Abstract:
Methods and apparatus for providing a tunable absorption band in a wavelength selective surface are disclosed. A device for selectively absorbing incident electromagnetic radiation includes an electrically conductive surface layer including an arrangement of multiple surface elements. The surface layer is disposed at a nonzero height above a continuous electrically conductive layer. An electrically isolating intermediate layer defines a first surface that is in communication with the electrically conductive surface layer. The continuous electrically conductive backing layer is provided in communication with a second surface of the electrically isolating intermediate layer. The arrangement of surface elements couples at least a portion of the incident electromagnetic radiation between itself and the continuous electrically conductive backing layer, such that the resonant device selectively absorbs incident radiation, and reflects a portion of the incident radiation that is not absorbed.

Description:
RELATED APPLICATIONS 
     This application is claims the benefit of priority under 35 U.S.C. §119 from U.S. Provisional Application Ser. No. 60/749,511, filed on Dec. 12, 2005, the contents of which are incorporated herein by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under DMI-0319284 awarded by the National Science Foundation. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to highly reflective and highly absorptive wavelength selective surfaces and more particularly such materials formed using multiple conductive elements over a ground plane. 
     BACKGROUND OF THE INVENTION 
     Frequency selective surfaces can be provided to selectively reduce reflections from incident electromagnetic radiation. Such surfaces are often employed in signature management applications to reduce radar returns. These applications are typically employed within the radio frequency portion of the electromagnetic spectrum. 
     As modern radar systems are often equipped with different and even multiple frequency bands, such signature management surfaces are preferably broad band, reducing reflections over a broad portion of the spectrum. Examples of known frequency selective surfaces providing such a response include one or more than one dielectric layers, which may be disposed above a ground plane. Thickness of the dielectric layers combined with the selected material properties reduce reflected radiation. The thickness of one or more of the layers is a predominant design criteria and is often on the order of one quarter wavelength. Unfortunately, such structures can be complicated and relatively thick, depending upon the selected dielectric materials and wavelength of operation, particularly since multiple layers are often employed. 
     The shapes can be selected to provide a resonant response having a preferred polarization. For example, surface features having an elongated shape provide a resonant response that is more pronounced in a polarization that is related to the orientation of the elongated shape. Thus, an array of vertically aligned narrow rectangles produces a response having a vertically aligned linear polarization. In general, preferred polarizations can be linear, elliptical, and circular. 
     The use of multiple frequency selective surfaces disposed above a ground plane, for radio frequency applications, is described in U.S. Pat. No. 6,538,596 to Gilbert. The frequency selective surfaces can include conductive materials in a geometric pattern with a spacing of the multiple frequency selective surface layers, which can be closer than a quarter wave. However, Gilbert seems to rely on the multiple frequency selective surfaces providing a virtual continuous quarter wavelength effect. Such a quarter wavelength effect results in a canceling of the fields at the surface of the structure. Thus, although individual layers may be spaced at less than one-quarter wavelength (e.g., λ/12 or λ/16), Gilbert relies on macroscopic (far field) superposition of resonances from three of four sheets, such that the resulting structure thickness will be on the order of one-quarter wavelength. 
     SUMMARY OF THE INVENTION 
     What is needed is a simple, thin, highly reflective and highly absorptive wavelength selective surface capable of providing a tunable absorption band. Preferably, the location of the absorption band as well as its bandwidth can be tuned. 
     Various embodiments of the present invention provide an apparatus and method for providing a tunable absorption band in a highly reflective wavelength selective surface. An array of surface elements are defined in an electrically conductive layer disposed above a continuous electrically conductive layer, or ground plane. 
     In one aspect, the invention relates to a device for selectively absorbing incident electromagnetic radiation. The device includes an electrically conductive surface layer including an arrangement of multiple surface elements. An electrically isolating intermediate layer defines a first surface in communication with the electrically conductive surface layer. A continuous electrically conductive backing layer is provided in communication with a second surface of the electrically isolating intermediate layer. The arrangement of surface elements selectively couples at least a portion of the incident electromagnetic radiation between itself and the continuous electrically conductive backing layer, such that the resonant device selectively reflects incident radiation responsive to the coupling. Alternatively or in addition, the device selectively absorbs incident radiation responsive to the coupling. 
