Patent Publication Number: US-2021165151-A1

Title: Optical element, production method thereof, and projection image display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese application No. 2019-215090, filed on Nov. 28, 2019 and incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an optical element, a production method thereof, and a projection image display device. 
     Description of the Related Art 
     As a light source used for a projector, a laser light source capable of emitting light of high luminance with high output has been prominent. 
     Conventionally, an optical element formed of an oblique angle vapor deposition film (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2012-256024). 
     However, there is a problem that such an optical element is deteriorated by light emitted from a laser light source. 
     SUMMARY OF THE INVENTION 
     &lt;1&gt; An optical element, including:
 
a substrate that is transparent to light at a using wavelength band;
 
an antireflection layer;
 
a matching layer; and
 
a birefringent layer formed of an oblique angle vapor deposition film,
 
wherein an optical loss in the optical element at the using wavelength band is 1.0% or less, and
 
the optical loss is calculated using the following formulae from intensity of transmitted light and intensity of reflected light measured by S-polarized light at an incident angle of 5° in the optical element,
 
       Transmittance=intensity of transmitted light/intensity of incident light (%) 
       Reflectance=intensity of reflected light/intensity of incident light (%) 
       Optical loss (%)=100%−transmittance (%)−reflectance (%).
 
     &lt;2&gt; The optical element according to &lt;1&gt;,
 
wherein the optical loss in the optical element at a wavelength of 455 nm is 1.0% or less.
 
&lt;3&gt; The optical element according to &lt;1&gt;,
 
wherein the antireflection layer is a multilayer film in which two or more inorganic oxide films having mutually different refractive indexes are deposited.
 
&lt;4&gt;The optical element according to &lt;3&gt;,
 
wherein at least one of the inorganic oxide films includes at least one selected from the group consisting of Ti, Si, Ta, Al, Ce, Zr, Nb, and Hf.
 
&lt;5&gt; The optical element according to &lt;4&gt;,
 
wherein at least one of the inorganic oxide films includes Nb.
 
&lt;6&gt; The optical element according to &lt;4&gt;,
 
wherein at least one of the inorganic oxide films includes Si.
 
&lt;7&gt; The optical element according to &lt;4&gt;,
 
wherein the antireflection layer is a multilayer film in which an inorganic oxide film including Nb and an inorganic oxide film including Si are deposited.
 
&lt;8&gt; The optical element according to &lt;1&gt;,
 
wherein the matching layer is a multilayer film in which two or more inorganic oxide films having mutually different refractive indexes are deposited.
 
&lt;9&gt; The optical element according to &lt;8&gt;,
 
wherein at least one of the inorganic oxide films includes at least one selected from the group consisting of Ti, Si, Ta, Al, Ce, Zr, Nb, and Hf.
 
&lt;10&gt; The optical element according to &lt;9&gt;,
 
wherein at least one of the inorganic oxide films includes Nb.
 
&lt;11&gt; The optical element according to &lt;9&gt;,
 
wherein at least one of the inorganic oxide films includes Si.
 
&lt;12&gt; The optical element according to &lt;9&gt;,
 
wherein the matching layer is a multilayer film in which an inorganic oxide film including Nb and an inorganic oxide film including Si are deposited.
 
&lt;13&gt; The optical element according to &lt;1&gt;,
 
wherein the antireflection layer is disposed on both sides of the substrate.
 
&lt;14&gt; A production method of an optical element, including:
 
providing a substrate that is transparent to light at a using wavelength band;
 
forming at least one antireflection layer;
 
forming a matching layer; and
 
forming a birefringent layer formed of an oblique angle vapor deposition film,
 
wherein an optical loss in the optical element at the using wavelength band is 1.0% or less,
 
the antireflection layer or the matching layer, or both are formed by reactive sputtering, and
 
the optical loss is calculated using the following formulae from intensity of transmitted light and intensity of reflected light measured by S-polarized light at an incident angle of 5° in the optical element,
 
       Transmittance=intensity of transmitted light/intensity of incident light (%) 
       Reflectance=intensity of reflected light/intensity of incident light (%) 
       Optical loss (%)=100%−transmittance (%)−reflectance (%).
 
     &lt;15&gt; The production method according to &lt;14&gt;,
 
wherein the reactive sputtering is reactive sputtering using mixed gas including inert gas and oxygen gas, and
 
an oxygen flow ratio in the mixed gas is set in a manner that the optical loss of the optical element is to be 1.0% or less relative to the light at the using wavelength band.
 
&lt;16&gt; The production method according to &lt;14&gt;,
 
wherein the forming at least one antireflection layer, the forming a matching layer, or both include laminating two or more inorganic oxide films having mutually different refractive indexes by reactive sputtering to form a multilayer film.
 
&lt;17&gt; The production method according to &lt;14&gt;,
 
wherein the forming at least one antireflection layer or the forming a matching layer, or both include forming an oxide film including Nb using mixed gas including inert gas and oxygen gas by reactive sputtering using Nb as a target, and
 
wherein an oxygen flow ratio in the mixed gas is greater than 18% when the oxide film including Nb is formed, where the oxygen flow ratio is represented by [an oxygen flow rate/(an inert gas flow rate+the oxygen gas flow rate)].
 
