JP2002277915A - Polarization inversion forming method and light wavelength converting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form thick polarization inversion of a high aspect ratio by a method of forming a polarization inversion structure in a dielectric by electric field application. SOLUTION: This polarization inversion forming method has unipolarized ferroelectric crystal 1, and a process of forming a polarization inversion part 3 in a 1st area of ferroelectric crystal and a process of forming a polarization inversion part 5 in a 2nd area of the ferroelectric crystal and is characterized by that the polarization inversion parts formed in the 1st and 2nd areas overlap with each other at least partially. Consequently, the formed polarization inversion can be increased in thickness while having its cycle directional expansion suppressed and the thick polarization inversion structure of the high aspect ratio can be formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a uniform and thick domain-inverted region in an arbitrary region when domain inversion is formed in a ferroelectric crystal substrate. In addition, the present invention relates to a method for forming a domain-inverted structure having a high aspect ratio, particularly on an X-cut substrate, a Y-cut substrate, and an off-cut substrate made of a ferroelectric crystal. Further, the present invention relates to an optical wavelength conversion element having periodic polarization inversion formed by the above method.

[0002]

2. Description of the Related Art A ferroelectric material has a phenomenon of inversion of spontaneous polarization in addition to having an insulating property and a high dielectric constant. Polarization reversal of a single-polarized ferroelectric crystal enables control of light waves such as non-linear optical effects, electro-optical effects, and acousto-optical effects. Optical switches, optical polarizers, surface acoustic wave devices, focusing elements, nonlinear optics It can be applied to many fields such as devices. As one of devices using the polarization inversion phenomenon, there is a wavelength conversion element (hereinafter, referred to as an SHG element) that obtains output light having a wavelength of λ / 2 by injecting laser light having a wavelength of λ. Various approaches have been taken for the development of SHG elements. For example, a ferroelectric crystal Li
The fact that a periodically poled structure is formed in NbO ₃ or LiTaO ₃ to form a pseudo phase matching state (a state in which the fundamental wave and the second harmonic have the same phase velocity) and high-efficiency wavelength conversion can be achieved. was suggested. In this SHG element,
It has the advantage that the output wavelength can be controlled by the period of the periodic domain inversion to be formed. For example, when obtaining blue light with a wavelength of 420 to 450 nm as the output wavelength, about 3 μm is required as the domain inversion cycle. .

Further, as an optical wavelength conversion device using a domain-inverted structure, an optical waveguide type SHG element capable of highly efficient wavelength conversion has been proposed. This forms a periodic domain-inverted structure in the light propagation direction. By confining the light in the waveguide, wavelength conversion with much higher efficiency than the bulk type SHG element is achieved. In the optical waveguide type SHG element, it is important to sufficiently overlap the guided light and the domain-inverted portion in order to increase the efficiency by confining the light. (Thickness) was desired to be larger than the cross-sectional thickness of the waveguide. In addition, in order to obtain a perfect phase matching state, it is important to form a domain inversion with a uniform period.

As one of effective methods for forming a domain-inverted region in a ferroelectric material, particularly a ferroelectric crystal substrate, a contact electrode having a pattern corresponding to a periodic domain-inverted structure is formed on a substrate surface. Methods for applying an electric field in a high vacuum or in an inert insulating liquid have been proposed. Specifically, Japanese Patent Application Laid-Open No. Hei 5-210132 proposes a polarization inversion forming method for forming a polarization inversion on a LiNbO ₃ crystal substrate which is monopolarized in an in-plane direction. A method in which counter electrodes are arranged at intervals of 7 to 200 μm in the polarization direction on the same main surface and an electric field is applied at a voltage of 15 kV or more / mm, and a method of applying two or more pulsed electric fields under these conditions. Proposed.

In Japanese Patent Application Laid-Open No. 9-218431, an off-cut substrate (the direction of spontaneous polarization of To the angle θ (0 °
An optical waveguide type wavelength conversion element using a substrate cut at a plane satisfying <θ <90 °) has been proposed. Among them, a method has been disclosed in which electrodes having a predetermined pattern are arranged on the same main surface of the off-cut substrate or in a direction parallel to the polarization direction, and an electric field is applied from the outside to form a periodically poled portion. . In an off-cut substrate, the direction of spontaneous polarization is inclined with respect to the crystal surface, and when an electric field is applied, the polarization inversion is formed so as to be parallel to the direction of spontaneous polarization, that is, to dive into the substrate along the crystal axis. As a result, in the domain where polarization inversion occurs near the surface of the offcut substrate, the Xcut substrate or the Y
A domain-inverted portion thicker than the cut substrate can be obtained.

