WO2016164961A1

WO2016164961A1 - Metamaterial-based devices

Info

Publication number: WO2016164961A1
Application number: PCT/AU2016/000127
Authority: WO
Inventors: Ilya V. SHADRIVOV; Willie J. Padilla; Whenchen CHEN; David A. Powell; Mingkai LIU; Lorenzo Faraone; Mariusz Martyniuk; Buddhika Dilusha SILVA
Original assignee: The Australian National University
Priority date: 2015-04-16
Filing date: 2016-04-15
Publication date: 2016-10-20

Abstract

A device for modulating incident electromagnetic (EM) radiation within a target wavelength range comprising: (a) an EM radiation absorber including: (i) a dielectric membrane; (ii) one or more meta-atoms disposed on a surface of said dielectric membrane, each of said one or more meta-atoms having a maximum dimension smaller than said target wavelength range; and (iii) a substrate in opposing spaced relationship with said one or more meta-atoms; and (b) displacement means for displacing said dielectric membrane and said substrate relative to each other.

Description

METAM ATE RIAL-BASED DEVICES

Cross-Reference to Related Application

[0001] The present application claims the benefit of Australian Provisional Application No. 2015901359, filed 16 April 2015, the entire specification of which is incorporated herein by cross-reference.

Field of the Invention

[0002] The present invention relates to metamaterial-based devices. Background

[0003] Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field.

[0004] Electromagnetic (EM) metamaterials are engineered materials fabricated from sub- wavelength unit cells (or "meta-atoms"), analogous to the atoms, molecules or ions of a conventional material. As in conventional materials, meta-atoms may be arranged in a metamaterial to form a regular or quasi-regular array or may be randomly distributed. As with conventional materials, the properties of a metamaterial are determined by the nature of its constituent elements as well as the arrangement of those elements. Thus, metamaterials having specific properties may be designed by engineering the structure and arrangement of their constituent meta-atoms.

[0005] Metamaterials are geometrically scalable to operate at most frequencies of the EM spectrum, including the technologically relevant terahertz (THz) gap (0.1 to 10 THz). The terahertz gap is difficult to access due to a lack of functional sources and detectors. THz radiation has the potential to provide significant contributions to the development of noninvasive tools in many diversified areas like biology, pharmacology, medical science, nondestructive evaluation, environment monitoring, security and astronomy. The benefit of THz radiation for some of these applications can be partially attributed to the presence of absorption lines of most organic molecules in the THz range. These signatures allow the identification of many materials through their unique transmission spectra, making it possible to distinguish illegal drugs and explosives from benign compounds. Another potential application of THz waves, skin cancer detection, has already become a reality, but has yet to find wide spread use due to the lack of sensitive, inexpensive and efficient THz cameras.

[0006] At present, THz frequencies remain under-utilised as THz radiation is difficult to generate, detect and manipulate. Existing metamaterials operate over a narrow band of frequencies. Furthermore, most existing metamaterials only allow for simultaneous tuning of a complete array of meta-atoms. Tunable metamaterials with dynamic EM responses may allow real-time manipulation of EM radiation. Applied to the THz frequency range, this may have significant application in areas such as short-range wireless THz communication and ultrafast THz switches/modulators.

[0007] Accordingly, there is a need for novel devices that may be used to interact with and manipulate EM radiation in the infrared and/or terahertz range and for devices able to detect EM radiation in the infrared and/or terahertz range. Further, there is a need for novel metamaterials having a tunable EM response.

Summary of the Invention

[0008] According to a first aspect of the invention, there is provided a device for modulating incident electromagnetic (EM) radiation within a target wavelength range comprising: (a) an EM radiation absorber including: (i) a dielectric membrane; (ii) one or more meta-atoms disposed on a surface of said dielectric membrane, each of said one or more meta-atoms having a maximum dimension smaller than said target wavelength range; and (iii) a substrate in opposing spaced relationship with said one or more meta-atoms; and (b) displacement means for displacing said dielectric membrane and said substrate relative to each other.

[0009] The following options may be used in combination with the above aspect, either individually or in any suitable combination.

[0010] Displacement of the dielectric membrane and the substrate relative to each other may change a wavelength range in which the EM absorber absorbs incident (EM) radiation.

[0011] The displacement means may be configured for altering the spacing between the dielectric membrane and the substrate in a direction normal to the dielectric membrane.

[0012] The displacement means may comprise a microelectromechanical system (MEMS). The MEMS may comprise one or more actuation systems, each of said one or more actuation systems comprising: (i) an actuation arm fixed to the dielectric membrane, said actuation arm comprising a first electrode; (ii) two dielectric posts fixed to and projecting from a surface of the substrate, wherein said actuation arm is fixed to and bridging said dielectric posts; and (iii) a second electrode disposed on said surface of the substrate in opposing spaced relationship with said first electrode; whereby application of an electrical voltage between said first electrode and said second electrode causes said actuation arm to bend towards the substrate. The MEMS may comprise a plurality of actuation systems. Each actuation system may be addressed individually or it may be addressed simultaneously with one or more other actuation arms or one or more other groups of actuation arms. The MEMs may comprise four actuation systems, the four actuation systems consisting of four dielectric posts, each of the posts being arranged at a point of a square on the surface of the substrate, and four actuation arms, each of the actuation arms being disposed parallel to a different side of the square.

[0013] The dielectric membrane and the substrate may be substantially parallel.

[0014] The substrate may comprise a dielectric material. The substrate may comprise a material selected from the group consisting of sapphire, silicon, silicon nitride and quartz.

