Radio wave control element

Abstract

The present invention addresses the problem of providing a radio wave control element that requires less power consumption to control radio waves. The radio wave control element solves the problem by comprising a first electrode, a liquid crystal composition layer, and a second electrode in this order, at least one of the first electrode and the second electrode having a metasurface structure, and further having a temperature adjustment mechanism for heating and cooling the liquid crystal composition layer, and the solid-liquid crystal phase transition temperature of the liquid crystal composition layer being 40°C or higher.

Description

Radio wave control element

[0001] The present invention relates to a radio wave control element using a metasurface structure and a liquid crystal layer.

[0002] The high-frequency radio waves (millimeter waves, terahertz waves) required for high-capacity wireless communication have a tendency to propagate in a direction of interest. Therefore, in order to deliver the radio waves to the desired location, a reflector that can bend the radio waves in any direction is required. However, for example, a normal radio wave reflector reflects the radio waves in a fixed direction, and the reflection direction is a specular reflection in which the angle of incidence and the angle of emission are equal. Therefore, with a normal reflector, there are significant limitations on the range in which the direction of the radio waves can be changed, making it difficult to deliver the radio waves to the desired location.

[0003] To solve this problem, radio wave control elements are used that can arbitrarily control the direction of radio waves. Known examples of such radio wave control elements include elements that use an integrated circuit (IC) to adjust the phase within the reflective surface, thereby reflecting radio waves in a desired direction.

[0004] However, radio wave control elements using ICs have the problem of high power consumption. In contrast, radio wave control elements using liquid crystals are known as radio wave control elements with low power consumption. These radio wave control elements apply a voltage to a liquid crystal layer sandwiched between electrodes to adjust the orientation of the liquid crystal, thereby adjusting the phase distribution within the reflective surface and reflecting radio waves in the desired direction.

[0005] As an example, Patent Document 1 describes a radio wave control element (tunable LC device) that includes a first substrate, a first reflector on the first substrate and including a first electrode layer, a liquid crystal layer (LC layer) on the first reflector, a second reflector on the liquid crystal layer and including a second electrode layer, and a second substrate on the second reflector, wherein the liquid crystal layer is adjustable by applying an electrical signal to at least one of the first or second electrode layers.

[0006] Special Publication No. 2023-513660

[0007] The radio wave control element described in Patent Document 1 uses a liquid crystal layer and a metasurface structure consisting of an array of conductive microstructures. In this radio wave control element, each microstructure serves as an electrode, and the orientation of the liquid crystal compound in the region corresponding to each electrode is adjusted by adjusting the voltage applied to each electrode. This adjusts the phase distribution within the reflective surface, making it possible to reflect radio waves in the desired direction.

[0008] Such a radio wave control element using a liquid crystal can control the direction of radio waves to the desired direction with less power consumption than the radio wave control element using the IC described above. However, conventional radio wave control elements using liquid crystals still consume a lot of power because they require a constant supply of power while in operation. Therefore, there is a demand for improvements to further reduce power consumption.

[0009] The object of the present invention is to solve the problems of the conventional technology and to provide a radio wave control element that controls the direction of radio waves using a metasurface structure and a liquid crystal layer, and that consumes little power to control radio waves.

[0010] To solve this problem, the present invention has the following configuration. [1] A radio wave control element having a first electrode, a liquid crystal composition layer, and a second electrode in this order, wherein at least one of the first electrode and the second electrode has a metasurface structure formed by arranging a plurality of microstructures, and further having a temperature control mechanism for heating and cooling the liquid crystal composition layer, wherein the liquid crystal composition layer has a solid-liquid crystal phase transition temperature of 40°C or higher. [2] The radio wave control element according to [1], wherein the liquid crystal composition layer can have a peak in a diffraction angle range of 15° or less in an X-ray diffraction spectrum measured at a temperature below 40°C. [3] The radio wave control element according to [1] or [2], wherein the liquid crystal composition layer contains a dichroic material. [4] The radio wave control element according to [3], wherein the content of the dichroic material is 30% by mass or more relative to the total mass of the liquid crystal composition layer.

[0011] According to the radio wave control element of the present invention, the traveling direction of radio waves can be controlled with less power consumption.

[0012] Fig. 1 is a conceptual diagram showing an example of a radio wave control element of the present invention. Fig. 2 is a conceptual diagram showing an example of a use of the radio wave control element of the present invention. Fig. 3 is a perspective view conceptually showing the metasurface structure of the radio wave control element shown in Fig. 1. Fig. 4 is a conceptual diagram for the operation of the radio wave control element of the present invention. Fig. 5 is a conceptual diagram for the operation of the radio wave control element of the present invention.

[0013] Hereinafter, the radio wave control element of the present invention will be described in detail based on the preferred embodiment shown in the accompanying drawings.

[0014] 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. In this specification, "the same" includes a margin of error generally accepted in the technical field.

[0015] The drawings shown below are conceptual diagrams for explaining the radio wave control element of the present invention. Therefore, the shape, size, thickness, positional relationship, etc. of each member do not necessarily match the actual ones.

[0016] 1 conceptually illustrates an example of a radio wave control element (electromagnetic wave control element) of the present invention. The radio wave control element 10 of the present invention is a reflective radio wave control element that uses a reflective metasurface structure and a liquid crystal layer to control the direction of propagation of radio waves (electromagnetic waves) to a desired direction. For example, such a radio wave control element of the present invention is used in active antennas and beam steering devices that can switch the direction of propagation of radio waves for transmission in indoor and outdoor radio wave communications.

[0017] As an example, the radio wave control element 10 of the present invention acts on radio waves with a frequency of 0.007 to 0.3 THz and reflects the radio waves in a desired direction. That is, the radio wave control element of the present invention reflects radio waves with a wavelength of 1 to 43 mm in a desired direction. Preferably, the radio wave control element 10 of the present invention reflects radio waves with a frequency of 0.1 to 0.3 THz, i.e., radio waves with a wavelength of 1 to 3 mm, in a desired direction.

[0018] As shown in Figure 1, the radio wave control element 10 of the present invention has, from the bottom in the figure, a first electrode layer 26, a liquid crystal composition layer 20, and a metasurface structure 12, in this order. The metasurface structure 12 is formed by two-dimensionally arranging microstructures 14 that serve as resonators on a support 16. The liquid crystal composition layer 20 is provided on a support 24. Furthermore, the first electrode layer 26 is provided so as to entirely cover the surface of the support 24 opposite to the liquid crystal composition layer 20.

[0019] Furthermore, in the radio wave control element 10 of the present invention, a temperature adjustment mechanism 30 is provided on the underside of the first electrode layer 26 in the figure. The temperature adjustment mechanism 30 heats and cools the liquid crystal composition layer 20. The temperature adjustment mechanism 30 in the illustrated example heats the first electrode layer 26 to heat the liquid crystal composition layer 20, and cools the first electrode layer 26 to cool the liquid crystal composition layer 20.

[0020] In the radio wave control element 10, the temperature adjustment mechanism 30 and the first electrode layer 26, the first electrode layer 26 and the support 24, and the liquid crystal composition layer 20 and the support 16 (metasurface structure 12) are attached using an adhesive (adhesive, adhesive) as necessary. There are no limitations on the attachment method, and various known methods that allow the radio waves targeted by the radio wave control element 10 to pass through can be used, such as a method using an OCA (Optical Clear Adhesive) that allows the radio waves targeted by the radio wave control element 10 to pass through.

