JP2009085974A - Polarizing element and method for fabricating the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small polarizing element suitable for mass production which can be fabricated through a simple method and employed in an optical pickup device or the like. <P>SOLUTION: The polarizing element comprises a lattice arranged on a substrate at a predetermined period Λ and satisfies a relation Λ(cosθ<SB>0</SB>)<λ, where λ is the wavelength of light and θ<SB>0</SB>is the incident angle to the lattice surface. On a protrusion (111) of the lattice, a layer (113) composed of a material having a refractive index higher than that of the protrusion is stacked and that layer is not stacked on the part other than the protrusion of the lattice. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polarization separation element, a polarization element including a quarter-wave plate, and a manufacturing method thereof. In particular, the present invention relates to a polarizing element suitable for mass production that can be used at two wavelengths for a compact disk and a digital versatile disk, and a manufacturing method thereof.

  In general, there are various types of small polarizing elements used in optical pickup devices and the like. (1) Glass element including multilayer film represented by cube type (for example, Patent Document 1), (2) Sub-wavelength grating (lattice having a period equal to or less than the wavelength of light) element utilizing structural birefringence, (3) an element using a diffraction grating made of an organic birefringent material (for example, Patent Document 2), (4) an element using a diffraction grating and a liquid crystal polymer material (for example, Patent Document 3), and the like.

  Regarding (1), in the example of Patent Document 1, a polarizing beam splitter is created by using a multilayer film to bring the incident angle of light closer to the Brewster angle (incident angle at which the reflectance of the P wave is 0). . In such a method, in order to form a multilayer film, a plurality of vapor deposition steps are required, the productivity is poor, and a plurality of complicated functions cannot be provided.

  (2) is a method of providing polarization dependence by creating a structure having a grating period that is approximately equal to or less than the wavelength. In this method, a large birefringence can be provided by the sub-wavelength grating, but in most cases, a grating structure with a high aspect ratio is required and the production is difficult.

  Regarding (3), in the example of Patent Document 2, an isotropic material is filled in a concave portion of a grating formed on a birefringent material. However, in any case, an etching process is required for forming the grating, so that mass productivity is poor, and in the case of a diffraction grating composed of both sides, a complicated process is required such that alignment is difficult.

  (4) is a method of creating a polarizing element by filling a concave portion of a diffraction grating with a material containing a liquid crystal polymer or forming a film with this material. This utilizes the fact that the optical properties have polarization dependence depending on the orientation direction of the liquid crystal polymer. Therefore, in order to obtain a predetermined performance, the alignment direction of the liquid crystal must be aligned, and an alignment process such as a rubbing process is required for each element, and the mass productivity is not excellent.

  Thus, any type of polarizing element is complicated in manufacturing method and is not suitable for mass production.

  Here, the principle of a polarizing element using a diffraction grating will be described. FIG. 11 is a diagram for explaining the principle of the beam separation function depending on the polarization by the diffraction grating. In FIG. 11, light travels from a medium having a refractive index of n1 to a medium of n2. A lattice having a period Λ is formed at the boundary.

  Light includes polarized light called TE polarized light (s polarized light) and TM polarized light (p polarized light). When light is incident on the diffraction grating, the polarized light in the direction in which the electric field vibrates in parallel with the groove of the grating is called TE polarized light, and the polarized light in the direction in which the electric field vibrates perpendicularly (in parallel with the magnetic field) is called TM polarized light. Call.

When the diffraction grating has the wavelength λ and the incident angle θ ₀ and period Λ are satisfied,
Λ cos θ ₀ <λ (1)
The diffraction grating structure is recognized as traveling in a thin film structure represented by an effective refractive index n _eff for light. At this time, the effective refractive index n _eff varies depending on the polarization direction of the incident light, and is expressed by the following equation in the first approximation.

ここで、fは周期Λに対する図１１における山側部分（凸部）の比を表す。
上式からfが0、1以外では、各々の偏光に対する有効屈折率の値が異なっていることがわかる。

Here, f represents the ratio of the peak portion (convex portion) in FIG. 11 to the period Λ.
From the above equation, it can be seen that when f is other than 0 and 1, the effective refractive index values for the respective polarized light are different.

