Polarization Selective Device

Abstract

A polarization selective device (10) of this invention is a polarization selective device that receives an incident electromagnetic wave and emits an electromagnetic wave, and includes a plurality of metamaterial elements (13), wherein among the plurality of metamaterial elements, at least two metamaterial elements are arranged in parallel to each other, each of the two metamaterial elements includes gaps (132), a shape of each of the two metamaterial elements has reflection symmetry, one of the two metamaterial elements includes the gaps in one direction, and the other of the two metamaterial elements includes the gaps in another direction vertical to the one direction. This invention can thus provide a polarization selective device capable of dynamically controlling selection of a polarized wave of an electromagnetic wave.

Description

Polarization Selective Device

[0001] The present invention relates to a polarization selective device using a metamaterial.In a wireless communication system, communication in a wide band at a high data rate has been increasingly required. To cope with this, the frequency multiplexing technology and the polarization multiplexing technology have been adopted.In the frequency multiplexing technology, there is disclosed a frequency selective surface (FSS) device (non-patent literature 1). The FSS device efficiently transmits, reflects, or absorbs an electromagnetic wave based on the frequency of an electric field.Furthermore, in the polarization multiplexing technology, there is disclosed a polarization-selective surface (PSS) device (non-patent literature 2). The PSS device separates different polarization modes of electromagnetic waves by reflecting or absorbing one polarization mode of the electromagnetic wave while transmitting the other polarization mode.Metamaterials are used for the FSS device and the PSS device. The metamaterial has an artificial structure and is configured to acquire a property from a subwavelength cell and simulate the atomic structure of a material in the natural world. By a specific configuration of metamaterial cells, an electromagnetic wave can be operated at a desired frequency via control of various characteristics such as the reflectance, permeability, and permittivity of the material. The electromagnetic wave and the metamaterial interfere with each other based on characteristics formed by the shape and dimensions of the metamaterial, the configuration of a periodical cell, the structure of the metamaterial, and peripheral media (a substrate, air, and the like).[NPL 1] Qin, Tao & Huang, Chenlu & Cai, Yang & Lin, Xianqi. (2023). Dual-Band Frequency Selective Surface with Different Polarization Selectivity for Wireless Communication Application. Sensors. 23. 4264. 10.3390 / s23094264.[NPL 2] Xian Wang, et al, "Design of a polarization selective surface for co-channel interference suppression", Electronics Letters, Volume 58, Issue 25, December 2022, pp. 934-936.The above-described PSS device can operate in a low frequency band of 20 GHz or lower.However, in a high frequency band (30 to 500 GHz) required by RFID, radar, and security, the operation of the PSS device is difficult due to difficulty of manufacturing and the limit of controllability. Especially, it is difficult to dynamically control the PSS device.This restricts the degree of freedom in design of a high-speed wireless network, thereby restricting an operating frequency region in the network.In order to solve the above-described problems, according to the present invention, there is provided a polarization selective device that receives an incident electromagnetic wave and emits an electromagnetic wave, comprising a plurality of metamaterial elements, wherein among the plurality of metamaterial elements, at least two metamaterial elements are arranged in parallel to each other, each of the two metamaterial elements includes gaps, a shape of each of the two metamaterial elements has reflection symmetry, one of the two metamaterial elements includes the gaps in one direction, and the other of the two metamaterial elements includes the gaps in another direction vertical to the one direction.According to the present invention, it is possible to provide a polarization selective device that can dynamically control the selection of a polarized wave of an electromagnetic wave.Fig. 1A is a bird's-eye perspective view showing the configuration of a metamaterial surface structure in a polarization selective device according to the first embodiment of the present invention;Fig. 1B is a bird's-eye perspective view showing the configuration of a metamaterial cell in the polarization selective device according to the first embodiment of the present invention;Fig. 2A is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 2B is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 2C is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 2D is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 3A is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 3B is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 3C is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 3D is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 4 is a schematic view showing the configuration of a metamaterial surface structure in the polarization selective device according to the first embodiment of the present invention;Fig. 5 is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 6A is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 6B is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 6C is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 6D is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 7 is a view for explaining the operation of the polarization selective device according to the first embodiment of the present invention;Fig. 8 is a view for explaining the effect of the polarization selective device according to the first embodiment of the present invention;Fig. 9A is a graph for explaining the effect of the polarization