[0015] The substrate may comprise a conductive material. The substrate may have disposed thereon one or more meta-atoms. Each meta-atom of the dielectric membrane may be in opposing spaced relationship with a meta-atom of the substrate. Each meta-atom of the substrate may be in opposing spaced relationship with a meta-atom of the dielectric membrane.

[0016] Each of the one or more meta-atoms may have a maximum dimension of about 3 mm. Each of the one or more meta-atoms may have a maximum dimension of about 1 mm. Each of the one or more meta-atoms may have a maximum dimension of about 0.1 mm. Each of the one or more meta-atoms may have a maximum dimension of about 10 μηι. Each of the one or more meta-atoms may have a maximum dimension of about 1 μηι.

[0017] The dielectric membrane may have disposed thereon from 2 to 100 meta-atoms. The dielectric membrane may have disposed thereon from 9 to 25 meta-atoms.

[0018] Each of the one or more meta-atoms may comprise a conductive material

independently selected from the group consisting of gold and copper. The conductive material may be disposed in a pattern independently selected from the group consisting of split rings, crosses and combinations thereof. [0019] The dielectric membrane may comprise a material selected from the group consisting of sapphire, silicon, silicon nitride and quartz.

[0020] According to a second aspect of the present invention there is provided an

electromagnetic (EM) radiation detector comprising: (a) one or more devices for modulating incident EM radiation within a target wavelength range, each being according to the first aspect of the invention; and (b) an infrared (IR) radiation detector, said IR radiation detector being in operative communication with said one or more devices.

[0021] The following options may be used in combination with the above aspect, either individually or in any suitable combination.

[0022] EM radiation having a wavelength within the target wavelength range incident on the EM radiation absorber of each of the one or more devices for modulating incident EM radiation may be absorbed by the EM radiation absorber and re-emitted as IR radiation, the IR radiation being detected by the EM radiation detector.

[0023] The detector may further comprise a high emissivity material, the high emissivity material being disposed on a surface of the dielectric membrane distal the substrate. The high emissivity material may have an emissivity of at least 0.9.

[0024] The detector may comprise a plurality of the devices for modulating incident EM radiation. Each of the devices for modulating incident EM radiation may be arranged to form a lattice, which may be regular, irregular or quasi-regular. Each of the devices for modulating incident EM radiation may be arranged to form a square lattice array or hexagonal lattice array.

[0025] The detector may further comprise a display means for imaging the EM radiation.

[0026] According to a third aspect of the present invention there is provided use of the device of the first aspect of the invention for modulating EM radiation within the target wavelength range.

[0027] According to a fourth aspect of the present invention there is provided use of the detector of the second aspect of the invention for detecting EM radiation within the target wavelength range. [0028] According to a fifth aspect of the present invention there is provided use of the detector of the second aspect of the invention for imaging EM radiation within the target wavelength range.

[0029] According to a sixth aspect of the present invention there is provided a method for modulating EM radiation within a target wavelength range, said method comprising exposing a device of the first aspect of the invention to said EM radiation such that said EM radiation is absorbed by the EM radiation absorber and emitted as IR radiation.

[0030] According to a seventh aspect of the present invention there is provided a method for detecting EM radiation within a target wavelength range, said method comprising exposing a detector of the second aspect of the invention to said EM radiation such that said EM radiation is absorbed by the EM radiation absorber and emitted as IR radiation, said IR radiation being detected by the IR radiation detector.

[0031] According to an eighth aspect of the present invention there is provided a method for imaging EM radiation within a target wavelength range, said method comprising exposing a detector of the second aspect of the invention to said EM radiation such that said EM radiation is absorbed by the EM radiation absorber and emitted as IR radiation, said IR radiation being detected by the IR radiation detector and imaged using the display means.

Brief Description of Figures

[0032] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures wherein:

[0033] Figures 1 (a)-(l) are schematic diagrams showing various types of meta-atom patterns;

[0034] Figure 2 is a schematic diagram of an embodiment of a device for modulating incident EM radiation including a microelectromechanical system (MEMS);

[0035] Figure 3 is a schematic diagram of an embodiment of an EM radiation absorber;

[0036] Figure 4 is a graph showing the calculated resonant absorptions of the EM absorber of Fig. 3 as a function of frequency of incident EM radiation for a series of distances "h" between the dielectric membrane and substrate; [0037] Figure 5 is a plan view of an embodiment of an EM radiation detector;

[0038] Figure 6 is an optical microscope image of the 100% scale EM radiation absorber of the the Example herein;

[0039] Figure 7 is a graph showing the measured THz absorption spectra of three fabricated EM radiation absorbers (90% scale, 100% scale and 1 10% scale) described in the Example herein;

[0040] Figure 8 (a) is a graph showing the measured absorption spectra under different peak- to-peak voltage (V_pp) for the 100% scale EM radiation absorber described in the Example herein; and (b) is a graph of the corresponding absorption change at the original resonant frequency and the change of resonant frequency f_r .

Description of Embodiments

[0041] Described herein are metamaterial-based devices for modulating EM radiation of a target wavelength range. As used herein in the relation to EM radiation, the term "modulate" and derivatives thereof refer to a change from one EM radiation frequency to another (e.g., from a THz frequency to an infrared (IR) frequency).

