Radio wave control element

Abstract

Provided is a radio wave control element that can shorten switching time and reduce radio wave loss. The radio wave control element comprises a first electrode, a liquid crystal composition layer, and a second electrode. The radio wave control element has a metasurface structure in which a plurality of fine structures are arranged. The metasurface structure constitutes at least a part of the first electrode and includes, between the liquid crystal composition layer and one of the first electrode or the second electrode, an intermediate layer in which a maximum refractive index change per unit voltage in an electromagnetic wave in a wavelength range of 300 μｍ - 30 cm is 0.001(1 / V) or less.

Description

Radio wave control element

[0001] The present invention relates to a radio wave control element.

[0002] Radio waves, such as the high-frequency radio waves (millimeter waves and terahertz waves) required for high-capacity wireless communication, tend to propagate in a straight line. Therefore, there is a need for radio wave control elements that can bend the direction of radio waves in any direction. However, for example, a conventional reflector reflects radio waves in a fixed direction, and the reflection direction is a specular reflection with the angle of incidence and the angle of emission being equal. This significantly limits the range in which the direction of radio waves can be changed, making it difficult to deliver radio waves to the desired destination.

[0003] In response to this, a radio wave control element has been proposed in which a liquid crystal layer is placed between a metasurface structure made of a conductor and a conductor (electrode), and by changing the voltage applied between the two conductors, the refractive index of the liquid crystal layer is changed, thereby changing the directionality of the electromagnetic waves.

[0004] For example, Patent Document 1 describes a structure including a first conductor, a plurality of second conductors that face the first conductor and are arranged repeatedly, a plurality of third conductors that face each of the plurality of second conductors, a dielectric constant variable layer that is provided at least between the plurality of third conductors and the plurality of second conductors or between the plurality of third conductors and the first conductors and whose dielectric constant changes depending on voltage, and a fourth conductor that connects the first second conductor and a second second conductor that is located next to the first second conductor.

[0005] International Publication No. 2011 / 152055

[0006] In radio wave control elements, it is desirable to reduce the switching time when changing the directionality of radio waves. In radio wave control elements in which a liquid crystal layer is disposed between conductors as described above, the switching time can be reduced by reducing the thickness of the liquid crystal layer. However, there is a problem in that reducing the thickness of the liquid crystal layer increases the loss of radio waves.

[0007] In view of the above circumstances, an object of the present invention is to provide a radio wave control element that can shorten the switching time and has small radio wave loss.

[0008] As a result of extensive research into solving the above problems, the present inventors have found that the problems can be solved by the following configuration.

[0009] [1] A radio wave control element having a first electrode, a liquid crystal composition layer, and a second electrode, the element comprising a metasurface structure formed by arranging a plurality of microstructures, the metasurface structure constituting at least a part of the first electrode, and an intermediate layer between one of the first and second electrodes and the liquid crystal composition layer, the intermediate layer having a maximum refractive index change per unit voltage of 0.001 (1 / V) or less for radio waves having a wavelength of 300 μm to 30 cm. [2] The radio wave control element according to [1], further comprising a waveguide for guiding radio waves on the side of the second electrode opposite the liquid crystal composition layer, the second electrode having an opening through which the radio waves pass. [3] The radio wave control element according to [2], wherein the second electrode also serves as a waveguide. [4] The radio wave control element according to any one of [1] to [3], wherein the thickness of the intermediate layer is 0.1 to 2 times the thickness of the liquid crystal composition layer. [5] The radio wave control element according to any one of [1] to [4], which controls radio waves having a wavelength of 300 μm to 30 cm.

[0010] According to the present invention, it is possible to provide a radio wave control element that can shorten the switching time and has little loss of radio waves.

[0011] 7 is a diagram showing an example of use of a radio wave control element. FIG. 8 is a diagram showing an example of a metasurface structure used in a radio wave control element. FIG. 9 is a diagram explaining the mechanism by which the emission direction of radio waves is changed in a radio wave control element. FIG. 10 is a diagram conceptually showing an example of a radio wave control element of the present invention. FIG. 11 is a diagram conceptually showing an example of a liquid crystal orientation pattern in a radio wave control element. FIG. 12 is a diagram showing the relationship between the applied voltage and the amount of phase delay of radio waves. FIG. 13 is a diagram conceptually showing another example of a radio wave control element. FIG. 14 is a perspective view of the radio wave control element shown in FIG.

[0012] The present invention will be described in detail below. The following description of the constituent elements may be based on a representative embodiment of the present invention, but the present invention is not limited to such an embodiment. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0013] In this specification, horizontal and vertical do not mean horizontal and vertical in the strict sense, but rather mean a range of horizontal ±5° and a range of vertical ±5°, respectively.

[0014] In addition, in this specification, for each component, a substance corresponding to the component may be used alone or in combination of two or more. Here, when two or more substances for each component are used in combination, the content of the component refers to the total content of the substances used in combination, unless otherwise specified.

[0015] In this specification, the bonding direction of a divalent group (e.g., -CO-O-, etc.) is not limited unless otherwise specified. For example, when Y is -CO-O- in a compound represented by the formula "X-Y-Z," the compound may be either "X-O-CO-Z" or "X-CO-O-Z."

[0016] [Radio wave control element] The radio wave control element of the present invention is a radio wave control element having a first electrode, a liquid crystal composition layer, and a second electrode, and has a metasurface structure formed by arranging a plurality of microstructures, the metasurface structure constituting at least a part of the first electrode, and an intermediate layer between one of the first and second electrodes and the liquid crystal composition layer, in which the maximum refractive index change per unit voltage in radio waves with a wavelength in the range of 300 μm to 30 cm is 0.001 (1 / V) or less.

[0017] The radio wave control element of the present invention acts on radio waves (electromagnetic waves). Examples of radio waves include those with frequencies of 1 GHz to 1000 GHz, i.e., wavelengths of 300 μm to 30 cm. Radio waves in this frequency band (RW) are also called high-frequency radio waves (centimeter waves, millimeter waves, terahertz waves), and are capable of high-capacity wireless communication while also having high directional properties.

[0018] In the radio wave control element of the present invention, by applying a voltage between the first electrode and the second electrode, the orientation state of the liquid crystal compound contained in the liquid crystal composition layer is controlled, and the refractive index anisotropy of the liquid crystal composition layer is adjusted, thereby adjusting the direction of propagation of radio waves.

[0019] Specific examples of radio wave control elements will be described below with reference to the drawings.

[0020] 1 uses a radio wave control element 10 according to the technique of the present disclosure. The radio wave reflecting device 2 is capable of reflecting highly directional radio waves RW radiated from an antenna ANT located behind a building BL toward an area AR1 in front of the building BL, which is in the shadow when viewed from the antenna ANT.

[0021] Furthermore, the radio wave reflecting device 2 can change the reflection direction of the radio wave RW in different directions between the multiple areas AR1 and AR2. For example, there are cases where the area where there are many users changes depending on the time of day, such as when many users using wireless communication are present in area AR1 during the daytime and many are present in area AR2 during the nighttime. In such cases, the radio wave reflecting device 2 can change the area to which the radio wave RW is supplied by changing the reflection direction of the radio wave RW depending on the time of day.

[0022] As shown in FIG. 2, the radio wave control element 10 has a metasurface structure 12 and is a reflective radio wave control element that reflects the traveling direction of the radio wave RW in a desired direction.

[0023] The metasurface structure 12 is a structure that utilizes metamaterials. Metamaterials are artificial materials that exhibit properties not found in natural materials, such as a negative refractive index for radio waves. The radio wave control element 10 is configured with a plurality of unit cells UC arranged two-dimensionally, and the two-dimensional plane formed by the arrangement of the plurality of unit cells UC serves as a reflection surface for the radio waves RW. Each unit cell UC includes a microstructure 14 as a metamaterial, and constitutes the smallest unit capable of actively changing the phase of the radio waves RW on the reflection surface. The microstructure 14 is made of metal, for example. The microstructure 14 has a size on the order of the wavelength of the incident radio waves RW or less and functions as a resonator that resonates through interaction with the incident radio waves RW. Electrically, the microstructure 14 can be considered equivalent to a resonant circuit, for example, a coil and a capacitor connected in series to resonate an alternating current. The phase of the incident radio waves RW changes due to the resonant effect of the microstructure 14. Furthermore, by actively changing the resonance conditions of the microstructure 14 by various methods, it is also possible to control the amount of phase delay of the radio wave RW.

