FOR CLOAKING THREE-DIMENSIONAL OBJECTS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional

Application No. 61 /430,792, filed on January 7, 201 1 . The entire disclosure of the above application is incorporated herein by reference.

FIELD

[0002] The present disclosure relates to a coating that absorbs electromagnetic radiation, particularly visible light and infrared radiation, for cloaking three-dimensional objects to render them undetectable or invisible.

BACKGROUND

[0003] This section provides background information related to the present disclosure which is not necessarily prior art.

[0004] Invisibility cloaking has attracted considerable research interest during the past few years due to the recent development of transformation optics and metamaterials. Metamaterials are artificial materials engineered to have properties from their structure, rather than composition of the material itself, by using small inhomogeneities to create effective macroscopic behavior. Many typical metamaterials consist of periodic structures of a guest material embedded in a host material. The optimization of component materials and geometries can yield metamaterials with unique optical properties which can allow such metamaterials to control light in unconventional ways with potential applications in photonic integration. For example, metamaterials can guide light around an object, rather than reflect or refract the light, or can be designed to be perfect reflectors. Thus, light waves falling upon an object with exposed metamaterials make it appear that the object is not present.

[0005] Recently metamaterials have been suggested for use for cloaking three-dimensional objects. For example, a ground plane cloak using a simplified isotropic metamaterials carpet has been suggested to render a three dimensional (3-D) object as a two dimensional (2-D) perfect reflecting surface. So far, however; only cloaking of wavelength-size objects has been realized at microwave and near-infrared frequencies with metamaterials. Although the metamaterials used in ground plane cloak are isotropic and do not involve singular values, such a metamaterial still requires the cloaking carpet to have inhomogeneous spatial distribution of effective index that varies with the shape of the object. Though cloaking of small bumps on a ground plane in 2D and 3D has been demonstrated in near infrared (NIR) range by structuring the dielectric host medium to produce specific index distribution, cloaking of arbitrary shaped 3-D objects in the visible band is almost impossible when using such an approach due to the extreme challenge in making inhomogeneous nanostructured metamaterials. Moreover, due to the tradeoff between the complexity of metamaterials and the cloaking area, most of the previous cloaking demonstrations were limited to an object size of only several wavelengths. Another method was introduced to use a specially designed tapered waveguide to emulate the anisotropic and inhomogeneous metamaterials, and broadband cloaking of object 100 times larger than the wavelength was demonstrated. However, such a strategy is confined into a 2-D waveguide and not extendable to 3-D objects. Therefore cloaking of large area 3D objects has not been possible, seriously limiting progress for a large number of practical applications at a wide range of electromagnetic spectrum radiation.

[0006] Owing to the extreme requirements for the metamaterials to be anisotropic and inhomogeneous, it is very challenging to extend this approach to higher frequencies and to achieve broadband cloaking. Further, the size of the object that can be cloaked based on such technology is limited due to the complexity of metamaterial realization. Thus, the cloaking of three-dimensional (3-D) objects at visible frequency demands challenging inhomogeneous 3-D nanostructured metamaterials, which is still unattainable and impractical. There is a need for cloaking of a three-dimensional (3-D) object having a wide variety of sizes with a feasible cloaking technique that operates through a wide spectrum of electromagnetic radiation and also in certain variations can provide acoustic cloaking, as well. SUMMARY

[0007] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0008] In various aspects, the present disclosure provides a radiation absorbing surface for cloaking a three-dimensional object. In certain variations, the radiation absorbing surface absorbs both electromagnetic radiation, as well as acoustic waves. In certain aspects, a radiation absorbing surface has a coating that comprises a plurality of axially shaped microparticles disposed on a substrate. Each of the axially shaped microparticles has an electrical conductivity of greater than or equal to about 50 S/cm. Furthermore, each of the axially shaped microparticles defines a major longitudinal axis that intersects with a plane defined by the substrate at an angle of greater than or equal to about 30° and less than or equal to about 150°, so that the plurality of axially shaped microparticles are substantially aligned to have a single orientation with respect to one another. The radiation absorbing surface has a refractive index (n _eff ) represented by the equation n _eff = Re(n _eff ) + lm(n _eff ) so that Re(n _eff ) is greater than or equal to about 0.9 to less than or equal to about 1 .1 and lm(n _eff ) is greater than or equal to about 0.0001 to less than or equal to about 0.1 , so that n _eff = (> about 0.9 to < about 1 .1 ) + i ^* (> about 0.0001 to < about 0.1 ).

[0009] In certain aspects, the inventive radiation absorbing cloaking surface is a perfect or near-perfect absorptive coating for certain wavelengths of radiation. In certain variations, the coating is a perfect absorption coating for at least one wavelength of electromagnetic radiation greater than or equal to about 300 nanometers to less than or equal to about 1 meter. In other aspects, the coating may be a perfect absorption coating for multiple different preselected wavelengths of electromagnetic radiation. Thus, the radiation absorbing surface can effectively absorb a wide spectrum of different electromagnetic waves and therefore is capable of broad-band radiation absorption. In certain aspects, such a material or coating comprising the plurality of axially shaped microparticles forms a low-density "forest" on a surface to be cloaked or concealed. In certain variations, at least one microparticle of the plurality of axially shaped microparticles comprises a single-walled carbon nanotube (SWNT) or a multi- walled carbon nanotube (MWNT). [0010] In other aspects, the present disclosure provides a method of cloaking a three-dimensional object. In one variation, the method comprises providing a surface shielding at least a portion of the three-dimensional object from a direction of detection. The surface is capable of absorbing greater than or equal to about 97% of at least one wavelength of electromagnetic radiation that is emitted towards the object. The at least one wavelength can range from greater than or equal to about 300 nanometers to less than or equal to about 1 meter. The surface comprises a radiation absorbing surface coating comprising a plurality of axially shaped microparticles. Each of the axially shaped microparticles of the plurality has an electrical conductivity of greater than or equal to about 50 S/cm and defines a major longitudinal axis that intersects with a plane defined by the surface at an angle of greater than or equal to about 70° and less than or equal to about 1 10°. Further, each of the axially shaped microparticles (in the plurality of axially shaped microparticles) is substantially aligned to have a single orientation with respect to one another.

[0011] In certain variations, the surface is capable of absorbing greater than or equal to about 97% of the at least one wavelength of electromagnetic radiation emitted from the object towards the direction of detection. The wavelength optionally ranges from greater than or equal to about 300 nanometers to less than or equal to about 1 meter. In additional variations, a background may be provided for the object that has a surface that also is capable of absorbing greater than or equal to about 97% of the at least one wavelength of electromagnetic radiation ranging from greater than or equal to about 300 nanometers to less than or equal to about 1 meter. In yet other variations, the object can be disposed in a non-reflective or an absorptive environment to the at least one wavelength of electromagnetic radiation ranging from greater than or equal to about 300 nanometers to less than or equal to about 1 meter. Further, in certain variations, the surface is also capable of absorbing at least one acoustic wave.

