Abstract:
A textile made by weaving while crossing stainless wires as warp yarn and silk yarns as weft yarn one by one alternately, and this textile is attached to a frame body to configure a wire grid. The pitch for the stainless wires is determined depending on a wavelength to be polarized and analyzed. Furthermore, the silk yarns are removed if necessary. This configuration avoids problems with cutting of metal wire or irregular intervals between metal wires because of long fine metal wires tightened parallel to each other in the frame body, and problems such as multiple reflection or interference on a substrate or a base material because of fine wire patterns made by etching or the like with the use of the substrate or base material, thereby obtaining an easily-manufactured, low-cost and high-precision wire grid for polarization and analysis of electromagnetic waves.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International Application No. PCT/JP2008/062821, filed Jul. 16, 2008, and claims priority to Japanese Patent Application No. JP2007-217567, filed Aug. 23, 2007, the entire contents of each of these applications being incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a wire grid which is, for example, a terahertz band or millimeter-wave band polarization component, and a method for manufacturing the wire grid. 
     BACKGROUND ART 
     Wire grids are polarization components commonly used in the field of optics or electromagnetic waves, and disclosed in, for example, Non-Patent Document 1. Furthermore, Patent Document 1 discloses the use of a wire grid for a terahertz band ellipsometer. 
       FIG. 1  is a diagram illustrating the structure of a wire grid. The terahertz band wire grid has, as shown in  FIG. 1 , thin and long single metal wires  1  arranged at regular intervals with respect to a frame body  2 . The metal wires  1  are, for example, tungsten wires of 5 μm to 50 μm in diameter, which are one by one attached to a metallic frame body with an adhesive at a pitch on the order of 10 to 100 μm. 
     The diameter of the metal wire and the pitch for the metal wire are determined depending on the wavelength used. In the terahertz band, the metal wire has a diameter on the order of 10 to 300 μm, and a pitch on the order of 30 μm to 1 mm.
     Patent Document 1: Japanese Patent Application Laid-Open No. 2003-14620   Non-Patent Document 1: Kunio Yoshihara, “Physical Optics”, (KYORITSU SHUPPAN CO., LTD., the first edition in 1966) p. 216   

     When such a wire grid is used as a terahertz band polarizer, the size of the wire grid ranges from 20 mm to approximately 100 mm in diameter. In the case of a wire grid with a diameter of 100 mm, metal wires have a length of about 100 mm, and it is thus difficult to tighten the long thin metal wires parallel to each other. More specifically, the tension easily cuts the metal wires, or causes an interval between the metal wires to get irregular, thus leading to difficulty in manufacture. Therefore, the wire grid will be very expensive as a component. 
     On the other hand, there are sold wire grids which are made by forming a thin film on a substrate through which electromagnetic waves are to be transmitted and making the thin film into fine wire patterns by etching or the like. Furthermore, there are also sold wire grids which are made by dispersing metal grain in a base material such as resin or glass, and making the base material into fine wires in the base material by stretching or the like. In the structures with these substrate or base material, phenomena such as multiple reflection or interference occur due to physical properties such as the index of refraction, reflectivity, and absorption index of the substrate or base material. Therefore, special treatment will be required in order to avoid the phenomena. 
     Furthermore, both the wire grid with the use of the metal wires and the wire grid with the use of the substrate or base material are limited in size up to a diameter on the order of 100 to 150 mm, and it is difficult to make larger wire grids over the size limit. 
     SUMMARY OF THE INVENTION 
     Thus, an object of the present invention is to solve the problem with the use of the wires described above and the problem with the use of the substrate or base material, and provide an easily-manufactured, low-cost, and even high-precision wire grid for polarization and analysis of electromagnetic waves and a manufacturing method for the wire grid. 
     A first aspect of the present invention provides a wire grid for polarization and analysis of electromagnetic waves, wherein a warp yarn and a weft yarn are crossed to make a textile, and one of the warp yarn and the weft yarn includes a conductive fine wire whereas the other includes an insulating yarn. 
     This aspect allows long conductive fine wires to be arranged parallel to each other at regular intervals, without the use of a frame body for applying a tension to fine metal wires as in conventional wire grids. Furthermore, with the textile as a whole, a wire grid of predetermined size can be easily manufactured by cutting the textile. Moreover, the textile can be shaped into a three-dimensional shape. More specifically, the degrees of freedom in size and shape are significantly improved. Furthermore, low-cost wire grids are obtained by virtue of ease in manufacture. 
     The conductive fine wire may be a metal wire, and the insulating yarn may be twine of an insulating fiber. 
     This aspect allows highly conductive fine wires to be made, resulting in favorable polarization and analysis characteristics. Furthermore, the insulating yarn including twine of an insulating fiber can enhance the compatibility with looms, and conventional looms can be thus used as they are for the manufacture. Therefore, lager wire grids can be made. 
