index metamaterials or negative index materials are artificial structures where the refractive index has a negative value over some frequency range. This does not occur in any known natural materials, and thus is only achievable with engineered structures known as metamaterials...
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
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
, lenses
Superlens
A superlens, super lens or perfect lens is a lens which uses metamaterials to go beyond the diffraction limit. The diffraction limit is an inherent limitation in conventional optical devices or lenses. In 2000, a type of lens was proposed, consisting of a metamaterial that compensates for wave...
, switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
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 modulators and sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s. This void also includes phase-shifting and beam-steering devicesBeam steering: is changing the direction of the main lobe
Main lobe
The main lobe, or main beam, of an antenna radiation pattern is the lobe containing the maximum power. This is the lobe that exhibits the greatest field strength....
of a radiation pattern
Radiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
. 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
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
radiation driving the elements. In optical systems, beam steering may be accomplished by changing the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
filters, optical switching modulators, and absorbers
Metamaterial absorber
A metamaterial absorber manipulates the loss components of the complex effective parameters, permittivity and magnetic permeability of metamaterials, to create a material with particularly high absorption...
. The recent existence of a terahertz radiating source in general are THz quantum cascade laser
Quantum cascade laser
Quantum cascade lasers are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.Unlike typical...
s. However, technologies to control and manipulate THz waves are lagging behind other frequency domains
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
of the spectrum of light.
Furthermore, research
Research
Research can be defined as the scientific search for knowledge, or as any systematic investigation, to establish novel facts, solve new or existing problems, prove new ideas, or develop new theories, usually using a scientific method...
into technologies which utilize THz frequencies show the capabilities for advanced sensing techniques
Wireless sensor network
A wireless sensor network consists of spatially distributed autonomous sensors to monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants and to cooperatively pass their data through the network to a main location. The more modern...
. In areas where other wavelengths are limited, THz frequencies appear to fill the near future gap for advancements in security, public health
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" . It is concerned with threats to health based on population health...
, biomedicine
Biomedicine
Biomedicine is a branch of medical science that applies biological and other natural-science principles to clinical practice,. Biomedicine, i.e. medical research, involves the study of physiological processes with methods from biology, chemistry and physics. Approaches range from understanding...
, defense
United States armed forces
The United States Armed Forces are the military forces of the United States. They consist of the Army, Navy, Marine Corps, Air Force, and Coast Guard.The United States has a strong tradition of civilian control of the military...
, communication
Communication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
, and quality control
Quality control
Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production. This approach places an emphasis on three aspects:...
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
Superlens
A superlens, super lens or perfect lens is a lens which uses metamaterials to go beyond the diffraction limit. The diffraction limit is an inherent limitation in conventional optical devices or lenses. In 2000, a type of lens was proposed, consisting of a metamaterial that compensates for wave...
the contents of a plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
container, penetrate a few millimeters of human skin tissue without ill effects, clothing to detect hidden objects on personnel, the detection of chemical
Chemical warfare
Chemical warfare involves using the toxic properties of chemical substances as weapons. This type of warfare is distinct from Nuclear warfare and Biological warfare, which together make up NBC, the military acronym for Nuclear, Biological, and Chemical...
and biological agent
Biological agent
A biological agent — also called bio-agent or biological threat agent — is a bacterium, virus, prion, or fungus which may cause infection, allergy, toxicity or otherwise create a hazard to human health. They can be used as a biological weapon in bioterrorism or biological warfare...
s as novel approaches for counter-terrorism
Counter-terrorism
Counter-terrorism is the practices, tactics, techniques, and strategies that governments, militaries, police departments and corporations adopt to prevent or in response to terrorist threats and/or acts, both real and imputed.The tactic of terrorism is available to insurgents and governments...
. 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
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
and lenses, tunable mirrors, isolator
Optical isolator
An optical isolator, or optical diode, is an optical component which allows the transmission of light in only one direction. It is typically used to prevent unwanted feedback into an optical oscillator, such as a laser cavity...
s, and converters
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
.