&lt;18&gt; The production method according to &lt;14&gt;,
 
wherein the forming at least one antireflection layer or the forming a matching layer, or both include forming an oxide film including Si using mixed gas including inert gas and oxygen gas by reactive sputtering using Si as a target, and
 
wherein an oxygen flow ratio in the mixed gas is greater than 8% when the oxide film including Si is formed, where the oxygen flow ratio is represented by [an oxygen flow rate/(an inert gas flow rate+the oxygen gas flow rate)].
 
&lt;19&gt; A projection image display device, including:
 
an optical element;
 
a light modulator;
 
a light source configured to emit light; and
 
a projection optical system configured to project modulated light,
 
wherein the light modulator and the optical element are disposed on a light path between the light source and the projection optical system, and
 
wherein the optical element includes:
 
a substrate that is transparent to light at a using wavelength band;
 
an antireflection layer;
 
a matching layer; and
 
a birefringent layer formed of an oblique angle vapor deposition film,
 
wherein an optical loss in the optical element at the using wavelength band is 1.0% or less, and
 
the optical loss is calculated using the following formulae from intensity of transmitted light and intensity of reflected light measured by S-polarized light at an incident angle of 5° in the optical element,
 
       Transmittance=intensity of transmitted light/intensity of incident light (%) 
       Reflectance=intensity of reflected light/intensity of incident light (%) 
       Optical loss (%)=100%−transmittance (%)−reflectance (%).
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a configuration example of an optical element; 
         FIG. 2  is a schematic cross-sectional view of an antireflection layer; 
         FIG. 3  is a perspective schematic view of an oblique angle vapor deposition film; 
         FIG. 4  is a schematic view illustrating one example of an oblique angle vapor deposition for forming an oblique angle vapor deposition film; 
         FIG. 5  is a schematic view illustrating one example of a direction in which a deposition direction from a deposition source is projected towards a deposition target surface; 
         FIG. 6  is a flowchart illustrating a production method of an optical element; 
         FIG. 7  is a schematic view illustrating one example of a structure of a projection image display device; 
         FIG. 8  is a view assisting descriptions of measuring methods of transmittance and reflectance; 
         FIG. 9A  is a graph depicting one transmittance of the sample of Example 1; 
         FIG. 9B  is a graph depicting one reflectance of the sample of Example 1; 
         FIG. 9C  is a graph depicting one optical loss of the sample of Example 1; 
         FIG. 10A  is a graph depicting one transmittance of the sample of Comparative Example 1; 
         FIG. 10B  is a graph depicting one reflectance of the sample of Comparative Example 1; and 
         FIG. 10C  is a graph depicting one optical loss of the sample of Comparative Example 1. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present technology will be described in details according to the following order with reference to drawings. 
     1. Optical element
 
2. Production method of optical element
 
3. Projection image display device
 
     4. Examples 
     The present invention can solve the above-described various problems existing in the art, and can achieve the following object. Specifically, the present invention has an object to provide an optical element having excellent durability even when a laser light source is used, a production method of the optical element, and a projection image display device including the optical element. 
     The present invention can solve the above-described various problems, in the art, and can provide an optical element having excellent durability even when a laser light source is used, a production method of the optical element, and a projection image display device including the optical element. 
     Optical Element 
     The optical element according to the present embodiment includes a substrate transparent to light at a using wavelength band, an antireflection layer, a matching layer, and a birefringent layer formed of an oblique angle vapor deposition film. The optical element may further include other members according to the necessary. 
     The optical element has an optical loss of 1.0 or less relative to light at a using wavelength band. 
     The optical loss is a value obtained by subtracting transmittance to the light at the using wavelength band and reflectance to the light at the using wavelength band from 100%, and is represented by the following formula (1). 
       Optical loss (%)=100%−transmittance (%)−reflectance (%)   Formula (1)
 
     The lower limit of the optical loss is not particularly limited and may be appropriately selected depending on the intended purpose. As the optical loss reduces, productivity may declines. Accordingly, the optical loss may be 0.1% or greater, 0.3% or greater, or 0.5% or greater. 
     The light at the using wavelength band may be, for example, light in a wavelength range of 400 nm or longer but 700 nm or shorter, or light of 455 nm. The light in the wavelength range of 400 nm or longer but 700 nm or shorter and the light of 455 nm are both light generally used with projection image display devices. 
     The optical loss is preferably an optical loss of 1.0% or less with all wavelengths in the using wavelength band. 
     The optical loss of the optical element is preferably 1.0% or less relative to light having a wavelength of 455 nm. 
     The optical loss is preferably 1.0% or less relative to the light of any wavelength in the wavelength range of 450 nm or longer but 700 nm or shorter. Note that, the optical loss is smaller as a wavelength is longer. 
     The transmittance and reflectance of the optical element to the light at the using wavelength band can be determined by applying S-polarized light to the optical element with an incident angle of 5° to measure intensity of the transmitted light and intensity of the reflected light, and calculating the transmittance and reflectance using the measured values according to the following formulae. 
       Transmittance=intensity of transmitted light/intensity of incident light (%) 
       Reflectance=intensity of reflected light/intensity of incident light (%) 
       Optical loss (%)=100%−transmittance (%)−reflectance (%)
 