[0042] The devices for modulating EM radiation include a metamaterial-based EM radiation absorber and a means for displacing the meta-atoms of the metamaterial-based EM absorber relative to an opposing substrate. The substrate optionally has one or more opposing meta- atoms disposed thereon. Displacement of meta-atoms within the metamaterial-based EM absorber relative to the opposing substrate and any opposing meta-atoms provides a strong modification of the near-field interaction between the meta-atoms and the opposing substrate and opposing meta-atoms (if present). This results in a strong change of the resonant frequency of the EM radiation absorber. This ability to modify the structure of the metamaterial- based EM absorber allows for real-time control of its EM absorption properties. Thus, metamaterial-based EM radiation modulators are described herein that may be tuned to absorb a specific frequency band of EM radiation. For example, the devices for modulating incident EM radiation may be tuned to absorb specific fractions of the IR and/or THz frequency ranges.

[0043] Radiation absorbed by the devices for modulating EM radiation described herein is emitted as infrared (IR) radiation, which may be detected using an IR radiation detector. In embodiments, the devices for modulating EM radiation are tunable to absorb specific fractions of the IR and/or THz frequency ranges, the radiation being emitted as infrared (IR) radiation and detected using an IR radiation detector. Thus, among other applications, the metamaterial- based devices for modulating EM radiation described herein may be used in devices for frequency-selective detection and/or imaging of EM waves. Thus, there are also described herein devices for detecting EM radiation and devices for imaging EM radiation, including EM radiation of the IR and THz frequency ranges.

[0044] The devices for modulating incident EM radiation described herein include an EM radiation absorber. The EM radiation absorber comprises a dielectric membrane having one or more meta-atoms disposed thereon and a substrate in opposing spaced relationship with the one or more meta-atoms of the dielectric membrane. The meta-atoms of the dielectric membrane may be disposed on a surface facing the substrate, or they may be disposed on a surface facing away from the substrate. Optionally, the substrate also has one or more meta- atoms disposed thereon. Where the substrate has one or more meta-atoms disposed thereon, the meta-atoms of the substrate may be disposed on a surface facing the dielectric membrane, or they may be disposed on a surface facing away from the dielectric membrane.

[0045] The dielectric membrane and substrate may be substantially parallel with respect to each other or they may be angled with respect to each other. In general, the dielectric membrane and substrate are separated at maximum spacing by a distance dictated by the desired wavelength range of operation. A larger maximum spacing generally corresponds to a greater wavelength of operation. The maximum achievable separation between the dielectric membrane and substrate may be any suitable distance. The maximum achievable separation between the dielectric membrane and substrate may be about may be about 1 μηι, about 1.5 μηι, about 2 μηι, about 2.5 μηι, about 3 μηι, about 4 μηι, about 5 μηι, or about 10 μηι. The minimum achievable separation between the dielectric membrane and substrate may be any suitable distance. The minimum achievable separation between the dielectric membrane and substrate may be about 1 μηι, about 0.75 μηι, about 0.5 μηι, about 0.25 μηι or about 0 μηι.

[0046] The surface of the dielectric membrane facing the substrate may have any suitable geometry. Where the one or more meta-atoms are not disposed on the surface facing the substrate, the surface on which the one or more meta-atoms are disposed may have any suitable geometry. Where the substrate has disposed thereon one or more meta-atoms, the surface of the substrate facing the dielectric membrane may have any suitable geometry. Where the one or more meta-atoms are not disposed on the surface facing dielectric membrane, the surface on which the one or more meta-atoms are disposed may have any suitable geometry. Each surface of the dielectric membrane and substrate may, for example, be individually selected from the group consisting of planar, concave and convex. In embodiments, the surface of the dielectric membrane on which the one or more meta-atoms are disposed is planar. In embodiments where the one or more meta-atoms are not disposed on the surface facing the substrate, the surface of the dielectric membrane facing the substrate may be planar. In embodiments, the surface of the substrate on which one or more meta-atoms are disposed is planar. In embodiments where the substrate does not have meta-atoms disposed on the surface facing dielectric membrane, the surface facing the dielectric membrane may be planar.

[0047] The dielectric membrane and substrate may have any suitable dimensions. The dielectric membrane and substrate may have the same dimensions or they may have different dimensions. The surface area of a face of the dielectric membrane or substrate on which one or more meta-atoms are disposed will generally be determined by the size, number and spacing of those meta-atoms. In embodiments, the majority of the surface area of a face of a dielectric membrane or substrate on which one or more meta-atoms are disposed is included within the area encompassed by those meta-atoms. Where the substrate does not have any meta-atoms disposed thereon, the substrate may have dimensions such that each meta-atom on the dielectric membrane is in opposing spaced relationship with a region on the substrate. In embodiments, the surface of the dielectric membrane facing the substrate may be smaller than, or the same size as, the surface of the substrate facing the dielectric membrane.

[0048] The dielectric membrane may comprise any suitable dielectric material(s). For example, the dielectric membrane may comprise or may consist of one or more materials selected from the group consisting of sapphire, silicon, silicon nitride, quartz and gallium arsenide.

[0049] The substrate may comprise any suitable material(s). The substrate may comprise one or more dielectric materials, one or more conductive materials or a combination thereof. For example, the substrate may comprise or may consist of one or more materials selected from the group consisting of sapphire, silicon, silicon nitride, quartz, gallium arsenide, metals and conductive polymers. In embodiments, the substrate comprises a dielectric material and a conductive material. In other embodiments, the substrate consists of one or more dielectric materials.

[0050] Where the substrate comprises a dielectric material, the dielectric material of the substrate may have disposed thereon one or more opposing meta-atoms. Each of the one or more meta-atoms disposed on the substrate may be in opposing spaced relationship with a meta-atom of the dielectric membrane, or may be in opposing spaced relationship with a portion of the dielectric membrane having no meta-atom disposed thereon.