selective device according to the first embodiment of the present invention;Fig. 9B is a graph for explaining the effect of the polarization selective device according to the first embodiment of the present invention;Fig. 9C is a graph for explaining the effect of the polarization selective device according to the first embodiment of the present invention;Fig. 9D is a graph for explaining the effect of the polarization selective device according to the first embodiment of the present invention;Fig. 10A is a graph for explaining the effect of the polarization selective device according to the first embodiment of the present invention;Fig. 10B is a graph for explaining the effect of the polarization selective device according to the first embodiment of the present invention;Fig. 11 is a view for explaining an application example of the polarization selective device according to the first embodiment of the present invention;Fig. 12 is a bird's-eye perspective view showing the configuration of a metamaterial surface structure in a polarization selective device according to the second embodiment of the present invention;Fig. 13A is a view for explaining the operation of the polarization selective device according to the second embodiment of the present invention;Fig. 13B is a view for explaining the operation of the polarization selective device according to the second embodiment of the present invention;Fig. 13C is a view for explaining the operation of the polarization selective device according to the second embodiment of the present invention;Fig. 13D is a view for explaining the operation of the polarization selective device according to the second embodiment of the present invention;Fig. 14A is a view for explaining the operation of a polarization selective device according to the third embodiment of the present invention; andFig. 14B is a view for explaining the operation of the polarization selective device according to the third embodiment of the present invention.<First Embodiment>A polarization selective device according to the first embodiment of the present invention will be described with reference to Figs. 1A to 11.<Configuration of Polarization Selective Device>A polarization selective device 10 according to the first embodiment of the present invention is a polarization-selective surface (PSS) device, and will also be referred to as a "PSS device" hereinafter. As shown in Fig. 1A, the polarization selective device 10 is formed by periodically arranging a plurality of metamaterial cells 12 in a two-dimensional array. In this example, 8 × 8 metamaterial cells 12 are arranged in a square shape. The present invention is not limited to this, and M × N metamaterial cells 12 may be arranged in a rectangular shape.As shown in Fig. 1B, the metamaterial cell 12 includes two metamaterial elements 13, a dielectric filling material 14, and active elements 15. In addition, the metamaterial cell 12 includes a control line and a ground line as electric wirings 16 for operating the active elements 15.In the metamaterial elements 13, two U-shaped metamaterial lines are arranged at a predetermined interval in reflection symmetry. As a result, the metamaterial element 13 includes gaps 132. The gap 132 is a space that separates the metamaterial lines forming the metamaterial element 13.The example in which the metamaterial element 13 is formed by two U-shaped metamaterial line 131 has been explained but the present invention is not limited to this. In the metamaterial lines 131 arranged in reflection symmetry, the gaps 132 may be arranged at reflection symmetry positions. For example, two semi-annular metamaterial lines 131 may be arranged at a predetermined interval in reflection symmetry.The two metamaterial elements 13 are arranged in parallel to each other. One of the metamaterial elements 13 is arranged as an upper layer 121 on the upper surface of the metamaterial cell 12, and the other metamaterial element 13 is arranged as a lower layer 122 on the bottom surface of the metamaterial cell 12.The one metamaterial element 13 is arranged at a position obtained by rotating the other metamaterial element 13 by 90° on the horizontal plane. As a result, for example, one metamaterial element 13 (upper layer) includes the gaps 132 in the y direction and the other metamaterial element 13 (lower layer) includes the gaps 132 in the x direction. A "gap direction" indicates a direction parallel to the end faces of the metamaterial lines 131 facing each other in the gaps 132.The metamaterial element 13 has a subwavelength resonant structure, and resonates at a millimeter wave frequency designated in advance, for example, 100 to 500 GHz. The size of the resonant structure is smaller than λ / 2 of the wavelength of the operating electromagnetic wave. The metamaterial element 13 couples to the electric field component or magnetic field component of the incident electromagnetic wave.The metamaterial element 13 is made of an electrically conductive material such as a metal, a high-conductivity polymer, a conductive oxide, a carbon nanotube, or graphene. Alternatively, the metamaterial element 13 may be made of a dielectric material having a permittivity higher than that of the dielectric filling material 14.The dielectric filling material 14 is arranged to surround the metamaterial elements 13, and is arranged between the two metamaterial elements 13.The dielectric filling material 14 is made of a non-conductive dielectric material. The dielectric filling material 14 may be made of a non-conductive insulating polymer, a resin, an epoxy, polyimide, benzocyclobutene, parylene, SU-8, polyethylene, or the like. As the dielectric filling material 14, a material having a low relative permittivity is preferably used.The active element 15 is embedded in the structure of the metamaterial element 13, and arranged in the gap 132. The active element 15 has a capacitance, and the capacitance changes depending on the voltage. The active element 15 is connected to one metamaterial line 131 and the other metamaterial line 131 in the gap 132 so that the capacitance is electrically parallel-connected to a capacitance generated from the gap 132.The active element 15 is a varactor diode, a transistor, or the like. As the active element 15, a liquid crystal-, graphene-, or semiconductor-based device (a diode, a transistor, a resistor, or the like) may be used. The active element 15 may electrically be controlled by a voltage, a current, an electric field, or the like.