Patent Grant
7826504
Not Applicable
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1. Field of the Invention
This invention pertains generally to high frequency detection and control, and more particularly to THz modulation with metamaterials.
2. Description of Related Art
Substantial research is being performed on metamaterials used for microwave radiation, and is in part due to the ease of fabrication of sub-wavelength structures at these frequencies. Indeed, negative refractive index media composed of negative permittivity (∈1<0) and negative permeability (μ1<0) metamaterial elements was first demonstrated at microwave frequencies. As a result intense theoretical, computational and experimental studies of exotic phenomena have arisen, such as perfect lensing and cloaking. Recently, researchers have ventured to create functional metamaterials at near-infrared and visible frequencies. Considerably less work has been directed in the THz frequencies, although the design flexibility associated with metamaterials may prove promising from a device perspective in filling the THz gap.
Metamaterials are geometrically scalable, which translates to operability over many decades of frequency. This engineering tunability is in fact a distinguishing and advantageous property of these materials. However, for many applications it is desirable to have real-time tunability. For instance, short-range wireless THz communication or ultrafast THz interconnects require switches and modulators. Current state-of-the-art THz modulators based on semiconducting structures have the desirable property of being broadband, which is of relevance to THz interconnects. However, these semiconducting structures are only capable of a few percent modulation and usually require cryogenic temperatures. Therefore, further improvement in the performance characteristics of these devices is required before practical application can be made of them.
Accordingly a need exists for apparatus and methods of performing switching and modulation at THz frequencies, including amplitude modulation of narrowband devices, for example THz quantum cascade lasers, thus enabling near-term practical applications. These needs and others are met within the present invention, which overcomes the deficiencies of previously developed apparatus and methods of THz generation and control.
The development of artificially structured electromagnetic materials, termed metamaterials, has led to the realization of phenomena that cannot be obtained with natural materials. This is especially important for the technologically relevant terahertz (1 THz=1012 Hz) frequency regime; many materials inherently do not respond to THz radiation, and the tools that are necessary to construct devices operating within this range—sources, lenses, switches, modulators and detectors—largely do not exist. Considerable efforts are underway to fill this ‘THz gap’ in view of the useful potential applications of THz radiation. Moderate progress has been made in THz generation and detection, with THz quantum cascade lasers being a recent example. However, techniques to control and manipulate THz waves are lagging behind.
Active metamaterial devices are demonstrated herein that are capable of efficient real-time control and manipulation of THz radiation. By way of example, and not limitation, one device consists of an array of gold electric resonator elements (the metamaterial) fabricated on a semiconductor substrate. The metamaterial array and substrate together effectively form a Schottky diode, which enables 50% modulation of THz transmissivity, which is an order of magnitude improvement over existing devices.
The invention is amenable to being embodied in a number of ways, including but not limited to the following.
One implementation of the invention is an apparatus for modulating terahertz radiation, comprising: (a) a substrate comprising a doped semiconductor material; (b) an array of multi-loop split-ring resonators on the substrate, wherein the multi-loop split ring resonators are of a size less that the wavelength of the incident terahertz radiation; (c) at least one gap in each of the split-ring resonators, the gap configured to modulate the circulation of current flow in the split-ring resonator; wherein the gap (at least one in each MM resonator) is configured for generating a resonant response with alternating charge accumulation in response to the presence of the gap; and (d) means for externally exciting electrons in the substrate for modulating the resonance of the array of multi-loop split-ring resonators to control the transmissive ratio of the apparatus.
The MM resonators can comprise either original MM resonators having multiple loops of conductive material within which at least one insulated gap is interposed, or complementary MM resonators, which are inverse of the original, and have multiple looping regions on the substrate which are non-conductive within which at least one conductive gap is interposed.
The gap in each split-ring resonators can operate to drive the capacitance toward zero, and thus dampens resonant response, when sufficient electrons are present near the gap. Alternatively, capacitance of the gaps in the resonators increases, and thus increases resonant response, in response to the depletion of electrons from the gap.
The modulator provides excellent transmissivity ratio of approximately 0.5, and can be operated at room temperature. The resonators in the modulator provide a resonant response although without a negative refractive index. At frequencies near the characteristic frequency of the resonant electron flow, the values of permittivity (∈) and permeability (μ) vary over a wide range in response to small changes in frequency. This modulator is easy to fabricate with a substrate comprising a sufficiently thin layer of semiconductor to allow the passage of a substantial portion of the terahertz radiation during portions of resonant cycles within the split-ring resonators. Preferably, the substrate comprises a lightly-doped semiconductor material.
The modulator may be incorporated within various devices in numerous fields, including imaging, communication and laser devices. The modulator technology can be incorporated in any terahertz devices, or of similar optical frequency range, when a means for externally exciting the substrate is provided, thus changing the depletion or enhancement of electrons near the MM resonator gaps. For example, the substrate can be configured with electrical contacts (e.g., ohmic contact and at least one Schottky contact) for receiving a modulating signal which is coupled through conductors to the multi-loop split-ring resonators. Alternatively, or additionally, the electrons can be excited optically, or by other means, to thus control the resonance and thus transmissivity.
The MM modulator can be incorporated within a Quantum Cascade Laser (QCL) to permit device operation in a surface emitting mode through the array of multi-loop split-ring resonators configured on an active region of the QCL device.
