Electrostrictive element

Abstract

An electrostrictive element (1) of this invention is an electrostrictive element through which an electromagnetic wave is transmitted, and includes a metamaterial cell (10) in which a layer (11) made of a material having an electrostrictive effect and one of a dielectric layer (12) and a conductive layer (13) are stacked. A dimension and a dielectric constant of the metamaterial cell change due to application of an external electric field, thereby changing a characteristic of the electromagnetic wave. Hence, this invention can provide an electrostrictive element that can accurately, actively control a metamaterial device.

Description

Electrostrictive Element

[0001] The present invention relates to an electrostrictive element using a metamaterial.

[0002] In recent years, a metamaterial has attracted attention in terms of a feature to interfere with an electromagnetic wave, and has been applied to a cloaking device and a device such as an absorber, a lens, a beamforming device, and a phase shifter. A metamaterial is an artificial medium which has subwavelength cells arranged like atoms in a normal material and presents unique characteristics. A metamaterial can control characteristics such as a dielectric constant and a magnetic permeability by designing the shape, size, and arrangement of periodic materials and the like, and operate electromagnetic radiation.

[0003] A metamaterial device has a passive characteristic that does not actively acts on interference with an incident electromagnetic wave or an active characteristic that actively acts on an electromagnetic wave. This active characteristic is required for applications associated with control of the direction, polarization, or intensity of the electromagnetic wave. Therefore, an active element including a metamaterial cell is necessary to actively change the device characteristics.

[0004] As a method of actively controlling a metamaterial device, there is provided a method using a varactor diode, a pin diode, a transistor, and the like based on a change in relative capacity of a metamaterial cell. In addition, there is provided a method using a thermally controlled material such as liquid crystal or vanadium oxide (VO2) based on a change in effective dielectric constant of a cell or a substrate. There is also provided a method using a change in magnetic permeability of a metamaterial cell. In the method of actively controlling a metamaterial device, liquid crystal, a thermally controlled material, or laser irradiation is used at high frequencies, in particular, from sub-THz to THz (from 100 GHz to 100 THz) (non-patent literature 1).

[0005] On the other hand, an electrostrictive material is an electromechanical material that converts electric energy into mechanical deformation or strain. For example, an electrostrictive material is used in a MEMS (Micro Electro Mechanical Systems) such as switches, micro-mirrors, micro-valves, micro-machines, or sensors. Strain is induced in an electrostrictive material by predetermined polarization of an electromagnetic wave or an applied electric field. As a result, the dimension (for example, a length such as a thickness or a width, or a volume) of an electrostrictive material can be changed. Along with a change in dimension of the electrostrictive material, the dielectric constant of the electrostrictive material also changes (non-patent literature 2).

[0006] Shuyuan, et al., "Active metamaterials and metadevices: a review", Journal of Physics D: Applied Physics, pp. 1-24, Issue Sep, 2020.Gildas, et al., "Physical behavior of electrostrictive polymers. Part1: Polarizationforces. Computational MaterialsScience", 2021, 190, pp. 110294. 10.1016 / j.commatsci.2021.110294. hal-03265512

[0007] However, the above-described method of actively controlling a metamaterial device has the following problems.

[0008] Liquid crystal is applicable to control of a metamaterial device from 0.2 to 0.3 THz without limitation. However, since the size of a metamaterial is inversely proportional to the frequency, the size of liquid crystal using a metamaterial becomes large from sub-THz to THz band. In addition, an incidence loss and electromagnetic wave absorption in the liquid crystal are relatively large.

[0009] When a thermally controlled material such as vanadium oxide (VO2) is used to control a metamaterial device, it is possible to readily control the size of an active region. However, it takes time in seconds or minutes to cool the material. Therefore, a method using this material is not suitable to high-speed control necessary for many applications.

[0010] As for diodes and transistors, a problem arises in terms of scaling of the size and a capacity inevitably formed in a manufacturing process. Furthermore, even if optimum diodes and transistors are used, they cannot operate at frequencies higher than 1 THz.

[0011] When controlling a metamaterial device using laser irradiation, a laser operation influences the active characteristic of a micro- or nano-scale metamaterial cell in a high-frequency band, thereby causing a problem.

[0012] To solve the above-described problems, an electrostrictive element according to the present invention is an electrostrictive element through which an electromagnetic wave is transmitted, and includes a metamaterial cell in which a layer made of a material having an electrostrictive effect and one of a dielectric layer and a conductive layer are stacked. A dimension and a dielectric constant of the metamaterial cell change due to application of an external electric field, thereby changing a characteristic of the electromagnetic wave.

[0013] According to the present invention, it is possible to provide an electrostrictive element that can accurately, actively control a metamaterial device.