[0012] In other alternative variations, a method of cloaking a three- dimensional object comprises providing a surface shielding at least a portion of the three-dimensional object from a direction of detection. The surface is capable of absorbing greater than or equal to about 97% of at least one wavelength of electromagnetic radiation emitted towards the object. In certain aspects, the at least one wavelength ranges from greater than or equal to about 300 nanometers to less than or equal to about 1 meter. The surface comprises a radiation absorbing surface coating having a thickness of greater than or equal to about 50 μιτι. The radiation absorbing surface coating comprises a plurality of carbon nanotubes disposed on a substrate present at less than or equal to about 1 volume % along the surface of the substrate. Thus, the coating provides the surface with a refractive index (n _eff ) represented by the equation n _eff = Re(n _eff ) + lm(n _eff ) so that Re(n _eff ) is greater than or equal to about 0.9 to less than or equal to about 1 .1 ) and lm(n _eff ) is greater than or equal to about 0.0001 to less than or equal to about 0.1 , so that n _eff = (> about 0.9 to < about 1 .1 ) + i ^* (> about 0.0001 to < about 0.1 ).

[0013] In yet other variations, a radiation absorbing surface for cloaking a three-dimensional object is provided that comprises a plurality of microparticles and a binder material disposed on a detectable surface region of the three-dimensional object. Each of microparticles has a plurality of carbon nanotubes disposed on a surface of the microparticle. The plurality of carbon nanotubes is substantially aligned to have a single orientation with respect to one another along the microparticle surface. Thus, the detectable surface region is capable of absorbing greater than or equal to about 97% of at least one wavelength of electromagnetic radiation ranging from greater than or equal to about 300 nanometers to less than or equal to about 1 meter emitted towards the three-dimensional object. In certain aspects, the coating comprising a plurality of carbon nanotubes has a refractive index (n _eff ) represented by the equation n _eff = Re(n _eff ) + lm(n _eff ), so that Re(n _eff ) is greater than or equal to about 0.9 to less than or equal to about 1 .1 and lm(n _eff ) is greater than or equal to about 0.0001 to less than or equal to about 0.1 , so that n _eff = (> about 0.9 to < about 1 .1 ) + i ^* (> about 0.0001 to < about 0.1

[0014] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. DRAWINGS

[0015] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0016] Figures 1 A-B schematically show an exemplary variation of a radiation absorptive material that can be used to cloak a three-dimensional object in accordance with the principles of the present disclosure;

[0017] Figure 2 shows transmission of electromagnetic radiation in the visible range having a wavelength of 633 nm through different radiation absorbing surface layers comprising a plurality of carbon nanotubes according to the principles of the present disclosure, where the surface layers have distinct thicknesses (z) for purposes of comparison;

[0018] Figures 3A-H show universal ground plane cloaking with a perfect radiation absorptive coating or carpet prepared in accordance with certain variations of the present disclosure. Figure 3A shows a schematic view of an object placed on top of a perfect absorbing ground or background that can be observed at far field. Figure 3A shows that when an object is placed on the perfect absorbing ground and then covered by a perfect absorption radiation absorptive coating or carpet prepared in accordance with certain variations of the inventive technology, the object along with the carpet cannot be seen at far field due to total absorption of incident light. Figure 3C shows a planar perfect absorption sheet or background without any object. Figure 3D shows a profile view of a simulated electric field intensity distribution reflected and scattered by the perfect electric conductor (PEC) object when it is illuminated with a Gaussian beam with wavelength of 632.8 nm, corresponding to the scenario in Figure 3A; Figure 3E shows a field distribution profile view of the shielded object like in Figure 3B, where the perfect absorption radiation absorptive coating covers the PEC object and has a refractive index (of homogeneous coating/carpet) of 1 +1 .07i. Figure 3F shows a field distribution for a planar perfect absorption sheet background similar to the scenario in Figure 3C, where only the perfect absorption background is present with no object. Figure 3G shows far field scattered field amplitudes for Figures 3D-F collected from the dashed line shown in Figure 3D. Figure 3H is a log scale plot of Figure 3G; [0019] Figures 4A-D show a radiation absorptive coating or carpet that comprises a plurality of carbon nanotubes (CNT) in a "forest" configuration according to certain variations of the present disclosure. Figure 4A shows a scanning electron microscope (SEM) image of a CNT "forest" carpet/coating with a low density of CNTs and having a rough surface (top view). Figure 4B shows a cross-sectional view of such a CNT "forest" coating/carpet with vertically aligned and long CNTs. Figure 4C is a calculated reflection at an air- metamaterial interface for normal incident as a function of the complex refractive index for such a radiation absorptive coating (CNT forest). Figure 4D is an effective index of a CNT "forest" with a volume ratio of 1 %, where the effective index range is marked with a cross symbol in Figure 4C;

[0020] Figures 5A-B show angle and wavelength dependent reflection of a radiation absorptive coating or carpet that comprises a plurality of carbon nanotubes (CNT) in a "forest" configuration according to certain variations of the present disclosure. Figure 5A has a calculated angle dependent reflection at air-CNT carpet interface for s and p polarization, where the wavelength is 632.8 nm and the reflection of air-silicon interface is also shown for comparison. Figure 5B shows wavelength dependent reflection at air- CNT carpet interface;

[0021] Figures 6A-F show an experimental demonstration of a perfect radiation absorptive coating or carpet according to certain variations of the present disclosure. Figure 6A is a scanning electron microscopy (SEM) image of a 65x22.5 μιτι "tank" pattern fabricated by focused ion beam (FIB) milling. Figure 6B is an SEM image of the same tank pattern sample surface covered by a 60 μιτι thick radiation absorption coating comprising a plurality of CNT particles. Figure 6C is prepared like the pattern in Figure 6B, but further includes a rectangular mark around the "tank," where FIB removes the CNT coating layer in a rectangular pattern. The corresponding optical reflection images taken under broadband visible illumination of the as-fabricated "tank" object are shown in Figures 6D-F; Figure 6D corresponds to the pattern of Figure 6A, the CNT coating/carpet covered "tank" sample of Figure 6B is shown in Figure 6E, and the rectangular mark surrounding the "tank" in Figure 6C is shown in Figure 6F; and [0022] Figures 7A-7C show images of a large area cloaking under a free-standing radiation absorptive coating or carpet comprising a plurality of CNT particles. Figure 7A is a toy airplane of 2.2x 1 .5 cm in size placed in front of the cloaking carpet, which is clearly visible. Figure 7B shows the airplane body entering the space covered by the free-standing radiation absorptive CNT forest carpet coating, where it is shown to gradually disappear and thus, is partially cloaked. In Figure 7C, the entire plane becomes invisible and cloaked when the object is placed completely under the space created beneath the free-standing radiation absorptive CNT forest carpet coating and thus is covered by the coating.

[0023] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

[0024] Example embodiments will now be described more fully with reference to the accompanying drawings.

[0025] Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

[0026] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a," "an," and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "including," and "having," are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0027] When an element or layer is referred to as being "on," "engaged to," "connected to," or "coupled to" another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly engaged to," "directly connected to," or "directly coupled to" another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., "between" versus "directly between," "adjacent" versus "directly adjacent," etc.). As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[0028] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as "first," "second," and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

[0029] Spatially relative terms, such as "inner," "outer," "beneath," "below," "lower," "above," "upper," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0030] As referred to herein, ranges are, unless specified otherwise, inclusive of endpoints and include disclosure of all distinct values and further divided ranges within the entire range. Thus, for example, a range of "from A to B" or "from about A to about B" is inclusive of A and of B. Disclosure of values and ranges of values for specific parameters (such as temperatures, molecular weights, weight percentages, etc.) are not exclusive of other values and ranges of values useful herein. It is envisioned that two or more specific exemplified values for a given parameter may define endpoints for a range of values that may be claimed for the parameter. For example, if Parameter X is exemplified herein to have value A and also exemplified to have value Z, it is envisioned that Parameter X may have a range of values from about A to about Z. Similarly, it is envisioned that disclosure of two or more ranges of values for a parameter (whether such ranges are nested, overlapping or distinct) subsume all possible combination of ranges for the value that might be claimed using endpoints of the disclosed ranges. For example, if Parameter X is exemplified herein to have values in the range of 1 -10, or 2-9, or 3-8, it is also envisioned that Parameter X may have other ranges of values including 1 -9, 1 -8, 1-3, 1 -2, 2-10, 2-8, 2-3, 3-10, and 3-9.

