Metamaterial device and method of converting between electromagnetic wave and orbital angular momentum wave

Abstract

A metamaterial device of this invention includes a metasurface on which a plurality of metamaterial cells are periodically arranged. The metasurface is divided into radial regions, one of an electromagnetic wave and an orbital angular momentum wave is transmitted through the metasurface and converted into the other of the electromagnetic wave and the orbital angular momentum wave, an electrical signal that is different for each region is applied to the metamaterial cells arranged in the region, the metamaterial cells have a different phase for each region, and the phase rotates about an axis in a propagation direction of the electromagnetic wave due to a change of the electrical signal.

Description

Metamaterial Device and Method of Converting between Electromagnetic Wave and Orbital Angular Momentum Wave

[0001] The present invention relates to a metamaterial device that converts between an electromagnetic wave and an orbital angular momentum (OAM) wave and a method of converting between an electromagnetic wave and an orbital angular momentum wave. Background Art

[0002] To increase the capacity of wireless communication, three approaches, that is, an increase of a spatial multiplexing order, an increase of a transmission bandwidth, and an increase of the number of modulation levels are considered.

[0003] One of the most common approaches for increasing the transmission bandwidth is to increase the operating frequencies to millimeter-wave bands, especially from 30 to 500 GHz. Another promising approach is to increase a spatial multiplexing order by applying orbital angular momentum (OAM) to the transmitted wave, thereby increasing the wireless transmission capacity.

[0004] OAM is one of physical quantities representing the properties of radio waves, indicating that a radio wave propagates while rotating the phase on a vertical plane in the propagation direction of the radio wave, and trajectories of identical phases have a spiral shape with respect to the propagation direction. Hence, OAM enables many independent data streams in the same spatial wireless medium and can thus provide a large capacity and a high data rate of wireless communication (non-patent literature 1).

[0005] Although the OAM is mainly used in an optical frequency domain, an application of OAM to wireless communication has been conducted recently. In wireless communication to about 100 GHz, a radially configured antenna set is used. A signal from a signal source is split into multiple channels, and an OAM wave is generated via phase modulation. Fig. 19 shows a typical configuration of a patch antenna 41 on a panel device used to generate an OAM wave. The patch antenna 41 is formed by uniform circular arrays (UCAs) 412 in which antenna elements 411 are arranged, and contribute to generation of various helical modes of beams (non-patent literature 2).

[0006] Normally, an OAM wave in an optical region is generated based on refraction, reflection, or diffraction of a beam wave. The wavefront of the OAM wave is controlled via control of propagation of light in a bulk material. Spatial phase modulation or polarization manipulation is used for phase control, and an OAM beam is thus generated. In this way, changes of phase and polarization are accumulated through propagation in refractive optical components such as lenses and wave plates.

[0007] Another method of generating an OAM wave is based on use of a metamaterial or a metasurface (non-patent literature 3). The phase is manipulated within the whole range of 2π by forming the structure of a metasurface unit cell. An incident light wave is thus converted into an OAM beam on the metasurface. Various metasurfaces are designed by rotating the direction of a metallic structure and controlling the configuration of unit cells in a spiral array. In this case, the degree of twist of OAM is determined by topological charge of the beam (non-patent literature 4).

[0008] A metamaterial or a metasurface is used for a flat lens, a collimator, or the like in a millimeter-wave frequency range, like the optical region. Many passive and active devices can generate an OAM beam in a low frequency band up to 30 GHz.

[0009] A metamaterial has an artificial structure and is configured to acquire properties from a subwavelength cell and imitate the atomic structure of a natural material. With a specific metamaterial cell configuration, an electromagnetic wave can be manipulated at a desired frequency via control of various characteristics such as the reflectance, permeability, and permittivity of the material.

[0010] The metamaterial interferes with the electromagnetic wave based on the shape, dimension, and configuration of a periodic metamaterial cell and the properties of the material, including the structure of the metamaterial and peripheral media (a substrate, air, and the like).

[0011] Device characteristics dynamically change by mounting metamaterial cells having various capacitance elements, typically varactor diodes, transistors, and pin diodes. When adjustment is performed for each cell or each set of cells, the permeability and permittivity of the metamaterial change. With these changes, the phase of a wave interfering with a device and the magnitudes of transmission and reflection can be changed.

[0012] [NPL 1] Y. Yan, et al., "High-capacity millimetre-wave communications with orbital angular momentum multiplexing" Nature Communications volume 5, Article number: 4876, Sept. 2014. [NPL 2] Hirofumi Sasaki, Doohwan Lee, Hiroyuki Fukumoto, Yasunori Yagi, Takana Kaho, Hiroyuki Shiba and Takashi Shimizu, "Experiment on Over-100-Gbps Wireless Transmission with OAM-MIMO Multiplexing System in 28-GHz Band" 2018 IEEE Global Communications Conference (GLOBECOM), DOI: 10.1109 / GLOCOM.2018.8647361, December 2018. [NPL 3] Yunfei Liu, Yueyi Yuan *, Qun Wu, Kuang Zhang, "Active metasurface in reflection manner for generation of reconfigurable OAM vortex beams," J. Information and Intelligence 1 (2023) 197-206. [NPL 4] Y. Shen, et al., "Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities" Light: Science & Applications volume 8, Article number: 90, October 2019.

[0013] As described above, when generating OAM in a low frequency range, a helical wave is generated by a specific configuration of antennas and phase-shifting circuits. On the other hand, if the operating frequency is increased up to a millimeter wave (100 to 500 GHz), various problems arise, as will be described below.

[0014] First, the size of an antenna necessary for generation of an electromagnetic wave at a high frequency is relatively small. The distance between antennas in the device is short. When generating an OAM beam with this configuration, the directivity and the gain decrease in the system, and the propagation distance shortens, resulting in degradation of overall performance.