Generating THz electromagnetic radiation
Without available terahertz sources, other applications are held back.Semiconductor device
Semiconductor device
Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced thermionic devices in most applications...
s have become integrated into everyday living. Commercial and scientific applications for generating the appropriate frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
bands of light, or the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
, commensurate with the semiconductor application or device are in wide use. Visible and infrared lasers are at the core of information technology
Information technology
Information technology is the acquisition, processing, storage and dissemination of vocal, pictorial, textual and numerical information by a microelectronics-based combination of computing and telecommunications...
, and 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 .1 to 10 THz, is an impoverished regime by comparison. Sources for generating the required THz frequencies (or wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
) exist, but other challenges hinder their usefulness. These laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
devices are not compact and therefore lack portability and are not easily integrated into systems. In addition, low-consumption, solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
terahertz sources are lacking. The current devices also have one or more shortcomings of low power output
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
, poor tuning abilities
Tunable metamaterials
A tunable metamaterial is a metamaterial with a variable response to an incident electromagnetic wave. This includes remotely controlling how an incident electromagnetic wave interacts with a metamaterial. This means the capablitity to determine whether the EM wave is transmitted, reflected, or...
, and may require cryogenic liquids for operation (liquid helium
Liquid helium
Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
).
This lack of appropriate sources hinders opportunities in spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
, remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
, free space communications, and medical imaging
Superlens
A superlens, super lens or perfect lens is a lens which uses metamaterials to go beyond the diffraction limit. The diffraction limit is an inherent limitation in conventional optical devices or lenses. In 2000, a type of lens was proposed, consisting of a metamaterial that compensates for wave...
.
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 (December 2009).
Magnetic field interaction
As briefly mentioned above, naturally-occurring materials such as conventional lenses and glass prismPrism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
s are unable to significantly interact with the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
of light. The significant interaction (permittivity
Permittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
) occurs with the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. In natural material
Natural material
A natural material is any product or physical matter that comes from plants, animals, or the ground. Minerals and the metals that can be extracted from them are also considered to belong into this category.* Biotic materials...
s any useful magnetic interaction
Inductive coupling
In electrical engineering, two conductors are referred to as mutual-inductively coupled or magnetically coupled when they are configured such that change in current flow through one wire induces a voltage across the ends of the other wire through electromagnetic induction...
will taper off in the gigahertz
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
range of frequencies. Compared to interaction with the electric field, the magnetic component is imperceptible when in terahertz, infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, and visible light
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
. 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 metamaterials
Negative index metamaterials or negative index materials are artificial structures where the refractive index has a negative value over some frequency range. This does not occur in any known natural materials, and thus is only achievable with engineered structures known as metamaterials...
. This is because the rudimentary elements of metamaterials have demonstrated a coupling and inductive response
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
to the magnetic component commensurate to the electric coupling and response. This demonstrated the occurrence of an artificial magnetism,See main article: Paramagnetism
Paramagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
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
Transmittance
In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength...
. This degree of choice is not possible with conventional materials
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
. 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
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
, which includes light, carries energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
and momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
that may be imparted to matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
with which it interacts. The radiation and matter have a symbiotic relationship. Radiation does not simply act on a material, nor does is it simply acted on upon by a given material. Radiation interacts with matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
.
The magnetic interaction, or induced coupling, of any material can be translated into permeability. The permeability of natural occurring materials is a positive value. A unique ability of metamaterials is to achieve permeability values less than zero (or negative values). These values not accessible in nature. Negative permeability was first achieved at microwave frequencies with the first metamaterials
Negative index metamaterials
Negative index metamaterials or negative index materials are artificial structures where the refractive index has a negative value over some frequency range. This does not occur in any known natural materials, and thus is only achievable with engineered structures known as metamaterials...
. A few years later negative permeability was demonstrated in the terahertz regime.
Materials which can couple magnetically
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
are particularly rare at terahertz or optical frequencies.