[0021] 1, as an example, the microstructures 14 are formed from a conductive material and serve as electrodes that form electrode pairs with the first electrode layer 26. Each microstructure 14 is connected to a power supply 28 for applying a voltage between the microstructure 14 and the first electrode layer 26. There are no limitations on the power supply 28, and various known AC power supplies can be used as long as they are capable of supplying the required power.

[0022] As described above, the radio wave control element 10 is a reflective radio wave control element that uses a reflective metasurface structure and a liquid crystal composition layer to control the propagation direction of radio waves (radio waves) to a desired direction. As an example, such a radio wave control element 10 is used in a radio wave reflection device RD (active antenna), as conceptually shown in FIG. 2 . Specifically, by using the radio wave control element 10, the radio wave reflection device RD switches the reflection direction of highly directional radio waves RW emitted from an antenna ANT located behind a building BL between a direction toward an area AR1 in front of the building BL, which is in the shadow when viewed from the antenna ANT, and a direction toward an area AR2 different from area AR1. For example, the area where many users are present may change depending on the time of day, such as when many wireless communication users are present in area AR1 during the daytime and in area AR2 during the nighttime. In such cases, the radio wave reflection device RD changes the reflection direction of the radio waves RW depending on the time of day, thereby changing the area to which the radio waves RW are supplied.

[0023] In the radio wave control element 10 of the present invention, the liquid crystal composition layer 20 undergoes a phase transition upon heating; it is solid (glassy) at room temperature and transitions to a liquid crystal phase upon heating. In the liquid crystal phase transition state, the alignment direction of the liquid crystal compound LC in the liquid crystal composition layer 20 can be changed by supplying power from the power source 28 to the microstructure 14, i.e., by applying a voltage between the microstructure 14 and the first electrode layer 26. Furthermore, if the liquid crystal composition layer 20 is cooled to room temperature while continuing to apply a voltage to maintain the changed alignment of the liquid crystal compound LC, the liquid crystal composition layer 20 will be solidified, maintaining the alignment state of the liquid crystal compound LC even after the application of the voltage is stopped. The temperature adjustment mechanism 30 is used for heating and cooling the liquid crystal composition layer 20 in this manner.

[0024] As described above, by supplying power to each microstructure 14 in a heated state, a voltage is applied to the liquid crystal composition layer 20 between the microstructure 14 and the first electrode layer 26, changing the alignment state of the liquid crystal compound LC. Furthermore, by adjusting the power supplied to each microstructure 14, the voltage applied to the region of the liquid crystal composition layer 20 corresponding to the microstructure 14 can be adjusted, thereby adjusting the alignment of the liquid crystal compound LC between the microstructure 14 and the first electrode layer 26.

[0025] As an example, in a heated state, i.e., in a liquid crystal phase state, with no voltage applied, the liquid crystal compound LC of the liquid crystal composition layer 20 is aligned in a direction parallel to the principal surface of the liquid crystal composition layer 20, as conceptually shown in the upper part of FIG. 5 (described later). The principal surface of the liquid crystal composition layer 20 is the X-Y plane (described later). In the following description, this alignment state is also referred to as "horizontal alignment." When a voltage is applied to the liquid crystal composition layer 20 in a heated state, as conceptually shown in the second top panel of FIG. 5 , the alignment state of the liquid crystal compound LC in the regions corresponding to the microstructures 14 changes depending on the strength 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. In this example, the liquid crystal compound LC is aligned in the thickness direction of the liquid crystal composition layer 20 at its maximum. The thickness direction of the liquid crystal composition layer 20 is the Z direction (described later). In the following description, this alignment state is also referred to as "vertical alignment."

[0026] The thickness direction is the stacking direction of the first electrode layer 26, the support 24, the liquid crystal composition layer 20, and the support 16. The main surface is the largest surface of the sheet-like material (film, plate, layer), and is usually both surfaces in the thickness direction of the sheet-like material. The normal direction is the direction perpendicular to the surface such as the main surface.

[0027] As described above, the radio wave control element 10 shown in FIG. 1 utilizes a reflective metasurface structure. As conceptually shown in FIG. 3 , in the illustrated metasurface structure 12, the microstructures 14 are, for example, rectangular flat plates with two sides aligned in the mutually orthogonal X and Y directions, and are arranged two-dimensionally in the X and Y directions. The planar shape refers to the shape when viewed from a direction perpendicular to the main surface of the sheet-like material; in this example, it refers to the shape when viewed from a direction perpendicular to the main surface of the support 16. In the metasurface structure 12, each microstructure 14 and the surrounding space extending to the middle of the adjacent microstructure 14 constitute a unit cell UC in the metasurface structure (see FIGS. 3 and 5 ).

[0028] When radio waves are incident on the radio wave control element 10 of the present invention having such a configuration, the radio waves have their phase modulated by resonance with the microstructures 14 (unit cells) as they pass through the metasurface structure 12, and are further phase modulated as they pass through the liquid crystal composition layer 20. The radio waves are then reflected by the first electrode layer 26, which also serves as a reflective layer. The radio waves reflected by the first electrode layer 26 are again phase modulated as they pass through the liquid crystal composition layer 20, and are further phase modulated by the metasurface structure 12, and are emitted from the radio wave control element 10 as reflected radio waves.

[0029] As described above, the orientation state, i.e., the refractive index, of the liquid crystal compound LC in the liquid crystal composition layer 20 varies depending on the voltage applied to each microstructure 14 in a heated state. That is, the effective refractive index for radio waves changes as the orientation state of the liquid crystal compound LC changes.

[0030] When no voltage is applied to the liquid crystal composition layer 20, the liquid crystal compound LC in the liquid crystal composition layer 20 is, for example, horizontally aligned. As described above, when a voltage is applied to the liquid crystal composition layer 20 in a heated state, the liquid crystal compound LC in the regions corresponding to the microstructures 14 is aligned at an angle relative to the main surface of the liquid crystal composition layer 20 depending on the magnitude of the applied voltage. Furthermore, when the liquid crystal composition layer 20 is returned to room temperature while maintaining the alignment state of the liquid crystal compound LC, i.e., while a voltage is applied, the liquid crystal composition layer 20 returns to a solid state, and the alignment of the liquid crystal compound LC is maintained even after the application of the voltage is stopped. In the radio wave control element 10, the greater the voltage applied to the liquid crystal composition layer 20, the closer the liquid crystal compound LC becomes to vertical alignment, and the refractive index of the liquid crystal composition layer 20 in that region changes from the state when no voltage is applied. In other words, in the liquid crystal composition layer 20, the refractive index of the corresponding region, i.e., the phase difference imparted to the transmitted radio waves, can be changed depending on the voltage applied to each microstructure 14.

[0031] As conceptually shown in Figure 4 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 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 multiple unit cells UC arranged in a one-dimensional manner from the unit cell UC on the right side of the figure to the unit cell UC on the left side of the figure. In this case, even if the line connecting the wavefronts of each incident radio wave RW is parallel to the reflecting surface, the line connecting the wavefronts of each radio wave 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 output 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.

[0032] That is, in the radio wave control element 10, by adjusting the power supplied to each microstructure 14 and the voltage applied to the corresponding region, the orientation of the liquid crystal compound in the liquid crystal composition layer 20 can be adjusted, thereby generating regions with different refractive indices in the in-plane direction of the liquid crystal composition layer 20. This allows incident radio waves to be reflected in a direction different from specular reflection. For example, when radio waves are incident from the normal direction of the liquid crystal composition layer 20, the radio waves are reflected in a direction tilted relative to the normal direction rather than in the normal direction. Furthermore, by changing the power supplied to each microstructure 14 and the voltage applied to the corresponding region of the liquid crystal composition layer 20, the orientation state of the liquid crystal compound at each position in the in-plane direction, i.e., the refractive index, or phase, can be adjusted, thereby switching the reflection direction of the incident radio waves. As described above, the radio wave control element 10 of the present invention is an active radio wave control element that can change the reflection direction of incident radio waves by adjusting the power supplied to each microstructure 14.