[0015] Second, the insertion loss of a phase-shifting circuit increases with the operating frequency. In association with the large loss of the phase-shifting circuit, the overall strength of signals generated by the system lowers.

[0016] Third, along with the increase of the operating frequency, downscaling of a device is necessary to operate the device with a shorter wavelength. In an operation at a low frequency, the sizes of chips and antennas are in the millimeter-scale order, and device downscaling is not needed. On the other hand, in an operation at a frequency band of 300 GHz or more, that is, with a wavelength of 1 mm or less, device downscaling is needed, and device fabrication is difficult.

[0017] As described above, there are problems in generating an OAM beam in the millimeter-wave band.

[0018] In order to solve the above-described problems, according to the present invention, there is provided a metamaterial device comprising a metasurface on which a plurality of metamaterial cells are periodically arranged, wherein the metasurface is divided into radial regions, one of an electromagnetic wave and an orbital angular momentum wave is transmitted through the metasurface and converted into the other of the electromagnetic wave and the orbital angular momentum wave, an electrical signal that is different for each region is applied to the metamaterial cells arranged in the region, and the metamaterial cells have a different phase for each region, and the phase rotates about an axis in a propagation direction of the electromagnetic wave due to a change of the electrical signal.

[0019] According to the present invention, there is also provided a method of converting between an electromagnetic wave and an orbital angular momentum wave, the method using a metamaterial device including a metasurface on which a plurality of metamaterial cells are periodically arranged, comprising a step of dividing the metasurface into radial regions, a step of applying an electrical signal that is different for each region to the metamaterial cells arranged in the region, and a step of applying the electrical signal applied to the metamaterial cells arranged in the region to the metamaterial cells arranged in a region adjacent on one side of the region, wherein one of the electromagnetic wave and the orbital angular momentum wave is transmitted through the metamaterial device and converted into the other of the electromagnetic wave and the orbital angular momentum wave.

[0020] According to the present invention, it is possible to provide a metamaterial device that converts between an electromagnetic wave and an orbital angular momentum (OAM) wave with a high gain and a method of converting between an electromagnetic wave and an orbital angular momentum wave.

[0021] Fig. 1 is a schematic view showing the configuration of a communication system using a metamaterial device according to the first embodiment of the present invention;Fig.2A is a schematic view showing the configuration of the metamaterial device according to the first embodiment of the present invention;Fig.2B is a schematic view showing the configuration of the metamaterial device according to the first embodiment of the present invention;Fig.3 is a view for explaining the operation of the metamaterial device according to the first embodiment of the present invention;Fig.4 is a view for explaining the operation of the metamaterial device according to the first embodiment of the present invention;Fig.5 is a view for explaining the operation of the metamaterial device according to the first embodiment of the present invention;Fig.6 is a schematic view showing the configuration of a metamaterial cell according to the first embodiment of the present invention;Fig.7 is a schematic view showing an example of the configuration of the metamaterial cell according to the first embodiment of the present invention;Fig.8A is a view for explaining the operation of the metamaterial cell according to the first embodiment of the present invention;Fig.8B is a view for explaining the operation of the metamaterial cell according to the first embodiment of the present invention;Fig.9 is a view for explaining the operation of the metamaterial device according to the first embodiment of the present invention;Fig.10 is a view for explaining the operation of the metamaterial device according to the first embodiment of the present invention;Fig.11A is a view for explaining an orbital angular momentum (OAM) wave generation method according to the first embodiment of the present invention;Fig.11B is a view for explaining the orbital angular momentum (OAM) wave generation method according to the first embodiment of the present invention;Fig.11C is a view for explaining the orbital angular momentum (OAM) wave generation method according to the first embodiment of the present invention;Fig.11D is a view for explaining the orbital angular momentum (OAM) wave generation method according to the first embodiment of the present invention;Fig.12 is a flowchart for explaining the orbital angular momentum (OAM) wave generation method according to the first embodiment of the present invention;Fig.13 is a schematic view showing the configuration of a metamaterial device according to the second embodiment of the present invention;Fig.14 is a schematic view showing the configuration of the metamaterial device according to the second embodiment of the present invention;Fig.15 is a schematic view showing the configuration of the metamaterial device according to the second embodiment of the present invention;Fig.16 is a schematic view showing an example of the configuration of the metamaterial device according to the second embodiment of the present invention;Fig.17 is a schematic view showing an example of the configuration of the metamaterial device according to the second embodiment of the present invention;Fig.18 is a view for explaining an application example of the metamaterial device according to the embodiment of the present invention; andFig.19 is a schematic view showing the configuration of a conventional OAM device.

[0022] <First Embodiment> A metamaterial device and a method of converting between an electromagnetic wave and an orbital angular momentum wave according to the first embodiment of the present invention will be described with reference to Figs. 1 to 12.

[0023] <Configuration of Metamaterial Device and Transmission System> Fig. 1 shows the configuration of a transmission system 10 using an OAM beam. The transmission system 10 includes a transmitter 11 and a receiver 12 as communication apparatuses. The transmitter 11 includes a radiation element such as a transmission antenna (Tx) 13, and a metamaterial device 14. The receiver 12 includes a radiation element such as a reception antenna (Rx) 15, and the metamaterial device 14. A high-gain horn antenna is used for both of the transmission antenna (Tx) 13 and the reception antenna (Rx) 15, and this provides a high gain and thus a large propagation distance.

[0024] The metamaterial device 14 according to this embodiment is a reconfigurable coding metasurface device (to be also referred to as a "metasurface device" hereinafter). As shown in Fig. 2A, the metamaterial device 14 includes metamaterial cells 141 having a subwavelength (< λ / 2) size. In the metasurface device 14, the subwavelength metamaterial cells 141 are arranged in an array and resonate at a predetermined millimeter-wave frequency (30 to 500 GHz). The structure in which the metamaterial cells 141 are periodically arranged in an array will be referred to as a "metasurface" here.