[0014] Fig. 1A is a schematic view of an electrostrictive element according to the first embodiment of the present invention;Fig. 1B is a schematic view of an electrostrictive element according to the first embodiment of the present invention;Fig. 1C is a schematic view of an electrostrictive element according to the first embodiment of the present invention;Fig. 2A is a schematic view showing an example of the configuration of a metamaterial cell in the electrostrictive element according to the first embodiment of the present invention;Fig. 2B is a schematic view showing an example of the configuration of a metamaterial cell in the electrostrictive element according to the first embodiment of the present invention;Fig. 2C is a schematic view showing an example of the configuration of a metamaterial cell in the electrostrictive element according to the first embodiment of the present invention;Fig. 2D is a schematic view showing an example of the configuration of a metamaterial cell in the electrostrictive element according to the first embodiment of the present invention;Fig. 2E is a schematic view showing an example of the configuration of a metamaterial cell in the electrostrictive element according to the first embodiment of the present invention;Fig. 2F is a schematic view showing an example of the configuration of a metamaterial cell in the electrostrictive element according to the first embodiment of the present invention;Fig. 3A is a view for explaining the operation of the electrostrictive element according to the first embodiment of the present invention;Fig. 3B is a view for explaining the operation of the electrostrictive element according to the first embodiment of the present invention;Fig. 3C is a view for explaining the operation of the electrostrictive element according to the first embodiment of the present invention;Fig. 4A is a view for explaining the effect of the electrostrictive element according to the first embodiment of the present invention;Fig. 4B is a view for explaining the effect of the electrostrictive element according to the first embodiment of the present invention;Fig. 5 is a view for explaining the effect of the electrostrictive element according to the first embodiment of the present invention;Fig. 6A is a graph for explaining the effect of the electrostrictive element according to the first embodiment of the present invention;Fig. 6B is a graph for explaining the effect of the electrostrictive element according to the first embodiment of the present invention;Fig. 7A is a graph for explaining the effect of the electrostrictive element according to the first embodiment of the present invention;Fig. 7B is a graph for explaining the effect of the electrostrictive element according to the first embodiment of the present invention;Fig. 8 is a graph for explaining the effect of the electrostrictive element according to the first embodiment of the present invention;Fig. 9 is a schematic view showing the configuration of an electrostrictive element according to Example 1 of the present invention;Fig. 10 is a schematic view showing the configuration of an electrostrictive element according to Example 2 of the present invention;Fig. 11A is a schematic view showing the configuration of an electrostrictive element according to Example 3 of the present invention;Fig. 11B is a schematic view showing the configuration of the electrostrictive element according to Example 3 of the present invention; andFig. 12 is a schematic view showing the configuration of an electrostrictive element according to Example 4 of the present invention.

[0015] <First Embodiment> An electrostrictive element according to the first embodiment of the present invention will be described with reference to Figs. 1A to 8.

[0016] <Configuration of Electrostrictive Element> As shown in Fig. 1A, an electrostrictive element 1 according to this embodiment includes a metamaterial cell 10. With respect to the electrostrictive element 1 and the metamaterial cell 10, among surfaces vertical to a direction (to be described later) in which an external electric field is applied, one surface is a "front surface" and the other surface is a "back surface".

[0017] As shown in Fig. 1A, the metamaterial cell 10 has a stacked structure in which a layer (to be referred to as an "electrostrictive layer" hereinafter) 11 made of an electrostrictive material and layers (to be referred to as "dielectric layers" hereinafter) 12 made of a dielectric material or the like are stacked. The metamaterial cell 10 may include only an electrostrictive layer. The metamaterial cell 10 may have a multilayer structure including a plurality of electrostrictive layers and a plurality of dielectric layers.

[0018] The electrostrictive material is a material having an electrostrictive effect. For example, a ceramic material having an electrostrictive effect is used. As a ceramic material, a Gd-doped ceria (cerium oxide) thin film may be used. Alternatively, (Nb,Y)-stabilized bismuth oxide (Bi7Nb2-xYxO2-2 / x) or La2Mo2O9may be used. As another material, methylammonium lead iodide (MAPbI3) as lead halide perovskite single crystal, a polymer matrix-based soft nanocomposite, a carbon nanotube, crystal graphene, or the like may be used. These materials exhibit an electrostrictive effect along the electric field direction.

[0019] The dielectric material or the like includes a dielectric or non-conductive material such as a ceramic, a polymer, TiO2, and SiO2.

[0020] Alternatively, as shown in Fig. 1B, the metamaterial cell 10 may have a stacked structure in which the electrostrictive layer 11 and conductive layers 13_1 and 13_2 made of a metal are stacked. For the conductive layers 13_1 and 13_2, conductive polymer or the like may be used instead of the metal.

[0021] Alternatively, as shown in Fig. 1C, the metamaterial cell 10 may include the conductive layers 13_1 and 13_2 on the front surface and back surface of the stacked structure in which the electrostrictive layer 11 and the dielectric layers 12 are stacked, respectively.

[0022] In the configurations shown in Figs. 1A to 1C, an external electric field E is applied by placing the electrostrictive element 1 in the external electric field.

[0023] In the configurations shown in Figs. 1B and 1C, the conductive layers 13_1 and 13_2 arranged on the front surface and back surface of the electrostrictive element 1 serve as electrodes and an electric field is applied between the electrodes, thereby applying the external electric field E. As shown in Fig. 1C, the electrostrictive layer 11 and the conductive layers 13_1 and 13_2 may be electrically isolated by the dielectric layer 12.

[0024] When metamaterial cells of different structures are used, changes in dimension and dielectric constant occur in different frequency ranges.

[0025] This embodiment has exemplified the metamaterial cell having a columnar structure. However, the present invention is not limited to this, and a metamaterial cell having another structure may be used. Figs. 2A to 2F show examples of the configuration of the metamaterial cell 10. The metamaterial cell 10 may have, for example, a U shape or split-ring shape (Fig. 2A or 2B), a cross shape (Fig. 2C), a columnar shape (Fig. 2D), a closed-ring shape (Fig. 2E), a split ring resonator shape (Fig. 2F), or the like.

