JPH11133229A - Polarizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a polarizer having high reliability and a broad absorption wavelength characteristic by laminating plural dielectric layers which are dispersed with metallic particles exhibiting light absorption anisotropy and very in refractive indices from each other on one main surface of a substrate having translucency. SOLUTION: The polarizer 1 is provided with polarizing layers 3 on at least one main surface of a dielectric substrate 2, such as glass having the translucency. The polarizing layers 3 are constituted by alternately laminating a plurality of metallic particle layers 4 also described as island-shaped metallic thin-film layers in which the many metallic particles 4a having the light absorption anisotropy are dispersed to a laminar form and the dielectric layers 5 having the translucency on the substrate 2. Unit laminates P are laminated in >=1 layer in such a manner that the compsns. and refractive indices of the dielectric layers 5 existing above and below vary. The refractive indices of the respective dielectric layers 5 of the case the unit laminates P to be cycled in particular are a tripolar structure are made to be at the optimized relation. As a result, the polarizer having the small insertion loss and excellent characteristics in the broad wavelength band of incident light is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarizer in which metal particles having shape anisotropy are dispersed in a dielectric, and more particularly, to a polarizer incorporated in an optical isolator used for optical communication. The present invention relates to a polarizer capable of performing a plurality of optical signal data processes.

[0002]

2. Description of the Related Art Polarizers are used to extract light polarized in a specific direction, and various structures of polarizers have been studied. Examples of this type of polarizer include a Glan-Thompson prism combining birefringent crystals, a rutile crystal having a large birefringence, and a polarizing film made by stretching a dichroic polymer material in the polarization direction. , A laminated polarizer (Ramipole) formed by alternately laminating dielectric layers and metal thin-film layers, and a metal-dispersed polarizer (Polar core) in which silver colloid is precipitated in borosilicate glass and stretched in the polarization direction. ), Island-shaped metal thin-film polarizers in which island-shaped metal particles are alternately laminated with a dielectric film, dispersed in the dielectric film, and stretched in the polarization direction are known.

[0003] These polarizers are used in sunglasses, filters for liquid crystal displays, filters for photographs, ski goggles, headlights for automobiles, anti-glare filters for displays, etc., as well as, for example, optical pickups, optical sensors, optical isolators. In recent years, especially in various fields such as optical recording and optical communication, the need for a small, high-performance and inexpensive polarizer has been increasing.

At present, a polar core of a metal-dispersed polarizer obtained by depositing silver colloid in borosilicate glass and extending in the polarization direction has been put to practical use, and this polarizer is most often used in the field of optical communication. Also known as This polarizer heat-treats a glass base having silver and halogen to precipitate silver halide grains, stretches the glass base under heating, and stretches the silver halide grains into a spheroid to obtain a halogen. Silver oxide particles are given anisotropy. Then, it is heated in a reducing atmosphere to reduce silver halide to metallic silver (for example, Japanese Patent Publication No. 2-4061).
No. 9, US Pat. Nos. 4,486,213 and 4,479,819).

However, in this polarizer, the aspect ratio of the silver particles (which indicates the degree of anisotropy of the particles and is usually represented by the length in the major axis direction / the length in the minor axis direction) is not sufficient. It is easy to be uniform. This is because it is difficult to precipitate silver particles having a uniform length in the short axis direction and the long axis direction.
Further, since it is difficult to reduce silver halide to the inside of the glass, opaque unreduced silver halide remains.
Further, the glass surface shrinks in the course of the reduction of silver halide, so that the glass surface tends to be porous and the long-term stability tends to decrease.

In order to solve such a problem, it has been proposed to manufacture a polarizer by using a thin film forming process such as a vacuum evaporation or sputtering method (1990 IEICE Autumn Meeting, lecture) Proceedings C-212).
In this proposal of an island-shaped metal thin film polarizer, an island-shaped metal thin film layer is provided on a dielectric substrate such as glass by vacuum evaporation, and a dielectric layer such as glass is laminated thereon by a sputtering method or the like. Then, several island-like metal thin film layers and dielectric layers are alternately formed. Next, the substrate is stretched under heating to give the metal particles of the island-shaped metal thin film layer anisotropy. In this way,
Each metal particle in the island-shaped metal thin film layer is extended in the elongating direction, becomes a spheroidal shape, and has polarization performance.

However, in the polarizer using the above-mentioned thin film forming process, since the refractive index of each dielectric layer is the same,
The wavelength characteristics of the metal particles were narrow, the wavelength characteristics could not be controlled by broadening the resonance absorption wavelength, and an optical signal having a plurality of wavelengths could not be processed by one polarizer. Further, the insertion loss for a wide range of light wavelengths is not satisfactory, and furthermore, the polarizer for an optical communication device does not have satisfactory quality and reliability.