     In the measurement above, the transmitted light to be measured is direct transmission light, and the reflected light to be measured is specular reflection light. 
     The transmittance of the optical element to the light at the using wavelength band and the reflectance of the optical element are, for example, measured by means of a spectrophotometer V-570, available from JASCO Corporation. 
     Examples of the optical element having the above-described structure include a retardation element configured to give a phase difference to incident light, and a retardation compensator element. 
       FIG. 1  is a cross-sectional view illustrating a configuration example of the optical element. As illustrated in  FIG. 1 , the optical element  10  includes a transparent substrate  11 , a matching layer  12  disposed on the transparent substrate  11 , where the matching layer includes high refractive index films and low refractive index films are alternately deposited, and a thickness of each film is equal to or less than a used wavelength, a birefringent layer  13  formed of an oblique angle vapor deposition film, disposed on the matching layer  12 , and a protective layer  14  formed of a dielectric film formed on the birefringent layer  13 . Moreover, the optical element includes a first antireflection layer  15 A at the side of the transparent substrate  11 , and a second antireflection layer  15 B at the side of the protective layer  14 . 
     Transparent Substrate 
     The transparent substrate  11  is transparent to light at a using wavelength band. The transparent substrate  11  has high transmittance to the light at the using wavelength band. Examples of a material of the transparent substrate  11  include glass, quartz, crystal, and sapphire. A shape of the transparent substrate  11  is typically a square, but the shape thereof is appropriately selected depending on the intended purpose. A thickness of the transparent substrate  11  is, for example, preferably 0.1 mm or greater but 3.0 mm or less. 
     Antireflection Layer 
     For example, the first antireflection layer  15 A is disposed to be in contact with a surface of the transparent substrate  11  opposite to a surface thereof facing the side of the birefringent layer  13 . 
     For example, the second antireflection layer  15 B is optionally disposed to be in contact with a surface of the protective layer  14  opposite to a surface thereof facing the birefringent layer  13 . 
     The antireflection layer  15 A and the second antireflection layer  15 B have an antireflection function in a desirable wavelength band for use. 
       FIG. 2  is a schematic cross-sectional view of the first antireflection layer. As illustrated in  FIG. 2 , the first antireflection layer  15 A is a multilayer film where two or more inorganic oxide films having mutually different refractive indexes are deposited. For example, the first antireflection layer  15 A is a multilayer film where first oxide films  151  and second oxide films  152  having mutually different refractive indexes are alternately deposited. The number of layers within the antireflection layer is appropriately determined according to the necessity, and the number of layers is preferably from about 5 layers to about 40 layers in view of productivity. Note that, the second antireflection layer  15 B has the similar configuration to the first antireflection layer  15 A. 
     The larger difference between the refractive index of the first oxide film  151  and the refractive index of the second oxide film  152  is more preferable. In view of readily availability of materials and film formability, the difference is preferably 0.5 or greater but 1.0 or less. The refractive index is, for example, a refractive index at a wavelength of 550 nm. 
     For example, the oxide films of the first antireflection layer  15 A and the oxide films of the second antireflection layer  15 B are each an oxide film including at least one selected from the group consisting of Ti, Si, Ta, Al, Ce, Zr, Nb, and Hf. 
     For example, the antireflection layer is a multiple film where first oxide films  151  formed of niobium oxide (refractive index at wavelength of 550 nm: 2.3) having a relatively high refractive index, and second oxide films  152  formed of silicon oxide (refractive index at wavelength of 550 nm: 1.5) having a relatively low refractive index are alternately deposited. 
     Note that, the oxides constituting the antireflection layer may be nonstoichiometric. Specifically, an atomic ratio of constitutional elements of the oxide is not necessarily a simple whole number ratio. When an oxide film is formed by sputtering etc., the formed oxide is often nonstoichiometric. Moreover, an elemental ratio of the oxide of the formed film cannot be easily measured stably, thus it is difficult to determine an elemental ratio of the oxide. 
     Considering the oxide being nonstoichiometric, for example, the oxide including Nb is represented by the following formula. 
       NbO X  (0&lt;X≤2.5)
 
     For example, the oxide including Si is represented by the following formula. 
       SiO Y  (0&lt;Y≤2)
 
