Polarizer and optical apparatus

This polarizer is a polarizer having a wire grid structure that includes a transparent substrate, a dielectric film which extends across one surface of the transparent substrate and has a lower refractive index than the transparent substrate, and a plurality of projections which extend in a first direction on top of the dielectric film and are arrayed periodically at a pitch that is shorter than the wavelength of the light in the used light region, wherein the transparent substrate has a thermal conductivity of at least 10 W/m·K but not more than 40 W/m·K, the plurality of projections each have, in order from the side closer to the dielectric film, a first dielectric layer, a reflective layer and a functional layer, the reflective layer contains a metal or a metal compound, and the functional layer is formed from a material different from the reflective layer.

BACKGROUND OF THE INVENTION

Claim for Priority

Priority is claimed to Japanese Patent Application No. 2019-152938, filed Aug. 23, 2019, the content of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a polarizer and an optical apparatus.

DESCRIPTION OF RELATED ART

Polarizers are used in liquid crystal displays and the like. In recent years, wire grid polarizers having reflective layers arranged at a period that is shorter than the wavelength of light in the used light region are attracting much attention.

Wire grid polarizers include reflective and absorptive polarizers. Reflective wire grid polarizers transmit light of a specific polarization component, and reflect light of other polarization components. In contrast, absorptive wire grid polarizers transmit light of a specific polarization component, and eliminate light of other polarization components by interference. Examples include the absorptive wire grid polarizers disclosed in U.S. Pat. Nos. 7,961,393, 7,813,039 and 8,027,087. Further, a reflective wire grid polarizer is disclosed in International Patent Publication No. WO2019/004435.

The wire grid polarizer disclosed in International Patent Publication No. WO2019/004435 uses sapphire for the substrate in order to enhance heat dissipation. Further, in order to reduce the stress caused by the difference between the coefficients of thermal expansion for the sapphire substrate and the reflective layers, grooves are formed in the sapphire substrate.

SUMMARY OF THE INVENTION

In recent years, as the brightness of liquid crystal projectors and the like has continued to increase, polarizers having superior heat dissipation are being demanded. One technique that can be considered for producing polarizers of superior heat dissipation involves using substrates that exhibit superior heat dissipation. However, a problems arises in that using a substrate of superior heat dissipation tends to cause a deterioration in the optical characteristics. If a polarizer having inferior optical characteristics (for example, a high reflectance) is used in a liquid crystal projector or the like, then the polarizer may cause malfunctions of the liquid crystal panel and a deterioration in the image quality due to stray light. As the brightness and definition of recent liquid crystal projectors and the like increase, additional reductions in the reflectance are required of polarizers.

The present invention has been developed in light of the above circumstances, and has an object of providing a polarizer and an optical apparatus having excellent optical characteristics and heat dissipation.

In order to achieve the above object, the present invention provides the following aspects.

A polarizer according to a first aspect is a polarizer having a wire grid structure, and includes a transparent substrate, a dielectric film which extends across a surface of the transparent substrate and has a lower refractive index than the transparent substrate, and a plurality of projections which extend in a first direction on top of the dielectric film and are arrayed periodically at a pitch that is shorter than the wavelength of the light in the used light region, wherein the transparent substrate has a thermal conductivity of at least 10 W/m·K but not more than 40 W/m·K, the plurality of projections each have, in order from the side closer to the dielectric film, a first dielectric layer, a reflective layer and a functional layer, the reflective layer contains a metal or a metal compound, and the functional layer is formed from a material different from the reflective layer.

In the polarizer according to the aspect described above, the functional layer may have, in order from the side closer to the transparent substrate, a second dielectric layer, an absorption layer containing an absorptive material, and a third dielectric layer.

In the polarizer according to the aspect described above, the functional layer may have a mixed layer containing a mixture of a dielectric material and an absorptive material.

In the polarizer according to the aspect described above, the refractive index of the transparent substrate may be at least 1.70 but not more than 1.80.

In the polarizer according to the aspect described above, a cross-section obtained by cutting the first dielectric layer through a section orthogonal to the first direction may be rectangular, trapezoidal, or pseudo-trapezoidal with curved sides.

In the cross-section of the polarizer according to the aspect described above, the angle between the side surfaces and the bottom surface of the first dielectric layer may be at least 20° but not more than 90°.

In the polarizer according to the aspect described above, if the refractive index of the transparent substrate is deemed ns, the refractive index of the dielectric film is deemed na, and the refractive index of an in-plane region including the first dielectric layer is deemed n1, then the relationship 1<n1<na<nsmay be satisfied.

In the polarizer according to the aspect described above, the thickness of the first dielectric layer may be at least 10 nm but not more than 100 nm.

In the polarizer according to the aspect described above, the thickness of the dielectric film may be at least 40 nm but not more than 120 nm.

In the polarizer according to the aspect described above, the transmission axis reflectance in the wavelength region from at least 430 nm to not more than 680 nm may be 1% or less.