  The physical meaning of the difference in effective refractive index depending on each polarization state is regarded as a shield that causes scattering or the like when light passes through a structure extremely smaller than the wavelength of light. As a result, energy loss occurs when passing through the shield, and it can be considered that the effect appears as an effective refractive index.

を満たすと、その偏光方向をもつ入射光は有効屈折率n_effのもつ薄膜層を通過できなくなる。この状態は図１１において、有効屈折率n_effのもつ薄膜層での屈折角度θ1がほぼ90°に達しており、n2側への層に光が移動できない状態に相当する。結果的に、入射したエネルギーの発散先として、反射光が生じることとなる。 Under this condition, the effective refractive index n _eff = n _TE or n _eff = n _TM (where n _TE ≠ n _TM ) for each polarization component is a relational expression for the refraction of light traveling through different media (Snell The following equation transformed from

When the above condition is satisfied, incident light having the polarization direction cannot pass through the thin film layer having the effective refractive index n _eff . This state corresponds to a state in FIG. 11 in which the refraction angle θ1 in the thin film layer having the effective refractive index n _eff reaches approximately 90 °, and light cannot move to the layer toward the n2 side. As a result, reflected light is generated as a destination of incident energy.

As described above, when the expression (4) is established by the effect of the effective refractive index n _eff in which light in one of the polarization directions is recognized from the grating structure, a polarizing element with a minute period is realized.

  As described above, by setting the period in the grating portion to be equal to or less than the wavelength, the light expressed as an electromagnetic wave is recognized as a diffraction effect expressed as a superposition of waves because no diffracted wave is generated as it progresses. Disappear. The grating portion is regarded as a target of refractive index change with respect to the wave progression, and the effect on the electromagnetic wave gives the same property as the progression in a material having a virtual refractive index. As a result, an effect similar to that of the thin film layer is brought about in a specific wavelength band. A method of assuming that the grating portion is a material having a virtual refractive index is called an effective refractive index method. For example, Non-Patent Document 1 describes an equation for obtaining an effective refractive index from a lattice shape. In the effective refractive index layer, the value of the effective refractive index is determined by the ratio of the peak portion to the period of the grating portion. Further, the value of the effective refractive index recognized by the incident light varies depending on the polarization direction. Therefore, by using a grating having a period equal to or shorter than the wavelength, the diffraction grating can have a separation function depending on polarization.

Thus, although the sub-wavelength grating functions as a polarizing element, as described above, a sub-wavelength grating as a polarizing element that has a simple manufacturing method and is suitable for mass production has not been developed.
Japanese Patent Laid-Open No. 2004-117760 JP 2003-43254 A Japanese Patent Laid-Open No. 10-265531 Wanji Yu et al., “Polarization-Multiplexed Diffractive Optical Elements Fabricated by Subwavelength Structures”, Vol.41, Issue 1 / January 2002 / Appl.Opt.96-100 (2002)

  Under the background described above, there is a need for a small polarizing element that is used in an optical pickup device and the like that is simple in manufacturing method and suitable for mass production.

A polarizing element according to the present invention includes a grating arranged on a substrate with a predetermined period Λ, where the wavelength of light is λ and the incident angle with respect to the grating surface is θ _0.

Λ (cos θ ₀ ) <λ

It is. A layer made of a material having a refractive index higher than the refractive index of the convex portion is laminated on the convex portion of the lattice, and a layer made of the high refractive index material is not laminated on a portion other than the convex portion of the lattice.

  In the present invention, a layer made of a material having a refractive index higher than the refractive index of the convex portion is laminated on the convex portion of the grating, and a layer made of the high refractive index material is laminated on the portion other than the convex portion of the lattice. Therefore, the ratio (aspect ratio) of the grating depth to the width of the grating protrusion can be reduced.

The method of manufacturing a polarizing element according to the present invention is a grating arranged on a substrate with a predetermined period Λ, where the wavelength of light is λ and the incident angle with respect to the grating plane is θ _0.

Λ (cos θ ₀ ) <λ

Forming a grating with a mold, and laminating a layer made of a material having a refractive index higher than the refractive index of the material constituting the convex part on the convex part of the grating. The layer made of the material having the high refractive index is not laminated on the portion other than the convex portion of the lattice.

