JP2001083321A - Polarizer and method for its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a polarizer comprising a two-dimensionally periodic struc ture whose period is about 1 μm or below. SOLUTION: The polarizer comprises a two-dimensionally nearly periodic structure obtained by nearly periodically and alternately laminating two or more filmlike materials each having one-dimensionally nearly periodic ruggedness. For example, the polarizer comprises materials 1 and 2 different from each other in refractive index. The two-dimensionally periodic structure whose period is about 1 μm or below is obtained by such an easy production method. The polarizer transmits incident light having a specified polarization plane and reflects the incident light having the polarization plane perpendicular to the specified plane.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarizer used for an optical device utilizing a polarization phenomenon, which is a characteristic of light, and which transmits only linearly polarized light in a specific direction and reflects linearly polarized light in an orthogonal direction. It relates to a manufacturing method.

[0002]

2. Description of the Related Art A polarizer is an element that converts unpolarized or elliptically polarized light whose electromagnetic field vibrates in an unspecified direction into linearly polarized light by transmitting only a vibration component in a certain direction. It is one of the most basic optical elements, and is widely used in optical communication devices, optical disc pickups, liquid crystal displays, optical applied measurement, and the like. The operation modes are roughly classified into two types: (1) a type that absorbs unnecessary polarized waves, and (2) a type that separates two orthogonal polarization components incident on the same optical path into separate optical paths. Large opening area, high performance,
It is desired to realize characteristics such as thinness, and it is important to be able to supply at low cost industrially.

[0003] Polarizers currently in practical use include:
In the operation of (1), a polymer film containing dichroic molecules such as iodine is generally used. This is inexpensive and has a large area, but has the disadvantage that the extinction ratio is low and the temperature stability is poor.

To solve this problem, a polarizer using a highly stable material has been developed. That is, absorbers such as metals and semiconductors are arranged in a thin line or thin film in one direction in a transparent body such as glass. A polarized component parallel to the thin wire or the thin film is absorbed or reflected, and a polarized beam orthogonal to the transmitted component is transmitted. This type of polarizer is characterized by having a high extinction ratio, but requires steps such as cutting and polishing, and it is difficult to reduce the manufacturing cost. Also, it is difficult to make the device large and thin.

On the other hand, the structure using a birefringent single crystal in (2) has a structure in which two triangular prisms made of a material having a large birefringence such as calcite are bonded. A typical example is a Glan-Thompson prism. This type of polarizer generally has a high extinction ratio and a high transmittance, but it is difficult to reduce the area and thickness, and the price is inevitably high because the material is expensive.

In the case of utilizing the Brewster angle of a transparent body, a polarizing beam splitter using a dielectric multilayer film can be cited. This is a low price due to its high productivity,
There are problems that a high degree of polarization cannot be obtained, miniaturization is difficult, and a wavelength band to be used is narrow.

[0007] Each of the above-mentioned polarizers has been put to practical use. On the other hand, recently, a polarizer utilizing the anisotropy of the propagation characteristics of a transparent periodic structure having a period of less than the wavelength has been theoretically proposed. (Tetsuko Hamano, Masayuki Izutsu, Hideki Hirayama, "2
Possibility of Polarizer Using 3D Photonic Crystal, "
Proceedings of the 8th Autumn Meeting, paper2a-W-7, 1997., Akira Sato, Masahiro Takebe, "Optical Anisotropic Multilayer Film by Structural Birefringence," Optics
Japan'97, Proceedings of papers, paper3OpDO1,1997). In each of these structures, transparent columns having a refractive index different from that of the base material are two-dimensionally and periodically arranged in a transparent base material.
If the structure satisfies the condition that the period is, for example, about half a wavelength, for polarized light parallel to the pillar and polarized light perpendicular to the pillar, one can propagate inside and the other can be blocked, and thus operates as a polarizer. Let me do. However, in practice, no method for industrially producing such a structure has been found, and there is no experimental example.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a small optical path length, an excellent extinction ratio and an insertion loss characteristic, and a large aperture. An object of the present invention is to provide a low-cost, industrially-manufacturable polarizer having a large area.