[0026] The metamaterial cell 10 shown in each of Figs. 2A and 2B includes a gap 101.

[0027] Each of Figs. 2A to 2F shows a stacked structure of the electrostrictive layer 11 and the dielectric layers 12 corresponding to Fig. 1A. However, a stacked structure of the electrostrictive layer 11 and the conductive layers 13_1 and 13_2 corresponding to Fig. 1B may be adopted. A configuration including a stacked structure of the electrostrictive layer 11 and the dielectric layers 12 and the conductive layers 13_1 and 13_2, which corresponds to Fig. 1C, may be adopted.

[0028] The metamaterial cell 10 has a subwavelength resonant structure. At a predetermined frequency, the metamaterial cell 10 couples with an electric component or a magnetic component of an incident electromagnetic wave and resonates. For resonance excitation, the size of the metamaterial is not more than a wavelength λ of the incident electromagnetic wave (millimeter wave), and is desirably not more than the λ / 2.

[0029] A filling material may be arranged around the metamaterial cell 10. As the filling material, an insulating polymer (parylene, SU-8, polyimide, or the like) or a dielectric material (SiO2, ZnO, or the like) may be used.

[0030] <Principle of Electrostrictive Element> The operation principle of the electrostrictive element 1 according to this embodiment will be described with reference to Figs. 3A to 3C.

[0031] Figs. 3A to 3C are views for explaining the principle of the electrostrictive effect in an electrostrictive material. As shown in Fig. 3A, when no electric field is applied to the electrostrictive material (E = 0), the electrostrictive material maintains the dimension and material constant (state 1).

[0032] The "dimension" includes the dimension (length) such as a width and thickness, the area, and the volume of the electrostrictive material. In this embodiment, the dimension indicates the length in a direction in which the electric field is applied, for example, the thickness of the electrostrictive material.

[0033] As shown in Figs. 3B and 3C, when an external electric field (E ≠ 0) is applied, the electrostrictive material changes the dimension by aligning electric dipoles along the direction of the electric field. A change in the alignment and stretching of the dipoles changes the dielectric constant of the electrostrictive material. For example, the dielectric constant increases by several times. The external electric field causes compressive electrostriction (a decrease in dimension, Fig. 3B) or tensile electrostriction (an increase in dimension, Fig. 3C) with respect to different materials.

[0034] In the electrostrictive material, polarization P induced by the electric field is given by equation (1) below.

[0035]

[0036] where ε0represents a dielectric constant in vacuum, and χijrepresents an electric susceptibility.

[0037] Strain x induced by a stress X is based on Hooke's law by equation (2) below.

[0038]

[0039] where s represents an elastic compliance tensor.

[0040] In accordance with equations (1) and (2), the electromechanical properties associate the strain and stress with the electric field and the polarization, respectively.

[0041] Four electrostrictive tensors will be described below. Among the four electrostrictive tensors, two electrostrictive tensors are associated with the strain generated by the electric field E and the strain generated by the polarization P, respectively, and are given by equations (3) and (4) below, respectively.

[0042]

[0043]

[0044] where Mijkl(m2 / V2) and Qijkl(m4 / C2) are an electric field tensor and a polarization electrostrictive tensor each associated with the strain.

[0045] The other two electrostrictive tensors are associated with the stress generated by the electric field E and the stress generated the polarization P, respectively, and are given by equations (5) and (6) below, respectively.

[0046]

[0047]

[0048] where mijkl(m2 / V2) and qijkl(m4 / C2) are an electric field tensor and a polarization electrostrictive tensor each associated with the stress.

[0049] Based on the relationship among the four electrostrictive tensors, electric field components are applied along the same direction, thereby providing a quadratic relationship between the strain / stress and the electric field / polarization. This means that control of the electric field E and the polarization P changes the dimension and the dielectric constant of the electrostrictive material.

[0050] The selection guide of the metamaterial cell 10 is based on a condition that the cell main body couples with an external electromagnetic wave to change the characteristic of an output wave. According to the present invention, the size, shape, and period of the metamaterial cell are designed to be able to implement a predetermined characteristic within a requested frequency range. A change in size normally depends on a piezoelectric coefficient Q, and is thus represented using a Newnham's relational expression by expression (7) below.

[0051]

[0052] where s represents the elastic compliance tensor, ε0represents the dielectric constant in vacuum, and εrrepresents a relative dielectric constant.

[0053] The Newnham's relational expression defines an experimental (empirical) scaling law that represents the properties of a dielectric electrostrictive material.

[0054] The elongation / strain of the electrostrictive material is expressed as the percentage of the first dimension. Scaling of the metamaterial with respect to different frequencies requires a change in thickness of the electrostrictive material. Since a change in dimension is proportional to the size, the metamaterial can be applied at various frequencies within a wide THz frequency range. To apply the metamaterial at different frequencies, it is necessary to adjust (tune) a resonant frequency by rescaling the dimension of the metamaterial cell and / or adjusting the shape.

[0055] <Effect> The effect of the electrostrictive element 1 according to this embodiment will be described with reference to Figs. 4A to 8.