The present invention has been devised in view of the above-mentioned problems, and has as its object to provide an excellent polarizer having high reliability and wide absorption wavelength characteristics.

[0009]

In order to solve the above-mentioned problems, a polarizer of the present invention has a structure in which metal particles exhibiting light absorption anisotropy are dispersed on at least one principal surface of a light-transmitting substrate. A plurality of dielectric layers having different refractive indexes are laminated.

Further, k, k with respect to the laminating direction of the substrate.
+1, k + 2nd dielectric layer refractive index _nk , _{nk + 1} , n
_{k + 2} has a region satisfying the following expression.

_{Nk + 2} <ns < _nk < _{nk + 1} (where k = 3h-2; h is a natural number, ns is the refractive index of the substrate) Further, each dielectric layer has an aspect dispersed therein. The ratio is 3
It is characterized in that the number density of up to 30 metal particles is ² to 37 particles / μm ² .

Here, the metal particles are distributed in a plane, and the number density is an average number density measured at a large number of locations. In particular, a glass substrate is most suitable for the substrate having a light-transmitting property, and is made of, for example, quartz glass, borosilicate glass, silicate glass, or the like. The metal particles are A
Noble metal elements such as u, Ag, Pt, and other metal elements such as C
The main component is at least one of u, Fe, Ni, Cr, Al and W.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1 which explains an embodiment of the present invention with reference to the drawings, a polarizer 1 is made of a dielectric substrate such as a light-transmitting glass (hereinafter simply referred to as a substrate). 2, a polarizing layer 3 is provided on at least one principal surface of the substrate 2, and the polarizing layer 3 has a large number of metal particles 4a having light absorption anisotropy (having shape anisotropy) dispersed on a substrate 2. A plurality of metal particle layers 4, which may be called layered island-like metal thin film layers, and dielectric layers 5 having translucency are alternately laminated. Further, as shown in FIG. 2, one or more unit laminates P in which the compositions and the refractive indices of the upper and lower dielectric layers are different are laminated.

Note that having translucency means being transparent to the wavelength used. The number density of the metal particles is a density in the direction of the substrate surface S, and is a density including a major axis of at least one metal particle 4a and measured when cut along a plane parallel to the substrate surface S.

The polarizer 1 is manufactured, for example, as follows. First, a series of steps B to D are repeatedly performed in the following step A, and then the following step E is performed. However, in order to change the refractive index of each dielectric layer, for example, the composition of the dielectric layer is appropriately changed in step D.

A: a step of preparing a substrate 2 made of glass; B: a step of applying metal fine particles to the substrate 2 by a thin film forming method; C: heating the metal fine particles at a temperature lower than the softening point of the substrate 2 A step of forming a large number of metal particles 4a by agglomeration; D: a step of applying a dielectric layer 5 made of glass on the many metal particles 4a by a thin film forming method; E: thermoplastic deformation of the substrate 2 in a predetermined direction. And giving a shape anisotropy to many metal particles 4a and orienting them.

In the polarizer 1, the number density of the metal particles 4a in each dielectric layer 5 is ² to 37 / μm ² ,
In addition, since the refractive index of each dielectric layer constituting the unit laminate is made different from each other, there are a plurality of absorption wavelength characteristics corresponding to each dielectric layer, and light in an extremely wide wavelength band is absorbed. It becomes possible.

The substrate 2 is made of, for example, borosilicate glass such as Pyrex glass (Pyrex is a trade name of Corning Glass Industry) or BK glass (BK is a trade name of Hoya Corporation), or high melting point silicate such as quartz glass. Low melting glass such as salt glass and soda glass can be used. Further, instead of such a glass material, another transparent material may be used.

Au, Ag, Pt, R
h, Ir and other noble metal elements, Cu, Fe, Ni, Cr, A
It is preferably at least one metal selected from transition metals such as l and W, and is a metal that has poor wettability with the substrate 2 and the dielectric layer 5 and is easy to aggregate, and is hardly oxidized. What can exist as metal particles 4a is preferred.
Of these, particularly preferred are those having a low melting point, which facilitates aggregation, has poor wettability with glass, and is hardly oxidized.
u and Cu are inexpensive and have poor wettability with glass. The metal particles 4a are not limited to a single metal, but may be an alloy.

The metal particles 4a are spheroidal and anisotropic, and in FIG. 1 (however, the traveling direction of light is the Z direction and the plane orthogonal to this is the XY plane), the metal particles 4a The major axis direction is the Y direction, and the minor axis direction is the X direction.