Published research pertaining to some natural magnetic materials
Ferrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
state 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 poasible 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
Coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.The...
at the atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
ic level. This drawback can be overcome by using metamaterials that mirror atomic magnetic coupling
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
, on a scale 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 materialsPassivity (engineering)
Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems...
. 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.
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 metmaterials 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 metamterial. Since it was designed to operate at terahertz frequencies, photolithography is used to etch the elements onto a substrate.
Magnetic response from metamaterials at 1.8 THz
Permeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
) at terahertz frequencies can be achieved with a structure composed of non-magnetic elements, such as SRRs, which demonstrate different responses at resonant frequencies and near resonant frequencies. The desired, artificially fabricated, magnetic response is realized over a relatively large bandwidth, and can be tuned throughout the terahertz frequency spectrum. The periodic array
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
allows the material to behave as a medium with an effective magnetic permeability µeff(ω), where ω is frequency. In other words, at resonance µeff is achieved.
Effective permeability µ-eff is boosted from the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
of the rings and the capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
occurs at the gaps of the split rings. In prior microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
frequency experiments bulk metamaterial is used, such as waveguides to transmit the source of radiation. In this terahertz experiment ellipsometry
Ellipsometry
Ellipsometry is an optical technique for the investigation of the dielectric properties of thin films....
is applied. 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. The change in polarization of the radiation reflected off the sample material is then measured. This is used to obtain phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
information and the polarization state of the emitted and reflected radiation
Reflection coefficient
The reflection coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A reflection coefficient describes either the amplitude or the intensity of a reflected wave relative to an incident wave...
. This information is then is used to demonstrate the boost in effective magnetic permeability at terahertz frequencies.
An external magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
is applied with the THz radiation. Then the radiation induces a current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
in the looped wire of the SRR cell. This current then induces a local magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
(a vector quantity). The local magnetic field can be understood as a magnetic response
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
. Well below the resonance frequency ω0 the local magnetic field increases over time
Coordinate time
In the theory of relativity, it is convenient to express results in terms of a spacetime coordinate system relative to an implied observer. In many coordinate systems, an event is specified by one time coordinate and three spatial coordinates...
corresponding to increasing frequency. This magnetic response stays in phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
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. All this has to do with alignments and spins at the atomic level.
-
-
-
-
- For more information see:ParamagnetismParamagnetismParamagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
and Split-ring resonator
- For more information see:Paramagnetism
-
-
-
As the frequency continues to increase, approaching resonance, 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 above ω0, 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, over time - including values less than zero. The linear coupling
Coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.The...
between the induced local field and the fluctuating applied field is in contrast to the non-linear characteristics of ferromagnetism
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
, hence no permanent magnetic effect is achieved.
Three different SRR samples were compared. The wavelength of the resonant excited field is λ
and the material is able to scale 1/7 λ. These are the necessary conditions for the metamaterial to become a medium with µeff. The sample was placed inside a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
produced inside a compartment. A mercury arc lamp was used as the electromagnetic source, and shined onto the sample, at an angle
Angle
In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...
of 30°. The SRRs are expected to respond magnetically when the magnetic field penetrates the rings (S-polarization
Brewster's angle
Brewster's angle is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly...
) and to exhibit no magnetic response when the magnetic field is parallel to the plane of the SRR (P-polarization
Brewster's angle
Brewster's angle is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly...
). The frequency range of 0.6 THz to 1.8 THz was used for the measurements. The reflectance ratio of S- and P- polarizations was matched with strong magnetic responses of SRRs when the magnetic field penetrates the rings (S-polarization). Three different artificial magnetic structures are designated D1, D2, and D3. See the graphical representation of the magnetic responses here. D1 strong magnetic response at 1.25 THz, with a ratio of just below 1.5. To show that it is the material that is used to vary the effective permeability, two other samples are used to show that this resonance should scale with dimensions in accordance to Maxwell's equations
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
. Therefore D2 has a strong magnetic response at peaks at 0.95 THz, and the D3 sample peaks at 0.8 THz. This demonstrates the scalability of these magnetic metamaterials throughout the THz range and potentially into optical frequencies. To further demonstrate verification of the results, a mathematical simulation was performed which repeated the demonstration. The results of the simulation were in good agreement with the actual results for materials D1, D2, and D3.
