Reflectarray

Abstract

This reflectarray comprises: a plurality of liquid crystal reflection plates each having a reflection region in which reflection elements that are obtained by sandwiching a liquid crystal layer between a pair of electrodes are arranged; and a dielectric substrate disposed on a radio wave incidence surface side of the plurality of liquid crystal reflection plates. The plurality of liquid crystal reflection plates are disposed side-by-side so as to be adjacent to each other. The dielectric substrate has a size so as to cover all of the plurality of liquid crystal reflection plates in plan view.

Description

Reflectarray 【0001】 One embodiment of the present invention relates to the structure of a reflectarray that reflects radio waves (electromagnetic waves) in a specific direction. 【0002】 In the fifth-generation communication system (5G), millimeter-wave electromagnetic waves are used. However, since electromagnetic waves have higher directivity as the frequency increases, dead zones where communication is impossible are likely to occur in the shadows of buildings and the like. Although this problem can be solved by increasing the number of base stations, it is not practical considering costs and securing installation locations. Therefore, a reflectarray that can asymmetrically reflect electromagnetic waves (radio waves) of a specific frequency has been proposed. For example, a reflectarray is disclosed in which a reflective element having a liquid crystal layer sandwiched between a pair of electrodes is used, and these reflective elements are arranged in a matrix to form a radio wave reflection surface (see Patent Document 1). 【0003】 International Publication No. 2023 / 058399 【0004】 In the reflectarray disclosed in Patent Document 1, the thickness of the dielectric substrate arranged on the incident surface side of the radio wave is adjusted to be one-fourth of the effective wavelength of the radio wave so that the reflected wave does not attenuate. The thickness of the dielectric substrate can be adjusted by adding an additional dielectric substrate in addition to the glass substrate sandwiching the liquid crystal layer. However, when arranging a plurality of liquid crystal reflector plates side by side (tiling) for the enlargement of the reflectarray, if the accuracy of bonding the dielectric substrates is low, the individual liquid crystal reflector plates cannot be arranged with high precision, and the reflection characteristics of electromagnetic waves deteriorate, which becomes a problem. 【0005】 The reflectarray according to one embodiment of the present invention includes a plurality of liquid crystal reflector plates having a reflection region in which reflective elements having a liquid crystal layer sandwiched between a pair of electrodes are arranged, and a dielectric substrate arranged on the radio wave incident surface side of the plurality of liquid crystal reflector plates. The plurality of liquid crystal reflector plates are arranged adjacent to each other side by side, and the dielectric substrate has a size that covers all of the plurality of liquid crystal reflector plates in a plan view. 【0006】This is a plan view showing the configuration of a reflect array according to one embodiment of the present invention. It shows a cross-sectional view of the reflect array corresponding to the section A-B shown in Figure 1A. This is a plan view showing the configuration of a liquid crystal reflector constituting a reflect array according to one embodiment of the present invention. It shows a cross-sectional view of the liquid crystal reflector corresponding to the section C-D shown in Figure 2A. This is a cross-sectional view of a reflective element constituting a reflect array according to one embodiment of the present invention. 【0007】 Embodiments of the present invention will be described below with reference to the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described below. In order to make the explanation clearer, the drawings may schematically represent the width, thickness, shape, etc. of each part compared to the actual embodiment, but these are merely examples and do not limit the interpretation of the present invention. In addition, in this specification and each drawing, elements similar to those described above with respect to previously shown drawings are denoted by the same reference numerals (or numerals followed by A, B, etc.), and detailed explanations may be omitted as appropriate. Furthermore, the letters "First," "Second," etc., attached to each element are convenient indicators used to distinguish each element and have no further meaning unless specifically explained. 【0008】 In this specification, when a member or region is said to be "above (or below)" another member or region, unless otherwise specified, this includes not only cases where it is directly above (or directly below) the other member or region, but also cases where it is above (or below) the other member or region, that is, cases where another component is included between them above (or below) the other member or region. 【0009】The reflect arrays disclosed herein may also be called IRS (Intelligent Reflecting Surface) or RIS (Reconfigurable Intelligent Surface). An IRS or RIS is a thin, planar structure that electronically controls the reflection characteristics (at least phase, and possibly amplitude and polarization) of a plurality of reconfigurable passive (or quasi-passive) elements arranged on a surface to reconfigure the propagation environment of wireless communication, thereby forming a desired direction and wavefront of reflected (or scattered) waves relative to incident radio waves. 【0010】 Figures 1A and 1B show the structure of a reflect array 100 according to one embodiment of the present invention. Figure 1A is a plan view of the reflect array 100 as seen from the radio wave incident surface side, and Figure 1B is a cross-sectional view corresponding to the section A-B shown in the plan view. 