[0024] The radio wave control element 10 mainly acts on radio waves RW with frequencies of 1 GHz to 1000 GHz (1 THz). Therefore, in the radio wave control element 10, the metasurface structure 12 is configured to act on radio waves RW with frequencies of 1 GHz to 1000 GHz. Radio waves RW with frequencies of 1 GHz to 1000 GHz have wavelengths of 300 μm to 30 cm, and the size of the microstructures 14 that make up the metasurface structure 12 is, for example, on the order of about half the wavelength. By making the size of the microstructures 14 equal to or less than the wavelength of the radio waves RW, the microstructures 14 resonate with the radio waves RW that pass through them and function as a phase modulation element that modulates the phase of the radio waves RW.

[0025] As shown in FIG. 3 by the incident direction IN and the outgoing direction OUT, the overall traveling direction of the radio wave RW can be considered as the normal direction to the line connecting the wavefronts of the multiple radio waves RW. In the radio wave control element 10, for example, consider gradually increasing the phase delay of the radio wave RW incident on and reflected from each of the multiple unit cells UC arranged in a one-dimensional array from the right-hand unit cell UC to the left-hand unit cell UC. In this case, even if the line connecting the wavefronts of the individual incident radio waves RW is parallel to the reflecting surface, the line connecting the wavefronts of the individual radio waves RW reflected by each unit cell UC is inclined with respect to the reflecting surface. In other words, the outgoing direction OUT, which is the traveling direction of the radio wave RW emitted from the reflecting surface, changes by an angle θ with respect to the incident direction IN of the radio wave RW. In this way, the traveling direction of the radio wave RW can be controlled by performing phase modulation, i.e., controlling the phase delay, for each unit cell UC.

[0026] As a result, while a normal reflector can only change the direction of propagation of the radio wave RW in the direction of specular reflection, the radio wave reflecting device 2 can change the direction of propagation of the radio wave RW in a direction other than specular reflection by using the metasurface structure 12. In addition, by actively changing the amount of phase delay in each unit cell UC, it becomes possible to actively change the direction of propagation of the radio wave RW.

[0027] As an example, as conceptually shown in Figure 4, a radio wave control element 10 uses a liquid crystal composition layer 20 as an element that actively changes the resonance conditions of the microstructures 14 of the metasurface structure 12. The radio wave control element 10 has, from the bottom in the figure, a second electrode 26, an intermediate layer 32, a liquid crystal composition layer 20, and a metasurface structure 12, in this order. The second electrode 26, the liquid crystal composition layer 20, and the intermediate layer 32 are provided on a support 24.

[0028] Each unit cell UC is configured to include a microstructure 14, a liquid crystal composition layer 20, an intermediate layer 32, and a second electrode 26. Of these, the microstructure 14 is provided individually for each unit cell UC. The remaining components, the liquid crystal composition layer 20, the intermediate layer 32, the support 24, and the second electrode 26, are not configured independently for each unit cell UC, but are integrally formed in regions corresponding to a plurality of unit cells UC.

[0029] In the radio wave control element 10, the second electrode 26 and the support 24, the second electrode 26 and the intermediate layer 32, and the intermediate layer 32 and the liquid crystal composition layer 20 are each attached using an adhesive (pressure-sensitive adhesive or adhesive) as necessary. There are no limitations on the attachment method, and various known methods that are permeable to the radio waves targeted by the radio wave control element 10, such as a method using an OCA (Optical Clear Adhesive) that is permeable to the radio waves targeted by the radio wave control element 10, can be used.

[0030] The microstructure 14 is formed of, for example, a conductive material, and serves as a first electrode that constitutes an electrode pair together with the second electrode 26 .

[0031] Furthermore, a power supply 28 is connected to each microstructure 14 to apply a voltage between the microstructure 14 and the second electrode 26. This makes it possible to control the magnitude of the voltage applied to each unit cell UC. The second electrode 26 is a common electrode shared by each unit cell UC, and the microstructure 14 of each unit cell UC functions as an individual electrode. The second electrode 26 functioning as a common electrode is an example of a "second electrode" according to the technology of the present disclosure, and the individual electrode shared by the microstructure 14 is an example of a "first electrode." The microstructure 14 as the first electrode and the second electrode 26 as the second electrode are an example of an "electrode pair for applying a voltage."

[0032] There are no particular limitations on the voltage application means for applying a voltage to each microstructure 14, and examples thereof include a thin film transistor (TFT).

[0033] The radio wave control element 10 is of a reflective type, and the second electrode 26 also serves as a reflective layer that reflects the radio wave RW.

[0034] In the liquid crystal composition layer 20, the alignment state (hereinafter also referred to as the alignment pattern) of the liquid crystal compound LC changes upon application of a voltage. The alignment direction of the microstructures 14 of each unit cell UC is a direction (X or Y direction in the figure) perpendicular to the thickness direction of the liquid crystal composition layer 20 (Z direction in the figure). Here, the microstructures 14 and the second electrode 26 are disposed on both sides of the liquid crystal composition layer 20 in the thickness direction. By supplying power from a power source 28 to the microstructures 14 and the second electrode 26, a voltage is applied between the microstructures 14 and the second electrode 26 of each unit cell UC. The application of the voltage generates an electric field in the thickness direction of the liquid crystal composition layer 20, changing the alignment state of the liquid crystal compound LC in each unit cell UC. Furthermore, the alignment state of the liquid crystal compound LC in each unit cell UC can be adjusted by adjusting the voltage applied to each unit cell UC.

[0035] In the example shown in Fig. 5, the liquid crystal compound LC is a rod-shaped liquid crystal compound and has a long axis and a short axis. As an example, when no voltage is applied between the microstructure 14 and the second electrode 26, which function as an electrode pair, no electric field is generated in the liquid crystal composition layer 20. In this state, as conceptually shown in the upper part of Fig. 5, the liquid crystal compound LC is aligned such that its long axis is aligned along the thickness direction of the liquid crystal composition layer 20. In the following description, this alignment state will also be referred to as "vertical alignment."

[0036] The thickness direction is the stacking direction of the microstructure 14 (first electrode), the liquid crystal layer 20, the intermediate layer 32, and the second electrode 26. The main surface is the largest surface of the sheet-like material (film, plate-like material, layer), and is usually both surfaces of the sheet-like material in the thickness direction.

[0037] When a voltage is applied between the microstructures 14 and the second electrode 26 from this state, an electric field is generated in the liquid crystal composition layer 20, and the alignment state of the liquid crystal compound LC changes. Specifically, as conceptually shown in the lower part of Figure 5, the alignment state of the liquid crystal compound LC in the region corresponding to the microstructures 14 changes depending on the magnitude of the applied voltage, and the liquid crystal compound LC is tilted with respect to the thickness direction of the liquid crystal composition layer 20. The example shown in the lower part of Figure 5 shows a state in which the tilt angle of the liquid crystal compound LC is maximum. When the tilt angle is maximum, the liquid crystal compound LC is aligned so that its major axis is aligned in a direction perpendicular to the thickness direction of the liquid crystal composition layer 20. In the following description, the alignment state in which the tilt angle is maximum is also referred to as "horizontal alignment."

[0038] The refractive index of the liquid crystal composition layer 20 increases as the tilt of the liquid crystal compound LC increases, i.e., as the angle of the long axis of the liquid crystal compound LC approaches the principal surface direction of the liquid crystal composition layer 20 (the X or Y direction in FIG. 5 ). Conversely, the refractive index of the liquid crystal composition layer 20 decreases as the tilt of the liquid crystal compound LC decreases, i.e., as the angle of the long axis of the liquid crystal compound LC approaches the thickness direction of the liquid crystal composition layer 20 (the Z direction in FIG. 5 ). Such changes in the refractive index of the liquid crystal composition layer 20 of each unit cell UC change the resonance conditions of the microstructure 14, thereby changing the amount of phase delay of the incident radio wave RW. In this example, the amount of phase delay of the unit cell UC in the lower row of FIG. 5 is greater than that of the unit cell UC in the upper row of FIG. 5 .

[0039] That is, when the alignment state of the liquid crystal compound LC changes in the liquid crystal composition layer 20 located at positions corresponding to and around the microstructures 14 of each unit cell UC, the refractive index of the liquid crystal composition layer 20 changes with respect to the radio wave RW transmitted through each unit cell UC. Because the refractive index and the dielectric constant are positively correlated, a change in the refractive index of the liquid crystal composition layer 20 changes the resonance conditions of the microstructures 14, which function as a resonator. The change in the resonance conditions of the microstructures 14 manifests as a change in the phase delay of the radio wave RW. Therefore, changing the refractive index of the liquid crystal composition layer 20 can change the phase delay of the radio wave RW. Furthermore, a change in the refractive index of the liquid crystal composition layer 20 itself also causes a change in the phase delay of the radio wave RW. Because the refractive index of the liquid crystal composition layer 20 of each unit cell UC changes depending on the voltage V applied to each unit cell UC, the relationship between the voltage V and the phase delay of the radio wave RW is, for example, as shown in FIG. 6 .