Magnetic response of metamaterials at 100 terahertz
From this analysis and demonstration the electrical susceptibilityPermittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
and magnetic permeability - the parameters of normal materials, are artificially expanded. In normal materials, resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
s fade away above gigahertz frequencies
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
. Instead, resonances at terahertz frequencies have been effectively demonstrated for metamaterial
Metamaterial
Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior....
s. This now allows for interesting new effects in linear optics as well as in nonlinear optics
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
. Furthermore, a negative magnetic permeability would allow for negative-index materials at optical frequencies, which seemed totally out of reach just a few years ago.
To fulfill a need to achieve localized magnetic resonant responses for terahertz optical frequencies
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
, an array of single nonmagnetic metallic split rings can be used to implement a magnetic resonance at 100 THz. The split-ring resonator mimicked an LC oscillator which generated waves with frequency ωLC = (LC)−1/2.
See the image to the right:
An LC circuit
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
is a resonant circuit or tuned circuit that consists of an inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
, and a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
. When connected together, an electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
can alternate between them at the circuit's resonant frequency. LC circuits are used either for generating signals at a particular frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, or picking out a signal at a particular frequency from a more complex signal. They are key components in many applications such as oscillators, filter
Filter (optics)
Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....
s, tuners
Electrical resonance
Electrical resonance occurs in an electric circuit at a particular resonance frequency where the imaginary parts of circuit element impedances or admittances cancel each other...
and frequency mixer
Frequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
s.
To couple an incident light beam to the LC resonance one of two conditions must be met. The first condition is that electric field vector E of the incident light source has a component that is normal to the plates of the capacitor. The second condition is the magnetic field vector H of the incident light has a component normal to the plane of the coil
Coil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
. When the second condition is met, a localized magnetic field is created which counteracts the magnetic field of the light source and can result in a negative permeability. Such metamaterials were first realized at frequencies around 10 GHz (3-cm wavelengths) - and could be fabricated on stacked electronic circuit boards. In this case another two orders of magnitude, to 100 THz, had been achieved. This puts visible frequencies for negative refraction index much closer.
The first responses are shown with a lattice constant of a = 450 nm. Additionally, this corresponds to a total number of 56 × 56 = 3136 SRR microstructures. Coupling is controlled through the polarization of the incident light - the interaction of the electric field components with the capacitor and the interaction of the magnetic field components with the inductor. Other lattice constants shown will have a different total number of SRR microstructures.
The LC resonance occurs at 3 µm. Resonant responses occur at lattice constants of 450 nm, 650 nm, and 900 nm. Two distinct resonant responses occur for all three of these lattice constants. Additionally, all three lattice constants are notably smaller than the LC resonant frequency. Coupling to the LC resonance can only occur if there is a component normal from the polarized electric field to the plates of the capacitance. If the electric field is rotated 90° then resonance around the 3-µm wavelength completely disappears.
Next, closed rings rather than split rings are radiated to compare results. Linear polarization does not occur for either position of the metamaterial. Hence, unlike the split ring resonators, no resonance occurs at 3-µm. Finally, measurements are performed under an angle of up to 40° with respect to the surface normal, such that the magnetic field vector of the incident light acquires a component normal to the coils. As expected, the 3-µm resonance persists and does not shift.
Later, in 2005, resonant magnetic nanostructures were fabricated that experimentally exhibited a negative permeability in the mid-infrared range. This was the first practical demonstration to do so. This was seen as an important step toward achieving negative refractive index in the IR range.