[0033] In the radio wave control element 10 of the present invention, the metasurface structure 12, like known metasurface structures, is formed by two-dimensionally arranging microstructures 14, which are microstructures, on a support 16. As described above, in the illustrated metasurface structure 12, the microstructures 14 have a rectangular planar shape and are two-dimensionally arranged at equal intervals in the X and Y directions, which are orthogonal to each other.

[0034] There are no limitations on the support 16, and various known sheet-like materials can be used as long as they can support the microstructure 14 and transmit radio waves of the frequency targeted by the radio wave control element 10, for example, radio waves of 0.007 to 0.3 THz. Examples of the support 16 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, a liquid crystal polymer (LCP (Liquid Crystal Polymer)) film, and a glass plate.

[0035] There is no restriction on the thickness of the support 16, and the thickness can be set appropriately depending on the material from which the support 16 is made so that it can support the microstructure 14, provide sufficient transparency to the target radio waves, and provide sufficient strength depending on the application of the radio wave control element 10, etc.

[0036] In the radio wave control element 10 of the present invention, the metasurface structure 12 is not limited to having the support 16. That is, in the radio wave control element of the present invention, 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.

[0037] Microstructures 14 are arranged on one surface of the support 16. This forms the metasurface structure 12. The metasurface structure 12 is formed by arranging the microstructures 14 two-dimensionally at intervals on a plane, and is basically composed of an arrangement of unit cells formed by one microstructure 14 and the space surrounding the microstructure 14.

[0038] In the radio wave control element 10 of the present invention, the metasurface structure is basically a known metasurface structure (metamaterial). Therefore, various known metasurface structures can be used in the radio wave control element 10 of the present invention. That is, in the present invention, there are no limitations on the shape and material of the microstructures 14, the arrangement of the microstructures 14, or the spacing (pitch) between the microstructures 14. Furthermore, the metasurface structure 12 may be designed using a known method according to the radio wave reflection characteristics desired by the radio wave control element 10 of the present invention. As an example, the amplitude and phase of the radio waves reflected by the microstructures 14 used may be calculated using commercially available simulation software, and the arrangement of the microstructures 14 may be set so as to achieve the desired distribution of phase modulation amount (refractive index).

[0039] The radio wave control element 10 of the present invention is, for example, intended for radio waves with a frequency of 0.007 to 0.3 THz, preferably 0.1 to 0.3 THz. Therefore, the metasurface structure 12 has the microstructures 14 selected so as to impart a desired phase difference to radio waves of this frequency, and further, the arrangement of the microstructures is set.

[0040] The metasurface structure 12 is basically composed of an arrangement of unit cells formed by one microstructure 14 and the space surrounding the microstructure 14. The metasurface structure 12 modulates the phase of incident radio waves by utilizing the resonance of the microstructure 14 through the arrangement of the unit cells. Note that, in the radio wave control element 10 of the present invention, one unit cell basically has one microstructure 14, but the present invention is not limited to this. That is, in the radio wave control element of the present invention, one unit cell may have multiple microstructures 14 as necessary depending on the desired optical properties, the size, material, and shape of the microstructure 14, and the size of the unit cell. In this case, one unit cell may have different microstructures 14. However, when one unit cell has multiple microstructures 14, the amount of phase modulation in the space where each microstructure of the unit cell exists is basically equal.

[0041] In the radio wave control element 10 of the present invention, the material for forming the microstructures 14 constituting the metasurface structure 12 is not limited, and various materials used as microstructures in known metasurface structures can be used. Examples of materials for forming the microstructures 14 include metals and dielectrics. In the case of metals, copper, gold, and silver are preferred due to their low optical loss. Also, composites consisting of metal particles and binders, and oxide semiconductors can also be used as materials for forming the microstructures 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 achieve large phase modulation. Note that, as shown in FIG. 1 , when the microstructures 14 also serve as electrodes that form an electrode pair with the first electrode layer 26, the microstructures 14 are formed from a conductor.

[0042] Similarly, the shape of the microstructures 14 that make up the metasurface structure 12 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 rectangular parallelepipeds connected at their 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.

[0043] In the metasurface structure 12, only one such microstructure 14 may be used, or multiple types may be used in combination. Furthermore, the same microstructures 14 may be arranged in the same direction as shown in Figure 3, in different directions, or a mixture of microstructures 14 oriented in the same direction and those oriented in different directions may be present.

[0044] In the illustrated example, in a preferred embodiment, the metasurface structure 12 has identical microstructures 14, all of which have the same structure, arranged two-dimensionally in the X and Y directions, orthogonal to each other, at equal intervals, in the same orientation. However, the present invention is not limited to this, and multiple types of microstructures may be used in combination as described above. Furthermore, the arrangement and spacing of the microstructures 14 may also vary in the plane direction of the support 16. However, considering the controllability of the reflection direction of radio waves when a voltage is applied to the liquid crystal composition layer 20 to align the liquid crystal compound LC, it is preferable that the metasurface structure 12 use all of the same microstructures 14. Furthermore, it is more preferable that the metasurface structure 12 has identical microstructures 14 arranged two-dimensionally in the same orientation, at equal intervals, and even more preferably arranged two-dimensionally at equal intervals in the orthogonal X and Y directions.

[0045] Although the radio wave control element 10 shown in FIG. 1 has one metasurface structure 12, the present invention is not limited to this. That is, the radio wave control element of the present invention may also have a metasurface structure on the first electrode layer 26 side. In other words, the radio wave control element of the present invention may have two metasurface structures sandwiching a liquid crystal composition layer. In this case, the two metasurface structures may be the same or different. Furthermore, the two metasurface structures may be the same, with the positions of the microstructures shifted.

[0046] The liquid crystal composition layer 20 is a layer in which the liquid crystal compound LC is aligned in a predetermined state. It is solid (glassy) at room temperature and transitions to a liquid crystal phase upon heating, allowing the alignment of the liquid crystal compound LC to be changed by applying a voltage. The liquid crystal composition layer 20 preferably undergoes a phase transition from solid to nematic upon heating. Therefore, in the liquid crystal composition layer 20, once the phase transition to the liquid crystal phase has occurred upon heating, as described above, applying a voltage between the microstructures 14 and the first electrode layer 26 changes the alignment state of the liquid crystal compound LC depending on the power supplied to each microstructure 14. In the liquid crystal composition layer 20, the alignment state of the liquid crystal compound LC is horizontally aligned in the steady state. As the applied voltage increases, the liquid crystal compound LC is tilted toward the main surface of the liquid crystal composition layer 20, becoming closer to a vertical alignment. Furthermore, when the liquid crystal compound LC is cooled while maintaining its alignment by applying a voltage, and then returned to room temperature, it returns to a solid state while maintaining its alignment. In this state, the liquid crystal composition layer 20 maintains the alignment of the liquid crystal compound even after the application of a voltage is stopped.

