JPH05323407A - Formation of partial polarization inversion region and optical second higher harmonic generating element - Google Patents

Abstract

PURPOSE:To invert polarization down to the inside of a substrate while suppressing the spread in a lateral direction in the method for forming the polarization inversion regions for forming an optical waveguide for the second higher harmonic generating element. CONSTITUTION:This method includes a mask forming stage for forming a mask 5 consisting of a metal on the substrate 1 consisting of a ferroelectric substance, a proton exchange stage for exchanging the constituting ions of the substrate 1 with proton through the apertures 5a of the mask 5, an electrode forming stage for forming an electrode 6 on the surface of the substrate 1 on the side opposite from the surface on which the mask 5 is formed and a polarization inversion stage for impressing a DC voltage between the mask 5 and the electrode 6. The method is so constituted as to progress the polarization inversions down to the inside of the substrate 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming a partial polarization inversion region for producing an optical waveguide for a second harmonic generation element.

In recent years, laser printers, laser scanners,
Semiconductor lasers (LDs) are widely used as light sources for optical disks and the like. On the other hand, there is an increasing demand for shorter wavelengths, such as infrared light to visible light, for the purpose of expanding storage capacity and convenience of handling.

Although the wavelength of the LD has been shortened, it is extremely difficult to reduce the oscillation wavelength to 600 nm or less at the current technical level, and other techniques, for example, the second harmonic wave is used to generate the short wavelength. A second harmonic generation (SHG) element that obtains coherent light is receiving attention.

A well-known second harmonic generating element is one that allows laser light to pass through a bulk type nonlinear optical crystal.
However, since there is no optical crystal with a large nonlinear optical constant and the conversion efficiency is low, a large fundamental wave power is required and the LD cannot be used as a light source.

Therefore, recently, as a promising method that can utilize a large nonlinear optical constant and obtain a high conversion efficiency, an optical waveguide combining an optical waveguide formed on a ferroelectric substrate and a partial polarization inversion region is used. A waveguide type second harmonic generation element has been proposed.

[0006]

2. Description of the Related Art FIG. 16 is a diagram for explaining a conventional SHG element. The SHG element 111 _A in FIG.
The so-called Cerenkov radiation type, which is phase-matched with the radiation mode, is provided with an optical waveguide 113 on the surface of a substrate 112 made of a ferroelectric material such as LiNbO ₃ (lithium niobate) or LiTaO ₃ (lithium tantalate). It was formed.

The optical waveguide 113 is formed by exchanging the surface of the substrate 112 with protons such as benzoic acid and pyrophosphoric acid.
The fundamental wave is confined inside this to achieve phase matching with the radiation mode of the second harmonic.

However, in this method, the beam shape is distorted into a thin three-month shape, and it is not possible to collect light up to about μm, and it is difficult to apply it to an optical disk or the like.

Therefore, a quasi-phase matching (QPM) type SHG element as shown in FIG. 16B has been considered.
In FIG. 16B, in the SHG element 111 _B , the optical waveguide 113 is formed on the same substrate 112 as described above, and the polarization non-inversion region 114 formed by maintaining the polarization direction of the substrate 112 on the same surface as it is. The polarization inversion regions 115 formed by partially inverting the polarization direction are alternately arranged in the light traveling direction at a constant cycle.

The width and arrangement period of the polarization non-inversion region 114 and the polarization inversion region 115 are set so that the phases of the fundamental wave and the second harmonic wave are pseudo-matched, and the optical waveguide of the second harmonic wave generating element is set. For example, when a fundamental wave is incident on 113 from the left side of the drawing, it is converted into a second harmonic while propagating through the optical waveguide 113 and is emitted from the optical waveguide 113 to the right side of the drawing.

That is, by propagating the fundamental wave on the periodic polarization inversion, a pseudo phase matching is obtained,
Since the beam shape is not distorted and the light can be collected easily, it can be applied to an optical disc.

As a means for partially reversing the polarization direction, a method of diffusing Ti into a LiNbO ₃ (LN) substrate and reversing the polarization direction of the diffusion portion, LiNbO ₃ (LN)
A method of releasing Li ₂ O from the surface of the substrate to the outside and reversing the polarization direction of the emitting portion, a method of replacing Li ⁺ of the LiTaO ₃ (LT) substrate with proton H ⁺ by a so-called proton exchange method and reversing the polarization direction by heat treatment. There is a method in which the electron beam is narrowed down and this beam is moved to directly form a polarization inversion pattern.

Here, FIGS. 17 and 18 are schematic views showing a conventional method of forming a partially domain-inverted region. FIG. 17
Shows the above, and a titanium film 121 having an opening pattern is formed on the LN substrate 112a by vapor deposition or the like (FIG. 17).
(A)). Then, by heat treatment just below the Curie point (1210 ° C.), titanium is diffused to cause polarization inversion, and the polarization non-inversion region 11
4 and the polarization inversion region 115 are formed (FIG. 17).
(B)).

FIG. 18 shows the above, and the LN
A silicon oxide film 122 having an opening pattern is formed on the substrate 112a by sputtering or the like (FIG. 18A). Then, by heat treatment just below the Curie point, Li ₂ O is emitted from the surface to the outside, and the polarization direction of the emission portion is reversed to make the polarization non-inversion region 11
4 and the domain inversion region 115 are formed (FIG. 18).
(B)).