[0051] The dielectric membrane may have any suitable number of meta-atoms disposed thereon. In embodiments, there may be between 1 and about 1000 meta-atoms, between about 6 and about 100 meta-atoms, between about 8 and about 50 meta-atoms, or between about 9 and about 25 meta-atoms disposed on the dielectric membrane. Where the substrate has one or more meta-atoms disposed thereon, any suitable number of meta-atoms may be disposed on the substrate. In embodiments, there may be between 1 and about 1000 meta-atoms, between about 6 and about 100 meta-atoms, between about 8 and about 50 meta-atoms, or between about 9 and about 25 meta-atoms disposed on the substrate. In embodiments where the substrate has meta-atoms disposed thereon, the dielectric membrane and substrate may have the same number of meta-atoms or they may have a different number of meta-atoms.

[0052] Where the dielectric membrane, substrate or both has disposed thereon a plurality of meta-atoms, the meta-atoms may be arranged at points of a regular lattice, randomly arranged or combination thereof (i.e., arranged at points of a quasi-regular lattice). A regular lattice may, for example, be a square lattice or a hexagonal lattice (i.e., a triangular lattice or honeycomb lattice). In embodiments, the dielectric membrane and substrate both have a plurality of meta- atoms arranged at points of a square lattice, wherein the square lattices are superimposed such that each meta-atom of the dielectric membrane is in opposing spaced relationship with a meta-atom of the substrate and vice versa.

[0053] Each meta-atom will generally have dimensions smaller than the minimum wavelength of the EM radiation range of interest. For example, for applications in the "terahertz gap" of 0.1 to 10 THz (wavelengths of about 3 mm to about 30 μηι), a meta-atom may have a maximum dimension of 3 mm, 1 mm, 0.3 mm, 0.1 mm or 30 μηι. For applications where the EM radiation range of interest includes IR radiation (wavelengths of about 1 mm to about 0.7 μηι), a meta- atom may have a maximum dimension of 1 mm, 0.3 mm, 0.1 mm, 30 μηι, 10 μηι, 3 μηι, 1 μηι or 0.7 μηι. Where the dielectric membrane has disposed thereon a plurality of meta-atoms, the dimensions of each meta-atom on the dielectric membrane may be the same as or different to the dimensions of any other meta-atom disposed on the dielectric membrane. In embodiments, all meta-atoms disposed on the dielectric membrane have the same dimensions. In

embodiments where the substrate has disposed thereon a plurality of meta-atoms, the dimensions of each meta-atom on the substrate may be the same as or different to the dimensions of any other meta-atom disposed on the substrate. In embodiments where the substrate has disposed thereon on or more meta-atoms in opposing spaced relationship with a meta-atom of the dielectric membrane, the opposing meta-atoms may have substantially the same dimensions or they may have different dimensions. In embodiments, opposing meta- atoms have the same dimensions as each other. In embodiments, all meta-atoms disposed on the dielectric membrane and substrate have the same dimensions.

[0054] Where the dielectric membrane, substrate or both has a plurality of meta-atoms disposed thereon, the space between any two adjacent meta-atoms may be any suitable distance. The distance between any two adjacent meta-atoms is generally less than the wavelength of the target EM radiation. In embodiments, the distance between any two adjacent meta-atoms is less than half the wavelength of the target EM radiation. For example, for applications in the "terahertz gap" of 0.1 to 10 THz (wavelengths of 3 mm to 30 μιτι), any two adjacent meta-atoms may be separated by a distance of less than 1.5 mm, 0.5 mm, 0.15 mm, 50 μηι or 15 μηι.

[0055] Each meta-atom of the dielectric membrane and, where applicable, the substrate may be any suitable sub-wavelength element. The term "meta-atom" as used herein refers to a conductive material in the form of a specific pattern of sub-wavelength dimensions. Interaction of an electromagnetic field with the meta-atom and opposing substrate or meta-atom causes oscillation of the electrons within the conductive material of the meta-atom(s).

[0056] Each meta-atom may comprise any suitable conductive material. For example, a meta- atom may comprise a metal, a conductive polymer or both. Where the meta-atom comprises a metal, the metal may be a highly conducting metal, such as a metal selected from the group consisting of gold, silver, aluminium and copper. However, it will be understood that any suitable metal may be used. When depositing the metal on the dielectric membrane so as to form a meta-atom, a thin adhesion layer of another metal with poorer conductivity may be deposited on the dielectric membrane first. Where the dielectric membrane of the device has disposed thereon a plurality of meta-atoms, the conductive material of each meta-atom on the dielectric membrane may be the same as or different to the conductive material of any other meta-atom disposed on the dielectric membrane. In embodiments, all meta-atoms disposed on the dielectric membrane comprise the same material(s). In embodiments where the substrate has disposed thereon one or more meta-atoms, the conductive material of each meta-atom on the substrate may be the same as or different to the conductive material of any other meta- atom disposed on the substrate. In embodiments, all meta-atoms on the dielectric membrane and the substrate comprise the same material(s). [0057] The conductive material of a meta-atom may form any suitable pattern on a dielectric membrane or substrate. For example, the pattern may be a split ring, a cross, or a combination thereof. A number of suitable patterns are exemplified in Fig. 1. In some embodiments, the pattern may be formed by the conductive material itself. This is exemplified in Figs. 1 (a)-(f), wherein the darker regions represent the dielectric material and the lighter regions represent the conductive material. In other embodiments, the conductive material may define the outline of the pattern, such that the pattern itself is formed by the dielectric material. This is exemplified by Figs. 1 (g)-(l), wherein the darker regions represent the dielectric material and the lighter regions represent the conductive material. Where the dielectric membrane has a plurality of meta-atoms disposed thereon, each of the meta-atoms on the dielectric membrane may have the same pattern or combination of patterns as any other meta-atom on the dielectric membrane, or it may have a different pattern or combination of patterns. In embodiments, all meta-atoms disposed on the dielectric membrane have the same pattern or combination of patterns. Where the substrate has a plurality of meta-atoms disposed thereon, each of the meta-atoms on the substrate may have the same pattern or combination of patterns as any other meta-atom on the substrate, or it may have a different pattern or combination of patterns. In embodiments, all meta-atoms disposed on the substrate have the same pattern or combination of patterns. In embodiments, all meta-atoms disposed on the dielectric membrane and substrate have the same pattern or combination of patterns.

