Reconfigurable intelligent surface devices

Abstract

A reconfigurable intelligent surface device including a substrate comprising a first major surface and a second major surface opposite the first major surface, the first major surface having a plurality of RIS elements, each RIS element of the plurality of RIS elements including a plurality of concentric electrically conductive closed loops, each electrically conductive closed loop of the plurality of concentric electrically conductive closed loops separated from an adjacent electrically conductive closed loop and electrically connected thereto by a plurality of linking electrical conductors. A transmittance of the reconfigurable intelligent surface device over a wavelength range of about 400 nm to about 700 nanometers may be equal to or greater than about 50%.

Description

RECONFIGURABLE INTELLIGENT SURFACE DEVICESCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Serial No. 63 / 642366 filed on May 3, 2024, the content of which is relied upon and incorporated herein by reference in its entirety.FIELD

[0002] The disclosure relates generally to structures for electromagnetic wave reflection, and more specifically, reconfigurable intelligent surface (RIS) devices for wireless communication systems (WCS), which can include fifth generation (5G) for later (6G) systems, 5G new-radio (5G-NR) systems, and / or a distributed communications system (DCS).BACKGROUND

[0003] The millimeter-wave (mm-wave) wavelength band is increasingly utilized for various communication applications, including 5G and 6G wireless communications. Communications employing a mm-wave wavelength band may provide benefits over previous communication technologies, such as greater communication speeds, reduced communication latencies, and greater capacity. For instance, mm-wave systems may operate in the 30 - 300 GHz frequency range. However, there are challenges transmitting and receiving wireless signals within mm-wave bands. For instance, propagation loss, penetration loss, and reflection loss may degrade the link corresponding budgets.

[0004] To compensate for these challenges, some systems may employ high gain phased array antennae. However, high gain phased array antennae may have their own drawbacks. For instance, the antennas may become saturated as the number of array elements increases. Moreover, high gain phased array antennas increase system loss and provide limited gains when employed in devices with limited space to accommodate the antennas, such as mobile devices.

[0005] More recently, reconfigurable intelligent surface (RIS) devices have been proposed for improving propagation channel conditions between base stations and mobile devices. RIS devices, also known as metasurfaces, may include the use of sub -wavelength resonators that adaptively reflect, transmit, absorb, and / or convert the polarizations of incident waves with low power consumption. For instance, an RIS device as a mm-wave device can be configured tocontrol signal reflectance in a desired direction. Further, RIS devices can generally be fabricated at low cost due to low-cost components.SUMMARY

[0006] In a first aspect, a reconfigurable intelligent surface device, comprising a substrate comprising a first major surface and a second major surface opposite the first major surface, the first major surface comprising a plurality of RIS elements, each RIS element of the plurality of RIS elements comprising a plurality of concentric electrically conductive closed loops, each electrically conductive closed loop of the plurality of concentric electrically conductive closed loops separated from an adjacent electrically conductive closed loop and electrically connected thereto by a plurality of linking electrical conductors. A transmittance of the reconfigurable intelligent surface device over a wavelength range of about 400 nm to about 700 nanometers may be equal to or greater than about 50%, for example equal to or greater than about 60%, equal to or greater than 70%, equal to or greater than about 80%, equal to or greater than about 90%, equal to or greater than about 95%, equal to or greater than about 98%, or equal to or greater than about 99%, including all ranges and subranges therebetween.

[0007] In a second aspect, the plurality of concentric electrically conductive closed loops of the first aspect may comprise rectangular electrically conductive closed loops.

[0008] In a third aspect, the second major surface may comprises a ground plane.

[0009] In a fourth aspect, the ground plane of the third aspect may comprise a metallic mesh.

[0010] In a fifth aspect, the plurality of electrically conductive concentric closed loops may comprise circular electrically conductive closed loops.

[0011] In a sixth aspect, the plurality of linking electrical conductors of the fifth aspect may be evenly angularly spaced between adjacent electrically conductive closed loops.

[0012] In a seventh aspect, the plurality of linking electrical conductors may comprise a mesh.

[0013] In an eighth aspect, the substrate may comprise a dielectric material.

[0014] In a ninth aspect, the substrate may comprise a glass substrate.

[0015] In a tenth aspect, the substrate may comprise a pair of glass substrates and a liquid crystal material disposed therebetween.

[0016] In an eleventh aspect, the substrate may comprise a semiconductor material.

[0017] In a twelfth aspect, the reconfigurable intelligent surface device may comprise a first plurality of RIS elements having a first structure and a second plurality of RIS elements having a second structure different from the first structure.

[0018] In a thirteenth aspect, the plurality of RIS elements may be periodically arranged.

[0019] In a fourteenth aspect, a fill factor of each unit cell, defined as the ratio of the area of the RIS element to the area of the unit cell, may be less than 10%.

[0020] In a fifteenth aspect, at least two linking electrical conductors of the plurality of linking electrical conductors may be orthogonal.

[0021] In a sixteenth aspect, the RIS element of the second aspect may comprise at least four linking electrical conductors, the at least four linking electrical conductors comprising a pair of parallel linking electrical conductors and a pair of orthogonal linking electrical conductors.

[0022] Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0023] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments, and together with the description explain principles and operation of the various embodiments.BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. l is a schematic view of wave refection from a reconfigurable intelligent surface in a Cartesian coordinate system;

[0025] FIG. 2 is a cross-sectional view of an example RIS device according to embodiments of the present disclosure;

[0026] FIG. 3 is a top view of another example RIS device comprising a plurality of patchtype RIS elements;

[0027] FIG. 4 is a top view of a rectangular (square) unit cell of the RIS device of FIG. 3 comprising a patch-type RIS element having equal length sides;

[0028] FIG. 5 is a top view of a rectangular unit cell of an example RIS device having a patchtype RIS element with unequal length sides;

[0029] FIG. 6 is a top view of another RIS device comprising a plurality of circular RIS elements;

[0030] FIG. 7 is a plot of reflection loss and phase as a function of length for the unit cell of FIG. 4;

[0031] FIG. 8 is a top view of a unit cell of another RIS device according to embodiments of the present disclosure comprising a ring-type RIS element;

[0032] FIG. 9 is a plot of reflection loss and phase as a function of RIS element length for the unit cell of FIG. 8;

[0033] FIG. 10 is a bottom view of an RIS device according to embodiments of the present disclosure comprising a mesh-type ground plane;

[0034] FIG. 11 is a top view of a unit cell of an RIS device according to embodiments of the present disclosure comprising a ring-type RIS element including a mesh;