     When the antireflection layer is formed, light absorption of the antireflection layer can be reduced, and an optical loss in the optical element can be declined by reducing oxygen deficiency of oxide formed. 
     A thickness of the antireflection layer is not particularly limited and may be appropriately selected depending on the intended purpose. For example, the thickness of the antireflection layer is 250 nm or greater but 2,300 nm or less. In the present specification, a thickness of a layer (film thickness) means an average film thickness. 
     Matching Layer 
     The matching layer  12  is, for example, a multilayer film where two or more inorganic oxide films having mutually different refractive indexes are deposited. The matching layer  12  is disposed between the transparent substrate  11  and the birefringent layer  13 . The matching layer  12  is designed to cancel interface reflection light by interference, to thereby prevent reflection at an interface between the transparent substrate  11  and the birefringent layer  13 . Specifically, the matching layer  12  is designed to cancel out reflected light at an interface between the transparent substrate  11  and the matching layer  12  and reflected light at an interference between the matching layer  12  and the birefringent layer  13 . 
     For example, the matching layer  12  is formed of an oxide film including at least one selected from the group consisting of Ti, Si, Ta, Al, Ce, Zr, Nb, and Hf. 
     Note that, the oxides constituting the matching layer  12  may be nonstoichiometric. Specifically, an atomic ratio of constitutional elements of the oxide is not necessarily a simple whole number ratio. When an oxide film is formed by sputtering etc., the formed oxide is often nonstoichiometric. 
     When the matching layer  12  is formed, light absorption of the matching layer  12  can be reduced, and an optical loss in the optical element can be declined by reducing oxygen deficiency of oxide formed. 
     A thickness of the matching layer  12  is not particularly limited and may be appropriately selected depending on the intended purpose. For example, the thickness of the matching layer  12  is 140 nm or greater but 240 nm or less. 
     Birefringent Layer 
     The birefringent layer  13  is formed of an oblique angle vapor deposition film. 
     The birefringent layer  13  is a layer having a function of imparting a phase difference to the optical element of the present invention. 
     In the optical element  10  illustrated in  FIG. 1 , the birefringent layer  13  is disposed between the matching layer  12  and the protective layer  14 . 
     The birefringent layer  13  includes, for example, a birefringent film formed of an inorganic material. The inorganic material is preferably a dielectric material. Examples of the inorganic material include an oxide including at least one selected from the group consisting of Si, Nb, Zr, Ti, La, Ta, Al, Hf, and Ce. 
     The inorganic material is preferably tantalum oxide (e.g., Ta 2 O 5 ). 
     A thickness of the birefringent layer  13  is, for example, 200 nm or greater but 4,200 nm or less. 
       FIG. 3  is a perspective schematic view of an oblique angle vapor deposition film. As illustrated in  FIG. 3 , the oblique angle vapor deposition film  23  constituting the birefringent layer  13  is formed by depositing a deposition material in a direction slanting relative to a normal line S that is a direction perpendicular to a surface of the transparent substrate  11  or the deposition target surface  21 . The slanted angle relative to the normal line S of the deposition target surface  21  is preferably 60° or greater but 80° or less. 
     The birefringent layer typically has a structure where a plurality of the above-described birefringent films are deposited. 
     Each birefringent film is formed by depositing in the direction slanting relative to the normal line S, and an angle formed between the film formation direction of an inorganic material constituting the birefringent film and a surface of the transparent substrate is not 90°. 
     A method creating a state where an angle formed between the film formation direction of the inorganic material and the surface of the transparent substrate is not 90° is, for example, preferably a method where a deposition source is arranged in a position slanted relative to the normal line S and an oblique angle vapor deposition film is formed by oblique angle vapor deposition from the deposition source. When a birefringent layer is formed by performing oblique angle vapor deposition multiple times, the oblique angle vapor deposition is repeated with varying the deposition angle to thereby obtain a final birefringent layer. 
       FIG. 4  is a schematic view illustrating one example of an oblique angle vapor deposition for forming the oblique angle vapor deposition film. 
       FIG. 5  is a schematic view illustrating one example of a direction (vapor deposition direction) projecting a flying direction of the deposition material from a vapor deposition source to a vapor deposition target surface. 
     As illustrated in  FIG. 4 , a linear direction for projecting a film formation direction of the birefringent film on a surface of the transparent substrate is represented by d, when an oblique angle vapor deposition film is formed on the transparent substrate  11  in the deposition direction D from the deposition source R. 
     As illustrated in  FIGS. 4 and 5 , a film formed by alternately forming oblique angle vapor deposition films is formed by alternately repeating a film formation performed by vapor deposition in the first vapor deposition direction  31 , and film formation performed by vapor deposition in the second vapor deposition direction  32 . Specifically, after forming a film by vapor deposition in the first vapor deposition direction  31 , the vapor deposition target surface is rotated by 180° around a center line passing through a center of the vapor deposition surface in a vertical direction relative to the vapor deposition surface, to thereby perform film formation by vapor deposition from the second vapor deposition direction  32 . By repeating the above-described processes, a film, in which first oblique angle vapor deposition films each having a first slanting direction relative to a normal line of the vapor deposition target surface, and second oblique angle vapor deposition films each having a second slanting direction relative to the normal line of the vapor deposition target surface are alternately formed, is obtained. 
     Protective Layer 
     The protective layer  14  is formed of a dielectric film, and is disposed to be in contact with the oblique angle vapor deposition film of the birefringent layer  13 . The presence of the protective layer  14  can prevent warping of the optical element  10 , and can improve humidity resistance of the oblique angle vapor deposition film. 
     The dielectric material of the protective layer  14  is not particularly limited and may be appropriately selected depending on the intended purpose, as long as the dielectric material can adjust stress applied to the optical element  10 , and can exhibit an effect of improving humidity resistance. Examples of such a dielectric material include oxide including at least one selected from the group consisting of Si, Ta, Ti, Al, Nb, and La, and MgF 2 . 
     A thickness of the protective layer  14  is not particularly limited and may be appropriately selected depending on the intended purpose. The thickness of the protective layer  14  is, for example, 10 nm or greater but 100 nm or less. 
     Production Method of Optical Element 