One implementation of the invention is an apparatus for modulating terahertz radiation, comprising: (a) a substrate comprising a thin and lightly-doped semiconductor material, in which the substrate comprises a sufficiently thin layer semiconductor material to allow the passage of a substantial portion of the terahertz radiation during portions of resonant cycles within the split-ring resonators; (b) an array of multi-loop split-ring resonators on the substrate, in which the multi-loop split ring resonators are of a size less that the wavelength of the incident terahertz radiation; (c) at least one gap each of the split-ring resonators, the gaps configured to modulate the circulation of current flow in the split-ring resonator, in which each of the gaps are configured for generating a resonant response with alternating charge accumulation in response to the presence of the at least one gap, and in which the gap in the split-ring resonators drives the capacitance toward zero, and thus dampens resonant response, of the split-ring resonators in response to the presence of electrons in the gap; and wherein the capacitance of at least one gap in the split-ring resonators increases, and thus increases resonant response, of the split-ring resonators in response to the depletion of electrons from the gap; (d) electrical contacts coupled to the array of multi-loop split-ring resonators for exciting electrons in the substrate to control their position in relation to the at least gap in the split-ring resonators and thus modulate the resonance of the array of multi-loop split-ring resonators for controlling the transmissive ratio of the apparatus.
One implementation of the invention is a surface emitting (SE) quantum cascade laser (QCL), comprising: (a) a substrate comprising a doped semiconductor material; (b) a first electrode on the surface of the substrate; (c) an active region upon the first electrode; (d) a second electrode comprising an array of multi-loop split-ring resonators on the surface of the active region, wherein the multi-loop split ring resonators are of a size less that the wavelength of the characteristic terahertz radiation; (e) at least one gap in the split-ring resonators, the gap configured to modulate the circulation of current flow in the split-ring resonator, and wherein the at least one gap is configured for generating a resonant response with alternating charge accumulation in response to the presence of the at least one gap; and (f) contacts on the first electrode and the second electrode through which a modulation signal is received for exciting electrons in the active region for modulating the resonance of the array of multi-loop split-ring resonators to control the surface emission from the transmissive ratio of the quantum cascade laser.
One implementation of the invention is a method of modulating transmissivity of a metamaterial structure at terahertz frequencies, comprising: (a) establishing a periodic array of metamaterial elements upon a semiconductor device layer; (b) configuring each metamaterial element to have multiple loops with at least one gap; (d) receiving an electrical modulation signal for controlling the transmissivity of the metamaterial structure; (e) depleting the number of electrons near the gaps of the periodic array of metamaterial structures to provide a first level of transmissivity in response to a first level of electrical modulation; (f) enhancing the number of electrons near the gaps of the periodic array of metamaterial structures to provide a second level of transmissivity in response to a second level of electrical modulation; and (f) generating a resonant response with alternating charge accumulation near gaps in each the metamaterial element in the periodic array in response to the presence of electrons near the gaps in each the metamaterial element toward changing transmissivity.
The present invention can provide a number of beneficial aspects which can be implemented either separately or in any desired combination without departing from the present teachings.
Each of the foregoing aspects, and related modes, embodiments, variations, and features thereof, is considered of independent value and benefit as an invention presented hereunder, as are the various combinations therebetween as presented throughout this disclosure and otherwise readily apparent to one of ordinary skill having reviewed these disclosed contents. Further aspects of the present disclosure and inventions contained hereunder will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.
The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only:
Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus generally shown in
The following description is provided in a number of sections. As these sections are based on separate documents and articles, the descriptions overlap in many areas.
1. Introduction to Terahertz MM Modulators
Discussed in this introduction are the use of resonators (e.g., metal) for creating a material with tunable responses to an applied voltage. When combined with a semiconductor substrate, these materials can be used to control technologically promising terahertz radiation.
Electromagnetic radiation with frequencies lying between the microwave region and the infrared, referred to as terahertz radiation, holds promise for imaging and sensing applications. It is non-ionizing, and therefore causes less damage to biological tissue than conventional, higher-energy X-rays. It penetrates plastics and clothing, but not metal, and so is ideal for security screening and non-contact testing or inspection. But to realize the full potential of terahertz radiation, more sophisticated techniques for its generation, manipulation and detection are required.
In this issue the teachings of the present invention fill a number of important gaps in THz capabilities. The development of an efficient, electrically driven modulator for terahertz signals that functions at room temperature. These materials engineered to have electromagnetic responses that are impossible in naturally occurring materials, such as a negative refractive index. The refractive index of a material, n, is a measure of the speed of light in the material, and is given by n=(μ∈)1/2, where μ is the material's “permeability” to magnetic fields, and ∈ its ‘permittivity’ to electric fields. All naturally occurring materials have positive μ, while transparent materials also have positive ∈. In these normal materials, therefore, the refractive index is a real and positive number.
Soviet physicist Victor Veselago showed in 1968 that a hypothetical material with negative ∈ and μ would also have a real refractive index, meaning that light waves could propagate through the material, but would behave as though its refractive index were negative. This material would have unusual and potentially valuable properties. A flat slab of negative-index material, for example, would focus light in much the same way as a curved slab of ordinary material (i.e., a lens), however, directed to a smaller focal spot. Although negative-index materials do not violate any laws of physics, the absence of a medium with negative μ confined the idea to the realm of speculation. Yet in the late 1990s, John Pendry found, that by assembling a collection of appropriately designed metallic structures, a material could be fabricated that has both negative ∈ and negative μ for incident electromagnetic radiation of a particular frequency. Furthermore, if the metal structures are each much smaller than the wavelength of the incident radiation, the radiation interacts with them not individually, but collectively, according to their average properties. These are the engineered materials now known as metamaterials. Engineering a material with negative ∈ was easy, as it equates to opacity, a property of all metals for incident radiation below a certain frequency. It was necessary to show only that a discrete set of thin metallic structures could mimic this property of the bulk metal. A more difficult task was achieving a negative μ. It turned out that this could be done using a pair of concentric metallic rings with gaps that prevent current from circulating. Because these rings are both capacitors (they store electric charge) and inductors (they induce magnetic fields that self-sustain any current flowing through them), the presence of gaps leads to a resonant response, with charge accumulating alternately on one side of the gap and then the other, moving back and forth through the rings rather as a mass vibrates back and forth on a spring. At frequencies near the characteristic frequency of this resonant electron flow, ∈ and μ can vary dramatically as a function of frequency. Indeed, either one can become negative if the resonance is strong enough.