[0009]

Means for Solving the Problems The background art of the polarizer of the present invention will be described. In an artificial periodic structure composed of a high-refractive index medium and a low-refractive index medium, two polarization components orthogonal to each other have independent dispersion relations (relationship between frequency and wave vector). In the two-dimensional periodic structure deeply related to the present invention, these two polarization components are a TE wave and a TM wave, respectively, depending on whether an electric field or a magnetic field is parallel to the length direction. Eigenmodes are also generally classified into TE-like waves and TM-like waves in a general three-dimensional periodic structure. Therefore, in the present invention, for convenience, TE wave,
It is called a TM wave. The band gap, that is, the frequency band in which light does not propagate is also different between the TE wave and the TM wave.
In a certain frequency band, one polarization mode may be cut off and the other polarization mode may become a propagation wave. That is, in this frequency band, the periodic structure can operate as a polarizer that reflects or diffracts one polarized light and transmits the other polarized light. Also, a sufficiently high extinction ratio can be obtained by increasing the number of periods.

[0010] The central idea of the present invention is a three-dimensional rectangular coordinate system xyz composed of two or more kinds of transparent materials having different refractive indices.
In the above, the shape of a layer serving as a unit of lamination has a periodic structure in the x-axis direction, and has a structure that is uniform in the y-axis direction or has a period longer than the x-axis direction, and the shape is repeated. And that the structure of a planar polarizer exists in a structure laminated in layers in the z-axis direction, that is, a structure in which two or more types of thin films having periodic folds (undulations) are multilayered. And a method of manufacturing the structure by the periodic structure manufacturing method developed by the inventors. Light is incident on the surface perpendicularly or obliquely. The opening area is determined by the size of the substrate, and it is extremely easy to increase the size. Although the optical path length is determined by the thickness of the laminated layers, it is sufficient that the optical path length is several times as large as the wavelength (several μm), and can be made thinner by several digits than the conventional polarizer.

On the other hand, in a film forming method using both diffused irradiation of deposited particles represented by bias sputtering and sputter etching, the deposition and etching operations are controlled mutually so that the unevenness of the surface is repeated. A method of laminating in layers is possible. This mechanism has the following three effects: (1) the effect of slowing down the deposition rate of the concave portion that becomes a shadow due to the diffusion and incidence of deposited particles; and (2) the etching rate at a tilt angle of about 50 ° to 60 ° by sputter etching. It can be explained that the effect of maximization, (3) the effect of particles attached mainly by sputter etching re-adhering to another part of the substrate at an appropriate ratio (Shojiro Kawakami, Hisashi Sato) , Takayuki Kawashima, "Formation mechanism of 3D periodic nanostructures fabricated by bias sputtering," IEICE CI, vol.J81-CI, no.2, pp.108
-109, February 1998).

By using this technique, a thin film made of two types of transparent materials is periodically formed on a substrate on which a periodic groove array is formed while repeating uneven shapes at the same position without any complicated alignment. Can be laminated. That is,
By using this technique, the polarizer of the present invention can be easily manufactured.

As described above, the polarizer of the present invention has a small optical path length, has excellent extinction ratio and insertion loss characteristics, can have a large aperture area, and can be provided at a low cost.

[0014]

FIG. 1 shows the structure of a polarizer of the present invention. Hereinafter, the polarizer of the present invention will be described with reference to FIG.

A transparent medium having a high refractive index and a medium having a low refractive index are alternately laminated on the periodic groove or linear projection row while preserving the shape of the interface. This embodiment has a periodic structure in the x direction and the z direction, and has a uniform structure in the present embodiment in the y direction. Alternatively, the operation mechanism is similar even if the structure is changed to a periodic or aperiodic structure having a length larger than the x-axis direction. Non-polarized light or elliptically polarized light is incident on the periodic structure thus obtained from the z direction. Polarization parallel to the groove array, i.e., y-polarization;
Light of the TE mode and the TM mode is induced inside the periodic structure with respect to the polarization orthogonal to it, that is, the x polarization.
However, if the frequency of the light is in the band gap of the TE mode or TM mode, the mode cannot propagate in the periodic structure and the incident light will be reflected or diffracted. On the other hand, if the frequency of the light is within the energy band, the light passes through the periodic structure while preserving the wave vector. Therefore, it operates as a planar polarizer.