[0047] That is, the radio wave control element 10 of the present invention adjusts the orientation of the liquid crystal compound LC by heating and applying a voltage, and then cools it back to room temperature (below the phase transition temperature) while the voltage is still applied. The orientation of the liquid crystal compound LC can then be maintained even after the power supply is stopped. Therefore, the radio wave control element 10 can subsequently control the reflection direction of an incident radio wave to a desired direction without applying a voltage to the liquid crystal composition layer 20. Therefore, the radio wave control element 10 of the present invention can significantly reduce power consumption for radio wave control. Furthermore, by heating the element 10 from a state in which the reflection direction of the radio wave is controlled in a certain direction, applying power to each microstructure 14 again to change the orientation of the liquid crystal compound LC, and then cooling the element, the reflection direction of the incident radio wave can be changed and the radio wave can be controlled to another desired direction. That is, as described above, the radio wave control element 10 of the present invention is an active radio wave control element that can arbitrarily change the reflection direction (control direction) of the radio wave.

[0048] In the illustrated example, the liquid crystal compound LC is horizontally aligned in a steady state in the liquid crystal composition layer 20 of the radio wave control element 10. Therefore, when the power supply is stopped while the liquid crystal compound LC is heated to a temperature equal to or higher than the phase transition temperature, the liquid crystal compound LC returns to a horizontal alignment.

[0049] In the present invention, the liquid crystal composition layer 20 is a layer having a solid-liquid crystal phase transition temperature of 40°C or higher, preferably a solid-nematic phase transition temperature of 40°C or higher. If the solid-liquid crystal phase transition temperature of the liquid crystal composition layer 20 is lower than 40°C, a phase transition occurs even at temperatures close to room temperature, making it possible to change the alignment of the liquid crystal compound LC. That is, if the solid-liquid crystal phase transition temperature of the liquid crystal composition layer 20 is lower than 40°C, the alignment of the liquid crystal compound LC will return to a horizontal alignment when the voltage application to the liquid crystal composition layer 20 is stopped after cooling. Therefore, if the solid-liquid crystal phase transition temperature of the liquid crystal composition layer 20 is lower than 40°C, in order to control radio waves, it is necessary to apply a voltage constantly, as in the conventional radio wave control element described in Patent Document 1, which results in high power consumption.

[0050] The solid-liquid crystal phase transition temperature of the liquid crystal composition layer 20 is preferably 50° C. or higher, more preferably 60° C. or higher, and even more preferably 80° C. or higher. There is basically no upper limit to the solid-liquid crystal phase transition temperature of the liquid crystal composition layer 20. However, in consideration of the heating energy required to cause the phase transition of the liquid crystal composition layer 20, prevention of damage to other members due to heat, prevention of thermal expansion, etc., the phase transition temperature is preferably 120° C. or lower.

[0051] In the radio wave control element of the present invention, the solid-liquid crystal phase transition temperature of the liquid crystal composition layer 20 is measured, for example, by observing the liquid crystal composition forming the liquid crystal composition layer 20 with a polarizing microscope. That is, while observing with a polarizing microscope, the liquid crystal composition forming the liquid crystal composition layer 20 is heated to a temperature at which it becomes a liquid crystal phase, and then, while cooling, the temperature at which the liquid crystal phase transitions to another phase, such as a crystalline phase, is measured. Alternatively, the support 16 or 24 may be peeled off to expose the liquid crystal composition layer 20, and the liquid crystal composition layer 20 may be sampled by a method such as scraping, and the solid-liquid crystal phase transition temperature of the liquid crystal composition layer 20 may be measured using this sample by the method described above. Alternatively, the solid-liquid crystal phase transition temperature may be measured by thermal analysis such as differential scanning calorimetry (DSC) and thermogravimetry (TG). The solid-liquid crystal phase transition temperature may also be measured by identifying the phase using X-ray diffraction (XRD).

[0052] There is also no limitation on the thickness of the liquid crystal composition layer 20, and a thickness that can impart the necessary phase difference to radio waves may be appropriately set depending on the material for forming the liquid crystal composition layer 20. In the present invention, the liquid crystal composition layer 20 may be formed, for example, on the surface of an alignment film described below by a known method depending on the liquid crystal compound to be used, etc. The liquid crystal composition layer 20 will be described in detail later.

[0053] In the radio wave control element 10, the liquid crystal composition layer 20 is formed on a support 24. The support 24 is basically the same as the support 16 described above.

[0054] Here, the support 24 on which the liquid crystal composition layer 20 is formed may be a substrate similar to the support 16 described above, with an alignment film for aligning the liquid crystal compound LC in a predetermined state on the surface of this substrate on which the liquid crystal composition layer 20 is formed. 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 by the Langmuir-Blodgett method of organic compounds such as ω-tricosanoic acid, dioctadecylmethylammonium chloride, and methyl stearate. Furthermore, so-called photo-alignment films, which are formed by irradiating a photo-alignable material with polarized or unpolarized light, can also be used as alignment films. These alignment films can be formed by known methods depending on the material from which the substrate is formed.

[0055] The surface of the support 24 on which the liquid crystal composition layer 20 is formed opposite to the liquid crystal composition layer 20 is entirely covered with a first electrode layer 26. The first electrode layer 26 is an electrode that changes the orientation of the liquid crystal compound LC in the liquid crystal composition layer 20, and also functions as a reflective layer that reflects radio waves incident from the metasurface structure 12 side, as described above.

[0056] There are no limitations on the first electrode layer 26, and a sheet-like object made of various known materials can be used as long as it has sufficient conductivity and can reflect the target radio waves. Examples of the first electrode layer 26 include metal layers such as copper, aluminum, gold, and silver; inorganic conductive materials such as ITO (tin-doped indium oxide); organic conductive materials such as polythiophenes, 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 reflective layers for radio waves of the above-mentioned frequencies.

[0057] There is no limitation on the thickness of the first electrode layer 26, and the thickness may be set appropriately depending on the material from which the first electrode layer 26 is made so that the target radio waves can be reflected with the required reflectivity.

[0058] As described above, the radio wave control element 10 is provided with a temperature adjustment mechanism 30 on the underside of the first electrode layer 26 in the drawing. The temperature adjustment mechanism 30 heats and cools the liquid crystal composition layer 20, and heats the first electrode layer 26 to heat the liquid crystal composition layer 20, and cools the liquid crystal composition layer 20 by cooling the first electrode layer 26.

[0059] In the illustrated radio wave control element 10, the liquid crystal composition layer 20 is heated by the temperature adjustment mechanism 30, thereby transitioning the liquid crystal composition layer 20 from a solid (solid phase) to a liquid crystal phase. After adjusting the alignment of the liquid crystal compound LC in the liquid crystal phase, the liquid crystal composition layer 20 is cooled by the temperature adjustment mechanism 30, thereby returning the liquid crystal composition layer 20 to a solid state. As described above, the radio wave control element 10 of the present invention can maintain the alignment of the liquid crystal compound LC even when the application of voltage to the microstructure 14 is stopped in this state, thereby reducing the power consumption for radio wave control. In the radio wave control element 10 of the present invention, the temperature adjustment mechanism 30 is provided with not only a heating means for transitioning the liquid crystal composition layer 20 to the liquid crystal phase but also a cooling means for cooling the liquid crystal composition layer 20, thereby controlling the alignment of the liquid crystal compound and then performing rapid cooling. This prevents unnecessary crystallization of the liquid crystal composition layer 20 due to cooling, and allows the solidified liquid crystal composition layer 20 to be in a suitable state, in which the X-ray diffraction spectrum, described below, has a peak in the diffraction angle range of 15° or less.