[0040] As shown in FIG. 3 , when a radio wave RW enters the radio wave control element 10 from the microstructure 14 side, the radio wave RW passes through the microstructure 14 and the liquid crystal composition layer 20 in this order. Furthermore, the radio wave RW is reflected by the second electrode 26, which also serves as a reflective layer, and again passes through the liquid crystal composition layer 20 and the microstructure 14 in this order before exiting the radio wave control element 10. The radio wave RW is reflected along this incident / exit path. Along the incident / exit path, the radio wave RW passing through each unit cell UC is phase-modulated by resonance with the microstructure 14 and by passing through the liquid crystal composition layer 20. More specifically, in each unit cell UC, the resonance conditions of the microstructure 14 are determined according to the refractive index of the liquid crystal composition layer 20, and the phase modulation of the radio wave RW occurs due to resonance according to these conditions. In addition, the phase modulation of the radio wave RW also occurs according to the magnitude of the refractive index of the liquid crystal composition layer 20.

[0041] Based on the relationship shown in FIG. 6, the reflection direction of the radio wave RW reflected at the radio wave control element 10 is controlled by controlling the amount of phase delay of the radio wave RW for each unit cell UC through the applied voltage V.

[0042] 3, while a normal reflector can only change the direction of propagation of the radio wave RW in the direction of specular reflection, the radio wave control element 10 can change the direction of propagation of the radio wave RW in a direction other than specular reflection by using the metasurface structure 12. In addition, by actively changing the amount of phase delay in each unit cell UC, it is possible to actively change the direction of propagation of the radio wave RW.

[0043] Furthermore, various examples of controlling the direction of travel of the radio waves RW emitted from the radio wave control element 10 are possible other than controlling the reflected radio waves RW so that they travel in one straight direction as a whole, as in the example shown in Figure 3. For example, the radio waves RW emitted from the radio wave control element 10 may be made to converge toward a single focal point, or conversely, may be made to diverge. The direction of travel of the emitted radio waves RW can be controlled by adjusting the voltage applied to each unit cell UC, thereby adjusting the amount of phase delay of the radio waves RW for each unit cell UC.

[0044] For example, in the case of a plurality of unit cells UC arranged in one direction as shown in FIG. 3 , consider increasing the phase delay of the central unit cell UC and decreasing the phase delay toward both sides. In this case, connecting the wavefronts of the radio waves RW passing through each unit cell UC forms a V-shape, allowing the emitted radio waves RW to be focused. Conversely, consider decreasing the phase delay of the central unit cell UC and increasing the phase delay toward both sides. In this case, connecting the wavefronts of the radio waves RW passing through each unit cell UC forms a mountain-like shape (inverted V-shape), allowing the emitted radio waves RW to diverge. The degree of such focusing and divergence can also be adjusted by controlling the phase delay of the radio waves RW passing through each unit cell UC through adjusting the magnitude of the applied voltage.

[0045] The metasurface structure 12, like known metasurface structures, is formed by a two-dimensional arrangement of microstructures 14, which are metamaterials. In the illustrated metasurface structure 12, as shown in FIG. 2, the microstructures 14 are arranged two-dimensionally at equal intervals in the mutually orthogonal X and Y directions. In this metasurface structure 12, all of the microstructures 14 are, for example, identical. The metasurface structure 12 also constitutes at least a portion of the first electrode. That is, as described above, each of the multiple microstructures 14 constituting the metasurface structure 12 is the first electrode in each unit cell UC. The metasurface structure 12 (microstructures 14) may be supported by a support.

[0046] There are no limitations on the support that supports the microstructure 14, and various known sheet-like materials can be used as long as they can support the microstructure 14 and can transmit radio waves RW of 1 GHz to 1000 GHz, which are the frequencies targeted by the radio wave control element 10. Examples of the support include a metal substrate having an oxide insulating layer such as a silicon substrate having silicon oxide, a support made of an oxide such as silicon oxide, a support made of a semiconductor such as germanium and chalcogenide glass, a polyacrylic resin film such as polymethyl methacrylate, a cellulose resin film such as cellulose triacetate, a cycloolefin polymer film (for example, a product under the trade name "Arton" manufactured by JSR Corporation or a product under the trade name "Zeonor" manufactured by Zeon Corporation), a resin film such as a polyethylene terephthalate (PET) film, a polycarbonate film, and a polyvinyl chloride film, and a glass plate.

[0047] There is no limitation on the thickness of the support, as long as it can support the microstructure 14, has sufficient transparency to radio waves RW with frequencies of 1 GHz to 1000 GHz, and has sufficient strength depending on the application of the radio wave control element 10. The thickness of the support is set appropriately depending on the material from which the support is made so as to satisfy these conditions.

[0048] In the radio wave control element 10 according to the technology of the present disclosure, a support is not an essential component of the metasurface structure 12, and the support may be omitted. For example, if possible, the metasurface structure 12 may be formed by arranging the microstructures 14 directly on the surface of the liquid crystal composition layer 20. When a support is provided, the support may be provided between the metasurface structure 12 and the liquid crystal composition layer 20, or the support may be provided on the surface of the metasurface structure 12 opposite to the liquid crystal composition layer 20 side.

[0049] As described above, the metasurface structure 12 is composed of microstructures 14, which are metamaterials, spaced apart and arranged two-dimensionally on a plane, and more specifically, is composed of an arrangement of unit cells UC, each of which basically consists of one microstructure 14 and the space surrounding the microstructure 14.

[0050] In the radio wave control element 10 according to the technology of the present disclosure, the shape of the metasurface structure is basically the same as that of known metasurface structures. Therefore, various known metasurface structures can be used in the radio wave control element 10 according to the technology of the present disclosure. That is, the technology of the present disclosure does not limit the shape and material of the microstructures 14, the arrangement of the microstructures 14, or the pitch (the spacing between the microstructures 14). Furthermore, the metasurface structure 12 may be designed using known methods depending on the wavelength of the radio wave RW to be controlled by the radio wave control element 10 and the desired reflection characteristics (e.g., the range of controllable reflection directions). For example, the amplitude and phase of the radio wave RW reflected by the microstructures 14 used may be calculated using commercially available simulation software, and the arrangement of the microstructures 14 may be set to achieve the desired distribution of phase modulation amounts. When a liquid crystal composition layer 20 is used as in this example, phase modulation occurs due to the refractive index and the interaction between the refractive index and the microstructures 14. The amount of phase modulation is determined by the resonance characteristics of the microstructures 14, which change depending on the refractive index.

[0051] The radio wave control element 10 according to the technology of the present disclosure is intended to control radio waves RW with frequencies between 1 GHz and 1000 GHz. Therefore, the metasurface structure 12 has the microstructures 14 selected so as to impart a desired phase difference to the radio waves RW of this frequency, and the arrangement of the microstructures is also set. Specifically, when radio waves RW with frequencies between 1 GHz and 1000 GHz are to be controlled, the wavelength range of the radio waves RW is approximately 300 μm to 30 cm, so the size of the microstructures 14 is selected to be equal to or smaller than this wavelength range.

[0052] Although one unit cell UC basically has one microstructure 14, the technology of the present disclosure is not limited to this. That is, in the radio wave control element according to the technology of the present disclosure, one unit cell UC may have multiple microstructures 14 as necessary depending on the reflection characteristics, the size, material and shape of the microstructures 14, and the size of the unit cell UC. In this case, one unit cell UC may have different microstructures 14. However, because the unit cell UC is the smallest unit capable of actively changing the phase of the radio wave RW, even when one unit cell UC has multiple microstructures 14, the amount of phase modulation is determined for each unit cell UC.

[0053] Furthermore, there are no limitations on the material for forming the microstructure 14, and various materials used as microstructures in known metasurface structures can be used. Examples of materials for forming the microstructure 14 include metals and dielectrics. In the case of metals, copper, gold, and silver are preferred due to their low optical loss. In addition, composites consisting of metal particles and binders and oxide semiconductors can also be used as materials for forming the microstructure 14. On the other hand, in the case of dielectrics, silicon, titanium oxide, and germanium are preferred due to their high refractive index and the ability to increase the amount of phase modulation. Note that, as shown in FIG. 4 , when the microstructure 14 also serves as an electrode that forms an electrode pair with the second electrode 26, the microstructure 14 is formed from a conductor.