However, since the LiNbO ₃ substrate used in FIGS. 17 and 18 has a low optical damage threshold value, it is difficult to put it into practical use as a second harmonic generation element. On the other hand, the LiTaO ₃ substrate used therefor has a high optical damage threshold value and is promising as a second harmonic generation element because it can obtain a domain-inverted region 115 having a depth of about 1/4 of the array period.

The depth of the optical waveguide 113 formed by using the LiTaO ₃ substrate is 2 μm or more, and it is difficult to make it shallower than this. To convert the fundamental wave into the second harmonic with high efficiency while propagating through the optical waveguide 113, the polarization inversion region is used.
The depth of 115 must reach the bottom of the optical waveguide 113, or it must reach a sufficient depth even if it does not reach the bottom.

Here, a conventional method of forming a partial polarization inversion region using the above-described method will be described in detail with reference to FIG. The + C surface means a surface in the direction of polarization, and the -C surface means the opposite surface.

In the mask forming step, a Z-plate LiTaO ₃ is cut so that its main surface is orthogonal to the Z-axis and is single-polarized.
19 is used as the substrate 112c, the -C surface of the substrate 1 is mirror-polished as shown in FIG. 19 (A), and a metal film made of Ta is deposited thereon to form a mask 123. Mask 123 on substrate 11
A plurality of openings 123a parallel to the Y axis are provided at positions corresponding to the domain-inverted regions of 2c.

In the proton exchange process, the processed substrate 112 is
C is immersed in pyrophosphoric acid and subjected to proton exchange treatment at 250 ° C. for 30 minutes. As a result of such proton exchange treatment, FIG.
As shown in FIG. 9 (B), the Li ^{+ of the} substrate 112c has an opening 123a.
Is replaced by the proton H ⁺ .

In the polarization reversal process, the mask 123 of the processing substrate 112c is removed, and then immediately below the Curie temperature (up to 590).
By rapidly heating to (.degree. C.), the polarization direction in the high proton concentration region is inverted as shown in FIG. 19C, and the polarization non-inversion region 114 and the polarization inversion region 115 are formed.

On the other hand, the above-mentioned method can form a very deep domain-inverted pattern, but has a drawback that the work time is long because it is necessary to scan the electron beam over all regions where domain-inversion is desired.

Although not shown in FIGS. 17 to 19, the optical waveguide 113 for propagating the laser light is masked on the processing substrate 112 after the polarization non-inversion region 114 and the polarization inversion region 115 are formed. Are formed in parallel with the X axis of the substrate 112 by a proton exchange method.

[0023]

Problems to be Solved by the Invention FIG. 16 (B) and FIG.
As shown in FIG. 18B or FIG. 19C, the width of the domain-inverted region formed by the conventional method of forming the partial domain-inverted region is the width of the domain-non-inverted region on the surface of the substrate. And its cross section is almost semicircular. That is, the second harmonic generation element applied to the laser light having the fundamental wave wavelength of about 1 μm has an array period of about 6 μm, and the width of the domain inversion region is half of the array period to about 3 μm, and the depth is arrayed. It becomes about 1.5 μm in 1/4 of the cycle. However, for the wavelength of the semiconductor laser (~ 0.85 μm), the required array period (3
~ 4 μm), the depth of the domain inversion region is 1 /
There is a problem that it becomes less than 1 μm at 4 and is insufficient compared with the depth of the optical waveguide of 2 μm.

An object of the present invention is to provide a method for forming a partial polarization inversion region in which the polarization is inverted to the inside of the substrate while suppressing the lateral expansion.

[0025]

Means for Solving the Problems The above-mentioned problems are a mask forming step of forming a mask made of metal on a substrate made of a ferroelectric substance, and a proton exchange step of exchanging constituent ions of the substrate with protons through the openings of the mask. And a polarization inversion step of applying a DC voltage between the mask and the electrode, and an electrode formation step of forming an electrode on the surface of the substrate opposite to the surface on which the mask is formed. This is achieved by the method of forming a partially domain-inverted region according to the present invention, which is performed by advancing inside.

[0026]

The polarization can be partially inverted in the portion of the ferroelectric substrate having a high concentration of protons by applying an electric field. Moreover, at this time, since the protons move into the inside of the substrate under the same electric field, the domain-inverted region also advances into the inside of the substrate. As a result, it becomes possible to invert the polarization to the inside of the substrate, unlike the case where the polarization is inverted only by the normal heat treatment. That is, it is possible to realize a method of forming a partial polarization inversion region in which the polarization is inverted to the inside of the substrate while suppressing the expansion in the lateral direction.

[0027]

FIG. 1 shows a manufacturing process diagram of a first embodiment of the present invention. FIG. 1 illustrates a method of forming a partially domain-inverted region according to the present invention. The + C plane means the plane in the direction of polarization, and the -C plane means the opposite side.

In the mask forming step, a Z plate LiTaO ₃ is cut so that its main surface is orthogonal to the Z axis and is polarized.
Using (lithium tantalate LT) as the substrate 1, the + C surface of the substrate 1 is mirror-polished as shown in FIG. 1 (A), and a metal film made of Ta is deposited thereon to form a mask 5. The mask 5 has a plurality of openings 5a parallel to the Y axis at positions corresponding to the domain-inverted regions of the substrate 1.

In the proton exchange step, the treated substrate 1 is immersed in pyrophosphoric acid and a proton exchange treatment is performed at 250 ° C. for 30 minutes. As a result of such proton exchange treatment, FIG.
As shown in (B), Li ⁺ (lithium ion) of the substrate 1 is replaced with proton H ⁺ through the opening 5a.

