Abstract:
In various embodiments, an adaptive spectral surface is provided including an upper layer having a frequency selective surface, a lower layer being at least partially reflective, and an active dielectric material layer therebetween. The active dielectric material may include a dielectric material with an adjustable permittivity, permeability, or thickness. The active dielectric material may be a dielectric material adapted to change its dielectric constant in response to at an applied electric field, an applied magnetic field, or/and thermal stimulus. Some embodiments allow shifting of the resonance of the spectral absorptive/reflective emissions of the adaptive spectral surface. Some embodiments allow modification of the electromagnetic signature of an adaptive spectral surface apparatus.

Description:
BACKGROUND 
     A frequency selective surface or FSS has many useful applications. For example, U.S. Pat. No. 5,208,603, by James S. Yee, entitled: FREQUENCY SELECTIVE SURFACE (FSS), issued May 4, 1993, herein incorporated by reference, shows one possible type and application. Considerable work is being done in making an FSS with switchable or adaptive properties, most notably to switch it from being a band pass to a band-stop device. Typically this is accomplished with the fabrication of multiple MEMS switches into the FSS layer. 
     Such techniques, while being technologically very impressive, require enormously complex fabrication and testing. The MEMS FSS techniques are also very difficult to scale to frequencies much higher than 50-100 GHz because of the complexity of the MEMS switches. 
     What is needed is an adaptive FSS that is more easily fabricated. Further, what is needed is device that may be easily fabricated to operate at frequencies higher than 50-100 GHz. 
     SUMMARY 
     In various embodiments, an adaptive spectral surface apparatus is provided including an upper layer having a frequency selective surface, a lower layer being at least partially reflective, and an active dielectric material layer between the upper layer and the lower layer. 
     In some embodiments, the active dielectric material includes a dielectric material with an adjustable permittivity and/or permeability of the active dielectric layer or thickness. In some embodiments, the active dielectric material may be a dielectric material adapted to change its dielectric constant in response to an applied electric field, an applied magnetic field, or/and thermal stimulus. 
     It is possible in some embodiments to shift the resonance of the absorptive/reflective spectrum of the adaptive spectral surface apparatus. Further, it is possible in some embodiments to modify the electromagnetic signature of an adaptive spectral surface apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention will be better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  is a perspective view of an adaptive spectral surface, in accordance with an embodiment of the present invention; 
         FIG. 2A  is a plot showing an example of the emission spectrum, the emissivity verses frequency, of an adaptive spectral surface in accordance with an embodiment utilizing a series-resonant FSS for the frequency selective pattern; 
         FIG. 2B  is a plot illustrating the blackbody spectrum  210  corresponding to the emission spectrum of  FIG. 2A ; 
         FIG. 2C  is a plot showing an example of the emission spectrum, the emissivity verses frequency, of an adaptive spectral surface in accordance with an embodiment utilizing a parallel-resonant FSS for the frequency selective pattern; 
         FIG. 2D  is a plot illustrating the blackbody spectrum corresponding to the emission spectrum of  FIG. 2C ; 
         FIG. 3A  is a top view of a possible frequency selective surface; 
         FIG. 3B  is a top view of a possible frequency selective surface; 
         FIG. 3C  is a plot representative of a transmission spectrum of an electromagnetic wave incident on a series-resonant FSS; 
         FIG. 3D  is a plot representative of a transmission spectrum of an electromagnetic wave incident on a parallel-resonant FSS; 
         FIG. 3E  is a plot illustrating the reflective power corresponding to the plot of  FIG. 3C ; 
         FIG. 3F  is a plot illustrating the reflective power corresponding to the plot of  FIG. 3D ; and 
         FIG. 4  is a graph of a permittivity response, in accordance with an embodiment of the present invention. 
     
    
    
     DESCRIPTION 
     In various embodiments, an adaptive spectral surface includes a frequency selective surface (which may be a frequency selective layer) on a dielectric layer. The adaptive spectral surface alters the spectral properties of a surface. It reflects an incident electromagnetic wave, and/or alters an emitted radiation, according to a frequency response. The resonant frequency of the frequency response is based on the geometry of the frequency-selective surface, and the electromagnetic properties of the dielectric layer, such as the permittivity and the permeability. The resonant frequency can be a frequency of maximum reflection or absorption of electromagnetic radiation. The permittivity of the dielectric layer may be modified to change the frequency response of the adaptive spectral surface by changing the resonant frequency of the frequency response. 