An optical apparatus according to a second aspect includes the polarizer according to the aspect described above.

The polarizer and the optical apparatus according to the aspects described above can improve the heat dissipation and the optical characteristics.

PREFERRED EMBODIMENTS

Embodiments of the present invention are described below in detail with appropriate reference to the drawings. The drawings used in the following description may sometimes be drawn with specific portions enlarged as appropriate to facilitate comprehension of the features of the present invention, and the dimensional ratios and the like between the constituent elements may differ from the actual values. Further, the materials and dimensions and the like presented in the following description are merely examples, which in no way limit the present invention, and may be altered as appropriate within the scope of the present invention.

FIG.1is a cross-sectional schematic view of a polarizer100according to a first embodiment. The polarizer100is a polarizer with a wire grid structure. The polarizer100includes a substrate10, a dielectric film20and a plurality of projections30. In the following description, the plane across which the substrate10extends is deemed the xy plane, and the direction orthogonal to the xy plane is deemed the z direction. In the following description, the +z direction is sometimes described using the terms “up” or “top”, and the −z direction is sometimes described using the terms “down” or “bottom”. These up and down directions do not necessarily correspond with the direction in which gravity acts. Further, within the xy plane, the direction along which the projections30extend is deemed they direction. They direction is one example of the first direction. In this description, the expression “extend along the y direction” means that, for example, the dimension in the y direction is greater than the shortest dimension among the dimensions in the x direction, the y direction and the z direction.FIG.1is a cross-sectional view cut through the xz plane of the polarizer100.

The polarizer100attenuates TE waves (S waves) and transmits TM waves (P waves). TE waves are polarized light waves having an electric field component parallel to the y direction along which the projections30extend. TM waves are polarized light waves having an electric field component perpendicular to the y direction. In the polarizer100, they direction is the absorption axis and the x direction is the transmission axis.

The substrate10extends across the xy plane. The substrate10is the base of the polarizer100. The substrate10has transparency relative to light of wavelengths in the used light region for the polarizer100. The substrate10is one example of a transparent substrate. The expression “has transparency” does not necessarily mean 100% transmittance of light of wavelengths in the used light region, and a level of transmittance that enables retention of the functionality as a polarizer is sufficient. The average thickness of the substrate10is preferably at least 0.3 mm but not more than 1 mm.

The substrate10has a thermal conductivity of at least 10 W/m·K but not more than 40 W/m·K. For example, a polarizer for the optical engine of a projector is irradiated with intense light, and therefore requires superior light resistance and heat dissipation. If the substrate10has high thermal conductivity, then heat dissipation occurs efficiently, and the light resistance improves.

The substrate10is, for example, formed from rock crystal, sapphire, a Mg2O single crystal, or a crystal having a spinel structure (such as MgAl2O4). For example, the thermal conductivity of a crystal having a spinel structure (such as MgAl2O4) is 16.2 W/m·K, and the thermal conductivity of sapphire is 33 W/m·K. Alkali-free glass (such as Eagle XG manufactured by Corning Inc.), which is widely used as a substrate for optical elements, has a thermal conductivity of 1.1 W/m·K, but the substrate10requires superior heat dissipation.

The refractive index of the substrate10is, for example, at least 1.1 but not more than 2.2, and is preferably at least 1.7 but not more than 1.8. For example, the refractive index of a crystal having a spinel structure (such as MgAl2O4) is 1.72, and the refractive index of sapphire is 1.77.

The dielectric film20extends over one surface of the substrate10. The dielectric film20has a refractive index lower than that of the substrate10. For example, if the refractive index of the substrate10is deemed ns, and the refractive index of the dielectric film20is deemed na, then these refractive indices satisfy ns>na.

The dielectric film20is, for example, formed from a metal oxide, magnesium fluoride (MgF2), cryolite, germanium, silicon, boron nitride, carbon, or a mixture of these materials. Examples of the metal oxide include Si oxides such as SiO2, Al2O3, beryllium oxide, bismuth oxide, boron oxide, and tantalum oxide. The dielectric film20is, for example, a Si oxide.

The thickness h20of the dielectric film20is, for example, at least 40 nm but not more than 120 nm, preferably at least 80 nm but not more than 120 nm, and more preferably at least 90 nm but not more than 110 nm.

The projections30are formed on top of the dielectric film20. When viewed in plan view from the z direction, the plurality of projections30exist on top of the dielectric film20. Each of the projections30extends along they direction. In a plan view viewed from the z direction, the projections30extending in they direction are arrayed periodically across the x direction.

The pitch Pin the x direction between adjacent projections30is shorter than the wavelength of light in the used light region for the polarizer100. For example, the pitch P is preferably at least 100 nm but not more than 200 nm. Provided the pitch P falls within this range, production of the projections30is simple, and the mechanical stability and the stability of the optical characteristics of the projections30can be enhanced.