【0011】 The reflect array 100 includes a plurality of liquid crystal reflectors 102. The plurality of liquid crystal reflectors 102 are arranged in a tile-like pattern. Figure 1A shows an example of the reflect array 100, in which a first liquid crystal reflector 102A, a second liquid crystal reflector 102B, a third liquid crystal reflector 102C, and a fourth liquid crystal reflector 102D are arranged in a row. In the reflect array 100, these liquid crystal reflectors 102 form a reflective surface together and function as a reflector against radio waves. 【0012】 Let us focus on one liquid crystal reflector 102 that constitutes the reflect array 100. The liquid crystal reflector 102 has a reflective region 110 within a flat surface. The reflective region 110 is formed by a plurality of reflective elements 112. The plurality of reflective elements 112 are arranged in a matrix at a predetermined pitch. The reflective element 112 is composed of a first substrate 1121 on which a first electrode 1123 is provided, a second substrate 1122 on which a second electrode 1124 is provided, and a liquid crystal layer LC1 between the first substrate 1121 and the second substrate 1122. Here, the first substrate 1121 is arranged on the side facing the incident surface of the radio waves, and the second substrate 1122 is arranged on the side opposite to the incident surface of the radio waves. 【0013】The liquid crystal reflector 102 includes a peripheral region 114 that surrounds the reflective region 110. The peripheral region 114 can also be described as a region where no reflective elements 112 are placed. Ideally, the entire surface of the flat surface of the liquid crystal reflector 102 should be a reflective region. However, since the reflective element 112 has a structure in which a liquid crystal layer LC1 is sandwiched between a first electrode 1123 on the first substrate 1121 side and a second electrode 1124 on the second substrate 1122 side (see Figure 1B), there is a region along the edge of the substrate where it is not possible to form a reflective element 112. Therefore, the liquid crystal reflector 102 inevitably includes a peripheral region 114. 【0014】 The dimensions of the first electrode 1123 constituting the reflective element 112 are designed taking into consideration the frequency (wavelength) of the radio waves to be reflected. For example, if the radio wave frequency is in the 28 GHz band, the dimensions of one side of the first electrode 1123 can be set to approximately 2 mm to 4 mm. 【0015】 The liquid crystal reflector 102 also includes a sealing material for bonding the first substrate 1121 and the second substrate 1122, a spacer for maintaining the distance between the first substrate 1121 and the second substrate 1122, and an alignment film for controlling the initial orientation state of the liquid crystal, but these are omitted in Figures 1A and 1B. 【0016】 The liquid crystal reflector 102 further has a structure in which a dielectric substrate 104 is added. The dielectric substrate 104 is placed on the side where the radio waves are incident. That is, the dielectric substrate 104 is placed on top of the first substrate 1121. The dielectric substrate 104 is large enough to cover all of the arranged liquid crystal reflectors 102. Figure 1A shows a structure in which the dielectric substrate 104 overlaps all of the first liquid crystal reflector 102A, the second liquid crystal reflector 102B, the third liquid crystal reflector 102C, and the fourth liquid crystal reflector 102D. The dielectric substrate 104 is not simply placed on top, but is provided fixed on top of the liquid crystal reflectors 102 with an adhesive or the like. 【0017】As shown in Figures 1A and 1B, when the liquid crystal reflectors 102 are arranged adjacent to each other, the area of ​​the dielectric substrate 104 in a plan view is the same as, or larger than, the area of ​​the tiled arrangement of the liquid crystal reflectors 102. In other words, the multiple liquid crystal reflectors 102 are arranged within the plane of the dielectric substrate 104 in a plan view. Based on the configuration shown in Figure 1A, the first liquid crystal reflector 102A, the second liquid crystal reflector 102B, the third liquid crystal reflector 102C, and the fourth liquid crystal reflector 102D are arranged within the plane of the dielectric substrate 104. To put it another way, the first liquid crystal reflector 102A, the second liquid crystal reflector 102B, the third liquid crystal reflector 102C, and the fourth liquid crystal reflector 102D are arranged densely without gaps, and the dielectric substrate 104 extends to cover not only the reflective region 110 of each liquid crystal reflector but also the entire peripheral region 114. 【0018】 The reflect array 100 according to this embodiment has a structure in which a dielectric substrate 104 covers an array of multiple liquid crystal reflectors 102. However, in order to make the boundaries less noticeable when the liquid crystal reflectors 102 are tiled, a transparent resin material may be filled into the boundary portions. 【0019】 As shown in the inset of Figure 1A, when the first liquid crystal reflector 102A and the second liquid crystal reflector 102B are placed side by side, the first peripheral region 114A and the second peripheral region 114B are adjacent. Reflective elements 112 are arranged in the first reflective region 110A and the second reflective region 110B at a predetermined pitch P1. For example, the pitch P1 can be about 3 mm to 4 mm when the first electrode 1123 has the aforementioned size. 【0020】As mentioned above, the reflect array 100 functions as a single unit comprised of multiple liquid crystal reflectors 102. Therefore, when the first liquid crystal reflector 102A and the second liquid crystal reflector 102B are placed side by side, it is preferable that the reflective elements 112 are arranged at the same pitch from the first reflective region 110A to the second reflective region 110B. Of course, reflective elements cannot be arranged in the first peripheral region 114A and the second peripheral region 114B, but in order to avoid degrading the reflective characteristics of the reflect array 100, it is preferable that the liquid crystal reflectors 102 are tiled so as not to disrupt the periodicity of the arrangement of the reflective elements 112. 