[0056] Figs. 4A and 4B are schematic views each showing interference between the electrostrictive material and an incident electromagnetic wave with an amplitude A1and a phase φ1. An incident wave transmitted through the metamaterial in a state (state 1) in which no bias (electric field) is applied to the electrostrictive material changes the amplitude and phase by absorption, reflection, and the dielectric characteristic of the material, thereby outputting an electromagnetic wave with an amplitude A2and a phase φ2(Fig. 4A).

[0057] In a state (state 2) in which a bias (electric field) is applied to the electrostrictive material, the dimension and dielectric constant of the electrostrictive material change to change the amplitude and phase of the transmitted electromagnetic wave, thereby outputting an electromagnetic wave with an amplitude A3and a phase φ3(Fig. 4B). A change in thickness of the electrostrictive material, that is, an increase or decrease in height of the electrostrictive material shifts the resonant frequency, thereby changing the transmission characteristic. By controlling the dimension and the dielectric constant, the characteristic of the incident electromagnetic wave can be controlled correctly.

[0058] Fig. 5 shows the metamaterial cell 10 of the columnar structure used in a simulation. Fig. 5 shows shape parameters used in this embodiment. The parameters are adjusted to 275 GHz frequency. A dielectric material used in the simulation is a microwave ceramic dielectric material, and is, for example, a lead chalcogenide-based material having a relative dielectric constant of 90. This relative dielectric constant is a typical value of a material of this type in the millimeter-wave band.

[0059] The electrostrictive material 11 is arranged between two ceramic plates 12. As shown in Fig. 5, the metamaterial cell 10 is arranged horizontally, and is arranged vertically to an electromagnetic wave emitted from port 1 and received by port 2. In this embodiment, since the metamaterial cell 10 is centrosymmetric, the polarization of the incident electromagnetic wave is not important.

[0060] In an asymmetric metamaterial cell including the gap 101 (for example, Fig. 3A or 3B), the direction of the electric field E of the electromagnetic wave needs to be vertical to the gap to induce the flow of a current in the metamaterial cell.

[0061] The simulation of the metamaterial cell is executed by a time-domain simulation using the vertical incidence and periodic boundary conditions with respect to the electric field components of the electromagnetic wave along the x-axis.

[0062] The shape parameters of the cell are optimized to implement a high transmission coefficient by assuming that the period of the metamaterial cell is constant. As a filling material, air with ε0= 1 is used.

[0063] Materials other than air may be used as a filling material. For the filling material, for example, various types of resins (SU-8, polyimide, benzocyclobutene, and the like), ceramic materials, TiO2, SiO2, and the like may be used. The shape of the simulated metamaterial needs to be optimized in consideration of the dielectric constant of the filling material to achieve a high transmission characteristic.

[0064] In the first embodiment, the metamaterial cell has the above-described columnar structure, and can change the characteristic of the incident electromagnetic wave by changing the thickness and dielectric constant of the electrostrictive material.

[0065] In this simulation, a thickness (dd) of a dielectric plate is 30 μm, a relative dielectric constant εdis 90, and a magnetic permeability μ is 1. A radius r of the columnar structure is 130 μm. These parameters are constant in all simulations. A first thickness deof the electrostrictive material is 210 μm, and a relative dielectric constant εeis 2. In the present invention, the thickness deand the dielectric constant (relative dielectric constant εe) are changed in the electric field E in accordance with the intensity of the electric field E. The size and period of the metamaterial cell are constant, and a = 350 μm is set in all simulations.

[0066] Each of Figs. 6A and 6B shows the simulation result of a transmission coefficient (S parameter) S21 and the transmission phase with respect to a change in relative dielectric constant εeof the electrostrictive material. The frequency range is from 250 GHz to 300 GHz. Each of a plurality of S21 spectra (solid lines) shown in Fig. 6A is a spectrum observed along with a change in relative dielectric constant εe. Similarly, each of a plurality of phase spectra (solid lines) shown in Fig. 6B is a spectrum observed along with a change in relative dielectric constant εe.

[0067] As shown in Fig. 6A, in the S21 spectrum, resonant frequency peaks are observed near 280 GHz and 290 GHz. These resonant frequency peaks shift to the high-frequency side along with a change in relative dielectric constant εe. As shown in Fig. 6B, in accordance with the resonant frequency peaks, abrupt phase changes shift.

[0068] In this way, the shift of the resonant frequency is observed within the range of the simulated dielectric constant (relative dielectric constant εe= 2 to 13). The shift of the resonant frequency changes the transmission coefficient and the value of the phase.

[0069] Each of Figs. 7A and 7B shows the simulation result of a phase change and a change in transmission coefficient of the incident electromagnetic wave with respect to a change in relative dielectric constant of the electrostrictive material. In this example, the frequency is 276.5 GHHz and the relative dielectric constant εefalls within the range of 2 to 13. The thickness of the electrostrictive material is de= 220 μm that is constant.

[0070] As shown in Fig. 7A, the transmission coefficient S21 is observed at high values of 0 to 4 dB with respect to all the dielectric constants (relative dielectric constant εe= 2 to 13).

[0071] As shown in Fig. 7B, along with a change in dielectric constant of the metamaterial cell and the shift of the resonant frequency, a change in transmission phase is observed within a range of 360° (2π) in a high-transmission region. The high transmission coefficient and a phase change within the range of 360° (2π) indicate that the metamaterial cell is suitable to various applications such as a beamformer, lens, and sensor.