[0031] As discussed above, conventional techniques for cloaking three-dimensional objects have focused on metamaterials and focal optics to predominantly rely upon the concept of perfect reflection of incident electromagnetic radiation. Owing to the extreme requirements for conventional metamaterials to be anisotropic and inhomogeneous, such an approach has posed significant challenges for higher frequencies and to achieve broadband cloaking. The present technology however, does not require such anisotropic and inhomogeneous metamaterials, but instead employs an inventive cloaking surface capable of absorbing radiation that is versatile for a wide range of applications. Such a radiation absorbing surface can optionally be formed as a coating or a carpet on a substrate, as a layer, a material, and the like. In various aspects, the inventive radiation absorbing surface comprises a plurality of electrically conductive, axial-geometry particles. In certain variations, the plurality of electrically conductive, axial-geometry particles are microparticles or nanoparticles. The radiation absorbing surface is capable of cloaking large areas and complex three-dimensional shapes for a variety of different electromagnetic waves. In certain variations, the inventive radiation absorbing cloaking surface is a perfect or near-perfect absorptive coating for certain wavelengths of radiation. In other variations, the radiation absorbing surface effectively absorbs a wide spectrum of different electromagnetic waves and therefore is capable of broad- band radiation absorption. Such embodiments enable cloaking of an object from broad-band radiation. In yet other aspects, the present disclosure provides methods of cloaking three-dimensional objects from at least one viewing direction, so that in certain variations the three-dimensional object is undetectable and appears to be invisible.

[0032] In various aspects of the present disclosure, the radiation absorbing surface comprises a plurality of electrically conductive axial geometry particles disposed on a substrate. In various embodiments, the axial-geometry particles are anisotropic and have a cylindrical, rod, or fibrous shape with an evident elongated longitudinal axis, which is longer than the other dimensions (e.g., diameter or width), thus having an axial anisotropic geometry.

[0033] Generally, an aspect ratio (AR) for cylindrical shapes (e.g., a rod, tube, or fiber) is defined as AR = L/D where L is the length of the longest axis (here the major longitudinal axis) and D is the diameter of the cylinder or fiber. Suitable axial geometry particles for use in the present technology generally have high aspect ratios, for example, ranging from at least about 100 to in excess of 1 ,000, for example. In yet other aspects, such axial geometry particles may have an aspect ratio of 5,000 or more and in certain variations 10,000 or more. In certain embodiments of the present disclosure, the axial- geometry particles include tubes, fibers, wires, whiskers, filaments, and the like.

[0034] Further, in certain preferred aspects, the plurality of electrically conductive axial geometry particles comprises microparticles or nanoparticles. A "microparticle" as used herein encompasses "nanoparticles," as discussed below. In certain variations of the present teachings, a microparticle component has an axial geometry with an evident longitudinal axis, as defined above, and further has at least one spatial dimension that is less than about 1 ,000 μιτι (i.e., 1 mm), optionally less than or equal to about 100 μιτι (i.e., 100,000 nm). The term "micro-sized" or "micrometer-sized" as used herein is generally understood by those of skill in the art to mean less than about 500 μιτι (i.e., 0.5 mm). As used herein, a microparticle component has at least one spatial dimension that is less than about 100 μιτι (i.e., 100,000 nm), optionally less than about 50 μιτι (i.e., 50,000 nm), optionally less than about 10 μιτι (i.e., 10,000 nm), and in certain aspects less than or equal to about 5 μιτι (i.e., 5,000 nm). In certain variations, a microparticle component has at least one spatial dimension that is less than or equal to about 1 ,000 μιτι, optionally less than or equal to about 100 μιτι, optionally less than or equal to about 50 μιτι, and in certain embodiments, less than or equal to 10 μιτι.

[0035] In certain preferred aspects, the plurality of electrically conductive axial geometry particles comprises nanoparticles. Particles that are "nano-sized" or "nanometer-sized" as used herein are generally understood by those of skill in the art to have at least one spatial dimension that is less than about 50 μιτι (i.e., 50,000 nm), optionally less than about 10 μιτι (i.e., 10,000 nm), optionally less than about 2 μιη (i.e., less than about 2,000 nm), optionally less than or equal to about 1 μιτι (i.e., less than about 1 ,000 nm), optionally less than about 0.5 μιτι (i.e., 500 nm), and in certain aspects, less than about 200 nm. Accordingly, a nanoparticle component has at least one spatial dimension that is greater than about 1 nm and less than about 50,000 nm (50 μιτι). In certain variations, a nanoparticle may have at least one spatial dimension of about 5 nm to about 5,000 nm. In certain variations, at least one spatial dimension of the nanoparticle component is about 20 nm to about 2,000 nm. In still other variations, nanoparticle components have at least one spatial dimension of about 50 nm to about 500 nm. Such nanoparticle components are intended to encompass components having a micro-scale, so long as at least one dimension of the particle is less than about 50 μιτι. It should be noted that so long as at least one dimension of the nanoparticle falls within the above-described nano- sized scale (for example, diameter), one or more other axes may well exceed the nano-size (for example, length and/or width). As used herein, unless otherwise indicated, the terms micro-component and nano-component, or microparticle and nanoparticle, are used interchangeably.

[0036] In various aspects, an axial geometry particle in accordance with the present teachings is electrically conductive. Suitable axially-shaped particles optionally have an electrical conductivity of greater than or equal to about 50 S/cm, optionally greater than or equal to about 75 S/cm, optionally greater than or equal to about 100 S/cm, optionally greater than or equal to about 1 ,000 S/cm, optionally greater than or equal to about 95 S/cm, optionally greater than or equal to about 10,000 S/cm, optionally greater than or equal to about 105 S/cm, and in certain variations, optionally greater than or equal to about 100,000 S/cm. In certain preferred aspects, a radiation absorbing surface that comprises a plurality of axially-shaped particles has an electrical conductivity of greater than or equal to about 100 S/cm.

[0037] In certain preferred aspects, the axially-shaped particles of the plurality of axially-shaped microparticles have substantially the same orientation relative to a substrate on which they are disposed. For example, in certain aspects, each axially-shaped microparticle of the plurality respectively defines a major longitudinal axis that intersects with a plane defined by a substrate and has substantially the same vertical orientation relative to a horizontal plane formed by the substrate. In certain preferred aspects, the axially-shaped particles of the plurality have substantially the same vertical orientation relative to a horizontal plane formed by the substrate.