     The conductive fine wire has a pitch determined within the range of 30 μm to 3 mm. Thus, a frequency with electromagnetic waves of 100 GHz to 10 THz can be polarized and analyzed. 
     A second aspect of the present invention provides a wire grid for polarization and analysis of electromagnetic waves, wherein a warp yarn and a weft yarn are crossed to make a textile, the warp yarn and the weft yarn include conductive fine wires, and the interval from one weft yarn to another is made 5 or more times as long as a wavelength of an electromagnetic wave to be transmitted. 
     This aspect makes the wavelength at which the conductive weft yarn acts as a grid 5 or more times as long as the target wavelength, and thus has almost no adverse effect on polarization and analysis characteristics for the target wavelength. 
     A third aspect of the present invention provides a method for manufacturing a wire grid for polarization and analysis of electromagnetic waves, the method including the steps of:
         preparing a conductive fine wire;   preparing an insulating yarn; and   making the conductive fine wire and the insulating yarn into a textile by using a loom.       

     This aspect allows wire grids for polarization and analysis of electromagnetic waves to be manufactured by using a loom for weaving with twine of a normal fiber. 
     The method for manufacturing a wire grid further includes the step of removing the insulating yarn after the textile is made. 
     When the textile is held as it is in the frame body, the conductive fine wire can be held even after the insulating yarn is removed. Thus, the removal of the insulating yarn provides a wire grid for polarization and analysis of electromagnetic waves with its electromagnetic characteristics unaffected by the insulating yarn. 
     According to the present invention, an easily-manufactured, low-cost and high-precision wire grid for polarization and analysis of electromagnetic waves is obtained without problems with cut metal wires or irregular intervals between metal wires because of long fine metal wires tightened parallel to each other in a frame body or problems such as multiple reflection or interference on a substrate or a base material because of fine wire patterns made by etching or the like with the use of the substrate or base material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a conventional wire grid. 
         FIG. 2  is a plan view of a wire grid according to a first embodiment. 
         FIG. 3(A)  is a plan view (photograph) of a wire grid fabric for use in the wire grid, and  FIG. 3(B)  is an enlarged view (photograph) of the wire grid fabric. 
         FIG. 4  is a diagram showing characteristics of the wire grid. 
         FIG. 5  is a plan view of a wire grid according to a second embodiment. 
         FIG. 6  is a plan view of a wire grid according to a third embodiment. 
     
    
    
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
         
           
               1  metal wire 
               2 ,  20  frame body 
               10  wire grid fabric 
               11  stainless wire (warp yarn) 
               12  silk yarn (weft yarn) 
               13  stainless wire (weft yarn) 
               20  frame body 
               100 ,  101 ,  102  wire grid 
           
         
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     A metal plate for a wire grid and a method for manufacturing the wire grid according to a first embodiment will be described with reference to  FIGS. 2 to 4 . 
       FIG. 2  is a plan view of a wire grid  100  according to the first embodiment. This wire grid  100  is obtained by attaching a textile to a ring frame body  20 , the textile including a plurality of stainless steel wires (hereinafter, simply referred to as stainless wires)  11  extending vertically and silk yarns  12  extending horizontally while crossing one by one alternately with respect to the stainless wires  11 . 
     When the wire grid  100  shown in  FIG. 2  is used as a polarizer or analyzer, electromagnetic waves are made incident in a direction perpendicular to a surface of the wire grid  100 . With incident electromagnetic waves having a plane of polarization parallel to the stainless wires  11  of the wire grid  100 , the electric field component of the electromagnetic waves generates an electric current in the stainless wires  11 . Therefore, the electric field component is reflected or absorbed and the electromagnetic waves are not transmitted. On the other hand, since the width (diameter) of the stainless wires ( 11 ) is significantly shorter than (about 1/10 or less of) the wavelength, the loss due to an electric current flowing in the stainless wires  11  can be mostly ignored with incident electromagnetic waves having a plane of polarization perpendicular to the stainless wires  11 . Therefore, the electromagnetic waves are transmitted as they are. In this way, the wire grid  100  acts as a polarizer or analyzer. 
       FIG. 3(A)  is a plan view (photograph) of a wire grid fabric for use in the wire grid  100  shown in  FIG. 2 , whereas  FIG. 3(B)  is an enlarged view (photograph) of the wire grid fabric. 
     The wire grid fabric  10  shown in  FIG. 3(A)  is a textile made by using a loom with a stainless wire as warp yarn and a silk yarn as weft yarn. 
     This wire grid fabric  10  is manufactured as follows. 
