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. [note 1]
This bandwidth is also known as the terahertz gap.[note 2]
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.[1] Applications of frequencies in the terahertz radiation range hold the promise of efficient advancement in notably important technologies.
Currently, a fundamental lack in naturally occurring materials that allow for the desired electromagnetic response has led to constructing new artificial composite materials, termed metamaterials. The metamaterials are based on a lattice structure which mimics crystal structures. However, the lattice structure of this new material consists of rudimentary elements much larger than atoms or single molecules, but is an artificial, rather than a naturally occurring structure. Yet, the interaction achieved is below the dimensions of the terahertz radiation wave. In addition, the desired results are based on the resonant frequency of fabricated fundamental elements. The appeal and usefulness is derived from a resonant response that can be tailored for specific applications, and can be controlled electrically or optically. Or the response can be as a passive material.[2][3][4][5]
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. 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.[6]
The development of electromagnetic, artificial-lattice structured materials, termed metamaterials, has led to the realization of phenomena that cannot be obtained with natural materials. This is observed , for example, with a natural glass lens, which interacts with light (the electromagnetic wave) in a way that appears to be one-handed, while light is delivered in a two-handed manner. In other words, light consists of an electric field and magnetic field. The interaction of a conventional lens, or other natural materials, with light is heavily dominated by the interaction with the electric field (one-handed). The magnetic interaction in lens material is essentially nil. This results in common optical limitations such as a diffraction barrier. Moreover, there is a fundamental lack of natural materials that strongly interact with light's magnetic field. Metamaterials, a synthetic composite structure, overcomes this limitation. In addition, the choice of interactions can be invented and re-invented during fabrication, within the laws of physics. Hence, the capabilities of interaction with the electromagnetic spectrum. which is light, are broadened.[4]
Development of metamaterials has traversed the electromagnetic spectrum up to terahertz and infrared frequencies, but does not yet include the visible light spectrum. This is because, for example, it is easier to build a structure with larger fundamental elements that can control microwaves. The fundamental elements for terahertz and infrared frequencies have been progressively scaled to smaller sizes. In the future, visible light will require elements to be scaled even smaller, for capable control by metamaterials.[7][8][9]
Along with the ability to now interact at terahertz frequencies is the desire to build, deploy, and integrate THz metamaterial applications universally into society. This is because, as explained above, components and systems with terahertz capabilities will fill a technologically relevant void. Because no known natural materials are available that can accomplish this, artificially constructed materials must now take their place.
Research has begun with first, demonstrating the practical terahertz metamaterial. Moreover, since, many materials do not respond to THz radiation naturally, it is necessary then to build the electromagnetic devices which enable the construction of useful applied technologies operating within this range. These are devices such as directed light sources, lenses, switches,[note 3] modulators and sensors. This void also includes phase-shifting and beam-steering devices[note 4] Real world applications in the THz band are still in infancy[4][7][9][10]
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 lasers. However, technologies to control and manipulate THz waves are lagging behind other frequency domains of the spectrum of light.[7][9][10]
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 agents as novel approaches for counter-terrorism.[5] Terahertz metamaterials, because they interact at the appropriate THz frequencies, seem to be one answer in developing materials which use THz radiation.[5]
Researchers believe that artificial magnetic (paramagnetic) structures, or hybrid structures that combine natural and artificial magnetic materials, can play a key role in terahertz devices. Some THz metamaterial devices are compact cavities, adaptive optics and lenses, tunable mirrors, isolators, and converters.[4][8][11]
Without available terahertz sources, other applications are held back.
Semiconductor devices 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.[12]
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).[12]
This lack of appropriate sources hinders opportunities in spectroscopy, remote sensing, free space communications, and medical imaging.[12]
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).[2][10][11][13]
As briefly mentioned above, naturally-occurring materials such as conventional lenses and glass prisms are unable to significantly interact with the magnetic field of light. The significant interaction (permittivity) occurs with the electric field. In natural materials any useful magnetic interaction will taper off in the gigahertz range of frequencies. Compared to interaction with the electric field, the magnetic component is imperceptible when in terahertz, infrared, and visible light. So, a notable step occurred with the invention of a practical metamaterial at microwave frequencies.[note 5] 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,[note 6] 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.[8][14][15]
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.[14][15]
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.[8][16]
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.[8]
This has to do with magnetic coupling at the atomic level. This drawback can be overcome by using metamaterials that mirror atomic magnetic coupling, on a scale magnitudes larger than the atom.[8][17][17]
The first terahertz metamaterials able to achieve a desired magnetic response, which included negative values for permeability, were passive materials. Because of this, "tuning" was achieved by fabricating a new material, with slightly altered dimensions to create a new response. However, the notable advance, or practical achievement, is actually demonstrating the manipulation of terahertz radiation with metamaterials.
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.[8]
The Split-Ring Resonator (SRR) is a common metamaterial in use for a variety of experiments.[2] 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.[8]
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.[8]
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.[8]
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.[8]
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.[8]
From this analysis and demonstration the electrical susceptibility and magnetic permeability - the parameters of normal materials, are artificially expanded. In normal materials, resonances fade away above gigahertz frequencies. Instead, resonances at terahertz frequencies have been effectively demonstrated for metamaterials. 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.[11]
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.[11]
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, filters, tuners and frequency mixers.
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.[11]
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.[11] 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.[11]
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.[11]
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.[18]
The two previous sections discussed a magnetic response 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.[19]
This accomplishment was in agreement with prior theoretical predictions that a layer of pairs of parallel metal nanorods can produce a negative refractive index.[19]
Electromagnetic metamaterials 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.[20][21]
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.[22]
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.[20][21]
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.[23]
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.[22]
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.[20]
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.[20]
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.[21]
The metamaterial was composed of an array of gold crosses fabricated on top of an n-doped semiconductor (GaAs) layer.[21]
The crossbars were effectively electric dipoles. 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.[21]
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.
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.[24]
By combining metamaterial elements - specifically, split ring resonators - with Microelectromechanical systems technology - has enabled the creation of non-planar flexible composites and micromechanically active structures where the orientation of the electromagnetically resonant elements can be precisely controlled with respect to the incident field.[25]
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).[26]
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 wire 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.[27][28]
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, 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.[1]
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).[29]
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.[29]
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 electromagnetic 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.[30]
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.
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.[10]
One of the most critical applications of such a filter is to block unwanted radiation from nearby military high-power lasers, while still allowing the sensor to conduct necessary battlefield.[31]
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 vibrational 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.[32]