[0072] Fig. 8 shows a phase change while an electromagnetic wave is transmitted through the metamaterial cell. With respect to a constant value of the dielectric constant, the thickness deof the electrostrictive material changes within the range of 210 to 220 μm, that is, by 5% of the first thickness in the 276.5 GHz frequency band. A change amount of 5% is a value that can be estimated based on the properties (physical constant and the like) of the electrostrictive material. As shown in Fig. 8, the change in thickness causes a phase change within the range of about 120° (60 to -60°).

[0073] These simulation results indicate that the phase of the electromagnetic wave changes along with a change in thickness of the metamaterial cell caused by applying the electric field.

[0074] In the electrostrictive material, its thickness and dielectric constant change at the same time. As a result, with respect to the electrostrictive material, it is possible to change the phase within a wide frequency range, and decrease the intensity of the electric field necessary to control the electrostrictive material.

[0075] In the electrostrictive element according to this embodiment, it is possible to change the strain and dielectric constant of the electrostrictive material by applying the electric field. This can correctly control a response by the electrostrictive effect, thereby improving the transmission characteristic of the electromagnetic wave.

[0076] The electrostrictive element according to this embodiment is implemented in a metamaterial-based device (to be referred to as a "metamaterial device" hereinafter) that changes the electromagnetic wave characteristic based on transmission of an electromagnetic wave, for example, a beamformer, a phase shifter, a lens, an absorber, or the like. This can accurately control the metamaterial device.

[0077] This embodiment has explained the example in which a single electrostrictive layer is used in a metamaterial cell. However, a multilayer stacked structure formed by a plurality of electrostrictive layers and a plurality of dielectric layers may be used.

[0078] <Example 1> An electrostrictive element according to Example 1 of the present invention will be described with reference to Fig. 9.

[0079] <Configuration of Electrostrictive Element> As shown in Fig. 9, an electrostrictive element 2 according to Example 1 includes a metamaterial cell 20 having a columnar structure. The metamaterial cell 20 includes a multilayer stacked structure and conductive layers (electrodes) 13_1 and 13_2. In the multilayer stacked structure, an electrostrictive layer 11 and a dielectric layer 12 are alternately stacked, and the dielectric layers 12 are arranged as a layer nearest to the front surface and a layer nearest to the back surface.

[0080] The material of the dielectric layer 12 is a dielectric material or the like, and increases the total strain and a change in dielectric constant in the metamaterial cell 20.

[0081] The conductive layers 13_1 and 13_2 are arranged as electrodes on the front surface and the back surface of the multilayer stacked structure. One electrode 13_1 is a bias electrode and the other electrode 13_2 is a ground electrode. The material of the conductive layers (electrodes) 13_1 and 13_2 is the same as in the first embodiment.

[0082] In the electrostrictive element 2, electric dipoles are aligned for each electrostrictive layer 11 by applying an external electric field E.

[0083] By applying the external electric field E, the thickness of each electrostrictive layer 11 changes, thereby increasing the total strain. Furthermore, the dielectric constant of each electrostrictive layer 11 changes.

[0084] In the electrostrictive element 2, the multiple electrostrictive layers 11 increase the phase change in transmission of an incident electromagnetic wave to a range of 360° (2π), as compared with a phase change (for example, a phase change of 120° shown in Fig. 8) in a single electrostrictive layer.

[0085] <Effect> In the electrostrictive element of Example 1, it is possible to increase strain and a change in dielectric constant by using the multilayer structure for the electrostrictive element. This can control a material response more correctly, and further improve the transmission characteristic of the electromagnetic wave. Furthermore, since resonance is generated in the metamaterial cell, for example, it is possible to improve the transmission characteristic by more satisfactory impedance matching between the metamaterial and air. In addition, many electrostrictive layers can decrease the intensity of the electric field E necessary to control the electrostrictive element.

[0086] The electrostrictive element according to Example 1 is effective in a case where the electrostrictive element with the single electrostrictive layer is applied to the metamaterial device and a sufficiently high transmission coefficient and a phase change cannot be induced.

[0087] <Example 2> An electrostrictive element according to Example 2 of the present invention will be described with reference to Fig. 10.

[0088] <Configuration of Electrostrictive Element> As shown in Fig. 10, an electrostrictive element 3 according to Example 2 includes metamaterial cell 30 having a columnar structure. The metamaterial cell 30 includes a multilayer stacked structure and conductive layers (electrodes) 13_1 and 13_2. In the multilayer stacked structure, an electrostrictive layer 11 and a dielectric layer are alternately stacked, and the dielectric layers are arranged as a layer nearest to the front surface and a layer nearest to the back surface.

[0089] The material of the dielectric layer is, for example, a dielectric material or the like, and increases the total strain and a change in dielectric constant in the metamaterial cell 30.

[0090] The conductive layers 13_1 and 13_2 are arranged as bias application electrodes on the front surface and the back surface of the multilayer structure. In this configuration, the dielectric layers 12 are arranged between the electrostrictive layer 11 and the electrode (to be also referred to as a "front electrode" hereinafter) 13_1 arranged on the front surface and between the electrostrictive layer 11 and the electrode (to be also referred to as a "back electrode" hereinafter) 13_2 arranged on the back surface.