[0006]

However, when a domain inversion is formed on a ferroelectric crystal X-cut substrate, Y-cut substrate, or off-cut substrate by the above-described conventional domain-inverted formation method, a domain-inverted portion is not formed. The aspect ratio (the ratio d / W of the width (polarization reversal width) W of the domain reversal portion to the domain reversal width W) and the thickness d of the domain reversal is small. For example, when the thickness of the optical waveguide is about 2 μm in the optical waveguide type wavelength conversion element, the periodic polarization inversion structure is not sufficiently formed with respect to the cross section of the optical waveguide. Was causing the decline. In particular, in the case of an X-cut substrate or a Y-cut substrate, it is difficult to form a thick domain inversion because an electrode pair is formed in the same main surface, and it has been a problem to form a thick periodic domain inversion. .

Further, when the desired polarization inversion period is as short as about 3 to 4 μm, there is a problem that the polarization inversion shape becomes non-uniform. In the method proposed in JP-A-5-210132, the electrode spacing is limited to 200 μm or less in a LiNbO ₃ crystal substrate, so that the above crystal breakage is likely to occur when a high voltage electric field is applied. Voltage is limited. Therefore, it was difficult to form a thick polarization inversion. In addition, Japanese Patent Application Laid-Open No. 9-2184
No. 31 describes the formation of a domain inversion having a thickness of 2 to 3 μm in the off-cut substrate in the example, but the domain inversion cycle at this time is as large as 4.75 μm, and
The width of the domain inversion in the periodic direction was 2.5 μm. In this method, it is difficult to form a polarization inversion having a high aspect ratio, especially in a short-period domain inversion,
Period 3μm required for generation of short wavelength light of about 450nm
There is a problem that it is difficult to form a domain-inverted structure to a certain extent. In particular, in an optical waveguide type optical wavelength conversion element, the polarization inversion period required to generate a shorter wavelength of violet to blue-violet light having a wavelength of 400 to 420 nm is 3 μm or less, and the uniform and thick polarization required to achieve high conversion efficiency is obtained. There was a problem that formation of inversion was more difficult.

[0008]

In order to solve the above-mentioned problems, the present invention provides a single-polarized ferroelectric crystal and a step of forming a domain-inverted portion in a first region of the ferroelectric crystal. ,
Forming a domain-inverted portion in the second region of the ferroelectric crystal, wherein at least a part of the domain-inverted portion formed in the first region and the second region overlaps with each other. This is a polarization inversion forming method.

Further, a single-polarized ferroelectric crystal,
Forming a domain-inverted portion in a different region of the ferroelectric crystal, wherein the shape of the domain-inverted portion formed between the regions in the different regions is controlled by a distance between the regions. It is a forming method.

A single-polarized ferroelectric crystal;
A step of forming a periodic domain-inverted portion having a period of an integral multiple of a desired length Λ in the ferroelectric crystal a plurality of times, wherein positions at which the periodic domain-inverted portions are formed are different from each other; This is a polarization inversion forming method.

In addition, the device has a domain-inverted portion or a periodic domain-inverted portion formed by any of the above-described domain-inverted forming methods, and has an arbitrary point A in the thickness direction in a domain-inverted cross section perpendicular to the polarization direction. The polarization inversion width w1 at the point B, the polarization inversion width w2 at the point B inside the crystal from the point A, and the polarization inversion width w3 at the point C inside the crystal from the point B are w1 ≧ w
An optical wavelength conversion element having a domain-inverted shape satisfying a relationship of 3 ≧ w2 or w3 ≧ w1 ≧ w2.

When the off-cut angle θ is 0.5 ° ≦ θ ≦
In the ferroelectric crystal of 7 °, there is provided a periodic domain-inverted structure with a period of 3 μm or less formed by any one of the above-mentioned domain-inverted formation methods. The ratio between the inversion thickness d and the polarization inversion width w is d
/W≧1.0 is a light wavelength conversion element.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. Here, the MgO-doped LiNbO ₃ crystal (hereinafter abbreviated as MgO: LN crystal) will be described as an example of the ferroelectric crystal, but the present invention is not limited to the MgO: LN crystal.

(Embodiment 1) A polarization inversion forming method of applying an electric field opposite to a spontaneous polarization of a single-polarized ferroelectric material, particularly a ferroelectric crystal substrate, and partially inverting the spontaneous polarization. Has been performed more widely than before. In order to form an electric field opposing spontaneous polarization (hereinafter abbreviated as polarization), positive and negative electrodes arranged substantially parallel to the + C side and the -C side of the polarization direction are formed. An electric field was applied to the ferroelectric crystal substrate. Also,
In some cases, an electric field component perpendicular to the polarization direction is added to the electric field in the polarization direction to achieve uniform polarization reversal.However, in either method, in order to reverse the polarization, A paired electrode arrangement is required.

For example, in the case of a Z-cut substrate whose polarization direction is perpendicular to the main surface of the substrate, electrodes are formed on the two main surfaces of the substrate facing each other, and an X-cut substrate whose polarization direction is parallel to the main surface of the substrate is formed. , Y-cut, or off-cut (a substrate cut out on a plane that is not parallel to the Z-axis and either the X-axis or the Y-axis, and has an angle θ (0) with respect to the direction of spontaneous polarization of the ferroelectric crystal. ° <θ <90 °) In the case of a substrate, the positive and negative electrodes are formed on the same main surface of the substrate. At this time, by forming the electrode arranged on the + C side into a periodic pattern, a periodic domain-inverted structure can be formed, and is used for, for example, an optical wavelength conversion element.