[0038] Figure 1 A depicts an exemplary schematic of a radiation absorbing surface 20 according to the present teachings. The radiation absorbing surface 20 includes a substrate 22 and a plurality of axially-shaped microparticles 24 that form a coating thereon. An average spacing between the microparticles 24 is designated "d" and an average diameter of the microparticles 24 is designated "e." The radiation absorbing surface 20 receives one or more waves of electromagnetic radiation 26 from one or more directions (for simplicity, shown here as a single wave). The wave of electromagnetic radiation 26 has a wavelength designated "c." In preferred aspects, the radiation absorbing surface 20 has a low density of axially-shaped microparticles 24 disposed on the substrate 22. [0039] In certain variations, each axially-shaped microparticle 24 of the plurality respectively defines a major longitudinal axis ("a") that intersects with a horizontal plane (corresponding to the x-y axes) defined by substrate 22 at an angle designated "b." In certain preferred embodiments, the plurality of axially-shaped microparticles 24 is substantially vertically oriented with respect to the horizontal plane formed by the substrate 22. However, in certain variations, any angle b may be employed if the volume ratio is kept sufficiently low. In certain embodiments, the major longitudinal axis "a" defines an average angle "b" for the plurality of axially-shaped microparticles 24 that ranges from greater than or equal to about 30° to less than or equal to about 150°; optionally greater than or equal to about 35° to less than or equal to about 145°; optionally greater than or equal to about 40° to less than or equal to about 140°; optionally greater than or equal to about 45° to less than or equal to about 135°; optionally greater than or equal to about 50° to less than or equal to about 130°; optionally greater than or equal to about 55° to less than or equal to about 125°; optionally greater than or equal to about 60° to less than or equal to about 120°; optionally greater than or equal to about 65° to less than or equal to about 1 15°; optionally greater than or equal to about 70° to less than or equal to about 1 10°; optionally greater than or equal to about 75° to less than or equal to about 105°; optionally greater than or equal to about 80° to less than or equal to about 100°; optionally greater than or equal to about 85° to less than or equal to about 95°; optionally greater than or equal to about 86° to less than or equal to about 94°; optionally greater than or equal to about 87° to less than or equal to about 93°; optionally greater than or equal to about 88° to less than or equal to about 92°; optionally greater than or equal to about 89° to less than or equal to about 91 °; and in certain preferred aspects, angle b is equal to about 90°.

[0040] As shown in Figure 1 A, a first axially-shaped microparticle 30 has a first major longitudinal axis designated "a- and a first diameter "e-i . " A second axially-shaped microparticle 32 likewise has a second major longitudinal axis designated "a ₂ " and second diameter "e ₂ ," while a third axially-shaped microparticle 34 has a third major longitudinal axis designated "a ₃ " and a diameter of "e ₃ ." As can be seen from the schematic in Figure 1 B, the first major longitudinal axis a- _\ (of first axially-shaped microparticle 30 of Figure 1 A) defines a first angle bi with respect to the horizontal plane of substrate 22. Further, the second major longitudinal axis a ₂ (of second axially-shaped microparticle 32 of Figure 1 A) defines a second angle b ₂ with respect to the horizontal plane of substrate 22. In Figure 1 B, first and second major longitudinal axes a^ and a ₂ have slightly differing orientation to one another; therefore the first angle bi and second angle b ₂ define different angles with respect to substrate 22. More specifically, bi is approximately 90°, as where b ₂ is approximately 120° from horizontal.

[0041] Thus, first and second major longitudinal axes a- _\ and a ₂ are considered to be "substantially aligned" with one another along a predetermined orientation when the angle formed between them (shown as offset angle Θ in the inset of Figure 1 B) is less than or equal to about 15°, optionally less than or equal to about 10°, optionally less than or equal to about 9°; optionally less than or equal to about 8°; optionally less than or equal to about 7°; optionally less than or equal to about 6°. In certain preferred embodiments, offset angle Θ formed between first and second major longitudinal axes a- _\ and a ₂ is less than or equal to about 5°; optionally less than or equal to about 4°; optionally less than or equal to about 3°; optionally less than or equal to about 2°; and optionally less than or equal to about 1 °. As can be appreciated by those of skill in the art, a plurality of axially-shaped microparticles 24 can be considered to be "substantially aligned" in a predetermined single orientation when an average offset angle Θ is small and less than the amounts specified above, although individual microparticles may have major longitudinal axes that intersect with a horizontal plane of the substrate at a greater angle "b" and therefore have a greater angle of deviation from the longitudinal axes of the other microparticles than those specified.

[0042] Selection of the electrically conductive axial-geometry particles depends in part on the cloaking application (the wavelength of electromagnetic radiation 26 to which the object to be hidden is exposed). For example, electrically conductive axial-geometry particles 24 that can form the cloaking radiation absorbing surface 20 include various kinds of suitable conductive materials tailored to specific frequencies of electromagnetic radiation. For visible and infrared electromagnetic radiation, for example where visible light has wavelengths ranging from about 390 to about 750 nm and infrared radiation (IR) (including near infrared (NIR) ranging from about 0.75 to about 1 .4 μιη; short wave infrared (SWIR) ranging from about 1 .4 to about 3 μιτι; mid wave infrared (MWIR) ranging from about 3 to about 8 μιτι; long wave infrared (LWIR) ranging from about 8 to about 15 μιτι; and far infrared (FIR) ranging from about 15 μιτι to 1 mm)), the following materials are particularly suitable for axially-shaped conductive particles 24. First, the axially-shaped conductive particles 24 optionally comprise a metal material, such as silver (Ag), gold (Au), aluminum (Al), and the like, or combinations thereof, or a non-metallic material having metallic conductivity levels, like graphite or other similar materials known in the art; or semiconductor materials, such as those comprising silicon (Si), germanium (Ge), and the like, or combinations thereof; polymers at infrared, such as cellulose and cellulose derivatives, synthetic rubber, polytetrafluoroethylene, other polymers known or to be developed in the art, or combinations thereof.

[0043] Other suitable electrically conductive axial-geometry particles 24 can be selected to form the cloaking radiation absorbing surface 20 for specific frequencies of electromagnetic radiation at terahertz (THz) and microwave spectra. Terahertz waves have wavelengths of about 0.1 mm (or 100 μιτι) to about 1 mm (or 1 ,000 μιτι) overlapping with the far infrared (FIR) range discussed above and microwave wavelengths range from about 1 mm to about 1 m. Thus, electrically conductive axial-geometry particles 24 suitable for cloaking for specific frequencies of terahertz (THz) and microwave radiation, include metals, such as copper (Cu), aluminum (Al), and the like, and a non- metallic material having metallic conductivity levels, like graphite; or an epoxide resin, foam, carbon fiber, and the like.

[0044] In certain preferred aspects, the axial geometry particles comprise graphite in the form of a graphene material. "Graphene" generally refers to a single layer of carbon in a graphite structure, where the carbon is hexagonally arranged to form a planar condensed ring system. The stacking of graphite layers can be either hexagonal or rhombohedral, although graphite predominantly has hexagonal stacking. Thus, as used herein, the term "graphene" comprises both single layers of elemental bonded carbon having graphite structure (including impurities), as well as graphite where carbon is bonded in three-dimensions with multiple layers. Suitable axial geometry microparticles that comprises graphene include carbon whiskers (cylindrical filaments where graphene layers are arranged in scroll-like manner with no three-dimensional stacking order) and carbon nanotubes (tubes or cylinders formed of one or more graphene layers), which exhibit excellent electrical conductivity.

[0045] Certain preferred axially-shaped microparticles that comprise graphene are single-walled carbon nanotubes (SWNT) or multi-walled carbon nanotubes (MWNT). Carbon nanotubes have high electrical conductivity, for example, preferred carbon nanotubes may have a bulk electrical conductivity of greater than or equal to about 100 S/cm, while having a relatively small diameter ("e" in Figure 1 A) of less than about 1 nm to 50 nm. Carbon nanotubes may be a single-walled carbon nanotube species (SWNT) comprising one graphene sheet or a multi-walled carbon nanotube (MWNT) species comprising multiple layers of graphene sheets concentrically arranged or nested within one another. A single-walled nanotube (SWNT) resembles a flat sheet that has been rolled up into a seamless cylinder. The wall thickness of the SWNT can vary, but an exemplary range of wall thicknesses is from about 5 nm to about 250 nm. A multi-walled nanotube (MWNT) resembles stacked graphite/graphene sheets that have been rolled up into seamless cylinders and may resemble multiple SWNTs that have been nested concentrically inside one another.