[0058] The pattern of a meta-atom may be formed by any suitable means. For example, the pattern of a meta-atom may be formed using microlithography. The conductive material of each meta-atom may be deposited on a dielectric membrane or substrate by any suitable means. For example, the conductive material may be deposited by an electron beam deposition process, sputtering or thermal evaporation.

[0059] The devices for modulating incident EM radiation described herein also include a means of displacing the meta-atom(s) and the substrate of the EM radiation absorber relative to each other. The displacement means allows for the resonant frequency of the EM radiation absorber to be tuned by altering the geometry of the absorber. Significant absorption of EM radiation by the EM absorber only occurs near the resonance frequency of the absorber. Thus, switching between On' and 'off' may be achieved by displacing the meta-atoms of the absorber relative to the substrate so that absorption frequency is close to or far from resonance, respectively. The displacement means may be capable of displacing the meta-atom in a direction that is normal to the surface of the dielectric membrane, a direction that is parallel to the surface of the dielectric membrane and/or a direction that has both normal and parallel components relative to the surface of the dielectric membrane. [0060] The device for modulating incident EM radiation may comprise any suitable displacement means. For example, the displacement means may comprise be a

microelectromechanical system (MEMS), a piezoelectric device, or a combination thereof. The displacement means may be operatively coupled with the dielectric membrane, the substrate or both.

[0061] In embodiments of the device for modulating incident EM radiation described herein, the displacement means comprises a MEMS. MEMS are micro-scale devices that can produce micro-scale motion with the application of an electrical actuation voltage or charge. Due to their size, and almost negligible inertia, MEMS devices are typically fast, require little power to operate, and are highly robust and vibration resistant. In addition, where the substrate has disposed thereon one or more meta-atoms, utilisation of MEMS as the displacement means allows for accurate alignment of opposing meta-atoms.

[0062] Where the displacement means comprises a MEMS, the MEMS may include one or more actuation systems. A suitable actuation system comprises an actuation arm fixed to and bridging two dielectric posts. The dielectric posts may be fixed to and project from the substrate and the actuation arm may be fixed to the dielectric membrane, such that the actuation arm is in spaced relationship with the substrate. Where the MEMS comprises a plurality of actuation systems, two adjacent actuation systems may share a dielectric post. In embodiments, a MEMS comprises four actuation systems, the four actuation systems including four dielectric posts and four actuation arms. The four dielectric posts are disposed at points of a square on a surface of the substrate, and each of the four actuation arms is disposed parallel to a different side of the square. In order to actuate the activation arm, the actuation arm may comprise a first electrode, such as a conductive coating on the actuation arm, and the substrate may have disposed thereon a second electrode, such as a conductive pad. Application of an electrical voltage between the first electrode and the second electrode causes the actuation arm to bend towards the substrate, decreasing the distance between the dielectric membrane and substrate. Where a MEMS comprises a plurality of actuation systems, each actuation arm may be addressed individually or simultaneously with one or more other actuation arms. The ability to individually control each actuation arm allows for greater control of the position of the membrane and can be used to keep the membrane parallel with the substrate, compensate for any inherent non-parallelism between the membrane and substrate or intentionally introduce non-parallelism between the membrane and substrate. The ability to individually control each actuation arm thus allows for greater tunability of the device. An actuation system may be individually addressed by application of voltage between the first electrode and the second electrode of that actuation arm. By simultaneous application of a voltage between the first electrode and the second electrode of two or more actuation arms, those actuation arms may be actuated in concert.

[0063] The dielectric posts and actuation arms of a MEMS may have any suitable dimensions. The dimension of the actuation arms will generally be determined by the size of the dielectric membrane to which they are fixed. For example, where the substrate is attached to four actuations arms forming a square, the dimensions of the actuation arms may be such that the substrate fits within that square. The dimensions of the dielectric posts will generally be determined by the maximum achievable spacing between the dielectric membrane and substrate. The dimensions of the dielectric posts will generally be greater than or approximately the same as the maximum achievable spacing between the dielectric membrane and substrate. In embodiments, the minimum achievable separation between the dielectric membrane and substrate is approximately 1/3 of the maximum achievable spacing. Therefore, a greater maximum achievable separation may provide a greater wavelength operation range. However, a smaller separation at maximum spacing may provide faster tuning of the device to a specific absorption frequency.

[0064] The first electrode and second electrode may comprise any suitable conductive material(s). The conductive material(s) may be selected from metals, conductive polymers, polycrystalline silicon ("poly-silicon") and combinations thereof.