The development of the split-ring-resonator concept was significant not only because it permits a negative refractive index, but more generally because it represents a new technique for “designing” the optical response of a medium. The first experimental demonstration of a negative index, along with nearly all research into metamaterials until now, was performed in the microwave regime. This region encompasses gigahertz frequencies below the terahertz regime, with wavelengths of several millimeters or longer. It was almost immediately recognized, however, that the approach could be extended into the shorter-wavelength, terahertz regime simply by shrinking the size of the individual metallic components so that they remained smaller than the incident radiation's wavelength. At 1 THz, this is 300 μm, so the fabrication of a negative-index medium requires the technically challenging construction of three-dimensional objects with micrometer-scale features. Two-dimensional patterns on that scale, on the other hand, can be easily generated using conventional photolithography, and these patterns can be designed to exhibit a strong resonance in either ∈ or μ at any frequency of interest.
The present invention utilizes the split-ring-resonator concept as the starting point for a metamaterial that provides a resonant response in ∈ the terahertz range; although not a negative refractive index. Furthermore, the teachings show that this resonance can be externally controlled, and therefore can be exploited as a modulator for controlling the transmission of terahertz electromagnetic radiation. The principle of device operation is simple and elegant.
Considered another way, the electrons in the semiconductor substrate effectively short circuit the gap in the split rings, driving the capacitance of the rings to zero and damping their normal resonant response. In response to a negative voltage being applied to the metal structures, the electrons in the substrate beneath are repelled away from the gap. As electrons become depleted in the gap, current can no longer flow effectively and the gap behaves as a capacitor, storing electric charge. In this case, the resonance of the ring structure re-emerges, which gives rise to a pronounced change in the optical properties of the array at frequencies near its resonant frequency.
A number of benefits are shown for the inventive teachings. At the design frequency, the on-off transmission ratio of the device embodiment can provide more than a ten times greater transmission ratio over current state-of-the-art electrically operated modulators within this frequency range. These new designs are simpler to fabricate, and the modulator operates at room temperature. This excellent performance comes from the exquisite sensitivity of a metamaterial's response to the precise properties of its resonant substructures. The ability to electrically switch the properties of a metamaterial by fabricating it on a semiconductor substrate provides a new method for active control of terahertz devices.
Some challenges remain to be overcome, for example larger on-off transmission ratios will be required for many applications, while increasing the modulation speed would open a number of applications. The teachings point out that these difficulties can be addressed by optimizing both the pattern of the metal structures and the properties of the substrate, as well as the use of optical excitation of the MM material. The properties of the device are also dependent on the direction of the electric field vector (the polarization) of the incident radiation, wherein the resonant behavior of the split rings relies on this electric field driving current across the gaps, all of which are oriented in one particular direction. Designs of future devices may seek to eliminate or alternatively exploit this polarization sensitivity. Finally, one can imagine a structure for which not only the transmitted intensity, but also the resonant frequency, is externally controllable. Whatever the next stage of development might be, the resonator structures taught herein open a new and promising set of possibilities for the active control of terahertz radiation.
2. Active Terahertz Metamaterial Devices
By way of example embodiment, the metamaterial device described herein is based on an electric analogue to split-ring resonators (SRRs).
Referring again to
In this embodiment the metamaterial elements are electrically connected using conducting wires 18 such that the entire metamaterial array functions as a voltage gate, schematically depicted in
Terahertz time-domain spectroscopy (THz-TDS) was used to characterize the performance of the metamaterial device, and has been described elsewhere in detail. In our photoconductive THz TDS experiment, a polyethylene lens focuses the linearly polarized THz beam onto the metamaterial sample to a diameter of about 3 mm, and a second polyethylene lens recollimates the transmitted THz beam, which is directed to a photoconductive receiver. The experiments were performed at room temperature in a dry air atmosphere.
In THz-TDS, the time-varying electric field of the impulsive THz radiation is recorded, and the electric field spectral amplitude and phase are directly obtained by performing Fourier analysis.
Measurements of the metamaterial device with respect to a suitable reference, as illustrated in
All experiments were performed at normal incidence, with the THz magnetic field lying completely in-plane. The polarization of the THz electric field was either perpendicular or parallel to the split gaps (and connecting wires). The wires connecting the individual electric resonators are necessary (as described above) to provide electrical connectivity to the gate. Importantly, these connecting wires have little effect on the electromagnetic properties of the electric resonators when the THz electric field is normal to the connecting wires. This was confirmed by finite element simulations using commercial software, as shown in
It can be seen from these results that the electric field is strongly concentrated at the split gaps, and there is no significant surface current flowing along the connecting metal wires between electric resonators at the resonant frequency (˜0.72 THz).
Without an applied gate bias, the device is not expected to display resonant behavior associated with the electric resonators because the substrate free charge carriers short out the capacitive response associated with the gaps. Upon application of a voltage, a resonant transmission should result as carriers in the substrate are displaced from the gaps. The long dashed line in
With zero applied voltage to the gate (solid black), the metamaterial response is very weak and does not show significant frequency dependence near the 0.72 THz resonance, as the relatively conductive substrate shorts the capacitive split gap and no LC resonance can be established. Increasing reverse gate bias depletes an increasing fraction of electrons in the n-GaAs layer near the metallic gate, thus significantly reducing the conductivity of the substrate near the split gaps, thereby restoring the LC resonant response. This conclusion is verified by the experimental results shown in
From
Furthermore, in our metamaterial device only a small fraction of the n-GaAs layer is depleted by the reverse gate bias and thus the reduction of free carrier absorption is not nearly enough to account for the change in transmission in
This active metamaterial device was designed such that the LC resonant response occurs with the electric field polarized perpendicular to the connecting wires. This eliminates the Drude-like response that occurs when the THz electric field is parallel to the array of connecting wires. Nonetheless, with the electric field applied parallel to the connecting wires, a response that changes with applied bias is still observed.