[0046] Thus, in certain embodiments, a conductive axially-shaped microparticle comprising carbon can be selected from the group consisting of: nanotubes, nanofibers, whiskers, rods, filaments, caged structures, and combinations thereof. It should be noted that other suitable conductive axially- shaped microparticles do not necessarily include carbon and can be formed of conductive metals. Thus, in certain embodiments, the plurality of axially-shaped microparticles comprises a material selected from graphite, graphene, gold (Au), copper (Cu), silver (Ag), nickel (Ni), aluminum (Al), silicon (Si), germanium (Ge), epoxide resin, polymers, and co-polymers thereof, foam, and the like that exhibit metallic conductivities, and combinations thereof. It should be noted that other materials exhibiting such metallic conductivities known or to be discovered by those of skill in the art are contemplated for use as axially-shaped conductive particles of the radiation absorptive coatings in certain variations of the present disclosure.

[0047] Such materials have intrinsic absorption properties, and their refractive index can be expressed as N=n+\k, where n is the real part and k is the imaginary part. In various aspects, the refractive index for a radiation absorbing surface in accordance with the present disclosure (for example, in the form of a homogeneous perfect absorption metamaterial) is an important aspect of achieving universal cloaking, because the reflection of radiation or acoustic waves needs to be minimized or eliminated at an air-material (e.g., air- metamaterials) interface to ensure all the light is absorbed inside the material that creates the radiation absorbing surface coating. According to Fresnel equations, the reflection from air-metamaterials interface can be calculated and the reflection of normal incident is shown in Figure 5C as a function of the complex refractive index N=n+ik. The results indicate that the smaller the real part ("n") and imaginary part ("k") of refractive index ("N"), the lower the reflection at interface due to better impedance match to air. Although no natural material has such low index of refraction, metamaterials comprising low density cylindrical inclusions can create very low effective refractive index according to effective media theory -f)e _a ir, where f is the volume ratio of cylindrical particles, £ _ma teriai and £ _air are permittivity of the particle and air, respectively. The relation between permittivity and refractive index is ε=Ν ² .

[0048] By introducing the plurality of electrically conductive axially- shaped particles 24 to the radiation absorbing surface 20, the volume ratio of air and materials (e.g., axially-shaped microparticles) can be manipulated. The volume ratio of the particle materials determines the effective refractive index of the overall radiation absorbing surface 20, for example, the effective permittivity of cylindrical materials can be selected by the equations discussed just above.

[0049] The thickness of the radiation absorbing surface layer can be determined by the effective refractive index of the radiation absorbing surface layer of cloaking material (e.g., axially-shaped microparticles). Because the field along the propagation direction inside the microparticles can be expressed by

[0050] E(z)=E ₀ exp(i ^* n _eff ^* k ₀ ^* z) [0051] where E ₀ is the amplitude of incident light, n _eff is the effective refractive index of material, k ₀ is the wave vector of incident light. The transmission through the coating or layer thickness z can be obtained,

[0052] T=|E(z)/Eo| ² =| exp(i ^* n _eff ^* ko ^* z)| ²

[0053] For n _eff =1 .01 + 0.02 (approximate for carbon nanotubes

(CNTs), for example) and a wavelength of electromagnetic radiation at 633 nm (visible red-orange light), comparative testing results are shown in Figure 2. These results indicate that transmission through 20 μιτι thick radiation absorbing surface layer comprising a plurality of axially-shaped CNT particles is 0.04%, while transmission for a 30 μιτι thick radiation absorbing surface layer comprising CNT particles is 0.0007%, which demonstrates the suitability for cloaking visible light.

[0054] This result is scalable to other frequencies, for example, infrared, THz, and microwave. If the imaginary part of the refractive index is about 0.01 , the typical thickness of the cloaking material should be about 50 times the wavelength in order to get a transmission smaller than 0.1 %, which is suitable for a near-perfect radiation absorbing surface layer used in a cloaking application. Since energy decays exponentially inside the cloaking material, the required thickness of the cloaking material is readily determined by the cloaking material's effective index and the target transmission value.

[0055] The refractive index of a radiation absorbing surface can be expressed as n _eff = Re(n _eff ) + lm(n _eff ) (based on N = n+ik) so in certain embodiments of the present disclosure, a real part of the refractive index Re(n _eff ) can range from greater than or equal to about 0.9 to less than or equal to about 1 .1 , while the imaginary lm(n _eff ) can range from greater than or equal to about 0.0001 to less than or equal to about 0.1 . Thus, in certain preferred variations, a refractive index of a radiation absorbing surface can be in the range of n _eff =(about 0.9 to about -1 .1 ) + i ^* (about 0.0001 to about 0.1 ) to obtain a desirably low reflection surface and cloaking effect.

[0056] In one embodiment of the present disclosure, a radiation absorbing surface that absorbs the required amount of electromagnetic radiation is a low refractive index surface coating or "carpet" material comprising a plurality of carbon nanotubes (CNT), which are preferably ultra-long vertically aligned low density cylindrical CNT arrays that form a "forest." Such a radiation absorbing surface CNT forest can desirably function as a perfect absorber over a wide band of electromagnetic frequencies.

[0057] In various aspects, the surface of substrate 22 is sparsely populated with a plurality of axially-shaped microparticles 24 to form the radiation absorption material or coating, expressed by the density of the microparticles on the substrate 22. For example, the plurality of axially-shaped microparticles 24 that form the radiation absorption coating optionally are present at less than or equal to about 10 vol. % along the substrate 22 surface; optionally at less than or equal to about 5 vol. %; optionally at less than or equal to about 4 vol. % optionally at less than or equal to about 3 vol. %; optionally at less than or equal to about 2 vol. %; and in certain preferred aspects, optionally at less than or equal to about 1 vol. % along the substrate 22 surface.

[0058] Furthermore, as appreciated by those of skill in the art, the average diameter "e" of the microparticles 24 is selected based upon the wavelength(s) "c" of electromagnetic radiation 26 to be absorbed by the radiation absorbing surface 20. In certain embodiments, an average diameter of the axially-shaped conductive microparticles 24 ranges from greater than or equal to about 1 nm to less than or equal to about 10 mm and optionally greater than or equal to about 100 nm to less than or equal to about 10 mm. In certain embodiments, an average diameter of the axially-shaped conductive microparticles 24 ranges from greater than or equal to about 100 nm to less than or equal to about 100 μιτι. In other embodiments, an average diameter of the axially-shaped conductive microparticles 24 ranges from greater than or equal to about 100 μιτι to less than or equal to about 10 mm.

[0059] In certain variations, a length along a major longitudinal axis of the axially-shaped conductive microparticles 24 optionally ranges can range from greater than or equal to about 100 nm to less than or equal to about 1 m, optionally greater than or equal to about 100 nm to less than or equal to about 1 mm. In certain embodiments, a length along a major longitudinal axis of the microparticles 24 ranges from optionally greater than or equal to about 100 nm to less than or equal to about 1 μιτι. In yet other aspects, a length along a major longitudinal axis of the microparticles 24 optionally ranges from greater than or equal to about 10 μιτι to less than or equal to about 1 mm. In certain other embodiments, a length along a major longitudinal axis of the microparticles 24 ranges from optionally greater than or equal to 10 mm to less than or equal to about 1 m.