<Operation of Polarization Selective Device>Figs. 2A to 2D are schematic views of the polarization selective device (PSS device) 10 including a reconfigurable metasurface. A polarized beam enters the device (incident wave 1) and is transmitted through the device (transmitted wave 2). A portion surrounded by a broken line indicates a region where an electromagnetic wave is reflected or scattered. Transmission of the polarized electromagnetic wave is controlled via active control of the metasurface characteristic.According to the first aspect, both polarized electromagnetic waves pass through the PSS device 10 without any main absorption or scattering (Fig. 2A).According to the second aspect, an electromagnetic wave having a horizontally polarized electric field is reflected or scattered by the PSS device 10, and an electromagnetic wave having a vertically polarized electric field is transmitted through the PSS device 10 (Fig. 2B).According to the third aspect, a vertically polarized electromagnetic wave is reflected or scattered by the PSS device 10, and a horizontally polarized electromagnetic wave is transmitted through the PSS device 10 (Fig. 2C).According to the fourth aspect, the transmission of all incident waves is blocked (Fig. 2D).As shown in Figs. 3A to 3D, there are four modes for the operation of the polarization selective device 10. In Figs. 3A to 3D, a white arrow indicates a polarized electromagnetic wave (to be referred to as an "x-polarized electromagnetic wave" hereinafter) in the x direction, and a hatched arrow indicates a polarized electromagnetic wave (to be referred to as a "y-polarized electromagnetic wave" hereinafter) in the y direction. Furthermore, a portion surrounded by a broken line indicates a region where an electromagnetic wave is reflected or scattered.・Mode 0: A control signal in the PSS device 10 is in an OFF state. Transmission of the electromagnetic wave is not controlled. Thus, the x-polarized electromagnetic wave and the y-polarized electromagnetic wave are transmitted (Fig. 3A).・Mode 1: A control signal is supplied to the PSS device 10 in the y direction (ON state). Then, the x-polarized electromagnetic wave is transmitted through the PSS device 10 while the y-polarized electromagnetic wave is reflected or scattered by the PSS device 10 (Fig. 3B).・Mode 2: A control signal is supplied to the PSS device 10 in the x direction (ON state). Then, the y-polarized electromagnetic wave is transmitted through the PSS device 10 while the x-polarized electromagnetic wave is reflected or scattered by the PSS device 10. In this way, the operation in mode 2 is reversed to the operation in mode 1 (Fig. 3C).・Mode 3: A control signal is supplied to the PSS device 10 in both the x direction and the y direction. Then, the x-polarized electromagnetic wave and the y-polarized electromagnetic wave are reflected or scattered by the device (Fig. 3D).As shown in Fig. 4, the metamaterial cell 12 includes the upper layer 121 and the lower layer 122. Each of the upper layer 121 and the lower layer 122 is formed by the metamaterial elements 13 and the dielectric filling material 14.The polarization-sensitive metamaterial elements 13 are integrated into a single cell that acts with the incident electromagnetic wave. The ground line and the control line as the electric wirings 16 are arranged to apply a control voltage signal to the capacitive active elements 15.In the metamaterial cell 12, the metamaterial element of the upper layer 121 and that of the lower layer 122 have the same shape. Each metamaterial element 13 includes the gaps 132. The metamaterial cell 12 is formed by relatively rotating the respective metamaterial elements 13 by 90°.By rotating and arranging the metamaterial elements 13, as described above, linearly polarized electromagnetic waves including different polarized waves act on the gaps 132 of each metamaterial element 13 when these waves enter. For example, in the metamaterial cell 12, the x-polarized electromagnetic wave acts on the gaps 132 of the upper layer 121 (in Fig. Fig. 4, a solid line arrow), and the y-polarized electromagnetic wave acts on the gaps 132 of the lower layer 122 (in Fig. Fig. 4, a dotted line arrow).For example, Fig. 5 shows a configuration in which the metamaterial element 13 including the gaps 132 is arranged in each of the upper layer 121 and the lower layer 122 of the metamaterial cell 12. In Fig. 5, a black arrow indicates the direction (polarization direction) of the electric field component of the electromagnetic wave. In this configuration, the directions of the gaps 132 are vertical to each other.When the electric field component of the incident electromagnetic wave is vertical to the direction of the gaps 132, the electric field component couples to the gaps 132 of the metamaterial element 13. As a result, LC resonance modes are excited, and a current flows in two loops in separate loops (in Fig. 5, dotted line arrows).At this time, the electric excitation coupling to the magnetic resonance (EEMR) occurs in the metamaterial cell 12. If the same cell element acts on another polarized wave of the electromagnetic wave (the electric field parallel to the gap direction), short-wire-like electric resonance occurs at a much higher frequency than LC resonance. This limits the action of the same cell with different polarized electromagnetic waves.In the shape of the metamaterial cell 12, with respect to the resonant excitation of the x-polarized electromagnetic wave, the resonant excitation in the upper layer 121 is stronger than that in the lower layer 122 based on the gap direction. With