[0035] FIG. 12 is a plot of reflection loss and phase as a function of RIS element length comparing a unit cell with an RIS element having a meshed-type ring and a unit cell with a ring-type RIS element having a solid ring;

[0036] FIG. 13 is a top view of a unit cell of an RIS device, wherein the RIS element of the RIS unit cell includes a single electrically conductive loop;

[0037] FIG. 14 is a plot of reflection loss and phase as a function of RIS element length for a unit cell comprising a solid ring-type RIS element compared to a unit cell comprising a single electrically conductive loop with two different fill factors;

[0038] FIG. 15 is a plot of phase sensitivity as a function of RIS element length for a unit cell comprising a solid ring-type RIS element compared to a unit cell comprising a single electrically conductive loop with a fill factor of 0.2%;

[0039] FIG. 16 is a top view of a unit cell including an RIS element comprising two electrically conductive loops joined by a plurality of linking conductors;

[0040] FIG. 17 is a plot of reflection loss and phase as a function of RIS element length comparing a unit cell comprising a solid ring-type RIS element compared to a unit cell comprising a linked ring-type RIS element comprising two linked electrically conductive loops;

[0041] FIG. 18 is a top view of a linked ring-type RIS element comprising three rectangular (e.g., square) electrically conductive loops connected by a plurality of linking conductors;

[0042] FIG. 19 is a top view of another linked ring-type RIS element comprising two rectangular (e.g., square) electrically conductive loops joined by linking conductors located at corners of the electrically conductive loops;

[0043] FIG. 20 is a top view of another linked ring-type RIS element comprising two circular electrically conductive loops joined by a plurality of linking conductors;

[0044] FIG. 21 is a top view of a unit cell comprising another RIS element including two unlinked electrically conductive closed loops;

[0045] FIG. 22 is a plot of reflection loss and phase as a function of RIS element length for the unit cell of FIG. 21;

[0046] FIG. 23 is a cross-sectional edge view of another RIS device comprising an active layer, e.g., a liquid crystal material, disposed between two substrates;

[0047] FIG. 24 is a plot of reflection loss as a function of fill factor for RIS elements of a variety of configurations;

[0048] FIG. 25 is a plot of phase as a function of fill factor for the configurations of RIS elements used for FIG. 24;

[0049] FIG. 26 is a plot of reflection loss as a function of fill factor;

[0050] FIG. 27 is a plot of reflection loss and phase for an RIS element having two square conductive loops linked by four linking conductors, one linking conductor positioned at each side of the square ring and formed as illustrated in FIG. 15;

[0051] FIG. 28 is a plot of reflection loss and phase for an RIS element having three square electrically conductive loops linked by four linking conductors, one linking conductor positioned at each side of the square ring and formed as illustrated in FIG. 17;

[0052] FIG. 29 is a plot of phase as a function of RIS element length comparing the RIS element of FIG. 26 and the RIS element of FIG. 27;

[0053] FIG. 30 is plot of modeled reflection loss and phase for a linked ring structure shown in FIG. 30 having two electrically conductive loops and two linking conductors, the two linking conductors linking the outer conductive loop 102 to the inner conductive loop 104 but arranged orthogonal to each other;

[0054] FIG. 31 is a top view of a unit cell comprising a rectangular (e.g., square) ring-type RIS element comprising two electrically conductive closed loops joined by two orthogonal linking conductors;

[0055] FIG. 32 is a plot of modeled reflection loss and phase as a function of RIS element length for the unit cell of FIG. 30;

[0056] FIG. 33 is a top view of a unit cell comprising a rectangular (e.g., square) ring-type RIS element comprising two electrically conductive closed loops joined by two parallel linking conductors;

[0057] FIG. 34 is a plot of modeled reflection loss and phase as a function of RIS element length for the unit cell of FIG. 32

[0058] FIG. 35 is a top view of a unit cell comprising a rectangular (e.g., square) ring-type RIS element comprising two electrically conductive closed loops joined by two parallel linking conductors and a third linking conductor orthogonal to the two parallel linking conductors;

[0059] FIG. 36 is a plot of reflection loss and phase as a function of RIS element length for the unit cell of FIG. 35;

[0060] FIG. 37 is a plot of modeled reflection loss and phase as a function of RIS element length for the unit cell of FIG. 34, but scaled up (increased in dimensions) from the data shown in FIG. 36;

[0061] FIG. 38 is a top view of a unit cell comprising a ring-type RIS element comprising three circular electrically conductive closed loops joined by a plurality of linking conductors arranged radially at 90 degree spacings;

[0062] FIG. 39 is a top view of a unit cell comprising a ring-type RIS element comprising three closed circular electrically conductive closed loops wherein the outermost electrically conductive loop is joined to an intermediate electrically conductive loop by a first plurality of linking conductors arranged radially at 90 degree spacings, and wherein the intermediate electrically conductive loop and the innermost electrically conductive loop are joined by a second plurality of linking conductors arranged radially at 90 degree spacing, and wherein the second plurality of linking conductors are offset from the first plurality of linking conductors by 45 degrees;

[0063] FIG. 40 is a plot of modeled reflection loss and phase as a function of RIS element length for the RIS element of FIG. 37;

[0064] FIG. 41 is a plot of modeled reflection loss and phase as a function of RIS element length for the RIS element of FIG. 38;

[0065] FIG. 42 is a plot of modeled phase as a function of RIS element length for the RIS elements of FIGS. 38 and 39;

[0066] FIG. 43 is a top view of a ring-type RIS element comprising three circular electrically conductive closed loops joined by a single linking conductor;

[0067] FIG. 44 is a top view of a ring-type RIS element comprising three circular electrically conductive closed loops joined by a two parallel linking conductors;

[0068] FIG. 45 is a top view of a ring-type RIS element comprising three circular electrically conductive closed loops joined by a two orthogonal linking conductors;

[0069] FIG. 46 is a top view of a ring-type RIS element comprising three circular electrically conductive closed loops joined by a three linking conductors arranged radially at 120 degree spacings;

[0070] FIG. 47 is a plot of modeled reflection loss and phase for the RIS element of FIG. 42;

[0071] FIG. 48 is a plot of modeled reflection loss and phase for the RIS element of FIG. 43;

[0072] FIG. 49 is a plot of modeled reflection loss and phase for the RIS element of FIG. 44;

[0073] FIG. 50 is a plot of modeled reflection loss and phase for the RIS element of FIG. 45;

[0074] FIG. 51 is another ring-type RIS element comprising a plurality of mesh loops joined by a plurality of mesh linking conductors;

[0075] FIG. 52 is another ring-type RIS element comprising a plurality of solid loops joined by a plurality of solid linking conductors; and

[0076] FIG. 53 is another RIS device comprising a first plurality of RIS elements of a first construction and a second plurality of RIS elements of a second construction different from the first construction.DETAILED DESCRIPTION

[0077] In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of various principles of the present disclosure. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that the present disclosure may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of various principles of the present disclosure. Finally, wherever applicable, like reference numerals refer to like elements.