[0060] In certain embodiments, a perfect radiation absorbing surface (which encompasses a near-perfect absorbing surface) is capable of absorbing greater than or equal to about 97% of at least one wavelength of electromagnetic wave to which it is exposed. In certain embodiments, a perfect radiation absorbing surface (which encompasses a near-perfect absorbing surface) is capable of absorbing greater than or equal to about 97% of multiple electromagnetic wavelengths to which it is exposed. Such electromagnetic waves can either be generated by the three-dimensional object to be cloaked and/or can be emitted from the three-dimensional object towards a viewing direction (e.g., from reflection or diffraction). In yet other variations, a perfect radiation absorbing surface in accordance with the principles of the present disclosure is capable of absorbing greater than or equal to about 90% of the electromagnetic waves to which it is exposed; optionally greater than or equal to about 95%; optionally greater than or equal to about 98%; optionally greater than or equal to about 99%; optionally greater than or equal to about 99.1 %; optionally greater than or equal to about 99.5%; optionally greater than or equal to about 99.7%; optionally greater than or equal to about 99.8%; optionally greater than or equal to about 99.9%, and in certain preferred aspects, optionally greater than or equal to about 99.99%.

[0061] In certain variations, the perfect radiation absorbing surface is capable of absorbing greater than or equal to about 97% of electromagnetic waves with wavelengths ranging from greater than or equal to about 300 nm to less than or equal to about 1 m; optionally greater than or equal to about 98% of such electromagnetic waves, and in certain variations, greater than or equal to about 99% of electromagnetic waves having a wavelength ranging from about 390 nm to about 1 m (from visible light to microwave radiation). In yet other variations, the perfect radiation absorbing surface is capable of absorbing greater than or equal to about 97% of electromagnetic waves with wavelengths ranging from greater than or equal to about 300 nm to less than or equal to about 1 mm (from visible light to far infrared radiation); optionally greater than or equal to about 98% of such electromagnetic waves, and in certain variations, greater than or equal to about 99% of electromagnetic waves having a wavelength ranging from about 300 nm to about 1 mm ranging from about 390 nm to about 1 m (from visible light to microwave radiation). In other aspects, a perfect radiation absorbing surface is capable of absorbing greater than or equal to about 97% of electromagnetic waves with wavelengths ranging from greater than or equal to about 390 nm to less than or equal to about 750 nm (visible light); optionally greater than or equal to about 98% of such electromagnetic waves, and in certain variations, greater than or equal to about 99% of electromagnetic waves having a wavelength ranging from about 390 nm to about 750 nm. In yet other aspects, the perfect radiation absorbing surface is capable of absorbing greater than or equal to about 97% of electromagnetic waves with wavelengths ranging from greater than or equal to about 750 nm to less than or equal to about 1 mm (including infrared radiation to far infrared radiation); optionally greater than or equal to about 98% of such electromagnetic waves, and in certain variations, greater than or equal to about 99% of electromagnetic waves having a wavelength ranging from about 300 nm to about 1 mm. In yet other variations, the present radiation absorbing surface is capable of absorbing acoustic waves, such as sound waves. In one particular variation, the perfect radiation absorbing surface is capable of absorbing greater than or equal to about 97% of acoustic waves; optionally greater than or equal to about 98% of such acoustic waves, and in certain variations, greater than or equal to about 99% of acoustic waves.

[0062] In accordance with the principles of the present disclosure, an alternative type of ground plane cloak in the form of a radiation absorbing surface is realized at visible frequencies with large area, broadband and polarization independent characteristics. As described above, a universal cloaking surface in uses a principle of perfect or near-perfect absorption of radiation that suppresses reflection from all directions and visually compresses arbitrary three-dimensional (3-D) objects into a two-dimensional (2-D) plane, thereby making the objects invisible over the entire visible band. Moreover, the radiation absorbing surface includes homogeneous metamaterials and therefore a universal carpet capable of large area cloaking of arbitrary shaped object is readily and practically fabricated.

[0063] Figures 3A-H show a perfect absorption ground plane cloak (a radiation absorbing surface in accordance with the inventive technology) that works at visible frequencies of electromagnetic radiation and is capable of cloaking a large area, as well as arbitrarily shaped three-dimensional (3-D) objects. The cloaking of a centimeter-size 3D object can be directly observed by naked eyes under the illumination of unpolarized broadband visible light. In this example, an exemplary radiation absorbing surface is a homogeneous perfect absorption carpet (coating) made of a low density carbon nanotube (CNT) "forest," which comprises a plurality of CNTs. The CNT forest visually compresses arbitrary 3-D objects so that they appear as a 2-D perfect absorption sheet. For example, invisibility has been observed by naked eyes for unpolarized light through the entire visible frequency range with a cloaking area 105 times larger than the longest wavelength of electromagnetic radiation applied.

[0064] Figures 4A-B show the scanning electron microscope (SEM) images of a radiation absorbing surface according to one embodiment of the present disclosure that is a vertically aligned multi-walled CNT forest carpet, whose volume ratio is about 1 %, with average CNT diameter of 10 nm and a spacing between CNTs of about 100 nm that is grown by a plasma-enhanced chemical vapor deposition (PECVD) process. Such CNT growth techniques are taught by Meyyappan, et al., "Carbon nanotube growth by PECVD: a review," Plasma Sources Sci. Technol. 12, pp. 205-216 (2003); Ok et al., "Electro-Optical Materials: Electrically Addressable Hybrid Architectures of Zinc Oxide Nanowires Grown on Aligned Carbon Nanotubes" Adv. Funct. Mater. 20, pp. 2470-80 (2010), the respective relevant portions of which are expressly incorporated herein by reference. The calculated refractive index of this fabricated CNT forest is shown in Figure 4D, where the real part of effective index ranges from 1 .021 to 1 .026, and the imaginary part from 0.033 to 0.056 at visible frequency. Such value of index is marked with cross bar in Figure 4C, which can produce extremely low reflection at the interface. Moreover, the angle and wavelength dependent reflection shown in Figures 5A-B indicate that the low density CNTs forest is also an effective broadband low reflection metamaterials for large incident angle and unpolarized light, with reflection of less than 0.1 % across the entire visible band.

[0065] Considering the rough surface of the CNT forest and the resultant diffused reflection, any reflected light is redistributed to all directions and the observer (whether detection equipment or a human observer) only receives a small portion of the reflected energy (in the detection direction) that is several orders of magnitude lower than total reflected energy. On the other hand, the absorption for 10 μιτι thick CNT coating is 99.7% if the imaginary part of the effective index is 0.03. Thus, in certain embodiments, a CNT coating can be grown to have a thickness of greater than or equal to about 50 μιτι, so that the excellent low reflection and high absorption properties of CNT carpet provide a perfect radiation absorption surface for a cloaking application.

[0066] In certain alternate embodiments, a radiation absorbing surface for cloaking a three-dimensional object comprises a coating on a detectable surface of the object. The coating comprises a plurality of microparticles that are admixed or dispersed in binder material, such as a polymeric resin to form a matrix. In certain preferred aspects, the plurality of microparticles is homogenously mixed in the binder material to form a homogeneous matrix. The matrix of microparticles and binder material is optionally coated or otherwise applied to the object's one or more detectable surfaces. Any binder material that is capable of retaining the plurality of microparticles on the detectable surface of the object without unduly interfering with the optical properties or absorption characteristics of the coating which are well known to those of skill in the art is contemplated.