In the electrode forming step, the processed substrate 1 is then used.
1 is formed on the surface opposite to the mask formation surface of, that is, -C surface.
As shown in (C), a metal film made of Ta is deposited to form the electrode 6.

Then, in the polarization inversion step, the positive electrode of the DC power supply 7 is connected to the mask 5 and the negative electrode is connected to the electrode 6 to apply an electric field, as shown in FIG.
The polarization direction is inverted in the region where the proton concentration is high, and the polarization non-inversion region 3 and the polarization inversion region 4 are formed.

The electric field applied at that time is a negative electric field on the electrode 6 side, and the protons are attracted to the electrode 6 and the substrate 1
Move inside. As a result, as the voltage application time elapses, the domain inversion region 3 also advances inside the substrate as shown in FIG.

Note that the polarization is reversed even by applying a DC voltage between the mask 5 and the electrode 6, and by appropriately heating the processing substrate 1 at this time, the polarization reversal in the high proton concentration region is promoted. At the same time, the polarization inversion region 3 can easily move to the inside of the substrate.

The polarization inversion region 3 on the surface of the substrate 1 has a high concentration of protons and the polarization direction is inverted by application of a relatively low voltage. As the polarization inversion region 3 advances into the substrate, The concentration of protons decreases and it becomes difficult to reverse the polarization direction. Further, the polarization inversion region 3 may have a blurred outline and spread laterally as it advances inside the substrate.

In such a case, the DC voltage applied between the mask 5 and the electrode 6 is appropriately changed in accordance with the progress of the polarization inversion in the region where the protons are exchanged and the change in the cross-sectional shape. It is possible to control the shape of the.

As described above, the polarization direction can be reversed in the portion of the substrate 1 made of a ferroelectric material having a high proton concentration by applying an electric field. Moreover, at this time, the protons move to the inside of the substrate 1 under the same electric field, so that the domain inversion region 3 also advances to the inside of the substrate. As a result, it becomes possible to invert the polarization to the inside of the substrate, unlike the case where the polarization is inverted only by the normal heat treatment. That is, it is possible to realize a method of forming a partial polarization inversion region in which the polarization is inverted to the inside of the substrate while suppressing the expansion in the lateral direction.

Next, FIG. 2 and FIG. 3 show manufacturing process diagrams of the second embodiment of the present invention. In FIGS. 2 and 3, first, a mask 10 (3.4 μm period, 1.0 μm opening 10a) is formed by depositing a metal film made of Ta (tantalum) on the + C surface of the substrate 1 of LiTaO ₃ (FIG. 2).
(A)).

This is subjected to proton exchange in pyrophosphoric acid at 255 ° C. for 20 minutes (FIG. 2 (B)). At this time, mask 1
Li ⁺ and proton H ⁺ are exchanged in the proton exchange region 11 on the surface of the substrate 1 in the opening portion 10a of 0. This proton exchange region 11 lowers the inversion threshold value when the polarization is inverted after the voltage is applied.

Subsequently, a resist 12 is applied on the mask 10 (FIG. 2C), exposed from the + C surface side and developed (FIG. 2D). On top of that, a Ti (titanium) film 13a,
13b is formed by vapor deposition (FIG. 2 (E)), and washed with acetone to remove the Ti film 13a on the resist 12 together with the resist 12 (FIG. 3 (F)).

Thereafter, the Ta mask 10 is etched by RIE (reactive ion etching) using CF ₄ (fluorocarbon) + O ₂ gas to form a titanium film electrode 13b on the proton exchange region (see FIG. Three
(G)). Further, a Ti film is vapor-deposited on the entire −C surface of the substrate 1 to form the electrode 6 (FIG. 3 (H)).

Then, a DC power supply 7 applies an electric field of 200 kV / cm between the electrodes 13b and 6 on the ± C plane for about 1 minute. As a result, a domain-inverted region 4 having a depth of several tens of μm is formed under the electrode 13b on the + C plane (FIG. 3 (I)). That is, by making the proton exchange region 11 have a difference between the inversion thresholds of the region on the surface of the substrate 1 where the polarization should be inverted and the region where the polarization is not inverted, and applying an electric field by the electrodes on the inversion region,
The periodic domain inversion region is formed deep in the substrate.

After that, the electrodes 13b and 6 are removed and an optical waveguide (not shown) is formed on the + C plane to obtain a second harmonic generation (SHG) element.

On the other hand, only by patterning the electrodes,
The movement of charges on the crystal surface raises the potential between the electrodes,
Not only the lower part of the electrode but also the polarization between the electrodes is inverted, and it is difficult to form a periodic inversion pattern. According to the present invention, the inversion threshold value of the portion where the inversion region is to be formed is set lower than the inversion threshold value of the portion where the inversion region is not formed, and an electric field having an intensity that inverts only the portion where the inversion region is to be formed is applied. Thereby, the polarization of only the lower part of the inversion part electrode can be inverted.

Therefore, it becomes possible to form a periodic domain-inverted region that reaches deep in the substrate, and the interaction between the guided light and the domain-inverted becomes large without making the optical waveguide extremely shallow, and the conversion efficiency of the wavelength conversion element is improved. Can be improved. That is, even if the optical waveguide is embedded or the confinement is slightly weakened, the interaction between the guided light and the polarization inversion is maintained and the effect on the conversion efficiency is small, so a low-loss optical waveguide without scattering of the crystal surface, It is possible to fabricate an optical waveguide that is strong against optical damage caused by a decrease in the power density of the second harmonic.