     First, a warp yarn feeding mechanism is set so that a stainless wire (warp yarn) wound around a yarn feeder such as a bobbin is fed to a loom while keeping a predetermined tension, and plain weave is carried out by alternating pull up and down of a heddle and passing of a shuttle for a silk yarn (weft yarn). As this loom, Jacquard loom can be used. 
     The pitch for the stainless wires  11   a ,  11   b  is selected within the range of 30 μm to 3 mm so that electromagnetic waves of 10 GHz to 10 THz can be polarized and analyzed. When the pitch is 30 μm, the wavelength is 120 μm in the case of 30 μm for the ¼ wavelength, thus allowing a 2.5 THz frequency band to be polarized and analyzed. Alternatively, when the pitch is 3 mm, the wavelength is 12 mm in the case of 3 mm for the ¼ wavelength, thus allowing a 25 GHz frequency band to be polarized. 
     This textile is made by plain weave as shown in  FIG. 3(B) , in such a way that the stainless wires  11   a ,  11   b  are arranged at a predetermined pitch and the stainless wires  11   a ,  11   b  and silk yarns (twine of silk fibers)  12   a ,  12   b  are crossed one by one. In this example, the pitch for the stainless wires  11   a ,  11   b  as warp yarn and the pitch for the silk yarns  12   a ,  12   b  as weft yarn have a relationship of about 1:5. This pitch is determined in view of issues as described next. 
     The smaller the ratio of the pitch for the silk yarns  12   a ,  12   b  to the pitch for the stainless wires  11   a ,  11   b  is, the relatively more the stainless wires  11   a ,  11   b  is flexed. The degree of the flexing is affected by the thicknesses and degrees of hardness of the stainless wires  11   a ,  11   b  and silk yarns  12   a ,  12   b . For example, when the stainless wires and silk yarns can be woven by using a loom for Nishijin silk fabrics, it is preferable that the ratio between the pitches be 1:1 or more. 
     Furthermore, the larger the ratio of the pitch for the silk yarns  12   a ,  12   b  to the pitch for the stainless wires  11   a ,  11   b  is, the larger the interval between the positions of supporting the stainless wires  11   a ,  11   b  by the silk yarns  12   a ,  12   b  is, and the stainless wires  11   a ,  11   b  will be thus likely to undergo deflection. The degree of the deflection is affected by the thicknesses and degrees of hardness of the stainless wires  11   a ,  11   b  and silk yarns  12   a ,  12   b . For example, when the stainless wires and silk yarns can be woven by using a loom for Nishijin silk fabrics, it is preferable that the ratio between the pitches be 1:10 or less. 
     Furthermore, it is preferable that the diameter of the stainless wire be adjusted so that when the pitch dimension and diameter of the stainless wire are respectively denoted by d and a, the ratio d/a has a value on the order of 2 to 4. When this value is less than 2, the transmittance of electromagnetic waves in the polarization direction desired to be transmitted will be less than a transmittance generally required by a polarizer or analyzer for electromagnetic waves of 10 GHz to 10 THz. When this value is greater than 4, the transmittance of electromagnetic waves in the polarization direction desired to be blocked will be greater as compared with the transmittance generally required by a polarizer or analyzer for electromagnetic waves of 10 GHz to 10 THz. 
     The wire grid shown in  FIG. 2  is obtained by fitting the wire grid fabric  10  as shown in  FIG. 3(A)  in the frame body  20 . The wire grid fabric  10  may be any wire grid fabric as long as the size of the wire grid fabric is larger than the opening of the frame body  20 , and it is also possible to cut the wire grid fabric  10 . Thus, the wire grid  100  of arbitrary size can be easily made depending on the size of the frame body  20 . 
     According to this embodiment, because the warp yarn is able to keep the arrangement pitch constant with a high degree of accuracy as compared with the weft, a wire grid with a high-precision grid pitch is obtained by using a stainless wire (conductive yarn) for the warp yarn and a silk yarn (insulating yarn) for the weft yarn. 
     According to the present invention, for example, a textile (piece goods) with a width of 1 m or more and a length of 1 m or more can be made at one time. Thus, the present invention provides a quite high production efficiency, and allows reduction in cost. Furthermore, the present invention eliminates the need for a frame body for applying a tension to fine metal wires to arrange the wires, as in conventional wire grids. Furthermore, with the textile as a whole, a wire grid of predetermined size can be easily manufactured by cutting the textile. Moreover, the textile can be shaped into a three-dimensional structure such as, for example, a parabolic shape. 
       FIG. 4  is a diagram showing an example of polarization and analysis characteristics of the wire grid according to the first embodiment. In  FIG. 4 , the horizontal axis indicates the frequency of an incident electromagnetic wave, whereas the vertical axis indicates a transmittance %. In  FIG. 4 , A indicates the transmittance of electromagnetic waves (light) with the electric field direction of incident light perpendicular to the stainless wire, whereas B indicates the transmittance of electromagnetic waves (light) with the electric field direction of incident light parallel to the stainless wire. 