     Next, the production method of an optical element according to the present embodiment will be described. 
     In the production method of an optical element according to the present embodiment, the optical element according to the present embodiment is produced. 
     The production method of an optical element of the present embodiment includes forming antireflection layer or a matching layer, or both by reactive sputtering with setting an oxygen flow ratio to a predetermined range to produce an optical element having an optical loss of 1.0% or less relative to light at a using wavelength band. 
     The production method of an optical element according to the present embodiment preferably includes a step for forming an oxide film including Nb, or a step for forming an oxide film including Si, or both. 
     Step for Forming Oxide Film Including Nb 
     In the production method of an optical element of the present embodiment, for example, an antireflection layer or a matching layer, or both include oxide including Nb. 
     The oxide film including Nb functions, for example, as a high refractive index layer in the antireflection layer or matching layer. 
     The production method of an optical element according to the present disclosure includes, for example, forming an oxide film including Nb using mixed gas including inert gas and oxygen gas by reactive sputtering using Nb as a target. 
     The oxygen flow ratio in the mixed gas when the oxide film including Nb is formed is preferably greater than 18%, where the oxygen flow ratio is represented by [oxygen gas flow rate/(inert gas flow rate+oxygen gas flow rate)]. When the oxygen flow ratio is greater than 18%, oxygen deficiency of the oxide in the antireflection layer or matching layer can be reduced, and hence light absorption of the antireflection layer or matching layer is low. As a result, an optical loss of the optical element can be easily kept low. 
     Moreover, the upper limit of the oxygen flow ratio is not particularly limited and may be appropriately selected depending on the intended purpose. For example, the upper limit thereof may be 30% or 25%. When the oxygen flow ratio is high, a film formation duration for forming an oxide film including Nb tends to be long. Therefore, the oxygen flow ratio is preferably 25% or less. 
     A unit of the inert gas flow rate and a unit of the oxygen gas flow rate are each a gas volume per unit time (e.g., mL/min). 
     Step for Forming Oxide Film Including Si 
     In the production method of an optical element of the present embodiment, for example, the antireflection layer or matching layer, or both include an oxide including Si. 
     The oxide film including Si functions, for example, a low refractive index layer in the antireflection layer or matching layer. 
     The production method of an optical element of the present embodiment includes, for example, forming an oxide film including Si using mixed gas including inert gas and oxygen gas by reactive sputtering using Si as a target. 
     The oxygen flow ratio in the mixed gas when the oxide film including Si is formed is preferably greater than 8%, where the oxygen flow ratio is represented by [oxygen gas flow rate/(inert gas flow rate+oxygen gas flow rate)]. When the oxygen flow ratio is greater than 8%, oxygen deficiency of the oxide in the antireflection layer or matching layer can be reduced, and hence light absorption of the antireflection layer or matching layer is low. As a result, an optical loss of the optical element can be easily kept low. 
     Moreover, the upper limit of the oxygen flow ratio is not particularly limited and may be appropriately selected depending on the intended purpose. For example, the upper limit thereof may be 20% or 15%. When the oxygen flow ratio is high, a film formation duration for forming an oxide film including Si tends to be long. Therefore, the oxygen flow ratio is preferably 15% or less. 
     As a specific example of the production method of an optical element, the production method of an optical element having the configuration example illustrated in  FIG. 1  will be described hereinafter.  FIG. 6  is a flowchart depicting the production method of an optical element. 
     &lt;&lt;S 1 &gt;&gt; 
     First, a transparent substrate  11  is provided in Step S 1 . 
     &lt;&lt;S 2 &gt;&gt; 
     Next, a matching layer  12 , in which oxide films are deposited, is formed on the transparent substrate in order to prevent reflection at an interface between the birefringent layer  13  and the transparent substrate  11  in Step S 2 . 
     During the formation of the matching layer  12 , the matching layer  12  is formed by alternately performing a process for forming the above-described oxide film including Nb and a process for forming the above-described oxide film including Si. As a result, the matching layer  12  having low light absorption can be obtained. 
     &lt;&lt;S 3 &gt;&gt; 
     Next, a first antireflection layer  15 A [back antireflection (AR) layer] is formed on a surface of the transparent substrate  11 , on which the matching layer  12  is not formed, in Step S 3 . 
     In the formation of the first antireflection layer  15 A, the first antireflection layer  15 A is formed by alternately performing the step for forming an oxide film including Nb and the step for forming an oxide film including Si. As a result, the first antireflection layer  15 A having low light absorption can be obtained. 
     &lt;&lt;S 4 &gt;&gt; 
     Next, a birefringent layer  13  is formed on the matching layer  12  by oblique angle vapor deposition in Step S 4 . As illustrated in  FIGS. 4 and 5 , for example, after performing film formation by vapor deposition in the first vapor deposition direction  31 , the vapor deposition target surface is rotated by 180° around a center line passing through a center of the vapor deposition surface in a vertical direction relative to the vapor deposition surface, to thereby perform film formation by vapor deposition from the second vapor deposition direction  32 . By repeating the above-described processes, a film, in which first oblique angle vapor deposition films each having a first slanting direction relative to a normal line of the vapor deposition target surface, and second oblique angle vapor deposition films each having a second slanting direction relative to the normal line of the vapor deposition target surface are alternately formed, is obtained. 
     &lt;&lt;S 5 &gt;&gt; 
     Next, the birefringent layer  13  is subjected to annealing at a temperature of 200° C. or higher but 600° C. or lower in Step S 5 . The birefringent layer  13  is subjected to annealing more preferably at a temperature of 300° C. or higher but 500° C. or lower, further more preferably 400° C. or higher but 500° C. or lower. As a result, properties of the birefringent layer  13  can be stabilized. 
     &lt;&lt;S 6 &gt;&gt; 
     Next, a protective layer  14  is formed on the birefringent layer  13  in Step S 6 . When a film of SiO 2  is formed as the protective layer  14 , for example, tetraethoxysilane (TEOS) gas and O 2  are preferably used as a material of SiO 2 , and a plasma CVD device is preferably used. 
     A SiO 2  CVD film formed by a plasma CVD device uses a vaporized material gas for film formation different from physical vapor deposition, such as sputtering. Therefore, TEOS gas is relatively easily penetrated into gaps in the column structure to further improve adhesion of the protective layer  14  to the birefringent layer  13 . 
     &lt;&lt;S 7 &gt;&gt; 
     Next, a second antireflection layer  15 B (surface AR layer) is formed on the protective layer  14  in Step S 7 . 