[0054] Similarly, the shape of the microstructure 14 is not limited, and various shapes used as microstructures in known metasurface structures can be used. Examples include a cross-shaped solid like a crossed rectangular parallelepiped, a rectangular parallelepiped, a cylindrical shape, a V-shaped solid like a rectangular parallelepiped connected at its ends as shown in JP 2018-046395 A, an approximately H-shaped solid like an H-beam, and an approximately C-shaped solid like a C-channel. As shown in JP 2018-046395 A, various shapes can be used for the V-shaped solid and the cross-shaped solid by adjusting the angle between the two rectangular parallelepipeds. Other shapes, such as those shown in Figure 5 of "Appl. Sci. 2018, 8(9), 1689; https: / / doi.org / 10.3390 / app8091689," can also be used.

[0055] In the metasurface structure 12, the microstructures 14 may be of the same type, or multiple types may be used in combination. Furthermore, the same microstructures 14 may be arranged in the same orientation in the XY plane, or in different orientations. Furthermore, microstructures 14 of the same orientation and those of different orientations may be mixed. However, in the radio wave control element 10 according to the technology of the present disclosure, it is preferable to use only one type of microstructure 14 and to arrange all of the microstructures 14 in the same orientation.

[0056] As shown in FIG. 3 , the metasurface structure 12 preferably has identical microstructures 14, all of which have the same structure, arranged two-dimensionally at equal intervals in the mutually orthogonal X and Y directions. However, the technology of the present disclosure is not limited to this, and multiple types of microstructures may be used in combination as described above. Furthermore, the arrangement intervals and arrangement of the microstructures 14 may also differ in the plane direction. However, considering the controllability of the reflection direction of the radio wave RW when a voltage is applied to the liquid crystal composition layer 20, it is preferable that the metasurface structure 12 use all the same microstructures 14. Furthermore, it is more preferable that the microstructures 14 in the metasurface structure 12 are spaced at equal intervals, and even more preferably at equal intervals in both the orthogonal X and Y directions.

[0057] The liquid crystal composition layer 20 is a layer in which the liquid crystal compound LC is aligned in a preset state, and as described above, the alignment state of the liquid crystal compound LC changes when a voltage is applied.

[0058] In the liquid crystal composition layer 20 illustrated in Fig. 4, the liquid crystal compound LC is vertically aligned when no voltage is applied. When a voltage is applied to the liquid crystal composition layer 20, the liquid crystal compound LC is aligned at an angle relative to the thickness direction in accordance with the voltage, and reaches a maximum horizontal alignment. In the radio wave control element 10, the change in alignment of the liquid crystal compound LC is not limited to a change from vertical alignment to horizontal alignment or vice versa, but may be a change from a state tilted relative to the thickness direction to horizontal or vertical alignment, a change from horizontal or vertical alignment to a state tilted relative to the thickness direction, or a change at an angle from a state tilted relative to the thickness direction to a state tilted relative to the thickness direction.

[0059] The liquid crystal composition layer 20 may be formed, for example, on the surface of an alignment film to be described later by a known method.

[0060] In the radio wave control element 10, the liquid crystal composition layer 20 is laminated on the second electrode 26 or the intermediate layer 32 on the support 24. The support 24 is basically the same as the support described above.

[0061] Here, when the liquid crystal composition layer 20 is formed on the second electrode 26 or the intermediate layer 32, an alignment film for aligning the liquid crystal compound LC in a predetermined state may be provided on the surface of the second electrode 26 or the intermediate layer 32. Various known alignment films can be used. Examples include rubbed films made of organic compounds such as polymers, obliquely evaporated films of inorganic compounds, films with microgrooves, and films formed by accumulating LB (Langmuir-Blodgett) films made of organic compounds such as ω-tricosanoic acid, dioctadecylmethylammonium chloride, and methyl stearate using the Langmuir-Blodgett method. Furthermore, so-called photoalignment films, which are formed by irradiating a photoalignable material with polarized or unpolarized light, can also be used as the alignment film. These alignment films can be formed by known methods depending on the material used to form the main body.

[0062] Alternatively, when a support for supporting the microstructures 14 is provided, the liquid crystal composition layer 20 may be formed on this support.

[0063] The liquid crystal composition containing the liquid crystal compound LC for forming the liquid crystal composition layer 20 will be described later.

[0064] The entire surface of the support 24 facing the liquid crystal composition layer 20 is covered with a second electrode 26. The second electrode 26 is an electrode that changes the alignment of the liquid crystal compound LC in the liquid crystal composition layer 20, and also functions as a reflective layer that reflects radio waves RW having a frequency of 1 GHz to 1000 GHz that are incident from the metasurface structure 12 side, as described above. In the illustrated example, the second electrode 26 is configured to be disposed between the support 24 and the liquid crystal composition layer 20, but this is not limiting, and the second electrode 26 may be disposed on the surface of the support 24 opposite to the liquid crystal composition layer 20 side.

[0065] There are no limitations on the second electrode 26, and a sheet-like object made of various known materials can be used as long as it has sufficient conductivity and is capable of reflecting radio waves RW. Examples of the second electrode 26 include a metal layer such as copper, aluminum, gold, or silver; an inorganic conductive material such as ITO (tin-doped indium oxide); an organic conductive material such as polythiophene, typified by PEDOT (poly 3,4-ethylenedioxythiophene); and graphene. Inorganic conductive materials, organic conductive materials, and graphene are transparent to visible light but act as a reflective layer for radio waves of the above frequencies.

[0066] There is no limitation on the thickness of the second electrode 26, and the thickness may be set appropriately depending on the material from which the second electrode 26 is made so that the target radio waves can be reflected with the required reflectivity.

[0067] As described above, the radio wave control element 10 according to the technology of the present disclosure is a reflective radio wave control element having a metasurface structure 12 and a liquid crystal composition layer 20. In the radio wave control element 10, power is supplied to each microstructure 14 to change the alignment state of the liquid crystal compound LC in the corresponding region of the liquid crystal composition layer 20, thereby forming regions with different refractive indices for each unit cell UC, thereby reflecting the radio wave RW in a desired direction. Furthermore, by changing the power supplied to each microstructure 14, i.e., the voltage applied to the liquid crystal composition layer 20, the reflection direction of the incident radio wave RW can be switched.

[0068] In the radio wave control element 10 according to the technique of the present disclosure, the refractive index anisotropy Δn of the liquid crystal composition layer 20 with respect to radio waves is not limited, but is preferably large. Here, in the reflective radio wave control element 10 of this example, the refractive index anisotropy Δn of the liquid crystal composition layer 20 with respect to radio waves of 100 GHz is preferably 0.35 or more. By making the refractive index anisotropy Δn of the liquid crystal composition layer 20 with respect to radio waves of 100 GHz 0.35 or more, the liquid crystal composition layer 20 can be made thinner, which is preferable in that the reflection direction of the radio waves RW can be switched more quickly.

[0069] Furthermore, there is no limitation on the thickness of the liquid crystal composition layer 20; a thickness that provides the necessary phase difference for the radio wave RW can be appropriately set depending on the material forming the liquid crystal composition layer 20. As will be described later, the radio wave control element 10 according to the technology of the present disclosure includes an intermediate layer 32 whose refractive index does not change. Therefore, even if the liquid crystal composition layer 20 is thinned to shorten the time required to switch the reflection direction of the incident radio wave RW, i.e., even if the response speed is increased, the loss of the radio wave RW can be reduced. In consideration of this, the thickness of the liquid crystal composition layer 20 is preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 100 μm or less. Setting the thickness of the liquid crystal composition layer 20 to 200 μm or less is preferable in that the reflection direction of the radio wave RW can be switched more quickly.

[0070] Furthermore, although there are no limitations on the Δn (birefringence) of the liquid crystal composition layer 20, a larger value is preferable. The Δn of the liquid crystal composition layer is preferably 0.35 or more. By making the Δn of the liquid crystal composition layer 20 0.35 or more, the liquid crystal composition layer can be made thinner, which is preferable in that the propagation direction of radio waves can be switched more quickly.

[0071] The intermediate layer 32 is a layer in which the maximum refractive index change per unit voltage for electromagnetic waves in the wavelength range of 300 μm to 30 cm is 0.001 (1 / V) or less. In other words, the refractive index of the intermediate layer 32 does not change with voltage. The intermediate layer 32 is also an insulating layer. The maximum refractive index change per unit voltage for electromagnetic waves in the wavelength range of 300 μm to 30 cm is preferably 0.0005 (1 / V).