       FIG. 1  illustrates an adaptive spectral surface  100 , in accordance with an embodiment of the present invention. The adaptive spectral surface  100  includes an upper layer  105 , a lower layer  120 , and active dielectric layer  115  between the upper and lower layers  105  an  120 . The upper layer  105  is a frequency selective surface that includes a spatially-periodic pattern  110 . The upper layer  105  may be an electromagnetic crystal, a photonic band gap material, a metasurface, or the like. 
     The active dielectric layer  115  includes a dielectric material, such as, for example, a ferroelectric or a ferrite. Additionally, the active dielectric layer  115  has properties such as a permittivity, permeability, and a size (e.g., length, width, and thickness), which can be modified in response to a stimulus, such as heat or electromagnetic field. In various embodiments, the active dielectric layer  115  is comprised of a material that is a broadband absorber, which absorbs incident electromagnetic radiation in the spectrum of interest. 
     The upper layer  105  and the active dielectric layer  115  may be fabricated with conventional printed circuit board techniques, electrochemical etching techniques, or photochemical etching techniques. For example, the active dielectric layer  115  may be a thin dielectric layer, and the spatially-periodic pattern  110  of the upper layer  105  may be created by printing textured metallization onto the active dielectric layer  115 . For example, the active dielectric layer  115  may have a thickness of 100-500 nanometers. 
     The lower layer  120  can include or be, depending on the embodiment, a reflective ground plane, a transmissive medium, a neutral semiconductor substrate, or nonexistent. In some embodiments, the active dielectric layer  115  may be composed of ferroelectric materials such as BATiO 3 , SRTiO 3 , BaSrTi 3 , LiTaO 3 , LiNbO 3 , LaSrMnO 3  or one of several ferrite compositions. The upper layer  105 , the active dielectric layer  115 , and the lower layer  120  may be formed by using conventional semiconductor processing techniques. Moreover, the adaptive spectral surface  100  may be a laminated structure of the upper layer  105 , the active dielectric layer  115 , and the lower layer  120 . 
     In one embodiment, the spatially-periodic pattern  110  includes an arrangement of conductive traces. The shape of the conductive pattern may take many forms. For example, in  FIG. 1 , the conductive portion is substantially shaped like a square. In  FIGS. 3A and 3B , the conductive shape is substantially shaped like a Jerusalem cross. In other embodiments, the spatially periodic pattern may be composed of crosses, linear slots, rectangular patches, strips, spirals, etc. The effects of various geometric shapes in an FSS are well documented in current literature. The spatially periodic pattern  110  functions to establish a frequency response of the adaptive spectral surface  100  in response to an electromagnetic wave incident on the upper layer  105 . 
     The FSS pattern may also be composed of the inverse of any pattern mentioned above; the inverse is defined as being the case where the metal is replaced with empty space and the empty space is replaced with metal. Two major classifications of patterns exist in the state of the art, known as series-resonant and parallel-resonant. The names are derived from analogous resonant electronic circuits. The inverse of a series-resonant FSS pattern is a parallel-resonant FSS pattern and vice versa. 
     Turning to  FIGS. 3A and 3B , a series-resonant FSS pattern  300  is typically composed of patches of patterned metal  305  separated, and electrically isolated, from each other by an insulating material  312 .  FIG. 3A  is an example of a series-resonant FSS pattern with the metal patches  306  in the shape of Jerusalem crosses.  FIG. 3C  is representative of the transmission spectrum  310  of an electromagnetic wave incident on a series-resonant FSS; it features a sharp dip  311  in the transmitted power at the resonant frequency. The resonant frequency is defined by the details of the pattern shape and its spatial period. The reflected power  320 , shown in  FIG. 3E , is related to the transmitted power  310 , shown in  FIG. 3C , by r=1−t, where r is the reflected power and t is the transmitted power. 