The pitch P between adjacent projections30can be measured as an average value using a scanning electron microscope or a transmission electron microscope. For example, by measuring the distance in the x direction between adjacent projections30at four random locations, the pitch P can be determined as the arithmetic mean of the four measured distances. The measurement method in which the measured values are averaged from four random locations among the plurality of projections30is termed the “electron microscope method”.

The projections30rise from base on the dielectric film20. The main direction in which the projections30rise is the z direction. The average width in the x direction of the projections30is preferably at least 20% but not more than 50% of the pitch P. In this description, the average width of the projections30means the average value of the widths of the projections30measured at 10 points evenly spaced along the z direction. The height of the projections30is preferably at least 250 nm but not more than 400 nm. Further, the aspect ratio obtained by dividing the height of the projections30by the average width is preferably at least 5 but not more than 13.3.

In those cases where the substrate10is an optically active crystal, the direction along which the projections30extend is preferably parallel or perpendicular to the optical axis of the crystal. In this description, the optical axis is the axis in a direction for which the difference between the refractive indices of an ordinary ray of light (0) and an extraordinary ray of light (E) travelling along that direction is smallest. Having the projections30extend along that direction improves the optical characteristics.

Each of the projections30has a first dielectric layer40, a reflective layer50and a functional layer60in that order from the side closer to the dielectric film20. The projections30may also have one or more layers besides these layers.

The first dielectric layer40is positioned on the side of the dielectric film20in each projection30. The first dielectric layer40is positioned between the dielectric film20and the reflective layer50of each projection30.

The first dielectric layer40is formed from a similar material to the dielectric film20. For example, the first dielectric layer40may be formed from a Si oxide. The first dielectric layer40may be formed from the same material as the dielectric film20. There are no particular limitations on the refractive index of the first dielectric layer40.

The refractive index n1of an in-plane region including the first dielectric layer40is, for example, lower than the refractive index of the dielectric film20. In this description, the “in-plane region including the first dielectric layer40” is the region, within the area in which the substrate10exists when viewed in plan view from the z direction, between the xy plane that passes through a top surface40tof the first dielectric layer40and an xy plane that passes through a bottom surface40bof the first dielectric layer40. InFIG.1, the region made up of the combination of the first dielectric layer40and open portions40A represents the “in-plane region including the first dielectric layer40”.

The open portions40A are the spaces sandwiched between the first dielectric layers40in the x direction. Because the pitch P of the projections30is shorter than the wavelength of the light in the used light region, the refractive index experienced by light incident from the z direction is the average of the refractive index of the first dielectric layer40and the refractive index of the open portions40A.

The refractive index n1of the in-plane region including the first dielectric layer40, the refractive index naof the dielectric film20, and the refractive index nsof the substrate10may, for example, satisfy the relationship 1<n1<na<ns. The first dielectric layer40is selected, for example, so that the refractive index relationship with the dielectric film20satisfies the above relationship. When the refractive index decreases in order from the substrate10to the dielectric film20and then the in-plane region including the first dielectric layer40, the change in the refractive index becomes stepwise, and the transmission axis reflectance of the polarizer100can be suppressed to a low value.

The thickness h40of the first dielectric layer40is, for example, at least 10 nm but not more than 100 nm, preferably at least 20 nm but not more than 80 nm, and more preferably at least 40 nm but not more than 60 nm. The thickness h40of the first dielectric layer40is the length of a perpendicular line dropped from the top surface40tof the first dielectric layer40to the bottom surface40b. The top surface40tof the first dielectric layer40is, for example, the interface between the first dielectric layer40and the reflective layer50. The thickness h40of the first dielectric layer40is, for example, the average value of the thickness of the first dielectric layer40in ten projections30.

The thickness h40of the first dielectric layer40is preferably at least 20% but not more than 100%, more preferably at least 20% but not more than 80%, and even more preferably at least 20% but not more than 60%, of the thickness h20of the dielectric film20.

The cross-sectional shape of the first dielectric layer40when cut through an xz plane is, for example, a trapezoidal shape such as that illustrated inFIG.1. In the trapezoid illustrated inFIG.1, the length of the bottom surface40bof the first dielectric layer40is longer than the length of the top surface40t. The first dielectric layer40expands in width from the top surface40ttoward the bottom surface40b. The top surface40tand the bottom surface40bare connected by side surfaces40s. Trapezoid-shaped openings40A are formed between adjacent first dielectric layers40. In those cases where the cross-section of the first dielectric layer40expands in width from the top surface40ttoward the bottom surface40b, the change in the refractive index in the z-direction becomes smoother, enabling better suppression of reflection.

The internal angle between the bottom surface40band the side surfaces40sof the first dielectric layer40is, for example, at least 20° but not more than 90°, preferably at least 40° but less than 90°, and more preferably at least 50° but not more than 80°.