The small transmitted intensity at low frequencies results from the Drude-like response of the connecting wires. Superimposed on this is a resonance at 1.25 THz from the metamaterial elements. This resonant response is associated with dipolar currents in the elements analogous to the higher-lying resonance when the electric field is perpendicular to the connecting wires. As such, the variation in ∈1 (C) shown in
Substrates typically used to fabricate planar metamaterial structures (for example, Si, GaAs, Teflon) are insulators and are essentially lossless at THz frequencies. In this case, the metamaterial structure and the substrate can be modeled as an equivalent LCR resonant circuit as shown in
Although the metamaterial structure presented here is a first generation device (that is, no optimization has been attempted), the performance as a THz modulator already exceeds current state-of-the-art electrical THz modulators, which are based on semiconductor structures, by an order of magnitude on resonance while it operates at room temperature. Higher modulation efficiency for practical applications is expected to be achievable through device optimization; that is, by varying the doping concentration and/or the thickness of the doping layer. Presently, however, the design as currently constituted does not provide for high frequency modulation. Measurements were performed of the THz intensity as a function of modulation frequency by applying a rectangular AC reverse gate bias alternating between 0 and 16V.
This demonstration of an active metamaterial device relied on electrically connecting the individual metamaterial elements. It is important to emphasize that this does not compromise the resonant LC response of the elements, and thereby provides additional design flexibility for metamaterials in general. The approach presented here for active THz metamaterials naturally extends to magnetically resonant metamaterials. Finally, consideration of the substrate or embedding environment also offers considerable flexibility in the design of active metamaterial devices at any frequency range.
3. Electrically Resonant Terahertz Metamaterials
A class of artificial metamaterials is presented that exhibit a tailored response to the electrical component of electromagnetic radiation. These electric metamaterials are investigated theoretically, computationally, and experimentally using terahertz time-domain spectroscopy. These structures display a resonant response including regions of negative permittivity ∈1 (ω) ranging from approximately 500 GHz to 1 THz. Conventional electric media such as distributed wires are difficult to incorporate into metamaterials. In contrast, these localized structures will simplify the construction of future metamaterials, including those with negative index of refraction. As these structures generalize to three dimensions in a straightforward manner, they can be utilized in a wide range of applications for significantly enhancing the design and fabrication of functional terahertz devices.
Shaped dielectric and conducting materials which control the electric component of electromagnetic fields with a designed response have been known for some time. Recently, these “artificial dielectrics” have met with renewed interest in the burgeoning field of electromagnetic metamaterials (EM MMs). The excitement about EM MMs stems from the ability of these materials to exhibit an electromagnetic response not readily available in naturally occurring materials including, as examples, negative refractive index and artificial magnetism. However, such exotic phenomena only became possible following the realization that artificial materials could be designed to exhibit an effective material response to electric and magnetic fields. To date, artificial magnetic metamaterials have been experimentally demonstrated over several decades of frequency ranging from radio frequencies to terahertz and near infrared frequencies.
One form of element utilized for magnetic response is the split ring resonator (SRR). Since SRRs were first used to create negative index media, important advances have been realized. Researchers have demonstrated designs with higher symmetry, non-planar structures, and generalization to two and three dimensions. In contrast, purely electric metamaterials have experienced little improvement over the past 60 years and conducting wires have primarily been the medium of choice. Wires have a potential for some tunability, for example a modified plasma frequency can be obtained by making extremely thin wires or by adding loops, thus increasing their inductance. However, recent research has shown that wire arrays are have a number of undesirable characteristics. Limiting factors, for example, include the necessity of inter-unit-cell connections and specific surface terminations. For many applications, it would be preferable to have a localized element with finite extent from which one could construct materials with an electric response.
In principle, the SRR can also be used as an electrically resonant element as it exhibits a strong resonant permittivity at the same frequency as the magnetic resonance. However, the electric and magnetic resonant responses are coupled, resulting in rather complicated bianisotropic electromagnetic behavior. The development of more symmetric designs, which can be predicted by group theoretical methods, eliminates any magneto-optical coupling effects related to bianisotropy and yields electrically resonant structures. Furthermore, in the symmetric elements the magnetic response is suppressed. Thus, such elements will function as localized elements from which one can construct a purely electrical resonant response.
In this aspect of the invention a series of uniaxial and biaxial electric metamaterials are described. The design of these symmetric structures accomplishes the goal of creating a class of subwavelength elements which exhibit a resonant response to the electric field while minimizing or eliminating any response to the magnetic field. Planar arrays of these structures targeted for the THz frequency regime have been simulated, fabricated, and characterized in transmission. Each of the EM MM structures show a resonant response including a region of negative permittivity ∈1 (ω)<0. The advantages of these localized elements are discussed in comparison to conventional wire-segment electric media. In particular, benefits of these structures are discussed that will ease the burden of fabricating additional metamaterial devices including those exhibiting a negative index of refraction.
Embodiments of EM MMs taught herein may be readily fabricated in a planar array by conventional photolithographic methods and consist of 200 nm thick gold with a 10 nm thick adhesion layer of titanium on semi-insulating gallium arsenide (GaAs) substrates of 670 μm thickness. For the sake of simplicity of understanding and comparison all of the EM MM embodiments discussed in this section have an outer dimension of 36 μm, a lattice parameter of 50 μm, a linewidth of 4 μm, and a gap of 2 μm, although it will be appreciated that they may be fabricated of different sizes and geometric relationships. These arrays are characterized such as by utilizing THz time domain spectroscopy (TDS) at normal incidence. The time dependence of the electric field is measured and the complex transmissivity is obtained from which we calculate the complex dielectric function {tilde over (∈)}(ω)=∈1+i∈2, assuming a cubic unit cell. The group-theoretical analysis is supplemented with finite-element numerical simulations of the EM MMs using commercial code and Lorentz oscillator fits to the data.