[0060] There are no limitations on the temperature adjustment mechanism 30, and various temperature adjustment means using known heating and cooling means can be used. Examples of heating means include direct or indirect contact with a hot medium such as hot water, heating by radiant heat using a resistance heating heater using Joule heat, heating by hot air, and heating by a Peltier element. Examples of cooling means include direct or indirect contact with a refrigerant such as cold water, cooling using a Peltier element, cooling by cold air, etc. Multiple heating means and cooling means may be used in combination.

[0061] In the illustrated example, the temperature of the liquid crystal composition layer 20 is adjusted by adjusting the temperature of the first electrode layer 26, but the present invention is not limited to this. For example, the temperature of the liquid crystal composition layer 20 may be adjusted by adjusting the temperature of the microstructures 14 and / or the support 16 of the metasurface structure 12. Alternatively, the liquid crystal composition layer 20 may be heated and cooled directly. Furthermore, when the liquid crystal composition is heated and cooled indirectly as in the illustrated example, heating and cooling may be performed at separate locations, such as by using the first electrode layer 26 for heating and the metasurface structure 12 for cooling.

[0062] The operation of the radio wave control element 10 of the present invention will be described below with reference to the conceptual diagram of Fig. 5. Note that Fig. 5 simplifies the drawing and, in order to clearly show the operation of the radio wave control element 10 of the present invention, only shows the microstructure 14 (metasurface structure 12), liquid crystal composition layer 20, first electrode layer 26, and power source 28. In Fig. 5, the dashed lines indicate unit cells (unit cells UC) corresponding to each microstructure 14.

[0063] As described above, in the radio wave control element 10, in the steady state, the liquid crystal compound LC of the liquid crystal composition layer 20 is horizontally aligned (approximately horizontally aligned) as shown in the upper part of Fig. 5. In this state, the temperature control mechanism 30 starts heating the liquid crystal composition layer 20. When the heating of the liquid crystal composition layer 20 starts and the temperature exceeds the transition temperature, the liquid crystal composition layer 20 transitions from a solid to a liquid crystal phase. In this state, the power source 28 is not driven (off).

[0064] With the liquid crystal composition layer 20 transitioning to the liquid crystal phase, the power source 28 is driven (on) to supply power to the microstructures 14 acting as first electrodes, thereby applying a voltage to the liquid crystal composition layer 20. As a result, as shown in the second row from the top of FIG. 5 , the liquid crystal composition layer 20 changes the orientation state of the liquid crystal compound LC depending on the power supplied to each microstructure 14, i.e., the applied voltage. In the illustrated example, the applied voltage to the unit cell on the right side of the figure is the highest, and the liquid crystal compound LC is in a state similar to vertical orientation. Furthermore, almost no voltage is applied to the unit cell on the left side of the figure, and the liquid crystal compound LC remains in horizontal orientation. Furthermore, a potential intermediate between the two is applied to the unit cell in the middle of the figure, and the liquid crystal compound LC is in an orientation state approximately intermediate between horizontal and vertical orientation.

[0065] Once the orientation state of the liquid crystal compound LC has stabilized, as shown in the third diagram from the top in FIG. 5 , the liquid crystal composition layer 20 is cooled by the temperature control mechanism 30 while the power supply 28 remains turned on. When the temperature of the liquid crystal composition layer 20 reaches room temperature (below the transition temperature), the power supply 28 is turned off and the application of voltage is stopped, as shown in the bottom diagram in FIG. 5 . As described above, in this state, even without applying voltage, the orientation of the liquid crystal compound LC is maintained in the same state as when a voltage is applied. Note that the temperature control mechanism 30 is generally turned off when the temperature reaches room temperature, but if necessary, it controls the temperature to maintain room temperature. Therefore, when a radio wave RW is incident in this state, as previously described with reference to FIG. 4 , the incident radio wave RW is given a phase corresponding to the orientation state of the liquid crystal compound LC depending on the applied voltage, and the reflection direction is controlled so that it is reflected in a predetermined direction.

[0066] Therefore, the radio wave control element 10 of the present invention can significantly reduce the power consumption for radio wave control. Furthermore, by reheating the liquid crystal composition layer 20 from the state shown on the right side of Fig. 5, and then supplying power to each microstructure 14 again to adjust the orientation of the liquid crystal compound LC, and then cooling it, the reflection direction of the incident radio wave RW can be changed and the traveling direction of the radio wave RW can be controlled to another desired direction.

[0067] As described above, in the radio wave control element 10 of the present invention, the liquid crystal composition layer 20 is solid at room temperature and has a solid-liquid crystal phase transition temperature of 40°C or higher, preferably a solid-nematic phase transition temperature of 40°C or higher.

[0068] Here, in the radio wave control element of the present invention, the liquid crystal composition layer 20 is preferably capable of having a peak in the range of a diffraction angle of 15° or less in an X-ray diffraction spectrum (XRD spectrum) measured at a temperature of less than 40° C. The liquid crystal composition layer 20 having such a peak in the XRD spectrum has a high degree of orientation (=Δn) and is capable of suitably controlling radio waves with a frequency of 0.007 to 0.3 THz.

[0069] The presence of a peak in the XRD spectrum at a diffraction angle of 15° or less indicates the presence of periodic structures such as a crystalline phase and a smectic phase in the liquid crystal composition layer 20. When the liquid crystal composition layer 20, which transitions to a liquid crystal phase (nematic phase) at 40° C. or higher, is cooled from a temperature above the solid-liquid crystal phase transition temperature to room temperature, a glassy state is formed in which the molecular fluctuations of the liquid crystal phase are frozen. The presence of periodic structures such as a crystalline phase and a smectic phase in the glassy state results in a radio wave control element 10 with a high degree of orientation. It is generally known that crystalline phases and smectic phases, which are higher-order phases than a nematic phase, have a high degree of orientation. Therefore, it is believed that the presence of such higher-order phases in the glassy state causes the liquid crystal composition layer 20 to exhibit a high degree of orientation.

[0070] A high degree of orientation of the liquid crystal composition layer 20 can increase the refractive index anisotropy (Δn). Δn increases as the anisotropy (intrinsic birefringence) of the molecules contained in the liquid crystal composition layer 20 increases, but Δn also increases as the degree of orientation, which represents the degree to which the molecules are aligned in a certain direction, increases. Generally, nematic liquid crystals and glassy states solidified by cooling nematic liquid crystals exhibit degrees of orientation of approximately 0.6 to 0.85. On the other hand, smectic liquid crystals and crystalline phases, which are higher-order phases, exhibit degrees of orientation higher than 0.85. It is believed that the presence of such higher-order phases in the glassy state causes the degree of orientation of the liquid crystal composition layer 20 to be higher than 0.85.

[0071] The glassy state solidified by cooling a liquid crystal phase does not have a periodic structure, and therefore generally exhibits no periodicity. In contrast, a broad peak (halo) due to the average intermolecular distance is observed in the XRD spectrum. On the other hand, higher-order phases, such as crystalline and smectic phases, generally form periodic structures, such as layer structures, and peaks corresponding to the layer lengths are observed in the XRD spectrum. Generally, the halos observed in nematic and glassy phases are at angles greater than 15°. The layer length, i.e., the spacing between the periodic structures, is also referred to as the periodic length. In other words, the presence of a peak at an angle of 15° or less in the XRD spectrum indicates the presence of a higher-order phase with a periodic structure, such as a crystalline or smectic phase.

