JP2009192609A - Polarization control element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small polarization (polarized light) control element for generating large phase change such as 90° or 180°. <P>SOLUTION: First and second slits 14, 15 crossing each other are provided to a metal layer 2 on a substrate 1. The first and second slits 14, 15 are each formed at a period of a wavelength or less and a dielectric 3 is embedded in the second slit 15. As a result, the phase difference according to the refractive index of the dielectric 3 is generated between the polarization component passing through the first slit 14 and the polarization component passing through the second slit 15. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an element that controls the polarization of electromagnetic waves, and more particularly to an element that controls the polarization or polarization of microwaves, infrared light, and visible light.

  Elements for controlling the polarization are a polarizer that transmits or reflects only an electric field in a specific polarization direction, a phase shifter that changes an electric field in a specific polarization direction by a specific phase amount, and a specific polarization state that is randomly polarized It is classified as a depolarizing element that changes to a state.

  Among these polarization control elements, those using an organic or inorganic birefringent material, those using a phase shift in total reflection, and the like are conventionally known. Furthermore, recently, a phase modulation technique using a periodic structure smaller than the wavelength of light has been developed.

For example, in the technique disclosed in Japanese Patent Application Laid-Open No. 2006-330108 (Patent Document 1), a group of metal microstructures that are a set of metal microstructures are formed on a substrate. When such a substrate is irradiated with light, an interaction occurs between the plurality of metal microstructures due to the near-field light generated in each metal microstructure. At this time, when the group of metal microstructures is asymmetrically arranged with respect to the polarization of incident light, anisotropy occurs in the near-field light interaction between the metal microstructures. For this reason, a phase difference also occurs in the polarization component of the reflected light or transmitted light on which the light from each metal microstructure is superimposed, and the polarization state in the emitted light is converted.
JP 2006-330108 A

  A phaser using an organic birefringent material utilizes the anisotropy of the refractive index accompanying the anisotropy of the molecular structure unique to the polymer material. Although organic materials have the merit that they can be used at low cost, there is a problem that heat resistance and light resistance are inferior. Inorganic birefringent materials include those using crystals such as quartz and calcite. Although both have heat resistance and light resistance, there is a problem that the size of the element becomes large because the birefringence is small.

  On the other hand, a phase shifter using total reflection is called a Fresnel prism, and s-polarized light (polarized light whose electric field is in a plane parallel to the total reflection surface) and p-polarized light (polarized light perpendicular to s-polarized light). The fact that the phase shift amount is different is used. This type of phaser is advantageous in that a phase amount with small chromatic dispersion can be obtained, but it also has the disadvantage that the size of the element becomes large.

  Further, in the technique disclosed in the above-mentioned Japanese Patent Laid-Open No. 2006-330108 (Patent Document 1), the phase shift amount is small, such as 90 ° (¼ wavelength plate) or 180 ° (1/2 wavelength plate). It is difficult to obtain a phase shift amount for use in an actual device.

  Accordingly, an object of the present invention is to provide a small polarization control device that can cause a large phase change such as 90 ° or 180 °.

  In summary, the present invention is a polarization control element that controls the polarization of electromagnetic waves, and includes a substrate that transmits electromagnetic waves and a reflective layer that is provided on the substrate and reflects electromagnetic waves. Here, a plurality of first and second slits are formed in the reflective layer.

  The plurality of first slits are formed along the first direction. The distance between the center lines of each of the plurality of first slits and the adjacent first slit is smaller than the value obtained by dividing the wavelength of the electromagnetic wave by the refractive index of the substrate.

  The plurality of second slits are formed so as to extend along the second direction and to intersect the plurality of first slits. The distance between the center lines of each of the plurality of second slits and the adjacent second slit is smaller than the value obtained by dividing the wavelength of the electromagnetic wave by the refractive index of the substrate. In each of the plurality of second slits, a first dielectric is embedded in at least a partial region.

  According to the present invention, a polarization component whose electric field vibrates in a direction perpendicular to the first direction passes through the plurality of first slits. A polarization component whose electric field vibrates in a direction perpendicular to the second direction passes through the plurality of second slits. Here, since the first dielectric is embedded in the second slit, a phase difference corresponding to the refractive index of the dielectric can be generated between the polarization components in both directions. As a result, a small polarization control element having a large phase change can be realized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or corresponding parts are denoted by the same reference numerals, and the description thereof will not be repeated. Further, in order to facilitate the illustration, the same hatching is given to the same or corresponding parts in the figure.