[0072] In the present disclosure, the maximum refractive index change per unit voltage in an electromagnetic wave having a wavelength in the range of 300 μm to 30 cm is measured by terahertz time-domain spectroscopy (THz-TDS) as follows.

[0073] First, the intermediate layer was cut into a test piece measuring 10 mm x 10 mm and 1 mm thick. Next, tin oxide (SnO) films were formed on both sides of the intermediate layer by sputtering. Next, a transmission terahertz spectroscopy optical system was fabricated, and the refractive index of the test piece was measured from the change in the time waveform of the optical field transmitted through the test piece when a voltage was applied to the tin oxide film in an environment of 25°C and 10% RH.

[0074] In the case of a liquid crystal composition layer, first, a liquid crystal composition is sealed in a glass cell (10 mm × 10 mm, 1 mm thick) in which tin oxide (SnO) films are formed as electrodes by a sputtering method. Next, a transmission terahertz spectroscopy optical system is fabricated, and the refractive index of the liquid crystal composition sealed in the glass cell is measured from the change in the time waveform of the optical field transmitted through the test piece when a voltage is applied to the electrodes in an environment of a temperature of 25°C and a humidity of 10%.

[0075] The intermediate layer 32 may be disposed between one of the microstructures 14 (first electrodes) and the second electrode 26 and the liquid crystal composition layer 20. That is, as shown in Fig. 4, the second electrode 26, the intermediate layer 32, and the liquid crystal composition layer 20 are disposed in this order. Alternatively, the second electrode 26, the liquid crystal composition layer 20, and the intermediate layer 32 may be disposed in this order.

[0076] The radio wave control element 10 may have two or more intermediate layers 32. For example, an intermediate layer may be provided between the microstructure 14 (first electrode) and the liquid crystal composition layer 20, and between the liquid crystal composition layer 20 and the second electrode 26.

[0077] Furthermore, the intermediate layer 32 may be an alignment film for aligning the liquid crystal compound LC in a predetermined state.

[0078] The material for forming the intermediate layer 32 is not particularly limited, but is preferably a dielectric material as long as it has sufficient transparency to the radio waves RW. Examples include semiconductors such as silicon, silicon oxide (quartz (SiO2)), germanium, and chalcogenide glass, polyacrylic resins such as polymethyl methacrylate, cellulose resins such as cellulose triacetate, resins such as cycloolefin polymers, polyethylene terephthalate (PET), polycarbonate, and polyvinyl chloride, and glass.

[0079] As described above, in a radio wave control element in which a liquid crystal composition layer is disposed between a metasurface structure (first electrode) and a second electrode, and in which different voltages are applied to each of the unit cells corresponding to the multiple microstructures constituting the metasurface structure, thereby varying the refractive index of the liquid crystal composition layer for each unit cell, thereby controlling the reflection direction of the radio wave RW, there is a demand for a reduction in the switching time when changing the reflection direction of the radio wave RW. In such a radio wave control element, by reducing the thickness of the liquid crystal composition layer, it is possible to quickly change the orientation state of the liquid crystal compound when a voltage is applied. In other words, by reducing the thickness of the liquid crystal composition layer, it is possible to reduce the switching time of the reflection direction of the radio wave RW in the radio wave control element.

[0080] However, it was found that a thinner liquid crystal composition layer results in a larger loss of radio wave RW when it is reflected. The inventors investigated this issue and found that a shorter distance between the metasurface structure (first electrode) and the second electrode increases the absorption of radio wave RW by the liquid crystal composition, resulting in a larger loss of radio wave RW.

[0081] In contrast, the radio wave control element 10 of the present invention has an intermediate layer 32 between one of the metasurface structure 12 (first electrode) and the second electrode 26 and the liquid crystal composition layer 20, in which the maximum refractive index change per unit voltage for radio waves in the wavelength range of 300 μm to 30 cm is 0.001 (1 / V) or less. This allows the distance between the first electrode and the second electrode 26 to be increased even when the liquid crystal composition layer 20 is made thin. Therefore, the radio wave control element 10 of the present invention can reduce the thickness of the liquid crystal composition layer, shortening the switching time for the reflection direction of the radio wave RW in the radio wave control element, while suppressing absorption of the radio wave RW and reducing loss of the radio wave RW.

[0082] Here, from the viewpoint of shortening the time required to switch the reflection direction of the radio wave RW (improving the response speed) and reducing the loss of the radio wave RW, the thickness of the intermediate layer 32 is preferably 0.1 to 2 times, more preferably 0.2 to 1.8 times, and even more preferably 0.5 to 1.5 times the thickness of the liquid crystal composition layer 20.

[0083] Furthermore, there are no particular restrictions on the dielectric constant of the material constituting the intermediate layer 32 with respect to the applied voltage. When the liquid crystal composition layer and the intermediate layer are arranged in series between opposing electrodes, such as when the second electrode 26 and the microstructure 14 of the radio wave control element 10 are electrodes, it is preferable that the dielectric constant of the intermediate layer is high, since this makes it possible to efficiently increase the electric field applied to the liquid crystal composition layer.

[0084] In the above example, the radio wave control element 10 reflects the radio wave RW and controls the reflection direction of the radio wave RW, but the radio wave control element of the present invention is not limited to this.

[0085] Fig. 7 is a conceptual diagram showing another example of the radio wave control element of the present invention, and Fig. 8 is a perspective view of the radio wave control element shown in Fig. 7.

[0086] The radio wave control element 50 shown in Figures 7 and 8 has, from the bottom in the figure, a waveguide 52, an intermediate layer 32, a liquid crystal composition layer 20, and a metasurface structure 12 (microstructure 14), in this order. The intermediate layer 32 and the liquid crystal composition layer 20 are provided on part of the outer surface of the waveguide 52. In the radio wave control element 50 shown in Figures 7 and 8, the waveguide 52 is a waveguide made of a conductor such as metal, and the waveguide also serves as the second electrode.

[0087] In the radio wave control element 50 shown in Figures 7 and 8, the same components as those in the radio wave control element 10 shown in Figure 4 are denoted by the same reference numerals, and the following description will mainly focus on the different components. In addition, although the support is not shown in the radio wave control element 50 shown in Figures 7 and 8, a support may be provided.

[0088] In the illustrated example, the waveguide 52 is a cylindrical member having a rectangular cross section, and is a metallic waveguide that guides the radio waves RW within its hollow portion. The radio waves RW propagate through the waveguide 52 while forming an electromagnetic field according to the shape and dimensions of the waveguide 52, the wavelength (frequency) of the radio waves RW, and the like.

[0089] The waveguide 52 has a plurality of openings 54 that connect the hollow portion to the outside, in a wall portion on the side where the intermediate layer 32, the liquid crystal composition layer 20, and the microstructures 14 are laminated, i.e., in a surface that faces the microstructures 14 and acts as a second electrode. The openings 54 are each provided at a position corresponding to a unit cell UC (i.e., a microstructure 14).

[0090] Because the waveguide 52 has multiple openings 54, the radio waves RW guided within the hollow portion of the waveguide 52 seep out from the openings 54 and are emitted to the outside. At this time, the radio waves RW pass through the intermediate layer 32 and the liquid crystal composition layer 20. In the radio wave control element 50, as in the radio wave control element 10 shown in FIG. 4, the propagation direction of the radio waves RW can be controlled by performing phase modulation, i.e., controlling the amount of phase delay, for each unit cell UC. That is, the radio wave control element 50 can control the emission direction of the radio waves RW. Furthermore, by actively changing the amount of phase delay in each unit cell UC, it is possible to actively change the emission direction of the radio waves RW.

[0091] Here, the radio wave control element 50 of the present invention has an intermediate layer 32 between one of the metasurface structure 12 (first electrode) and the waveguide 52 functioning as the second electrode and the liquid crystal composition layer 20, in which the maximum refractive index change per unit voltage in radio waves with wavelengths in the range of 300 μm to 30 cm is 0.001 (1 / V) or less. This allows the distance between the first electrode and the second electrode to be increased even when the liquid crystal composition layer 20 is thin. Therefore, the radio wave control element 50 of the present invention can reduce the thickness of the liquid crystal composition layer, thereby reducing the time required to switch the emission direction of the radio wave RW in the radio wave control element, and suppressing absorption of the radio wave RW to reduce loss of the radio wave RW.

[0092] In the example shown in FIG. 8, the cross-sectional shape of the waveguide 52 is rectangular, but this is not limited to this and various shapes such as square, circular, polygonal, etc. may be used.