respect to the resonant excitation of the y-polarized electromagnetic wave, the resonant excitation in the lower layer 122 is stronger than that in the upper layer 121. With different sensitivities to the polarized waves of the incident electromagnetic waves, the transmission characteristic and / or the reflection characteristic is controlled by introducing an additional control element such as a capacitance diode or transistor.Figs. 6A to 6D are schematic views of the metamaterial cell 12 including a lumped active control element such as a diode. In Figs. 6A to 6D, a white arrow indicates an x-polarized electromagnetic wave, and a hatched arrow indicates a y-polarized electromagnetic wave. A dotted line arrow indicates a reflected electromagnetic wave. Figs. 6A to 6D, a white circle represents the active element in the ON state, and a black circle represents the active element in the OFF state.In mode 0, no control voltage is applied to the active elements 15 of the upper layer 121 and the lower layer 122 of the metamaterial cell 12 (V = 0 V). At this time, the capacitance of each active element 15 is high (Fig. 6A).In mode 1, the control voltage (V ≠ 0 V) is applied to the active elements 15 of the lower layer 122 of the metamaterial cell 12 to cause the transmission characteristic of the beam polarized along the y-axis (the beam including the electric field component along the y-axis) to shift to the reflection (or scattering) characteristic. At this time, no control voltage is applied to the active elements 15 of the upper layer 121 of the metamaterial cell 12 (Fig. 6B).In mode 2, the control voltage V is applied to the active elements 15 of the upper layer 121 of the metamaterial cell 12 to cause the transmission characteristic of the beam polarized along the x-axis (the beam including the electric field component along the x-axis) to shift to the reflection / scattering characteristic. No control voltage is applied to the active elements 15 of the lower layer 122 of the metamaterial cell 12. Because of the polarization sensitivity, the electromagnetic wave vertically polarized in the gap direction is not influenced by a change in capacitance caused by the application of the control signal (Fig. 6C).In mode 3, the control voltage is applied to the active elements 15 of the upper layer 121 and the lower layer 122 of the metamaterial cell 12, and both the polarized waves are reflected / scattered by the device (Fig. 6D).In this embodiment, when the electromagnetic wave is transmitted through the PSS device 10, the polarized wave is not converted (Fig. 7). In this case, localized equivalent electric admittance tensor Yesand magnetic impedance tensor Zmsare used. For the x-polarized electromagnetic wave, assuming that the PSS device 10 is surrounded by air, the localized Yes,xxand Zms,yyare normalized by the electromagnetic wave impedance η in a free space, given by:where Js represents an electric surface current density, Ms represents a magnetic surface current density, x bar represents a unit vector in the x direction, and y bar represents a unit vector in the y direction. Ex1 represents a tangential component of an electric field in region 1, Ex2 represents a tangential component of an electric field in region 2, Hy1 represents a tangential component of a magnetic field in region 1, and Hy2 represents a tangential component of a magnetic field in region 2.The transmission and reflection complex coefficients of a surface element S are given by:T = t・exp(φt) and R = r・exp(φr) represent the transmittance and the reflectance, respectively. t(φt) represents the intensity of the transmitted wave, r(φr) represents the intensity of the reflected wave, φtrepresents a transmission phase, and φrrepresents a reflection phase. Y and Z are pure imaginary values, given by:By optimizing the parameters Z and Y of the metamaterial cell 12 of the PSS device 10, it is possible to increase the transmittance. In particular, when Z = Y, a transmission of 100% of the incident wave is obtained.<Effect>The effect of the polarization selective device 10 according to this embodiment will be described with reference to Figs. 8 to 10B.With respect to the transmission characteristic of the polarization selective device 10, a simulation was conducted. Fig. 8 is a schematic view of the PSS metamaterial cell 12 used in the simulation and adjusted to a frequency of 300 GHz. In the simulation, an H-shaped metamaterial was used. For the sake of simplification, the control line and the ground line are not illustrated.As shown in Fig. 8, the metamaterial elements 13 are arranged horizontally and are vertical to an incident electromagnetic wave. The incident electromagnetic wave is radiated from port 1 and received by port 2. The upper layer 121 of the metamaterial cell 12 is oriented along the x-axis direction so as to allow coupling of the electric field component to the main body of the metamaterial cell 12 and induce LC resonance in the upper layer 121. On the other hand, the metamaterial element 13 of the lower layer 122 is oriented along the y-axis direction so as to allow coupling of the electromagnetic wave polarized vertically (along the y-axis) and induce LC resonance in the lower layer 122.In the simulation of the metamaterial cell 12 of the PSS device 10, a time-domain solver was used with normal incidence and periodic boundary conditions. Assuming that the period of the metamaterial cell 12 is constant, the shape (geometrical) parameters of the metamaterial cell 12, such as the size of the gap 132, a cell size, and a width were optimized to obtain transmission / reflection switching performance as a reaction (response) to the control signal and the capacitance change.The polarization-selective surface device includes a two-layer metasurface. The two-layer metasurface actively controls the transmission and reflection / scattering of the linearly polarized incident electromagnetic wave by the change of the capacitance of each active element 15. The optimized metamaterial cell 12 is used in the simulation of