In general, in the periodically poled structure, it is important to control the period thereof and the duty ratio of the poled and non-inverted portions. Since the overlap between the structure and the domain-inverted structure determines the characteristics of the device, it is required that the domain-inverted thickness from the substrate surface be large. For a Z-cut substrate, for example, 0.5 mm
Since the domain inversion is formed in the thickness direction of the thick substrate, the above-mentioned requirement can be satisfied by controlling the domain inversion cycle and the duty ratio. However, in the case of the X-cut, Y-cut, and off-cut substrates, it is particularly difficult to control the thickness of the domain inversion, and the thickness of the domain inversion formed by, for example, the electrode interval and the domain inversion cycle has been limited. Therefore, a short-period periodic domain-inverted structure is formed,
For example, in order to produce a highly efficient light wavelength conversion element, means for increasing the thickness of the domain-inverted portion has been required.

We have considered the increase in the thickness of the domain inversion as follows.
The relationship between the off-cut substrate angle (off-cut angle θ) and the aspect ratio in the conventional polarization inversion forming method was examined.
4A shows the relationship between the off-cut angle θ and the polarization inversion thickness d, and FIG. 4B shows the off-cut angle θ and the aspect ratio of the domain-inverted portion (the ratio of the domain-inverted thickness d to the domain-inverted width w = d). / W)
Shows the relationship. As a result, the duty ratio (the ratio between the domain-inverted portions and the non-inverted portions) of the periodic domain-inverted structure is substantially 1: 1.
It has been found that, when the domain inversion is formed so as to be as follows, the domain inversion thickness d increases as the substrate angle θ increases. On the other hand, the aspect ratio d / w increases as the substrate angle increases, but does not depend on the domain inversion period Λ. This means that the domain inversion thickness d is limited by the domain inversion period Λ.

Therefore, in order to form a short-period domain-inverted structure having a large domain-inverted thickness, (1) domain-inverted is formed using a substrate having a large off-cut angle θ.

(2) The thickness of the domain inversion is increased by a new domain inversion forming method. A method was conceived.
The method (1) using a substrate having a large off-cut angle θ has already been generally disclosed, and a conventional polarization inversion method by applying an electric field has also been proposed. However, for example, in an optical waveguide device that performs optical coupling with a semiconductor laser that oscillates in a TE mode, there is a problem that the coupling loss increases depending on the substrate angle. It was required to realize.

On the other hand, a feature of the present invention is that in a method of forming domain-inverted portions in different regions of a ferroelectric crystal, domain-inverted portions are formed so that at least a part of each region overlaps. This makes it possible to greatly increase the thickness of the domain-inverted portion while controlling the aspect ratio of the domain-inverted portion. In the present embodiment, a case where an MgO: LN crystal substrate is used as a ferroelectric crystal substrate will be described.

First, the polarization inversion method of the present invention will be described with reference to the drawings. FIG. 1 shows an example of a configuration diagram of a method for forming domain inversion on an off-cut MgO: LN crystal substrate according to the first embodiment of the present invention. FIG. 1B is a cross-sectional view when FIG. 1A is viewed from the −Y plane. Also,
FIG. 1D is a cross-sectional view when FIG. 1C is viewed from the −Y plane. FIGS. 1A and 1B show a configuration in a step of forming a domain-inverted portion in a first region, and FIGS.
(C) and FIG. 1 (d) are views showing a configuration in a step of forming a domain-inverted portion in the second region.

1 (a) to 1 (d), reference numeral 1 denotes a single-polarized off-cut MgO: LN crystal substrate;
Is the power supply. 1A and 1B, reference numeral 2a denotes a first main surface (+ X) of the MgO: LN crystal substrate 1.
Surface), a comb-shaped electrode having a periodic pattern formed on
b is a striped electrode paired with the comb-shaped electrode 2a, and 3 is a periodic domain-inverted portion formed in the first region. 1 (c) and 1 (d), the dotted lines indicate the formation positions of the comb-shaped electrode 2a and the striped electrode 2b in FIGS. 1 (a) and 1 (b).

4a is a comb-shaped electrode 2 shown in FIG.
A comb-shaped electrode 4b having a periodic pattern formed at a position different from a is a striped electrode paired with the comb-shaped electrode 4a, and 5 is a periodic domain-inverted portion formed in the second region. At this time, the comb electrodes 2a and 4a have a periodic pattern corresponding to the period of the domain-inverted portions 3 and 5 to be formed, and have the same period in the present embodiment. The power supply 6 is a pulse electric field generation source capable of controlling an applied voltage, an application time, and a rising speed.