  The polarization control element of the present invention is used for electromagnetic waves such as microwaves, infrared light, and visible light. In the following description of each embodiment, the terms electromagnetic wave and polarization are used. However, when the wavelength of the target electromagnetic wave is a light region, these terms can be read as light and polarization.

[Embodiment 1]
FIG. 1 is a perspective view schematically showing the configuration of the polarization control element 10 according to the first embodiment of the present invention.

  2 is a front view of the polarization control element 10 of FIG. 1, and FIG. 3 is a top view of the polarization control element 10 of FIG. 1 to 3, directions parallel to the main surface 1A of the substrate 1 are defined as an x direction and ay direction, and a direction perpendicular to the main surface 1A of the substrate 1 is defined as a z direction. In the following drawings, the same hatching is given to the same or corresponding parts for easy illustration.

  1 to 3, a polarization control element 10 includes a substrate 1 such as quartz glass and a metal layer 2 formed on a main surface 1 </ b> A of the substrate 1. The substrate 1 is preferably transparent to electromagnetic waves incident on the polarization control element 10. The metal layer 2 is used as a reflective layer that reflects incident electromagnetic waves.

  A plurality of first slits 14 along the x direction and a plurality of second slits 15 along the y direction are formed in the metal layer 2. The plurality of first slits 14 and the plurality of second slits 15 intersect with each other and are formed in a lattice shape. The metal layer 2 is divided by the first slit 14 and the second slit 15 into a plurality of metal blocks 2 (a reflection portion of the present invention) arranged in a matrix in the x and y directions. Hereinafter, the x direction is also referred to as the row direction of the metal block 2, and the y direction is also referred to as the column direction of the metal block 2.

  In FIG. 1, for the sake of simplicity, six first slits 14 and three second slits 15 are illustrated. A large number of first and second slits 14 and 15 are formed in the metal layer 2 of the actual polarization control element 10.

  In the polarization control element 10 of the first embodiment, the first slit 14 and the second slit 15 are orthogonal to each other. The first and second slits 14 and 15 are formed with a constant width and a constant period, respectively. The period of the first and second slits 14 and 15 is set to be smaller than a value obtained by dividing the wavelength of the electromagnetic wave incident on the polarization control element 10 by the refractive index of the substrate.

  The metal block 2 has a substantially rectangular parallelepiped shape in which the size in the x direction is Dx, the size in the y direction is Dy, and the thickness in the z direction is Lz. Here, if the width of the first slit 14 is Wy and the width of the second slit 15 is Wx, the distance between the center lines of the adjacent first slits 14 (period of the first slits 14). Becomes Ly = Dy + Wy. Similarly, the distance between the center lines of the second slits 15 adjacent to each other (the period of the second slit 15) is Lx = Dx + Wx.

  In each of the second slits 15, the first dielectric 3 is embedded in the entire region including the intersections 17 with the plurality of first slits 14. As will be described below, the dielectric 3 can cause a phase difference in the polarization component of the electromagnetic wave that passes through the first and second slits 14 and 15, respectively.

  FIG. 4 is a diagram for explaining the operation of the polarization control element 10 of the first embodiment. Hereinafter, the operation of the polarization control element 10 will be described in detail with reference to FIG.

  Polarized light (polarized light) indicates that the vibration direction of the electric field of electromagnetic waves (or a magnetic field orthogonal thereto) is spatially biased. There are two independent directions in which the electric field vibrates, which are perpendicular to the traveling direction of the electromagnetic wave and orthogonal to each other, and the polarization state of the electromagnetic wave can be expressed as a superposition of the electric fields in the two directions. Therefore, an arbitrary polarization state is determined by the ratio of the amplitude in these two independent directions and the phase difference. For example, the linearly polarized wave represents a state where the phase difference between vibrations in two directions is zero. Further, the circularly polarized wave shows a state where the amplitudes in the two directions are equal and the phase difference is 90 °.

  FIG. 4 shows a case where electromagnetic waves are incident in a direction 11 perpendicular to the substrate (a direction from the main surface 1A side to the back surface side of the substrate 1 on which the metal layer 2 is formed). In the case of FIG. 4, the electric field 12 of the incident electromagnetic wave is a linearly polarized wave that vibrates perpendicularly to the traveling direction k of the electromagnetic wave (k indicates the direction of the wave vector). At this time, the electric field 12 of the electromagnetic wave is represented by the sum of the polarization component Ex whose electric field vibrates in the x direction and the polarization component Ey whose electric field vibrates in the y direction. In the case of linear polarization, the phase difference between the polarization component Ex in the x direction and the polarization component Ey in the y direction is zero.