     In another aspect, the invention relates to a process of selectively absorbing incident radiation. A first electrically conductive layer is provided including multiple discrete surface elements. A continuous electrically conducting ground plane is also provided. The first electrically conductive layer is separated from the continuous electrically conductive ground plane using an intermediate layer. The resulting structure couples between at least one of the multiple surface elements and the continuous electrically conducting ground plane, at least a portion of electromagnetic radiation incident upon the first electrically conductive layer. At least a portion of the incident radiation that is not coupled is reflected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  shows a top perspective view of one embodiment of a wavelength selective surface having a rectangular array of electrically conductive surface elements. 
         FIG. 2  shows a top planar view of the wavelength selective surface of  FIG. 1 . 
         FIG. 3  shows a top planar view of another embodiment of a wavelength selective surface in accordance with the principles of the present invention having a hexagonal array of electrically conductive square surface elements. 
         FIG. 4  shows a top perspective view of an alternative embodiment of a wavelength selective surface having apertures defined in an electrically conductive surface layer. 
         FIG. 5A  shows a cross-sectional elevation view of the wavelength selective surface of  FIG. 1  taken along A-A. 
         FIG. 5B  shows a cross-sectional elevation view of the wavelength selective surface of  FIG. 4  taken along B-B. 
         FIG. 6A  shows a cross-sectional elevation view of an alternative embodiment of a wavelength selective surface having an over layer covering electrically conductive surface elements. 
         FIG. 6B  shows a cross-sectional elevation view of an alternative embodiment of a wavelength selective surface having an over layer covering an electrically conductive surface layer and apertures defined therein. 
         FIG. 7A  shows in graphical form, an exemplary reflectivity-versus-wavelength response of a narrowband wavelength selective surface constructed in accordance with the principles of the present invention. 
         FIG. 7B  shows in graphical form, an exemplary reflectivity-versus-wavelength response of a wideband wavelength selective surface constructed in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description of preferred embodiments of the invention follows. 
     An exemplary embodiment of a wavelength selective surface  10  is shown in  FIG. 1 . The wavelength selective surface  10  includes at least three distinguishable layers. The first layer is an electrically conductive outer or surface layer  12  including an arrangement of surface elements  20 . The surface elements  20  of the outer layer  12  are disposed at a height above an inner layer including a continuous electrically conductive sheet, or ground layer  14 . The arrangement of surface elements  20  and ground layer  14  is separated by an intermediate layer  16  disposed therebetween. At least one function of the intermediate layer  16  is to maintain a physical separation between the arrangement of surface elements  20  and the ground layer  14 . The intermediate layer  16  also provides electrical isolation between the two electrically conductive layers  12 ,  14 . 
     In operation, wavelength selective surface  10  is exposed to incident electromagnetic radiation  22 . A variable portion of the incident radiation  22  is coupled to the wavelength selective surface  10 . The level of coupling depends at least in part upon the wavelength of the incident radiation  22  and a resonant wavelength of the wavelength selective surface  10 , as determined by related design parameters. Radiation coupled to the wavelength selective surface  10  can also be referred to as absorbed radiation. At other non-resonant wavelengths, a substantial portion of the incident radiation is reflected  24 . 
     In more detail, the electrically conductive surface layer  12  includes multiple discrete surface features, such as the electrically conductive surface elements  20  arranged in a pattern along a surface  18  of the intermediate layer  16 . The discrete nature of the arrangement of surface features  20  requires that individual surface elements  20  are isolated from each other. This also precludes interconnection of two or more individual surface elements  20  by electrically conducting paths. Two or more individual surface elements which are connected electrically form a composite surface element which gives rise to a new resonance. 