PSS device 10.In the simulated device, the relative permeability and permittivity of the dielectric filling material 14 are εr1= 2.4 and μ = 1, respectively. The total layer thickness is ts= 120 μm. The loss tangent is tanδ = 0.003. The polyimide material parameters are based on commercially available data in a millimeter wave band (30 - 500 GHz). The active control elements are directly formed on the polyimide material in the regions of the gaps 132 of the metamaterial elements 13. The metamaterial elements 13 are composed of an Au film having a thickness of 400 nm, and formed directly on the polyimide filling material.The parameters of the metamaterial cell 12 optimized for the operation in the 300-GHz band are as follows. A cell period is given by a = 250 μm, the size of the metamaterial element 13 is given by x1= y1= 160 μm, the length of each of the gaps 132 in the upper layer 121 and the lower layer 122 of the metamaterial elements 13 is given by g = 30 μm, the width of the metamaterial element 131 is given by w = 40 μm.The two layers of the same shape are stacked via an intermediate layer of 100 μm while relatively rotating the layers by 90°. The above parameters are obtained by adjusting the metamaterial cell 12 with respect to a transmission coefficient S21 at 300 GHz for the OFF state of the control signal (capacitance C = 3 fF).Figs. 9A to 9D show simulated transmission spectra for the different operation modes of the device. In the simulation, with the change of capacitance values in the active elements 15 of the upper layer 121 and the lower layer 122, the reflection and transmission characteristics are controlled in a frequency range from about 240 GHz to 330 GHz. In Figs. 9A to 9D, the spectrum of a solid line indicates the characteristic of the x-polarized electromagnetic wave, and a spectrum of a dotted line indicates the characteristic of the y-polarized electromagnetic wave.Fig. 9A shows the transmission characteristic in operation mode 0. The control signal is in the OFF state. No control signal is applied. The capacitances in both the layers are C = 3 fF. The total transmission is over -3 dB at 240 GHz or higher. The peak observed at about 235 GHz is a resonance peak of the metamaterial, and indicates large reflection / scattering of the incident wave.Fig. 9B shows the transmission characteristic in operation mode 1. The control signal is applied to the lower layer 122 (ON state). On the other hand, the signal in the upper layer 121 of the metamaterial layer is in the OFF state. By gradually changing the capacitance of each active element 15, the resonance peak in the spectrum shifts from about 235 GHz to about 330 GHz within a capacitance range from 3 fF to 0 fF. The position of the resonance peak indicates the frequency band that is reflected by the PSS device 10. In mode 1, the incident wave polarized along the x direction is transmitted through the device while the incident wave polarized along the y direction is reflected / scattered.Fig. 9C shows the transmission characteristic in operation mode 2. The control signal is applied to the upper layer 121 (ON state). On the other hand, the signal in the lower layer 122 of the metamaterial layer is in the OFF state. By gradually changing the capacitance of each active element 15, the resonance peak in the spectrum shifts from about 235 GHz to about 330 GHz within a capacitance range from 3 fF to 0 fF. The position of the resonance peak indicates the frequency band that is reflected by the PSS device 10. In mode 2, the incident wave polarized along the y direction is transmitted through the device while the incident wave polarized along the x direction is reflected / scattered. In this way, mode 2 is reverse to mode 1.Fig. 9D shows the transmission characteristic in operation mode 3. The control signal is applied to the upper layer 121 and the lower layer 122 (ON state). The incident wave is reflected at a predetermined operating frequency with respect to both polarized waves. The maximum control signal forms a capacitance of 0 fF, resulting in the reflection of the incident wave at about 330 GHz. The frequency band is adjusted by optimizing the shape of the metamaterial cell 12.The intensity of the resonance peak is associated with the reflection of the incident wave within a range from -15 dB to over -30 dB, for C = 3 fF to 0 fF. Accordingly, the incident wave is reflected in about 96.8% in the OFF state, and in about 99.9% for the maximum control signal in the ON state.As described above, in this configuration, the device can be operated in modes 0 to 3 with respect to an electromagnetic wave of 330 GHz. By adjusting the voltage of the control signal, the device may be operated in modes 0 to 3 with respect to electromagnetic waves of 240 to 330 GHz.Figs. 10A and 10B each show the transmission characteristics of the incident wave polarized along the x-axis and the incident wave polarized along the y-axis on the metasurface PSS.Figs. 10A and 10B each show a transmission characteristic S21 of an electromagnetic wave propagating from port 1 to port 2 and a transmission characteristic S12 of an electromagnetic wave propagating from port 2 to port 1. The transmission characteristics S21 and S12 coincide with other to the extent that it is impossible to distinguish between them in Figs. 10A and 10B.The same transmission characteristic is obtained in both the propagation directions on the metasurface PSS. Due to the change of the control voltage signal that causes the capacitance change (C = 3 fF to 0 fF), the resonance peak shifts in both the propagation directions in the same way. The transmission coefficient and the intensity of the resonance peak remains unchanged.As described above, the metasurface PSS can operate in both the propagation directions.A single resonance peak is observed in the calculated S21 spectrum, due to the fact that the two metamaterial elements 13 of the metamaterial cell 12 have the same gap size, wiring width, element size, and the like, that is, due to the symmetry of the metamaterial elements 13.By generating additional resonance peaks at frequencies around the main resonance, the asymmetry of the shape of the metamaterial cell 12 may be introduced to cause broadening of the resonance peak, thereby increasing the range of the reflection.In the polarization selective device 10, when applying a voltage to one (for example, upper) metamaterial element, among the polarized waves of the electromagnetic waves, a polarized wave vertical to the gap direction of the one metamaterial element is emitted. When applying a voltage to the other (for example, lower) metamaterial element, among the polarized waves of the electromagnetic waves, a polarized wave vertical to the gap direction of the other metamaterial element is emitted.The polarization selective device according to this embodiment can dynamically control selection of the polarized wave of the electromagnetic wave.