Adaptive spectral surface

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.

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.

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. 1illustrates an adaptive spectral surface100, in accordance with an embodiment of the present invention. The adaptive spectral surface100includes an upper layer105, a lower layer120, and active dielectric layer115between the upper and lower layers105an120. The upper layer105is a frequency selective surface that includes a spatially-periodic pattern110. The upper layer105may be an electromagnetic crystal, a photonic band gap material, a metasurface, or the like.

The active dielectric layer115includes a dielectric material, such as, for example, a ferroelectric or a ferrite. Additionally, the active dielectric layer115has 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 layer115is comprised of a material that is a broadband absorber, which absorbs incident electromagnetic radiation in the spectrum of interest.

The upper layer105and the active dielectric layer115may be fabricated with conventional printed circuit board techniques, electrochemical etching techniques, or photochemical etching techniques. For example, the active dielectric layer115may be a thin dielectric layer, and the spatially-periodic pattern110of the upper layer105may be created by printing textured metallization onto the active dielectric layer115. For example, the active dielectric layer115may have a thickness of 100-500 nanometers.

The lower layer120can 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 layer115may be composed of ferroelectric materials such as BATiO3, SRTiO3, BaSrTi3, LiTaO3, LiNbO3, LaSrMnO3or one of several ferrite compositions. The upper layer105, the active dielectric layer115, and the lower layer120may be formed by using conventional semiconductor processing techniques. Moreover, the adaptive spectral surface100may be a laminated structure of the upper layer105, the active dielectric layer115, and the lower layer120.

In one embodiment, the spatially-periodic pattern110includes an arrangement of conductive traces. The shape of the conductive pattern may take many forms. For example, inFIG. 1, the conductive portion is substantially shaped like a square. InFIGS. 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 pattern110functions to establish a frequency response of the adaptive spectral surface100in response to an electromagnetic wave incident on the upper layer105.

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 toFIGS. 3A and 3B, a series-resonant FSS pattern300is typically composed of patches of patterned metal305separated, and electrically isolated, from each other by an insulating material312.FIG. 3Ais an example of a series-resonant FSS pattern with the metal patches306in the shape of Jerusalem crosses.FIG. 3Cis representative of the transmission spectrum310of an electromagnetic wave incident on a series-resonant FSS; it features a sharp dip311in 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 power320, shown inFIG. 3E, is related to the transmitted power310, shown inFIG. 3C, by r=1−t, where r is the reflected power and t is the transmitted power.

FIG. 3Bis an example of a parallel-resonant FSS pattern350that is the inverse pattern of the series-resonant FSS pattern300shown inFIG. 3A. It is composed of an array of Jerusalem-cross shaped holes355in a metallic sheet357.FIG. 3Dis representative of the transmission spectrum330of an electromagnetic wave incident on a parallel-resonant FSS; it features a sharp peak331in the transmitted power at the FSS's resonant frequency. The reflected power340, shown inFIG. 3F, is related to the transmitted power330by r=1−t.

Referring toFIG. 1, the active dielectric material115is a broadband absorber that absorbs incident electromagnetic radiation. The active dielectric material115works in conjunction with the patterned FSS layer110to modify the surface's emission spectrum (e.g.202, shown inFIG. 2A), and subsequently its blackbody radiation emission215, shown inFIG. 2B, and its reflective properties. When the active dielectric layer115is laminated with a patterned FSS layer110configured as a series-resonant FSS such as inFIG. 3A, then electromagnetic radiation incident at the resonant frequency corresponding to the transmission dip311, shown inFIG. 3C, is totally reflected. Incident radiation far from the resonant frequency is transmitted through the FSS layer110into the active dielectric115and is absorbed.

When the active dielectric layer115is laminated with a patterned FSS layer110configured as a parallel-resonant FSS such as inFIG. 3B, then electromagnetic radiation incident at the resonant frequency corresponding to the frequency of the transmission peak331, shown inFIG. 3D, is transmitted through the FSS layer110into the active dielectric115and is absorbed. Incident radiation far from the resonant frequency is reflected from the FSS layer110.