In this embodiment, a 1.5 ° cut X substrate having a thickness of about 500 μm (an off cut cut so that the X-axis direction is inclined by 1.5 ° with respect to the crystal substrate surface) is used as a MgO: LN crystal substrate. Substrate), the electrode fingers of the comb-shaped electrodes 2a and 4a are 40 to 100 μm in length, 1 μm in width,
It was formed with a period of 2-3 μm. Comb electrodes 2a and 4a,
Materials for forming the striped electrodes 2b and 4b include Ta (tantalum), Al (aluminum), Cu
Although conductive materials such as (copper), Au (gold), and Ag (silver) can be used, the resistivity is particularly 1.0 × 1.
By using a material of 0 ^-7 Ωm or less, the electric field intensity distribution immediately below the electrode when an electric field is applied can be made uniform, so that the uniformity of the domain-inverted portions 3 and 5 formed can be improved.

Hereinafter, a case where a Ta electrode is used in the present embodiment will be described. The thickness of the comb-shaped electrode 2a and the stripe-shaped electrode 2b used in the step of forming the domain-inverted portion in the first region was set to several hundreds to 2,000 degrees. Further, the comb-shaped electrode 4a and the striped electrode 4 used in the step of forming the domain-inverted portion in the second region are used.
Similarly, b was formed at a thickness of several hundreds to 2,000.
The comb electrodes 2a and 4a and the stripe electrodes 2b and 4b are formed by vapor deposition, sputtering, or the like.
This was easily performed by forming a Ta thin film on the surface of the LN crystal substrate 1 and using a photolithography process and an etching process, or a lift-off process. By applying an electric field to the MgO: LN crystal substrate 1 using the power source 6 via the comb-shaped electrode 2a and the stripe-shaped electrode 2b thus formed, the domain-inverted portion 3
Could be formed.

After the step of forming domain-inverted portions in the first region, the comb-shaped electrode 2a and the striped electrode 2b are removed by wet etching using hydrofluoric nitric acid (HF: HNO ₃ = 2: 1 mixture). Thereafter, a Ta thin film was formed again, and a comb-shaped electrode 4a and a striped electrode 4b were formed by the same process as described above. By applying a second electric field to the MgO: LN crystal substrate 1 using the power supply 6 via these electrodes, the domain-inverted portions 5 could be formed. The comb-shaped electrode 2a and the striped electrode 2 used in the step of forming the domain-inverted portions in the first region
Even when a Ta film was formed without removing b and photolithography and dry etching were performed, a similar domain-inverted portion 5 could be formed. In this case,
There is an advantage that the alignment between the comb-shaped electrode 2a and the comb-shaped electrode 4a can be easily performed in the photolithography process.

Further, in the polarization inversion forming method of the present invention, particularly, the positioning accuracy of the comb-shaped electrode 2a and the comb-shaped electrode 4a is important. ±
It was possible to suppress it to 0.05 μm or less. As a result, the domain-inverted portions 3 and 5 could be formed without impairing the domain-inverted width and duty ratio or the uniformity of the periodic domain-inverted structure. In particular, in the case of the Y-cut substrate and the off-cut Y plate, hydrofluoric acid (H
F: HNO ₃ = 2: 1 mixed solution) when removing the electrode by wet etching using YO of MgO: LN crystal.
The domain inversion / non-inversion region can be observed as a minute step due to the difference in the etching rate in the direction. By performing a photolithography process using this, there is an advantage that highly accurate positioning can be performed.

The shape of the domain-inverted nucleus generation region (domain inversion-generated region on the substrate surface) formed by the domain inversion forming method using electric field application is controlled by the shape of the electrode used and the applied electric field conditions. At this time, the cross-sectional shape of the domain-inverted portion is a shape obtained by projecting the shape of the domain-inverted nucleus generation region.

The domain-inverted nucleus generation region is generated in a region where the electric field intensity is large (when a comb-shaped electrode in which the electrode finger is arranged toward the -Z axis side is used, the region immediately below the tip of the electrode finger is used). appear).

The length of the domain-inverted nucleus generation region (X cut,
The Z direction in the Y-cut or off-cut substrate: the longitudinal direction of the electrode fingers of the comb-shaped electrode) and the thickness of the domain-inverted portion are controlled by the magnitude of the applied electric field (applied voltage) and the amount of electric charge (current and applied time). You.

The width of the domain-inverted nucleus generation region (periodic direction of the electrode fingers of the comb-shaped electrode) depends on the applied voltage, the amount of charge, and the application time (the effect of heat generated by the applied current). I knew that. Based on our previous studies,
Furthermore, the generation of heat at the time of applying an electric field can be suppressed by a technique such as multi-pulse application (a short pulse is applied a plurality of times at intervals), and the spread of the domain inversion width can be suppressed to some extent. That knowledge was obtained. However, there is a limit to the method of suppressing the spread of the domain inversion width (for example, application of multiple pulses). Although the spread is suppressed as compared with the application of a single-pulse electric field, the spread gradually increases. It is insufficient to form a sufficiently thick domain-inverted portion in the periodic domain-inverted formation.