  The polarization component of the incident electromagnetic wave is controlled by the polarization control element 10. In the case of FIG. 4, the electric field 13 of the electromagnetic wave transmitted through the polarization control element 10 is a right-handed polarized wave that turns clockwise in the traveling direction of the electromagnetic wave. Such characteristics of the polarization control element 10 are based on the propagation characteristics of electromagnetic waves in the slits 14 and 15 formed in the metal layer 2. First, the propagation characteristics of electromagnetic waves of a polarizer having slits formed in one direction will be described with reference to FIG.

  FIG. 5 is a perspective view schematically showing a configuration of a polarizer 50 as a comparative example of the polarization control element 10. The polarizer 50 of FIG. 5 is called a wire grid polarizer.

  Referring to FIG. 5, a polarizer 50 includes a translucent substrate 1 and a metal layer 2 </ b> A formed on the main surface of the substrate 1. The metal layer 2A is divided into a plurality of metal blocks 2A by forming slits 15 along the y direction.

  The characteristics of the polarizer 50 differ depending on the magnitude relationship between the wavelength λ of the incident electromagnetic wave and the period Lx of the slit 15. In the substrate 1, since the wavelength of the electromagnetic wave is shortened by the refractive index, it is necessary to accurately compare the value obtained by dividing the wavelength λ of the electromagnetic wave by the refractive index of the substrate with the period Lx of the slit. In the following description, the refractive index of the substrate 1 is 1 for simplicity.

  When the slit period Lx is larger than the wavelength λ of the electromagnetic wave, the polarizer 50 functions as a diffraction grating. When the slit period Lx is shorter than the wavelength λ of the electromagnetic wave, there is no θ that satisfies λ = Lx × sin θ, and thus the polarizer 50 has no spectral action. When the slit period Lx is shorter than the wavelength λ, the polarization action appears remarkably.

  When electromagnetic waves are incident on the slit, the polarized wave having the electric field Ey in the direction parallel to the slit 15 cannot propagate in the slit 15 unless the wavelength is shorter than twice the slit width Wx. Therefore, almost all the polarized light having the electric field Ey in the direction parallel to the slit 15 is reflected as the wavelength becomes longer than the slit width Wx. On the other hand, even if the wavelength of the polarized wave having the electric field Ex in the direction perpendicular to the slit 15 is longer than the width Wx of the slit 15, propagation in the slit is not limited. Here, even if the polarization can propagate through the slit, if the slit is single, most of it will be reflected by the surface of the metal layer, but many slits should be arranged periodically. Thus, the electromagnetic wave can be efficiently guided into the slit. In this way, as shown in FIG. 5, the electromagnetic wave travel direction k (k is the direction of the wave vector. The travel direction k is the direction 11 perpendicular to the substrate 1). The polarization component 16 in the vertical direction passes through the slit.

  The characteristics of the polarizer 50 can also be explained by the response of free electrons in the metal layer 2A. Free electrons in the conductor vibrate in synchronization with receiving an electromagnetic wave. Therefore, the linearly polarized wave having an electric field in the direction parallel to the slit 15 is almost reflected because it is shielded by the vibration of free electrons in each metal block 2A. On the other hand, in the case of linearly polarized waves having an electric field in a direction perpendicular to the slit 15, the movement of free electrons is blocked by the slit 15. Therefore, most of the linearly polarized wave having an electric field in the direction perpendicular to the slit 15 passes through the slit 15. Such characteristics hold even when slits are arranged in a lattice pattern in both the x and y directions.

  Referring to FIG. 4 again, the polarization control element 10 is provided with slits 14 and 15 parallel to the x and y directions orthogonal to each other. The periods Ly and Lx of the slits 14 and 15 are shorter than the wavelength λ of the electromagnetic wave in both the x and y directions. Here, consider a case where linearly polarized waves whose electric field vibrates in a direction perpendicular to the traveling direction is incident on the substrate 1 perpendicularly. In the following description, it is assumed that the electromagnetic wave is incident perpendicular to the two-dimensional plane (xy plane) on which the metal layer 2 is disposed, but the treatment is not changed even if the electromagnetic wave is inclined and incident on the two-dimensional plane.