[0072] The XRD of the liquid crystal composition layer 20 may be measured, for example, using the liquid crystal composition forming the liquid crystal composition layer 20. For example, the liquid crystal composition forming the liquid crystal composition layer 20 is dissolved in a solvent and spin-coated onto a common alignment film, such as a rubbed polyimide, to form a film. This film is heated to a temperature equal to or higher than 40°C, at which the liquid crystal phase transition occurs, to form a liquid crystal phase in which the liquid crystal compound LC in the liquid crystal composition is aligned, which is the same temperature process as that actually used in a radio wave control element. The film is then cooled to less than 40°C (below the transition temperature), thereby obtaining a sample of the liquid crystal composition layer 20 in which the liquid crystal composition is fixed. The XRD of this sample is measured using the liquid crystal composition layer as described below. Alternatively, the support 16 or 24 may be peeled off to expose the liquid crystal composition layer 20, and the liquid crystal composition layer 20 may be sampled by a method such as scraping, and the sample may be treated in the same manner as the liquid crystal composition described above to obtain a sample of the liquid crystal composition layer 20.

[0073] Next, the XRD spectrum is measured using an in-plane method. This measurement is performed while the liquid crystal composition layer is kept at 40° C. or less. Hereinafter, X-ray diffraction analysis performed using the in-plane method is also referred to as "in-plane XRD." In-plane XRD is performed by irradiating the sample surface with X-rays using a thin film X-ray diffractometer under the following conditions: (Conditions) Cu source used (CuKα, output 45 kV, 200 mA) X-ray incidence angle 0.2° Optical system used: parallel optical system (CBO (Cross Beam Optics)) (PB (Parallel Beam)) Incident side: Incident slit 0.2 mm, incident parallel slit In-plane PSC (Parallel Slit Collimator) 0.5 deg (degrees), length limiting slit 10 mm Receiving side: Receiving slit 20 mm, receiving parallel slit In-plane PSA (Parallel Slit Analyzer) 0.5 deg Detector: Rigaku HyPix3000 (0D mode) 2θχ / φ scan Scan conditions: 1 to 40 degree range at 0.008 degree / step, 2.0 degree / min (minute) φ scan Scan conditions: -120 to 120 degrees, 0.5 degree / step, 9.6 degree / min. The above conditions are the settings for the thin film X-ray diffractometer. Any known thin film X-ray diffractometer can be used. One example of a thin film X-ray diffractometer is the SmartLab manufactured by Rigaku Corporation.

[0074] The sample is placed on an X-ray diffraction apparatus so that the long axis direction (orientation axis direction) of the aligned liquid crystal compound LC is parallel to the incident X-ray, with the azimuth angle (φ) at this time being 0°.

[0075] The orientation axis direction is measured as follows: In-plane XRD (2θχ / φ scan) is performed so that a periodic structure in the orientation axis direction is observed. φ scans are performed on the observed peaks at 0.5° intervals, and the direction perpendicular to the direction in the substrate plane where the peak intensity is maximum is determined as the orientation axis direction.

[0076] In-plane XRD (2θχ / φ scan) is performed at 0.5° increments over the azimuth angle (φ) range from 0° to 90°, and the orientation in the substrate plane at which the peak intensity is maximized is determined by the φ scan performed on the observed peak. In-plane XRD (2θχ / φ scan) is performed in the orientation at which the peak intensity is maximized to obtain an XRD spectrum. The diffraction angle θ is the diffraction angle 2θχ / φ in the in-plane XRD 2θχ / φ scan.

[0077] In the XRD spectrum, a peak is considered to have a peak top intensity that is 80 or more times higher than the baseline intensity. The baseline intensity is defined by a known method. The unit of peak intensity is cps (counts per second). The peak intensity is the difference between the baseline intensity and the peak top intensity.

[0078] In the radio wave control element 10 of the present invention, when the liquid crystal composition layer 20 is at a temperature of less than 40° C., the peak of the XRD spectrum of the liquid crystal composition layer 20 measured in this manner preferably exists in a diffraction angle range of 15° or less. As described above, having a peak in a diffraction angle range of 15° or less means that a higher-order phase having a periodic structure, such as a crystalline phase or a smectic phase, is included. The peak preferably exists in a diffraction angle range of 1° to 15°, more preferably in a diffraction angle range of 1.5° to 15°, and even more preferably in a diffraction angle range of 1.8° to 15°.

[0079] Here, when a voltage is applied to the liquid crystal composition layer 20 to align the liquid crystal compound LC and then the layer is cooled to room temperature (phase transition temperature) to solidify, the amount and state of higher-order phases having a periodic structure, such as crystalline phases and smectic phases, present in the liquid crystal composition layer 20 at temperatures below 40° C. vary depending on the time it takes for the temperature to return to room temperature. Specifically, in order to adequately form the periodic structure of the higher-order phases, increase the degree of alignment of the liquid crystal composition layer 20, and increase the refractive index anisotropy (Δn), it is necessary to apply a voltage to the liquid crystal composition layer 20 to align the liquid crystal compound LC, and then cool the layer to room temperature relatively quickly.

[0080] As described above, in the radio wave control element 10 of the present invention, the temperature adjustment mechanism 30 heats and cools the liquid crystal composition layer 20 and has a cooling means in addition to a heating means. In the radio wave control element 10 of the present invention, the temperature adjustment mechanism 30 that adjusts the temperature of the liquid crystal composition layer 20 has a cooling means (cooling function), which enables rapid cooling to room temperature after applying a voltage to the liquid crystal composition layer 20 to align the liquid crystal compound LC. Therefore, the radio wave control element 10 of the present invention can stably increase the degree of orientation (= Δn) of the liquid crystal composition layer 20 when controlling radio waves. The radio wave control element 10 of the present invention can stably and suitably control radio waves with a frequency of 0.007 to 0.3 THz.

[0081] In the present invention, in order to achieve a higher degree of orientation, the half-width of the peak in a φ scan of the periodic structure corresponding to at least one peak with a diffraction angle of 15° or less in the liquid crystal composition layer 20 is preferably 30° or less, more preferably 3° to 23°, and even more preferably 3° to 20°. As is well known, the results of a φ scan indicate the direction and degree of distribution of the measured periodic structure. Therefore, the narrower the half-width of the peak, the more the measured periodic structure exists in the same direction, i.e., the higher the degree of orientation. The half-width of the peak can be determined by fitting the observed peak with a Gaussian function.

[0082] In addition, the liquid crystal composition layer 20 may have a peak half-width of 30° or less in a φ scan for a periodic structure corresponding to at least one peak with a diffraction angle of 15° or less, but it is preferable that all peak half-widths in a φ scan for periodic structures corresponding to peaks with an angle of 15° or less are 30° or less.

[0083] In the present invention, in the liquid crystal composition layer 20, it is preferable that Peak A is observed in a direction other than the range of ±5° of the direction perpendicular to the alignment axis of the liquid crystal composition. That is, it is preferable that Peak A is observed in the range of ±85° of the azimuth angle φ.

[0084] The degree of orientation of the liquid crystal composition layer 20 (sample) can be confirmed by the following method. With the liquid crystal composition layer 20 inserted on the light source side of an optical microscope (Nikon, ECLIPSE E600 POL), the sample is set on a sample stage, and the absorbance of the sample is measured using a multichannel spectrometer (Ocean Optics, QE65000), and the degree of orientation is calculated by the following formula: Orientation degree: S=[(Az0 / Ay0)-1] / [(Az0 / Ay0)+2] Az0: absorbance for polarized light in the absorption axis direction of the sample Ay0: absorbance for polarized light in the transmission axis direction of the sample

[0085] If the sample does not absorb visible light, the degree of orientation can be confirmed by a similar method using infrared spectroscopy. Regarding the peak of the wave number resulting from the vibration of the molecules in the direction of the orientation axis, the peak intensity for polarized light in the direction of the orientation axis is defined as Az0, and the peak intensity for polarized light in the direction perpendicular to that is defined as Ay0, and the degree of orientation can be calculated using the above formula.