[0065] A perspective view of an embodiment of a device for modulating incident EM radiation that utilises MEMS is shown in Fig. 2. The device for modulating incident EM radiation 210 comprises a silicon nitride membrane 220 in opposing spaced relationship with and

substantially parallel to a sapphire substrate 230. It will be understood that although the dielectric membrane and substrate of the present example are silicon nitride and sapphire, respectively, any suitable dielectric material(s) may be used. At maximum separation, the distance between the silicon nitride membrane 220 and sapphire substrate 230 is

approximately 3 μηι. Deposited on the silicon nitride membrane 220 is an array of gold meta- atoms 240. The gold meta-atoms 240 are shown as being deposited on the surface of the silicon nitride membrane 220 facing away from the sapphire substrate 230. However, it will be understood that the gold meta-atoms 240 may be deposited on the surface of side of the silicon nitride membrane 220 facing the sapphire substrate 230. Disposed on the sapphire substrate 230 and opposing each of the gold meta-atoms 240 is an opposing gold meta-atom (not shown). In other embodiments, the opposing meta-atoms are absent. It will be understood that although the meta-atoms of the present example are gold, any suitable conducting material(s) may be used. The silicon nitride membrane 220 is tethered at four points to an ultra-low stress silicon nitride support arm 250. Each support arm 250 has a conductive coating of either metal or poly-silicon. Each support arm 250 is fixed at both ends to a silicon nitride post 260 fixed to and projecting from the sapphire substrate 230. The support arms are configured so as to be in opposing spaced relationship with a conductive pad (not shown) disposed on the sapphire substrate 230. Actuation of an individual support arm 250 is achieved by applying an electrical voltage between the support arm 250 and the opposing conductive pad. Electrostatic attraction between the conductive pad and the support arm 250 causes the support arm 250 to bend towards the sapphire substrate 230. This causes the silicon nitride membrane 220 and the gold meta-atoms 240 disposed thereon to be pulled towards the opposing meta-atoms on the sapphire substrate 230. Simultaneous actuation of each of the four support arms 250 causes the silicon nitride membrane 220 to be pulled towards the sapphire substrate while maintaining parallel alignment of the silicon nitride membrane 220 and sapphire substrate 230.

[0066] In the embodiment described in the preceding paragraph, a phenomenon known as "snap-down" prevents control of the silicon nitride membrane in the lower half of the original spacing between the silicon nitride membrane and sapphire substrate. At the limiting voltage, the electrostatic force between the conductive pads and the support arms can no longer be balanced by the mechanical restoring force of the support arms and, as a result, the support arms snap down onto the sapphire substrate. Once snap-down occurs, the two surfaces can permanently adhere due to Van der Waals force. However, recovery from snap-down is possible by the use of anti-stiction bumps disposed on one or more of the support arms, the silicon nitride membrane and the sapphire substrate, which prevent permanent adherence of the surfaces. In the presence of anti-stiction bumps, the arms return to their un-deflected position when the actuation voltage is removed.

[0067] Fig. 3(a) is a schematic illustration of an embodiment of an EM radiation absorber. The EM radiation absorber 310 includes a square dielectric membrane 320 having approximate side length of 170 μηι. Disposed on the dielectric membrane 320 is a 3x3 square lattice of meta- atoms 330, each being approximately 40 μηι square. The meta-atoms 330 are of the type depicted in Fig 1 (c). The dielectric membrane 320 is tethered to four silicon nitride actuator arms 340 of a MEMS (not shown), each having a length of approximately 40 μηι. The silicon nitride actuator arms are fixed at each end to a dielectric post of a MEMS (not shown). Fig. 3(b) is perspective view of the EM absorber 310 showing the dielectric membrane 320 suspended above a dielectric substrate 350. Calculated resonant absorptions of the EM absorber 310 as a function of frequency of incident EM radiation for a series of distances "h" between the dielectric membrane 320 and dielectric substrate 350 are shown in Fig. 4. [0068] In use, a device for modulating EM radiation described herein absorbs incident EM radiation of a specific frequency range and converts it to heat, while transmitting and/or reflecting EM radiation outside that frequency range. Relative displacement of the meta-atoms and the opposing substrate (and any opposing meta-atoms thereon) by actuation of the displacement means modulates this absorption frequency range. In embodiments, as the separation between the meta-atom(s) and the substrate decreases, the device will absorb EM radiation in a lower frequency range. Conversely, as the separation between the meta-atom(s) and the substrate increases, the device will absorb EM radiation in a higher frequency range. In other embodiments, as the separation between the meta-atom(s) and the substrate decreases, the device will absorb EM radiation in a higher frequency range. Conversely, as the separation between the meta-atom(s) and the substrate increases, the device will absorb EM radiation in a lower frequency range.

[0069] The EM absorber of the device for modulating EM radiation described herein converts absorbed radiation to heat, which is emitted as IR radiation and may be detected by an I R radiation detector. The devices for modulating EM radiation described herein may therefore be utilised for frequency-selective imaging and/or detection of EM radiation when used in combination with one or more infrared (IR) detectors. Thus, there are provided metamaterial- based devices for frequency-selective imaging and/or detection of EM radiation comprising one or more of the devices for modulating EM radiation described herein in operative

communication with an IR detector. The EM radiation detectors described herein may, for example, be used for imaging and/or detection of EM radiation in the IR and/or THz frequency range.