     The electrically conductive surface layer  12  including an arrangement of surface elements  20  is typically flat, having a smallest dimension, height, measured perpendicular to the intermediate layer surface  18 . In general, each surface element  20  defines a surface shape and a height or thickness measured perpendicular to the intermediate layer surface  18 . In general, the surface shape can be any closed shape, such as closed curves, regular polygons, irregular polygons, star-shapes having three or more legs, and other closed structures bounded by piecewise continuous surfaces including one or more curves and lines. In some embodiments, the surface shapes can include annular features, such as ring shaped patch with an open center region. More generally, the annular features have an outer perimeter defining the outer shape of the patch and an inner perimeter defining the shape of the open inner region of the patch. Each of the outer an inner perimeters can have a similar shape, as in the ring structure, or a different shape. Shapes of the inner and outer perimeters can include any of the closed shapes listed above (e.g., a round patch with a square open center). 
     Each of the electrically conductive surface elements  20  is formed with an electrically conductive material. Such conductive materials include ordinary metallic conductors, such as aluminum, copper, gold, silver, iron, nickel, tin, lead, and zinc; as well as combinations of one or more metals in the form of a metallic alloy, such as steel, and ceramic conductors such as indium tin oxide and titanium nitride. Alternatively or in addition, conductive materials used in formation of the surface elements  20  include semiconductors. Preferably, the semiconductors are electrically conductive. Exemplary semiconductor materials include: silicon and germanium; compound semiconductors such as silicon carbide, gallium-arsenide and indium-phosphide; and alloys such as silicon-germanium and aluminum-gallium-arsenide. Electrically conductive semiconductors are typically doped with one or more impurities in order to provide good electrical conductivity. Similarly, the ground layer  14  can include one or more electrically conductive materials, such as those described herein. 
     The intermediate layer  16  can be formed from an electrically insulative material, such as a dielectric providing electrical isolation between the arrangement of surface elements  20  and the ground layer  14 . Some examples of dielectric materials include silicon dioxide (SiO 2 ); alumina (Al 2 O 3 ); aluminum oxynitride; silicon nitride (Si 3 N 4 ). Other exemplary dielectrics include polymers, rubbers, silicone rubbers, cellulose materials, ceramics, glass, and crystals. Dielectric materials also include: semiconductors, such as silicon and germanium; compound semiconductors such as silicon carbide, gallium-arsenide and indium-phosphide; and alloys such as silicon-germanium and aluminum-gallium-arsenide; and combinations thereof. As dielectric materials tend to concentrate an electric field within themselves, an intermediate dielectric layer  16  will do the same, concentrating an induced electric field between each of the surface elements  20  and a proximal region of the ground layer  14 . Beneficially, such concentration of the electric-field tends to enhance electromagnetic coupling of the arrangement of surface elements  20  to the ground layer  14 . 
     Dielectric materials can be characterized by parameters indicative of their physical properties, such as the real and imaginary portions of the index of refraction, often referred to as “n” and “k.” Although constant values of these parameters n, k can be used to obtain an estimate of the material&#39;s performance, these parameters are typically wavelength dependent for physically realizable materials. In some embodiments, the intermediate layer  16  includes a so-called high-k material. Examples of such materials include oxides, which can have k values ranging from 0.001 up to 10. 
     The arrangement of surface elements  20  can be configured in a preferred arrangement, or array on the intermediate layer surface  18 . Referring now to  FIG. 2 , the wavelength selective surface  10  includes an exemplary array of flattened, electrically conductive surface elements  20 . Multiple surface elements  20  are arranged in a square grid along the intermediate layer surface  18 . A square grid or matrix arrangement is an example of a regular array, meaning that spacing between adjacent surface elements  20  is substantially uniform. Other examples of regular arrays, or grids include oblique grids, centered rectangular grids, hexagonal grids, triangular grids, and Archimedean grids. In some embodiments, the grids can be irregular and even random. Each of the individual elements  20  can have substantially the same shape, such as the circular shape shown. 
     Although flattened elements are shown and described, other shapes are possible. For example, each of the multiple surface elements  20  can have non-flat profile with respect to the intermediate layer surface  18 , such as a parallelepiped, a cube, a dome, a pyramid, a trapezoid, or more generally any other shape. One major advantage of the present invention over other prior art surfaces is a relaxation of the fabrication tolerances. The high field region resides underneath each of the multiple surface elements  20 , between the surface element  20  and a corresponding region of the ground layer  14 . 