<Application Example of Polarization Selective Device>An application example of the polarization selective device 10 according to this embodiment will be described with reference to Fig. 11.As shown in Fig. 11, a transmission antenna of a base station 101 generates different polarized electromagnetic waves. The generated electromagnetic waves are emitted to different nodes in a network and enter the PSS device 10.In the polarization selective device 10, transmission of a beam is controlled depending on the polarization of the incident wave. One (the first polarized wave indicated by a solid line arrow in Fig. 11) of the electromagnetic waves is transmitted through the polarization selective device (PSS device) 10 and is transmitted to a user 103 via a repeater device 102. The other electromagnetic wave (the second polarized wave indicated by a dotted line arrow in Fig. 11) is reflected by the polarization selective device (PSS device) 10 and is transmitted to another user 103 via another repeater device 102.<Second Embodiment>A polarization selective device according to the second embodiment of the present invention will be described with reference to Figs. 12 to 13D.<Configuration of Polarization Selective Device>A polarization selective device 20 according to this embodiment is a reflection-type device. When applied to wireless communication in association with control of wave propagation in various environments, a reflection-type device that selectively reflects an electromagnetic wave as a different polarized wave is necessary.Fig. 12 is a schematic view of a reflection-type PSS metamaterial cell 22. The reflection-type PSS metamaterial cell 22 further includes a conductive reflecting layer 27 in a lower layer 122 of the structure of a two-layer metamaterial cell 12 in the first embodiment.The distance between the lower layer 122 and the reflecting layer 27 is optimized to increase the reflection of a polarized wave in the OFF state of a control signal and increase the absorption of a polarized wave in the ON state of the control signal, at a predetermined operating frequency.Figs. 13A to 13D are schematic views of the reflection-type PSS device 20 including a reconfigurable metasurface. In Figs. 13A to 13D, a white arrow indicates an x-polarized electromagnetic wave, and a hatched arrow indicates a y-polarized electromagnetic wave. Furthermore, a portion surrounded by a broken line indicates a region where an electromagnetic wave is absorbed. A solid line arrow indicates an incident electromagnetic wave, and a dotted line arrow indicates a reflected electromagnetic wave.The incident polarized electromagnetic wave enters the PSS device 20, and the reflection of the polarized waves is controlled by the active control of the characteristic of the metasurface.In the first aspect (mode 0), the control signal of the PSS device 20 is in the OFF state. The reflection of the polarized waves is not controlled (Fig. 13A).In the second aspect (mode 1), the control signal is applied to the metamaterial device in the y-axis direction (ON state), the x-polarized incident wave is fully reflected, and the y-polarized incident wave is absorbed by the device (Fig. 13B).In the third aspect (mode 2), the control signal is applied to the metamaterial device in the x-axis direction (ON state), the y-polarized incident wave is fully reflected, and the x-polarized incident wave is absorbed by the device (Fig. 13C).In the fourth aspect (mode 3), when the control signal is applied in both the x-axis direction and the y-axis direction of the device, both the polarized waves are absorbed (Fig. 13D).The incident polarized waves are absorbed by adjusting the impedance of the structure of the PSS device 20. The effective impedance is calculated based on an extracted reflection coefficient S11(ω) by setting S21(ω) = 0, in accordance with equation (6). Note that S11(ω) is obtained by the simulation.In the PSS device 20, when the real part of the relative impedance of the electromagnetic wave is set to about 1 and the imaginary part is set to about 0 on average in the operating frequency range, absorption of about 100% can be implemented and the reflection of the metamaterial cell 12 can be maximized.The polarization selective device according to this embodiment can dynamically control selection of the polarized reflected wave.<Third Embodiment>A polarization selective device according to the third embodiment of the present invention will be described with reference to Figs. 14A and 14B.