Next, FIG. 4 shows a block diagram of a third embodiment of the present invention. In FIG. 4, first, a mask of a Ta metal film with an opening width of 2 μm pattern (see 22 in FIG. 4B) is formed on the + C surface of the substrate 1 of LiTaO _{3, and} the proton is stored in pyrophosphoric acid at 250 ° C. for 30 minutes. Exchange is performed to form the optical waveguide 2. After that, the mask is removed, and annealing is performed in air at 360 ° C. for 2 hours to reduce the waveguide loss and restore the nonlinear optical constant.

Subsequently, a transparent insulating film 41a such as a SiO ₂ film is formed on the optical waveguide 2, and comb-shaped pattern electrodes 42a and 42b are formed thereon with a metal film such as Ti. The pattern electrodes 42a and 42b have a width of 0.7 μm and a pitch.
3.4 μm comb teeth are aligned with each other in a meshed state. Further, the electrodes 6 are formed on the opposite surfaces of the substrate 1. Further, the insulating film 41b similar to the above is formed on the pattern electrodes 42a and 42b. This insulating film 41b
Serves to prevent discharge between the pattern electrodes 42a and 42b when an electric field is applied.

Therefore, the DC power source 7 is applied to the pattern electrode 42a.
a is connected, and the DC power supply 7b is connected to the pattern electrode 42b.

Then, 210 kV / cm is provided between the pattern electrode 42a immediately above the region where the polarization is to be inverted and the electrode 6 on the back surface.
Is applied for about 1 minute. At this time, a potential (preferably 200 kV) is applied to the pattern electrode 42b immediately above the polarization non-inversion region 3 so that the electric field strength between the pattern electrode 42a and the back surface electrode 6 is smaller than the electric field strength between the pattern electrode 42a and the back surface electrode 6. /
cm or less).

The potential difference between the pattern electrodes 42a and 42b is preferably as large as possible unless the electrodes are damaged by discharge between electrodes or dielectric breakdown, and a technique such as pulsed electric field application may be used as necessary. Is valid,
It is also effective to apply an electric field in a vacuum.

In this case, the pattern electrode 4 immediately above the inversion region
A domain-inverted region 4 slightly wider than the width 2a is formed from the surface of the substrate 1 to a depth of several tens of μm. A wavelength of 840
When a semiconductor laser beam with a wavelength of
0 nm) was obtained.

As described above, it is known that the polarization of the ferroelectric substance is reversed by the application of an electric field. However, in the method using only the normal pattern electrode, the charge between the electrodes on the crystal surface on which the pattern electrode is formed is charged. This causes not only the lower part of the electrodes but also the polarization between the electrodes to be inverted, which makes it difficult to form a periodically inverted pattern. Therefore, the pattern electrodes 42a and 42b are independently formed on both the portion where the inversion region is to be formed and the other portion, and the potential difference between the pattern electrode 42b and the electrode 6 is calculated from the potential difference between the pattern electrode 42a and the electrode 6. By lowering also, it becomes possible to invert the polarization of only the lower part of the pattern electrode 42a deeply.

FIG. 5 shows a block diagram of another embodiment of the fourth embodiment. FIG. 5 shows a buried type optical waveguide 2a for forming the optical waveguide in FIG. In this case, the insulating film 41a formed on the substrate 1
Formation is omitted. This is the insulating film 41 in FIG.
This is because a is to prevent the absorption of light by the pattern electrodes 42a and 42b, and the influence of light can be prevented by using the embedded optical waveguide 2a.

In this way, it becomes possible to form the periodic domain-inverted regions 4 that reach deep into the substrate 1. Even if the optical waveguide 2 is not made extremely shallow, the interaction between the optical waveguide and the domain-inverted becomes large, and wavelength conversion is performed. The conversion efficiency of the device can be improved. Further, even if the optical waveguide 2a is of an embedded type or if the confinement is slightly weakened, the effect on the conversion efficiency is small, so that an optical waveguide with low loss can be manufactured and the optical damage can be strengthened. Next, FIG. 6 shows a manufacturing process diagram of a fourth embodiment of the present invention. In FIG. 6A, first, for example, + C of the substrate 1 of LiTaO _{3 having} a thickness of 500 μm is used.
A Ti metal pattern electrode 21 having a thickness of about 1000 Å, a period of 3.4 μm, and a width of 1 μm is formed on the surface by a normal photolithography technique. In addition, T for grounding on the -C surface
The i-film electrode 6 is formed. Next, this substrate is placed in a vacuum, and a positive potential is applied to the Ti metal pattern electrode 21 formed on the + C surface. That is, 210 kV from the DC power supply 7.
By applying an electric field of / cm for 1 minute, the domain-inverted region 4 slightly wider than the width of the metal part of the pattern electrode 21
It is formed to a depth of at least several tens of μm from the surface of the substrate 1. The reason why the electric field is applied in a vacuum is to suppress the movement of charges on the crystal surface and prevent the polarization reversal in the portion without the electrode pattern.

The applied voltage and time are adjusted so that the size ratio of the polarization inversion region 4 and the polarization non-inversion region 3 approaches 1: 1. In this way, after forming the periodically poled pattern, the pattern electrode 21 on the surface is removed. In this state, deep periodic domain inversion regions of the inversion layer are produced. A pulsed electric field (for example, a peak electric field of 400 kv /
(cm, pulse width 10 msec, 1 Hz for 10 seconds) is naturally effective in preventing discharge.