[0070] The EM radiation detectors described herein comprise one or more devices for modulating incident EM radiation. An EM radiation detector may comprise any suitable number of devices for modulating incident EM radiation. In embodiments, the EM radiation detector may comprise between 1 and about 10000, between about 10 and about 8000, between about 100 and about 6000, between about 1000 and about 4000 or between about 2000 and about 3000 devices for modulating incident EM radiation. Where an EM radiation detector comprises a plurality of devices for modulating incident EM radiation, the devices may be arranged at points of a regular lattice, randomly arranged or combination thereof (i.e., arrange at points of a quasi-regular lattice). A regular lattice may, for example, be a square lattice or a hexagonal lattice (i.e., a triangular lattice or honeycomb lattice). In embodiments, the EM radiation detectors comprise a plurality of devices for modulating incident EM radiation arranged at points of a square lattice. [0071] Where an EM radiation detector comprises a plurality of devices for modulating incident EM radiation, each may be provided with a separate displacement means such that each may be individually addressed. This allows independent tuning of the absorption frequency of each device for modulating incident EM radiation and for full spatially-inhomogeneous

reconfigurability of the EM radiation detector.

[0072] The IR detector may be any conventional detector suitable for detecting IR radiation within the range emitted by the EM absorber. The IR radiation emitted by the EM absorber may cover a broad spectral range. The IR detector may be suitable for detecting one or more wavelengths within that range. In embodiments, the IR detector may be a mercury cadmium telluride (MCT) detector. MCT detectors are capable of detecting broad spectrum IR radiation with sensitivity in the range of 5-12 microns. Typically, the emitted IR radiation spectrum is dependent on the temperature of the device and the emissivity of the surface material. The EM radiation detector may further comprise a high emissivity material configured so as to increase the amount of IR radiation emitted in the direction of the IR detector, thus improving the efficiency of detection. The high emissivity material may, for example, by graphitised carbon or a commercially available high emissivity paint or coating. In embodiments, the high emissivity material may be deposited on a surface of the dielectric membrane distal the substrate. The high emissivity material may have an emissivity of at least about 0.8, at least about 0.85, at least about 0.9, at least about 0.95 or at least about 0.98.

[0073] A plan view of an EM radiation detector that utilises MEMS is shown in Fig. 5. The EM radiation detector 510 comprises an array of fifteen sub-elements 520 in a 3x5 configuration. Each sub-element 520 is a device for modulating EM radiation (such as that as shown in Fig. 2) sharing a single sapphire substrate 530. The EM radiation detector also includes a mercury cadmium telluride IR radiation detector (not shown), positioned such that the sapphire substrate 530 is between the IR detector and the silicon nitride membrane 520 of the EM radiation detector 510. The IR detector is in operative communication with the array of sub-elements 520, such that, in use, radiation incident on the array of sub-elements 520 is absorbed and re- emitted as IR radiation, which is detected by the IR radiation detector. Each individual sub- element 520 of the EM radiation detector 510 may be actuated independently by a control voltage.

[0074] In utilising an EM radiation detector described herein, the EM absorber absorbs incident electromagnetic radiation of a particular frequency range and converts it to heat. The displacement means provides for dynamic tuning of the absorption frequency of the EM absorber, thus providing the device with spectral selectivity. Where the displacement means comprises a MEMS, the EM radiation absorption of the device may be switched between Όη^: and 'off'. By repeatedty switching the absorption between 'on' and 'off' and iocking the detection to this switching frequency, the contribution of many noise components can be eliminated, greatly enhancing the sensitivity of this terahertz detector. The IR detector detects the thermal signal (i.e., IR radiation) emitted by the EM absorber following absorption of the EM radiation of the selected frequency. The emitted IR radiation is detected by the IR detector, thus indirectly providing detection of the incident EM radiation of the selected frequency.

Exam pie

[0075] Three EM radiation absorbers were fabricated having the same design as EM radiation absorber 310 shown in Fig. 3, with the exception that the meta-atoms 330 were of the type depicted in Fig 1 (d). Each of the three EM radiation absorbers included a MEMS for varying the distance "h" between the dielectric membrane and dielectric substrate. The three EM radiation absorbers were fabricated to 90%, 100% and 110% of the scale of EM radiation absorber 310. An optica! microscope image of the meta-atoms of the 100% scale EM radiation absorber is shown in Fig 6.

[0076] The reflection spectrum of each of the three EM radiation absorbers was measured with a commercial THz time-domain spectrometer. The samples were placed in a chamber purged with nitrogen and their orientations tuned to match to incident polarisation. The measured resonant absorption of the three EM radiation absorbers is shown in Fig. 7 as a function of frequency of incident EM radiation, tt can be seen from these data that the absorption profile of the three EM radiation absorbers shifts to lower frequencies with increasing physical dimensions. The measured resonant absorption of the 100% scale EM radiation absorber as a function of frequency of incident EM radiation for a series of different actuation voltage values is in shown Fig. 8(a). The actuation voltage controls the distance "h" between the dielectric membrane and dielectric substrate. The corresponding absorption change at the original resonant frequency, as well as the change of resonant frequency f_r, are shown in Fig. 8{b). A large resonance shift (-40 GHz) is observed as the membranes are close to snap-down. A change in absorption of around 40% is achieved at the original resonant frequency.

Claims

Claims:

1. A device for modulating incident electromagnetic (EM) radiation within a target wavelength range comprising:

(a) an EM radiation absorber including:

(i) a dielectric membrane;

(ii) one or more meta-atoms disposed on a surface of said dielectric membrane, each of said one or more meta-atoms having a maximum dimension smaller than said target wavelength range; and

(iii) a substrate in opposing spaced relationship with said one or more meta- atoms; and

(b) displacement means for displacing said dielectric membrane and said substrate relative to each other.

2. The device of claim 1 , wherein displacing said dielectric membrane and said substrate relative to each other changes a wavelength range in which said EM absorber absorbs incident (EM) radiation.

3. The device of claim 1 or claim 2, wherein said displacement means is for configured for altering the spacing between said dielectric membrane and said substrate in a direction normal to said dielectric membrane.