<Configuration of Polarization Selective Device>A polarization selective device 30 according to this embodiment includes a metamaterial layer for a beam operation in addition to the metamaterial layer for polarized wave selection in each of the first and second embodiments. Thus, the number of metamaterial layers of the polarization selective device 30 is larger than that in each of the first and second embodiments so as to separate the polarized wave selectivity and the beam operation characteristic. In the polarization selective device 30, the phase is shifted depending on a control signal applied to active elements 15.Figs. 14A and 14B each show an example of the operation of the polarization selective device 30 that selectively transmits or reflects a polarized incident electromagnetic wave. In Figs. 14A and 14B, a white arrow indicates an x-polarized electromagnetic wave, and a hatched arrow indicates a y-polarized electromagnetic wave. Solid line arrows indicate an incident electromagnetic wave and a transmitted electromagnetic wave, and a dotted line arrow indicates a reflected electromagnetic wave. If no control signal voltage is applied to the polarization selective device 30, both the x-polarized electromagnetic wave and the y-polarized electromagnetic wave are transmitted through the polarization selective device 30, as shown in Fig. 14A.On the other hand, if a control signal voltage is applied to the polarization selective device 30, the x-polarized electromagnetic wave is transmitted through the polarization selective device 30, and operated in a desired direction, as shown Fig. 14B. The y-polarized electromagnetic wave is reflected by the polarization selective device 30, and operated in a desired direction.In this embodiment, a beamformer may be arranged near the back surface of the polarization selective device 30. In this configuration, a transmitted electromagnetic wave (beam) including a polarized wave selected by the polarization selective device 30 enters the beamformer, and is operated in a desired direction by active control.Alternatively, a beamformer may be arranged near the front surface of the polarization selective device 30. In this configuration, a reflected electromagnetic wave (beam) including a polarized wave selected by the polarization selective device 30 enters the beamformer and is operated in a desired direction by active control.In this embodiment, the beamformer device has a configuration based on polarization-insensitive metamaterial elements 13 with high symmetry and can act on both the polarized waves (x-polarized wave and y-polarized wave) selected by the polarization selective device 30.According to this embodiment, a beam operation can be performed together with a polarized wave selection operation. Thus, in a more complicated wireless communication system, a device that can selectively transmit or reflect a polarized wave and perform a beam operation can increase the data rate of the electromagnetic wave by the polarization characteristic, and increase a signal coverage by the beam operation.Each of the embodiments of the present invention has explained the example in which the polarization selective device includes the two metamaterial elements but the present invention is not limited to this. The polarization selective device may include three or more metamaterial elements, and at least two of the plurality of metamaterial elements may be arranged in parallel to each other.Each of the embodiments of the present invention has explained the example in which the polarization direction of the electromagnetic wave entering the polarization selective device is parallel or vertical to the gap direction of the metamaterial element. Even if the polarization direction of the electromagnetic wave entering the polarization selective device is not parallel or vertical to the gap direction of the metamaterial element and has a predetermined angle, the polarization selective device can operate.In the embodiments of the present invention, examples of the structure, dimensions, material, and the like of the constituent parts have been described concerning the configuration of the polarization selective device. However, the present invention is not limited to these, and the polarization selective device need only exhibit its functions and provide effects.Note that the present invention is not limited to the above-described embodiments, and it is obvious that a person who has normal knowledge in this field can make many modifications and combinations within the technical scope of the present invention. For example, the third embodiment may be combined with the first or second embodiment.Some or all of the above-described exemplary embodiments can also be described as in the following supplementary notes but are not limited to the followings.(Supplementary Note 1)There is provided a polarization selective device that receives an incident electromagnetic wave and emits an electromagnetic wave, comprising a plurality of metamaterial elements, wherein among the plurality of metamaterial elements, at least two metamaterial elements are arranged in parallel to each other, each of the two metamaterial elements includes gaps, a shape of each of the two metamaterial elements has reflection symmetry, one of the two metamaterial elements includes the gaps in one direction, and the other of the two metamaterial elements includes the gaps in another direction vertical to the one direction.(Supplementary Note 2)The polarization selective device according to supplementary note 1 or 9, further comprises a dielectric configured to surround the two metamaterial elements, and an active element arranged in the gap, wherein a capacitance of the active element changes by electric control.(Supplementary Note 3)In the polarization selective device according to supplementary note 2, the active element is connected so that the capacitance of the active element is electrically parallel-connected to a capacitance generated from the gap.(Supplementary Note 4)In the polarization selective device according to any one of supplementary notes 1 to 3 and 10, when applying a voltage to the one metamaterial element, among polarized waves of the incident electromagnetic waves, a polarized wave in a direction vertical to the one direction is emitted, and when applying a voltage to the other metamaterial element, among the polarized waves of the incident electromagnetic waves, a polarized wave in a direction vertical to the other direction is emitted.