FIG.1illustrates an example in which the cross-sectional shape of the first dielectric layer40is trapezoidal, but the cross-sectional shape of the first dielectric layer40is not limited to trapezoidal shapes.FIG.2toFIG.5are cross-sectional views of other examples of polarizers according to the first embodiment.FIG.2toFIG.5each represent a cross-sectional view illustrating the polarizer cut through an xz plane. In the polarizers101,102,103and104illustrated inFIG.2toFIG.5respectively, the shapes of the first dielectric layers41,42,43and44differ from the example illustrated inFIG.1. Those structures that are the same as the structures inFIG.1are labeled with the same symbols.

In the polarizer101illustrated inFIG.2, the shape of the first dielectric layer41is rectangular. In the first dielectric layer41, the lengths of a top surface41tand a bottom surface41bare equal, and the angle between the bottom surface41band the side surfaces41sis 90°. The cross-sectional shape of the open portions41A is rectangular. The cross-sectional shape of the first dielectric layer41may also be square.

In the polarizer102illustrated inFIG.3, the shape of the first dielectric layer42is pseudo-trapezoidal. The pseudo-trapezoidal shape differs from a trapezoid in that the side surfaces42sare curved. In the first dielectric layer42, the length of a bottom surface42bis longer than the length of a top surface42t. The first dielectric layer42expands in width from the top surface42ttoward the bottom surface42b. The top surface42tand the bottom surface42bare connected by side surfaces42s. The side surfaces42sare curved in the −z direction. When the side surfaces are curved, the angle between the bottom surface and the side surfaces can be taken as the angle between the bottom surface and an external tangent to the side surface. The cross-sectional shape of the open portions42A is a semicircular cylindrical shape having an arc in the −z direction.

In the polarizer103illustrated inFIG.4, the shape of the first dielectric layer43is pseudo-trapezoidal. The shape of the first dielectric layer43may also be termed a semicircular cylindrical shape. In the first dielectric layer43, the length of a bottom surface43bis longer than the length of a top surface43t. The first dielectric layer43expands in width from the top surface43ttoward the bottom surface43b. The top surface43tand the bottom surface43bare connected by side surfaces43s. The side surfaces43sare curved in the +z direction. The cross-sectional shape of the open portions43A is a pseudo-trapezoidal shape.

In the polarizer104illustrated inFIG.5, the shape of the first dielectric layer44is trapezoidal. The fact that the bottom surfaces44bof adjacent first dielectric layers44are joined differs from the shape illustrated inFIG.1. In the first dielectric layer44, the length of a bottom surface44bis longer than the length of a top surface44t. The first dielectric layer44expands in width from the top surface44ttoward the bottom surface44b. The top surface44tand the bottom surface44bare connected by side surfaces44s. The cross-sectional shape of the open portions44A is a triangular shape.

The reflective layer50is positioned between the first dielectric layer40and the functional layer60. The reflective layer50protrudes in the z direction relative to the dielectric film20, and extends in a belt-like shape along the y direction. The reflective layer50reflects TE waves (S waves) and transmits TM waves (P waves).

A material that has reflective properties relative to light of wavelengths in the used light region may be used as the reflective layer50. For example, the reflective layer50contains a metal or a metal compound. Examples of the material of the reflective layer50include simple metals such as Al, Ag, Cu, Mo, Cr, Ti, Ni, W, Fe, Si, Ge and Ta, and alloys of these metals. The reflective layer50is preferably formed from Al, Cu, or an alloy of these metals.

Further, the reflective layer50may also be an inorganic film or resin film for which the surface reflectance has been enhanced by coloration or the like.

The height of the reflective layer50may be designed freely. For example, the height of the reflective layer50is preferably at least 100 nm but not more than 300 nm. The height of the reflective layer50can be determined by the electron microscope method.

The width of the reflective layer50is preferably at least 20% but not more than 50% of the pitch P. Specifically, the width of the reflective layer50is preferably at least 10 nm but not more than 100 nm, and more preferably at least 20 nm but not more than 50 nm.

The functional layer60is positioned on the distant side (the +z direction side) of the reflective layer50from the substrate10. The functional layer60is formed from a different material from the reflective layer50. The expression “formed from a different material from the reflective layer50” does not mean that the functional layer60must not contain any of the material that constitutes the reflective layer50, but means that the composition of each of the layers that constitute the functional layer60differs from the composition of the reflective layer50.

The functional layer60has, for example, a second dielectric layer61, an absorption layer62, and a third dielectric layer63in order from the side closer to the substrate10. The second dielectric layer61, the absorption layer62and the third dielectric layer63use interference to attenuate the polarized waves (TE waves (S waves)) reflected by the reflective layer50.

The second dielectric layer61is, for example, stacked on top of the reflective layer50. The second dielectric layer61need not necessarily contact the reflective layer50, and another layer may exist between the second dielectric layer61and the reflective layer50. The second dielectric layer61extends in a belt-like shape along the y direction. The second dielectric layer61constitutes a portion of the functional layer60.