Subsequently, in the case of forming an optical waveguide, FIG.
In (B), a Ta metal film is deposited on the + C surface of the substrate 1 to provide a mask 22 having an opening 22a (width 2 μm) for forming a waveguide.

This is immersed in, for example, pyrophosphoric acid 23, and proton exchange is carried out at 250 ° C. for 30 minutes to form the optical waveguide 2.

After that, the mask 22 is removed, and annealing is performed to reduce the waveguide loss and recover the nonlinear optical constants.
When a semiconductor laser beam having a wavelength of 840 nm was made to enter the second harmonic generation element having a length of 10 mm thus manufactured,
Blue light (wavelength 420 nm) was obtained.

In FIG. 6, the polarization inversion shown in FIG. 6A may be performed after forming the optical waveguide 2 shown in FIG. 6B.

Now, referring to FIGS. 7 and 8, the fourth embodiment will be described.
The block diagram of the other Example in FIG. In FIG. 7, Li
TaO₃Ta mask pattern on + C surface of substrate 1
Then, the optical waveguide 2 is manufactured by the same procedure as in FIG.
Next, on the optical waveguide 2, SiO ₂Form insulating film 24a such as a film
After forming, pattern electrode 21 of Ti metal film is formed on top of it.
To achieve.

Further, an insulating film 24b such as a SiO ₂ film having a thickness of several thousand Å is covered on the pattern electrode 21 and an electric field similar to that shown in FIG. It is a thing.

Thus, since the space between the pattern electrodes 21 is covered with the insulating films 24a and 24b, the treatment in air is possible. Further, since it is not affected by the surface state of LiTaO ₃ , it is possible to form a domain inversion pattern with good reproducibility.

The insulating film 24a between the optical waveguide 2 and the pattern electrode 21 is for preventing absorption of light by the electrode.
Alternatively, as shown in FIG. 8, the optical waveguide 2a may be a buried type.

The optical waveguide 2a can be manufactured later, as in the case of FIG. In that case, it is not necessary to form the insulating film 24a between the substrate 1 and the pattern electrode 21, but it is necessary to remove the pattern electrode 21 and the upper insulating film 24b after the domain inversion pattern is formed.

As described above, as shown in FIGS. 6 to 8, it becomes possible to form a periodically domain-inverted region that reaches a deep portion of the substrate, and the guided light and the domain-inverted region can be mutually processed without making the optical waveguide extremely shallow. The action is increased, and the conversion efficiency of the wavelength conversion element can be improved. Further, even if the optical waveguide is embedded or the confinement is slightly weakened, there is little influence on the conversion efficiency, so that an optical waveguide with low loss can be manufactured and the optical waveguide can be strengthened against optical damage.

Next, FIG. 9 shows a manufacturing process diagram of a fifth embodiment of the present invention. In FIG. 9, first, LiTaO
₃ (or LiNbO ₃ ) on the ± C plane of the substrate 1, for example, T
The electrodes 51, 6 of the metal film of i are formed by vapor deposition or the like (FIG. 9A). Then, the + C surface electrode 51 (or the −C surface electrode 6) may be patterned, for example, in a cycle of 3.4 μm to form a pattern electrode 51a (FIG. 9B).

Then, the pattern electrodes 51a or the pattern electrodes 51a themselves are cut to form the groove 52 as a buffer portion.
Is formed on the + C surface of the substrate 1 (FIG. 9C). This groove 5
2 plays a role of preventing discharge when an electric field is applied. In this case, it is more preferable to evaporate the water content by heating.

Therefore, an electric field of, for example, 210 kV / cm is applied from the DC power supply 7 between the main pattern electrode 51a and the electrode 6 to periodically invert the polarization under the main pattern electrode 51a, and the polarization non-inversion region is obtained. 3 and the domain inversion region 4 are formed (FIG. 9D). For example, if the substrate 1 is LiTa with a thickness of 0.5 mm,
In the case of O ₃ , 10.5 is required to apply an electric field of 210 kV / cm.
It is necessary to apply a voltage of kv between the electrodes, but if the groove 52 is not formed, it cannot be applied because it exceeds the discharge electric field on the device surface in the air, but the groove 52 can raise the limit. It is possible.

Then, the pattern electrode 51a and the electrode 6 are removed, and the optical waveguide 2 is formed in the direction perpendicular to the formed polarization non-inversion region 3 and polarization inversion region 4 by a usual method (for example, proton exchange) (FIG. 9 (E)).

By forming the groove 52 in this way,
A high electric field can be applied in the air, and the domain-inverted region 4 having a depth of 20 μm or more can be formed. Therefore, sufficient overlap with the optical waveguide 2 can be ensured. Of course, the combined use of electric field application in a vacuum and pulsed electric field application is also effective.

Here, FIG. 10 shows a manufacturing process drawing of another embodiment of the fifth embodiment. In FIG.
9A and 9B are similar to FIGS. 9A and 9B.
Of the pattern electrode 51a, the unnecessary portion is removed to leave the main pattern electrode 51b, and the other portion is used as a space buffer region 53 which is a buffer portion (FIG. 10C). The space buffer region 53 plays a role of preventing discharge when an electric field is applied, as in the case of FIG. 9C. Further, in this case as well, it is better to evaporate the water content by heating.