(Supplementary Note 5)The polarization selective device according to any one of supplementary notes 1 to 4 and 10, further comprises a reflecting layer on an opposite side of an incident side of the electromagnetic wave with respect to the metamaterial element on an emission side of the electromagnetic wave out of the two metamaterial elements, wherein the emitted electromagnetic wave is an electromagnetic wave reflected by the polarization selective device.(Supplementary Note 6)In the polarization selective device according to supplementary notes 5 or 10, a real part of the relative impedance of the electromagnetic wave reflected by the polarization selective device is 1 and an imaginary part of the relative impedance is 0.(Supplementary Note 7)The polarization selective device according to any one of supplementary notes 1 to 6 and 10, further comprises a metamaterial element configured to perform beam forming.(Supplementary Note 8)In the polarization selective device according to any one of supplementary notes 1 to 4 and 10, the emitted electromagnetic wave is an electromagnetic wave transmitted through the polarization selective device.(Supplementary Note 9)In the polarization selective device according to supplementary note 8 or 10, a relative electric admittance and a relative magnetic impedance of the electromagnetic wave transmitted through the polarization selective device are equivalent.(Supplementary Note 10)There is provided a polarization selective device that receives an incident electromagnetic wave and emits an electromagnetic wave, comprising two metamaterial elements arranged in parallel to each other, wherein the metamaterial element includes gaps, a shape of the metamaterial element has reflection symmetry, one of the two metamaterial elements includes the gaps in one direction, and the other of the two metamaterial elements includes the gaps in another direction vertical to the one direction.(Supplementary Note 11)In the polarization selective device according to any one of supplementary notes 1 to 10, the metamaterial element has a subwavelength resonant structure, and the size of the metamaterial element is λ / 2 of a wavelength of the electromagnetic wave.(Supplementary Note 12)In the polarization selective device according to any one of supplementary notes 1 to 11, the metamaterial element couples to one of at least an electric field component and a magnetic field component of the electromagnetic wave.(Supplementary Note 13)In the polarization selective device according to any one of supplementary notes 1 to 12, the metamaterial element is made of an electrically conductive material.(Supplementary Note 14)In the polarization selective device according to any one of supplementary notes 1 to 13, the metamaterial element is made of at least one of a metal, a conductive polymer, a carbon nanotube, and graphene.(Supplementary Note 15)In the polarization selective device according to any one of supplementary notes 2 to 14, the metamaterial element is made of a dielectric material having a permittivity higher than a permittivity of the dielectric.(Supplementary Note 16)In the polarization selective device according to any one of supplementary notes 2 to 15, the dielectric includes at least one of a non-conductive insulating polymer, a resin, an epoxy, polyimide, benzocyclobutene, parylene, SU-8, and polyethylene.(Supplementary Note 17)In the polarization selective device according to any one of supplementary notes 1 to 16, the electromagnetic wave has a frequency in a millimeter wave band.The present invention relates to a polarization selective device, and can be applied to a communication system and a communication apparatus for wireless communication and the like.10...polarization selective device13...metamaterial element132...gap

Claims

1. A polarization selective device that receives an incident electromagnetic wave and emits an electromagnetic wave, comprising: 　　　　a plurality of metamaterial elements, wherein: 　　　　among the plurality of metamaterial elements, at least two metamaterial elements are arranged in parallel to each other, 　　　　each of the two metamaterial elements includes gaps, 　　　　a shape of each of the two metamaterial elements has reflection symmetry, 　　　　one of the two metamaterial elements includes the gaps in one direction, and 　　　　the other of the two metamaterial elements includes the gaps in another direction vertical to the one direction.

2. The polarization selective device according to claim 1, further comprising: 　　　　a dielectric configured to surround the two metamaterial elements; and 　　　　an active element arranged in the gap, wherein 　　　　a capacitance of the active element changes by electric control.

3. The polarization selective device according to claim 2, wherein 　　　　the active element is connected so that the capacitance of the active element is electrically parallel-connected to a capacitance generated from the gap.

4. The polarization selective device according to claim 1 or 2, wherein 　　　　when applying a voltage to the one metamaterial element, among polarized waves of the incident electromagnetic waves, a polarized wave in a direction vertical to the one direction is emitted, and 　　　　when applying a voltage to the other metamaterial element, among the polarized waves of the incident electromagnetic waves, a polarized wave in a direction vertical to the other direction is emitted.

5. The polarization selective device according to claim 1 or 2, further comprising: 　　　　a reflecting layer on an opposite side of an incident side of the electromagnetic wave with respect to the metamaterial element on an emission side of the electromagnetic wave out of the two metamaterial elements, wherein 　　　　the emitted electromagnetic wave is an electromagnetic wave reflected by the polarization selective device.

6. The polarization selective device according to claim 5, wherein a real part of a relative impedance of the electromagnetic wave reflected by the polarization selective device is 1 and an imaginary part of the relative impedance is 0.

7. The polarization selective device according to claim 1 or 2, further comprising a metamaterial element configured to perform beam forming.

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