[0086] In the radio wave control element 10 of the present invention, there are no limitations on the liquid crystal compound constituting the liquid crystal composition layer 20. Therefore, both low-molecular-weight liquid crystal compounds and high-molecular-weight liquid crystal compounds can be used. Here, "low-molecular-weight liquid crystal compound" refers to a liquid crystal compound that does not have a repeating unit in its chemical structure. Furthermore, "high-molecular-weight liquid crystal compound" refers to a liquid crystal compound that has a repeating unit in its chemical structure. Examples of low-molecular-weight liquid crystal compounds include those described in JP 2013-228706 A. Examples of high-molecular-weight liquid crystal compounds include those described in JP 2011-237513 A. The liquid crystal compound is preferably a thermotropic liquid crystal and may exhibit either a nematic or smectic phase, but preferably exhibits at least a nematic phase. As mentioned above, the temperature range in which the nematic phase is exhibited is 40°C or higher. To prevent polymerization reactions, the liquid crystal compound preferably does not contain a polymerizable group. To prevent a decrease in voltage holding ratio, the liquid crystal compound preferably does not contain ionic components. Furthermore, from the viewpoint of preventing a slowdown in response speed, it is preferable that the viscosity of the liquid crystal is low, and therefore low-molecular-weight liquid crystal compounds are more preferable than high-molecular-weight liquid crystal compounds.

[0087] The content of the liquid crystal compound in the liquid crystal composition layer 20 is preferably 30% by mass or more, and more preferably 50% by mass or more.

[0088] In the radio wave control element 10 of the present invention, the liquid crystal composition layer 20 preferably contains a dichroic material in addition to a liquid crystal compound. By containing a dichroic material in the liquid crystal composition layer 20, the Δn of the liquid crystal composition layer 20 can be more suitably increased when controlling radio waves.

[0089] In the present invention, the dichroic material is not particularly limited. A dichroic material is a material that exhibits dichroism, and dichroism refers to the property of having different absorbance depending on the polarization direction. That is, examples of dichroic materials include visible light absorbing materials (dichroic dyes), luminescent materials (fluorescent materials, phosphorescent materials), ultraviolet absorbing materials, infrared absorbing materials, nonlinear optical materials, carbon nanotubes, and inorganic materials (e.g., quantum rods), and conventionally known dichroic materials (dichroic dyes) can be used. Specifically, for example, paragraphs

[0067] to

[0071] of JP 2013-228706 A, paragraphs

[0008] to

[0026] of JP 2013-227532 A, paragraphs

[0008] to

[0015] of JP 2013-209367 A, paragraphs

[0045] to

[0058] of JP 2013-14883 A, paragraphs

[0045] to

[0058] of JP 2013-109090 A, paragraphs

[0012] to

[0029] of JP-A No. 2013-101328, paragraphs

[0009] to

[0017] of JP-A No. 2013-37353, paragraphs

[0051] to

[0065] of JP-A No. 2012-63387, paragraphs

[0049] to

[0073] of JP-A No. 2011-305036, paragraphs

[0016] to

[0018] of JP-A No. 2001- Paragraphs

[0009] to

[0011] of JP 133630 A,

[0030] to

[0169] of JP 2011-215337 A,

[0021] to

[0075] of JP 2010-106242 A,

[0011] to

[0025] of JP 2010-215846 A, and

[0017] to

[0069] of JP 2011-048311 A Examples of suitable dichroic materials include those described in paragraphs

[0013] to

[0133] of JP 2011-213610 A, paragraphs

[0074] to

[0246] of JP 2011-237513 A, paragraphs

[0005] to

[0041] of WO 2016 / 060173 A, and paragraphs

[0008] to

[0062] of WO 2016 / 136561 A. Furthermore, dichroic materials having liquid crystallinity are preferably used. Dichroic azo dye compounds are preferred as dichroic materials. A dichroic azo dye compound refers to an azo dye compound whose absorbance varies depending on the direction. The dichroic azo dye compound may or may not exhibit liquid crystallinity.When the dichroic azo dye compound exhibits liquid crystal properties, it may exhibit either nematic or smectic properties.

[0090] The content of the dichroic material in the liquid crystal composition layer 20 is not limited, but is preferably 30% by mass or more. By setting the content of the dichroic material in the liquid crystal composition layer 20 to 30% by mass or more, the Δn of the liquid crystal composition layer 20 when controlling radio waves can be more suitably increased. The content of the dichroic material in the liquid crystal composition layer 20 is more preferably 40% by mass or more, and even more preferably 50% by mass or more. When the dichroic material exhibits liquid crystallinity, it is not necessary to contain any liquid crystal compound other than the dichroic material, and since the Δn can be increased by the dichroic material, the content is more preferably 80% by mass or more.

[0091] 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.

[0092] For example, the radio wave control element 10 of the present invention shown in FIG. 1 sandwiches a liquid crystal composition layer 20 between supports 16 and 24, with a microstructure 14 (second electrode) and a first electrode layer 26 provided on the side of the support opposite the liquid crystal composition layer 20. However, the present invention is not limited to this. As an example, the radio wave control element of the present invention may be configured as described in B. Kang, et al., SID 2023 DIGEST (2023) p. 993, in which microstructures are two-dimensionally arranged on two supports, with the microstructures facing the liquid crystal composition layer and the liquid crystal composition layer sandwiched between the two supports. In this configuration, one of the supports may also be provided with a planar electrode layer instead of microstructures. Alternatively, rather than both microstructures (electrodes) facing the liquid crystal composition layer, one microstructure may face the liquid crystal composition layer and the other microstructure may be provided on the opposite side of the support from the liquid crystal composition layer.

[0093] 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.

[0094] [Preparation of Liquid Crystal Composition] <Liquid Crystal Compound> The following Compound 1 was prepared as a liquid crystal compound. Compound 1

[0095] <Dichroic Substance> The following compounds 2-1 and 2-2 were prepared as dichroic substances. Compound 2-1 Compound 2-2

[0096] The above-described Compound 1, Compound 2-1, and Compound 2-2, and Liquid Crystal Compound A (RDP-A3123, manufactured by DIC Corporation) were mixed to obtain the compositions shown in Table 1 below, to prepare liquid crystal compositions corresponding to Examples 1 to 5 and Comparative Example 1.

[0097] <Measurement of solid-liquid crystal phase transition temperature> The prepared liquid crystal composition was placed on a hot stage. The liquid crystal composition placed on the hot stage was observed under a polarizing microscope. The liquid crystal composition was heated to a temperature at which it became liquid crystal phase, and then the temperature at which the liquid crystal phase transitioned to crystal (referred to as the solid-liquid crystal phase transition temperature) was measured while the temperature was being lowered. As a result, the solid-liquid crystal phase transition temperatures of the liquid crystal compositions corresponding to Examples 1 to 5 were all 40°C or higher, while the solid-liquid crystal phase transition temperature of the liquid crystal composition corresponding to Comparative Example 1 (containing 100% by mass of Compound A) was less than 40°C. The results are also shown in Table 1.