     In more detail, each of the circular elements  20  has a respective diameter D. In the exemplary square grid, each of the circular elements  20  is separated from its four immediately adjacent surface elements  20  by a uniform grid spacing A measured center-to-center. An alternative embodiment of another wavelength selective surface  40  including a hexagonal arrangement, or array of surface elements  42  is shown in  FIG. 3 . Each of the discrete surface elements includes a square surface element  44  having a side dimension D′. Center-to-center spacing between immediately adjacent elements  44  of the hexagonal array  42  is about A′. For operation in the infrared portion of the electromagnetic spectrum, D will generally be between about 0.5 microns for near infrared and 50 microns for the far infrared and terahertz, understanding that any such limits are not firm and will very depending upon such factors as n, k, and the thickness of layers. 
     Array spacing A can be as small as desired, as long as the surface elements  20  do not touch each other. Thus, a minimum spacing will depend to some extent on the dimensions of the surface feature  20 . Namely, the minimum spacing must be greater than the largest diameter of the surface elements (i.e., A&gt;D). The surface elements can be separated as far as desired, although absorption response suffers from increased grid spacing as the fraction of the total surface covered by surface elements falls below 10%. 
     An exemplary embodiment of an alternative family of wavelength selective surfaces  30  is shown in  FIG. 4 . The alternative wavelength selective surfaces  30  also include in intermediate layer  16  stacked above a ground layer  14 ; however, an electrically conductive surface  32  layer includes a complementary feature  34 . The complementary feature  34  includes the electrically conductive layer  32  defining an arrangement of through apertures  36 , holes, or perforations. 
     The electrically conductive layer  32  is generally formed having a uniform thickness. The arrangement of through apertures  34  includes multiple individual through apertures  36 , each exposing a respective surface region  38  of the intermediate layer  16 . Each of the through apertures  36  forms a respective shape bounded by a closed perimeter formed within the conductive layer  32 . Shapes of each through aperture  36  include any of the shapes described above in reference to the electrically conductive surface elements  20  ( FIG. 1 ),  44  ( FIG. 3 ). 
     Additionally, the through apertures  36  can be arranged according to any of the configurations described above in reference to the electrically conductive surface elements  20 ,  44 . This includes a square grid, a rectangular grid, an oblique grid, a centered rectangular grid, a triangular grid, a hexagonal grid, and random grids. Thus, any of the possible arrangements of surface elements  36  and corresponding exposed regions of the intermediate layer surface  18  can be duplicated in a complementary sense in that the surface elements  20  are replaced by through apertures  36  and the exposed regions of the intermediate layer surface  18  are replaced by the electrically conductive layer  32 . 
     A cross-sectional elevation view of the wavelength selective surface  10  is shown in  FIG. 5A . The electrically conductive ground layer  14  has a substantially uniform thickness H G . The intermediate layer  16  has a substantially uniform thickness H D , and each of the individual surface elements  20  has a substantially uniform thickness H P . The different layers  12 ,  14 ,  16  can be stacked without gaps therebetween, such that a total thickness H T  of the resulting wavelength selective surface  10  is substantially equivalent to the sum of the thicknesses of each of the three individual layers  14 ,  16 ,  12  (i.e., H T =H G +H D +H P ). A cross-sectional elevation view of the complementary wavelength selective surface  30  is shown in  FIG. 5B  and including a similar arrangement of the three layers  14 ,  16 ,  32 . 
     In some embodiments, the intermediate insulating layer has a non-uniform thickness with respect to the ground layer. For example, the intermediate layer may have a first thickness H D  under each of the discrete conducting surface elements and a different thickness, or height at regions not covered by the surface elements. It is important that a sufficient layer of insulating material be provided under each of the surface elements to maintain a design separation and to provide isolation between the surface elements and the ground layer. In at least one example, the insulating material can be substantially removed at all regions except those immediately underneath the surface elements. In other embodiments, the insulating layer can include variations, such as a taper between surface elements. At least one benefit of the inventive design is a relaxation of design tolerances that results in a simplification of fabrication of the devices. 
     The thickness chosen for each of the respective layers  12 ,  32 ,  16 ,  14  (H P , H D , H G ) can be independently varied for various embodiments of the wavelength selective surfaces  10 ,  30 . For example, the ground plane  14  can be formed relatively thick and rigid to provide a support structure for the intermediate and surface layers  16 ,  12 ,  32 . Alternatively, the ground plane  14  can be formed as a thin layer, as long as a thin ground plane  14  forms a substantially continuous electrically conducting layer of material providing the continuous ground. Preferably, the ground plane  14  is at least as thick as one skin depth within the spectral region of interest. Similarly, in different embodiments of the wavelength selective surfaces  10 ,  30 , the respective surface layer  12 ,  32  can be formed with a thickness H P  ranging from relatively thin to relatively thick. In a relatively thin embodiment, the surface layer thickness H P  can be a minimum thickness required just to render the intermediate layer surface  18  opaque. Preferably, the surface layer  12 ,  32  is at least as thick as one skin depth within the spectral region of interest. 