The thickness of the second dielectric layer61can be determined in accordance with the polarized waves reflected by the absorption layer62. The thickness of the second dielectric layer61is determine so that the phase of the polarized waves reflected by the absorption layer62and the phase of the polarized wave reflected by the reflective layer50deviate by half a wavelength. Specifically, the thickness of the second dielectric layer61is preferably at least 1 nm and not more than 500 nm. Provided the thickness falls within this range, the relationship between the phases of the two types of reflected polarized waves can be adjusted, and the interference effect can be enhanced. The thickness of the second dielectric layer61can be measured using the electron microscope method described above.

The second dielectric layer61may be formed using a similar material to the dielectric film20. For example, the second dielectric layer61may be a Si oxide such as SiO2.

The refractive index of the second dielectric layer61is preferably greater than 1.0 but not more than 2.5. The optical characteristics of the reflective layer50are also affected by the surrounding refractive indices (for example, the refractive index of the second dielectric layer61). By adjusting the refractive index of the second dielectric layer61, the characteristics of the polarized light can be controlled.

The absorption layer62is positioned between the second dielectric layer61and the third dielectric layer63. The absorption layer62extends in a belt-like shape along they direction. The absorption layer62constitutes a portion of the functional layer60.

The thickness of the absorption layer62is, for example, preferably at least 10 nm but not more than 100 nm. The thickness of the absorption layer62can be measured using the electron microscope method described above.

The absorption layer62contains at least one substance having a light absorption action for which the optical constant extinction coefficient is not zero.

For example, the absorption layer62may contain a metal material or a semiconductor material. The material used for the absorption layer62may be selected appropriately in accordance with the wavelength range for the light in the used light region.

In those cases where a metal material is used for the absorption layer62, the metal material is preferably a simple metal such as Ta, Al, Ag, Cu, Au, Mo, Cr, Ti, W, Ni, Fe or Sn, or an alloy containing one or more of these elements. Further, in those cases where a semiconductor material is used for the absorption layer62, the semiconductor material is preferably Si, Ge, Te, ZnO, or a silicide material. Examples of silicide materials include β-FeSi2, MgSi2, NiSi2, BaSi2, CrSi2and TaSi. A polarizer100produced using one of these materials for the absorption layer62has a high extinction ratio in the visible light region. Further, it is particularly preferable that the absorption layer62contains Fe or Ta, and Si.

In those cases where the absorption layer62is a semiconductor material, the band gap energy of the semiconductor material contributes to the light absorption action. As a result, the band gap energy of the semiconductor material must be no higher than the energy-equivalent value of the wavelength in the used light region. For example, in the case where the used light region is the visible light region, a semiconductor material having a band gap energy of not more than 3.1 eV, which is the equivalent of the absorption energy at wavelengths of 400 nm or greater, is preferably used.

The absorption layer62is not limited to a single layer, and may be composed of two or more layers. In those cases where the absorption layer62is composed of two or more layers, the materials of the various layers may be different. The absorption layer62can be formed using methods such as vapor deposition or sputtering methods.

The third dielectric layer63is, for example, stacked on top of the absorption layer62. The third dielectric layer63extends in a belt-like shape along the y direction. The third dielectric layer63constitutes a portion of the functional layer60.

The third dielectric layer63is formed from a similar material to the second dielectric layer61. The third dielectric layer63may be formed from the same material as the second dielectric layer61or from a different material. For example, the third dielectric layer63may be formed from a Si oxide. The refractive index of the third dielectric layer63is preferably within a similar range to that of the second dielectric layer61described above. The thickness of the third dielectric layer63is, for example, typically at least 10 nm but not more than 100 nm. The thickness of the third dielectric layer63can be measured using the electron microscope method described above.

FIG.1illustrates a case in which the functional layer60has a 3-layer structure, but the functional layer is not limited to this particular case.FIG.6is a cross-sectional view of another example of a polarizer according to the first embodiment.FIG.6represents a cross-sectional view illustrating the polarizer cut through an xz plane. In the polarizer105illustrated inFIG.6, the configuration of the functional layer70differs from the example illustrated inFIG.1. Those structures that are the same as the structures inFIG.1are labeled with the same symbols.

The functional layer70illustrated inFIG.6is a mixed layer containing a mixture of a dielectric material and an absorptive material. The functional layer70is positioned on the distant side (the +z direction side) of the reflective layer50from the substrate10. In the functional layer70, for example, the absorptive material content varies along the z direction.

Examples of the dielectric material include oxides containing elements such as Si, Al, Be, Bi, Ti, Ta or B, nitrides containing elements such as Si or B, fluorides containing elements such as Mg or Ca, as well as silicon, germanium, carbon and cryolite. The functional layer70may contain one, or two or more, dielectric materials. When two or more dielectric materials are used, the two or more dielectric materials may be dispersed uniformly within the functional layer70, or different dielectric materials may be used at different positions along the z direction.