[0091] A ground electrode 13_3 is arranged on the outer periphery of the electrostrictive layer 11 at an intermediate point between the front surface and the back surface. In this way, the ground electrode 13_3 is arranged to be electrically connected to the electrostrictive layer 11 at an intermediate point between the front surface and the back surface. The material of the ground electrode 13_3 is the same as in the first embodiment.

[0092] Example 2 has explained the example in which the ground electrode 13_3 is arranged on the whole outer periphery of the electrostrictive layer 11 but the present invention is not limited to this. The ground electrode 13_3 may be arranged in part of the electrostrictive layer 11. The conductive layer may be used as a ground electrode instead of the electrostrictive layer 11 in which the ground electrode 13_3 is arranged.

[0093] Example 2 has explained the example in which the ground electrode 13_3 is arranged to be electrically connected to the central electrostrictive layer 11 but the present invention is not limited to this. The ground electrode 13_3 may be arranged to be electrically connected to any of the electrostrictive layers 11 in the multilayer structure.

[0094] <Operation of Electrostrictive Element> In the electrostrictive element 3, a bias applied to the front electrode 13_1 induces an electric field E only in a region between the front electrode 13_1 and the ground electrode 13_3, that is, a region on the front surface side of the metamaterial 30. This induces changes in dimension and dielectric constant in the region on the front surface side.

[0095] On the other hand, when a bias is applied to the back electrode 13_2, the electric field E is induced only in a region between the back electrode 13_2 and the ground electrode 13_3, that is, a region on the back surface side of the metamaterial cell 30. This induces changes in dimension and dielectric constant in the region on the back surface side.

[0096] As described above, in the electrostrictive element 3, it is possible to electrically isolate the region on the front surface side and the region on the back surface side and control them. That is, by applying different electric fields to the region on the front surface side and the region on the back surface side, respectively, it is possible to induce different transmission characteristics and phase changes in the region on the front surface side and the region on the back surface side.

[0097] Since the electrostrictive element of Example 2 can individually control different regions of the metamaterial structure, it is possible to accurately control the phase and amplitude by applying, to the respective metamaterial cells, biases that induce different electric fields E.

[0098] This can accurately control a device such as a metamaterial-based phase shifter or beamformer.

[0099] <Example 3> An electrostrictive element according to Example 3 of the present invention will be described with reference to Figs 11A and 11B.

[0100] <Configuration of Electrostrictive Element> Figs. 11A and 11B respectively show a schematic view and a top perspective view of an electrostrictive element 4 according to Example 3. In Fig. 11B, solid lines represent a front electrode structure 14_1 arranged on the front surface of the electrostrictive element 4. In Fig. 11B, alternate long and short dashed lines represent a back electrode structure 14_2 arranged on the back surface of the electrostrictive element 4. In Fig. 11B, a dotted line represents a metamaterial cell 30.

[0101] As shown in Figs. 11A and 11B, the electrostrictive element 4 according to Example 3 includes the metamaterial cells 30 arranged in a grid pattern. The configuration of the metamaterial cell 30 is the same as in Example 2.

[0102] A plurality of metamaterial cells 30 are periodically arranged at equal intervals in each of the x-axis direction and the y-axis direction in Figs. 11A and 11B.

[0103] A filling material may be arranged between the side surfaces of the plurality of metamaterial cells 30.

[0104] The front electrode structure 14_1 and the back electrode structure 14_2 are arranged on the front surface and the back surface of the electrostrictive element 4, respectively. In the front electrode structure 14_1, a plurality of band-like electrodes 14_1_1 to 14_1_n extend in the x direction in Figs. 11A and 11B to be arranged in an array, and are arranged to be connected to a front electrode (conductive layer) 13_1 provided for each column of the metamaterial cells 30 arrayed in the y direction.

[0105] On the other hand, in the back electrode structure 14_2, a plurality of band-like electrodes 14_2_1 to 14_2_n extend in the y direction in Figs. 11A and 11B to be arranged in an array, and are arranged to be connected to a back electrode (conductive layer) 13_2 provided for each column of the metamaterial cells 30 arrayed in the x direction.

[0106] Furthermore, ground electrodes 13_3 of the plurality of metamaterial cells 30 are connected by an electrode 14_3 arranged on a horizontal surface.

[0107] In the electrostrictive element 4, voltages (V1 to Vn) that increase from above to below in Fig. 11B are applied to the band-like electrodes 14_1_1 to 14_1_n in the front electrode structure 14_1, respectively. This applies an electric field corresponding to the voltage (V1 to Vn) to a region (a region on the front surface side) between the front electrode 13_1 and the ground electrode 13_3 of each metamaterial cell 30.

[0108] On the other hand, the voltages (V1 to Vn) that increase from left to right in Fig. 11B are applied to the band-like electrodes 14_2_1 to 14_2_n in the back electrode structure 14_2, respectively. This applies an electric field corresponding to the voltage (V1 to Vn) to a region (a region on the back surface side) between the back electrode 13_2 and the ground electrode 13_3 of each metamaterial cell 30.

[0109] The voltages V1 to Vn are increased by an equal voltage difference. The voltages V1 to Vn may be changed by different voltage differences.

[0110] With combinations of voltages (signals) applied to the front electrode structure 14_1 and the back electrode structure 14_2, the electric fields E of various values are applied to the metamaterial cells 30.