     In the formation of the second antireflection layer  15 B, the second antireflection layer  15 B is formed by alternately performing the step for forming an oxide film including Nb and the step for forming an oxide film including Si. As a result, the second antireflection layer  15 B having low light absorption can be obtained. 
     &lt;&lt;S 8 &gt;&gt; 
     Finally, scribe cutting is performed to obtain a size matched to a specification in Step S 8 . 
     According to the production method as described above, an optical element having excellent durability against light of high luminance and high output emitted from a laser light source etc. can be obtained. 
     Projection Image Display Device 
     Since the above-described optical element has excellent durability against light of high luminance and high output, a projection image display device including the optical element can be suitably used as a projector, such as a liquid crystal projector, a digital light processing (DLP) (registered trademark) projector, a liquid crystal on silicon (LCOS) projector, and a grating light valve (GLV) (registered trademark) projector. 
     Specifically, the projection image display device according to the present embodiment includes the optical element, a light modulator, a light source configured to emit light, and a projection optical system configured to project modulated light, where the light modulator and the optical element are disposed on a light path between the light source and the projection optical system. 
     Light Modulator 
     Examples of the light modulator include a liquid crystal display device including a transmissive liquid crystal panel etc., a micro-mirror display device including a digital micro-mirror device (DMD) etc., a reflective liquid crystal display device including a reflective liquid crystal panel etc., and a one-dimensional diffraction display device including a one-dimensional light modulator (grating light valve [GLV]) etc. 
     In the projection image display device using the liquid crystal display device, for example, the liquid crystal display device includes at least a liquid crystal panel, a first polarizing plate, and a second polarizing plate, and may further include other members according to the necessity. 
     Liquid Crystal Panel 
     The liquid crystal panel is not particularly limited. For example, the liquid crystal panel includes a substrate, and a VA-mode liquid crystal layer including liquid crystal molecules having pre-tilt relative to the orthogonal direction to the main surface of the substrate, and modulates the entered luminous flux entered. The VA-mode (vertical alignment mode) means a system where liquid crystal molecules aligned vertical (or with pre-tilt) to the substrate are moved using a longitudinal electric field in a vertical direction. 
     First Polarizing Plate and Second Polarizing Plate 
     A first polarizing plate is a polarizing plate disposed at the inlet side of the liquid crystal panel, and a second polarizing plate is a polarizing plate disposed at the outlet side of the liquid crystal panel. The first polarizing plate and the second polarizing plate are preferably inorganic polarizing plates in view of durability. 
     Optical Element 
     An optical element is the optical element of the present invention. 
     For example, the optical element is the optical element of the structural example illustrated in  FIG. 1 , and the optical element is disposed in a desirable position on a light path constituting the projection image display device. 
     In the projection image display device using the micro-mirror display device, the optical element is disposed on the same light path in combination with a diffuser, a polarization beam splitter, etc. 
     Light Source 
     A light source is not particularly limited and may be appropriately selected depending on the intended purpose, as long as the light source is a member that emits light. Since the liquid crystal display device includes the optical element having excellent durability in the present embodiment, a laser light source that emits light of high luminance and high output can be used. 
     The wavelength of the laser light source is, for example, 455 nm. 
     Projection Optical System 
     The projection optical system is not particularly limited and may be appropriately selected depending on the intended purpose, as long as the projection optical system is a member for projecting modulated light. Examples of the projection optical system include a projection lens configured to project the modulated light onto a screen. 
     The projection image display device having the above-described structure can display an image of high luminance and high output using light of high luminance and high output emitted from a laser light source etc. 
       FIG. 7  is a schematic view illustrating one example of the structure of the projection image display device according to the present embodiment. The projection image display device  115 A is a so-called 3-panel liquid crystal projector, which displays a color image using 3 liquid crystal panels of red, green, and blue. As illustrated in  FIG. 7 , the projection image display device  115 A includes liquid crystal display devices  101 R,  101 G, and  101 B, a light source  102 , dichroic mirrors  103  and  104 , a total reflection mirror  105 , polarization beam splitters  106 R,  106 G, and  106 B, a beam-combining prism  108 , and a projection lens  109 . 
     The light source  102  is configured to emit light-source light (white light) L including blue light LB, green light LG, and red light LR for forming an image display. Examples of the light source  102  include a halogen lamp, a metal halide lamp, a xenon lamp, and a laser light source. 
     The dichroic mirror  103  has a function of separating the light-source light L into blue light LB and light of other colors LRG. The dichroic mirror  104  has a function of separating the light passed LRG through the dichroic mirror  103  into red light LR and green light LG. The total reflection mirror  105  reflects the blue light LB separated by the dichroic mirror  103  towards the polarization beam splitter  106 B. 
     The polarization beam splitters  106 R,  106 G, and  106 B are prism-type polarized light separators disposed on light paths of the red light LR, the green light LG, and the blue light LB, respectively. The polarization beam splitters  106 R,  106 G, and  106 B have polarized light splitting surfaces  107 R,  107 G, and  107 B, respectively. The polarization beam splitters  106 R,  106 G, and  106 B have a function of splitting the entered light of each color into two polarized light components orthogonal to each other at the polarized light splitting surfaces  107 R,  107 G, and  107 B, respectively. The polarized light splitting surfaces  107 R,  107 G, and  107 B reflect one polarized light component (e.g., an S-polarized light component) and transmit the other polarized light component (e.g., a P-polarized light component). 
     The color light of the certain polarized light component (e.g., an S-polarized light component) separated by each of the polarized light splitting surfaces  107 R,  107 G, and  107 B of the polarization beam splitters  106 R,  106 G, and  106 B enters each of the liquid crystal display devices  101 R,  101 G, and  101 B. The liquid crystal display devices  101 R,  101 G, and  101 B are driven by driving voltage applied according to an image signal to modulate the incident light, and also have a function of reflecting the modulated light to the polarization beam splitters  106 R,  106 G, and  106 B. 