Examples of the absorptive material include simple metals (excluding simple silicon) or alloys containing at least one element selected from the group consisting of Fe, Ta, Si, Ti, Mg, W, Mo and Al. Examples of the alloys include FeSi alloys and TaSi alloys. From the viewpoint of the etching properties, the Fe content of the FeSi alloy is preferably not more than 50 atm %, and more preferably 10 atm % or less. From the viewpoints of the reflectance and transmittance, the Ta content of the TaSi alloy is preferably not more than 40 atm %. The functional layer70may contain one, or two or more, absorptive materials. When two or more absorptive materials are used, the two or more absorptive materials may be dispersed uniformly within the functional layer70, or different absorptive materials may be used at different positions along the z direction.

In the mixture of the dielectric material and the absorptive material, it is preferable that the dielectric material contains at least one of Si and a Si oxide (such as silica), and the absorptive material contains a metal. Examples of the metal include at least one simple metal selected from the group consisting of Fe, Ta, W, Mo and Al, or an alloy of such metals. By combining Si or a Si oxide with a metal and generating a cermet, the heat resistance of the polarizer105can be improved.

The absorptive material content in the functional layer70may, for example, vary along the z direction. By varying the absorptive material content, the optical characteristics of the polarizer105can be improved. Furthermore, by controlling the degree of variation in the absorptive material content along the z direction, the wavelength at which the absorption axis reflectance reaches a minimum can be adjusted.

Furthermore, the absorptive material content preferably increases with increasing distance from the reflective layer50. For example, the absorptive material content at a first end of the functional layer70may be at least 0 atm % but not more than 20 atm %, while the absorptive material content at a second end of the functional layer70may be at least 45 atm % but not more than 98 atm %. The first end is the end of the functional layer70on the side of the reflective layer50, whereas the second end is the end at the opposite end from the first end. The variation in content may be linear or non-linear (for example, stepwise). Further, the absorptive material content at the first end may be set, for example, to at least 0.1 atm % but not more than 20 atm %, or to at least 3 atm % but not more than 20 atm %, whereas the absorptive material content at the second end may be set to at least 45 atm % but not more than 98 atm %, at least 45 atm % but not more than 80 atm %, or at least 45 atm % but not more than 60 atm %.

The thickness of the functional layer70is, for example, thinner than the thickness of the reflective layer50. The thickness of the functional layer70is, for example, at least 10 nm but not more than 100 nm. The thickness of the functional layer70can be measured using the electron microscope method described above.

Up until this point, the widths in the x direction of the functional layers60and70have been illustrated as having substantially the same width in the x direction as the reflective layer50, but the widths in the x direction of the functional layers60and70may be shorter than the width in the x direction of the reflective layer50. If the width in the x direction of the reflective layer50is shorter than the widths in the x direction of the functional layers60and70, then the transmittance of the polarizer increases. The widths in the x direction of the functional layers60and70may, for example, expand with increasing proximity to the reflective layer50, so that, for example, the cross-sectional shapes of the functional layers60and70are substantially triangular.

The polarizer may have other layers besides the structures described above.

For example, the polarizer may have a diffusion barrier layer between the second dielectric layer61or third dielectric layer63and the absorption layer62. A diffusion barrier layer prevents light diffusion in the absorption layer62. A metal film of Ta, W, Nb, or Ti or the like may be used as the diffusion barrier layer.

Furthermore, a protective film may be formed on the light incident side of the polarizer. For example, in the case where the light is incident from the +z direction toward the −z direction, a protective film may be formed so as to cover the area surrounding the projections30. For example, a similar material to the dielectric film20may be used for the protective film. The protective film suppresses unnecessary oxidation of metal films such as the reflective layer50. The protective film can be formed using CVD (Chemical Vapor Deposition) or ALD (Atomic Layer Deposition) or the like.

Further, a water-repellent film may be formed on the light incident side of the polarizer. For example, a fluorine-based silane compound such as perfluorodecyltriethoxysilane (FDTS) may be used for the water-repellent film. The water-repellent film may be formed using CVD or ALD or the like. The water-repellent film enhances the moisture resistance of the polarizer and improves the reliability.

Next is a description of a method for producing the polarizer usingFIG.1as reference. The polarizer is produced via a film formation step and an etching step.

First, the substrate10on which film formation is to be conducted is prepared. The substrate10is a substrate having a thermal conductivity of at least 10 W/m·K but not more than 40 W/m·K, and is, for example, formed from sapphire.

Subsequently, in the film formation step, a layer that becomes the dielectric film20and the first dielectric layer40, a layer that becomes the reflective layer50, a layer that becomes the second dielectric layer61, a layer that becomes the absorption layer62, and a layer that becomes the third dielectric layer63are stacked in that order on the substrate. In those cases where the dielectric film20and the first dielectric layer40are formed from different materials, the layers that becomes the dielectric film20and the first dielectric layer40may be formed separately. These layers can be formed using a sputtering method or a vapor deposition method.

In those cases where the functional layer70is a mixed layer containing a mixture of a dielectric material and an absorptive material, a layer that becomes the functional layer70is stacked instead of the layer that becomes the second dielectric layer61, the layer that becomes the absorption layer62, and the layer that becomes the third dielectric layer63. For example, by adjusting the respective sputtering rates of a target composed of the dielectric material and a target composed of the absorptive material, the absorptive material content in the functional layer70can be varied along the z direction.