A number of different resonant element structures are discussed herein, as shown in
Simulations on the structures shown in
Two graphs associated with each element structure show the experimentally measured field transmission T(ω) and the real part of the dielectric function ∈1 (ω), respectively. Each structure exhibits a very strong resonance with the transmission decreasing to as little as approximately 10%. Additionally, all of the EM MMs characterized in this study display regions of negative permittivity.
Table 1 summarizes a few characteristic parameters related to the ∈(ω) response, with important parameters that aid in quantifying the electric response of the metamaterials characterized in this study. Column 1 (leftmost column) gives the name of the element structure (as shown in
A Drude-Lorentz model was used to fit the ∈1 (ω) data, from which ω0 is extracted, which is the center frequency, and ωp, which is the frequency of the zero crossing of ∈1 (ω). In addition, the minimum value of ∈1 is listed, the oscillator strength S=ωp2/ω02, the percentage bandwidth over which ∈1<0 is achieved, and the ratio of the free space wavelength to the unit-cell length, λ0/a.
Next we compare these electrically resonant elements with wires, which are the canonical electric metamaterial. As mentioned, prior use of electric metamaterials relied upon straight wires which can be thought of as a bulk metal with a reduction in the effective electron density due to the reduction in volume fraction of the metal. A bulk metal has free electrons that screen external EM fields from penetrating inside the material for frequencies below the plasma frequency, defined as ωp2=4πne2/m*, where n is the effective electron density and m* is the effective mass. For an electric field polarized along the wire axis an effective medium response is obtained with a modified plasma frequency. Wires can also be formed with loops to add additional inductance to increase carrier mass and reduce the plasma frequency. Lastly, cuts can be added periodically along the wires to obtain a Drude-Lorentz response for added tunability. However, as discussed above, the disadvantages of wires limit their functionality as electric metamaterials.
Now discussed are several advantageous features of the inventive electric metamaterials which principally derive from their symmetry and localized extent. For THz metamaterials with characteristic lengths a=50 μm, the samples are easily fabricated with standard optical lithographic methods. The EM MMs shown in
As mentioned above, E1-E6 do not exhibit a magnetic response near the resonant frequency (i.e., μ=1). Thus, these elements are natural complements to magnetically active SRRs indicating that it is possible to create negative index materials through appropriate combinations of these two varieties of subwavelength elements. Although the thickness of the samples characterized in this study was relatively thin, for example 200 nm, E1-E6 yielded regions of negative permittivity and a substantial bandwidth. It should be noted that several simple ways exist in which to improve the response of the EM MMs in this study, namely, thicker samples, increased filling fraction, extension to multiple layer structures, and utilization of higher-conductivity metals. However, the results presented in
Accordingly, designs for metamaterials have been presented which exhibit a tailored resonant electrical response investigated using THz time domain spectroscopy. The example embodiments offer significant advantages over current electric metamaterials, in terms of both fabrication as well as characteristics. Each of the EM MMs characterized in this study exhibits a negative dielectric response, the benefits for device fabrication one of ordinary skill in the art will appreciate. Furthermore, EM-MMs may be constructed to exhibit polarization-sensitive responses. These electric metamaterials can significantly reduce the burden of constructing negative index metamaterial devices, and the initial demonstrations of these devices at THz frequencies highlights their capabilities as functional electromagnetic materials.
4. Complementary Planar Terahertz Metamaterials
Planar electric split ring resonator (eSRR) metamaterials and their corresponding inverse structures are designed and characterized computationally and experimentally utilizing finite element modeling and THz time domain spectroscopy. A complementary response is observed in transmission. Specifically, for the eSRRs a decrease in transmission is observed at resonance whereas the inverse structures display an increase in transmission. The frequency dependent effective complex dielectric functions are extracted from the experimental data and, in combination with simulations to determine the surface current density and local electric field, provide considerable insight into the electromagnetic response of our planar metamaterials. These structures may find applications in the construction of various THz filters, transparent THz windows, or THz grid structures ideal for constructing THz switching/modulation devices.
4.1 Introduction
In recent years, terahertz (1 THz=1012 Hz) science and technology has been a significant focus of many scientists and researchers worldwide. For instance, the development of THz sources and detectors have led to applications such as THz imaging, semiconductor characterization, chemical and biological sensing, as well as personnel and baggage security screening at airports. However, compared to the well established neighboring infrared and microwave regions, the THz region is still in need of fundamental technological advances. The challenge to bring THz applications to fruition arises in large part because it is difficult to find naturally occurring materials that display a usable electronic response at THz frequencies. This deficiency is often referred to in the industry as the “THz gap”, and so far only moderate progress has been made in THz generation, detection, and manipulation. Considering the prominent potential applications of THz radiation, further improvements are necessary in developing THz devices by using novel materials.
A new type of artificial electromagnetic composites termed metamaterials have been developed in recent years. These new materials have yielded a number of phenomena not achievable with natural materials, including negative refractive index and cloaking. Composite metamaterial elements with sub-wavelength scale of about (λ0/10), such as split ring resonators (SRRs), are patterned in a periodic array to form metamaterials. Thus metamaterials can be considered as an effective medium and are well described by magnetic permeability μ(ω) and/or electric permittivity ∈(ω), in accordance with the macroscopic form of Maxwell's equations. Metamaterials are easily fabricated for response in the microwave regime, including three dimensional (3-D) structures, owing to the relatively large unit cell size (˜1 cm) and smallest required feature size (˜1 mm). With modest effort metamaterials have been extended to THz frequencies. Planar THz electric and magnetic metamaterial elements have typical lateral dimensions of ˜30 μm and are often fabricated upon substrates with dielectric constant ranging from 8-15 ∈0.