[0111] For example, in Figs. 11A and 11B, in a metamaterial cell M11, with the voltage V1 applied to the front electrode and the voltage V1 applied to the back electrode, a total E11 of electric fields corresponding to the voltages V1 is applied to the metamaterial cell M11. Similarly, by applying electric fields E22, E33, E44,..., Enn to metamaterial cells M22, M33, M44,..., Mnn, an electric field gradient (a gradual change in electric field) can be generated from the metamaterial cell Mnn to the metamaterial cell M11. This can induce a phase gradient (a gradual change in phase) between the metamaterial cells Mnn and M11 (a solid line arrow in Fig. 11B).

[0112] For example, in Figs. 11A and 11B, by applying electric fields to the metamaterial cells M43 and M1n, respectively, an electric field gradient can be generated from the metamaterial cell M43 to the metamaterial cell M1n, thereby inducing a phase gradient between the metamaterial cells Mn43 and M1n (a dotted line arrow in Fig. 11B).

[0113] As described above, the electric fields E of various values can be applied to the metamaterial cells, respectively, to induce various changes in transmission characteristics, thereby obtaining a phase gradient distribution in the overall device.

[0114] In the electrostrictive element according to Example 3, with control bias signals applied along the x-axis and the y-axis, various electric fields are applied to the metamaterial cells, respectively, thereby obtaining the two-dimensional gradient (gradual change) of the phase. This can accurately control a device such as a metamaterial-based phase shifter or beamformer.

[0115] Example 3 has explained the example in which the plurality of metamaterial cells are periodically arranged at equal intervals but the present invention is not limited to this. The plurality of metamaterial cells need not be arranged periodically.

[0116] <Example 4> An electrostrictive element according to Example 4 of the present invention will be described with reference to Fig. 12.

[0117] <Configuration of Electrostrictive Element> As shown in Fig. 12, an electrostrictive element 5 according to Example 4 includes metamaterial cells 10 and a filling material 15, and the metamaterial cells 10 and the filling material 15 form a mesh structure.

[0118] The metamaterial cell 10 has a columnar structure, and is formed by a stacked structure of an electrostrictive layer 11 and a dielectric layer 12. The plurality of metamaterial cells 10 are periodically arranged at equal intervals in each of the x-axis direction and the y-axis direction in Fig. 12. The plurality of metamaterial cells 10 need not be arranged periodically. The metamaterial cell 10 may have a structure including only the electrostrictive layer. The metamaterial cell 10 may have a multilayer structure formed by a plurality of electrostrictive layers and a plurality of dielectric layers.

[0119] The filling material 15 is arranged between the plurality of metamaterial cells 10.

[0120] The electrostrictive element 5 operates in a state in which it is arranged in an external electric field E. By applying the external electric field, the dimension and dielectric constant of the metamaterial cell 10 change, thereby shifting the resonant frequency. With the design of the mesh structure, for example, the design of the thickness of an electrostrictive material, the metamaterial cell, and the like, it is possible to adjust the characteristic of the electrostrictive element, and change transmission of an electromagnetic wave into reflection or absorption by the external electric field E in accordance with the operating frequency.

[0121] Since the electrostrictive element according to Example 4 includes the plurality of metamaterial cells, it is possible to increase the strain and a change in dielectric constant, as compared to an electrostrictive element including a single metamaterial cell, and to decrease the intensity of the external electric field E necessary to control the electrostrictive element. Furthermore, with a simple configuration including no electrode, it is possible to accurately control a device such as a metamaterial-based phase shifter or beamformer.

[0122] In the embodiment and examples of the present invention, a columnar metamaterial cell is used. However, the present invention is not limited to this. The cross-sectional shape of the columnar structure may be an elliptical shape, a triangular shape, a rectangular shape, or another polygonal shape instead of a circular shape.

[0123] In the embodiment and examples of the present invention, the conductive layer of the metamaterial cell is used as an electrode. However, the present invention is not limited to this. An electrode may be arranged in the conductive layer of the metamaterial cell.

[0124] In the embodiment and examples of the present invention, in the configurations of the electrostrictive element, examples of the structure, dimension, material, and the like of each constituent part have been described. However, the present invention is not limited to these. It is only necessary that the functions and effects of the electrostrictive element can be obtained.

[0125] Note that the present invention is not limited to the above-described embodiment and examples, and it is obvious that various modifications and combinations can be made by those who have normal knowledge in the field without departing from the technical scope of the present invention. For example the first embodiment may be combined with Example 4.

[0126] Some or all of the above-described exemplary embodiments and examples can also be described as in the following supplementary notes but are not limited to the followings.

[0127] (Supplementary Note 1) There is provided an electrostrictive element through which an electromagnetic wave is transmitted, comprising a metamaterial cell in which a layer made of a material having an electrostrictive effect and one of a dielectric layer and a conductive layer are stacked, wherein a dimension and a dielectric constant of the metamaterial cell change due to application of an external electric field, thereby changing a characteristic of the electromagnetic wave.

[0128] (Supplementary Note 2) In the electrostrictive element according to supplementary note 1, the metamaterial cell has a stacked structure in which the layer made of the material having the electrostrictive effect and the dielectric layer are stacked, and further includes a first conductive layer arranged on one surface vertical to a direction in which the external electric field is applied, and a second conductive layer arranged on another surface parallel to the one surface, and the dielectric layers are arranged between the first conductive layer and the layer made of the material having the electrostrictive effect and between the second conductive layer and the layer made of the material having the electrostrictive effect.