In the polarizer of the present invention, the TE mode and the TM mode are controlled by controlling the period Lx of the groove row and the period Lz in the stacking direction.
The wavelength band in which the mode band gap occurs can be changed arbitrarily. That is, it is possible to arbitrarily set a wavelength band operated as a polarizer.

As the low refractive index medium, a material containing SiO ₂ as a main component is most common. SiO ₂ has a wide transparent wavelength range, is chemically, thermally, and mechanically stable, and can be easily formed into a film. As the high refractive index material, TiO ₂
Oxide or a semiconductor such as Si or GaAs can be used. TiO _{2 and the} like have a wide transparent wavelength range and can be used in the visible light region. On the other hand, semiconductors are limited to the near infrared region, but have the advantage of a large refractive index.

Incidentally, it is desirable to use a multipurpose polarizer in a wide frequency band. By appropriately determining the shapes of the high-refractive-index medium layer and the low-refractive-index medium layer, the frequency band used as a polarizer can be widened. Conversely, for monochromatic light such as a specific laser beam, the degree of freedom with respect to the shapes of the high-refractive index medium and the low-refractive index medium is large, and it is possible to select a shape that can be easily repeated in film formation. .

In the following, in the examples, the shape of the layer, the repeating structure, and the manufacturing method thereof will be described.

[0020]

(Embodiment 1) FIG. 1 shows the structure of an embodiment of the present invention.
FIG. In this figure, reference numeral 1 denotes an amorphous
SiO_TwoReference numeral 2 denotes an amorphous Si layer.
It is. The period Lx in the x-axis direction is 0.4 μm,
The period Lz is 0.32 μm. SiO _TwoLayer and Si
The layer bends periodically with a slight change in thickness t
It has a sharp shape. Next, the manufacturing method will be described.
You. First, electron beam lithography and dry etching
A periodic groove was formed by the etching. Figure 2 shows the model.
FIG. Reference numeral 3 is a quartz glass substrate, reference numeral 4 is non-reflective
The coating layer 5 is a periodic groove portion. one
Generally, 4 and 5 are different from the substrate depending on the size of the periodic structure.
Material, but keep the same material as the substrate.
Grooves can also be formed thereon. Here is an example of the latter
You. The width of the groove is 0.4 μm, the depth is 0.2 μm,
The period is 0.4 μm. On this substrate, SiO_TwoAnd
Bias sputtering using Si and Si targets
By the method_TwoLayers and Si layers were alternately stacked. That
When saving the shape of the periodic irregularities in the x-axis direction of each layer,
It is important to carry out film formation. The conditions are as follows
It was cage: SiO_TwoAr film pressure
1.9mTorr, target high-frequency power 400W, base
Plate high frequency power 60W; Ar gas pressure for Si film formation
3.6 mTorr. Target high-frequency power 400W
Was. SiO_TwoAnd Si layers were deposited by 10 layers.

Under these conditions, the reason why the laminated structure shown in FIG. 1 is formed on the substrate having the rectangular groove shown in FIG. 2 will be explained by the following three superpositions. (1) Deposition of neutral particles from the target by dispersed incidence; (2) Sputter etching by perpendicular incidence of Ar ions; (3) Reattachment of deposited particles.

FIGS. 3A and 3B show the TE wave at a wavelength of 1.0 μm and the T wave in the periodic structure thus obtained.
It is a figure which shows the intensity distribution in near field of the transmitted light with respect to M wave. The horizontal axis indicates the position on the substrate wafer. The central portion is the polarizer portion, and on both sides thereof are portions where the substrate wafer has no grooves and parallel layers of Si and SiO ₂ are deposited.
The vertical axis represents the transmitted light intensity at each point on the substrate wafer. It can be seen that the polarizer almost blocks the TE wave. On the other hand, for the TM wave, the difference in transmitted light intensity between the portion of the film deposited on the substrate having no grooves on both sides and the polarizer is small. In other words, if the antireflection coating is applied to the polarizer, the TM wave can be transmitted with a small loss.