     The optical elements  10  and the ¼-wave plates  113 R,  113 G, and  113 B are disposed between the polarization beam splitters  106 R,  106 G, and  106 B, and the liquid crystal panels of the liquid crystal display devices  101 R,  101 G, and  101 B, respectively. The ¼-wave plates  113 R,  113 G, and  113 B each function as a ½-wave plate as the ¼-wave plates  113 R,  113 G, and  113 B allow to pass the light twice, i.e., when the light enters the liquid crystal panel, and when the light is emitted from the liquid crystal panel (for example, converting an S-polarized light component into a P-polarized light). Moreover, the ¼-wave plates  113 R,  113 G, and  113 B have a function of correcting a reduction of the contrast owing to the incident light angle dependency the polarization beam splitters  106 R,  106 G, and  106 B have. The optical elements  10  have a function of compensating the residual phase difference of the liquid crystal panels constituting the liquid crystal display devices  101 R,  101 G, and  101 B, respectively. In one aspect, the ¼-wave plate is the optical element of the present embodiment. In another aspect, moreover, the optical element  10  is the optical element of the present embodiment. 
     The beam-combining prism  108  has a function of combining color light of the certain polarized light components (e.g., P-polarized light components) emitted from the liquid crystal display devices  101 R,  101 G, and  101 B and passed through the polarization beam splitter  106 R,  106 G, and  106 B. The projection lens  109  has a function of projecting the synthesized light emitted from the beam-combining prism  108  towards the screen  110 . 
     Next, an operation of the projection image display device  115 A constituted in the above-described manner will be described. 
     First, white light L emitted from the light source  102  is split into blue light LB and other color light (red light and green light) LRB by a function of the dichroic mirror  103 . The blue light LB is reflected to the polarization beam splitter  106 B by a function of the total reflection mirror  105 . 
     Meanwhile, other color light (red light and green light) LRG is further split into red light LR and green light LG by a function of the dichroic mirror  104 . The split red light LR and green light LG enters the polarization beam splitters  106 R and  106 G, respectively. 
     The polarization beam splitters  106 R,  106 G, and  106 B are configured to split the entered color light into two polarized light components orthogonal to each other by the polarized light splitting surfaces  107 R,  107 G, and  107 B, respectively. The polarized light splitting surfaces  107 R,  107 G, and  107 B reflect one polarized light component (e.g., an S-polarized light component) to the liquid crystal display devices  101 R,  101 G, and  101 B. The liquid crystal display devices  101 R,  101 G, and  101 B are driven by driving voltage applied according to an image signal, and modulate color light of the entered certain polarized light by pixel. 
     The liquid crystal display devices  101 R,  101 G, and  101 B reflect the modulated color light to the polarization beam splitters  106 R,  106 G, and  106 B, respectively. The polarization beam splitters  106 R,  106 G, and  106 B only pass through the certain polarized light component (e.g., P-polarized light components) within the reflected light (modulated light) from the liquid crystal display devices  101 R,  101 G, and  101 B, and emit towards the beam-combining prism  108 . 
     The beam-combining prism  108  synthesize the color light of the certain polarized light components passed through the polarization beam splitters  106 R,  106 G, and  106 B, and emits towards the projection lens  109 . The projection lens  109  projects the synthesized light emitted from the beam-combining prism  108  to the screen  110 . As a result, an image corresponding to the light modulated by the liquid crystal display devices  101 R,  101 G, and  101 B is projected on the screen  110 , and a desired image display is achieved. 
     EXAMPLES 
     Specific example of the present invention will be described hereinafter. However, the present invention is not limited to the example below. Note that, formed films are described as a SiO 2  film and a Nb 2 O 5  film for the matter of convenience, but the films are highly likely nonstoichiometric. 
     Example 1 
     Production of Optical Element 
     On one surface of a glass substrate (average thickness: 0.7 mm), a SiO 2  film and an Nb 2 O 5  film were alternately deposited to form 5 layers in total by sputtering to thereby form a matching layer. 
     The SiO 2  film was formed by reactive sputtering using a Si target with introducing Ar gas and O 2  gas. The O 2  gas flow ratio was set to 12%. 
     Note that, the O 2  gas flow ratio can be determined as follows. 
       O 2  gas flow ratio=O 2  gas flow rate/(Ar gas flow rate+O 2  gas flow rate) 
     The Nb 2 O 5  film was formed by reactive sputtering using an Nb target with introducing Ar gas and O 2  gas. The O 2  gas flow ratio was 22%. 
     Subsequently, on the other surface of the glass substrate, an Nb 2 O 5  film and a SiO 2  film were alternately deposited to form 7 layers in total by sputtering to thereby form an antireflection layer. 
     The SiO 2  film was formed by reactive sputtering using a Si target with introducing Ar gas and O 2  gas. The O 2  gas flow ratio was set to 12%. 
     The Nb 2 O 5  film was formed by reactive sputtering using an Nb target with introducing Ar gas and O 2  gas. The O 2  gas flow ratio was 22%. 
     Subsequently, a deposition source was arranged in a position slanted relative to a normal line of the glass substrate by 70°, and oblique angle vapor deposition was performed on the matching layer using a Ta 2 O 5  deposition material with alternating between a first vapor deposition direction of 0°, and a second vapor deposition direction of 180° to thereby obtain a birefringent layer formed of an oblique angle vapor deposition film. 
     After the vapor deposition, annealing was performed at 400° C. to stabilize the properties of the birefringent layer. After the annealing, a SiO 2  film was formed by plasma CVD using tetraethoxysilane (TEOS) gas and O 2 . 
     Subsequently, an Nb 2 O 5  film and a SiO 2  film were alternately deposited to form 7 layers in total by sputtering to thereby form an antireflection layer. 
     The SiO 2  film was formed by reactive sputtering using a Si target with introducing Ar gas and O 2  gas. The O 2  gas flow ratio was set to 12%. 
     The Nb 2 O 5  film was formed by reactive sputtering using an Nb target with introducing Ar gas and O 2  gas. The O 2  gas flow ratio was 22%. 
     In the manner as described above, an optical element was obtained. 
     Comparative Example 1 
     An optical element was produced in the same manner as in Example 1, expect that the O 2  gas flow ratio was changed to 8% for the formation of the SiO 2  film and the O 2  gas flow ratio was changed to 18% for the formation of the Nb 2 O 5  film during formation of the matching layer and the antireflection layer. 
     Measurements of Transmittance and Reflectance 
     As illustrated in  FIG. 8 , S-polarized light having wavelengths of 400 nm to 700 nm was applied at an incidence angle of 5°, and the intensity of the transmitted light and the intensity of the reflected light were measured to calculate transmittance and reflectance. In  FIG. 8 , IL is incident light, S is a normal line, RL is reflected light,  10  is an optical element, and TL is transmitted light. 
       Transmittance=intensity of transmitted light/intensity of incident light (%) 
       Reflectance=intensity of reflected light/intensity of incident light (%) 
       Optical loss (%)=100%−transmittance (%)−reflectance (%)
 