[0078] Ranges can be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints ofeach of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0079] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow; plain meaning derived from grammatical organization or punctuation; the number or type of embodiments described in the specification.

[0080] As used herein, the singular forms “a,” “and,” and “the” include plural referents unless the context clear dictates otherwise. Thus, for example, reference to “an” element includes aspects having two or more such elements, unless the context clearly indicates otherwise.

[0081] For brevity, ranges of values disclosed herein, including compositional ranges or attribute (performance) ranges, or series of ranges, may be appended by the phrase “including all ranges and subranges therebetween,” which is to be interpreted as including whole number or decimal subranges as though explicitly presented. Thus, by way of example, a range between 6 and 8 (units omitted) implicitly includes a subrange between 6.4 and 8, or a subrange between 6 and 7.2, or a subrange between 6 and 7, and so forth. Additionally, a series of ranges, such as “in a range from 6 to 11 or in a range from 6 to 8” implicitly includes a range from 7 to 10, or subranges therebetween, such as 7.2 to 10.4, as though explicitly presented, provided the range does not exceed the minimum or maximum endpoints of the explicitly presented range or series of ranges. Thus, for example, “in a range from 6 to 11 or in a range from 6 to 8” has as endpoints 6 and 11.

[0082] An RIS device comprises a large number of discrete electrically conductive elements, disposed on a substrate, that act as electromagnetic wave scatterers. Typically, these conductive elements are arranged in a regular (e.g., periodic) array, for example a rectangular array of row and columns, although circular arrays may be formed as well. In embodiments, these conductive elements may be electronically reconfigurable to change the device properties, such as the surface reflectance, absorption, and / or permittivity. Reconfiguration of the elements may be controllable in real-time, allowing the RIS device to adapt to changing environmental conditions or system requirements. Beam forming can be achieved through the interference of scattered waves at a receiver. Such interference can be dynamically altered bythe RIS device. Moreover, characteristics of the RIS device elements may be configured to produce a desired outcome, for example a wave direction.

[0083] Functionality of the RIS device relies on unit cells that comprise the electrically conductive elements. Unit cells serve as phase distributors that enable the adjustment of outgoing (e.g., reflected) beams of electromagnetic energy. By choosing the dimensions (e.g., RIS length, diameter) of the unit cells, the resonance frequency of the unit cell can be selected, facilitating phase tuning. The RIS phase can be calculated when the directions of the incident wave and the desired reflected wave are known. For example, the RIS phase C|)RIS can be calculated as the difference between the reflection phase (|)R and the phase of the incident electromagnetic energy c|)i (incident phase) according to the following in reference to FIG. 1 :n (in cartesian coordinates) of the ithRIS element, yi is the y position of the ithRIS element, k = a constant, Ri = incident wave vector, n = distance between the point of incidence and the cartesian origin, and ro = the reflected wave vector.

[0084] The phase distribution curve of the RIS device can exhibit an S-shape, characterized by rapid phase changes near resonance and slower changes at the extremes of the distribution. A complete phase cycle of 360 degrees is typically needed to create an effective RIS device. Since the RIS device is a phase-controlling device, achieving 360 degrees of phase change can be used to minimize phase errors. However, phase changes of less than 360 degrees may be suitable for some applications. For example, in practical applications, a phase variation exceeding 300 degrees may often be sufficient. Unit cells for an RIS device can be designed with variable sizes that exhibit a satisfactory phase range and an appropriate slope of the phase distribution curve.

[0085] A steep slope of the S-shaped phase curve denotes increased phase sensitivity, measured in degrees per millimeter (degrees / mm) relative to the unit cell size (e.g., length, L). If phase sensitivity is too high compared to fabrication tolerance, reflection errors may occur. Various approaches, such as employing a multi-layered structure or a single-layered structure with multi-resonant elements, have been proposed to achieve both a sufficient phase range and phase sensitivity. However, these methods may not be suitable for reducing optical transparency of an RIS device, whereas an optically transparent RIS device may be desired in some scenarios. For example, an optically transparent RIS device may be used to attach anRIS device to a cover glass, a glass window, a windshield, or a billboard, to name several examples. An optically transparent RIS device can be designed to selectively filter out certain wavelengths of electromagnetic waves while allowing visual wavelengths to pass, thereby enabling the RIS device to be used as an optically transparent antenna or filter.

[0086] Accordingly, an RIS device capable of manipulating beams in the radio frequency (RF) range with sufficient fabrication tolerance is disclosed.

[0087] Referring to FIG. 2, a cross-sectional edge view of an example RIS device 10 is shown comprising a plurality of electrically conductive layers 12 positioned on first major surface 14 of substrate 16. In some embodiments, an electrically conductive ground plane 18 may be disposed on the opposing second major surface 20 of the substrate 16. As used herein, a ground plane is an electrically conductive surface that serves as a reflecting surface for radio frequency waves. The ground plane is typically large compared to the wavelength of the incident electromagnetic wave. In practical terms, the ground plane may generally extend across substantially all of the second major surface of substrate 16, or at least that portion opposite, (e.g., having the same general footprint as) the cumulative electrically conductive layers 12. However, the ground plane need not be continuous and may, in various embodiments, comprise multiple openings (areas of non-electrical conductivity), such as comprising a mesh, as described later. First and second major surfaces 14, 20 may be parallel, or substantially parallel, for example within 5 degrees of parallel, or within 4 degrees, within 3 degrees, within 2 degrees, within 1 degree, within 0.5 degrees, or even within 0.25 degrees or less. Substrate 16 may comprise various materials, including glass types such as Pyrex, fused silica, and / or amorphous glass; dielectric substrates like FR4 (a glass-reinforced epoxy laminate), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), ZEONEX®, a cyclo olefin polymer, and / or polyimide (PI); semiconductor materials such as Si and / or GaAs; as well as hybrid substrates like Rogers printed circuit board material manufactured by the Rogers Corporation. The material used for electrically conductive layer 12 may include one or more metals (e.g., copper, gold, titanium), a transparent conductor (e.g., transparent oxide conductor), and / or graphene. The thickness T of electrically conductive layer 12 can be greater than the skin depth, 6, of electrically conductive layer 12, where 5 = (2p / o p)1 / 2and the parameters p, o, and p represent the resistivity, angular frequency, and permeability of the electrically conductive layer, respectively. If the RIS device is to be optically transparent, a substrate that exhibits the desired transparency may be selected.