Therefore, an electric field of 210 kV / cm, for example, is applied from the DC power supply 7 between the main pattern electrode 51b and the electrode 6 to periodically invert the polarization under the main pattern electrode 51b, and the polarization non-inversion region is obtained. 3 and the domain inversion region 4 are formed (FIG. 10 (D)). Then, as in the case of FIG. 9E, the optical waveguide 2 is formed (FIG. 10E).

Thus, even with this method, 20 μm
The domain-inverted region 4 having the above depth can be formed, and sufficient overlap with the optical waveguide 2 can be ensured.

Next, FIG. 11 shows a manufacturing process drawing of a sixth embodiment of the present invention. In FIG. 11, FIG.
The process of (B) is the same as that of FIGS. 9 (A) and 9 (B), and FIG. 11 (B) shows a state in which the substrate 1 is inverted.

Therefore, this is placed in the vacuum chamber 54,
The pattern electrode 51a is grounded, and the electron beam 5 is applied to the electrode 6.
5 (FIG. 11C). In this case, the polarization is inverted by irradiating it with the electron beam 55 until the applied electric field reaches 210 kV / cm to form the polarization non-inversion region 3 and the polarization inversion region 4. Then, similarly to FIG. 9E, the optical waveguide 2 is formed (FIG. 11D).

Although the case where the electron beam 55 is irradiated is shown in FIG. 11C, the ion beam may be irradiated. Further, similar to FIGS. 9 and 10, a groove of the buffer portion or a space buffer region may be formed on the substrate 1 on which the pattern electrode 51a is formed.

As described above, the polarization inversion having a sufficient depth of 20 μm or more in the period of 3.4 μm can be made long in the propagation direction of the optical waveguide 2. Thereby, the optical waveguide 2
The interaction region of and polarization inversion can be made long both in the depth direction and the length direction, and the conversion efficiency of the wavelength conversion element can be improved. Moreover, the manufacturing time of the periodic polarization inversion can be made shorter than that of the conventional method of forming a narrow aperture pattern of the electron beam itself.

Next, a method of manufacturing the second optical harmonic generating element according to the seventh embodiment of the present invention will be described with reference to FIG.

Referring to FIG. 12, in the seventh embodiment of the present invention, a metal electrode layer 61 made of Ti is formed on the + C surface corresponding to the upper main surface of the ferroelectric substrate 1 made of LiTaO _3. , -C which also corresponds to the lower main surface of the substrate 1
The back electrode layer 6 is also formed on the surface. Next, the electrode layer 61
Is patterned to repeatedly form a number of elongated rectangular openings 61a extending in the Y direction in the X direction.

In this embodiment, a direct current power supply is connected between the electrode 61 and the electrode 6, and a region where the polarization is inverted is formed in the substrate except the portion where the opening 61a is formed. That is, by applying a DC voltage between the electrodes 61 and 6, a rectangular domain inversion region corresponding to the opening 61a is formed in the substrate. Further, by forming the optical waveguide extending in the X direction so as to cross the rectangular polarization inversion region, the interaction between the fundamental wave and the repetition of the periodic polarization inversion region is increased, and the optical second harmonic It becomes possible to form an SHG element with improved generation efficiency.

By the way, in such an SHG element, the rectangular polarization inversion region formed corresponding to the opening 61a has a straight and clear front edge portion and rear edge portion, so that efficient wavelength conversion is performed. Important above. However, in such a rectangular domain inversion region, the size in the longitudinal direction,
Therefore, it has been found that when the length L of the opening 61a is large, the front and rear edges of the domain-inverted region deviate from the straight line and become wavy.

In FIG. 15, the vertical axis represents the amplitude of deviation from the straight line at the front and rear edges of the domain-inverted region, while the horizontal axis represents the longitudinal size L of the rectangular opening 61a. As can be seen from FIG. 15, the wave amplitude greatly increases when the size L of the mask opening 61a exceeds 50 μm. That is, it is understood from FIG. 15 that the size L is preferably 50 μm or less. In particular, it can be seen that better results are obtained when the size L is 20 μm or less. In addition, a similar relationship to this is that the pattern of the electrode 61 is reversed and the opening 61
This also applies when an electrode piece is formed instead of a.

In the example of FIG. 12, it can be seen that the electrode 6 formed on the lower main surface of the substrate 1 is patterned so that the portions 6a and 6b on both sides thereof are removed. By patterning the electrode 6 in this way, it is possible to eliminate the possibility of discharge occurring between the upper electrode and the lower electrode via the side wall of the substrate.

FIG. 13 shows a modification of the seventh embodiment.
In this modification, a pair of elongated openings 61b extending in the x direction are formed on both sides of the opening 61a. In this embodiment, by forming the opening 61a in this way, it becomes possible to concentrate the electric field on both ends of the opening 61a,
This makes it possible to more clearly form the domain-inverted region.

FIG. 14 shows another modification of the seventh embodiment of the present invention. In this embodiment, the electric field is further concentrated on both ends of the rectangular domain inversion region, and at the same time, in the structure of FIG. 13, it is possible to prevent the discharge that occasionally occurs on the substrate surface exposed by the opening 61b. That is, the opening 61b which is a single continuous opening in the modification of FIG.
Is divided into a plurality of smaller openings 61c by providing the bridging conductor portion 61d. In addition, the bridging conductor portion 61
By forming d, charges are supplied to the portion where the opening 61a is formed.