[0025] As shown in Fig. 2B, the metamaterial cell 141 includes an electrically conductive metamaterial element 142, a dielectric base material 143, and an active control element 144. The active control element 144 is embedded in the metamaterial element 142.

[0026] The metamaterial element 142 is formed by a thin film of a high conductive material. Examples of the high conductive material are metals, high conductive polymers, conductive oxides, and other high conductive materials such as carbon nanotubes and graphene.

[0027] The dielectric base material 143 is formed by a nonconductive dielectric material of any type. Examples of the nonconductive dielectric material are nonconductive polymers, resins, epoxy, polyimide (PI), benzocyclobutene (BCB), parylene, polyethylene (PE), polytetrafluoroethylene (PTFE), and SU-8.

[0028] The active element 144 is formed by various semiconductor-based devices. Examples of the semiconductor-based devices are pin diodes, rectifying diodes, varactor diodes, and various kinds of transistors. It may be a device using another material such as a liquid crystal-based device or a vanadium oxide-based device.

[0029] The characteristics (for example, the capacitance and the permittivity) of the active element 144 are changed by an applied control signal. The characteristics of the active element 144 may be controlled by an electric signal such as a control voltage, a current, or an electric field.

[0030] The metamaterial cell 141 may actively be controlled by a capacitance change of the metamaterial element 142 or a change of the relative permittivity of the dielectric material around the metamaterial element 142.

[0031] <Operation of Metamaterial Device> The operation of the metamaterial device 14 according to this embodiment will be described with reference to Figs. 3 to 10.

[0032] As shown in Fig. 3, the metamaterial device 14 is connected to an external controller 16 (FPGA, PLC, or the like). The external controller 16 controls a phase change on the metasurface formed by the active metamaterial cells 141. In each cell, the phase can thus be changed in a whole range of 2π. An electric control signal is individually applied to the different metamaterial cells 141. As a result, a phase change is induced.

[0033] The phase changes based on specific coding of the metasurface supplied by the controller 16. Thus, the phase of an incident wave passing through the device changes, and an OAM wave is generated. The pattern of the phase obtained using the reconfigurable coding metasurface is controlled to exhibit a coding scheme necessary to obtain different modes of the OAM wave.

[0034] The OAM wave has an orbital characteristic associated with the degree of freedom of electromagnetic rotation and a rotation characteristic associated with the energy of an electromagnetic wave that rotates about an axis.

[0035] In the OAM wave, the electromagnetic wave has an angular momentum. That is, the electromagnetic wave rotates about an axis in the propagation direction. The OAM wave is generated by a plane electromagnetic wave having a single-phase rotation factor. The single-phase rotation factor is represented by exp(ilφ). Here, i = √(-1), l is the order of the OAM mode, and φ is the azimuthal angle defined as an angular position on a plane perpendicular to the axis of propagation.

[0036] In wireless communication, a pure OAM mode has a characteristic indicated by a positive or negative integer value or 0, and different modes are orthogonal to each other. As a result, the isophase plane of the OAM wave has a spiral structure. The phase distribution on the plane perpendicular to the beam propagation direction changes continuously at 2πl per rotation with respect to the axis of the beam propagation direction.

[0037] Fig. 4 shows various coding schemes used to generate the OAM wave. Each individual metamaterial cell 141 on the metasurface is controlled to exhibit a desired phase value. Thus, a phase distribution having various steps and a phase change Δφ between regions are obtained. Since the regions are controlled in real time, the coding scheme changes with certain time steps according to a desired space-time coding scheme sequence for various incident waves.

[0038] As shown in Fig. 4, 3-bit coding 1001 and 4-bit codings 1002 and 1003 are used to divide the metasurface into 8 regions and 16 regions, respectively. Each of adjacent regions exhibits a different phase shift. In this device, an additional phase coding scheme is used to simultaneously generate OAM beams of multiple modes.

[0039] Fig. 5 shows a change of the coding scheme for the reconfigurable metasurface. The coding scheme is represented by a phase distribution for a topological charge l. The topological charge l indicates the rotation direction of the OAM wave and the number of twist. For example, if l = +1, the phase changes clockwise, and the number of twists in one wavelength is 1. The topological charge l corresponds to the mode (OAM mode) of the OAM wave.

[0040] Fig. 5 shows a case of 3-bit coding as an example. In the phase distribution of the coding scheme, the phase values are φ1 to φ8, and these are set at intervals of 45° from φ1 = 0° to φ8 = 315°. The topological charge l is l = 0, 1, 2, 3, 4 (-4), -3, -2, and -1. Each arrow in Fig. 5 indicates a vector indicating the same point on the wavefront that rotates about the axis.

[0041] For example, when topological charge l = 1, in the phase distribution, the phase of one zone is 0, and the phase of the signal delays by 45° for each adjacent zone. Also, when topological charge l = 4, in the phase distribution, the phase of the signal delays by 180° for each adjacent zone. In this way, the phase distribution changes along with the change of the topological charge l.

[0042] A specific mode in the OAM wave is generated by a phase shift 2πl / N between zones, in which l is the OAM mode, and N is the number of zones. For example, the number l of modes generated for an array divided into 8 zones on the metasurface is l = -4 to 4. To the elements of each array, a phase having the same strength and having a phase shift between the elements is supplied.

[0043] In this way, the reconfigurable coding metasurface can generate various OAM modes depending on the configuration of coding regions having various phases.

[0044] The OAM wave can be generated by changing the OAM mode over time in accordance with a sequence of changing the change of the above-described coding scheme in a predetermined time step. At this time, the phase distribution in the coding scheme changes. For the OAM wave that is changed over time, a time-dependent phase gradient (for example, the difference between θ1 and θ2 in Fig. 5) of azimuthal angle having a topological charge l(t) is needed. In addition, a vector (an arrow in Fig. 5) indicating the wavefront of an OAM wave rotates in accordance with a predetermined time sequence. Thus, the reconfigurable coding metasurface can generate the OAM wave by changing the OAM modes over time depending on the configuration of coding regions having various phases.