  Due to the propagation characteristics of the slit described above, the polarization component parallel to the x direction propagates through the second slit 15 parallel to the y direction, and the polarization component parallel to the y direction corresponds to the first polarization component parallel to the x direction. Propagates through the slit 14. Furthermore, the dielectric 3 is embedded only in the second slit 15 parallel to the y direction. Therefore, when the refractive index of the dielectric is n, the wavelength of the electromagnetic wave is λ, and the thickness of the metal block 2 (equal to the propagation distance of the electromagnetic wave in the slit) is Lz, the second dielectric 3 embedded The amount of phase shift φx accompanying propagation in the slit 15 is

It becomes. Here, π is the circumference ratio. On the other hand, the phase shift amount φy accompanying the propagation in the first slit 14 where the dielectric is not embedded is

It becomes. Therefore, the phase difference between the polarized wave component parallel to the x direction and the polarized wave component parallel to the y direction generated on the back surface of the plurality of metal blocks 2 due to propagation in the first and second slits 14 and 15. Δφ is

It becomes. From this result, for example, as shown in FIG. 4, when electromagnetic waves of linearly polarized waves 12 are incident, the thickness Lz of the metal block 2 and the dielectric 3 so that Δφ = 0.5φ in equation (3). If an element having a refractive index n is produced, the emitted electromagnetic wave becomes a circularly polarized wave 13.

  The above-described result is established when the slit widths Wx and Wy are sufficiently smaller than the slit periods Lx and Ly. In order to derive a more accurate relational expression of the phase difference Δφ, it is necessary to consider the intersecting portion 17 between the first slit 14 and the second slit 15.

  According to the study by the present inventors, the polarization component parallel to the y direction propagates to the intersecting portion 17 of the first and second slits with a width Wy equal to the width Wy of the first slit 14. Is elucidated. Therefore, the average effective refractive index of the first slit 14 is that of the refractive index 1 of the air of the length Dx sandwiched between the metal blocks 2 and the dielectric 3 of the length Wx corresponding to the intersecting portion 17. A value obtained by averaging the refractive index n. Therefore, the above equations (2) and (3) are respectively corrected as the following equations (4) and (5).

  As described above, according to the polarization control element 10 of the first embodiment, an incident electromagnetic wave having an arbitrary polarization has a polarization component that passes only through a gap in which a dielectric is embedded, and air that is orthogonal to the polarization component. It is divided into polarization components that pass only through the air gap. Since the effective refractive index is different for each polarization component, a large phase difference occurs between both polarization components after passing through the polarization control element 10. Furthermore, the refractive index anisotropy can be designed by changing the type and thickness of the dielectric. The polarization control element 10 can be used, for example, as a near-infrared or visible light polarization control plate (1/2 wavelength plate, 1/4 wavelength plate).

  In the above description, even if the thickness of the metal block 2 and the thickness of the dielectric 3 are different, the polarization control element 10 functions as a phaser. When the thickness of the dielectric 3 is larger than the thickness of the metal block 2, the electromagnetic wave spreads outside the slit, so that the portion of the dielectric 3 protruding from the slit hardly affects the function of the phaser. Therefore, in this case, Lz in the above formulas (3) and (5) may be the thickness of the metal block 2. Conversely, when the thickness of the dielectric 3 embedded in the slit is smaller than the thickness of the metal block 2, Lz in the above formulas (3) and (5) may be the thickness of the dielectric 3.

  Further, even if the periods of the first and second slits 14 and 15 are not constant, a polarization (polarization) action occurs. However, when the slit period is constant, the transmittance of the electromagnetic wave is increased, and the transmittance of the electromagnetic wave is also spatially uniform, so that preferable phaser characteristics can be obtained.

  Further, the directions of the first and second slits 14 and 15 function as a phase shifter even if they are not necessarily orthogonal. However, it is considered that the conversion efficiency of the phase of the electromagnetic wave is better when the directions of the first and second slits 14 and 15 are orthogonal to each other.

  In addition, the lower limit of the thickness Lz of the metal block 2 needs to be a thickness that does not transmit electromagnetic waves. The upper limit of the thickness Lz of the metal block 2 is determined by the waveguide loss of the slit. Waveguide loss includes scattering due to structural fluctuations depending on the slit fabrication system, and electromagnetic wave absorption by metals depending on the wavelength of electromagnetic waves.

Next, a specific numerical calculation example of the polarization control element 10 will be described.
FIG. 6 is a graph showing a numerical calculation result of the phase shift by the polarization control element 10. The vertical axis in FIG. 6 represents the phase shift difference Δφ between the polarization component parallel to the x direction and the polarization component parallel to the y direction. The horizontal axis of FIG. 6 represents the refractive index n of the dielectric 3.