[0067] Each of the microparticles of the plurality comprises a plurality of carbon nanotubes (or other axially-shaped conductive microparticles or nanoparticles) disposed on a surface of the microparticle itself. In preferred aspects, the carbon nanotubes are disposed on a surface of the microparticle similar to the manner described above for the plurality of axially-shaped microparticles grown or otherwise disposed on a substrate of an object. Thus, the plurality of carbon nanotubes is preferably substantially aligned to have a single orientation or direction with respect to one another along the microparticle surface. For example, each of the plurality of carbon nanotubes defines a major longitudinal axis that intersects with a plane defined by the microparticle surface at an angle of greater than or equal to about 30° and less than or equal to about 150° or any of the other angles discussed above. Such an orientation is optionally a substantially vertical orientation.

[0068] As described above, in certain aspects, the carbon nanotubes formed on the surface of the microparticle have a low volume density (e.g., less than or equal to about 10 vol. %, optionally less than or equal to about 5 vol. %, optionally less than or equal to about 1 vol. %) to provide a desired refractive index to the coating for cloaking (in accordance with the principles discussed above). Thus, in various aspects, such a coating is designed to form a radiation absorbing coating on a detectable surface of the object to be cloaked, where it can have an optional refractive index (n _eff ) represented by the equation n _eff = Re(n _eff ) + lm(n _eff ), where Re(n _eff ) is greater than or equal to about 0.9 to less than or equal to about 1 .1 and lm(n _eff ) is greater than or equal to about 0.0001 to less than or equal to about 0.1 , so that n _eff = (> about 0.9 to < about 1 .1 ) + i ^* (> about 0.0001 to < about 0.1 ).

[0069] In various other aspects, the present disclosure provides for methods of cloaking a three-dimensional object. In one variation, the method comprises providing a radiation absorbing surface that shields at least a portion of the object from a viewing direction. See, for example, Figures 3A and 3B, showing visible light being applied to an object at an angle of about 40° and a receiver in a viewing direction at an angle of about 125° with respect to the object. In Figure 3A, the object reflects or diffracts a large portion of the visible light applied to it, so that from the viewing direction, the object is observed. As can be seen in Figures 3B and 3E for example, a perfect radiation absorbing surface (which encompasses a near-perfect absorbing surface) of the inventive technology is capable of absorbing the electromagnetic radiation described above in the context of the radiation absorptive coatings, for example, greater than or equal to about 98% of the electromagnetic waves either generated by the three-dimensional object to be cloaked and/or 98% of the electromagnetic waves emitted in the viewing direction (e.g., from reflection or diffraction) to the object. In certain preferred variations, greater than or equal to about 99%, optionally greater than or equal to about 99.5%, and in certain preferred aspects, greater than or equal to about 99.9% of the electromagnetic waves either generated by the three-dimensional object to be cloaked and/or greater than or equal to about 99%, optionally greater than or equal to about 99.5%, and in certain preferred aspects, greater than or equal to about 99.9% of the electromagnetic waves emitted in the viewing direction (e.g., from reflection or diffraction) to the object. The cloaking radiation absorbing surface comprises a plurality of axially shaped nanoparticles having an electrical conductivity of greater than or equal to about 50 S/cm disposed on a substrate, where the plurality of the axially shaped nanoparticles each define a major longitudinal axis that intersects with a plane defined by the substrate at an angle of greater than or equal to about 70°. Further, the plurality of axially-shaped nanoparticles is substantially aligned to have a single orientation with respect to one another and to a horizontal plane defined by the substrate.

[0070] In certain aspects, the methods of the present disclosure are particularly suitable for cloaking a three-dimensional object in a water-containing environment like the ocean or sea; in the outer strata of Earth's atmosphere (e.g., in outer space) in a vacuum, or in other environments where electromagnetic radiation is primarily absorbed or typically not reflected (a non- reflective background). In other aspects, the methods of the present disclosure may further comprise disposing the object to be cloaked on a highly absorptive background, for example, a perfect or near-perfect absorbing surface for the target electromagnetic or acoustic waves. When the object likewise has a cloaking radiation absorbing surface disposed thereon (or is a protective layer over the object), the three-dimensional object cannot be detected against the highly absorbent background from a viewing or detection direction. Preferably, when a background is employed in conjunction with a radiation absorbing surface for cloaking a three-dimensional object, the background has a substantially similar capability for absorbing target electromagnetic or acoustic waves to the radiation absorbing surface, so that no discernible difference can be detected.

[0071] In certain variations, where a background is disposed behind the object having a radiation absorbing surface from the viewing direction, both the background and the radiation absorbing surface absorb at least 97% of the electromagnetic or acoustic waves emitted to the object and background, thereby rendering the three-dimensional object invisible against the background from a detection direction. In certain variations, both the background and the radiation absorbing surface absorb greater than or equal to about 98% of the electromagnetic or acoustic waves emitted to the object and background, and optionally greater than or equal to about 99% of the electromagnetic or acoustic waves emitted to the object and background, for cloaking the three-dimensional object.

[0072] Figures 6A-F demonstrate cloaking of an arbitrarily shaped object, which is fabricated on a silicon substrate by focused ion beam (FIB) milling. In this case, a "tank" pattern of 65x22.5 μιτι in size (Figure 6A) was made, and its reflection image was taken under an optical microscope illuminated by unpolarized broadband visible light (Figure 6D). To cover the object with the cloaking carpet, a 60 μιτι-thick CNT forest was grown on top of the whole silicon sample and follows the profile of the original "tank" object (Figure 6B). More specifically, a "tank" object is milled on top of a 500 μιτι thick silicon substrate, using a dual beam focused ion beam (FIB) workstation combined with scanning electron microscope (SEM) (FEI Nova Nanolab). The accelerating voltage for ion beam milling is 30 kV and current is 20 nA. The fabricated tank pattern measures 65 μιτι χ 22.5 μιτι and 2 μιη in depth. The SEM image for the "tank" in Figure 6A is taken at a tilt angle of 45 degrees to view its 3-D perspective. To obtain a radiation absorbing surface comprising a CNT carpet, first a 300 nm-thick Si0 ₂ layer is deposited on the silicon sample by plasma-enhanced chemical vapor deposition and then a 1 nm-thick Fe catalyst layer is deposited by electron beam evaporation. The sample is loaded in a single-zone tube furnace, which is heated to 775 °C under the gas mixture of C ₂ H ₄ /H ₂ /He. More detailed CNT growth process can be found at the previously incorporated by reference article: Meyyappan, et al. "Carbon nanotube growth by PECVD: a review," Plasma Sources Sci. Technol. 12, pp. 205-216 (2003). The SEM image of Figure 6B taken after growth of the CNT forest is also taken at 45 degree tilting angle. [0073] For the control structure in Figure 6C, the rectangular pattern (200 μιη long and 8 μιη wide) around the "tank" is made by removing the CNT layer using FIB milling. The SEM image in Figure 6C is a top view without tilt. The visible optical images in Figures 6D-F were taken under optical microscope (Nikon TE-300) at reflection mode, with broadband illumination that covers the entire visible band (Techni-Quip Corp., T-Q/FOI-1 ). The optical images are almost the same using objective lens of 10x, 20x, and 40x magnification with numerical aperture ranging from 0.25 to 0.55.