A reflecting groundplane120can be laminated to the backside of the dielectric layer115in 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 layer115and 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's emissivity with respect to frequency.

Shown inFIG. 2Ais an example of the emission spectrum, i.e. the emissivity vs. frequency200of an adaptive spectral surface100in accordance with an embodiment utilizing a series-resonant FSS for the frequency selective pattern110. The emission spectrum200is 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 everywhere201, and an imperfect emitter, i.e. a “gray” body, has a constant emissivity less than 1 at all frequencies. The emission spectrum200has a minimum202and approaches 1 at frequencies far from202. The deviation in the emission spectrum from the constant blackbody emissivity201is caused by the resonance of the frequency selective pattern110. The arrows indicate that the minimum in the emissivity is variable due to changes in the active dielectric material115caused by the application of external stimulus such as an applied electric field, mechanical strain, or a change in temperature.

FIG. 2Billustrates the blackbody spectrum210corresponding to the emission spectrum ofFIG. 2A. and compares it to the emission from a perfect emitter205. The dip in the blackbody radiation215corresponds to the dip in the emissivity202.

Shown inFIG. 2Cis an example of the emission spectrum, i.e. the emissivity verses frequency220of an adaptive spectral surface100, shown inFIG. 1, in accordance with an embodiment utilizing a parallel-resonant FSS for the frequency selective pattern110, shown in FIG.1. The emission spectrum220has a maximum222and approaches zero at frequencies far from222. The deviation in the emission spectrum from the constant blackbody emissivity221is caused by the resonance of the frequency selective pattern110. The arrows indicate that the maximum in the emissivity is variable due to changes in the active dielectric material115caused by the application of external stimulus such as an applied electric field or a change in temperature.

FIG. 2Dillustrates the blackbody spectrum230corresponding to the emission spectrum220ofFIG. 2C. and compares it to the emission from a perfect emitter231. The peak in the blackbody radiation232corresponds to the peak in the emissivity222.

FIG. 4corresponds to particular embodiments where the active dielectric layer115consists of the commercially available ferrite materials FAIR-RITE NiZn44and NiZn51, available from Fair-Rite Products, Corp. Wallkill, N.Y.FIG. 4illustrates the permeability of the active dielectric layer115(FIG. 1) as a function of temperature, in accordance with embodiments of the present invention. The permeability response405is for a dielectric material composed of FAIR-RITE NiZn44, and the permeability response410is for a dielectric material composed of FAIR-RITE NiZn51. Each permeability response405and410increases 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 layer115changes with a change in the temperature of the active dielectric layer115. In turn, the change in permeability causes the resonant frequency of the frequency response of the adaptive spectral surface100to shift as indicated by arrows216inFIG. 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 frequency215(FIG. 2B) is selected to be a frequency in the visible spectrum of electromagnetic radiation. In this embodiment, changing the resonant frequency215causes the apparent color of the adaptive spectral surface100(FIG. 1) to change.

In another embodiment, the resonant frequency215is selected in the infrared spectrum of electromagnetic radiation. In this embodiment, changing the resonant frequency of the adaptive spectral surface100changes an infrared signature of the adaptive spectral surface100. Thus, in some embodiments, the surface100may 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 frequency215is selected in the microwave spectrum of electromagnetic radiation. In this embodiment, changing the resonant frequency changes a microwave signature of the adaptive spectral surface100. For example, the reflective properties of the adaptive spectral surface100can be controlled.

In general, changing the resonant frequency changes the electromagnetic signature of the adaptive spectral surface100. 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 layer115(FIG. 1) may change in response to an electric field. Thus, in some embodiments, the upper layer105(FIG. 1) and the lower layer120(FIG. 1) are electrically conductive layers. The electric field may be a voltage applied between the upper layer105and the lower layer120across the active dielectric layer115. 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 layer115changes with a change in the voltage between the upper layer105and the lower layer120. In turn, the change in permittivity causes the resonant frequency of the frequency response of the adaptive spectral surface100(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 layer115may be composed of piezoelectric materials whose electrical properties are altered with the application of pressure.