In this aspect of the invention results of experimental and computational studies are presented on planar THz eSRR metamaterials and their corresponding inverse structures where the metallic patterns are replaced with open areas and open areas are replaced with metallic conducting regions (see complementary structures of
Original electric metamaterials reveal a resonant decrease in transmission (T(ω)) characteristic of a Lorentz-like effective permittivity with resonant frequency ω0. Off resonance, they yield relatively high values of T(ω). In contrast, the complementary metamaterials display a resonantly enhanced T(ω). The permittivity in this case is the sum of an effective Drude-like response (with an effective plasma frequency ωcp) associated with the interconnected metallic regions and a Lorentz-like oscillator with resonant frequency ωc0. The enhanced transmission occurs near ωcp but below ωc0. Results are also presented for numerical simulations of the local electric field and surface current density, (both on resonance), which provide additional insight into the origin of the electromagnetic response of the original and complementary metamaterial structures.
The complementary transmissive properties of these metamaterials are consistent with Babinet's principle, and provide a foundational understanding useful in the construction of functional THz devices. Such metamaterials may be of particular interest in applications such as FIR spectroscopy, astronomy, laser cavity output couplers, and Fabry-Perot interferometers. In consideration of the critical dependence of devices on materials for use in the mm-wave, THz and infrared frequency regimes, the design flexibility associated with metamaterials offers considerable advantages in construction of low loss, state-of-the-art, high performance frequency agile devices. This is expected to be especially important where conventional designs of FIR filters or frequency selective surface using metallic grids and meshes, or other similar structures, are not ideal. Recently, both optical and electrical methods have demonstrated switching of THz radiation utilizing metamaterial elements thus evincing the versatile and functional electromagnetic response that these exotic artificial structures afford.
4.2 Sample Design and Experimental Technique
The metamaterial structures employed in this work are based upon an electric analogue to the split-ring resonators. Example embodiments of the unit cell geometries and notations are shown in
Terahertz time-domain spectroscopy (THz-TDS) is utilized herein for characterizing the electromagnetic responses of these electric metamaterials. The photoconductive THz-TDS test system has been detailed elsewhere. The test setup includes a polyethylene lens to focus the linearly polarized THz beam onto the metamaterial sample to a diameter of about 3 mm and a second polyethylene lens to recollimate the transmitted THz beam back to a photoconductive receiver. For simplicity, these tests were performed at room temperature in a dry air atmosphere.
4.3 Complementary THz Transmission
With normally incident THz radiation (i.e., the electric and magnetic field is in-plane with the metamaterial structures), the polarization is configured in accordance with the dual sources in Babinet's principle as shown in
Ec−Z0H=E0c, Hc+E/Z0=H0c, (1)
where (E, H) is the transmitted field of the original metamaterial, and (Ec, Hc) is the transmitted field of its complement. This means that, if the polarizations of the THz sources are dual, the field transmission coefficients of the original and complementary planar metamaterials should satisfy:
t(ω)+tc(ω)=1. (2)
As will be shown, THz transmission results through the planar metamaterials demonstrate good agreement with Babinet's principle and Eq. (2) despite the presence of the supporting GaAs substrate. In measurements the complementary samples were rotated (with respect to the orientation of the original structures) by 90° with the THz polarization fixed. However, this rotation is only required for the OE1, OE2 and CE1, CE2 samples, as these structures lack π/2 rotational symmetry.
In addition to the low frequency resonant response, there is a second transmission minimum (original metamaterials) or maximum (complementary metamaterials), which originates from the excitation of electric dipoles similar to that in cut wires or their complements. For these higher frequency resonances, the dimension of the unit cell is on the order of half wavelength of THz wave in GaAs (refractive index 3.6) meaning that the structures are no longer in the effective medium limit. Therefore, the focus in the remainder of this section is on the lower frequency resonant response which can be described in the effective medium limit.
Here it should be emphasized that the enhanced resonant transmission in the complementary metamaterials is due purely to the electric response. It has been found during our tests that the normal incidence of the THz radiation ensures that the electric and magnetic fields are configured to be completely in-plane, which indicates that there is no component of the magnetic field capable of causing a magnetic response by driving circulating currents. Thus, only an electric resonant response is obtained, in accordance with previous results on electric metamaterials and the simulation results shown in the following section.
4.4 Numerical Simulations
The design of the original and complementary planar metamaterial structures (with the aforementioned counter-circulating currents) and the field configuration in these THz transmission tests suggest that the THz resonant transmission is, as described, purely an electric response. In order to further elucidate the nature of the resonances observed in transmission, numerical simulations have been performed for the original and complementary metamaterials using commercially available finite element modeling software. The THz transmission simulations (not shown) are in excellent agreement with the results shown in
To discern surface current density and electric field norms for the original and complementary metamaterials simulations were performed and plotted at the low lying resonance frequencies (not shown). It was found that for the original structures the incident THz field drives circulating surface currents in the inductive loops resulting in charge accumulation at the capacitive split gaps. Thus, in the vicinity of the gaps the electric field is resonantly enhanced. The simulations revealed that the electric metamaterial structures have counter-circulating currents in the loops which cancels the electrically driven magnetic dipoles (oriented normal to the metamaterial plane) which would otherwise result in a magnetoelectric response as occurs in conventional SRRs. That is, these structures are not bianisotropic.