[0074] Figure 6E shows that the tank completely disappears and the surface looks exactly the same as a flat CNT sheet. As a further proof, a control experiment was performed where a rectangle mark around the "tank" was made by FIB milling that removed the CNT (Figure 6C). The optical image now clearly shows the rectangle mark, but the tank pattern inside the mark remains invisible (Figure 6F). All the optical reflection images in Figures 6D-F are taken using 10x objective lens with numerical aperture of 0.25, i.e., incident angle ranging from 0 to 14.5 degrees. The images taken by 40x magnification objective lens with numerical aperture of 0.55 showed the similar performance, which indicate that the cloaking carpet works equally well for large incident angle range from 0 to 33.4 degrees. Because the incident visible light is broadband and unpolarized, the cloaking effect is directly observed by naked eyes under conventional microscope.

[0075] The schematic of a perfect absorption ground plane cloak in accordance with the present teachings is shown in Figures 3A-H For an arbitrary object placed on top of a perfect absorption background (provided as a perfect absorption sheet), the scattering and reflection from the object make it detectable in a detection direction by observers (Figure 3A). When covered by a layer of perfect radiation absorbing surface according to the present technology, the entire incident light applied to the object is absorbed by the carpet without causing any reflection or scattering, regardless of the angle of incidence and wavelength of the light (Figure 3E). Therefore the object under the perfect radiation absorbing carpet surface in Figure 3B is indistinguishable from the perfect absorption background and has the same appearance as a flat perfect absorption sheet shown in Figure 3C. This simple approach visually compresses any arbitrarily-shaped 3-D object into a 2-D plane and thereby making the object invisible. Because the radiation absorbing surface of the present teachings is made of a homogeneous material and can absorb the light incident from all directions, it can be made to follow the profile of the object (as a surface coating, for example) or create an arbitrarily shaped hollow spaces (as a shield, for example) to conceal the object under it. In contrast to current conventional cloaking principles, the inventive technology provides flexible invisibility cloaking, without requiring specific design of a metamaterial with intricate parameters for different objects.

[0076] Figures 3D-F show numerical simulation results for a perfect electric conductor (PEC) object, a perfect absorption carpet covering the PEC object, and a planar perfect absorption sheet, respectively. The simulations are performed using finite element method (FEM) with incident wavelength of 632.8 nm, where the 3-D object has a width of 9.0 μιτι and a height of 3.0 μιτι. The perfect conductor is assumed to contain both sharp edges and curved surfaces, because any arbitrarily shaped object can be considered as a combination of generalized edges and curves. The calculated field distribution presented in Figure 3D shows strong scattering and reflection by the object that can be easily identified by a far field observer (shown in a detection direction in Figure 3A). In Figure 3E, a perfect absorption radiation absorbing surface is placed at the top of the object to follow its profile (as a shield), where a thickness of the coating on the radiation absorbing surface is 2.5 μιτι and the refractive index used is n= 1+\0.07. The simulation results indicate that the reflection and scattering at the air-carpet interface is extremely low, meanwhile the electromagnetic energy decays exponentially inside the radiation absorbing surface shield and thus cannot reach the perfect conductor object to create any reflection from the object. Furthermore, it should be noted that the radiation absorbing surface on the shield is capable of absorbing radiation or acoustic waves generated by the object, thus such a coating can likewise shields any radiation emitted from the object (cloaking any thermal signatures or acoustic noise). Consequently, there is almost no detectable reflected field at far field— this behavior is identical to that of flat perfect absorption surface shown in Figure 3F. [0077] Far field scattering amplitudes collected from the dashed line (shown in Figure 3D) for all three cases are shown in Figure 3G, which clearly indicates that cloaking carpet works effectively and indeed absorbs almost all the light without reflection. Figure 3H further plots the log scale of the far field distribution, and the reflection from cloaking carpet and planar surface are at the same level when taking into account the calculation error. Such an approach is highly scalable, because increasing the cloaking area does not increase the complexity of the radiation absorbing surface/cloaking carpet, and the same refractive index for the homogeneous radiation absorbing surface works equally well for cloaking larger areas.

[0078] Thus, the methods of the present teachings are equally effective in providing universal cloaking of large area objects of arbitrary shapes and with sizes many orders of magnitude larger than a wavelength(s) applied to the object. For example, a macroscopic object placed under a hollow shield coated with a CNT forest coating like that shown in Figure 3B. Furthermore, Figures 7A-C show still images taken from a movie showing cloaking and complete concealment of a toy airplane (having dimensions of 2.2 cm by 1 .5 cm) under a space that is covered by a suspended shield having a surface with a radiation absorbing surface in the form of a CNT coating. The configuration includes a radiation absorbing sheet as background and the shield (having the radiation absorbing surface) is placed over the background (so that an open space is defined below the shield and above the background). First, Figure 7A shows a fully visible airplane being placed over the background from the detection position. In Figure 7B, the airplane enters the empty space below the shield, thus, as the airplane enters the space covered by the radiation absorbing surface (CNT forest carpet), the airplane body gradually disappears (the airplane is partially concealed in Figure 7B, where the front of the plane is invisible). Finally, the entire plane is cloaked and becomes invisible when the airplane is slid into the open space beneath the shield covered by the radiation absorbing surface carpet in Figure 7C. There is no observable boundary of the cloaking radiation absorbing surface on the shield due to the perfect absorption characteristics of both the radiation absorbing coating carpet and the ground plane background. [0079] The cloaking radiation absorbing surface CNT carpet is made of a 500 μιτι thick CNT forest grown on a 1 inch square silicon substrate, which is peeled off to form a free-standing carpet sheet. The plastic airplane is held by a 3-D stage through a stripe of CNT carpet with a cantilever under it. The CNT coating carpet is positioned 1 .5 cm above another perfect light absorbing CNT substrate acting as ground plane to create an invisible space to conceal the airplane. The still images in Figures 7A-C are from a movie taken under natural light that contains all the visible wavelengths with various incident angles and polarization states. This movie demonstrates both cloaking of the airplane by the radiation absorbing surface on the shield, but further the airplane appears to hover over the ground/background, which is due to the cloaking of cantilever by another radiation absorbing surface in the form of a CNT carpet. Such techniques demonstrate the flexibility of such homogeneous radiation absorbing surface materials, capable of cloaking large arbitrarily-shaped 3-D objects.

[0080] Thus, in certain embodiments, a ground plane cloaking method employs a perfect radiation absorption coating comprising a plurality of conductive axially-shaped multi-walled carbon nanotubes provided at a low density to form a CNT forest. However, such a cloaking approach is not restricted to CNT-based coatings embodiments, but rather contemplates a wide variety of radiation absorption coatings. Cloaking of three-dimensional objects at visible frequency with broadband unpolarized illumination and other wavelengths five orders of magnitude larger than those of visible light are contemplated by the present teachings. It can be applied to a broader frequency range from ultraviolet to THz, or for underwater acoustic wave cloaking, by using the techniques of the present disclosure. Because the refractive index of the inventive cloaking radiation absorbing surface is homogeneously distributed, this approach provides a universal carpet for optically cloaking arbitrary objects. Thus, the present technology provides the ability to practically cloak large areas and objects over extensive broad band frequencies for electromagnetic and acoustic waves.

[0081] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. All possible combinations of the enumerated features specifically disclosed as embodiments of these radiation absorbing surfaces and methods for cloaking three-dimensional objects are contemplated. Any one or any combination of more than one of the enumerated features is also contemplated. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

[0082] This description of the technology is merely exemplary in nature and, thus, variations that do not depart from the gist of the present disclosure are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.