Of course, it is possible to combine the various features described in the previous embodiments with the seventh embodiment. For example, the isolation region such as the groove 52 shown in FIG. 9 or the exposed surface 53 shown in FIG. 10 is formed in the embodiment of FIG. 13, and the direct current voltage is further applied in vacuum as in the case of FIG. It is possible to maximize the effects of the present invention.

Furthermore, the present invention is not limited to the above embodiments, and various modifications and changes can be made.

[0087]

As described above, according to the present invention, the polarization can be inverted deeply inside the substrate while suppressing the lateral expansion.

[Brief description of drawings]

FIG. 1 is a manufacturing process diagram of a first embodiment of the present invention.

FIG. 2 is a manufacturing process diagram of a second embodiment of the present invention.

FIG. 3 is a manufacturing process diagram of a second embodiment of the present invention.

FIG. 4 is a configuration diagram of a third embodiment of the present invention.

FIG. 5 is a configuration diagram of another embodiment of the third embodiment.

FIG. 6 is a manufacturing process drawing of the fourth embodiment of the present invention.

FIG. 7 is a configuration diagram of another embodiment of the fourth embodiment.

FIG. 8 is a configuration diagram of another embodiment of the fourth embodiment.

FIG. 9 is a manufacturing process drawing of the fifth embodiment of the present invention.

FIG. 10 is a manufacturing process diagram of another example of the fifth example.

FIG. 11 is a manufacturing process drawing of the sixth embodiment of the present invention.

FIG. 12 is a manufacturing process drawing of the seventh embodiment of the present invention.

FIG. 13 is a diagram showing a modification of the seventh embodiment of the present invention.

FIG. 14 is a diagram showing another modification of the seventh embodiment of the present invention.

FIG. 15 is a diagram showing the effect of the seventh embodiment of the present invention.

FIG. 16 is a diagram for explaining a conventional SHG element.

FIG. 17 is a schematic view showing a conventional method of forming a partially domain-inverted region.

FIG. 18 is a schematic diagram showing a conventional method of forming a partially domain-inverted region.

FIG. 19 is a schematic diagram showing a conventional method of forming a partially domain-inverted region.

[Explanation of symbols]

1 substrate 2, 2a optical waveguide 3 polarization non-inversion region 4 polarization inversion region 5, 10, 22 mask 5a, 10a, 22a, 61a, 61b, 61c opening 6 electrode 6a, 6b exposed region 7, 7a, 7b DC power supply 11 Proton exchange region 21 Pattern electrode 23 Pyrophosphoric acid 24a, 24b, 41a, 41b Insulating film 42a, 42b, 51a Pattern electrode 52 Groove 53 Space buffer region 55 Electron beam

Claims (22)