[0045] Fig. 6 shows a plan view of an example of the active metallic metamaterial cell 141 formed by overlaying shape parameters, an equivalent circuit, and lumped capacitive elements used to control the coding metasurface.

[0046] A control signal is applied to the metamaterial cell 141 via a bias line, thereby changing the capacitance of the active element 144 (CL). The equivalent capacitance (Ceq) of the metamaterial element 142 is thus changed. The change of the capacitance results in a change in the transmission characteristics (the transmission intensity and the transmission phase) of the metamaterial cell 141.

[0047] The resonance frequency of the metamaterial cell 141 is calculated by

[0048]

[0049] Here, L is the equivalent inductance, and Ceqis the equivalent capacitance of the metamaterial cell 141. The equivalent inductance L is given by

[0050]

[0051] Here, μ0is the permeability in a free space, a and b are the lengths of the sides of the rectangular metamaterial cell 141, which indicate the size of the metamaterial cell 141, and t is the thickness of a metal that forms the metamaterial cell 141.

[0052] The equivalent capacitance Ceqis the sum of the capacitance (to be referred to as a "gap capacitance" hereinafter) Cgof the gap of the metamaterial element 142 and the variable lumped capacitance CLof the active element 144. The equivalent capacitance Ceqand the gap capacitance Cgare given by

[0053]

[0054]

[0055] Here, ε0is the permittivity in a free space, εcis the relative permittivity of the material (for example, dielectric filling material) forming the gap, w is the width of a band structure that forms the metamaterial element 142, and g is the length (width) of the gap of the metamaterial element 142.

[0056] Ceqand L are proportional to the size of the metamaterial cell 141, and the resonance frequency is inversely proportional to the size.

[0057] The metamaterial cell 141 is arranged in a direction to couple with an incident external electric field (E) to induce a current in the conductive metamaterial cell 141. Parameters such as a size, a shape, and a distance between cells are optimized to achieve coupling at the millimeter-wave frequency band. In the metamaterial cell 141, the electric field component of an electromagnetic wave may couple with a magnetic field component.

[0058] Fig. 6 shows an example in which the metamaterial cell 141 has the metamaterial element 142 having an H shape, but the present invention is not limited to this. For example, configurations of the metamaterial cells 141 shown in Fig. 7 may be used. In the metamaterial cells 141, the structure of the metamaterial element 142 includes a gap g. In the configurations, a control line and a ground line necessary for control of the active element are connected.

[0059] In this embodiment, an example in which two-dimensional metamaterial elements are used has been described. However, three-dimensional metamaterial elements may be used if the operation principles are maintained.

[0060] Also, in this embodiment, an example in which active control is based on lumped capacitive elements has been described. However, another control method based on a liquid crystal, a thermal material, or the like may be used. At this time, the design of the metamaterial cell 141 may be changed.

[0061] Fig. 8A shows a schematic view of the active metamaterial cell 141 used for simulations.

[0062] The metamaterial cell 141 is adjusted to a 300-GHz frequency band. The metamaterial cell 141 includes five layers of metamaterial elements 142 each having an H shape. The metamaterial element 142 has a gap in the structure of the metamaterial element 142. The metamaterial element 142 is formed by a copper (Cu) metal film. To increase the phase change range, patches may be added in the design. The signal lines (ground and control) are omitted in a simulation.

[0063] As the dielectric filling material 143, SU-8 epoxy resin is used. As the relative permittivity εrand the permeability μ of the SU-8 epoxy resin, εr= 2.86 and μ = 1, which are typical values in this material, are used.

[0064] A total thickness ts of the metamaterial cell 141 is 300 μm. The dielectric loss tangent is tanσ = 0.002. The characteristics listed here are based on commercially available products for the millimeter-wave frequency band.

[0065] The metamaterial element 142 is formed by a thin film of copper (Cu, σ = 5.8 × 107S / m) having a thickness of 350 nm, and is used to form each layer of the patterns of the thin metamaterial elements 142.

[0066] The parameters of the metamaterial cell 141 are as follows. The period of the metamaterial cells 141 is px = py = 300 μm, the gap size is g = 60 μm, the width of the band structure of the metamaterial element 142 is w = 30 μm, the distance between layers in the multilayer metamaterial cell 141 is 50 μm, and the thickness of each of the additional upper layer (top layer) and lower layer (bottom layer) of the dielectric material is 50 μm. The total thickness is 0.3 mm. Furthermore, the size of an impedance patch is 70 × 80 μm. The above-described parameters are obtained by performing a metamaterial cell optimization process for a high transmission phase shift of 360° or more at 300 GHz, with a relatively high transmission coefficient S21, for various values of the lumped capacitance.

[0067] In Fig. 8A, each metamaterial element 142 is arranged horizontally, that is, perpendicular to an incident electromagnetic wave that is radiated from port 1 and received by port 2. The metamaterial cells 141 are arranged along the x-axis direction such that the electric field (E) is coupled with the gap in the structure of the metamaterial element 142 to induce an LC resonance.

[0068] In the simulation of the metamaterial cell 141, a time-domain solver was used using normal incidence conditions and periodic boundary conditions. Assuming that the period of the metamaterial cells 141 is constant, the shape parameters of the metamaterial such as the gap size, the cell size, and the width were optimized such that the transmission phase is shifted within the whole range of 2π (360°) at high transmission in response to the change of the capacitance of lumped elements caused by an external control signal.

[0069] Fig. 8B shows the overall change of the transmission phase with respect to the change of the lumped capacitive elements in the transmission-type metamaterial cells 141 of five layers.