[0093] There is also no particular limitation on the dimensions of the waveguide 52. The length of the waveguide 52 is preferably 1 to 10,000 mm, and more preferably 3 to 3,000 mm.

[0094] Furthermore, as in the examples shown in Figures 7 and 8, when the waveguide 52 also serves as the second electrode, the waveguide 52 can be formed from the same material (conductor) as the material used to form the second electrode 26 described above.

[0095] 7 and 8, the waveguide 52 also serves as the second electrode, but this is not limiting, and the waveguide and the second electrode may be separate. When the waveguide and the second electrode are separate, the waveguide is disposed on the surface of the second electrode opposite the liquid crystal composition layer. Even when the waveguide and the second electrode are separate, the second electrode has an opening through which the radio wave RW passes, at a position corresponding to each cell unit. Furthermore, the waveguide has an emission portion for emitting the radio wave RW, at a position corresponding to the opening of the second electrode.

[0096] When the waveguide and the second electrode are separate bodies, in addition to the waveguide formed from the above-mentioned conductor, conventionally known waveguides capable of guiding radio waves, such as a stripline line, a microstripline line, or a coplanar line, can be used as appropriate as the waveguide.

[0097] The size of the opening formed in the second electrode is 0.01 to 100,000 mm 2 is preferable, and 0.02 to 80,000 mm 2 More preferably, 0.05 to 50,000 mm 2 is more preferable.

[0098] In the illustrated example, one opening is formed in the second electrode for each unit cell UC, but this is not limitative, and two or more openings may be formed for each unit cell UC.

[0099] The radio wave control element may also have a temperature adjusting member for adjusting the temperature of the liquid crystal composition layer. In this embodiment, the alignment state of the liquid crystal compound in the liquid crystal composition layer can be fixed. This embodiment will be described in detail below.

[0100] The temperature adjusting member is not particularly limited in configuration as long as it is a member that adjusts the temperature of the liquid crystal composition layer 20. There are also no limitations on the location where the temperature adjusting member is disposed, as long as it can adjust the temperature of the liquid crystal composition layer 20. The temperature adjusting member may be disposed in the form of a layer between any of the members that constitute the radio wave control element, or may be disposed outside the radio wave control element.

[0101] The temperature adjusting member may also include a heating device for increasing the temperature of the liquid crystal composition layer 20 and a cooling device for decreasing the temperature of the liquid crystal composition layer 20 .

[0102] The procedure for fixing the alignment state of the liquid crystal compound in the liquid crystal composition layer 20 using a radio wave control element having a temperature adjusting member is as follows. Note that the liquid crystal compound contained in the liquid crystal composition layer 20 is a compound that exhibits liquid crystallinity when subjected to a heat treatment using a temperature adjusting member. Specifically, it is preferable to use a composition that exhibits a nematic phase at any temperature between 50 and 150°C and a glassy state or a smectic phase at any temperature below 50°C.

[0103] First, the liquid crystal composition layer 20 is heated by the temperature adjusting member in the radio wave control element, causing it to transition to a liquid crystal phase. Next, while maintaining the heating process, a voltage is applied between the second electrode 26 and the microstructure 14 to control the alignment direction of the liquid crystal compound. At this time, the voltage applied to each unit cell UC may be changed to vary the alignment state of the liquid crystal compound. When the heating process and voltage application process are then stopped, the temperature drops below the transition temperature of the liquid crystal phase, and the alignment state of the liquid crystal compound is fixed. In other words, the aligned state of the liquid crystal compound can be maintained even without applying a voltage.

[0104] Furthermore, in the radio wave control element, a state with higher alignment can be created depending on the type of liquid crystal compound used. For example, when the liquid crystal compound exhibits a nematic phase as well as a higher-order liquid crystal phase such as a smectic phase, the higher-order liquid crystal phase can be fixed by rapidly cooling the radio wave control element using a temperature adjusting member.

[0105] Materials for forming the liquid crystal composition layer will be described below.

[0106] The liquid crystal composition layer is made of a liquid crystal composition containing a liquid crystal compound, and is a layer in which the alignment state of the liquid crystal compound contained in the liquid crystal composition layer changes depending on the magnitude of the applied voltage. The liquid crystal composition may also contain components other than the liquid crystal compound. Preferably, the liquid crystal composition is substantially free of a solvent. "Substantially free of a solvent" means that the solvent content is 5% by mass or less, and preferably 1% by mass or less, relative to the total mass of the liquid crystal composition.

[0107] The liquid crystal composition preferably exhibits a nematic phase over the entire temperature range of 10 to 50° C. It is also preferable that the liquid crystal composition exhibits a nematic phase at any temperature between 50 and 150° C., and exhibits a glassy state or smectic liquid crystallinity at any temperature below 50° C.

[0108] Furthermore, the liquid crystal composition preferably contains an azo compound, and the liquid crystal compound is preferably a liquid crystal compound having an azo structure. By containing an azo compound in the liquid crystal composition, the Δn (birefringence) of the liquid crystal composition layer can be increased. Therefore, the thickness of the liquid crystal composition layer required to provide the required refractive index, i.e., the required phase difference, to radio waves can be made thinner. By making the liquid crystal composition layer thinner, the orientation of the liquid crystal compound changes more quickly when the applied voltage is changed. As a result, the response speed to changes in voltage applied to the liquid crystal composition layer can be increased, and the propagation direction of incident radio waves can be switched in a shorter time.

[0109] The azo compound is not particularly limited as long as it contains an azo structure (-N=N-). The number of azo structures contained in the azo compound is not particularly limited as long as it is 1 or more, and preferably 2 or more. There is no particular upper limit on the number of azo structures, but it is often 5 or less, and more often 3 or less.

[0110] The azo compound may be a compound that exhibits liquid crystallinity or a compound that does not exhibit liquid crystallinity, but is preferably a compound that exhibits liquid crystallinity, i.e., is preferably a liquid crystal compound having an azo structure.

[0111] The azo compound is preferably a compound represented by formula (1).

[0112]

[0113] In formula (1), Ar 1 represents an (m1+1)-valent aromatic ring.

[0114] The (m1+1)-valent aromatic ring may be a monocyclic ring or a condensed ring of two or more rings, or may be a ring in which a plurality of monocyclic rings are bonded together via single bonds (for example, a biphenyl ring or a terphenyl ring).

[0115] The (m1+1)-valent aromatic ring includes an aromatic hydrocarbon ring or an aromatic heterocyclic ring.

[0116] Examples of the aromatic hydrocarbon ring include a benzene ring, a naphthalene ring, an azulene ring, a fluorene ring, a biphenyl ring, and an anthracene ring. Of these, a benzene ring is preferred.

[0117] Examples of the aromatic heterocycle include a pyridine ring, a thiophene ring, a quinoline ring, an isoquinoline ring, and a thiazole ring.

[0118] For example, when m1 is 1, Ar １ represents a divalent aromatic ring.

[0119] In formula (1), Ar 2 represents an (m2+2)-valent aromatic ring.

[0120] The (m2+1)-valent aromatic ring may be a monocyclic ring or a condensed ring of two or more rings, or may be a ring in which a plurality of monocyclic rings are bonded together via single bonds (e.g., a biphenyl ring, a terphenyl ring).

[0121] The (m2+1)-valent aromatic ring includes an aromatic hydrocarbon ring or an aromatic heterocyclic ring.

[0122] Examples of the aromatic hydrocarbon ring include a benzene ring, a naphthalene ring, an azulene ring, a fluorene ring, a biphenyl ring, and an anthracene ring. Of these, a benzene ring is preferred.

[0123] Examples of the aromatic heterocycle include a pyridine ring, a thiophene ring, a quinoline ring, an isoquinoline ring, and a thiazole ring.

[0124] For example, when m2 is 1, Ar 2 represents a trivalent aromatic ring.

[0125] In formula (1), Ar 3 represents an (m3+1)-valent aromatic ring.

[0126] The (m+1)-valent aromatic ring may be a monocyclic ring or a condensed ring of two or more rings, or may be a ring in which a plurality of monocyclic rings are bonded together via single bonds (e.g., a biphenyl ring, a terphenyl ring).

[0127] The (m3+1)-valent aromatic ring includes an aromatic hydrocarbon ring or an aromatic heterocyclic ring.

[0128] Examples of the aromatic hydrocarbon ring include a benzene ring, a naphthalene ring, an azulene ring, a fluorene ring, a biphenyl ring, and an anthracene ring. Of these, a benzene ring is preferred.