[Claims] 1. A step of forming electrodes on both surfaces of a substrate (1) made of a ferroelectric material, and applying a predetermined electric field between the electrodes while suppressing the growth of a domain-inverted region in the lateral direction, A step of forming a polarization non-inversion region (3) and a polarization inversion region (4), and suppressing the growth of the polarization inversion region in the lateral direction is performed only in a portion where the polarization inversion region (4) is to be formed. A method for forming a partially domain-inverted region, which comprises applying a predetermined electric field after exchanging constituent ions of the substrate (1) with protons. 2. A mask forming step of forming a mask (5) made of metal on a substrate (1) made of a ferroelectric material, and the substrate (1) through an opening (5a) of the mask (5).
And a step of forming an electrode (6) on the surface of the substrate (1) opposite to the surface on which the mask (5) is formed, The part according to claim 1, further comprising at least a polarization reversal step of applying a DC voltage between the electrode (6) and the electrode (6), to advance a polarization reversal region into the inside of the substrate (1). Of forming a static domain inversion region. 3. A method for forming a partial domain-inverted region, which comprises heating the substrate (1) in the domain-inverting step according to claim 2. 4. The DC voltage applied between the mask (5) and the electrode (6) is changed in accordance with the progress of polarization inversion in the region where protons are exchanged in the polarization inversion step according to claim 2 or 3. A method for forming a partially domain-inverted region, characterized by: 5. A mask (10) made of metal is formed on a substrate (1) made of a ferroelectric material, and the constituent ions of the substrate (1) are protonated through an opening (10a) of the mask (10). Exchanging to form the proton exchange region (11), and removing the mask (10) to form the proton exchange region (1).
1) forming a pattern electrode (13b) on the substrate, and forming an electrode (6) on the surface of the substrate (1) opposite to the surface on which the electrode (13b) is formed; The step of applying a predetermined voltage between two electrodes (13b, 6) to cause polarization inversion under the electrode (13b) of the substrate (1), the partial process according to claim 1. Method of forming domain inversion region. 6. A pattern electrode (42a, 42b) adjacent to each other is formed on the surface of a ferroelectric substrate (1) on which a domain-inverted pattern is formed, and an electrode (6) is formed on the opposing surface. Independently of the step of forming and applying a predetermined electric field with a potential difference to the two pattern electrodes (42a, 42b), a polarization inversion region (4) is formed under one of the pattern electrodes (42a). And forming a polarization non-inversion region (3) under the other pattern electrode (42b), the partial polarization inversion region forming method. 7. The two patterned electrodes (42a, 42a)
7. The method for forming a partially domain-inverted region according to claim 6, wherein in step b), the comb-teeth-shaped electrodes are formed in alignment with each other in a meshed state. 8. A step of forming a pattern electrode (21) on one surface of a substrate (1) made of a ferroelectric substance and forming an electrode (6) on the other surface thereof, and the pattern electrode (21) and the electrode ( 6) between the steps of applying a predetermined electric field in a vacuum to form the polarization non-inversion region (3) and the polarization inversion region (4), and forming a partially polarization inversion region. Method. 9. The method for forming a partially domain-inverted region according to claim 5, wherein the predetermined voltage is applied in a vacuum. 10. A step of forming a pattern electrode (21) on one surface of a substrate (1) made of a ferroelectric material and forming an electrode (6) on the other surface of the substrate (1), the pattern electrode (21) being formed. The substrate (1)
A step of forming an insulating film (24b) on at least the exposed portion, and applying a predetermined electric field between the pattern electrode (21) and the electrode (6) to obtain the polarization non-inversion region (3) and the polarization non-inversion region (3). A method of forming a partially domain-inverted region, comprising the step of forming an inversion region (4). 11. An insulating film (2) is formed on at least an exposed portion of the substrate (1) on which the pattern electrode is formed.
4b) is included.
10. A method of forming a partially domain-inverted region according to any one of 9 to 10. 12. A step of forming electrodes (51, 6) on both surfaces of a substrate (1) made of a ferroelectric material, and preventing discharge when an electric field is applied on the substrate (1) on which the electrodes are formed. A step of forming a buffer part (52, 53) for forming a polarization non-inversion region (3) and a polarization inversion region (4) by applying a predetermined electric field between the electrodes. A method for forming a characteristic partially domain-inverted region. 13. The substrate (1) on which the electrode is formed.
A buffer part (52, 5) for preventing discharge when an electric field is applied.
The method of forming a partially domain-inverted region according to claim 1, further comprising the step of forming 3). 14. The partial polarization inversion region according to claim 12, wherein the buffer portion is a groove (52) or a space buffer region (53) or both a groove and a space buffer region. Method. 15. A step of forming a pattern electrode (51a) on one surface of a substrate (1) made of a ferroelectric substance and forming an electrode (6) on the other surface, and one electrode (51a or 6). While being grounded, the opposite electrode (6 or 51a) is charged with a charged beam (5
5) and applying a predetermined electric field to form a polarization non-inversion region (3) and a polarization inversion region (4), and a method for forming a partial polarization inversion region. 16. The charged beam is an electron beam (5
5) or an ion beam.
5. The method for forming a partially domain-inverted region according to 5. 17. A first electrode (61) is formed on one surface of a ferroelectric substrate (1) and a second electrode is formed on the other surface.
Forming an electrode (6), and patterning the first electrode so that a plurality of elongated patterns (61a) extend parallel to each other on the one surface of the substrate; A step of repeatedly forming in a direction different from the extending direction; applying a predetermined direct current voltage between the first electrode and the second electrode to form the ferroelectric substrate in the ferroelectric substrate; In the method of forming a partial polarization inversion region, which comprises the step of inducing polarization reversal in a portion corresponding to a plurality of elongated patterns, the step of patterning the first electrode comprises a step of patterning the plurality of elongated patterns (61a). Each in its longitudinal direction,
A method characterized by forming so as to extend with a length of 50 μm or less. 18. The method of patterning the first electrode, wherein each of the plurality of elongated patterns is formed so as to extend in the longitudinal direction by a length of 20 μm or less. 19. The step of patterning the first electrode (61) comprises a plurality of elongated openings (61a) in the first electrode as the elongated pattern, each elongated opening of the elongated pattern. Forming the elongated openings so as to extend in line with the extending direction, and the elongated openings are repeated in the different direction different from the extending direction to form a row; 2. A pair of different elongated openings are formed on both sides of each row of the rows so as to extend in the extending direction of the row.
8. The method according to 8. 20. A first electrode (61) is formed on one surface of a ferroelectric substrate (1) and a second electrode is formed on the other surface.
Forming an electrode (6), and patterning the first electrode so that a plurality of elongated patterns (61a) extend parallel to each other on the one surface of the substrate; A step of repeatedly forming in a direction different from the extending direction; applying a predetermined direct current voltage between the first electrode and the second electrode to form the ferroelectric substrate in the ferroelectric substrate; A method of forming a partial domain-inverted region, which comprises a step of inducing polarization reversal in a portion corresponding to a plurality of elongated patterns, wherein a structure for preventing discharge between the first electrode and the second electrode ( 6a, 6b), wherein the step of patterning the first electrode is such that each of the plurality of elongated patterns (61a) extends in its longitudinal direction by a length of 50 μm or less. Formed into a plurality; As a long pattern, a plurality of elongated openings (61a) are provided in the first electrode such that each elongated opening extends in a direction corresponding to an extending direction of the elongated pattern, and the elongated openings are formed. It is formed so as to be repeated to form a row in the different direction different from the extending direction; and on each side of the row of the elongated openings, a pair of different elongated openings is provided in the extending direction of the row. The method of applying a DC voltage is performed in a vacuum. 21. The structure for preventing the discharge is the second
The electrode (6) is patterned so that the electrode patterns extending substantially in the direction of the columns of the elongated openings (61a) are exposed so that the other surface (6a, 6b) of the substrate (1) is exposed on both sides thereof. 21. The method of claim 20, wherein the method is formed by forming. 22. A substrate (1) having a partially domain-inverted region (4) formed thereon, and incident light formed by being embedded in the substrate (1) in a direction perpendicular to the domain-inverted region (4). An optical second harmonic element comprising: an optical waveguide (2a) that further generates a second harmonic.