[0070] In the metamaterial cell 141, when the lumped capacitance is changed from 0 to 3 fF, the whole transmission phase changes from 370° to 0°. The individually designed and simulated metamaterial cell 141 has the lumped capacitance value CLof 0 to 3 fF and causes the shift of the transmission phase.

[0071] The metamaterial cells 141 are optimized to exhibit a phase change of 360° (2π) in a transmission range higher than -3 dB in response to the change of the lumped capacitance.

[0072] The continuous phase distribution in the coding scheme is discretized to fabricate a metasurface by the metamaterial cells 141 having a finite size. For example, a phase range of 360° is divided into a finite number of steps, such as 4, 8, 16, or the like. As shown in Fig. 8B, the values of the phase of the continuous distribution are assigned to the finite metamaterial cells 141 that exhibit a predetermined transmission phase. When the phase difference between the metamaterial cells 141 is reduced by increasing the number of steps to divide the phase range, the efficiency of the device can be improved.

[0073] Fig. 9 shows the simulation result of the coding patterns of the phase distribution for various OAM modes. As the metasurface, a configuration in which 101 metamaterial cells 141 were arranged in each of the x and y directions (101 × 101 metamaterial cells 141 were arranged) was used. The coding patterns were simulated based on equations of focus of OAM for 2-bit (4 phase steps), 3-bit (8 phase steps), and 4-bit (16 phase steps) quantizations.

[0074] The number of steps is arbitrarily selected based on the design of the control scheme. If the number of steps to divide the phase range (phase gradient) is large, an OAM beam can accurately be generated especially in a high OAM mode.

[0075] To generate various OAM modes, each metamaterial cell 141 is individually controlled to exhibit a desired phase pattern.

[0076] In Fig. 9, coding patterns of the phase distribution in modes l = -3, -2, -1, 0, 1, 2, and 3 are shown for different degrees of quantization (different number of phase steps). For l = 0, an incident plane wave is transmitted through the metasurface device 14 and output without any changes or twists. In the OAM modes l = 1, 2, and 3, a twisted structure occurs, and an OAM wave is generated. The higher the value of the OAM mode is, the larger the number of twists included in the observed output wave is. In the OAM modes of negative values l = -1, -2, and -3, twists that are the same as those in the positive OAM modes (l = 1, 2, and 3,) but in the opposite direction are observed.

[0077] Fig. 10 shows the phase distributions of coding patterns for a 4-bit (16 phase steps) metasurface pattern (left view) and simulated transmission intensity spectra for different OAM modes. The transmission intensity spectra were simulated, for the modes l = -3, -2, -1, 0, 1, 2, and 3, at z = 0 cm in a plane (x-y plane) perpendicular to the metasurface (center view) and at y = 10 cm in a plane (x-z plane) parallel to the metasurface (right view).

[0078] For the mode l = 0, an incident wave is transmitted through the device as a collimated beam without generating the OAM mode.

[0079] For the modes l = 1, 2, and 3, OAM waves that rotate counterclockwise are observed. In the transmission intensity spectrum, the light intensity is low near the center of the beam (parts indicated by dark color in the center and right views) and high in a ring-shaped region around the center of the beam (parts indicated by light color in the center and right views). Thus, as for the power of the beam, ideally, the power does not propagate along the center of the beam and the beam is focused in a ring shape.

[0080] When the OAM mode increases from l = 1 to l = 3, the size of the diameter of the ring with the maximum power increases. On the other hand, the diameter of the central spot with minimum power also increases accordingly.

[0081] On the other hand, in the OAM modes l = -1, -2, and -3, OAM waves that rotate clockwise are observed. In the OAM modes l = -1, -2, and -3, the tendencies of changes other than the rotation direction are the same as in the positive OAM modes.

[0082] <Method of Converting between Electromagnetic Wave and Orbital Angular Momentum Wave> In a method of converting between an electromagnetic wave and an orbital angular momentum wave (OAM wave) according to this embodiment, a case where an electromagnetic wave such as a plane wave or a spherical wave is transmitted through the metamaterial device 14 and thus converted into an OAM wave, that is, a case where an OAM wave is generated will be described with reference to Figs. 11A to 11D and Fig. 12. Fig. 11A shows the configuration of the metasurface device 14 in which the metamaterial cells 141 are arranged in an array. Fig. 11B shows the voltage dependence of the phase in the metamaterial cell 141. Fig. 11C shows the time-rate change of the voltage applied to each region of the metasurface device 14. Fig. 11D shows the time-rate change of the phase in each region of the metasurface device 14. Fig. 12 is a flowchart showing an OAM wave generation method.

[0083] A case of 3-bit coding (8 phase steps) will be described as an example.

[0084] First, the number of bits is determined in accordance with a desired OAM mode. The metasurface device 14 is radially divided into regions according to the number of bits (step S1). Here, as shown in Fig. 11A, the metasurface device 14 is divided into eight regions A to H in accordance with 3 bits.

[0085] In the metamaterial cells 141 that form the metasurface device 14, as shown in Fig. 11B (based on Fig. 8B), phases φ1 to φ8 change in correspondence with control voltages (signals) V1 to V8, respectively.

[0086] Next, the different control voltages (signals) V1 to V8 are applied to the metamaterial cells 141 in the regions A to H, respectively (1011 in Fig. 11C, step S2). Thus, the phases φ1 to φ8 of the metamaterial cells 141 are assigned to the regions A to H (1021 in Fig. 11D).

[0087] Next, the control voltages V2 to V8 and V1 are applied to the regions A to H, respectively. That is, the control voltage applied to the metamaterial cell 141 in each region in step S2 is shifted to the metamaterial cells 141 in the region adjacent on one side of the region (1012 in Fig. 11C, step S3). In other words, the control voltage applied to the region adjacent on the other side of each region in step S2 is applied to the metamaterial cells 141 in each region. Thus, the phases assigned to the regions A to H change to φ2 to φ8 and φ1 (1022 in Fig. 11D).