We have considered that the cause of this "slight spread of the domain inversion width" is the presence or absence of an electrode. The coercive electric field (the threshold electric field strength at which the polarization inversion is formed) decreases in the peripheral region of the region where the polarization inversion is formed, and the polarization is easily inverted.
Therefore, when the same electric field pulse is applied using the same electrode, the polarization inversion length and the polarization inversion width are enlarged. We pay attention to this point, and it is possible to increase the domain inversion thickness by eliminating the spread of domain inversion width by adopting a method in which the electrode position is shifted and a high electric field is not applied to the region where the domain inversion is already formed. I thought I could.

Based on this idea, as shown in FIG. 1C, an electrode pair used in a step of forming a domain-inverted portion in the second region is formed by a step of forming a domain-inverted portion in the first region. (For example, 15 to 40 in a 1.5 ° cut X plate)
(μm) with respect to the electrode pair used in the step of forming the domain-inverted portion in the first region, and shifted in the Z direction to form a domain-inverted portion in the second region. This allows
In the domain-inverted nucleus generation region of the domain-inverted portion formed in the first region, the portion from which the electrode has been removed is not affected by the electric field when the domain-inverted portion is formed in the second region. The domain-inverted portion could be enlarged in the Z direction while suppressing the extension of the domain-inverted portion in the periodic direction.

At this time, the domain-inverted portions 3 formed in the first region and the domain-inverted portions 5 formed in the second region were fused inside the substrate to form a domain-inverted region having a high aspect ratio. Further, when the domain-inverted portions 3 and the domain-inverted portions 5 are formed so as to at least partially overlap each other, it is possible to form a structure having a substantially uniform domain-inverted width especially in the domain-inverted cross-sectional shape. was gotten. This is due to the fact that the shape of the domain-inverted nucleus generation region of the off-cut substrate shown in the present embodiment has a feature that the width decreases from directly below the tip of the electrode finger toward the + Z direction.

Therefore, for example, not the length of the domain-inverted nucleus generation region formed in the first step but the domain-inverted width from the tip (the end on the −Z side) of the domain-inverted nucleus generation region is larger than the electrode finger width. An electrode pair used in the second step is defined as +
Shift in the Z direction.

Estimate the distance from the tip of the domain-inverted nucleus generation region (the end on the -Z side) formed in the second step to the portion where the domain-inverted width is smaller than the electrode finger width (can be estimated by the applied electric field conditions) ), The electrode pair used in the second step is shifted in the −Z direction as a shift amount of the electrode position. By adopting such a method, it was possible to form a structure having a substantially uniform polarization inversion width in the polarization inversion cross-sectional shape. Also, by repeating the above steps,
It has been found that a periodically poled structure with a high aspect ratio can be formed over a large area in the Z direction.

FIG. 2 shows details of the polarization inversion formed in the first region and the second region of the MgO: LN crystal substrate using the configuration shown in FIG. Figure 2 shows MgO: LN
An electric field was applied to the crystal off-cut substrate using electrodes arranged in a direction substantially parallel to the polarization direction, and a new polarization inversion was formed so as to at least partially overlap the region where the polarization inversion was already formed. FIG. 4 is a schematic diagram showing a domain-inverted region in a substrate at the time. Here, it is assumed that the electrode pair used in the step of forming the domain-inverted portion in the second region is shifted in the −Z direction with respect to the electrode pair used in the step of forming the domain-inverted portion in the first region. Show. FIG. 2B is a cross-sectional view of FIG. 2A taken along line AA ′ and viewed from the −Z plane. FIG. 2D is a cross-sectional view of FIG. 2C taken along line BB ′ and viewed from the −Z plane. 2 (a) and 2 (b) show the domain-inverted portions formed in the first electric field application step, and FIGS. 2 (c) and 2 (d) show the case where the second electric field is further applied. FIG. 4 is a diagram illustrating a domain-inverted portion formed in FIG.

2A to 2D, reference numeral 7 denotes a single-polarized off-cut MgO: LN crystal substrate;
Is a periodically poled portion formed in the first region.
2 (c) and 2 (d), reference numeral 9 denotes a periodic domain-inverted portion formed in the second region. First, the domain-inverted portions 8 were formed in the first region of the off-cut MgO: LN crystal substrate 7 by the above-described steps. Specifically, for example, it was possible to form a periodic polarization inversion having a period of 3 μm and a reversal thickness of about 1.3 μm on a 1.5 ° cut X plate. At this time, the domain inversion width was about 1.7 μm, and the aspect ratio of the domain inversion portion 8 (ratio d / w of domain inversion thickness d to domain inversion width w) was about 0.75. In addition,
As described above, the domain-inverted portions 8 formed in the off-cut substrate are formed so as to dive inside the substrate, and the thickness d of the domain-inverted portions 8 becomes smaller toward the inside of the substrate. Therefore,
The polarization inversion thickness near the tip of the electrode finger (the part where the polarization inversion portion 8 is separated from the substrate surface: AA ′ cross section in FIG. 2A) is the largest.