[0129] Examples of the aromatic heterocycle include a pyridine ring, a thiophene ring, a quinoline ring, an isoquinoline ring, and a thiazole ring.

[0130] For example, when m3 is 1, Ar 3 represents a divalent aromatic ring.

[0131] In formula (1), R 1 , R 2 and R 3 each independently represents a substituent.

[0132] If m1≧2, multiple R 1 may be the same or different, and when m2 ≥ 2, a plurality of R 2 may be the same or different, and when m3≧2, a plurality of R 3 may be the same or different from each other.

[0133] The above-mentioned substituent is a monovalent substituent, and can be exemplified by alkyl group, alkenyl group, aralkyl group, aryl group, heterocyclic group, halogen atom, cyano group, nitro group, mercapto group, hydroxy group, alkoxy group, aryloxy group, alkylthio group, arylthio group, acyloxy group, amino group, alkylamino group, dialkylamino group, carbonamido group, sulfonamido group, sulfamoylamino group, oxycarbonylamino group, oxysulfonylamino group, ureido group, thioureido group, acyl group, oxycarbonyl group, carbamoyl group, sulfonyl group, sulfinyl group, sulfamoyl group, carboxy group (including salt), sulfo group (including salt), and the group that combines these groups.These groups can also be substituted with these groups.

[0134] In formula (1), m1, m2, and m3 each independently represent an integer of 0 to 5. m1 is preferably 1 to 3, m2 is preferably 0 to 1, and m3 is preferably 1 to 3.

[0135] In formula (1), n1 represents an integer of 1 to 4, preferably 1 to 3, and more preferably 2 or 3.

[0136] The liquid crystal composition preferably contains a dichroic dye, and the liquid crystal compound is preferably a liquid crystal dichroic dye. By orienting the dichroic dye in the liquid crystal composition, the refractive index anisotropy with respect to radio waves can be increased.

[0137] A dichroic dye is a substance that exhibits dichroism, and dichroism means a property in which the absorbance varies depending on the direction of polarization.

[0138] The dichroic dyes may be used alone or in combination of two or more. In particular, the liquid crystal composition preferably contains two or more dichroic dyes. When the liquid crystal composition contains two or more dichroic dyes, the liquid crystal composition preferably contains two to four types of dichroic dyes, and more preferably contains two to three types of dichroic dyes.

[0139] The dichroic dye preferably exhibits liquid crystallinity, that is, a liquid crystalline dichroic dye is preferred.

[0140] Among these, the compound represented by formula (X) is preferred as the dichroic dye in terms of the superior effect of the present invention.

[0141]

[0142] In formula (X), R １ and R ２ each independently represents a linear or branched hydrocarbon group having 1 to 10 carbon atoms, and the hydrocarbon group may contain an oxygen atom, a nitrogen atom, or a sulfur atom.

[0143] The hydrocarbon group has 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, and more preferably 2 to 6 carbon atoms, in that the effects of the present invention are more excellent.

[0144] The hydrocarbon group may be linear or branched, preferably linear.

[0145] The hydrocarbon may be a saturated hydrocarbon group or an unsaturated hydrocarbon group.

[0146] The hydrocarbon group may contain an oxygen atom, a nitrogen atom, or a sulfur atom, or may contain a plurality of atoms selected from the group consisting of oxygen atoms, nitrogen atoms, and sulfur atoms.

[0147] For example, the hydrocarbon group may have, between carbon atoms or at the terminal, -O-, -S-, -CO-, -CS-, -CO-O-, or -CO-NR １０ -, -NR １０ -, or a combination thereof.

[0148] R １０ represents a hydrogen atom or an alkyl group.

[0149] The hydrocarbon group may include, between carbon atoms or at the terminal, -O-, -S-, -CO-, -CS-, -CO-O-, -CO-NR １０ -, -NR １０ -, or alkyl groups which may contain a combination of these groups are preferred.

[0150] In formula (X), R １ and R ２ may be bonded to each other to form a ring.

[0151] The ring formed may be an aliphatic ring or an aromatic ring.

[0152] In formula (X), A and B each independently represent a divalent aromatic ring group.

[0153] The divalent aromatic ring group includes a divalent aromatic hydrocarbon ring group and a divalent aromatic heterocyclic group.

[0154] A divalent aromatic hydrocarbon ring group is a group formed by removing two hydrogen atoms from an aromatic hydrocarbon ring. The aromatic hydrocarbon ring may be a single ring or a condensed ring. Examples of the aromatic hydrocarbon ring include a benzene ring, a naphthalene ring, an anthracene ring, a pyrene ring, a phenanthrene ring, and a fluorene ring.

[0155] A divalent aromatic heterocyclic group is a group formed by removing two hydrogen atoms from an aromatic heterocycle. The aromatic heterocycle may be a monocycle or a condensed ring. Examples of the aromatic heterocycle include a pyridine ring, a pyrimidine ring, a pyridazine ring, a pyrazine ring, a triazine ring (for example, a 1,2,3-triazine ring, a 1,2,4-triazine ring, and a 1,3,5-triazine ring), a tetrazine ring (for example, a 1,2,4,5-tetrazine ring), a quinoxaline ring, a pyrrole ring, a furan ring, a thiophene ring, an imidazole ring, an oxazole ring, a thiazole ring, a benzopyrrole ring, a benzofuran ring, a benzothiophene ring, a benzimidazole ring, a benzoxazole ring, a benzothiazole ring, a naphthopyrrole ring, a naphthofuran ring, a naphthothiophene ring, a naphthoimidazole ring, a naphthoxazole ring, a pyrroloimidazole ring (for example, a 5H-pyrrolo[1,2-a]imidazole ring), an imidazooxazole ring (for example, an imidazo[2,1-b]oxazole ring), a thienoyl group, a benzophenone ring, a benzophenone ring, a benzoisopropyl group ... Thiazole rings (for example, thieno[2,3-d]thiazole rings, etc.), benzothiadiazole rings, benzodithiophene rings (for example, benzo[1,2-b:4,5-b']dithiophene rings, etc.), thienothiophene rings (for example, thieno[3,2-b]thiophene rings, etc.), thiazolothiazole rings (for example, thiazolo[5,4-d]thiazole rings, naphthodithiophene rings (for example, naphtho[2,3-b: [6,7-b']dithiophene ring, naphtho[2,1-b:6,5-b']dithiophene ring, naphtho[1,2-b:5,6-b']dithiophene ring, and 1,8-dithiadicyclopenta[b,g]naphthalene ring, etc.), benzothienobenzothiophene ring, dithieno[3,2-b:2',3'-d]thiophene ring, and 3,4,7,8-tetrathiadicyclopenta[a,e]pentalene ring.

[0156] In formula (X), L represents a single bond, —CR═CR—, —C≡C—, —CR═N—, or —N═N—.

[0157] Each R independently represents a hydrogen atom, a halogen atom, or an alkyl group having 1 to 6 carbon atoms.

[0158] In formula (X), n represents an integer of 1 to 3, and when n is 2 or 3, a plurality of As and Ls may be the same or different.

[0159] R ３ represents a hydrogen atom or a substituent.

[0160] The type of the substituent is not particularly limited, and examples thereof include halogen atoms (e.g., fluorine atoms, chlorine atoms, bromine atoms, iodine atoms, etc.), hydrocarbon groups (alkyl groups (including cycloalkyl groups, bicycloalkyl groups, and tricycloalkyl groups), alkenyl groups (including cycloalkenyl groups and bicycloalkenyl groups), alkynyl groups, and aryl groups), heterocyclic groups, cyano groups, isothiocyanate groups, nitro groups, alkoxy groups, aryloxy groups, silyl groups, silyloxy groups, heterocyclic oxy groups, acyloxy groups, carbamoyloxy groups, alkoxycarbonyloxy groups, aryloxycarbonyl groups, etc. Examples of such an amino group include an oxy group, a primary, secondary, or tertiary amino group (including an anilino group), an alkylthio group, an arylthio group, a heterocyclic thio group, an alkyl or arylsulfinyl group, an alkyl or arylsulfonyl group, an acyl group, an aryloxycarbonyl group, an alkoxycarbonyl group, an aryl or heterocyclic azo group, an imido group, a phosphino group, a phosphinyl group, a phosphinyloxy group, a phosphinylamino group, a phosphono group, a carboxy group, a phosphate group, a sulfonic acid group, a hydroxy group, a thiol group, an acylamino group, a carbamoyl group, a ureido group, a boronic acid group, and combinations thereof.