[0088] Thickness T of electrically conductive layer 12 may be, for example, in a range from about 100 nanometers (nm) to about 1 micrometer (pm), for example in a range from about 200 nm to about 1 pm, in a range from about 300 nm to about 1 pm, in a range from about 400 nm to about 1 pm, in a range from about 500 nm to about 1 pm, in a range from about 600 nm to about 1 pm, in a range from about 700 nm to about 1 pm, in a range from about 800 nm to about 1 pm, or in a range from about 900 nm to about 1 pm, including all ranges and subranges therebetween. In embodiments, electrically conductive layer 12 may be a uniform layer. That is, electrically conductive layer 12 may, in some embodiments, be substantially uniform in thickness and everywhere continuous (not include gaps, holes, etc.) within the boundaries of the electrically conductive layer. As used hereinafter, electrically conductive layer 12 disposed on outward facing first major surface 14 of substrate 16, opposite ground plane 18, will be referred to as RIS element 12. A substantially uniform (e.g., continuous and contiguous, solid) RIS element 12 such as described above will be referred to as a patch. However, as described later, RIS element 12 may be in a variety of different forms that are not uniformly continuous and may be patterned (e.g., comprising a plurality of open areas).

[0089] As shown in FIGS. 2 and 3, the plurality of RIS elements 12 may be arranged on substrate 16 in a periodic manner, e.g., a periodic array. For example, FIG. 3 depicts a top view of an example RIS device 10 comprising a plurality of RIS elements 12 configured as patches and arranged as an 8 x 8 array on first major surface 14, thereby forming a square array of 64 RIS elements arranged in 64 unit cells 24. That is, an RIS element 12 is separated from an immediately adjacent RIS element by a gap G. Each unit cell comprises an RIS element and a portion of the substrate 16 surrounding the RIS element within gap G (see FIG. 1). Unit cells 24 are arranged with a periodicity P, wherein the periodicity P can be determined between the midpoint of the gap G between a selected unit cell 24 and a first adjacent unit cell to the midpoint of another gap G between the selected RIS element and a second adjacent unit cell positioned opposite the first adjacent unit cell (i.e., the selected unit cell is positioned between the first and second adjacent unit cells with no other unit cells therebetween). Accordingly, P may also be used to designate the length of a unit cell. Gap G may be uniform between the plurality of unit cells. Thus, for example, for an array of rectangular RIS elements, an RIS element selected from the array may have four adjacent RIS elements, and the gap G between each of the four adjacent RIS elements and the selected RIS element will be equal. Where the RIS element is an edge element with no adjacent RIS element on one side, the period P may be determined based on an equal distance on both sides (e.g., all sides) of the RIS element,provided the amount of substrate 16 extending past the edge RIS element is greater than i G. For example, still referring to FIG. 2, period P of the left-most unit cell is G / 2 on the right side of the RIS element + the length L of the RIS element + G / 2 to the left of the left-most RIS element. More generally, unit cells of the array have a period of Pl and P2, where, as used herein, Pl is the period of columns of the array and P2 refers to the period of rows of the array. In some embodiments, unit cells may be rectangular, where Pl does not equal P2. For example, a length of the RIS element may be greater than or less than a width of the RIS element. For a square array, such as the array depicted in FIG. 3, Pl = P2 = P, and the area of the unit cell is P2.

[0090] FIG. 4 is a top view of a single unit cell 24 having a period (length) P for the RIS device of FIG. 3 and depicts a single RIS element 12 configured as a square patch with a side length L positioned within unit cell 24. As noted above, dimensions of the unit cell 24 in the embodiment of FIGS. 3 and 4, which comprises a square RIS device comprising a square array of square RIS elements with length L, are the length L of the RIS element, plus (G / 2) x 2, where G represents the gap between adjacent RIS elements. Thus, the dimensions of this particular unit cell are G + L = P, or an area of P x P = P2.

[0091] FIG. 5 is a top view of another example unit cell 24 comprising a rectangular RIS element 12. In this more general case, for example when RIS element 12 is a rectangle with dimensions length La dn width W, where L does not equal W and Pl does not equal P2, the gap G between adjacent RIS elements may be the same as for square RIS elements. Accordingly, the area of the unit cell 24 of FIG. 4 is Pl x P2, where Pl = L + G and P2 = W + G. Rectangular RIS elements (and unit cells), where L does not equal W, may be successfully used to build an RIS device. However, the device may exhibit polarization dependence with respect to the long axis of the unit cells.

[0092] FIG. 6 is a top view of an example RIS device 10 comprising a plurality of RIS elements 12 arranged in an 8 x 8 array, wherein the RIS elements are circular patches, each RIS element 12 having a diameter equal to L of FIG. 4. FIG. 6 illustrates that the dimensional unit cell representations of FIGS. 3 and 4 may also be applied to circular RIS elements. In this instance, for circular IRS elements, G represents the shortest distance between adjacent RIS elements and the area of the unit cell 24 is still P2(P x P).

[0093] A square patch design for the RIS elements of FIGS. 3 and 4 may offer numerous desirable attributes, including polarization insensitivity, low phase sensitivity, ease of design, and low fabrication tolerances. The period P for such square unit cells may be in a range fromabout X / 5 ~ X, where X is the wavelength of the incident electromagnetic energy. To analyze electromagnetic wave properties within the unit cells and RIS elements described herein, simulations were conducted using a HFSS (High-Frequency Structural Simulator) from ANSYS. The results of such an analysis for the unit cell depicted in FIG. 4 are shown in FIG. 7, where Lo was 1.8 mm and conductor thickness was 600 nm.

[0094] In the case of polarization parallel to the y-axis shown in FIG. 4, it has been observed that the induced electric field at the resonance frequency is amplified at the edges of the two sides of the RIS element along the y-axis.