  In the present invention, a layer made of a material having a refractive index higher than the refractive index of the convex portion is laminated on the convex portion of the grating, and a layer made of the high refractive index material is laminated on the portion other than the convex portion of the lattice. Therefore, the ratio (aspect ratio) of the grating depth to the width of the grating protrusion can be reduced.

  FIG. 1 is a diagram illustrating a shape of a polarizing element according to an embodiment of the present invention. A polarizing element consists of a grating | lattice arrange | positioned on a board | substrate. The convex portion 111 of the lattice has a flat head, and a layer 113 made of a material having a refractive index higher than that of the convex portion is laminated on the flat head. A layer made of the material having a high refractive index is not laminated on a portion other than the flat head portion of the lattice.

  The grating period, the height of the grating protrusion (lattice depth), the thickness of the layer made of a material having a high refractive index, and the width of the grating protrusion are represented by a, b, c, and d, respectively.

When the incident angle of light incident on the diffraction grating is θ ₀ and the wavelength is λ, the grating period a needs to satisfy the following conditions.

a (cos θ ₀ ) <λ

When the light is incident perpendicularly to the diffraction grating, the incident angle is 0 degree.

a <λ

It becomes. Therefore, a for visible light is approximately 0.4 micrometers or less.

  The ratio of the grating width d to the grating period a is called the duty ratio.

  In FIG. 1, the cross section of the lattice convex portion 111 is rectangular, but may be trapezoidal, for example. When the cross section of the lattice convex portion 111 is a trapezoid, the lattice width d may be an average value of the length of the upper base and the length of the lower base of the trapezoid.

  The lattice may be formed of a synthetic resin such as acrylic or polyolefin. The refractive index of the grating protrusion is in the range of 1.48 to 1.62.

The layer 113 made of a material having a refractive index higher than that of the convex portion may be formed of titanium oxide (for example, Ti ₂ O ₅ ). The refractive index of the layer 113 is in the range of 2.0 to 2.6. A layer made of a material having a refractive index higher than the refractive index of the convex portion is laminated on the head of the lattice convex portion so that the ratio (aspect ratio) of the grating depth b to the width d of the lattice convex portion is as much as possible. This is to make it smaller. The reason for reducing the aspect ratio will be described later. It is preferable that the difference between the high refractive index and the refractive index of the grating protrusion is 0.65 or more.

  FIG. 2 is a flowchart illustrating a method for manufacturing a polarizing element according to an embodiment of the present invention. Moreover, FIG. 3 is a figure for demonstrating the manufacturing method of a polarizing element by one Embodiment of this invention.

In step S010 in FIG. 2, the lattice mold is processed. The lattice mold is made of metal, glass, or the like, such as Ni, NiP, Si, WC, SiC, and SiO ₂ . For processing, a fine processing apparatus such as an electronic drawing apparatus or an etching apparatus is used. (A) of FIG. 3 is a figure which shows the structure of the metal mold | die 121 after a process.

  In step S020 in FIG. 2, a lattice is formed by a mold. FIG. 3B is a diagram showing the relationship between the mold 121 and the lattice material when the shape of the mold 121 is transferred to the lattice material to form the lattice. Examples of the transfer method include an injection molding method, a melt retransfer technique, and an imprint method. In order to perform transfer by the above method, it is preferable that the ratio (aspect ratio) of the lattice depth b to the width d of the lattice convex portion is 3.0 or less. When the aspect ratio is larger than 3, it is difficult to perform transfer appropriately by the above method.

  In step S030 in FIG. 2, the grid is released from the mold. FIG. 3C is a diagram showing the shape of the grating 111 after release.

  In step S040 of FIG. 2, a material having a refractive index higher than the refractive index of the material forming the grating is deposited on the heads of the convex portions of the grating. The material should not be deposited in the valleys of the grid. The width of the valley of the lattice is (ad). If the ratio of the lattice depth b to the valley width (ad) is 1.0 or more, no material is deposited on the valley of the lattice. Examples of the vapor deposition method include a sputtering vapor deposition method and a vacuum vapor deposition method. (D) of FIG. 3 is a figure which shows the shape of the grating | lattice after vapor deposition.