[0088] Next, the control voltages are shifted to the adjacent regions. The shift of the control voltages is repeated (1013 to 1018 in Fig. 11C, step S4). Thus, the phases assigned to the regions A to H sequentially change (1023 to 1028 in Fig. 11D).

[0089] In this way, the control voltages applied to the metamaterial device 14 are sequentially changed, and the phases of the metamaterial cells 141 are changed and rotated about the beam propagation direction. As a result, an electromagnetic wave such as a plane wave or a spherical wave is transmitted through the metamaterial device 14 and converted into an OAM wave having a twisted structure.

[0090] Also, the number of regions is changed in accordance with the OAM mode in time steps and steps S1 to S4 are executed, thereby changing the OAM mode over time and generating an OAM wave. For example, the OAM mode is changed over time in accordance with the time sequence of the coding scheme shown in Fig. 5, thereby generating an OAM wave.

[0091] In this embodiment, an example in which an OAM wave is generated, that is, an incident electromagnetic wave such as a plane wave or a spherical wave is converted into an OAM wave has been described. However, the present invention is not limited to this. Like the generation of an OAM wave, an incident OAM wave may be converted into an electromagnetic wave such as a plane wave or a spherical wave by controlling the metamaterial cells by an electrical signal. In this way, one of the electromagnetic wave and the orbital angular momentum wave is transmitted through the metamaterial device, and one of the electromagnetic wave and the orbital angular momentum wave is converted into the other.

[0092] <Effect> In a conventional UCA antenna, a number of small horn antennas or small patch antennas are configured radially, thereby generating an OAM wave. As a result, since the gain and directivity of the small antennas are low, the gain and directivity of the UCA antenna is low.

[0093] On the other hand, in the transmission / reception device according to this embodiment, an antenna larger than the conventional small antenna is used, and a metamaterial device that has a high gain and can dynamically be adjusted is arranged before the antenna. As a result, since the large antenna and the metamaterial device have a high gain, the gain of the transmission / reception device is high.

[0094] Also, in the transmission / reception device according to this embodiment, the phase of a transmitted / received wave is changed by an active element embedded in a metamaterial element. Since the transmission / reception device includes no phase-shifting circuit, a loss generated in the phase-shifting circuit can be reduced.

[0095] <Second Embodiment> A metamaterial device and a method of converting between an electromagnetic wave and an orbital angular momentum wave according to the second embodiment of the present invention will be described with reference to Figs. 13 to 15.

[0096] A metamaterial device 24 according to this embodiment is a reconfigurable coding metasurface device and is an actively adjustable device.

[0097] As shown in Fig. 13, the metamaterial device 24 includes groups (to be also referred to as supercells hereinafter) 241 of metamaterial cells 141 formed in an array, and metal pads 242. In addition, the metamaterial device 24 includes control lines 243 that connect the metamaterial supercells 241 and the metal pads 242, and a ground line 244.

[0098] A control signal and a ground signal are connected to each metamaterial supercell. The control signal is supplied to the metamaterial supercell via a metal pad or the like.

[0099] In the metasurface device 24, active signal control is executed for the group (supercell) 241 of the metamaterial cells 141 formed in an array.

[0100] Fig. 14 shows a schematic view of detailed implementation of the metamaterial supercells 241 and electrical connection. In the schematic view, as an example, the single metamaterial supercell 241 is formed by 4 × 4 metamaterial cells 141. The configuration is not limited to 4 × 4 metamaterial cells, and another configuration may be used.

[0101] The metal pads 242 are arranged near the metamaterial cells 141. For example, the metal pads 242 may be arranged near an end portion of the metasurface device.

[0102] The metamaterial supercells 241 are controlled by the different control lines 243 and different applied control signals. Thus, each metamaterial supercell is individually biased to exhibit a predetermined transmission phase value.

[0103] In this configuration, the ground line 244 is common to all the metamaterial supercells 241.

[0104] Distances X and Y between the metamaterial supercells 241 are set to be larger than the wavelength of the operating frequency by organizing the metamaterial cells 141 of a subwavelength to the larger supercells 241. The distances X and Y between the metamaterial supercells 241 are adjusted in accordance with the space for the control lines 243 and the size of the whole supercells 241 on the reconfigurable metasurface.

[0105] Fig. 15 shows a schematic view of the metamaterial supercells 241 on the reconfigurable coding metasurface.

[0106] Each supercell 241 is formed by 4 × 4 metamaterial cells 141 configured as in the first embodiment. The period of the distance of the metamaterial cells 141 is less than λ / 2 of the operating frequency.

[0107] Considering active control of a response by changing the capacitance of lumped elements in the cells, a half 142_1 of a metamaterial element electrically connected in each metamaterial cell 141 is connected to the control signal line 243, and the other half 142_2 is connected to the common ground signal line 244. A part of the metamaterial element electrically connected in each metamaterial cell 141 may be connected to the control signal line 243, and the other part may be connected to the common ground signal line 244.

[0108] The control signal line 243 is directly connected to one of control signal pads via the outside of the supercells 241 and used to apply the control signal. The ground line 244 is connected to other supercells and one of the connection pads via the outside of the metamaterial supercells 241.

[0109] A method of converting between an electromagnetic wave and an orbital angular momentum wave can be implemented, as in the first embodiment, using the metamaterial device according to this embodiment.

[0110] <Effect> In the first embodiment, active signal control is executed for individual metamaterial cells on the metasurface. In the first embodiment, along with the rise of the operating frequency and reduction of the size of the metamaterial cells, spaces for control lines, vias, and other electrical connections are limited, it is difficult to fabricate the metamaterial cells, and control for individual cells is more difficult.