[0095] The resonance frequency of unit cell 24 is influenced by the length L of the resonant RIS element, which subsequently affects the reflected phase at a specific frequency. FIG. 7 illustrates variations in reflection loss (curve 26) and phase (curve 28) of the reflected electromagnetic energy as the length L of RIS element 12 (and therefore the dimensions of the unit cell) is changed (e.g., the unit cell of FIG. 4). This is relevant because variations in length due to manufacturing tolerances can affect the phase of the reflected electromagnetic wave. A unit cell having low phase sensitivity can be more forgiving of such manufacturing variations and therefore exhibit more predictable performance. On the other hand, selection of the length of the unit cell can be used to provide phase tuning of the unit cell. Thus, by appropriate selection of the RIS element and unit cell dimensions, a specific phase response can be achieved. Optimization of amplitude and phase can be realized, for example, through graphbased techniques.

[0096] Still referring to FIG. 7, in the case of a square patch-containing unit cell, such as a unit cell shown in FIGS. 3 and 4, the phase curve 28 exhibits a smooth, low slope, indicating low phase sensitivity resulting from low Q-factor resonance. However, as shown, the attained phase range may be limited to less than 300 degrees, which may consequently lead to a degradation in radiation performance. The observed phase behavior aligns with that of microstrip reflectarrays employing single resonance phasing elements of varying sizes.

[0097] The phase range of a single-layer microstrip patch element can be defined as 360 degrees multiplied by (1.0 — where srrepresents the relative permittivity of thesubstrate, X denotes the wavelength, and t represents the substrate thickness.

[0098] Referring now to the structure depicted in FIG. 8, the phase range is increased when utilizing a unit cell comprising an RIS element with a ring structure in contrast to unit cells comprising patch-type RIS elements comprising uniformly solid rectangular (e.g., square)structures. The width AL of the ring structure can be defined as the difference between the ring outer dimension (Lo) and the ring inner dimension (Li), with AL = (Lo - Li) / 2 in a range from about X / 10 to about X / 100. This can be attributed to the presence of multiple resonances at both the outer and inner portions of the ring (i.e., the outer perimeter of the RIS element and the inner perimeter of the RIS element). However, the modeled data presented in FIG. 9 show the phase range attained by the ring structure exceeds 360 degrees, making the ring RIS element structure satisfactory for the design of an RIS device.

[0099] Analysis further found the sensitivity of the phase increases as the width AL of the ring structure decreases. This phenomenon is due to the thinner resonators (smaller width AL) exhibiting a rapid change in phase distribution.

[0100] In some embodiments, one side of the dielectric substrate may be patterned with an electrically conducting material forming a plurality of unit cells. The opposing surface may be covered with a conducting material to form a ground plane 18. Ground plane 18, if present, may be a uniform layer, for example a deposited metal layer or a transparent metal oxide layer, or ground plane 18 may be a mesh ground plane as shown in FIG. 10. FIG. 10 depicts the bottom surface 20 of an RIS device 10 comprising a plurality of interconnecting electrical conductors 30 that form the mesh. The RIS device may operate in the RF range and function as an effective reflector in the visual frequency range (e.g., 400 nm to about 700 nm) due to the presence of conducting materials. To achieve simultaneous improvement in optical transparency and RF performance, the RIS element, and the ground plane when present, of the unit cell may comprise a mesh. For example, FIG. 11 depicts a unit cell 24 comprising a ringshaped RIS element 12 comprising a metal mesh.

[0101] The electrically conductive (e.g., metal) mesh structure of the RIS element 12 shown in FIG. 11 comprises mesh conductors 30 extending between and in electrical communication with a first innermost electrically conductive loop 34 and a second outermost electrically conductive loop 36, the RIS element mesh conductors 30 connecting the electrically conductive loops 34, 36. Mesh conductors 30, first electrically conductive loop 34, and second electrically conductive loop 36 may be thin metallic wires. As in the previous solid ring example, the ring width AL of the RIS element of FIG. 11 is defined as (Lo - Li) / 2. Pm is the period of the mesh (e.g., distance between parallel mesh conductors) and the mesh linewidth Wm (i.e., width of individual mesh conductors) may be in a range from about X / 400 to about X / 5000. For RF electromagnetic energy, a typical wavelength may be about 10 mm. Referring to FIG. 12, the modeled phase variation exhibited by the meshed ring structure of FIG. 11 is comparable tothat of the solid ring structure depicted in FIG. 8. However, the reflection loss of the meshed ring structure is greater than the reflection loss of the solid ring structure of FIG. 8. Optical transmittance Tr of the unit cell structure of FIG. 11 is defined as comprising a ratio of the mesh areas,where the mesh area refers to the area of the conductors of the mesh such that Amesh is the cumulative area occupied by the mesh conductors themselves while Atot represents the total area of the unit cell. Rgiass represents the reflectance of the glass substrate.

[0102] Transmittance Tr of the unit cell can be regulated by adjusting the parameters of the RIS element mesh, such as the mesh period Pm and / or the linewidth Wm. Due to the mesh structure having a linewidth on the order of a few hundred micrometers, it may be perceived visually as an opaque plate. That is, when a meshed ring structure is present in the unit cell, the ring structure remains visible. Although the mesh structure itself may not be distinguishable, it still affects transmittance. This can lead to blurring of the colors of objects located behind the RIS device, which may limit its applications in windows, cover glass for information technology products (e.g., electronic displays), and elements of interior building architecture.

[0103] To mitigate color distortion resulting from the RIS element, the width AL of the solid ring structure can be significantly reduced, leaving a thin conductor (e.g., wire). The fill factor, denoted as f, is defined as the ratio of the area of the RIS element structure (Amesh) to the total area of the unit cell (Atot), i.e.,When a ground plane is positioned opposite the RIS plane, in the case of a simple wire structure, the fill factor f is low, resulting in optical transmittance similar to that of a mesh ground plane alone if a ground plane is present. That is, obstruction by the wire ring is negligible and transmittance of the unit cell may be a function of the mesh ground plane characteristics. Optical transmittance is determined by the electrically conductive area, given by the expression,When the area of the unit cell structure decreases, the transmittance of the ground plane structure (e.g., ground plane mesh) decreases as well. However, wire structures can maintain high transmittance due to their low fill factor.

[0104] FIG. 13 depicts a unit cell 24 having an RIS element 12 comprising a single thin electrical loop conductor 40 (e.g., wire). As depicted, the RIS element is square, as is the unit cell 24 it comprises, although other RIS element shapes are contemplated, such as rectangular (e.g., unequal side lengths) or circular.