Subsequently, in the etching step, each of the formed layers is etched. The etching is conducted through a patterned mask. The mask pattern is formed using a photolithography method or nanoimprinting method or the like. The etching is preferably performed using dry etching.

In those cases where the dielectric film20and the first dielectric layer40are formed from the same material, the dielectric film20and the first dielectric layer40are formed by stopping the etching partway through the thickness direction of the layer that becomes the dielectric film20and the first dielectric layer40. In those cases where the dielectric film20and the first dielectric layer40are formed from different materials, the etching is stopped when the dielectric film20is reached. The degree of progression of the etching can be adjusted by optimizing etching conditions such as the gas flow rate, the gas pressure, the voltage output used for generating the ions or radicals, and the cooling temperature for the substrate10.

By using the polarizer according to an embodiment of the present invention, the transmission axis reflectance can be suppressed to a low level even in those cases where a high heat dissipation substrate having superior heat resistance is used. In other words, the polarizer according to an embodiment of the present invention exhibits excellent heat resistance and optical characteristics.

Although described in detail in the following examples, in those cases where a high heat dissipation substrate such as sapphire is used, even if unevenness is formed on the surface of the high heat dissipation substrate, satisfactory optical characteristics cannot be achieved. Further, high heat dissipation substrates are often hard, meaning actual processing of the surface is difficult. However, in the polarizer according to an embodiment of the present invention, by forming a dielectric film and a first dielectric layer of a specific shape on one surface of a high heat dissipation substrate, the optical characteristics can be improved even in those cases where a high heat dissipation substrate is used.

The polarizer according to an embodiment of the present invention preferably exhibits a transmission axis reflectance of not more than 1% in the wavelength region from at least 430 nm to not more than 680 nm. By using the polarizer according to an embodiment of the present invention, the superior optical characteristics described above can be realized even in those cases where a high heat dissipation substrate such as sapphire is used.

Preferred embodiments of the present invention have been described above in detail, but the present invention is not limited to any specific embodiments, and various modifications and alterations are possible within the scope of the present invention as disclosed within the claims. For example, the characteristic structures illustrated inFIG.1toFIG.6may be combined as desired.

An optical apparatus according to a second embodiment includes the polarizer of the aspect described above. Examples of the optical apparatus include liquid crystal projectors, head-up displays, and digital cameras and the like. The polarizer of the aspect described above exhibits high transmittance of light polarized along the transmission axis direction, and low reflectance of light polarized along the absorption axis direction. Accordingly, the polarizer can be used in a variety of applications. Further, the polarizer has a high heat dissipation substrate, and can therefore be used particularly favorably in liquid crystal projectors and head-up displays and the like which require superior heat resistance.

In those cases where the optical apparatus contains a plurality of polarizers, at least one of the plurality of polarizers is preferably the polarizer of the aspect described above. For example, in the case where the optical apparatus is a liquid crystal projector, polarizers are disposed at the incident side and the exit side of the liquid crystal panel. The polarizer of the aspect described above may be used for one of these polarizers.

EXAMPLES

In Example 1, changes in the optical characteristics were measured for the polarizer101illustrated inFIG.2as the thickness of the dielectric film20and the thickness of the first dielectric layer41were varied. The optical characteristics of the various polarizers were investigated by electromagnetic field simulation using Rigorous Coupled Wave Analysis (RCWA). A grating simulator GSolver V51 manufactured by Grating Solver Development Co. was used for the simulations.

The specific structure of the polarizer used in Example 1 is described below.

Thickness of reflective layer50: 250 nm

Thickness of second dielectric layer61: 5 nm

Thickness of absorption layer62: 25 nm

Thickness of third dielectric layer63: 15 nm

Pitch P between adjacent projections30: 141 nm

Width of projections30in x direction: 35 nm

The measurement results for Example 1 are shown inFIG.7andFIG.8. The transmission axis reflectance represents the value for incident light having a peak wavelength of 550 nm and a wavelength region of 430 to 680 nm. As illustrated inFIG.8, as the thickness of the dielectric film20approached 100 nm, the transmission axis reflectance decreased. Further, as illustrated inFIG.7, in those cases where the thickness of the first dielectric layer41was within a range from at least 20% to not more than 80% of the thickness of the dielectric film20, the transmission axis reflectance tended to decrease more than the reference example.

In Example 2, changes in the optical characteristics were measured for the polarizer104illustrated inFIG.5as the thickness of the dielectric film20and the thickness of the first dielectric layer44were varied.

The specific structure of the polarizer used in Example 2 is described below.

Items that are the same as Example 1 are omitted.