  As described above, the polarization state can be controlled by changing the refractive index n of the dielectric 3 or the thickness Lz of the metal block 2. FIG. 6 shows the phase shift Δφ when the refractive index n of the dielectric 3 is changed. This data (black dots in FIG. 6) is calculated by the finite difference time domain method when 500 mm in the microwave domain is used as the wavelength of the electromagnetic wave. Similar results were confirmed when electromagnetic waves having a wavelength of 500 nm in the optical region were used.

  The parameters used for the numerical calculation are as follows. In the case of the optical region, the dimensions of the metal block are Dx = 230 nm, Dy = 110 nm, Lz = 300 nm, and the widths of the first and second slits are both 20 nm. In the microwave region, the metal block dimensions are Dx = 230 mm, Dy = 110 mm, Lz = 300 mm, and the widths of the first and second slits are both 20 mm. In both the microwave region and the light region, the electromagnetic wave is incident on the substrate 1 perpendicularly. Moreover, the thickness of the dielectric 3 and the thickness of the metal block 2 are equal.

  The complex dielectric function of the metal block is given according to the Drude model. According to the Drude model, the relative dielectric constant ε with respect to the angular frequency ω of the electromagnetic wave is expressed by the following equation, where the plasma frequency is ωp, the electron relaxation time is 1 / Γ, and the imaginary unit is i.

In the numerical calculation, in the above equation, Γ = 1.94e + 13 rad / s and ωp = 3.57e + 15 rad / s (where e represents a power of 10). In this case, the characteristics of the metal block can be considered as a metal having a finite conductivity in the optical region and a perfect conductor in the microwave region.

  As shown in FIG. 6, the dependency of the phase difference Δφ on the refractive index n of the dielectric 3 is in agreement with the equation (3) shown by the solid line in FIG. Here, in Equation (3), the thickness Lz of the metal block 2 is set to 0.6 times the wavelength λ in accordance with the numerical calculation parameters.

  FIG. 7 is a diagram for explaining an example of the manufacturing process of the polarization control element 10. Hereinafter, the manufacturing process of the polarization control element 10 will be described with reference to FIGS.

  First, as shown in FIGS. 7A and 7B, a dielectric 3 is formed on the entire surface of the main surface 1A of the substrate 1. Thereafter, a photoresist 4A is applied to the surface of the dielectric 3 and patterned into a line having a width Wx by electron beam lithography or the like. FIG. (B) is a top view of FIG. (A).

  The substrate 1 is preferably transparent and non-birefringent with respect to the wavelength of the electromagnetic wave used. For example, in the microwave, a substrate containing plastics or ceramics, silicon in the infrared region, and quartz glass in the visible light region can be used. As with the substrate 1, the dielectric 3 is preferably transparent to the wavelength used. For example, ceramics that can be sintered at low temperatures in the microwave, sol-gel glass in the infrared or visible region, polymer materials in which nanoparticles are dispersed, and the like can be used as transparent dielectrics with adjustable refractive index.

  Next, the dielectric 3 is etched using the patterned photoresist 4A as a mask. As an etching method, for example, anisotropic etching such as reactive plasma etching is used. By removing the photoresist 4A after the etching, a line-shaped dielectric 3 is formed on the substrate 1 as shown in FIG.

  Next, the metal block 2 is formed by a lift-off method. Therefore, as shown in FIG. 7D, a resist pattern 4B is formed at a position (position opposite to the metal block 2) where the lattice-like slits are formed by electron beam lithography or the like. The vertical resist pattern in the figure is formed on the line-shaped dielectric 3 formed in FIG.

  Next, a metal material used as the metal block 2 is formed on the entire main surface side of the substrate 1 by vacuum deposition or the like. The metal material may be a material having a high electrical conductivity, such as gold, silver, copper, aluminum, nickel, chromium, platinum, and an alloy made of these materials. If the wavelength of the electromagnetic wave is in the microwave region, a semiconductor with a high carrier density can be used.

  Next, the unnecessary metal layer deposited on the resist 4B is removed together with the resist, whereby the polarization control element 10 shown in FIG. 7E is formed.

[Modification of Embodiment 1]
FIG. 8 is a front view schematically showing a configuration of a polarization control element 10A of a modification of the first embodiment. The polarization control element 10A of FIG. 8 is shown in FIGS. 1 to 3 in that the dielectric 3A is embedded in a region of the second slit 15 excluding the intersection 17 with the first slit 14. Different from the polarization control element 10. Since other points are the same as those of the polarization control element 10 described above, common portions are denoted by the same reference numerals and description thereof will not be repeated.