[0105] The presence of a narrow wire prevents the occurrence of double resonances and consequently shifts the resonance to a shorter frequency range. This leads to a more rapid phase change for the singular wire loop than for the solid ring structure, as illustrated in FIG. 14 showing modeled phase response for the solid ring (curve 42), and two additional unit cells including a singular wire conductive loop having respective fill factors of 0.2 (curve 44) and 0.4 (curve 46). The rapid phase change (steep slope) of the singular wire loop structures (curves 44, 46) resulted in a twofold increase in phase sensitivity for the singular wire loop (f = 0.2%) compared to that of the solid ring structure, as shown in FIG. 15, where the dashed line represents phase sensitivity in degrees / mm (i.e., the first derivative of the phase distribution curve). The singular wire loop structure with high sensitivity described above exhibits inherent drawbacks, including elevated loss and limited fabrication tolerance.

[0106] Accordingly, a structure that can achieve high optical transmittance compared to a wired structure while simultaneously minimizing phase sensitivity, like the ring structure, is described. The structure consists of multiple concentric, closed, electrically conductive loop structures, wherein adjacent electrically conductive loops are connected with multiple linking electrical conductors.

[0107] FIG. 16 depicts an example unit cell 24 comprising two electrically conductive closed loops, an innermost electrically conductive closed loop 102 and an outermost electrically conductive loop 104, connected by a plurality of linking conductors 106, e.g., four linking conductors 106, a linking conductor 106 connecting each of the adjacent sides of the electrically conductive loops 102, 104. However, additional linking conductors 106 may be used, such as two linking conductors per side, three linking conductors per side, or four or more linking conductors per side. The width AL of the ring formed by the wire loops and linking conductors is controlled to be the same as that of the solid ring structure of FIG. 8, with AL being the difference between the outer and inner lengths, represented by Loand Li, respectively. The induced electric field in this structure was found to exhibit a similar pattern to the solid ring structure. The electric field is induced both in the innermost electrically conductive loop 102 and the outermost electrically conductive loop 104. As shown in the modeled data of FIG. 17, the phase distribution of the proposed structure of FIG. 16 is shown to resemble that of asolid ring structure, and resonance occurs at the same frequency. The phase sensitivity of this structure is equivalent to that of a solid ring structure, indicating that the fabrication tolerance remains stable. However, the performance shown in FIG. 17 exhibits lower reflection loss compared to the solid ring structure of FIG. 8. Despite its hollowed-out configuration, the linked loop structure of the RIS element of FIG. 16 offers performance similar to a solid ring structure. Additional contemplated looped RIS element structures are shown in FIGS. 18-20. For example, FIG. 18 depicts a square RIS element comprising three concentric electrically conductive loops, a first innermost loop 102, a second outermost loop 104, and a third intermediate loop 108 positioned between first loop 102 and second loop 104, wherein all three electrically conductive loops are electrically connected by four linking conductors 106, one linking conductor for each side of the concentric square loops. In embodiments, the gap G13 between first conductive loop 102 and third conductive loop 108 may be the same as the gap G32 between third conductive loop 108 and second conductive loop 104. That is, the electrically conductive loops may be concentric and equally spaced. The structure of FIG. 18 may have more than three loop conductors. FIG. 19 illustrates a square RIS element comprising two concentric electrically conductive loops 102, 104 joined together by linking conductors 106 arranged through the comers of the concentric electrically conductive loops. The structure of FIG. 19 may have more than two loop conductors. FIG. 20 shows a circular RIS element comprising two concentric electrically conductive loops 102, 104 joined together by linking conductors 106 arranged at 0 degrees, 90 degrees, 180 degrees, and 270 degrees. That is, radially joined to the outermost and innermost loops at 90 degree offsets. The structure of FIG. 20 may have more than two loop conductors.

[0108] Increasing the number of loops in the RIS element, while keeping the loop area constant, can result in a lower reflection loss due to the increased surface area of the electrical conductors. However, the optical transmittance of the unit cell will decrease as the number of loop conductors increases. The trade-off between reflection loss and optical transmittance can be optimized when designing the unit cell.

[0109] The fill factor of the structure of FIG. 16 is similar to that of the wired ring structure of FIG. 13, as evidenced by the small difference between the transmittance of the RIS unit cell TR and the transmittance of a ground plane Tg(shown in Table 1). This indicates that TR and Tgare within 1% of each other. The hollow region between the two closed loops of the RIS element of FIG. 16 does not cause interference with the color or image displayed behind thetransparent substrate printed with the RIS unit cell structure. Therefore, the structure of FIG. 16 achieves both high optical transmittance and optimal phase sensitivity.Table 1

[0110] The double but unlinked wired ring structure having first innermost loop 102 and second outermost loop 104, without linking conductors therebetween depicted in FIG. 21 exhibits dual resonances and phase variations corresponding to each loop. These dynamics differ significantly from a ring structure (e.g., linked or meshed ring structures) with the same width. The characteristics of the structure are determined by the absence of linking conductors. Unlinked multiple loops exhibit two rapidly changing sections in phase variation, as shown in the modeled data of FIG. 22, which hinders achieving a one-to-one correspondence between the outer loop size (length Lo) and phase.

[0111] Turning to FIG. 23, in embodiments, the RIS devices disclosed herein may include an active layer. FIG. 23 shows an example RIS device 200 comprising an RIS element 12, a substrate 16, and a ground plane 18. Substrate 16 comprises a first glass sheet 202, a second glass sheet 204, and a liquid crystal layer 206 that acts as the active layer disposed between first glass sheet 202 and second glass sheet 204. Liquid crystal layer 206 can be connected with a controller configured to control the polarization state of the liquid crystal layer, thereby altering a permittivity of substrate 16. Altering the permeability of substrate 16 can cause a variation in the reflectivity of RIS device 200 and the phase of the reflected RF wave. This variation in properties can be used to direct (e.g., steer) the reflected electromagnetic wave dynamically. RIS device 200 may include any of the RIS elements described herein.