       FIG. 3B  is an example of a parallel-resonant FSS pattern  350  that is the inverse pattern of the series-resonant FSS pattern  300  shown in  FIG. 3A . It is composed of an array of Jerusalem-cross shaped holes  355  in a metallic sheet  357 .  FIG. 3D  is representative of the transmission spectrum  330  of an electromagnetic wave incident on a parallel-resonant FSS; it features a sharp peak  331  in the transmitted power at the FSS&#39;s resonant frequency. The reflected power  340 , shown in  FIG. 3F , is related to the transmitted power  330  by r=1−t. 
     Referring to  FIG. 1 , the active dielectric material  115  is a broadband absorber that absorbs incident electromagnetic radiation. The active dielectric material  115  works in conjunction with the patterned FSS layer  110  to modify the surface&#39;s emission spectrum (e.g.  202 , shown in  FIG. 2A ), and subsequently its blackbody radiation emission  215 , shown in  FIG. 2B , and its reflective properties. When the active dielectric layer  115  is laminated with a patterned FSS layer  110  configured as a series-resonant FSS such as in  FIG. 3A , then electromagnetic radiation incident at the resonant frequency corresponding to the transmission dip  311 , shown in  FIG. 3C , is totally reflected. Incident radiation far from the resonant frequency is transmitted through the FSS layer  110  into the active dielectric  115  and is absorbed. 
     When the active dielectric layer  115  is laminated with a patterned FSS layer  110  configured as a parallel-resonant FSS such as in  FIG. 3B , then electromagnetic radiation incident at the resonant frequency corresponding to the frequency of the transmission peak  331 , shown in  FIG. 3D , is transmitted through the FSS layer  110  into the active dielectric  115  and is absorbed. Incident radiation far from the resonant frequency is reflected from the FSS layer  110 . 
     A reflecting groundplane  120  can be laminated to the backside of the dielectric layer  115  in another embodiment. The presence of the backplane does not change the qualitative function of the adaptive spectral surface. However, it can be advantageous because (1) it enhances the resonant character of the spectral surface, (2) it enables making the surface thinner, (3) an voltage can be applied to the groundplane in order to apply an electric field to the active dielectric layer  115  and modify its electrical properties, and (4) it enables the spectral surface to be fabricated in a stand-alone sheet that can be applied to existing structures. 
     The adaptive spectral surface modifies the spectrum of the electromagnetic radiation reflected from the surface. It also modifies the spectrum of blackbody radiation emitted by the surface by modifying the surface&#39;s emissivity with respect to frequency. 
     Shown in  FIG. 2A  is an example of the emission spectrum, i.e. the emissivity vs. frequency  200  of an adaptive spectral surface  100  in accordance with an embodiment utilizing a series-resonant FSS for the frequency selective pattern  110 . The emission spectrum  200  is characteristic of what is known as a selective radiator; a selective radiator is a body for which the emissivity varies with frequency. In contrast, a perfect emitter, i.e. a blackbody, has emissivity=1 everywhere  201 , and an imperfect emitter, i.e. a “gray” body, has a constant emissivity less than 1 at all frequencies. The emission spectrum  200  has a minimum  202  and approaches 1 at frequencies far from  202 . The deviation in the emission spectrum from the constant blackbody emissivity  201  is caused by the resonance of the frequency selective pattern  110 . The arrows indicate that the minimum in the emissivity is variable due to changes in the active dielectric material  115  caused by the application of external stimulus such as an applied electric field, mechanical strain, or a change in temperature. 
       FIG. 2B  illustrates the blackbody spectrum  210  corresponding to the emission spectrum of  FIG. 2A . and compares it to the emission from a perfect emitter  205 . The dip in the blackbody radiation  215  corresponds to the dip in the emissivity  202 . 
     Shown in  FIG. 2C  is an example of the emission spectrum, i.e. the emissivity verses frequency  220  of an adaptive spectral surface  100 , shown in  FIG. 1 , in accordance with an embodiment utilizing a parallel-resonant FSS for the frequency selective pattern  110 , shown in FIG.  1 . The emission spectrum  220  has a maximum  222  and approaches zero at frequencies far from  222 . The deviation in the emission spectrum from the constant blackbody emissivity  221  is caused by the resonance of the frequency selective pattern  110 . The arrows indicate that the maximum in the emissivity is variable due to changes in the active dielectric material  115  caused by the application of external stimulus such as an applied electric field or a change in temperature. 