  Referring to FIG. 8, when the slit widths Wx and Wy are sufficiently smaller than the slit periods Lx and Ly, the characteristics of the polarization control element 10A are the same as the characteristics of the polarization control element 10 described above. is there. That is, when linearly polarized light is incident on the substrate 1 perpendicularly, the phase difference Δφ between the polarized wave component parallel to the x direction and the polarized wave component parallel to the y direction of the transmitted electromagnetic wave is given by equation (3). It is done.

  When the intersection 17 between the first slit 14 and the second slit 15 is taken into consideration, the above equations (1) and (3) are respectively corrected as the following equations (7) and (8). .

  In the derivation of the above equation, it is considered that an electromagnetic wave having a polarization component parallel to the x direction propagates to the intersecting portion 17 of the first and second slits with a width Wx equal to the width Wx of the second slit 15. Then, the average effective refractive index of the second slit 15 is expressed by the refractive index n of the dielectric 3 having a length Dy sandwiched between the metal blocks 2 and the refractive index of air having a length Wy corresponding to the intersecting portion 17. It is given as an average with 1.

  FIG. 9 is a front view schematically showing a configuration of a polarization control element 10B of another modification of the first embodiment. The polarization control element 10B of FIG. 9 is shown in FIGS. 1 to 3 in that the dielectric 3B is embedded in a region of the first slit 14 excluding the intersection 17 with the second slit 15. Different from the polarization control element 10. Since other points are the same as those of the polarization control element 10 described above, common portions are denoted by the same reference numerals and description thereof will not be repeated.

  Referring to FIG. 9, the refractive index of dielectric 3 embedded in second slit 15 is n1, and the refractive index of dielectric 3B embedded in first slit 14 is n2. The refractive index n1 is assumed to be larger than the refractive index n2. In this case, when linearly polarized light enters the substrate 1 perpendicularly, the phase difference Δφ between the polarized wave component parallel to the x direction and the polarized wave component parallel to the y direction of the transmitted electromagnetic wave is given by the following equation: . However, it is assumed that the slit widths Wx and Wy are sufficiently smaller than the slit periods Lx and Ly.

  As described above, various variations are conceivable for the manner in which the dielectric is embedded in the slit. For example, even when the dielectric portion and the air portion are repeated with a period shorter than the wavelength of the electromagnetic wave, the effective refractive index of the second slit 15 is different from that of the first slit 14. Therefore, even when the dielectric is partially embedded in the second slit 15 as described above, the polarization control element 10 functions as a phaser.

[Embodiment 2]
In the second embodiment, a material whose refractive index changes with an electric field is used as the dielectric embedded in the slit. Each metal block 2 is used as an electrode, and by applying a voltage between the metal blocks sandwiching the dielectric, the polarization state of the electromagnetic wave passing through the polarization control element can be changed according to the applied voltage.

  FIG. 10 is a top view schematically showing the configuration of the polarization control element 10C of the second embodiment of the present invention.

  FIG. 11 is a perspective view schematically showing the configuration of the polarization control element 10C. FIG. 12 is a perspective view showing portions of the substrate 1 and the transparent electrode 5 in the polarization control element 10C of FIG. 10 to 12, the directions parallel to the main surface 1A of the substrate 1 are the x direction and the y direction, and the direction perpendicular to the main surface 1A of the substrate 1 is the z direction. 10 and 11 show the metal blocks 2 in 4 rows and 6 columns for the sake of simplicity, but in the actual polarization control element 10C, a large number of metal blocks 2 are arranged in a matrix.

  Referring to FIGS. 10 to 12, polarization control element 10 </ b> C further includes a plurality of transparent electrodes 5 having the same width Dx as metal block 2 and extending in the y direction. Different from the element. Each transparent electrode 5 is provided corresponding to the columns 2.1 to 2.4 of the metal blocks 2 arranged in the y direction, and is arranged between the metal block 2 and the substrate 1 for each column 2.1 to 2.4. It is formed. Therefore, since each transparent electrode 5 is connected only to the metal block 2 in the corresponding column, a common voltage can be applied to the plurality of metal blocks 2 arranged in the column direction.

  The transparent electrode 5 is desirably transparent to the target electromagnetic wave. As the transparent electrode 5, for example, ITO (Indium Tin Oxide) can be used in the visible light region. In the microwave and infrared regions, a material doped with an impurity having an appropriate concentration can be used. For example, a material obtained by doping boron into a semiconductor crystal (Ge: Ga) in which germanium is doped with a small amount of gallium by ion implantation can be used.