Further, the domain-inverted portions 9 were formed in the second region by re-forming the electrode pairs and applying an electric field in the above-described steps. At this time, the amount of displacement of the electrodes used for forming the domain-inverted portions 8 and 9 in the Z direction is such that the domain-inverted portions are formed in the second region with respect to the electrodes in the step of forming the domain-inverted portions in the first region. The electrode used in the forming process is-
The thickness was about 25 μm in the Z direction. In the BB 'cross section, the domain inversion 8 (the domain inversion section 8 formed in the first region shown in FIG. 2D) is inside the substrate, and the domain inversion thickness of only the domain inversion section 8 is about 0.8 μm. However, by overlapping with the domain-inverted portion 9, the domain-inverted thickness as a whole (FIG. 2)
(Polarization inversion thickness at BB ′ section in (c)) is about 1.8
μm. At this time, the domain inversion width was about 1.7 μm, and the aspect ratio of the domain inversion cross section was able to be greatly increased to about 1.05.

In FIG. 2C and FIG. 2D, the domain-inverted portion 8 and the domain-inverted portion 9 are separately illustrated for easy understanding, but actually formed and observed. The domain-inverted portions are indistinguishable in their characteristics (such as etching rate). A domain-inverted portion formed and observed only by a process of forming a domain-inverted portion in the first region;
In the comparison with the domain-inverted portions when the step of forming domain-inverted portions in the region and the second region is performed, the existence thereof can be confirmed by the difference in shape and the difference in the thickness of domain-inverted portions.

We further examined and considered the amount of shift between the electrode pair used in the step of forming the domain-inverted portion in the second region and the electrode pair used in the step of forming the domain-inverted portion in the first region. Hereinafter, characteristics of domain inversion formed by a conventional domain inversion forming method using electric field application will be described. The applied electric field condition and the electrode structure were as follows: applied voltage 5 kV, applied time 2 ms, electrode interval 400 μm, domain inversion period 3.0 μm.
m. The electric field condition and the electrode structure are conditions under which the thickest and uniform domain-inverted structure can be obtained. The following (Table 1) shows the relationship between the off-cut angle and the polarization reversal characteristics in the off-cut X-plate.

[0042]

[Table 1]

FIG. 5 shows an off-cut X plate.
Relationship between the amount of positional deviation (-Z direction) between the electrode used for forming the domain-inverted portion in the first region and the electrode used for forming the domain-inverted portion in the second region, and the domain-inversion characteristic Is shown. From FIG. 5: There is an optimum amount of deviation that maximizes the thickness of the domain-inverted portion to be formed.

The optimum shift amount at which the polarization reversal thickness becomes maximum changes depending on the substrate angle. I understood that.
According to the results of this study, the fusion of the domain-inverted portion formed by applying the first electric field and the domain-inverted portion formed by applying the second electric field is performed when the off-cut angle θ is in the range of 0 ° <θ ≦ 7 °. When the displacement amount L of the formation position of the electrode pair is L × tan θ ≦
It occurred when the relationship of 1.5 was satisfied.

In the present embodiment, only the step of forming the domain-inverted portions in the first and second regions has been described as an example of the domain-inverted formation method, but the same electrode re-formation and electric field application steps are repeated. By doing so, Mg
O: Over almost the entire surface of the LN crystal substrate, it was possible to produce a region where thick periodic domain inversion was formed near the substrate surface. As a result, for example, in the manufacture of an optical waveguide device, the position of the optical waveguide can be easily controlled, and the interval between the optical waveguides that can be manufactured can be reduced, so that the optical waveguide device can be integrated. there were. Further, in the present embodiment, 1.5 ° cut M
The gO: LN crystal substrate has been described as an example, but the same method can be applied to a substrate having 0 ° <off-cut angle θ ≦ 7 ° to increase the domain-inverted thickness and increase the size of the thick domain-inverted formation region. The area could be increased.

In the present embodiment, the position (shifting direction) of the electrode pair used for applying the second electric field is set in the −Z direction with respect to the electrode pair used for applying the first electric field, but the position is shifted in the + Z direction. Also in this case, the same domain-inverted portions 3 and 5 can be formed, and a periodic domain-inverted structure having a high aspect ratio can be formed. When the formed periodic domain-inverted structure is used, for example, as an optical waveguide device, it is necessary to align the domain-inverted portion with the optical waveguide. At this time, for example, the domain-inverted portion is formed in the second region. By shifting the electrode pair used in the process in the −Z direction, alignment is performed particularly with reference to the comb-shaped electrode portion used for applying the electric field, and an optical waveguide can be formed accurately at a position several μm from the tip of the electrode finger. There was an advantage that.