     In this example, the diameter and pitch of the stainless wire were 30 μmm and 10 wires/mm. 
     Thus, polarized waves were allowed to be transmitted at a transmittance of 80% or more over a range of 10 GHz to 1000 GHz, and polarized waves were blocked at a transmittance of 5% or less over a range of 100 GHz to 650 GHz. 
     It is to be noted that while the single stainless wire is used as the conductive fine line in the example described above, other metal wires such as tungsten wires, carbon twine including carbon fibers, further, conductive organic fibers, etc. can be also be similarly applied. 
     Furthermore, while the silk yarn is used as the insulating yarn, in addition, natural fibers such as cotton yarn and wool, as well as synthetic fibers such as polyethylene and polyester can also be similarly applied. In general, the use of a synthetic fiber increases the durability, and suppresses changes in characteristics with respect to environmental changes since the synthetic fiber is less hygroscopic. 
     Moreover, while the conductive yarn and the insulating yarn are used respectively for the warp yarn and weft yarn, the insulating yarn and the conductive yarn may be reversely used respectively for the warp yarn and weft yarn. More specifically, weaving may be carried out by using a loom with the use of the insulating yarn such as a silk yarn for the warp yarn and of the conductive yarn such as a stainless wire for the weft yarn. 
     Second Embodiment 
     A metal plate for a wire grid and a method for manufacturing the wire grid according to a second embodiment will be described with reference to  FIG. 5 . 
     While the wire grid fabric with the insulating yarns left in place is used in the first embodiment, this second embodiment provides a wire grid fabric fixed in a frame body with insulating yarns removed. 
       FIG. 5  is a plan view of a wire grid  101  according to the second embodiment. This wire grid  101  is obtained by attaching a textile to a ring frame body  20  and then removes the silk yarns, the textile including a plurality of stainless wires  11  extending vertically and silk yarns  12  extending horizontally while crossing one by one alternately with respect to the stainless wires  11 . 
     Specifically, processing such as burning the insulating yarns causes the yarns to substantially disappear. Alternatively, chemically soluble yarns are used as the insulating yarns, and the insulating yarns are dissolved in a liquid, with the wire grid fabric fixed in the frame body. 
     As described above, the wire grid  101  configured to have only the conductive yarns arranged at a predetermined pitch is not to be electromagnetically affected by the insulating yarns. Therefore, the electromagnetically influential insulating yarns can also be used to weave textiles, and the wire grid  101  thus has higher degrees of freedom for the material, thickness, arrangement pitch, and the like of the insulating yarns. 
     Third Embodiment 
     While the textile with the conductive yarns and the insulating yarns is used in the first and second embodiments, the third embodiment uses conductive yarns for both warp yarn and weft yarn. 
       FIG. 6  is a plan view of a wire grid  102  according to the third embodiment. This wire grid  102  is obtained by attaching a textile to a ring frame body  20 , the textile including a plurality of stainless wires  11  extending vertically and stainless wires  13  extending horizontally while crossing one by one alternately with respect to the stainless wires  11 . 
     The pitch for the stainless wires  11  as warp yarn is made to have the ¼ wavelength of a wavelength to be polarized and analyzed, whereas the pitch for the stainless wires  13  as weft yarn is made 10 times as long as a wavelength to be polarized. When the pitch for the stainless wires  13  as weft yarn is 5 or more times as long as the wavelength, the increase in loss due to the existence of the stainless wires  13  can be suppressed to an increase on the order of several %, and the loss is further reduced by making the pitch 10 or more times as long as the wavelength. 
     With this configuration, the stainless wires  13  as weft yarn have no adverse effect on polarization and analysis characteristics for the target wavelength, the increase in loss due to the existence of the stainless wires  13  can be mostly ignored, and the wire grid can be directly used for a wire grid for polarization and analysis of electromagnetic waves. 
     While the pitch for the weft yarn is greater than the pitch for the warp yarn in the example shown in  FIG. 6 , the reverse relationship may be employed. 
     It is to be noted that while the plain-woven textile includes warp yarn and weft yarn in each embodiment described above, twill weave or satin weave may be employed besides plain weave when the conductive yarn is used as the warp yarn. Alternatively, even when the conductive yarn is used as the weft yarn, twill weave or satin weave may be employed in such a way that the conductive yarn is arranged at a predetermined pitch. 
     Furthermore, a plating process such as gold plating may be applied to the conductive yarns after the textile is made. The application of the plating process allows the conductivity to be increased, and suppresses degradation in characteristics due to aged deterioration and environment.