  The polarization control element 10C is further different from the polarization control element 10 of the first embodiment in that a material whose refractive index is changed by an electric field is used as the dielectric 3C embedded in the second slit 15. The dielectric 3 </ b> C is embedded between the adjacent metal blocks 2 and the adjacent transparent electrodes 5 including the boundary portion 17 of the first and second slits 14 and 15 until reaching the surface of the substrate 1. . As the dielectric 3C, for example, an organic material such as liquid crystal or an inorganic nonlinear optical crystal such as lithium niobate (LiNbO3) can be used.

  Since the other points of polarization control element 10 are the same as those of polarization control element 10 of the first embodiment, common portions are denoted by the same reference numerals and description thereof will not be repeated.

  By providing the transparent electrode 5, if a voltage is alternately applied to each column 2.1 to 2.4 of the metal block 2, an electric field is generated in the second slit 15 provided between adjacent columns. Uniformly formed. Specifically, a DC power supply 6 is provided, the positive electrode of the DC power supply 6 and the odd-numbered columns 2.1 and 2.3 are connected, and the negative electrode of the DC power supply 6 and the even-numbered columns 2.2, 2.. 4 is connected. Thereby, an electric field can be applied to the dielectric 3C without impairing the polarization control characteristics of the two-dimensional metal block 2. Since the refractive index of the dielectric 3C is changed by applying an electric field, the polarization state can be controlled.

  Note that the electromagnetic wave that has passed through the first slit 14 spreads when passing through the transparent electrode 5, so that the refractive index of the transparent electrode 5 hardly affects the polarization control characteristics.

  Further, the width of the transparent electrode 5 is not necessarily the same width Dx as the metal block. Since the metal blocks 2 in each of the columns 2.1 to 2.4 may be electrically connected, the width may be smaller than the width Dx of the metal block.

  FIG. 13 is a diagram for explaining an example of the manufacturing process of the polarization control element 10C. Hereinafter, the manufacturing process of the polarization control element 10C will be described with reference to FIGS.

  First, as shown in FIG. 13A, a material for the transparent electrode 5 such as ITO is formed on the entire main surface 1A of the substrate 1. Further, a photoresist 4C patterned into the shape of the transparent electrode 5 having a width Dx and extending in the y direction is formed on the formed ITO layer by electron beam lithography or the like.

  Next, the ITO layer is etched by reactive plasma etching or the like using the photoresist 4C as a mask. By removing the photoresist 4C after the etching, a plurality of transparent electrodes 5 arranged in the column direction are formed on the substrate 1 as shown in FIG. 13B.

  Next, as shown in FIG. 13C, a dielectric film 3C is formed on the entire surface on the main surface 1A side of the substrate 1 on which the transparent electrode 5 is formed. A photoresist 4A patterned in a line shape is further formed on the deposited dielectric film 3C by electron beam lithography or the like. Since the photoresist 4A has a reverse pattern of the photoresist 4C, it is not formed in the upper layer of the transparent electrode 5.

  Thereafter, the dielectric 3 is etched by reactive plasma etching or the like using the patterned photoresist 4A as a mask. By removing the photoresist 4A after the etching, a shape in which the line-shaped dielectrics 3C and the transparent electrodes 5 are alternately arranged on the substrate 1 is formed as shown in FIG.

  Next, the metal block 2 is formed by a lift-off method. For this reason, as shown in FIG. 13E, a resist pattern 4B is formed at a position (position opposite to the metal block 2) where a lattice-like slit is formed by electron beam lithography or the like. Next, a metal material used as the metal block 2 is formed on the entire surface on the main surface side of the substrate 1 by vacuum deposition or the like from above the resist pattern 4B. Thereafter, an unnecessary metal layer deposited on the resist 4B is removed together with the resist, thereby forming the polarization control element 10C as shown in FIG.