FIG. 4 shows the relationship between the frequency and the limb movement vector in this periodic structure by the FDTD using the periodic boundary condition.
The result calculated by the method (finite difference time domain method) is shown.
Analysis of the band structure and light transmission characteristics of a photonic crystal by the FDTD method is described by S. Fan et al. In Physical Review B, vo.
l.54, no.16, pp.11245-11251 (1996). In FIG. 4, the horizontal axis represents the magnitude of the wave vector represented by a relative value, and the vertical axis represents the frequency Lx / λ represented by a relative value. Where λ is the wavelength of the incident light,
kz is the z component of the wave vector. Solid lines and broken lines show dispersion curves for TE waves and TM waves, respectively. Here, from Lx = 0.4 μm and wavelength 1 μm, the frequency Lx / λ =
0.4. As can be seen from this figure, Lx / λ =
The straight line of 0.4 does not intersect with the dispersion curve of the TE wave (solid line) but intersects with the dispersion curve of the TM wave (dashed line). This is TE
The wave is cut off and reflected, and the TM wave is transmitted. That is, this periodic structure has a frequency Lx / λ of 0.5.
T in the frequency band of code 6 located between 39 and 0.43
It acts as a polarizer that transmits M waves.

(Embodiment 2) In this embodiment, parameters such as the in-plane uniformity of the thickness of each dielectric layer, the shape of the groove, and the value of the Lz / Lx ratio are changed from those shown in the first embodiment. It is illustrated that excellent polarizer characteristics can be obtained even when the above method is used.

FIG. 5 is a diagram showing the configuration of another embodiment of the present invention. Reference numeral 7 is amorphous Si0 ₂ layer, reference numeral 8 is an amorphous Si layer. Period Lx in x-axis direction
Is 0.4 μm, and the period Lz in the z-axis direction is 0.32 μm. The SiO ₂ layer has a thickness t of O.D. While varying between 9 Lz and 0.3 Lz, the Si layer has a thickness of 0.1 Lz.
While changing between z and 0.7 Lz, it has a periodically bent shape. In the production of the laminated film, the substrate was the same as in Example 1, except that the SiO ₂ layer and the S
The bias sputtering conditions for forming the i-layer are different.

FIG. 6 shows the result of calculating the relationship between the frequency and the wave vector in the periodic structure by the FDTD method. The horizontal axis represents the magnitude of the wave vector represented by a relative value, and the vertical axis represents the frequency represented by a relative value. The solid line and the broken line are dispersion curves for the TE wave and the TM wave, respectively.
As can be seen from this figure, compared to the case of the first embodiment,
The frequency band acting as a polarizer is widened. By the way, when focusing on one band gap, it is desirable that the frequency width of a polarizer used at a single optical frequency is also wide. This is because at a frequency not sufficiently distant from the end of the band gap, the number of periods in the z direction required to increase the extinction ratio increases.

In the first and second embodiments, the ratio Lz / Lx of the repetition period in the z-axis direction and the x-axis direction is 0.8, but from other calculation results by the FDTD method, it is about 0.2. Even so, it has been found that it can function as a polarizer. When used as a normal polarizer, the period Lx in the x direction is selected to be about the wavelength of light or less,
It has been found that in a polarizing element for transmitting one polarized light straight and diffracting the other polarized light, it is preferable to select a period Lx longer than the wavelength of light. further,
The grooves need not necessarily be uniform in the y-axis direction, and may have a different periodic structure with respect to the width and spacing of the grooves in the x-axis direction, or grooves of a random length sufficiently long in the y-direction. It is known from other calculations that this may be the case.

In this case, the bias sputtering method is used as a means for laminating while repeating the shape of the unit layer. However, a method in which the deposition process and the sputtering etching process are separated at the same time but not simultaneously is added. Thereby, the degree of freedom in designing the shape of the layer serving as the unit of lamination can be increased. Further, as the low refractive index medium, optical glass such as Pyrex (registered trademark) can be used other than amorphous SiO ₂ . On the other hand, as the high refractive index medium, in addition to Si, TiO
₂ , Ta ₂ O ₅ or the like can also be used. Although the cross-sectional shape of the groove of the substrate is V-shaped this time, it is apparent that a rectangular groove may be used. Also, by appropriately selecting the conditions of bias sputtering, various cross-sectional shapes of the groove can be obtained.