Internal angle θ between bottom surface44band side surfaces44s: one of 0°, 21°, 37°, 49° or 57° (with the angle changing depending on the thickness of the first dielectric layer44)

The measurement results for Example 2 are shown inFIG.9andFIG.10. The transmission axis reflectance represents the value for incident light having a peak wavelength of 550 nm and a wavelength region of 430 to 680 nm. In Example 2, because the cross-sectional shape of the first dielectric layer was a trapezoidal shape, reflectance was able to be better suppressed than in Example 1. Further, the optical characteristics changed as a result of varying the thicknesses of the first dielectric layer44and the dielectric film20.

In Example 3, changes in the optical characteristics were measured for the polarizer100illustrated inFIG.1as the thickness of the first dielectric layer40was varied.

The specific structure of the polarizer used in Example 3 is described below. Items that are the same as Example 1 are omitted.

Thickness of dielectric film20: 80 nm

Internal angle θ between bottom surface40band side surfaces40s:70° or 80°

The measurement results for Example 3 are shown inFIG.11. The transmission axis reflectance represents the value for incident light having a peak wavelength of 550 nm and a wavelength region of 430 to 680 nm. In Example 3, because the cross-sectional shape of the first dielectric layer was a trapezoidal shape, reflectance was able to be better suppressed than in Example 1. In both the case where the internal angle θ was 70° and the case where the internal angle θ was 80°, the transmission axis reflectance was lower than 1%.

Furthermore,FIG.12is a graph obtained by combining the results of Example 1 to Example 3 and determining the relationship with the internal angle θ. The transmission axis reflectance values represent values for incident light having a peak wavelength of 550 nm and a wavelength region of 430 to 680 nm.

The results for 0=0° correspond with the reference example having no first dielectric layer. The results for 0=21°, 37°, 49° and 57° correspond with Example 2. The results for 0=70° and 80° correspond with Example 3. The results for 0=90° correspond with Example 1. The data for the various thicknesses of the dielectric film20were plotted. The thickness of the dielectric film20was, in order, 20 nm, 40 nm, 100 nm, 60 nm or 80 nm.

Comparative Example 1

FIG.13is a cross-sectional schematic view of a polarizer110according to Comparative Example 1. Comparative Example 1 has a substrate11, a reflective layer50and a functional layer60. The substrate11has protrusions11A in the z direction corresponding with the positions of the projections30. Rectangular recesses are formed in the spaces between these protrusions11A. The reflective layer50and the functional layer60are the same as those of the polarizer101illustrated inFIG.2. Comparative Example 1 corresponds with the case where the dielectric film20and the first dielectric layer40of the polarizer101illustrated inFIG.2are formed from the same sapphire as the substrate11.

The specific structure of the polarizer used in Comparative Example 1 is described below. Items that are the same as Example 1 are omitted.

First dielectric layer41: none

FIG.14illustrates the results for measuring the transmission axis reflectance for the polarizer110of Comparative Example 1.

Comparative Example 2

FIG.15is a cross-sectional schematic view of a polarizer111according to Comparative Example 2. Comparative Example 2 has a substrate11, a reflective layer50and a functional layer60. The substrate11has protrusions11B that expand in width in the x direction from the projections30. Triangular recesses are formed in the spaces between these protrusions11B. The reflective layer50and the functional layer60are the same as those of the polarizer104illustrated inFIG.5. Comparative Example 2 corresponds with the case where the dielectric film20and the first dielectric layer44of the polarizer104illustrated inFIG.5are formed from the same sapphire as the substrate11.

The specific structure of the polarizer used in Comparative Example 2 is described below. Items that are the same as Example 2 are omitted.

First dielectric layer44: none

FIG.16illustrates the results for measuring the transmission axis reflectance for the polarizer111of Comparative Example 2.

Further,FIG.17illustrates the relationship between the heights of the protrusions11A and11B and the transmission axis reflectance. The transmission axis reflectance represents the value for incident light having a peak wavelength of 550 nm and a wavelength region of 430 to 680 nm. As illustrated inFIG.17, in Comparative Example 1, the transmission axis reflectance tended to increase as the height of the protrusions11A increased. In contrast, as illustrated inFIG.17, in Comparative Example 2, the transmission axis reflectance improved when the height of the protrusions11B was increased to 160 nm or greater. However, carving out the sapphire substrate to a depth of 160 nm to form the protrusions11B is difficult.

Finally, the optical characteristics were compared for the case in Example 1 where the thickness of the dielectric film20was 100 nm and the thickness of the first dielectric layer41was 40 nm, and the case in Comparative Example 2 where the height of the protrusions11B was 60 nm. The results are shown inFIG.18toFIG.21.FIG.18illustrates the results of measuring the transmission axis transmittance.FIG.19illustrates the results of measuring the transmission axis reflectance.FIG.20illustrates the results of measuring the absorption axis transmittance.FIG.21illustrates the results of measuring the absorption axis reflectance.

As illustrated inFIG.18andFIG.19, the structure of Example 1 exhibits improved transmission axis transmittance and reduced transmission axis reflectance compared with Comparative Example 2. On the other hand, as illustrated inFIG.20andFIG.21, there was little difference in the absorption axis transmission and reflectance.