The ratio of the length in the major axis direction to the length in the minor axis direction of the metal particles 4a is defined as an aspect ratio. Here, the average value of the aspect ratios of many metal particles 4a is simply referred to as the aspect ratio. .

The reason why the metal particles 4a become spheroidal is as follows.
This is because the metal particles 4a are stretched in the stretching direction together with the substrate 2 during stretching after the polarizing layer 3 is formed on the substrate 2. The extinction ratio increases as the aspect ratio increases, but at the same time, the stretching rate of the substrate 2 increases, making stretching difficult. In addition, the rate of increase in the extinction ratio decreases in a region having a large aspect ratio. The aspect ratio (length in the major axis direction / length in the minor axis direction) of the metal particles 4a in 3 is 3 to 3.
30 is suitable, and particularly preferably 15 to 25.
The extinction ratio is defined as the energy ratio between transmitted light in the X direction and transmitted light in the Y direction in decibels when incident light that is not polarized at a predetermined wavelength is used. At 10 dB.

On the other hand, the insertion loss is defined as the ratio of the total energy of incident light to the energy of transmitted light in the Y direction in decibels when incident light that is not polarized at a predetermined wavelength is used. 0.1 when the ratio is 0.1
dB.

The number density of the metal particles 4a in the metal particle layer 4 is ² to 37 / μm ^{2 in the} direction of the substrate surface. This is because the number density is less likely to appear the characteristics of the polarizer and falls below two / [mu] m ^2, for example, because the extinction ratio is lower than 20 dB, also, in the metal particles exceeds than 37 / [mu] m ² Is large, and the insertion loss increases more than 1 dB. That is, if the ratio is out of the above range, the metal particles are too close to each other to increase the insertion loss, or are too far from each other to obtain an extinction ratio.

For each layer, the metal particles after stretching a certain layer have the same anisotropy, and if the spacing between the metal particles is short, the absorption peak wavelength caused by the interaction between the particles will be smaller than that in the case where the spacing is long. It occurs on the short wavelength side. When the length of the metal particles 4a in the minor axis direction increases, the insertion loss with respect to the polarized light in the Y direction to be transmitted increases, which indicates that the aspect ratio is 3 or more, preferably 15 or more, and It is preferable that the length is short and the insertion loss is small. Metal particles 4
When the average length in the major axis direction of a increases, the absorption peak wavelength in the X direction increases and approaches the wavelength used in optical communication.

However, there is a limitation on the aspect ratio of the metal particles 4a in terms of manufacturing, and taking into account that an increase in the length in the short axis direction causes an insertion loss, the length in the long axis direction is also limited.

The preferred conditions for the metal particles 4a are that the aspect ratio is 3 to 30, the average length in the long axis direction is 100 to 300 nm, and the average length in the short axis direction is 10 to 50 nm. , More preferably the aspect ratio is 10-30, most preferably the aspect ratio is 15-25.
It is.

In the case of FIG. 1, the incident light L1 incident in the Z direction
In, the polarized component in the X direction is absorbed by resonance with the free electrons of the metal particles 4a, and the polarized component in the Y direction has a high transmittance and becomes polarized outgoing light L2. Further, there is a difference in the absorption peak wavelength between the X direction and the Y direction, and there is an absorption peak on the longer wavelength side in the X direction than in the Y direction. Unless otherwise specified, the extinction ratio is determined by the wavelength at which the absorption peak in the X direction occurs.

According to the polarizer of the present invention, the refractive indices n _k , k k, k + 1 and k +
_{nk + 1} and _{nk + 2} have a region satisfying the following expression.

_{Nk + 2} <ns < _nk < _{nk + 1} (where k = 3h-2; h is a natural number, and ns is the refractive index of the substrate) For example, a unit laminate P having a periodic unit structure is In the case of three layers, when the unit laminate P is formed by sequentially laminating a dielectric layer having a refractive index of n1, a dielectric layer having a refractive index of n2, and a dielectric layer having a refractive index of n3, The refractive index of the dielectric layer is n
It was found that when the relationship of 3 <ns <n1 <n2 was satisfied, the insertion loss was reduced over a wide optical wavelength range.

That is, as shown in FIG. 3, when all the dielectric layers have the same refractive index, n1 <ns <n2 <n
In the case of 3, if n2 <ns <n1 <n3,
And in all other possible combinations, 1
When the insertion loss in a wide wavelength band of 200 to 1700 nm was measured, only when the relationship of n3 <ns <n1 <n2 was satisfied, a very small insertion loss of 0.1 dB or less was obtained in a wide wavelength band of 1350 to 1700 nm. It turned out to be. Note that one or more antireflection films may be appropriately formed on the uppermost stacked body P. Also,
Dielectric layer 5 in which metal particles 4a are dispersed on both main surfaces of substrate 2
May be stacked, but in this case, the dielectric layer 5 is stacked so as to satisfy the above relational expression in the direction of stacking on the substrate 2.