[0112] Examples

[0113] Example 1

[0114] In a first example, several different RIS element designs were evaluated for reflection loss and phase dependence on fill factor. Each design comprised a unit cell with a period P of 2.85 mm, a mesh period Pm of 175 micrometers, and a conductor thickness of 600 nanometers. Each design was disposed in a unit cell with a mesh ground plane (Gnd) having a mesh conductor linewidth Wm of 20 micrometers. In a first instance (#1), a single thin conductiveloop was modeled (various parameters for the different RIS designs are presented in Table 2). In a second instance (#2), the same single conductive loop was modeled, however in the second instance, the linewidth Wm of the conductive loop was increased. In a third instance (#3), a linked ring comprising two linked loops, as illustrated in FIG. 16, was modeled. In a fourth instance (#4), a single mesh ring as shown in FIG. 11 was modeled. In a fifth instance (#5), a second single mesh ring was modeled, wherein the linewidth Wm of the conductive loop and mesh conductors was increased (e.g., doubled). In a sixth instance (#6), another single mesh ring was modeled, wherein the linewidth Wm of the loop and mesh conductors was further increased. Finally, in a seventh instance (#7), a solid ring, such as depicted in FIG. 8, was modeled. The data show that as the linewidth Wm of the RIS element conductors increases, the fill factor of the unit cell increases and the transmittance of the unit cell decreases. Plots of the modeled reflection loss and phase for these seven instances (#l-#7) are shown in FIGS. 24- 25 on the basis of fill factor, respectively. FIG. 26 depicts reflection loss as a function of fill factor in percent from FIG. 24.Table 2

[0115] Example 2

[0116] To determine the effect of increasing the number of loops in a linked ring configuration, a first structure having two square conductive loops linked by four linking conductors, one linking conductor positioned at each side of the square ring and formed as illustrated in FIG. 16, was compared to a second similar structure having three conductive loops and four linking conductors electrically linked all three conductive loops (as illustrated in FIG. 18), were modeled and compared. A plot of reflection loss (curve 300) and phase (curve 302) for the first structure (FIG. 16) are shown in FIG 27 while a plot of reflection loss (curve 304) and phase (curve 306) for the second structure (FIG. 18) are shown in FIG. 28. The physical parameters of the conductors in the two rings were maintained the same between the two structures. That is, the widths of both linked ring structures were the same with equal outerlengths and equal inner lengths (AL = (Lo - Li) / 2 = 0.8 mm), Lo was 0.9 - 4 mm, and the period P was 5 mm. The data show that both structures exhibit essentially the same resonance at an outer length (Lo) in a range from about 1.8 mm to 2.1 mm, and, while reflection loss varied somewhat between the two (with the three-loop structure exhibiting less reflection loss due to an increase in metal surface area), the phase response of the two structures was virtually identical because of the same ring area, as illustrated in FIG. 29, where curve 308 (solid line) represents the phase of the first structure and curve 310 (dashed line) represents the phase of the second structure.

[0117] Example 3

[0118] In a third example, RIS elements comprising a linked ring structure having two electrically conductive loops are shown in FIGS. 30 - 33. FIG. 30 depicts modeled reflection loss (curve 312) and phase (curve 314) for a linked ring structure shown in FIG. 31 having two electrically conductive loops and two linking conductors, the two linking conductors linking the outer conductive loop 102 to the inner conductive loop 104 but arranged orthogonal to each other. FIG. 32 depicts modeled reflection loss (curve 316) and phase (curve 318) for a linked ring structure shown in FIG. 33 having two electrically conductive loops and two linking conductors, the two linking conductors linking the outer conductive loop to the inner conductive loop, but arranged parallel to each other, in this case opposing each other. FIG. 34 depicts modeled reflection loss (curve 320) and phase (curve 322) for a linked ring structure shown in FIG. 35 having two electrically conductive loops and three linking conductors 106, the three linking conductors 106 linking the outer conductive loop to the inner conductive loop, with two linking conductors arranged in parallel (e.g., opposite each other), and a third linking conductor 106 orthogonal to the first two parallel linking conductors. The data show that when two linking conductors are perpendicular to each other, the phase distribution includes a region near resonance where the phase exhibits a low slope and a phase angle that exceeds more than 300 degrees, indicating usefulness to serve as a RIS device. Conversely, in structures with parallel linking conductors, the phase distribution resembles that of a non-linked double conductor structure, like that of FIG. 21, due to its symmetry.

[0119] Example 4

[0120] In a fourth example, modeling results for an RIS element depicted in FIG. 35 are shown in the plots of FIGS. 36 and 37, wherein the physical parameters for the second case are scaled up relative to the physical parameters of the first case. In FIG. 36, reflection loss is represented by curve 324 and phase is represented by curve 326. In the first case (FIG. 36) theimpinging wavelength was 10.7 mm, the period P of the unit cell was 5 mm, the width AL of the ring was 0.8 mm, Lo was evaluated over a length from 0.9 mm to 4 mm, linewidth Wm was 20 micrometers, and the thickness of the conductors (both loop and link) was 600 nm. In the second case (FIG. 37), where reflection loss is represented by curve 328 and phase is represented by curve 330, the wavelength was 83.3 mm, the period was 28 mm, the ring width AL was 12 mm, Lo was evaluated over a length from 12 mm to 20 mm, Wm was 400 micrometers, and the thickness of the conductors was 2000 nanometers. The data show that the phase responses of the two cases are similar, indicating scalability of the RIS element configuration.

[0121] Example 5

[0122] In a fifth example, two circular RIS elements were evaluated. The first circular RIS element, shown in FIG. 38, had three concentric circular loops, innermost electrically conductive loop 102, outermost electrically conductive loop 104, and an intermediate electrically conductive loop 108 therebetween. Four linking conductors 106 spaced apart by 90 degrees linked the outermost conductive loop to the middle conductive loop and the innermost conductive loop. Here, the length Lo is equivalent to the diameter of the outermost electrically conductive loop 104. In the second instance, shown in FIG. 39, intermediate electrically conductive loop 108 was linked to innermost electrically conductive loop 102 by four linking conductors 106 arranged orthogonal to each other (i.e., radially spaced apart by 90 degrees). That is, the four outer linking conductors 106 linking outermost conductive loop 104 with intermediate electrically conductive loop 108, were also arranged 90 degrees apart but offset from the four inner linking conductors 106 by 45 degrees. Modeled data, shown in FIGS. 40-41, respectively, show no difference in the behavior of the two structures, showing that reflection loss and phase characteristics are unaffected by the angle of the linking conductors. In FIG. 40, curve 328 represents reflection loss (in dB) and curve 330 represents phase (in degrees), both as a function of length Lo (equivalent to the diameter of the outermost loop) in millimeters. In FIG. 41, curve 332 represents reflection loss (in dB) and curve 334 represents phase (in degrees), both as a function of length Lo in millimeters. FIG. 42 shows the two phase curves on the same plot, again as a function of length Lo in millimeters of the RIS element.