[0129] (Supplementary Note 3) In the electrostrictive element according to supplementary note 2, the metamaterial cell further includes either of a third conductive layer or one of the layers made of the material having the electrostrictive effect to which a ground electrode is connected.

[0130] (Supplementary Note 4) The electrostrictive element according to supplementary note 3, further comprises a plurality of metamaterial cells, wherein the plurality of metamaterial cells are arranged in a grid pattern along one direction and another direction orthogonal to the one direction, the first electrodes of the metamaterial cells are connected for each column of the metamaterial cells arrayed along the one direction, the second electrodes of the metamaterial cells are connected for each column of the metamaterial cells arrayed along the other direction, and either of the third conductive layers or the layers made of the material having the electrostrictive effect to which a ground electrode of the plurality of metamaterial cells are connected.

[0131] (Supplementary Note 5) The electrostrictive element according to any one of supplementary notes 1 to 4, further comprises a plurality of metamaterial cells, and a filling material arranged between side surfaces of the plurality of metamaterial cells.

[0132] (Supplementary Note 6) In the electrostrictive element according to any one of supplementary notes 2 to 5, the layer made of the material having the electrostrictive effect and the dielectric layer are alternately stacked.

[0133] (Supplementary Note 7) In the electrostrictive element according to any one of supplementary notes 2 to 6, the layer on the side of the one surface of the stacked structure and the layer on the side of the other surface are the dielectric layers.

[0134] (Supplementary Note 8) In the electrostrictive element according to any one of supplementary notes 1 to 7, the metamaterial cell has a subwavelength resonant structure, and couples with an electric field component of an incident electromagnetic wave.

[0135] (Supplementary Note 9) In the electrostrictive element according to any one of supplementary notes 1 to 8, a size of the metamaterial cell is shorter than a wavelength of the incident electromagnetic wave.

[0136] (Supplementary Note 10) In the electrostrictive element according to any one of supplementary notes 1 to 9, the material having the electrostrictive effect is formed by ceramic having the electrostrictive effect.

[0137] (Supplementary Note 11) In the electrostrictive element according to supplementary note 10, the ceramic is one of Gd-doped cerium oxide, (Nb,Y)-stabilized bismuth oxide, and La2Mo2O9.

[0138] (Supplementary Note 12) In the electrostrictive element according to any one of supplementary notes 1 to 9, the material having the electrostrictive effect is one of methylammonium lead iodide as a lead halide perovskite single crystal and a polymer matrix-based soft nanocomposite.

[0139] (Supplementary Note 13) In the electrostrictive element according to any one of supplementary notes 1 to 9, the material having the electrostrictive effect is one of a carbon nanotube and crystal graphene.

[0140] (Supplementary Note 14) In the electrostrictive element according to any one of supplementary notes 1 to 12, a material of the dielectric layer is an insulating polymer.

[0141] (Supplementary Note 15) In the electrostrictive element according to any one of supplementary notes 1 to 12, a material of the dielectric layer is a dielectric material.

[0142] The present invention is related to an electrostrictive element using a metamaterial, can be adapted to a metamaterial-based device such as a beamformer, a phase shifter, a lens, or an absorber, and can be applied to a millimeter-wave antenna or the like in high-frequency wireless communication.

[0143] 1...electrostrictive element 10...metamaterial cell 11...layer (electrostrictive layer) made of material having electrostrictive effect 12...dielectric layer 13...conductive layer

Claims

1. An electrostrictive element through which an electromagnetic wave is transmitted, comprising: 　　　　a metamaterial cell in which a layer made of a material having an electrostrictive effect and one of a dielectric layer and a conductive layer are stacked, 　　　　wherein a dimension and a dielectric constant of the metamaterial cell change due to application of an external electric field, thereby changing a characteristic of the electromagnetic wave.

2. The electrostrictive element according to claim 1, wherein 　　　　the metamaterial cell has a stacked structure in which the layer made of the material having the electrostrictive effect and the dielectric layer are stacked, and 　　　　further includes a first conductive layer arranged on one surface vertical to a direction in which the external electric field is applied, and a second conductive layer arranged on another surface parallel to the one surface, and 　　　　the dielectric layers are arranged between the first conductive layer and the layer made of the material having the electrostrictive effect and between the second conductive layer and the layer made of the material having the electrostrictive effect.

3. The electrostrictive element according to claim 2, wherein the metamaterial cell further includes either of a third conductive layer or one of the layers made of the material having the electrostrictive effect to which a ground electrode is connected.

4. The electrostrictive element according to claim 3, further comprising: 　　　　a plurality of metamaterial cells, 　　　　wherein the plurality of metamaterial cells are arranged in a grid pattern along one direction and another direction orthogonal to the one direction, 　　　　the first conductive layers of the metamaterial cells are connected for each column of the metamaterial cells arrayed along the one direction, 　　　　the second conductive layers of the metamaterial cells are connected for each column of the metamaterial cells arrayed along the other direction, and 　　　　either of the third conductive layers or the layers made of the material having the electrostrictive effect to which a ground electrode of the plurality of metamaterial cells are connected.

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