[0032]

Next, more specific and preferred embodiments will be described. First, as the substrate 2 in FIG.
m, quartz glass substrate with an area of about 760 mm ² (softening point: 1
000-1200 ° C., a refractive index of 1.49), a thin film layer of MgO, ZnS, and MgF ₂ as a dielectric layer 5 and a thin film layer of gold (Au) as a metal particle layer 4 on the substrate 2.
The MgO layer containing Au fine particles (refractive index 1.7), the ZnS layer containing Au fine particles (refractive index 2.35), and the MgF ₂ layer containing Au fine particles (refractive index 1.
Three unit laminates P having the three-layer structure of 7) as a periodic unit were laminated.

That is, the degree of vacuum was 2.0 × 10 ⁻³ Toor and the film formation rate was 1 on the substrate by magnetron sputtering film formation.
An Au thin film layer, which is a first metal thin film layer having a thickness of 30 nm and a thickness of 0.6 nm / sec, is formed, and a vacuum degree of 2.0 × 10 ⁻³ Toor and a film forming rate of 0.2 nm are formed on the Au thin film layer. An MgO thin film layer, which is a first dielectric thin film layer having a thickness of 982.5 m, is formed at a thickness of 982.5 m / sec. Thereafter, an Au thin film layer, which is a second metal thin film layer, is formed in the same manner as described above, and a vacuum degree of 2.0 × 10 ⁻³ Toor and a film formation rate of 0.4 nm are formed on the Au thin film layer.
A second dielectric thin film layer of ZnS having a thickness of 1965 nm is formed at a rate of 1 second / second. Thereafter, an Au thin film layer, which is a third metal thin film layer, is formed in the same manner as described above, and a vacuum degree of 2.0 × 10 ⁻³ Toor and a film formation rate of 0.3 nm / sec are formed on the Au thin film layer. An MgF ₂ thin film layer as a third dielectric thin film layer having a thickness of 982.5 nm is formed.

The above series of steps is repeated three times to obtain a substrate in which three unit laminates each including three types of dielectric layers containing Au fine particles are laminated.

Next, the above-mentioned substrate body is subjected to thermoplastic deformation. Heating and stretching are performed at 1500 ° C., which is a temperature near the softening point of quartz glass, and Au metal fine particles are aggregated by heating to form Au metal particles. The metal particles are given shape anisotropy by stretching, and the Au metal particles are also oriented.

As a result, the Au metal particles form a substantially spheroid having an aspect ratio of 10 to 30, and the thickness of each dielectric layer is 327.5 nm for the MgO layer, 655 nm for the ZnS layer, and
The gF ₂ layer became 327.5 nm.

With respect to the obtained polarizer, a wavelength of 1200 to 1200
When the extinction ratio and the insertion loss were measured using light of 1700 nm, excellent characteristics with an extinction ratio of 40 dB and an insertion loss of 0.1 dB or less in a wide wavelength band of 1350 to 1700 nm were exhibited.

[0038]

As described above in detail, the polarizer of the present invention has a structure in which the refractive index and the composition of the upper and lower dielectric layers are different from each other, particularly when the unit laminate to be periodic has a three-layer structure. Since the refractive index of each dielectric layer is optimized, a polarizer having excellent characteristics with small insertion loss in a wide wavelength band of incident light can be provided. It is possible to provide a polarizer that can be suitably used for an optical communication device capable of performing processing.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view for explaining a polarizer according to the present invention.

FIG. 2 is a cross-sectional view for explaining a polarizer according to the present invention, and is a cross-sectional view taken along line AA in FIG.

FIG. 3 is a graph illustrating light absorption wavelength and insertion loss.

[Explanation of symbols]

 1: polarizer 2: substrate 3: polarizing layer 4: metal particle layer 4a: metal particle 5: dielectric layer P: unit laminate

Claims (2)

[Claims] 1. A polarizer comprising a plurality of dielectric layers, each of which has metal particles exhibiting light absorption anisotropy dispersed therein and has a different refractive index, on at least one principal surface of a substrate having a light transmitting property. . 2. The method according to claim 1, wherein k, k +
Refractive index _nk , _{nk + 1} , n of the first, k + 2nd dielectric layer
The polarizer according to claim 1, wherein _{k + 2} has a region satisfying the following expression. _{nk + 2} <ns < _nk < _{nk + 1} (where k = 3h-2; h is a natural number, and ns is the refractive index of the substrate)