[0123] Example 6

[0124] In a sixth example, four different RIS elements (FIGS. 43-46) comprising three concentric circular conductive loops were connected by a varying number of linking conductors 106. In the first case (FIG. 43), three concentric electrically conductive loops, innerelectrically conductive loop 102, outermost electrically conductive loop 104, and intermediate electrically conductive loop 108 were joined by a single linking conductor 106. In the second case (FIG. 44), the three concentric electrically conductive loops 102, 104, and 108 were joined by two parallel linking conductors 106. In the third case (FIG. 45), the three concentric electrically conductive loops 102, 104, 108 were joined by two orthogonal linking conductors 106. And in the fourth case (FIG. 46), the three concentric electrically conductive loops 102, 104, 108 were joined by three linking conductors 106 arranged along radii 120 degrees from each other. Reflection loss and phase for the four RIS structures of FIGS. 43-46 are shown in FIGS. 47-50, respectively. The data show that at least two linking conductors are needed to obtain an acceptable phase distribution for an RIS element. That data further show that while the phase distribution remained similar between the second, third, and fourth cases, indicating the phase was not dependent on the angle of the links, the number of links did affect the reflection loss. To wit, the greater the number of links, the lower the reflection loss.

[0125] In another embodiment shown in FIG. 51, an RIS element 400 is disclosed similar to the linked ring structure of FIG. 18 and having a first, innermost ring 402, a second, outermost ring 404, a third intermediate ring 406 positioned between first ring 402 and second ring 404, and a plurality of linking electrically conductive elements 408, but wherein individual conductive components (e.g., electrically conductive rings and linking conductors) of the RIS element are comprised of a mesh. For example, each ring 402, 404, 406 comprises an innermost electrically conductive loop and an outermost electrically conductive loop, with an electrically conductive mesh extending therebetween. Rings of the RIS element are electrically connected by similarly constructed linking conductors. Each conductive element had a width W2 (in this instance all element widths W2 are the same, but such consistency is not required). RIS element 400 comprises an overall width AL determined between the innermost length Li of the innermost ring 402 and the outermost length Lo of the outermost ring 404. First RIS element 400 was compared to a similarly constructed second RIS element 500, shown in FIG. 51, with the exception that the conductive elements of the second structure of FIG. 52, i.e., innermost ring 502, outermost ring 504, intermediate ring 506, and linking conductors 508, were solid instead of mesh. Modeled physical parameters of the two RIS elements 400, 500 were made at a wavelength of 83.3 mm, a unit cell period P of 28 mm, AL was 14 mm, Lo was 16.8 mm, mesh period Pm (for RIS element 400) was 20 micrometers, linewidth Wm (for RIS element 400) was 175 micrometers, and the thickness of the conductive elements was 2000 nm. Table 3 discloses the fill factor in percent, and the transmittance of the unit cells as apercent, for the two structures of FIGS. 51 and 52. The data show that by forming the electrically conductive components of RIS element 400 of a mesh, transmittance of a unit cell comprising the same can be improved compared to a construction (RIS element 500) utilizing solid, non-mesh conductors.Table 3

[0126] RIS devices described here may not be homogeneous in the sense that each and every RIS element contained therein is identical. For example, RIS devices may include different RIS elements comprising different numbers of electrically conductive loops, different numbers of linking conductors, different outer lengths (Lo), different inner lengths (Li), and / or different orientations of linking conductors. RIS devices disclosed herein may include different RIS element shapes, for example a mix of rectangular (e.g., square) and / or circular RIS elements 12. For example, FIG. 53 depicts an RIS device 600 comprising a first plurality of two electrically conductive loop RIS elements with four linking conductors and a second plurality of three electrically conductive loop RIS elements with four linking conductors. Other combinations of RIS elements and their characteristic constructions are contemplated.

[0127] It will be apparent to those skilled in the art that various modifications and variations can be made to embodiments of the present disclosure without departing from the spirit and scope of the disclosure. Thus, it is intended that the present disclosure cover such modifications and variations provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A reconfigurable intelligent surface device, comprising: a substrate comprising a first major surface and a second major surface opposite the first major surface, the first major surface comprising a plurality of RIS elements, each RIS element of the plurality of RIS elements comprising a plurality of concentric electrically conductive closed loops, each electrically conductive closed loop of the plurality of concentric electrically conductive closed loops separated from an adjacent electrically conductive closed loop and electrically connected thereto by a plurality of linking electrical conductors; and wherein a transmittance of the reconfigurable intelligent surface device over a wavelength range of about 400 nm to about 700 nanometers may be equal to or greater than about 50%.

2. The reconfigurable intelligent surface device of claim 1, wherein the plurality of concentric electrically conductive closed loops comprise rectangular electrically conductive closed loops.

3. The reconfigurable intelligent surface device of claim 1, wherein the second major surface comprises a ground plane.

4. The reconfigurable intelligent surface device of claim 1, wherein the ground plane of the third aspect comprises a metallic mesh.

5. The reconfigurable intelligent surface device of claim 1, wherein the plurality of electrically conductive concentric closed loops may comprise circular electrically conductive closed loops.

6. The reconfigurable intelligent surface device of claim 5, wherein the plurality of linking electrical conductors are evenly angularly spaced between adjacent electrically conductive closed loops.

7. The reconfigurable intelligent surface device of claim 1, wherein the plurality of linking electrical conductors may comprise a mesh.

8. The reconfigurable intelligent surface device of claim 1, wherein the substrate may comprise a dielectric material.

9. The reconfigurable intelligent surface device of claim 1, wherein the substrate may comprise a glass substrate.

10. The reconfigurable intelligent surface device of claim 1, wherein the substrate may comprise a pair of glass substrates and a liquid crystal material disposed therebetween.

11. The reconfigurable intelligent surface device of claim 1, wherein the substrate may comprise a semiconductor material.

12. The reconfigurable intelligent surface device of claim 1, wherein the reconfigurable intelligent surface device comprises a first plurality of RIS elements having a first structure and a second plurality of RIS elements having a second structure different from the first structure.

13. The reconfigurable intelligent surface device of claim 1, wherein the plurality of RIS elements may be periodically arranged.

14. The reconfigurable intelligent surface device of claim 1, wherein a fill factor of each unit cell, defined as the ratio of the area of the RIS element to the area of the unit cell, is less than 10%.

15. The reconfigurable intelligent surface device of claim 1, wherein at least two linking electrical conductors of the plurality of linking electrical conductors are orthogonal.

16. The reconfigurable intelligent surface device of claim 2, the RIS element comprises at least four linking electrical conductors, the at least four linking electrical conductors comprising a pair of parallel linking electrical conductors and a pair of orthogonal linking electrical conductors.

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