【0021】 Furthermore, as shown in the inset of Figure 1(A), when the first liquid crystal reflector 102A and the second liquid crystal reflector 102B are adjacent, peripheral regions 114A and 114B are adjacent. In this case, deterioration of the reflection characteristics can be prevented by making the width of each peripheral region 114A and 114B an integer multiple of half the wavelength λ of the radio wave to be reflected. 【0022】 As shown in Figure 1B, the liquid crystal reflector 102 may be supported by a support member 118. In other words, the liquid crystal reflector 102 may be tiled on the support member 118. There are no limitations on the structure and material of the support member 118. The support member 118 may be a flat plate or it may be composed of a frame. 【0023】 In this embodiment, the reflect array 100 has a dielectric substrate 104 that extends throughout, so when tiling the liquid crystal reflector 102, the dielectric substrate 104 may also function as a support member. That is, the support member 118 shown in Figure 1B may be omitted. 【0024】Furthermore, a metal layer 116 may be provided on the back surface of the liquid crystal reflector 102. Preferably, the metal layer 116 is large enough to cover the entire surface of the support member 118. The metal layer 116 may be formed from a metal plate or from a metal film deposited on a glass substrate or the like. Preferably, the metal layer 116 is grounded. In such a configuration, the metal layer 116 can be considered as a grounding conductor plate within the reflect array 100. By providing the metal layer 116, the attenuation of reflected waves can be suppressed. 【0025】 The details of the liquid crystal reflector 102 will be explained with reference to Figures 2A and 2B. Figure 2A shows a plan view of the liquid crystal reflector 102 as seen from the radio wave incident surface side. Figure 2B shows a cross-sectional view of the liquid crystal reflector 102 corresponding to the section C-D shown in Figure 2A. 【0026】 As shown in Figure 2A, the liquid crystal reflector 102 has a reflective region 110 on which reflective elements 112 are arranged, and a peripheral region 114 outside the reflective region 110. The reflective element 112 includes a first electrode 1123 arranged on the side of the first substrate 1121, a second electrode 1124 arranged on the side of the second substrate 1122, a liquid crystal layer LC1 between the first electrode 1123 and the second electrode 1124, a first alignment film AL1 covering the first electrode 1123, and a second alignment film AL2 covering the second electrode 1124. The peripheral region 114 is provided with a sealing material SEA that bonds the first substrate 1121 and the second substrate 1122 and encapsulates the liquid crystal layer LC1. The sealing material SEA is provided in the peripheral region 114 so as to surround the reflective region 110. Since the liquid crystal layer LC1 is not provided in the portion of the sealing material SEA, this becomes a region where reflective elements 112 are not formed. 【0027】 In the liquid crystal layer LC1, the initial orientation state of the liquid crystal molecules (when no voltage is applied) is controlled by the first orientation film AL1 and the second orientation film AL2, and the orientation state of the liquid crystal molecules changes due to the electric field generated between the first electrode 1123 and the second electrode 1124. As the orientation state of the liquid crystal molecules changes, the dielectric constant of the liquid crystal layer LC1 changes. The reflecting element 112 has the function of controlling the phase of the reflected radio waves by utilizing this dielectric anisotropy of the liquid crystal layer LC1. 【0028】Figures 2A and 2B show a structure in which the first electrode 1123 is interconnected by strip wiring SL1, and the second electrode 1124 is individually connected to a switching element SWT. In other words, the reflective element 112 has a configuration in which the first electrode 1123 is fixed at a constant potential, and a signal that controls the orientation state of liquid crystal molecules (liquid crystal control signal) is applied to the second electrode 1124 via the switching element SWT. The second electrode 1124 can also be called a liquid crystal control electrode. The voltage applied to the second electrode 1124 may be a DC voltage, or it may be an AC signal that can periodically reverse the orientation direction of the liquid crystal. The control signal applied to the second electrode 1124 is a DC voltage signal, or a polarity reversal signal in which a positive DC voltage and a negative DC voltage alternately reverse. The potential of the first electrode 1123 may be ground or an intermediate potential between the polarity reversal signal. 【0029】 The switching element SWT is controlled by drive circuits 1125A and 1125B provided in the peripheral region 114. Drive circuit 1125A is a circuit that outputs a signal to select the switching element SWT, and drive circuit 1125B is a circuit that outputs a control signal to the switching element SWT for driving the liquid crystal layer LC1. 【0030】 In this embodiment, the first electrode 1123, which is positioned on the radio wave incident surface side, is held at a constant potential, and a control signal for driving the liquid crystal layer LC1 is applied to the second electrode 1124. However, the arrangement of these two electrodes may be reversed. That is, the second electrode 1124 for driving the liquid crystal layer LC1 may be provided on the radio wave incident surface side (first substrate 1121), and the first electrode 1123 may be provided on the second substrate 1122 side. 【0031】There are no limitations on the materials of the components constituting the liquid crystal reflector 102, but glass substrates are used for the first substrate 1121 and the second substrate 1122. The glass substrates used are made of alkali-free glass such as aluminosilicate glass and aluminoborosilicate glass. The dielectric substrate 104 is made of the same glass as the glass substrate. Alternatively, resin substrates such as sapphire substrates, ceramic substrates, acrylic resin, epoxy resin, and polyimide resin can be used as the dielectric substrate 104. The first electrode 1123 and the second electrode 1124 are made of metallic materials such as aluminum (Al), copper (Cu), titanium (Ti), and molybdenum (Mo). The first electrode 1123 and the second electrode 1124 may have a titanium (Ti) / aluminum (Al) / titanium (Ti) laminated structure or a molybdenum (Mo) / aluminum (Al) / molybdenum (Mo) laminated structure. Furthermore, the first electrode 1123 and the second electrode 1124 may be formed from a transparent conductive film such as indium tin oxide or zinc oxide. As the liquid crystal layer LC1, nematic liquid crystal, smectic liquid crystal, cholesteric liquid crystal, or discotic liquid crystal can be used. 