  FIG. 4 is a diagram illustrating a configuration of a polarization beam splitter according to an embodiment of the present invention. In the polarization separation element, two types of grating groups 101 and 103 are alternately arranged. Since the effective refractive indexes for the TE wave and TM wave in the two types of grating groups 101 and 103 are generally different, the two types of grating groups 101 and 103 can be regarded as materials having birefringence. Further, the effective refractive index changes depending on the lattice shape. Therefore, the grating shapes of the two types of grating groups 101 and 103 are changed so that the effective refractive indexes of the two types of grating groups 101 and 103 are substantially equal to a certain polarization state.

  For example, an array of two types of lattice groups 101 and 103 is regarded as one virtual lattice, and a TM wave for the virtual lattice is considered. The effective refractive indexes of the two types of grating groups 101 and 103 are made substantially equal to this polarization state. In this case, for the TM wave, the polarization separation element is equivalent to a uniform material, and the TM wave is transmitted through the polarization separation element. Since the effective refractive indexes of the two types of grating groups 101 and 103 are different for the TE wave, the polarization separation element is equal to the grating, and the TE wave is diffracted by the polarization separation element. At this time, if the depth b of the diffraction grating and the deposited film thickness c are designed so that there is no transmission of 0th-order light, a polarization separation element that transmits TM polarized light and diffracts TE polarized light can be configured. .

  The configuration of the polarization separation element of this embodiment will be described with reference to FIG.

  The polarization separation element is provided with two types of grating groups 101 and 103 on a substrate. Arbitrary XY orthogonal coordinates are defined on the substrate surface. In the first lattice group 101, lattice convex portions extending in the Y direction are arranged on the substrate with a period of 0.25 micrometers in the X direction. In the second grating group 103, grating convex portions extending in the X direction are arranged on the substrate with a period of 0.4 micrometers in the Y direction. The first grating group 101 and the second grating group 103 are alternately arranged with a period of 3.7 micrometers in the X direction. The periodic arrangement of the first lattice group 101 and the second lattice group 103 is such that the first lattice group 101 is a lattice convex portion extending in the Y direction, and the second lattice group 103 is a portion other than the lattice convex portion. Can be regarded as a virtual lattice.

Assuming that the wavelength of the incident light is λ and the incident angle with respect to the grating plane is θ ₀ , the grating period of the first grating group 101 and the grating period of the second grating group 103 are

λ / cos θ ₀

To be smaller. For example, the wavelength λ of the incident light is assumed to be a wavelength of 0.66 micrometers for a digital versatile disk or a wavelength of 0.785 micrometers for a compact disk. If the incident angle θ ₀ with respect to the lattice plane is 0 degree,

λ / cos θ ₀

Becomes 0.66 micrometers or 0.785 micrometers. The grating period 0.25 micrometers of the first grating group 101 and the grating period 0.4 micrometers of the second grating group 103 are smaller than 0.66 micrometers or 0.785 micrometers.

  Table 1 is a table | surface which shows the parameter of the polarization splitting element of this embodiment. The right four columns show the effective refractive index of the deposited layer in each lattice group. In Table 1, TE polarization and TM polarization are the polarization directions for a virtual grating with a period of 3.7 micrometers. In addition, the TM polarized light is incident from the surface of the substrate having the grating, and the TE polarized light is incident from the surface of the substrate having no grating.

  In the polarization separation element of this embodiment, the grating depth (lattice height) a is 0.46 micrometers, and the thickness b of the layer (deposition film layer) made of a material having a high refractive index is 0.51 micrometers. It was. Here, the lattice edge angle is an angle indicated by α in FIG.

  In the first lattice group, the ratio (aspect ratio) of the lattice depth b to the width d of the lattice convex portion is 2.3. Even in the second lattice group, the aspect ratio is 2.3. Thus, since the aspect ratio of the lattice convex portion is 3 or less, the production of the lattice is relatively easy. On the other hand, when the vapor deposition film layer is not provided, the aspect ratio of the lattice convex portion is 9 or more, and it becomes difficult to manufacture the lattice.

  The refractive index of the first grating group and that of the second grating group are substantially equal to the TM wave of light having a wavelength of 0.66 micrometers for a digital versatile disk and 0.785 micrometers for a compact disk. Indicates. Therefore, the TM wave passes through the polarization separation element.