     Likewise, the intermediate layer thickness H D  can be formed as thin as desired, as long as electrical isolation is maintained between the outer and inner electrically conducting layers  12 ,  32 ,  14 . The minimum thickness can also be determined to prevent electrical arcing between the isolated conducting layers under the highest anticipated induced electric fields. Alternatively, the intermediate layer thickness H D  can be formed relatively thick. The concept of thickness can be defined relative to an electromagnetic wavelength λ c  of operation, or resonance wavelength. For example, the intermediate layer thickness H D  can be selected between about 0.01λ c  in a relatively thin embodiment to about 0.5λ c  in a relatively thick embodiment. 
     The wavelength selective surfaces  10 ,  30  can be formed using standard semiconductor fabrication techniques. Alternatively or in addition, the wavelength selective surfaces  10 ,  30  can be formed using thin film techniques including vacuum deposition, chemical vapor deposition, and sputtering. In some embodiments, the conductive surface layer  12 ,  44  can be formed using printing techniques. The surface features can be formed by providing a continuous electrically conductive surface layer and then removing regions of the surface layer to form the surface features. Regions can be formed using standard physical or chemical etching techniques. Alternatively or in addition, the surface features can be formed by laser ablation, removing selected regions of the conductive material from the surface, or by nano-imprinting or stamping, or other fabrication methods known to those skilled in the art. 
     Referring to  FIG. 6A  a cross-sectional elevation view of an alternative embodiment of a wavelength selective surface  50  is shown having an over layer  52 . Similar to the embodiments described above, the wavelength selective surface  50  includes an electrically conductive outer layer  12  having an arrangement of surface elements  20  ( FIG. 1 ) disposed at a height above a ground layer  14  and separated therefrom by an intermediate layer  16 . The over layer  52  represents a fourth layer, or superstrate  52  provided on top of the electrically conductive surface layer  12 . 
     The over layer  52  can be formed having a thickness H C1  measured from the intermediate layer surface  18 . In some embodiments, the over layer thickness H C1  is greater than thickness of the surface elements  20  (i.e., H C1 &gt;H P ). The over layer  52  can be formed with varying thickness to provide a planar external surface. Alternatively or in addition, the over layer  52  can be formed with a uniform thickness, following a contour of the underlying electrically conductive surface  12 . 
     An over layering material  52  can be chosen to have selected physical properties (e.g., k, n) that allow at least a portion of incident electromagnetic radiation to penetrate into the over layer  52  and react with one or more of the layers  12 ,  14 , and  16  below. In some embodiments, the overlying material  52  is optically transparent in the vicinity of the primary absorption wavelength, to pass substantially all of the incident electromagnetic radiation. For example, the overlying material  52  can be formed from a glass, a ceramic, a polymer, or a semiconductor. The overlaying material  52  can be applied using any one or more of the fabrication techniques described above in relation to the other layers  12 ,  14 ,  16  in addition to painting and/or dipping. 
     In some embodiments, the over layer  52  provides a physical property chosen to enhance performance of the wavelength selective device in an intended application. For example, the overlaying material  52  may have one or more optical properties, such as absorption, refraction, and reflection. These properties can be used to advantageously modify incident electromagnetic radiation. Such modifications include focusing, de-focusing, and filtering. Filters can include low-pass, high-pass, band pass, and band stop. 
     The overlaying material  52  can be protective in nature allowing the wavelength selective surface  50  to function, while providing environmental protection. For example, the overlaying material  52  can protect the surface conductive layer  12  from corrosion and oxidation due to exposure to moisture. Alternatively or in addition, the overlaying material  52  can protect either of the exposed layers  12 ,  16  from erosion due to a harsh (e.g., caustic) environment. Such harsh environments might be encountered routinely when the wavelength selective surface is used in certain applications. At least one such application that would benefit from a protective overlaying material  52  would be a marine application, in which a protective over layer  52  would protect the electrically conductive layer  12  or  32  from corrosion. 