     As a result of the measurements performed on 30 samples, the optical loss of the samples of Example 1 was from 0.5% to 0.9%, and the optical loss of the samples of Comparative Example 1 was from 1.2% to 1.6%. 
     Moreover, one transmittance, one reflectance, and one optical loss of the sample of Example 1 are presented in Figures. 
       FIG. 9A  is a graph depicting one transmittance of the sample of Example 1. 
       FIG. 9B  is a graph depicting one reflectance of the sample of Example 1. 
       FIG. 9C  is a graph depicting one optical loss of the sample of Example 1. 
     Moreover, one transmittance, one reflectance, and one optical loss of the sample of Comparative Example 1 are presented in Figures. 
       FIG. 10A  is a graph depicting one transmittance of the sample of Comparative Example 1. 
       FIG. 10B  is a graph depicting one reflectance of the sample of Comparative Example 1. 
       FIG. 10C  is a graph depicting one optical loss of the sample of Comparative Example 1. 
     Laser Irradiation Test 
     Laser irradiation conditions
 
Wavelength:  455  nm-CW
 
Laser power: 50 W
 
Power density: 8.3 W/mm 2  
 
Irradiation duration: 3 minutes
 
     Thirty samples of each of Example 1 and Comparative Example 1 were irradiated with laser under the above-described laser irradiation conditions, and the presence of damages was visually observed. The results are presented below. 
       Example 1: the number of damages [0]/the number of tests [30] 
       Comparative Example 1: the number of damages [10]/the number of tests [30] 
     It was found that there was no damage from the laser irradiation test and excellent durability against laser was obtained when the optical loss was 1.0% or less. 
     INDUSTRIAL APPLICABILITY 
     Since the optical element of the present invention has excellent durability even when a laser light source is used, the optical element is suitably used for a projection image display device using a laser light source.