In the present embodiment, as the polarization inversion method by applying an electric field, the application of an electric field via an electrode pair formed on the same main surface of an off-cut MgO: LN crystal substrate has been described. For example, a Ta thin film is formed on the other main surface (back surface) opposite to the paired surface to form a back electrode, and an electric field is applied simultaneously between the comb electrode and the stripe electrode and between the comb electrode and the back electrode. (Two-dimensional electric field application method). In this case, by forming at least the electrode pair, it is possible to form a domain-inverted portion having a high aspect ratio, which is an effect of the present invention.

As described in the present embodiment, a method including a step of forming a domain-inverted portion so that a part of the domain-inverted formation region overlaps, for example, after forming a first domain-inverted region, Several to several tens of μm from the tip of the electrode finger of the comb-shaped electrode
May be etched and an electric field is applied. The advantage of this method is that the first polarization inversion forming step and the second
In this case, there is no need to align the comb electrodes used in the domain inversion forming step in the periodic direction. However, unlike the method of the present invention, since the electrode pair spacing is different in the first polarization inversion forming step and the second polarization inversion forming step, it is necessary to change the applied electric field condition depending on the etching amount. At this time, since the electrode spacing in the second polarization inversion forming step is always larger than the electrode spacing in the first polarization inversion forming step, the applied voltage tends to increase in the second polarization inversion forming step. Therefore, there is a demerit that the possibility of crystal destruction due to application of a high-voltage electric field appears. Also, the domain inversion forming region can be expanded only in the + Z direction (it is difficult to increase the area).

Since the tip of the electrode finger is removed by etching, it is difficult to align the tip of the domain-inverted region (the end on the -Z side) with the optical waveguide when used as an optical waveguide device. There are drawbacks. It can be said that the polarization inversion forming method in the present invention is an effective means which does not have these disadvantages.

Next, as another example of the present embodiment, a domain-inverted portion is formed in a first region of an X-cut MgO: LN crystal substrate by the same configuration as that of the above-mentioned off-cut substrate. The details of the domain-inverted portion formed in the step of forming the domain-inverted portion in the second region and the step of forming the domain-inverted portion in the second region will be described in detail. Although the X-cut substrate will be described here, the same effect can be obtained with a Y-cut substrate. FIG. 3 shows X of the MgO: LN crystal which is a ferroelectric crystal.
An electric field is applied to the cut substrate by using an electrode arranged in a direction substantially parallel to the polarization direction, and a substrate in which a new polarization inversion is formed so as to at least partially overlap a region where the polarization inversion has already been formed. FIG. 3 is a schematic diagram showing a domain-inverted region in FIG. Here, it is assumed that the electrode pair used in the step of forming the domain-inverted portion in the second region is shifted in the −Z direction with respect to the electrode pair used in the step of forming the domain-inverted portion in the first region. Show.

FIG. 3B and FIG. 3C show FIG.
It is sectional drawing at the time of seeing from the -Z plane cut | disconnected by the line segment LL 'and MM' of (a), respectively. FIG.
3 (e) and FIG. 3 (f) show the state of FIG.
It is sectional drawing when it cut | disconnects at -O 'and sees from -Z plane.
FIGS. 3A, 3B, and 3C show the domain-inverted portions formed in the first region, and FIGS.
(E) and FIG. 3 (f) are diagrams showing the domain-inverted portions further formed in the second region, wherein the line segments NN ′ and O−
The positions of O 'correspond to the positions of line segments LL' and MM ', respectively. 3A to 3F, reference numeral 10 denotes a single-polarized X-cut MgO: LN crystal substrate, and reference numeral 11 denotes a periodic domain-inverted portion formed in the first region. 3 (d), 3 (e) and 3
In (f), reference numeral 12 denotes a periodically poled portion formed in the second region.

In general, when a domain-inverted portion is formed on an X-cut or Y-cut substrate by the above-described method, the domain-inverted thickness decreases along the Z-axis direction (as compared with the domain-inverted thickness in FIG. 3B). 3 (c) is small in polarization inversion thickness). This is because the electric field strength due to the electrode pair formed on the substrate surface is smaller in the portion far from the electrode than in the vicinity of the tip of the comb-shaped electrode. Therefore, for example, there is a problem that the thickness of the domain-inverted structure is not uniform in the Z direction when the electrode interval is set to several 100 μm. However, by using the polarization inversion forming method of the present invention, it becomes possible to form a polarization inversion portion having a uniform thickness over a large area.

First, the domain-inverted portions 11 were formed in the first region of the MgO: LN crystal substrate 10 by the above-described steps.
Specifically, for example, it was possible to form the periodic domain-inverted portions 11 having a period of 3.2 μm and a maximum value of domain-inverted thickness of about 1.0 μm. The position where the polarization reversal thickness is maximum (FIG. 3
The polarization inversion width in the line segment LL ′ section (a) is about 1.5.
μm, and the aspect ratio was about 0.7. However, the thickness of the domain-inverted portion gradually decreases along the Z-axis direction,
−A position at 200 μm in the Z direction (the line segment M− in FIG.
The domain-inverted thickness at M ′ cross section was about 0.75 μm. At this time, the domain inversion width was about 1.5 μm, and the aspect ratio of the domain inversion section 11 was about 0.5.