  The embodiment disclosed this time must be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a perspective view which shows typically the structure of the polarization control element 10 of Embodiment 1 of this invention. It is a front view of the polarization control element 10 of FIG. It is a top view of the polarization control element 10 of FIG. FIG. 6 is a diagram for explaining the operation of the polarization control element 10 of the first embodiment. 3 is a perspective view schematically showing a configuration of a polarizer 50 as a comparative example of the polarization control element 10. FIG. 4 is a graph showing a numerical calculation result of phase shift by the polarization control element 10. 5 is a diagram for explaining an example of a manufacturing process of the polarization control element 10. FIG. FIG. 7 is a front view schematically showing a configuration of a polarization control element 10A of a modification of the first embodiment. FIG. 10 is a front view schematically showing a configuration of a polarization control element 10B of another modification of the first embodiment. It is a top view which shows typically the structure of 10 C of polarization control elements of Embodiment 2 of this invention. It is a perspective view which shows typically the structure of 10 C of polarization control elements. It is a perspective view which shows the part of the board | substrate 1 and the transparent electrode 5 among the polarization control elements 10C of FIG. It is a figure for demonstrating an example of the manufacturing process of 10 C of polarization control elements.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Substrate, 1A main surface, 2 Metal layer (metal block), 3, 3A, 3B, 3C Dielectric, 5 Transparent electrode, 10, 10A-10C Polarization control element, 14 1st slit, 15 2nd slit , 17 Intersections of slits, 22, 23 connecting portions, Lx, Ly slit periods, Wx, Wy slit widths.

Claims (10)

A polarization control element for controlling the polarization of electromagnetic waves,
A substrate that transmits the electromagnetic wave;
A reflective layer provided on the substrate and reflecting the electromagnetic wave;
A plurality of first slits are formed in the reflective layer along a first direction,
The distance between the center lines of each of the plurality of first slits and the adjacent first slit is smaller than a value obtained by dividing the wavelength of the electromagnetic wave by the refractive index of the substrate,
The reflective layer is further formed with a plurality of second slits along the second direction and each intersecting the plurality of first slits,
The distance between the center lines of each of the plurality of second slits and the adjacent second slit is smaller than a value obtained by dividing the wavelength of the electromagnetic wave by the refractive index of the substrate,
Each of the plurality of second slits is a polarization control element in which a first dielectric is embedded in at least a partial region.   The polarization control element according to claim 1, wherein the electromagnetic wave is any one of microwave, infrared light, and visible light. The width of each of the plurality of first slits and the interval between the center lines of the first slits adjacent to each other are constant,
The polarization control element according to claim 1 or 2, wherein a width of each of the plurality of second slits and a distance between center lines of the second slits adjacent to each other are constant.   4. The polarization control element according to claim 3, wherein each of the second slits is embedded with the first dielectric material in an entire region including an intersection with the plurality of first slits. The first direction and the second direction are orthogonal to each other,
As the electromagnetic wave, two linearly polarized waves having the same phase of wavelength λ in which electric fields vibrate in the first and second directions respectively pass through the polarization control element when perpendicularly incident on the substrate. In order to obtain a desired phase difference Δφ between two linearly polarized waves, the phase difference Δφ, the refractive index n of the first dielectric, the thickness Lz of the reflective layer and the first dielectric, The width Wx of each of the plurality of second slits and the distance Lx between the center lines of the adjacent second slits are expressed by the following equation:

The polarization control element according to claim 4, wherein the relationship is satisfied.   4. The polarization control element according to claim 3, wherein each of the second slits is embedded with the first dielectric material in an entire region except for an intersection with the plurality of first slits. The first direction and the second direction are orthogonal to each other,
As the electromagnetic wave, two linearly polarized waves having the same phase of wavelength λ in which electric fields vibrate in the first and second directions respectively pass through the polarization control element when perpendicularly incident on the substrate. In order to obtain a desired phase difference Δφ between two linearly polarized waves, the phase difference Δφ, the refractive index n of the first dielectric, the thickness Lz of the reflective layer and the first dielectric, The width Wy of each of the plurality of first slits and the interval Ly between the center lines of the adjacent first slits are expressed by the following equation:

The polarization control element according to claim 6, satisfying the relationship:   In each of the first slits, a second dielectric having a refractive index different from that of the first dielectric is embedded in at least a part of a region excluding an intersection with the plurality of second slits. The polarization control element according to any one of claims 1 to 3. The refractive index of the first dielectric changes according to the electric field,
The reflective layer is separated into a plurality of reflective portions arranged in a matrix by the plurality of first and second slits,
Each of the plurality of reflecting portions is an electrode for applying an electric field to the first dielectric provided between the reflecting portions adjacent to each other in the first direction. 2. The polarization control element according to item 1. The polarization control element further includes a plurality of transparent electrodes provided corresponding to each row of the plurality of reflecting portions arranged along the second direction,
Each of the plurality of transparent electrodes is connected only to a plurality of reflective portions in a corresponding row,
The polarization control element according to claim 9, wherein each of the plurality of transparent electrodes transmits the electromagnetic wave.

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