[0111] On the other hand, in this embodiment, active signal control is executed for the group (supercell) of metamaterial cells formed in an array. As a result, electrical connection lines such as control lines and biases can be decreased. For this reason, when the operating frequency rises, and the size of the metamaterial cells is reduced, spaces for electrical connections are not limited, the metamaterial cells can easily be fabricated, and each supercell can easily be controlled.

[0112] As described above, in this embodiment, all supercells are individually controlled to exhibit various transmission phases. The supercells are coded to exhibit a predetermined transmission pattern, and various OAM modes are thus generated.

[0113] <Third Embodiment> A metamaterial device according to the third embodiment of the present invention will be described with reference to Figs. 16 and 17.

[0114] A metamaterial device 34 according to this embodiment is a reconfigurable coding metasurface device and is an actively variable device.

[0115] Fig. 16 shows a schematic view of the reconfigurable coding metasurface of a metasurface device 34_1 used to generate an OAM beam.

[0116] In the metasurface device 34_1, metamaterial cells 141 are formed based on a usable coding scheme. The pattern of the metamaterial cells 141 is a radial pattern, like a uniform circular antenna (UCA) pattern. As shown in Fig. 16, the metamaterial cells 141 are arranged in four square rings (regions indicated by the dashed line in Fig. 16)and used to generate different OAM modes l = 1 to 4. Each ring may be divided into eight sections (the regions divided by white lines in the left view of Fig. 16), or may be divided into 16 sections (the regions divided by white lines in the right view of Fig. 16). In these rings, control is performed on a section basis to generate an OAM beam changed over time.

[0117] Fig. 17 shows a schematic view of the reconfigurable coding metasurface of a metasurface device 34_2 used to generate an OAM beam.

[0118] In the metasurface device 34_2, the metamaterial cells 141 form square groups (supercells) 241 that are individually controlled by an applied signal exhibiting various transmission phase values.

[0119] As shown in Fig. 17, the groups (supercells) 241 of metamaterial cells are arranged in five circular rings (regions indicated by the dashed line in Fig. 17)and used to generate different OAM modes l = 1 to 5.

[0120] The number of metamaterial cells in each section, the number of sections, the distance between the sections, and the like may arbitrarily be selected to provide an optimum operation in a generated OAM beam.

[0121] Each ring is divided into a predetermined number of sections and individually controlled to exhibit a change in the transmission phase. The value of a phase exhibited by each section is designed by a predetermined phase coding scheme, as described above.

[0122] The design is thus made to individually control the groups (supercells) of metamaterial cells separated by circular patterns.

[0123] According to this embodiment, it is possible to simplify manufacturing and control scheme of the reconfigurable coding metasurface device.

[0124] A method of converting between an electromagnetic wave and an orbital angular momentum wave can be implemented, as in the first embodiment, using the metamaterial device according to this embodiment.

[0125] <Application Example of Metasurface Device> Fig. 18 shows a schematic view of OAM beam transmission / reception in a wireless communication system 10 to which the metamaterial device according to the embodiment of the present invention is applied.

[0126] As the metamaterial device, the metasurface device 14 according to the first embodiment is used. The metasurface device 24 or 34 according to the second or third embodiment may be used.

[0127] In the wireless communication system 10, a radiation element 11 such as a transmission antenna (Tx) radiates a millimeter wave that is an incident wave to the reconfigurable coding metasurface device 14 at a predetermined frequency (30 to 500 GHz). The incident wave that enters the metasurface is transmitted via the metasurface device 14.

[0128] On the reconfigurable coding metasurface, an incident wave is converted into an OAM wave that changes over time by the coding scheme supplied by the metasurface device 14. The twists of the spiral of the electromagnetic wave are controlled by the metasurface. Different spiral modes (llto ln) are generated as positive and negative modes.

[0129] Propagation of the OAM wave is received by the reconfigurable coding metasurface device 14 via air. The OAM wave is converted into a plane wave and received by a reception antenna (Rx) 15.

[0130] In the embodiment of the present invention, examples of the structures, dimensions, materials, and the like of the constituent parts in the configurations of the metamaterial device and the method of converting between an electromagnetic wave and an orbital angular momentum wave, the manufacturing method, and the like have been described. However, the present invention is not limited to this. Any configuration capable of implementing the function of the metamaterial device and obtaining the effect can be used.

[0131] Note that the present invention is not limited the above-described embodiments, and it is obvious that many modifications and combinations can be done by any person ordinarily skilled in the art without departing from the technical scope of the present invention. The metamaterial device according to the second or third embodiment may be used in the method of converting between an electromagnetic wave and an orbital angular momentum wave according to the first embodiment.

[0132] Some or all of the above-described embodiments or examples thereof can also be described as in the following supplementary notes but are not limited to the followings.

[0133] (Supplementary Note 1) There is provided a metamaterial device comprising a metasurface on which a plurality of metamaterial cells are periodically arranged, wherein the metasurface is divided into radial regions, one of an electromagnetic wave and an orbital angular momentum wave is transmitted through the metasurface and converted into the other of the electromagnetic wave and the orbital angular momentum wave, an electrical signal that is different for each region is applied to the metamaterial cells arranged in the region, and the metamaterial cells have a different phase for each region, and the phase rotates about an axis in a propagation direction of the electromagnetic wave due to a change of the electrical signal.

[0134] (Supplementary Note 2) The metamaterial device according to Supplementary Note 1 further comprises a control line and a ground line, wherein the plurality of metamaterial cells form a group, the metamaterial cell comprises a metamaterial element, and a part of the metamaterial element is connected to the control line, and the other part of the metamaterial element is connected to the ground line.

[0135] (Supplementary Note 3) There is provided a method of converting between an electromagnetic wave and an orbital angular momentum wave, the method using a metamaterial device including a metasurface on which a plurality of metamaterial cells are periodically arranged, comprising a step of dividing the metasurface into radial regions, a step of applying an electrical signal that is different for each region to the metamaterial cells arranged in the region, and a step of applying the electrical signal applied to the metamaterial cells arranged in the region to the metamaterial cells arranged in a region adjacent on one side of the region, wherein one of the electromagnetic wave and the orbital angular momentum wave is transmitted through the metamaterial device and converted into the other of the electromagnetic wave and the orbital angular momentum wave.