Similarly, in the complementary metamaterial structures with the polarization configured as shown in CE1 of
4.5 Dielectric Function
According to the above discussion coupled with the simulation results, the THz transmission of the original and complementary structures arises from a purely electric resonant response. Thus, the planar THz metamaterials and their complements, in the effective medium approximation, can be described by a frequency dependent complex dielectric function, {tilde over (∈)}(ω)=∈1 (ω)+i∈2 (ω) which is derived from the experimentally determined frequency dependent complex transmission coefficient, {tilde over (t)}(ω)=t(ω)eiφ(ω). This is accomplished through numerical inversion of the Fresnel equations (with the permeability correctly set to unity) and assuming a cubic unit cell.
In the complementary metamaterials, an effective Drude response arising from the interconnected metallic regions occurs at low frequencies, as shown in
It is noted that the resonance frequency, ω0, of the original metamaterial occurs at the lower frequency crossing, ωcp, of the complementary metamaterial, as indicated in
4.6 Observations
A series of novel planar electric metamaterials and their complements which are resonant at THz frequencies have been designed, modeled, and characterized according to this aspect of the invention. These metamaterial structures show complementary transmission properties as characterized by THz time domain spectroscopy, consistent with Babinet's principle. The highly symmetric structures and finite element modeling simulations elucidate the origin of the resonance, which is purely due to the electric response of the metamaterials. THz time domain data permits the extraction of the frequency dependent complex dielectric function, which shows negative values in the original metamaterials and positive values in the complementary metamaterials over a narrow frequency range. While the Lorentz-like resonant permittivity describes the THz transmission dips in the original metamaterials, it modifies the effective Drude response of the complementary metamaterials. Thus for the complementary metamaterials, the result is a combination of the Drude and Lorentz responses, which culminates in the real part of the dielectric function crossing one at the same frequency of the Lorentz oscillator of the original structure. This explains why the transmission minimum of the original metamaterial and the transmission maximum of the complementary metamaterial occur at the same frequency. These novel metamaterial structures will be of potential use in the construction of functional devices operating at THz frequencies.
5. Ultrafast Optical Switching of Terahertz Metamaterials
Optical switching of electrically resonant terahertz planar metamaterials fabricated on ErAs/GaAs nanoisland superlattice substrates is taught and demonstrated in this section. The use of photoexcited charge carriers in the superlattice shunt the capacitive regions of the constituent elements, thereby modulating the resonant response of the metamaterials. A switching recovery time of 20 ps results from fast carrier recombination in the ErAs/GaAs superlattice substrates.
Metamaterials consisting of periodically patterned split ring resonators (SRRs) display a strong resonant electromagnetic response. Importantly, the resonant frequency scales with the SRR dimensions. This characteristic is of particular importance for the terahertz (1 THz=1012 Hz) regime, where naturally occurring materials rarely have a useful electronic or photonic response. Recently, it has been shown that the resonance strength, and hence the transmission, can be controlled via external stimuli for planar metamaterials fabricated on semiconducting substrates. This has resulted in efficient switching and modulation of freely propagating THz radiation, which is essential for many potential applications such as secure short-range communication.
Switching/modulation of THz radiation with an applied voltage was recently demonstrated using a hybrid structure consisting of a Schottky diode and a planar metamaterial array. An external voltage controlled the conductivity of the semiconducting substrate at the capacitive split gaps, thereby modulating the metamaterial resonance strength. Owing to the large capacitance and series resistance, the device operates at a low switching/modulation rate (up to ˜10 kHz). An optically based approach has also been demonstrated where photoexcitation of free charge carriers in the semiconducting substrate shunts the metamaterial resonance. The advantage of the optical approach is the possibility of extremely fast switching using femtosecond optical pulses. In this case, although the switching rise time can be very fast, the recovery time must also be minimized to achieve high bit rate modulation. For intrinsic GaAs this recovery time is of the order of nanoseconds due to carrier recombination. In this section ultrafast switching of THz metamaterial with recovery time as short as 20 ps through engineering the substrate carrier lifetime using molecular beam epitaxy grown ErAs/GaAs superlattices is demonstrated.
The recent progress in THz switching and modulation is noted based on one-dimensional photonic crystals using GaAs as a defect layer. Switching is achieved through photogeneration of carriers in GaAs, with a demonstrated switching time of approximately 100 ps limited by the carrier recombination time.
Various methods to shorten the carrier lifetime in semiconductors have been employed including radiation damage and low-temperature growth to introduce defects. By way of example and not limitation a ErAs/GaAs superlattice structure is grown by molecular beam epitaxy on an intrinsic GaAs wafer. It should be appreciated that in the ErAs/GaAs superlattice structure, the carrier lifetime is strongly correlated with the period L.
The unit cells for these tests are illustrated in the insets to
The optical-pump THz-probe test system has been utilized in the industry. The linearly polarized impulsive THz radiation is focused to about a 3 mm diameter spot onto the metamaterials at normal incidence, with the field polarization indicated in the insets. Synchronized femtosecond optical pulses with 100 fs pulse duration, 1 kHz repetition rate, and 800 nm center wavelength are expanded to about 8 mm diameter, larger than the THz focal spot, to excite free charge carriers in the ErAs/GaAs superlattice substrates. At various time delays between the arrival of THz pulses and optical excitation, the transmitted THz electric field transient is measured. A second measurement of the transmitted THz pulse through an ErAs/GaAs superlattice without the photoexcitation and the metamaterial serves as a reference. The frequency-dependent transmission of the metamaterial structure is obtained by dividing the Fourier transform of the sample scan by the Fourier transform of the reference scan. This is performed at each delay such that the temporal evolution of the frequency-domain metamaterial response can be determined. We purposely engineered the ErAs/GaAs superlattices to have a lifetime of approximately 10 ps to minimize artifacts that arise in the analysis when the sample response varies quickly in comparison with the THz pulse duration. However, ErAs/GaAs nanoisland superlattices can be engineered to have subpicosecond response times, making it possible, in principle, to have even faster switching and recovery times than what is demonstrated below.