【0032】 Figure 3 shows a cross-sectional view of one reflective element 112. As shown in Figure 3, the reflective element 112 has a structure in which a dielectric substrate 104, a first substrate 1121, a first electrode 1123, a first alignment film AL1, a liquid crystal layer LC1, a second alignment film AL2, a second electrode 1124, and a second substrate 1122 are stacked on top of each other from the radio wave incident surface side. Within the reflective element 112, the dielectric substrate 104 and the first substrate 1121 can also be considered as a single dielectric layer. 【0033】The first electrode 1123 preferably has a shape that is symmetrical with respect to the vertical and horizontal polarization of the incident radio waves, and can be, for example, a square or circular shape in a plan view. There are no particular limitations on the shape of the second electrode 1124, but it is preferable that it has a larger area than the first electrode 1123 in a plan view. The distance between the first substrate 1121 and the second substrate 1122 is 20 to 100 μm, for example, 50 μm. The first electrode 1123, the second electrode 1124, the first alignment film AL1, and the second alignment film AL2 are interposed between the first substrate 1121 and the second substrate 1122, but the film thickness of these components is 1 μm or less, which is sufficiently small compared to the substrate distance, so the distance between the first substrate 1121 and the second substrate 1122 can be considered substantially as the thickness of the liquid crystal layer LC1. 【0034】 The reflecting element 112 can change the dielectric constant of the liquid crystal layer LC1 by a control signal applied to the second electrode 1124, thereby changing (delaying) the phase of the reflected radio waves. The frequency bands of the radio waves reflected by the reflecting element 112 are the very high frequency (VHF), ultra-high frequency (UHF), super high frequency (SHF), submillimeter wave (THF), and extra high frequency (EHF) bands. The liquid crystal molecules in the liquid crystal layer LC1 change their orientation in response to the control signal applied to the second electrode 1124, but they hardly follow the frequency of the irradiated radio waves. Therefore, the reflecting element 112 can change the dielectric constant of the liquid crystal layer LC1 without being affected by the radio waves, and control the phase of the reflected radio waves. 【0035】 In Figure 3, when T is the thickness from the surface of the first electrode 1123 to the upper surface of the dielectric substrate 104, and λ is the wavelength of the radio wave, the amplitude of the reflected wave can be increased if the following equation (1) is satisfied (see Patent Document 1: International Publication No. 2023 / 058399). However, λ / 4=(c / f) / ε ０．５The formula is √4, where c is the speed of light, f is the frequency of the radio wave, and ε is the relative permittivity of the dielectric substrate 104. Note that the thickness of the first electrode 1123 is 1 μm or less, which is extremely small compared to the thickness of the first substrate 1121 and the dielectric substrate 104; therefore, the thickness T can be considered to be the thickness of the first substrate 1121 and the dielectric substrate 104. 【0036】 The thickness T shown in equation (1) is the effective thickness with respect to the radio waves incident on the reflector element 112. Therefore, if the frequency of the radio waves is different, the magnitude of the thickness T will also be different. The reflect array 100 is designed with the size of the reflector elements 112 and the pitch of the array to match the frequency of the target radio waves. However, even if the thickness of the first substrate 1121 is constant, the thickness T can be adjusted by changing the thickness of the dielectric substrate 104, thereby optimizing the reflection characteristics. 【0037】 In this case, by making the size of the dielectric substrate 104 in plan view the same as or larger than the overall size when the multiple liquid crystal reflectors 102 are arranged side by side, the liquid crystal reflectors 102 can be precisely arranged and no gaps can be left. In other words, by positioning the peripheral edge of the dielectric substrate 104 outside the peripheral edge of the multiple liquid crystal reflectors 102, the liquid crystal reflectors 102 can be tiled without disturbing the pitch in which the reflectors 112 are arranged. As a result, the reflective characteristics of the reflect array 100 can be kept good. 【0038】 The various configurations of the reflect array exemplified as one embodiment of the present invention can be combined as appropriate, as long as they do not contradict each other. Furthermore, reflect arrays based on those disclosed herein and in the drawings, with additions, deletions, or design modifications of components, or additions, omissions, or changes in processes, as modified by those skilled in the art, are also included within the scope of the present invention, as long as they retain the essence of the present invention. 【0039】 Any effects or benefits other than those brought about by the embodiments disclosed herein are to be understood to be brought about by the present invention if they are clear from the description herein or can be easily predicted by a person skilled in the art. 【0040】 100: Reflect array, 102: Liquid crystal reflector, 104: Dielectric substrate, 110: Reflection region, 112: Reflecting element, 1121: First substrate, 1122: Second substrate, 1123: First electrode, 1124: Second electrode, 1125A, 1125B: Driving circuit, 114: Peripheral region, 116: Metal layer, 118: Support member, AL1, AL2: Alignment film, LC1: Liquid crystal layer, SL1: Strip wiring, SEA: Sealing material, SWT: Switching element