     In another embodiment shown in  FIG. 6B , a wavelength selective surface  60  includes an overlying material  62  applied over a conductive layer  32  defining an arrangement of through apertures  34  ( FIG. 4 ). The overlying material  62  can be applied with a maximum thickness H C2  measured from the intermediate layer surface  18  to be greater than the thickness of the conductive layer  32  (i.e., H C2 &gt;H P ). The overlaying material  62  again can provide a planar external surface or a contour surface. Accordingly, a wavelength selective surface  60  having apertures  36  defined in an electrically conductive layer  32  is covered by an overlying material  62 . The performance and benefits of such a device are similar to those described above in relation to  FIG. 6A . 
     Referring to  FIG. 7A , an exemplary reflectivity versus wavelength response curve  70  of a representative narrow-resonance response is shown in graphical form. The response curve  70  is achieved by exposing a wavelength selective surface  10  ( FIG. 1 ) constructed in accordance with the principles of the present invention to incident electromagnetic radiation  22  ( FIG. 1 ) within a band including a resonance. As shown, the reflectivity to incident electromagnetic radiation varies according to the curve  70  within the range of 0% to 100%. As the wavelength of the incident radiation  22  is varied from 2 to 20 microns, the reflectivity starts at a relatively high value of about 75%, increases to a value of over 85% at about 3 microns, reduces back to about 75% at about 3.5 microns, and increases again to nearly 100% between about 3.5 and 7 microns. Between 7 and 8 microns, the reflectivity response curve  70  incurs a second and more pronounced dip  72  to less then 20% reflectivity. The second dip  72  is steep and narrow, corresponding to absorption of incident electromagnetic radiation by the surface  10 . The reflectivity response curve  70  at wavelengths beyond about 8 microns rises sharply back to more than 90% and remains above about 80% out to at least 20 microns. This range, from 2 to 20 microns, represents a portion of the electromagnetic spectrum including infrared radiation. 
     The second and much more pronounced dip  72  corresponds to a primary resonance of the underlying wavelength selective surface  10 . As a result of this resonance, a substantial portion of the incident electromagnetic energy  22  is absorbed by the wavelength selective surface  10 . A measure of the spectral width of the resonance response  70  can be determined as a width in terms of wavelength normalized to the resonant wavelength (i.e., Δλ/λ c  or dλ/λ c ). Preferably, this width is determined at full-width-half-maximum (FWHM). For the exemplary curve, the width of the absorption band at FWHM is less than about 0.2 microns with an associated resonance frequency of about 7 microns. This results in a spectral width, or dλ/λ c  of about 0.03. Generally, a dλ/λ c  value of less than about 0.1 can be referred to as narrowband. Thus, the exemplary resonance is representative of a narrowband absorption response. 
     Results supported by both computational analysis of modeled structures and measurements suggest that the resonant wavelength associated with the primary resonance response  72  is sensitive to a maximum dimension of the electrically conductive surface elements (e.g., a diameter of a circular patch D, or a side length of a square patch D′). As the diameter of the surface elements is increased, the wavelength of the primary absorption band  72  also increases. Conversely, as the diameter of the surface elements is decreased, the wavelength of the primary absorption band  72  also decreases. 
     The first, less pronounced dip  74  in reflectivity corresponds to a secondary absorption band of the underlying wavelength selective surface  10 . Results supported by both computational analysis of modeled structures and measurements suggest that the wavelength associated with the secondary absorption band  74  corresponds at least in part to a center-to-center spacing of the multiple electrically conductive surface elements. As the spacing between surface elements  20  in the arrangement of surface elements  20  is reduced, the wavelength of the secondary absorption band  74  decreases. Conversely, as the spacing between the arrangement of surface elements  20  is increased, the wavelength of the secondary absorption band  74  increases. The secondary absorption band  74  is typically less pronounced than the primary absorption band  72 , such that a change in reflectivity ΔR can be determined between the two absorption bands  74 ,  72 . A difference in wavelength between the primary and secondary absorption bands  72 ,  74  is shown as ΔW. 
     In general, the performance may be scaled to different wavelengths according to the desired wavelength range of operation. Thus, by scaling the design parameters of any of the wavelength selective surfaces as described herein, resonant performance can be obtained within any desired region of the electromagnetic spectrum. Resonant wavelengths can range down to visible light and even beyond into the ultraviolet and X-ray. At the other end of the spectrum, the resonant wavelengths can range into the terahertz band (e.g., wavelengths between about 1 millimeter and 100 microns) and even up to radio frequency bands (e.g., wavelengths on the order of centimeters to meters). Operation at the shortest wavelengths will be limited by available fabrication techniques. Current techniques can easily achieve surface feature dimensions to the sub-micron level. It is conceivable that such surface features could be provided at the molecular level using currently available and emerging nanotechnologies. Examples of such techniques are readily found within the field of micro-mechanical-electrical systems (MEMS). 