Negative index of refraction at 200 THz
The two previous sections discussed a magnetic responseMagnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
at terahertz frequencies, but not a negative index of refraction. These two studies are nevertheless important because a negative magnetic permeability is necessary to achieve negative refraction. In addition, these experiments demonstrated that optical negative index metamaterials
Photonic metamaterials
Photonic metamaterials, also known as Optical metamaterials, are a type of electromagnetic metamaterial, which are designed to interact with optical frequencies which are terahertz , infrared , and eventually, visible wavelengths. As a type of metamaterial, the periodic structures are made up of...
are possible because of the acquired magnetic response (permeability). In 2005 experimental observation of a negative refractive index for the optical range, specifically, for the wavelengths close to 1.5 μm (200 THz frequency) was accomplished.
This accomplishment was in agreement with prior theoretical predictions that a layer of pairs of parallel metal nanorods can produce a negative refractive index.
Reconfigurable terahertz metamaterials
Electromagnetic metamaterialsMetamaterial
Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior....
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
Terahertz radiation
In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz...
. Second is the inability to translate the successes with EM metamaterials in the microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
and optical domain
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, to the terahertz domain.
Moreover, the majority of research has focused on the passive properties
Passivity (engineering)
Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems...
of artificial periodic THz transmission
Terahertz radiation
In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz...
, 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
Magnetostatics
Magnetostatics is the study of magnetic fields in systems where the currents are steady . It is the magnetic analogue of electrostatics, where the charges are stationary. The magnetization need not be static; the equations of magnetostatics can be used to predict fast magnetic switching events that...
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.
Surface-plasmon-enhanced terahertz transmission
In August 2003, measurements of the transmission of terahertz radiation through periodic arrays of holes made in highly doped silicon wafers were reported. The unusual transmission was attributed to the resonant tunneling of surface-plasmon polaritons that can be excited on doped semiconductors at terahertz frequencies.Electronic control of THz transmission properties
Electronic switching of the extraordinary THz transmission was demonstrated with subwavelength metal hole arrays fabricated on doped semiconductor substrates. The passive resonance properties are mainly determined by the geometry and dimensions of the metal holes as well as the array periodicity. By electronically altering the substrate conductivity via an external voltage bias, switching of the extraordinary THz transmission is accomplished in real time.Hybrid metamaterial modulation of terahertz radiation
Terahertz modulators based on semiconductor structures often require cryogenic temperatures. This particular modulator is electrically modulated at room temperature. The bandwidth of the hybrid structure is proactively controlled by semiconductor conduction.Semiconductor-SRR metamaterial-based terahertz electrical modulators will be useful for real-time terahertz imaging, fast sensing and identification, and even in short range secure terahertz communications.
High-frequency modulation of terahertz radiation
In 2008, a metamaterial based modulator for THz radiation, was designed, fabricated and experimentally demonstrated. It was electrically tunable. The metamaterial is constructed with symmetric unit cell structures to ensure the material is not affected by the arbitrary polarizations of a radiated source.The metamaterial was composed of an array of gold crosses fabricated on top of an n-doped
N-type semiconductor
N-type semiconductors are a type of extrinsic semiconductor where the dopant atoms are capable of providing extra conduction electrons to the host material . This creates an excess of negative electron charge carriers....
semiconductor (GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
) layer.
The crossbars were effectively electric dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
s. In the vicinity of the resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
frequency the crossbars create a negative effective permittivity for this metamaterial. Upon reaching negative permittivity
Permittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
, a major fraction of the electromagnetic wave is reflected from the metamaterial surface. The other part is of course transmitted, hence a stop band occurs around the dipole resonance frequency. Here is where the n-doped GaAs layer comes into play. The conductivity of the semiconductor layer is the tuning device for the transmitted part of the EM wave. And the semiconductor layer can be purposely tuned.