       FIG. 2D  illustrates the blackbody spectrum  230  corresponding to the emission spectrum  220  of  FIG. 2C . and compares it to the emission from a perfect emitter  231 . The peak in the blackbody radiation  232  corresponds to the peak in the emissivity  222 . 
       FIG. 4  corresponds to particular embodiments where the active dielectric layer  115  consists of the commercially available ferrite materials FAIR-RITE NiZn  44  and NiZn  51 , available from Fair-Rite Products, Corp. Wallkill, N.Y.  FIG. 4  illustrates the permeability of the active dielectric layer  115  ( FIG. 1 ) as a function of temperature, in accordance with embodiments of the present invention. The permeability response  405  is for a dielectric material composed of FAIR-RITE NiZn  44 , and the permeability response  410  is for a dielectric material composed of FAIR-RITE NiZn  51 . Each permeability response  405  and  410  increases with an increase in temperature, reaches a peak at a Curie temperature of the dielectric material, and then decreases with a further increase in temperature. Thus, the permeability of the active dielectric layer  115  changes with a change in the temperature of the active dielectric layer  115 . In turn, the change in permeability causes the resonant frequency of the frequency response of the adaptive spectral surface  100  to shift as indicated by arrows  216  in  FIG. 2 . The material shown is an example of an active dielectric that may be used. Other active dielectric materials are possible. 
     In one embodiment, the resonant frequency  215  ( FIG. 2B ) is selected to be a frequency in the visible spectrum of electromagnetic radiation. In this embodiment, changing the resonant frequency  215  causes the apparent color of the adaptive spectral surface  100  ( FIG. 1 ) to change. 
     In another embodiment, the resonant frequency  215  is selected in the infrared spectrum of electromagnetic radiation. In this embodiment, changing the resonant frequency of the adaptive spectral surface  100  changes an infrared signature of the adaptive spectral surface  100 . Thus, in some embodiments, the surface  100  may be a variable selective emitter, which has an emissivity that changes with frequency. As such, in some embodiments, blackbody/gray-body radiation may be controlled. 
     In still another embodiment, the resonant frequency  215  is selected in the microwave spectrum of electromagnetic radiation. In this embodiment, changing the resonant frequency changes a microwave signature of the adaptive spectral surface  100 . For example, the reflective properties of the adaptive spectral surface  100  can be controlled. 
     In general, changing the resonant frequency changes the electromagnetic signature of the adaptive spectral surface  100 . Although specific frequency ranges are discussed for in the examples above, embodiments are not limited to those frequencies. 
     In some embodiments, the permittivity of the active dielectric layer  115  ( FIG. 1 ) may change in response to an electric field. Thus, in some embodiments, the upper layer  105  ( FIG. 1 ) and the lower layer  120  ( FIG. 1 ) are electrically conductive layers. The electric field may be a voltage applied between the upper layer  105  and the lower layer  120  across the active dielectric layer  115 . For example, the voltage may be supplied by a power source (not shown). The voltage may be in a range of zero to two-hundred and fifty volts. Thus, the permittivity of the active dielectric layer  115  changes with a change in the voltage between the upper layer  105  and the lower layer  120 . In turn, the change in permittivity causes the resonant frequency of the frequency response of the adaptive spectral surface  100  ( FIG. 1 ) to change. 
     In other embodiments, thermal plates may be used to change the temperature of the active dielectric layer to shift the resonant frequency as discussed above. In yet other embodiments, a magnetic field may be generated to shift the resonant frequency of the active dielectric layer. In still other embodiments, the active dielectric layer  115  may be composed of piezoelectric materials whose electrical properties are altered with the application of pressure. 
     The embodiments described herein are illustrative of the present invention. As these embodiments of the present invention are described with reference to illustrations, various modifications or adaptations of the methods and/or specific structures described may become apparent to those skilled in the art. All such modifications, adaptations, or variations that rely upon the teachings of the present invention, and through which these teachings have advanced the art, are considered to be within the spirit and scope of the present invention. Hence, these descriptions and drawings should not be considered in a limiting sense, as it is to be understood that the present invention is not limited to only the embodiments illustrated.