4. The device of any one of claims 1 to 3, wherein said displacement means comprises a microelectromechanical system (MEMS).

5. The device of claim 4, wherein the MEMS comprises one or more actuation systems, each of said one or more actuation systems comprising:

(i) an actuation arm fixed to the dielectric membrane, said actuation arm

comprising a first electrode;

(ii) two dielectric posts fixed to and projecting from a surface of the substrate,

wherein said actuation arm is fixed to and bridging said dielectric posts; and

(iii) a second electrode disposed on said surface of the substrate in opposing

spaced relationship with said first electrode;

whereby application of an electrical voltage between said first electrode and said second electrode causes said actuation arm to bend towards the substrate.

6. The device of claim 5, wherein said MEMS comprises a plurality of actuation systems.

7. The device of claim 6, wherein each actuation system may be addressed individually or simultaneously with one or more other actuation arms.

8. The device of any one of claims 5 to 7, wherein said MEMs comprises four actuation systems, the four actuation systems consisting of four dielectric posts, each of said posts being arranged at a point of a square on the surface of the substrate, and four actuation arms, each of said actuation arms being disposed parallel to a different side of said square.

9. The device of any one of claims 1 to 8, wherein said dielectric membrane and said substrate are substantially parallel.

10. The device of any one of claims 1 to 9, wherein said substrate comprises a dielectric material.

11. The device of claim 10, wherein said substrate comprises a material selected from the group consisting of sapphire, silicon, silicon nitride and quartz.

12. The device of any one of claims 1 to 11 , wherein said substrate comprises a conductive material.

13. The device of claim 12, wherein said substrate has disposed thereon one or more meta- atoms.

14. The device of claim 13, wherein each meta-atom of said dielectric membrane is in opposing spaced relationship with a meta-atom of said substrate.

15. The device of claim 13 or claim 14, wherein each meta-atom of said substrate is in opposing spaced relationship with a meta-atom of said dielectric membrane.

16. The device of any one of claims 1 to 15, wherein each of said one or more meta-atoms has a maximum dimension of about 3 mm.

17. The device of any one of claims 1 to 16, wherein each of said one or more meta-atoms has a maximum dimension of about 1 mm.

18. The device of any one of claims 1 to 17, wherein each of said one or more meta-atoms has a maximum dimension of about 0.1 mm.

19. The device of any one of claims 1 to 18, wherein each of said one or more meta-atoms has a maximum dimension of about 10 μηι.

20. The device of any one of claims 1 to 19, wherein each of said one or more meta-atoms has a maximum dimension of about 1 μηι.

21. The device of any one of claims 1 to 20, wherein said dielectric membrane has disposed thereon from 2 to 100 meta-atoms.

22. The device of claim 21 , where said dielectric membrane has disposed thereon from 9 to 25 meta-atoms.

23. The device of any one of claims 1 to 22, wherein each of said one or more meta-atoms comprises a conductive material independently selected from the group consisting of gold and copper.

24. The device of claim 23, wherein said conductive material is disposed in a pattern independently selected from the group consisting of split rings, crosses and combinations thereof.

25. The device of any one of claims 1 to 24, wherein said dielectric membrane comprises a material selected from the group consisting of sapphire, silicon, silicon nitride and quartz.

26. An electromagnetic (EM) radiation detector comprising:

(a) one or more devices for modulating incident EM radiation within a target wavelength range, each being according to of any one of claims 1 to 25; and

(b) an infrared (IR) radiation detector, said IR radiation detector being in operative communication with said one or more devices.

27. The detector of claim 26, wherein EM radiation having a wavelength within the target wavelength range incident on the EM radiation absorber of each of the one or more devices for modulating incident EM radiation is absorbed by said EM radiation absorber and re-emitted as IR radiation, said IR radiation being detected by the EM radiation detector.

28. The detector of claim 26 or claim 27, further comprising a high emissivity material, said high emissivity material being disposed on a surface of said dielectric membrane distal said substrate.

29. The detector of claim 28, wherein said high emissivity material has an emissivity of at least 0.9.

30. The detector of any one of claims 26 to 29 comprising a plurality of said devices for modulating incident EM radiation.

31. The detector of claim 30, wherein each said devices for modulating incident EM radiation is arranged to form a regular, irregular or quasi-regular lattice.

32. The detector of claim 30 or 31 , wherein each said devices for modulating incident EM radiation is arranged to form a square lattice array or hexagonal lattice array.

33. The detector of any one of claims 26 to 32 further comprising a display means for imaging the EM radiation.

34. Use of the device of any one of claims 1 to 25 for modulating EM radiation within said target wavelength range.

35. Use of the detector of any one of claims 26 to 33 for detecting EM radiation within said target wavelength range.

36. Use of the detector of claim 33 for imaging EM radiation within said target wavelength range.

37. A method for modulating EM radiation within a target wavelength range, said method comprising exposing a device of any one of claims 1 to 25 to said EM radiation such that said EM radiation is absorbed by the EM radiation absorber and emitted as IR radiation.

38. A method for detecting EM radiation within a target wavelength range, said method comprising exposing a detector of any one of claims 26 to 33 to said EM radiation such that said EM radiation is absorbed by the EM radiation absorber and emitted as IR radiation, said IR radiation being detected by the IR radiation detector.

39. A method for imaging EM radiation within a target wavelength range, said method comprising exposing a detector of claim 33 to said EM radiation such that said EM radiation is absorbed by the EM radiation absorber and emitted as IR radiation, said IR radiation being detected by the IR radiation detector and imaged using the display means.