  The first and second grating groups have different refractive indexes for the TE wave of light having a wavelength of 0.66 micrometers for digital versatile disks and 0.785 micrometers for compact disks. Show. Therefore, the TE wave is diffracted by the virtual grating.

  FIG. 5 is a diagram showing a change in transmission efficiency of the TM wave and the TE wave when the grating depth a is changed by about 10% from the reference value.

  FIG. 6 is a diagram showing a change in transmission efficiency of the TM wave and the TE wave when the thickness b of the layer made of a material having a high refractive index is changed by about 10 percent from the reference value.

  FIG. 7 is a diagram showing a change in transmission efficiency of the TM wave and the TE wave when the incident angle is changed from 0 degree to 7 degrees.

  From FIG. 5 to FIG. 7, the TM wave of light having a wavelength of 0.66 micrometers for a digital versatile disk and a wavelength of 0.785 micrometers for a compact disk transmits almost 90%, and is used for a digital versatile disk. The TE wave of light having a wavelength of 0.66 micrometers and a compact disk wavelength of 0.785 micrometers does not transmit approximately 90 to 95 percent. In addition, when the incident angle is in the range from 0 degree to 7 degrees, the incident angle dependency of the transmittance is hardly seen.

  FIG. 8 is a diagram illustrating a configuration of a quarter-wave plate according to an embodiment of the present invention. The quarter-wave plate is composed of a grating having the parameters shown in Table 2. The right four columns of Table 2 show the effective refractive index in each part of the grating. As shown in FIG. 8, the direction of the grating is inclined 45 degrees with respect to the polarization direction of the incident light.

  The ratio (aspect ratio) of the grating depth b to the width d of the grating protrusions is 1.0. Thus, since the aspect ratio of the lattice convex portion is 3 or less, the production of the lattice is relatively easy. On the other hand, when the vapor deposition film layer is not provided, the aspect ratio of the lattice convex portion is 8 or more, and it becomes difficult to manufacture the lattice.

  FIG. 9 is a diagram illustrating a phase change of incident light with respect to a thickness c of a layer (deposition film) made of a material having a high refractive index. If c is 0.375 micrometers, the phase change is almost 90 degrees with respect to light having a wavelength of 0.66 micrometers for a digital versatile disk and 0.785 micrometers for a compact disk.

  FIG. 10 is a diagram showing a configuration of an optical pickup system using the polarizing element of the present invention. In FIG. 10, the beam emitted from the laser light source 201 passes through the polarization separation element 203 as 0th-order diffracted light, and reaches the disc (DVD or CD) 209 through the quarter-wave plate 205 and the objective lens 207. The beam reflected by the disk 209 returns to the polarization separation element 203 via the objective lens 207 and the quarter wavelength plate 205. The beam is diffracted as first-order diffracted light by the polarization separation element 203 and detected by the photodiodes 211 and 213.

  The light emitted from the laser light source 201 is TM-polarized (p-polarized) and passes through the polarization separation element 203 as zero-order diffracted light with an efficiency of approximately 90%. This light is converted into circularly polarized light by the quarter wavelength plate 205. When this light is reflected by the disk 209 and passes through the quarter-wave plate 205 in the reverse direction, it becomes TE-polarized (s-polarized) light. In the polarization separation element 203, the TE polarized light is about 30% for the first order diffracted light, about 30% for the −1st order diffracted light, 10% or less for the 0th order diffracted light, and the rest becomes higher order diffracted light.

  Here, the laser light source 201 may be a two-wavelength semiconductor laser unit. The polarization separation element 203 and the quarter-wave plate 205 according to the embodiment of the present invention have substantially the same characteristics as those of the digital versatile disk wavelength 0.66 micrometers and the compact disk wavelength 0.785 micrometers. Can be used for both wavelengths.