[0098] [Preparation of Liquid Crystal Composition Layer] A polyimide resin (Hitachi Chemical Co., Ltd., LX-1400) was coated on a 3 cm square quartz glass substrate as an alignment layer to a thickness of approximately 30 nm, and then subjected to a rubbing treatment (rotation speed: 1000 rpm, movement speed: 20 mm / s, one round trip). A solution of a liquid crystal composition dissolved in chloroform solvent to a solids concentration of 1% by mass was cast onto this alignment layer, and a sample with a film thickness of 90 nm was obtained by spin coating. The liquid crystal composition was heated to or above the nematic transition temperature to form a nematic phase in which the liquid crystal composition was aligned, which was the same temperature process as that actually used in radio wave control devices. The liquid crystal composition was then cooled to room temperature to obtain aligned liquid crystal composition layers corresponding to Examples 1 to 5 and Comparative Example 1 (see Table 1).

[0099] <X-ray diffraction measurement> The prepared liquid crystal composition layer was subjected to X-ray diffraction (XRD) measurement by the method described above. In the XRD spectrum, a peak was observed at a diffraction angle of 15° or less, and a peak was not observed at a diffraction angle of 15° or less, and the result was evaluated as A, and the result was evaluated as B.

[0100] <Measurement of Degree of Orientation> The degree of orientation of the prepared liquid crystal composition layer was measured by the method described above. An orientation degree of more than 0.85 was rated A, and an orientation degree of 0.85 or less was rated B.

[0101] <Measurement of Refractive Index Anisotropy (Δn)> The refractive index anisotropy Δn of the prepared liquid crystal composition layer at 30 GHz radio waves was measured according to the method described in Applied Optics, Vol. 44, No. 7, p. 1150 (2005). The refractive index anisotropy Δn was measured by filling a variable short-circuit waveguide with the liquid crystal composition and aligning it. A 30 GHz radio wave was input into the waveguide, and the amplitude ratio of the reflected wave to the incident wave was measured. Measurements were performed by changing the direction of the static magnetic field and the length of the short-circuit tube, and the refractive indices n e and n o were determined. The refractive index anisotropy (Δn @ 30 GHz) was calculated from n e - n o . Δn of 0.40 or greater was rated A; Δn of less than 0.40 and 0.25 or greater was rated B; Δn of less than 0.25 and 0.20 or greater was rated C; and Δn of less than 0.20 was rated D. The results are shown in Table 1 below.

[0102] [Fabrication of Radio Wave Control Element] Using the prepared liquid crystal composition layer, radio wave control elements of Examples 1 to 5 and Comparative Example 1 were fabricated using the method described in B. Kang, et al., SID 2023 DIGEST (2023) p. 993. This radio wave control element has a metasurface structure in which circular microstructures are arranged two-dimensionally at equal intervals in the orthogonal X-Y directions. This radio wave control element is configured such that the microstructures are sandwiched between two supports, each with a microstructure formed on one side, facing the liquid crystal composition layer. Glass plates were used as the supports. Furthermore, an AC power source was connected to each of the microstructures constituting the electrode pair. In this radio wave control element, a temperature control mechanism was provided in contact with the entire surface of one of the supports. A Peltier element was used as the temperature control mechanism.

[0103] <Evaluation of Alignment Maintenance> The prepared radio wave control element was heated to a temperature equal to or higher than the liquid crystal phase transition temperature of the liquid crystal composition layer using a temperature control mechanism. Next, while maintaining the temperature, a bias voltage was applied equally to each microstructure to change the alignment state of the liquid crystal compound. The element was then cooled to room temperature (20°C) while the driving voltage was still applied, and the power supply from the power source was stopped to stop the application of voltage to the liquid crystal composition layer. During this process, the reflected radio waves from the radio wave control element were measured using a horn antenna connected to a network analyzer at each stage: before heating to a temperature equal to or higher than the liquid crystal phase transition temperature and applying the bias voltage, while applying the bias voltage, and after cooling and stopping the application of the bias voltage, using the method described in B. Kang, et al., SID 2023 DIGEST (2023) p. 994. From the measured reflected radio waves, those whose reflected radio wave characteristics after cooling and stopping the application of voltage maintained the reflected radio wave characteristics of the state when the voltage was applied, i.e., those whose orientation state of the liquid crystal compound was maintained, were classified as A, and those whose reflected radio wave characteristics of the state when the voltage was applied were not maintained, i.e., those whose orientation state of the liquid crystal compound was relaxed, were classified as B. The results are shown in Table 1 below.

[0104]

[0105] As shown in Table 1, with a radio wave control element of the present invention in which the liquid crystal composition layer has a liquid crystal phase transition temperature of 40°C or higher, the liquid crystal composition layer can be heated, a voltage applied to align the liquid crystal compound, and then the liquid crystal composition layer can be maintained in an oriented state even after cooling and the application of voltage is stopped. That is, with the radio wave control element of the present invention, after heating and applying a voltage to align the liquid crystal compound, the propagation direction of radio waves can be controlled to the desired direction after cooling without applying a voltage. Therefore, with the radio wave control element of the present invention, the power consumption required for controlling radio waves can be reduced.

[0106] <Evaluation of Frequency Tunability (Tunability)> The fabricated radio wave control elements were heated to a temperature equal to or higher than the liquid crystal phase transition temperature of the liquid crystal composition layer using a temperature control mechanism. Next, while maintaining the temperature, a bias voltage was applied equally to each microstructure to change the alignment state of the liquid crystal compound. The elements were then cooled to room temperature (20°C) while still applying the driving voltage, and the power supply from the power source was stopped to terminate the application of voltage to the liquid crystal composition layer. This operation was repeated with the bias voltage varied from 0 to 25 V. The frequency tunability of each radio wave control element was measured using a horn antenna connected to a network analyzer according to the method described in B. Kang, et al., SID 2023 DIGEST (2023) p. 994. The radio wave control element of Example 5 had the highest frequency tunability, followed by the radio wave control element of Example 4, Examples 2 and 3, and Example 1 had the lowest. While the difference between Examples 2 and 3 was slight, Example 3 was higher. From these results, it can be seen that the larger Δn is, the higher the frequency tunability of the radio wave control element, i.e., the wider the range of possible characteristic changes. The higher the frequency tunability, the wider the frequency range of radio waves that the radio wave control element can accommodate. Furthermore, the higher the frequency tunability, the wider the movable phase range, and therefore the wider the reflection angle range.

[0107] The present invention can be suitably used in active antennas, beam steering devices, etc. in radio wave communications.

[0108] REFERENCE SIGNS LIST 10 Radio wave control element 12 Metasurface structure 14 Microstructure 16, 24 Support 20 Liquid crystal composition layer 26 First electrode layer 28 Power supply 30 Temperature control mechanism LC Liquid crystal compound ANT Antenna AR1, AR2 Area BL Building RD Radio wave reflecting device RW Radio wave

Claims

1. A radio wave control element having a first electrode, a liquid crystal composition layer, and a second electrode in this order, wherein at least one of the first electrode and the second electrode has a metasurface structure formed by arranging a plurality of microstructures, and further has a temperature control mechanism for heating and cooling the liquid crystal composition layer, and the solid-liquid crystal phase transition temperature of the liquid crystal composition layer is 40°C or higher.

2. The radio wave control element according to claim 1, wherein the liquid crystal composition layer can have a peak in the diffraction angle range of 15° or less in an X-ray diffraction spectrum measured at a temperature below 40°C.

3. The radio wave control element according to claim 1 or 2, wherein the liquid crystal composition layer contains a dichroic material.

4. The radio wave control element according to claim 3, wherein the content of said dichroic material is 30% by mass or more based on the total mass of said liquid crystal composition layer.

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