Adaptive metamaterials (THz)
With adaptive metamaterials the unit cell's response is reorientation. Adaptive metamaterials offer significant potential to realize novel electromagnetic functionality ranging from thermal detection to reconfigurable electromagnetic radiation absorbers.Reconfigurable terahertz metamaterials
The first demonstrations of negative refractive index with metamaterials were anisotropic metamaterials. Reconfigurable metamaterials at terahertz frequencies are anisotropic materials where the artificial dipole, which comprises the unit cell, is reoriented when responding to the external EM source field. The split ring resonators are designed in a cantilever configuration, which allows bending out of plane in response to stimulus. A distinctive capability to tune the electric and magnetic response as the split ring resonators reorient within their unit cells.Employing MEM technology
By combining metamaterial elements - specifically, split ring resonators - with Microelectromechanical systemsMicroelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
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
The current trend of metamaterial research aims for design of nanostructures that are capable of manipulating electromagnetic waves at the visible frequency regime. A metamaterial mimicking the Drude-Lorentz model can be straightforwardly achieved by an array of wireelements into which cuts are periodically introduced. At frequencies above the resonant frequency and below plasma frequency, the permittivity is negative and, because the resonant
frequency can be set to virtually any value in a metamaterial, phenomena usually associated with optical frequencies including negative ε can be reproduced at
low frequencies.
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 amplifiers. These are designs that involve folded waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
, slow-wave circuits, in which the terahertz wave
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
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 planar circuit designs are also being investigated.
In-house work at the NASA Glenn Research Center has investigated
Research
Research can be defined as the scientific search for knowledge, or as any systematic investigation, to establish novel facts, solve new or existing problems, prove new ideas, or develop new theories, usually using a scientific method...
the use of metamaterials—engineered
Metamaterial
Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior....
materials with unique electromagnetic properties
Metamaterial
Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior....
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.
Novel terahertz sensors
Device design is quickly becoming a large part of metamaterial research. In the short half decade since its conception, understanding of the physics behind tailored electromagneticresponses in metamaterials has progressed far enough to where application demonstrations are surfacing.
A process is demonstrated for tuning the magnetic resonance frequency of a fixed split-ring resonator array, by way of adding material near the split-ring elements. The sensitivity of the fine tuning suggests possible applications as a sensor device. The resonant frequency responds to silicon nanospheres.
Applying drops of a silicon-nanospheres/ethanol solution to the surface of the sample decreases the magnetic resonance frequency of the split-ring array in incremental steps of 0.03 THz. This fine tuning is done post fabrication and is demonstrated to be reversible. The exhibited sensitivity of the split-ring resonance frequency to the presence of silicon nanospheres also suggests further application possibilities as a sensor device.
A metamaterial solid-state terahertz phase modulator
The terahertz phase modulator uses a voltage-controlled metamaterial of a single unit cell layer. This new device achieves a voltage-controlled linear phase shift of π /6 radians at 16 V.Moreover, the causal relation between amplitude switching and phase shifting enables broadband modulation.
THz metamaterial IR sensor
One of the most critical applications of such a filter is to block unwanted radiationfrom nearby military high-power lasers, while still allowing the sensor to conduct necessary battlefield.
Biomolecular sensing at THz frequencies
Recently, it has been proposed in a numerical study to use THz-FSS based on asymmetric split ring resonators as a sensor for detecting biomolecular sample films with a thickness of only 10 nm. Because large biomolecules, e.g. DNA, exhibit a multitude of inherent vibrationalmodes, terahertz radiation is ideal to excite and probe these modes and to detect DNA by its terahertz properties at a specific binding state. This is a proposal for a rapid processing and reading of up to 100 arrayed gene sensors for diagnostic applications.
See also
- Acoustic metamaterialsAcoustic metamaterialsAcoustic metamaterials are artificially fabricated materials designed to control, direct, and manipulate sound in the form of sonic, infrasonic, or ultrasonic waves, as these might occur in gases, liquids, and solids. The hereditary line into acoustic metamaterials follows from theory and research...
- MetamaterialMetamaterialMetamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior....
- Metamaterial absorberMetamaterial absorberA metamaterial absorber manipulates the loss components of the complex effective parameters, permittivity and magnetic permeability of metamaterials, to create a material with particularly high absorption...