It has been shown that a substrate carrier density of 1016 cm−3 (corresponding to fluence of about 1 μJ/cm2) is sufficient to shunt the resonant metamaterial response. The transmission changes of THz intensity in the composite metamaterial ErAs/GaAs structures are displayed in
Following a procedure described elsewhere the effective frequency-dependent complex dielectric function of the metamaterials is determined assuming a cubic unit cell. The real portion of the dielectric function is shown in
The present approach is different from previously demonstrated THz waveform synthesis, in which the switching is accomplished during the THz pulse generation. The freely propagating THz waves are switched and modulated by modifying the resonance strength of the planar metamaterials. The engineered carrier lifetime in the ErAs/GaAs superlattice substrates provides an advantageous ultrafast switching recovery time. This would also be very useful in THz pulse shaping. In contrast, many other resonant switches have slow recovery time due to unavoidable thermal effects. For instance, a similar phenomenon arises in the ultrafast switching of the mid-infrared surface-plasmon resonance through photoinduction of an insulator-to-metal phase transition. However, it takes tens of nanoseconds to recover to the insulating phase. In the present approach metamaterials may simply be scaled to operate at other target frequency ranges, though to obtain a usable response the carrier density in the substrate must also be carefully considered. For applications such as short-range THz communication, the bit rate will, of course, be determined by the repetition rate of the optical pulses.
In summary, optical modification of the resonant response of metamaterials and appropriate engineering design of the supporting substrates enables construction of efficient THz functional devices such as THz switches and modulators. We have demonstrated ultrafast optical switching of the resonant response of THz metamaterials. A switching recovery of approximately 20 ps was demonstrated by engineering design of the carrier lifetime of the ErAs/GaAs superlattice substrates upon which the planar metamaterial arrays were constructed.
6. Surface Emission Terahertz QCL with MM Gates
Some of the most important improvements needed for Quantum Cascade Lasers (QCLs) operating at terahertz (THz) frequencies are related to (i) beam quality, and (ii) imperfect output impedance, both of which could be mitigated with surface emitting QCL structures.
In general, QCLs utilize nano spaced semiconducting layers to form quantum-wells in which the energy levels (and hence the output wavelength) can be chosen by modifying the thicknesses of the individual layers. Previously disclosed QCLs are edge emitting devices.
The active region 116 is typically about 10 μm thick, e.g. between electrodes 114, 118 by about 25 μm wide, such as between opposite transverse edges 122, 124 that are shown to run transverse (and in the particular embodiment shown perpendicular to) the laser emitting edge 120. This configuration results in an overall rectangular structure shown over bottom substrate 112, according to the particular illustrative embodiment shown in the figure. At these typical dimensions and arrangement, the emerging radiation is strongly divergent. This has been observed to lead to problems with beam quality, and imperfect output impedance.
Another type of MM based grid structure, such as in particular examples according to various embodiments provided hereunder, involves using the “inverse” (complement) of the elements shown in
A well established “Babinet's” principle dictates that the “stop-band” provided by the MM structure should turn into a “pass-band” when modified according to the inverse structure shown in rectangular array form.
However, by providing the top electrode to the structure as the “inverse” MM band-pass top gate 180 as shown and described, the radiation 186 passes through the MM and emits from the top surface 180 of the active region 176. This provides the laser emission from the overall SE-QCL 170, in the direction of the bolded arrow shown in the
According to these present embodiments, such QCL devices may be readily fabricated, such as via Molecular Beam Epitaxy (MBE), and operated at THz frequencies utilizing MM top gates. As would be appreciated by one of ordinary skill upon a complete review of the entirety of the present disclosure, and in context of other available information, such MM top gated QCL structures may be readily adapted via ordinary experimentation with respect to gate size, impedance, beam shape, and conversion efficiency for particular applications and specific implementations pursued. Such adaptations may be further developed for example using theoretical and numerical simulations, in addition to ordinary experimental design.
The various benefits provided by the present embodiments include permitting QCL devices to be made larger, limited only by the fabrication quality and wafer size.
The most recent present embodiment provides particular benefit, as strong diffraction observed for beams in modern QCLs (at least as observed prior to the advent of the presently disclosed improvements) is due to the small size of the emission window (e.g., 10 μm×25 μm), such as shown at edge 20 in
Another important consideration in QCLs involves their output impedance. Current state-of-the-art QCLs yield a 1% output due to the large mismatch of the device with free space. This severely limits the performance of QCLs and output powers are typically less than 5 mW. As mentioned, MMs allow a designed electromagnetic response, and thus the impedance of the gate can be carefully tuned to yield maximum output powers.
The various aspects, modes, and embodiments just described for MM top gated SE-QCLs are readily experimentally verified. For example, one approach first fabricates the active quantum well structures on a standard 4″ wafer. A small portion of the wafer is used to fabricate a conventional QCL, for example a similar to the configuration shown in
Notwithstanding the particular benefits of the specific embodiments provided hereunder, further verification and fundamental understanding of these novel SE-QCLs may be expanded upon by one of ordinary skill. In any case, the present disclosure represents a substantial advance over the existing state-of-the-art. A QCL operating in surface emission mode will allow the device to be made significantly larger thus permitting diffraction limited sized (or larger) lasers to be constructed, such as shown in
Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.”
This application is a 35 U.S.C. §111 (a) continuation of, co-pending PCT international application serial number PCT/US2007/082023, filed on Oct. 19, 2007, incorporated herein by reference in its entirety, which claims priority from U.S. provisional application Ser. No. 60/853,503 filed on Oct. 19, 2006, incorporated herein by reference in its entirety. Priority is claimed to each of the listed applications.
This invention was made with Government support under Contract No. DE-AC52-06NA25396 awarded by the U.S. Department of Energy. The Government has certain rights in this invention.
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