In order to use the thus-produced laminated film as a polarizer, it is only necessary to apply an anti-reflection coating on the surface opposite to the surface of the substrate and then cut it. Not only can a large number of devices be manufactured at once, but also no polishing is required, and the cutting process is simple. As a result, a low-cost polarizer can be provided. The thickness of the laminated film excluding the substrate is several microns, and can be used at normal incidence or at a small incident angle. Therefore, it can be widely applied to small optical communication isolators and the like. In addition, when used as a polarization splitting element used in an optical circulator or the like, the light may be used with a large inclination with respect to incident light, but in this case, no light is transmitted through the cut surface, and therefore, polishing is unnecessary. .

[0030]

The polarizer manufactured by the film forming method including the sputtering etching action of the present invention has a small thickness in the light transmission direction, and a large-area laminated film can be obtained by one film forming process. When the device is manufactured, polishing is not required and cutting is easy. On the other hand, it is possible to design to have excellent polarization characteristics according to the wavelength range to be used. Such a polarizer is most suitable as a polarizer for an optical isolator. Other industrial applications such as optical circulators and optical switches are also wide and can replace conventional polarizers.

[Brief description of the drawings]

FIG. 1 is a diagram showing a structure of a first embodiment.

FIG. 2 is a diagram showing a substrate having a groove on a surface.

3A is a diagram showing a near-field intensity distribution of transmitted light with respect to a TE wave; FIG. 3B is a diagram showing a near-field intensity distribution of transmitted light with respect to a TM wave;

FIG. 4 is a diagram showing a relationship between a frequency and a wave vector in the first embodiment.

FIG. 5 is a diagram showing a structure of a second embodiment.

FIG. 6 is a diagram showing a relationship between a frequency and a wave vector in the second embodiment.

[Explanation of symbols]

1 SiO ₂ layer 2 Si layer 3 substrate 4 one 7 SiO ₂ layer 8 Si layer of the band acting as a polarizer for transmitting non-reflective coating layer 5 periodic grooves 6 TM wave

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takayuki Kawashima No.45, Village No. 203, Kawachi-Sannincho 45-5 Kawauchi, Aoba-ku, Sendai, Miyagi Prefecture

Claims (4)

[Claims] 1. In three-dimensional rectangular coordinates x, y, z,
A multilayer periodic structure in the z-axis direction composed of a high-refractive-index amorphous transparent material and a low-refractive-index amorphous transparent material, and the shape of a layer serving as a unit of lamination for each transparent material is v-shaped in the x-axis direction. Or a periodic structure of a square or waveform, having a periodic uneven structure whose wavelength is equal to or less than the wavelength of the light used, a uniform structure in the y-axis direction, or a periodic or non-uniform structure having a length greater than the x-axis direction. Has a periodic uneven structure,
It is incident perpendicularly or obliquely to the xy plane, and the electric field acts on light having a wavelength and an incident direction in which either the polarization orthogonal to the y-axis or the polarization orthogonal to the x-axis is included in the cutoff band. The polarizer characterized by the above-mentioned. 2. The polarizer according to claim 1, comprising a high refractive index medium mainly composed of Si or TiO ₂ and a low refractive index medium layer mainly composed of SiO ₂ . 3. In three-dimensional rectangular coordinates x, y, z,
A multilayer periodic structure in the z-axis direction comprising a material having a high refractive index and a material having a low refractive index, wherein the materials are formed by sputtering, and at least a part of the period is sputter-etched, and each transparent material is formed. Each of the layers serving as a unit of lamination has a periodic uneven structure whose wavelength is equal to or less than the wavelength of light whose period is a v-shaped or square or a wave-shaped periodic structure in the x-axis direction, and is uniform in the y-axis direction. Structure or a periodic or aperiodic uneven structure with a length greater than the x-axis direction, perpendicularly or obliquely incident on the xy plane, and an electric field orthogonal to the y-axis or orthogonal to the x-axis. A polarizer characterized by acting on light having a wavelength and an incident direction in which one of polarized waves is included in a cutoff band. 4. On a substrate having periodic grooves or periodic linear projections or elongated projections or elongated depressions,
high refractive index medium containing i or TiO ₂ as a main component and SiO ₂
A polarizer produced by laminating a low-refractive-index medium having as a main component a film by a film forming method including dry etching at least in part while repeating the shape in each cycle.