     Referring to  FIG. 7B , an exemplary reflectivity versus wavelength response curve  80  of a wide-resonance wavelength selective surface is shown in graphical form. This wideband response curve  80  can also be achieved with the wavelength selective surface  10  ( FIG. 1 ) constructed in accordance with the principles of the present invention, but having a different selection of design parameters. Here, a primary absorption band  82  occurs at about 8 microns, with wavelength range at FWHM of about 3 microns. This results in a spectral width Δλ/λ c  of about 0.4. A spectral width value Δλ/λ c  greater than 0.1 can be referred to as broadband. Thus, the underlying wavelength selective surface  10  can also be referred to as a broadband structure. 
     One or more of the physical parameters of the wavelength selective surface  10  can be varied to control reflectivity response of a given wavelength selective surface. For example, the thickness of one or more layers (e.g., surface element thickness H P , dielectric layer thickness H D , and over layer thickness H C ) can be varied. Alternatively or in addition, one or more of the materials of each of the different layers can be varied. For example, the dielectric material can be substituted with another dielectric material having a different n and k values. The presence or absence of an over layer  52  ( FIG. 6A ), as well as the particular material selected for the over layer  52  can also be used to vary the reflectivity or absorption response of the wavelength selective surface. Similar performance changes may be achieved by changing the material of the ground plane, change the dimension D of the surface elements, or by changing the shape of the surface elements. 
     In a first example, a wavelength selective surface includes an intermediate layer formed with various diameters of surface patches. The wavelength selective surface includes a triangular array of round aluminum patches placed over an aluminum film ground layer. The various surfaces are each formed with surface patches having a different respective diameter. A summary of results obtained for the different patch diameters is included in Table 1. In each of these exemplary embodiments, the patch spacing between adjacent patch elements was about 3.4 microns, and the thickness or depth of the individual patches and of the ground layer film were each about 0.1 micron. An intermediate, dielectric layer having thickness of about 0.2 microns was included between the two aluminum layers. It is worth noting that the overall thickness of the wavelength selective surface is about 0.4 microns—a very thin material. The exemplary dielectric has an index of refraction of about 3.4. Table 1 includes wavelength values associated with the resulting primary absorptions. As shown, the resonant wavelength increases with increasing patch size. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Primary Absorption Wavelength Versus Patch Diameter 
               
             
          
           
               
                   
                 Patch Diameter 
                 Resonant Wavelength (λ c ) 
               
               
                   
                   
               
               
                   
                 1.25 μm 
                 4.1 μm 
               
               
                   
                 1.75 μm 
                 5.5 μm 
               
               
                   
                 2.38 μm 
                 7.5 μm 
               
               
                   
                 2.98 μm 
                 9.5 μm 
               
               
                   
                   
               
             
          
         
       
     
     In another example, triangular arrays of circular patches having a uniform array spacing of 3.4 microns and patch diameter of 1.7 microns are used. A dielectric material provided between the outer conducting layers is varied. As a result, the wavelength of the primary absorption shifts. Results are included in Table 2. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Resonance Versus Dielectric Material 
               
             
          
           
               
                   
                 Dielectric material 
                 Resonant Wavelength (λc) 
               
               
                   
                   
               
               
                   
                 Oxide 
                 5.8 μm 
               
               
                   
                 Nitride 
                 6.8 μm 
               
               
                   
                 Silicon 
                 7.8 μm 
               
               
                   
                   
               
             
          
         
       
     
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.