- Metamaterial antennasMetamaterial antennasMetamaterial antennas are a class of antennas which use metamaterials to increase performance of miniaturized antenna systems. Their purpose, as with any electromagnetic antenna, is to launch energy into free space...
- Metamaterials (journal)Metamaterials (journal)Metamaterials is a peer-reviewed scientific journal that was established in March 2007. It is published by Elsevier in association with the Metamorphose Network of Excellence. The Coordinating Editor is Mikhail Lapine. The journal is published quarterly, with occasional special issues...
- Metamaterial cloakingMetamaterial cloakingMetamaterial cloaking is the scientific application of metamaterials in order to achieve invisibility-cloaking. This is accomplished by manipulating the paths traversed by light through a novel optical material....
- Negative index metamaterialsNegative index metamaterialsNegative index metamaterials or negative index materials are artificial structures where the refractive index has a negative value over some frequency range. This does not occur in any known natural materials, and thus is only achievable with engineered structures known as metamaterials...
- Nonlinear metamaterialsNonlinear metamaterialsA 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 material permeability. The product of the permittivity and permeability results in the refractive...
- Photonic metamaterialsPhotonic metamaterialsPhotonic metamaterials, also known as Optical metamaterials, are a type of electromagnetic metamaterial, which are designed to interact with optical frequencies which are terahertz , infrared , and eventually, visible wavelengths. As a type of metamaterial, the periodic structures are made up of...
- Photonic crystalPhotonic crystalPhotonic crystals are periodic optical nanostructures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons...
- Seismic metamaterialsSeismic metamaterialsSeismic metamaterials, are metamaterials which are designed to counteract the adverse effects of seismic waves on artificial structures, which exist on or near the surface of the earth...
- Split-ring resonator
- SuperlensSuperlensA superlens, super lens or perfect lens is a lens which uses metamaterials to go beyond the diffraction limit. The diffraction limit is an inherent limitation in conventional optical devices or lenses. In 2000, a type of lens was proposed, consisting of a metamaterial that compensates for wave...
- Tunable metamaterialsTunable metamaterialsA tunable metamaterial is a metamaterial with a variable response to an incident electromagnetic wave. This includes remotely controlling how an incident electromagnetic wave interacts with a metamaterial. This means the capablitity to determine whether the EM wave is transmitted, reflected, or...
- List of symbols in physics
Electromagnetic interactions
- Chirality (electromagnetism)Chirality (electromagnetism)The term chiral describes an object, especially a molecule, which has or produces a non-superimposeable mirror image of itself. In chemistry, such a molecule is called an enantiomer or is said to exhibit chirality or enantiomerism...
- DielectricDielectricA dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
- EM radiation
- Frequency domainFrequency domainIn electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....
- Frequency range or Frequency band (on Wikipedia)
- Infrared radiationInfraredInfrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
- electromagnetic radiation with a wavelength between 0.7 and 300 micrometres. - Intrinsic impedance, Electrical impedanceElectrical impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
, and Impedance of free space - Optical medium
- Permeability (electromagnetism)Permeability (electromagnetism)In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
* and Magnetic constant - PermittivityPermittivityIn electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
* and Electric constantElectric constantThe physical constant ε0, commonly called the vacuum permittivity, permittivity of free space or electric constant is an ideal, physical constant, which is the value of the absolute dielectric permittivity of classical vacuum... - Periodic functionPeriodic functionIn mathematics, a periodic function is a function that repeats its values in regular intervals or periods. The most important examples are the trigonometric functions, which repeat over intervals of length 2π radians. Periodic functions are used throughout science to describe oscillations,...
- Ultraviolet radiationUltravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
- are frequencies above visible light but at a wavelength shorter than that of visible light ranging from 10 nm to 400 nm.
General references
- Federal Standard 1037C to "byte"
- Federal Standard 1037C Glossary of Telecommunication Terms
- Military Acronyms, Initialisms, and Abbreviations
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 Dictionary of Units of Measurement (A - Z) - University of North Carolina at Chapel Hill. See nanometer here.