[0161] Among these, the substituent is preferably an alkyl group, an alkoxy group, a cyano group, or an isothiocyanate group, each of which may contain an oxygen atom, a nitrogen atom, or a sulfur atom. The alkyl group may contain an oxygen atom, a nitrogen atom, or a sulfur atom. For example, the alkyl group and alkoxy group may have —O—, —S—, —CO—, —CS—, —CO—O—, or —CO—NR １０ -, -NR １０ -, or a combination thereof. １０ represents a hydrogen atom or an alkyl group.

[0162] The alkyl group and alkoxy group are each independently —O—, —S—, —CO—, —CS—, —CO—O—, —CO—NR １０ -, -NR １０ -, or a combination thereof.

[0163] The number of carbon atoms in the alkyl group and alkoxy group is not particularly limited, but is preferably 1 to 10, and more preferably 1 to 6.

[0164] R represents a hydrogen atom or an alkyl group.

[0165] The total content of the dichroic dyes in the liquid crystal composition is 30% by mass or more relative to the total mass of the liquid crystal composition. When the liquid crystal composition contains only one type of dichroic dye, the total content of the dichroic dyes refers to the content of that one type of dichroic dye relative to the total mass of the liquid crystal composition. When the liquid crystal composition contains two or more types of dichroic dyes, the total content of the dichroic dyes refers to the combined amount of these two or more types of dichroic dyes.

[0166] A liquid crystal composition layer made of a liquid crystal composition containing a dichroic dye can exhibit a state in which the refractive index anisotropy with respect to radio waves is large. Although the details of the reason for this are unclear, it is presumed that increasing the total content of the dichroic dyes in the liquid crystal composition layer increases the interaction between the dichroic dyes, thereby increasing the degree of orientational order of the absorption axes caused by the absorbing skeleton in the dichroic dyes, and as a result, the absorption anisotropy of the dichroic dyes affects the increase in the refractive index anisotropy at wavelengths longer than the absorption wavelength.

[0167] In order to obtain a more excellent effect of the present invention, the total content of the dichroic dyes in the liquid crystal composition is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, based on the total mass of the liquid crystal composition. The upper limit is not particularly limited, but may be 100% by mass or less.

[0168] The radio wave control element of the present invention has been described in detail above, but the present invention is not limited to the above examples, and various improvements and modifications may of course be made within the scope of the gist of the present invention.

[0169] The present invention will be described in more detail below with reference to examples. The materials, amounts used, ratios, treatment details, and treatment procedures shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the examples shown below.

[0170] Using optical simulation software, a model of the radio wave control element was created as shown in Figure 4. For the simulation, COMSOL Multiphysics, a finite element method simulation software from COMSOL, was used.

[0171] [Example 1-1] The modeled radio wave control element had a structure in which a second electrode, an intermediate layer, a liquid crystal composition layer, and a microstructure (metasurface structure) were stacked in this order. The size of one unit cell was 11 mm × 11 mm, and by applying periodic boundary conditions, the unit cells were arranged infinitely in the in-plane direction.

[0172] The second electrode and the microstructure were made of copper and had a thickness of 2 μm. Each microstructure was a square measuring 8 mm × 8 mm, and was positioned approximately at the center of the unit cell in the in-plane direction.

[0173] The liquid crystal composition layer was a mixture of the following liquid crystal compounds 1-1 and 1-3 in a ratio of 50%:50%, and the thickness of the liquid crystal composition layer was 300 μm.

[0174] A sample of a liquid crystal composition layer was prepared by mixing the following liquid crystal compounds 1-1 and 1-3, and the maximum refractive index change per unit voltage in radio waves having wavelengths in the range of 300 μm to 30 cm was measured by the above-mentioned method, and was found to be 0.12 (1 / V).

[0175]

[0176]

[0177] The intermediate layer was made of quartz (SiO2) and had a thickness of 300 μm, so the distance between the microstructure and the second electrode was 600 μm.

[0178] A sample of the intermediate layer made of quartz was prepared, and the maximum change in refractive index per unit voltage in radio waves with wavelengths in the range of 300 μm to 30 cm was measured by the above-mentioned method, and was found to be 0.0001 (1 / V).

[0179] Comparative Example 1-1 A radio wave control element similar to that of Example 1-1 was modeled, except that it did not have an intermediate layer and the thickness of the liquid crystal composition layer was 600 μm. That is, the distance between the microstructure and the second electrode was 600 μm.

[0180] Comparative Example 1-2 A radio wave control element similar to that of Example 1-1 was modeled, except that it did not have an intermediate layer. That is, the distance between the microstructure and the second electrode was 300 μm.

[0181] [Evaluation] Using the simulation software "COMSOL Multiphysics," the loss of 10 GHz radio waves was calculated when 10 GHz radio waves were incident on and reflected from the models of the radio wave control elements of the above examples and comparative examples. Note that the loss of radio waves in the liquid crystal composition layer is approximately the same regardless of the orientation state of the liquid crystal compound.

[0182] It is also known that the switching time when a voltage is applied to a liquid crystal composition layer to change the alignment state of the liquid crystal compound is proportional to the square of the thickness. The switching time of the radio wave control element of Comparative Example 1 was set to 1, and the switching times of the Examples and Comparative Examples were calculated. The results are shown in Table 1.

[0183]

[0184] It can be seen from Table 1 that the examples of the present invention can achieve both a shorter switching time and a reduced loss of radio waves compared to the comparative examples.

[0185] [Example 2-1] The radio wave control element having a modeled waveguide had a structure in which the waveguide, intermediate layer, liquid crystal composition layer, and microstructure (metasurface structure) were stacked in this order. The waveguide was a copper metal waveguide with a width of 25.4 mm, a height of 12.7 mm, and a length of 80 mm, and three openings, each 13.7 mm wide and 1 mm long, were installed on the top surface at 25 mm intervals.

[0186] The microstructures were made of copper and had a thickness of 2 μm. Each microstructure was a square measuring 17 mm×17 mm and was placed directly above the opening on the top surface of the waveguide.

[0187] The liquid crystal composition layer and the intermediate layer were the same as those in Example 1-1. That is, the thickness of the liquid crystal composition layer was 300 μm, and the thickness of the intermediate layer was 300 μm. Therefore, the distance between the microstructure and the second electrode was 600 μm.

[0188] Comparative Example 2-1 A radio wave control element similar to that of Example 2-1 was modeled, except that it did not have an intermediate layer and the thickness of the liquid crystal composition layer was 600 μm. That is, the distance between the microstructure and the second electrode was 600 μm.

[0189] Comparative Example 2-2 A radio wave control element similar to that of Example 2-1 was modeled, except that it did not have an intermediate layer. That is, the distance between the microstructure and the second electrode was 300 μm.

[0190] [Evaluation] Using the simulation software "COMSOL Multiphysics," the loss of 10 GHz radio waves emitted from the aperture when 10 GHz radio waves are incident on and propagated through the waveguide was calculated for the models of the radio wave control elements of the above examples and comparative examples.

[0191] The results are shown in Table 2.

[0192]

[0193] From Table 2, it can be seen that the examples of the present invention can achieve both a shorter switching time and a reduced loss of radio waves compared to the comparative examples.

[0194] The above results clearly demonstrate the effectiveness of the present invention.

[0195] 2 Radio wave reflecting device 10, 50 Radio wave control element 12 Metasurface structure 14 Microstructure (first electrode) 24 Support 20 Liquid crystal composition layer 26 Second electrode 28 Power supply 32 Intermediate layer 52 Waveguide 54 Opening ANT Antenna AR1, AR2 Area BL Building LC Liquid crystal compound RW Radio wave UC Unit cell

Claims

1. A radio wave control element having a first electrode, a liquid crystal composition layer, and a second electrode, the element having a metasurface structure formed by arranging a plurality of microstructures, the metasurface structure constituting at least a part of the first electrode, and an intermediate layer between one of the first electrode and the second electrode and the liquid crystal composition layer, the maximum refractive index change per unit voltage in radio waves having a wavelength in the range of 300 μm to 30 cm being 0.001 (1 / V) or less.

2. The radio wave control element according to claim 1, further comprising a waveguide for guiding radio waves on the side of said second electrode opposite said liquid crystal composition layer, said second electrode having an opening through which radio waves pass.

3. The radio wave control element according to claim 2, wherein said second electrode also serves as said waveguide.

4. The radio wave control element according to claim 1, wherein the thickness of said intermediate layer is 0.1 to 2 times the thickness of said liquid crystal composition layer.

5. The radio wave control element according to any one of claims 1 to 4, which controls radio waves in the wavelength range of 300 μm to 30 cm.

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