[0136] (Supplementary Note 4) In the method of converting between an electromagnetic wave and an orbital angular momentum wave according to Supplementary Note 3, the number of regions is changed over time.

[0137] (Supplementary Note 5) In the metamaterial device according to Supplementary Note 1 or 2, the electromagnetic wave is a millimeter wave, and the metamaterial cells exist within a range of 100 μm square or more and 900 μm square or less.

[0138] (Supplementary Note 6) In the metamaterial device according to any one of Supplementary Notes 1, 2, and 5, the number of regions corresponds to the number of modes of the orbital angular momentum wave.

[0139] (Supplementary Note 7) In the metamaterial device according to any one of Supplementary Notes 1, 2, 5, and 6, the metamaterial device is a transmission-type device.

[0140] (Supplementary Note 8) In the metamaterial device according to any one of Supplementary Notes 1, 2, and 5 to 7, the electromagnetic wave is a plane wave or a spherical wave.

[0141] (Supplementary Note 9) In the metamaterial device according to any one of Supplementary Notes 1, 2, and 5 to 8, the metamaterial cell includes an electrically conductive metamaterial element, a dielectric material surrounding the metamaterial element, and an active element embedded in the metamaterial element.

[0142] (Supplementary Note 10) In the metamaterial device according to Supplementary Note 9, the metamaterial element is formed by a thin film of a high conductive material.

[0143] (Supplementary Note 11) In the metamaterial device according to Supplementary Note 9 or 10, the metamaterial element is at least one of a metal, a high conductive polymer, a conductive oxide, a carbon nanotube, and graphene.

[0144] (Supplementary Note 12) In the metamaterial device according to Supplementary Note 9, the dielectric material is formed by a nonconductive dielectric material.

[0145] (Supplementary Note 13) In the metamaterial device according to Supplementary Note 9 or 12, the dielectric material is at least one of a nonconductive polymer, a resin, epoxy, polyimide, benzocyclobutene, parylene, polyethylene, polytetrafluoroethylene, and SU-8.

[0146] (Supplementary Note 14) In the metamaterial device according to Supplementary Note 9, the active element is at least one of a semiconductor-based device, a liquid crystal-based device, and a vanadium oxide-based device.

[0147] (Supplementary Note 15) In the metamaterial device according to Supplementary Note 9 or 14, a characteristic of the active element is controlled by an electric signal of one of a control voltage, a current, and an electric field.

[0148] (Supplementary Note 16) In the metamaterial device according to any one of Supplementary Notes 9 to 15, the metamaterial cell is actively controlled based on a capacitance change of the metamaterial element or a change of a relative permittivity of the dielectric material around the metamaterial element.

[0149] (Supplementary Note 17) There is provided a transmitter including a radiation element configured to radiate the electromagnetic wave, and a metamaterial device according to any one of Supplementary Notes 1, 2, and 5 to 16, to which the electromagnetic wave enters from the radiation element and which converts the electromagnetic wave into the orbital angular momentum wave.

[0150] (Supplementary Note 18) There is provided a receiver including a metamaterial device according to any one of Supplementary Notes 1, 2, and 5 to 16, to which the orbital angular momentum wave enters and which converts the orbital angular momentum wave into the electromagnetic wave, and a radiation element to which the electromagnetic wave enters.

[0151] (Supplementary Note 19) In the method of converting between an electromagnetic wave and an orbital angular momentum wave according to Supplementary Note 3 or 4, the number of regions is determined in accordance with the number of modes of the orbital angular momentum wave.

[0152] (Supplementary Note 20) In the method of converting between an electromagnetic wave and an orbital angular momentum wave according to any one of Supplementary Notes 3, 4, and 19, the number of regions is determined based on a coding scheme.

[0153] The present invention can be applied to a wireless communication apparatus and a wireless communication system.

[0154] 14...metamaterial device 141...metamaterial cell

Claims

1. A metamaterial device comprising a metasurface on which a plurality of metamaterial cells are periodically arranged, 　　　　wherein the metasurface is divided into radial regions, 　　　　one of an electromagnetic wave and an orbital angular momentum wave is transmitted through the metasurface and converted into the other of the electromagnetic wave and the orbital angular momentum wave, 　　　　an electrical signal that is different for each region is applied to the metamaterial cells arranged in the region, and the metamaterial cells have a different phase for each region, and 　　　　the phase rotates about an axis in a propagation direction of the electromagnetic wave due to a change of the electrical signal.

2. The metamaterial device according to claim 1, further comprising: 　　　　a control line; and 　　　　a ground line, 　　　　wherein the plurality of metamaterial cells form a group, 　　　　the metamaterial cell comprises a metamaterial element, and 　　　　a part of the metamaterial element is connected to the control line, and the other part of the metamaterial element is connected to the ground line.

3. A method of converting between an electromagnetic wave and an orbital angular momentum wave, the method using a metamaterial device including a metasurface on which a plurality of metamaterial cells are periodically arranged, comprising: 　　　　a step of dividing the metasurface into radial regions; 　　　　a step of applying an electrical signal that is different for each region to the metamaterial cells arranged in the region; and 　　　　a step of applying the electrical signal applied to the metamaterial cells arranged in the region to the metamaterial cells arranged in a region adjacent on one side of the region, 　　　　wherein one of the electromagnetic wave and the orbital angular momentum wave is transmitted through the metamaterial device and converted into the other of the electromagnetic wave and the orbital angular momentum wave.

4. The method of converting between an electromagnetic wave and an orbital angular momentum wave according to claim 3, wherein the number of regions is changed over time.

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