It is a figure which shows the shape of the polarizing element by one Embodiment of this invention. 3 is a flowchart illustrating a method for manufacturing a polarizing element according to an embodiment of the present invention. It is a figure for demonstrating the manufacturing method of a polarizing element by one Embodiment of this invention. It is a figure which shows the structure of the polarization separation element by one Embodiment of this invention. It is a figure which shows the change of the transmission efficiency of TM wave and TE wave when changing the grating | lattice depth a about 10 percent from a reference value. It is a figure which shows the change of the transmission efficiency of TM wave and TE wave when thickness b of the layer which consists of material of a high refractive index is changed about 10 percent from the reference value. It is a figure which shows the change of the transmission efficiency of TM wave and TE wave when an incident angle is changed from 0 degree to 7 degrees. It is a figure which shows the structure of the quarter wavelength plate by one Embodiment of this invention. It is a figure which shows the phase change of the incident light with respect to the thickness b of the layer which consists of material with a high refractive index. It is a figure which shows the structure of the optical pick-up system using the polarizing element of this invention. It is a figure explaining the principle of the beam separation function depending on the polarization by a diffraction grating.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 ... 1st grating | lattice group, 103 ... 2nd grating | lattice group, 111 ... Convex part of a grating | lattice, 113 ... Layer which consists of material of refractive index higher than the refractive index of a convex part

Claims (16)

Provided with a grating arranged on the substrate with a predetermined period Λ, the wavelength of light is λ, and the incident angle to the grating surface is θ ₀

Λ (cos θ ₀ ) <λ

A layer made of a material having a refractive index higher than the refractive index of the convex portion is laminated on the convex portion of the grating, and a layer made of the material having a high refractive index is laminated on a portion other than the convex portion of the lattice. No polarizing element.   The polarizing element according to claim 1, wherein the grating is made of a synthetic resin.   The polarizing element according to claim 1, wherein the high refractive index material is titanium oxide.   The polarizing element according to claim 1, wherein the layer made of the material having a high refractive index is laminated by vapor deposition.   The polarizing element according to claim 1, wherein the ratio of the grating depth to the width of the grating protrusion is 3 or less.   The polarizing element according to any one of claims 1 to 5, wherein a ratio of the lattice depth to the width of the lattice valley portion is 1.0 or more.   The polarizing element according to any one of claims 1 to 6, wherein a difference between the high refractive index and the refractive index of the grating convex portion is 0.65 or more. A grating arranged on a substrate with a predetermined period Λ, where the wavelength of light is λ and the incident angle with respect to the grating surface is θ ₀

Λ (cos θ ₀ ) <λ

A step of forming a lattice with a mold,
Laminating a layer made of a material having a refractive index higher than the refractive index of the material constituting the convex portion on the convex portion of the lattice, and the material other than the convex portion of the lattice has the high refractive index material The manufacturing method of the polarizing element which does not laminate | stack the layer which consists of.   The manufacturing method according to claim 8, wherein the lattice is made of a synthetic resin.   The method according to claim 8 or 9, wherein the material having a high refractive index is titanium oxide.   The manufacturing method according to claim 8, wherein the layer made of the material having a high refractive index is laminated by vapor deposition.   The manufacturing method according to claim 8, wherein the ratio of the lattice depth to the width of the lattice convex portion is 3 or less.   The manufacturing method according to claim 8, wherein a ratio of the lattice depth to the width of the lattice valley is 1.0 or more.   The manufacturing method according to any one of claims 8 to 13, wherein a difference between the high refractive index and the refractive index of the grating convex portion is 0.65 or more. When arbitrary XY orthogonal coordinates are defined on the substrate surface, a first lattice group in which lattice convex portions extending in the Y direction are arranged in the X direction with a first period Λ ₁ on the substrate, and extending in the X direction. A polarization separation element in which a second grating group in which grating convex portions are arranged in the Y direction with a second period Λ ₂ and alternately arranged in a _third period Λ ₃ in the X direction, the first grating group And the second grating group comprises the polarizing element according to claim 1, and the first grating group with respect to a predetermined polarized wave of light whose traveling direction is in a plane including the X axis. And the effective refractive index of the second grating group are substantially equal, and the effective refractive indexes of the first and second grating groups are different for different polarized waves. 1 of grating group and the second grating group is configured to function as a virtual grid, as ₀ the incident angle θ with respect to the virtual lattice plane, the third period lambda ₃

λ / cos θ ₀

Larger polarization separation element. The polarization separating element according to claim 15, which can be used at two wavelengths for a compact disk and a digital versatile disk.

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