Claims

1. A reflect array comprising: a plurality of liquid crystal reflectors having a reflective region in which reflective elements, each having a liquid crystal layer sandwiched between a pair of electrodes, are arranged; and a dielectric substrate disposed on the radio wave incident surface side of the plurality of liquid crystal reflectors, wherein the plurality of liquid crystal reflectors are arranged adjacent to each other, and the dielectric substrate is sized to cover all of the plurality of liquid crystal reflectors in a plan view.

2. The reflect array according to claim 1, wherein the dielectric substrate has a larger area in a plan view than the area where the plurality of liquid crystal reflectors are arranged adjacent to each other.

3. The reflect array according to claim 1, wherein each of the plurality of liquid crystal reflectors has a peripheral region surrounding the reflective region, and the dielectric substrate covers the peripheral region of each of the plurality of liquid crystal reflectors.

4. The reflect array according to claim 1, wherein the plurality of liquid crystal reflectors have a first substrate disposed on the side of the incident surface of radio waves and having insulating properties, and a second substrate disposed on the opposite side of the incident surface of radio waves and facing the first substrate, and the reflecting element includes a first electrode provided on the first substrate and a second electrode provided on the second substrate facing the first electrode, and the liquid crystal layer is disposed between the first electrode and the second electrode.

5. The reflect array according to claim 4, wherein the combined thickness of the first substrate and the dielectric substrate is one-quarter the wavelength of the target radio wave.

6. The reflect array according to claim 3, wherein the width of the peripheral region is an integer multiple of half the wavelength λ of the radio wave to be reflected.

7. The reflect array according to claim 4, wherein the first substrate and the dielectric substrate are glass substrates.

8. The reflect array according to claim 1, wherein the reflect array has a metal layer disposed on the back side of the plurality of liquid crystal reflectors and extending over the entire surface of the plurality of liquid crystal reflectors.

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