As described above, in the X-cut substrate, the aspect ratio of the domain-inverted portion changes along the Z direction. Then, when the domain-inverted portion 12 is formed in the second region by re-forming the electrode pair and applying an electric field, which are the domain-inverted formation methods of the present invention, the domain-inverted portion 12 overlaps the domain-inverted portion 11. The polarization inversion thickness of the OO ′ cross section in 3 (d) could be reduced to about 1.0 μm. At this time, the domain-inverted width is about 1.5 μm, and the aspect ratio of the domain-inverted portion cross section is about 0.7 over the entire domain-inverted formation region, so that a uniform periodic domain-inverted structure can be formed. Was.

Next, the effects and advantages of the case where the domain inversion forming method of the present invention is used in comparison with the conventional domain inversion forming method by applying an electric field will be described. The problems in the conventional domain inversion forming method solved by the present invention include (1) an increase in the domain inversion aspect ratio in the off-cut substrate and (2) a uniform domain inversion thickness in the X-cut substrate and the Y-cut substrate. Further, as an advantage of the configuration of the present invention, (1)
There is an improvement in the degree of integration of the domain-inverted structure. Hereinafter, each will be described in detail.

First, an increase in the aspect ratio of a domain-inverted portion when domain inversion is formed on an MgO: LN crystal off-cut substrate by application of an electric field will be described. A method using an off-cut substrate has been proposed as one means for increasing the thickness of polarization inversion in a MgO: LN crystal substrate.
As shown in FIG. 4, it was possible to increase the polarization inversion thickness in the case of the same period (for example, period of 3 μm) by increasing the off-cut angle. Specifically, at a period of 3 μm, a 0 ° cut substrate (X cut plate or Y cut substrate)
Is about 1.3 μm for a 1.5 ° cut substrate, 1.8 μm for a 3 ° cut substrate,
With a 5 ° cut substrate, a substrate of 2.2 μm could be formed.

However, an increase in the off-cut angle causes a problem of an increase in coupling loss particularly in an optical waveguide device on the premise of optical coupling with a semiconductor laser oscillating in the TE mode. Therefore, in the present invention, as shown in FIG. 1, a domain-inverted portion is formed in the first region of the off-cut MgO: LN crystal substrate, and the electrode pair is formed again. An attempt was made to increase the domain-inverted thickness by forming a domain-inverted portion in the second region so as to partially overlap the formed portion.

As described above, the domain-inverted thickness d of the domain-inverted portion formed in the off-cut substrate increases as the substrate angle θ increases. At this time, the aspect ratio increases as the substrate angle increases, but does not depend on the polarization inversion period. This is due to the mechanism of generation and growth of the domain-inverted portions. In general, a domain-inverted region is generated in a portion where an electric field in a direction opposite to spontaneous polarization of a crystal is applied, and grows along a polarization axis (Z-axis) direction depending on an applied charge amount. However, the electric field intensity distribution in the substrate rapidly decreases from the vicinity of the electrode on the substrate surface toward the inside of the substrate, so that the domain-inverted thickness is limited. Further, the spread (in the thickness direction and the width direction) of the domain-inverted portion is also affected by heat generated by electron movement (inversion current) when an electric field is applied. Therefore, when, for example, a high electric field is applied for a long time to increase the polarization inversion thickness, the inversion thickness and the inversion width increase.

However, in particular, in forming a periodically poled structure used for a wavelength conversion device or the like, it is necessary to control the duty ratio of the poled portion and the non-inverted portion. It is limited by the period. We believe that in order to increase the aspect ratio of the domain-inverted portion in the formation of the short-period domain-inverted structure, it is necessary to further increase the domain-inverted thickness. In the present invention, as shown in FIG. And applying an electric field using the re-formed electrode pair to form a domain-inverted region that at least partially overlaps the domain in which the domain inversion has already been formed. , It is possible to significantly increase the thickness of the domain-inverted layer.

In particular, the effect of increasing the thickness of the domain-inverted domain while maintaining the width of the domain-inverted domain constant is great, and a periodic domain-inverted structure having a high aspect ratio can be formed. As a result, a domain inversion with a period of 2.0 to 3.0 μm can be formed on an off-cut substrate of 0 ° <θ ≦ 7 °, and a domain inversion thickness of 1.3 times or more as compared with the related art can be obtained. Was. In particular, the effect is remarkable in an off-cut substrate of 3 ° or less where low-loss optical coupling with a TE mode semiconductor laser can be expected. For example, when used in an optical waveguide type optical wavelength conversion element, the most efficient quasi-phase matching state is obtained. It is possible to increase the overlap between the domain-inverted structure portion and the cross-section of the optical waveguide and to increase the overlap between the guided light and the domain-inverted portion, thereby producing an optical wavelength conversion element with higher conversion efficiency. Was completed.

Next, a description will be given of the homogenization of the domain inversion thickness when domain inversion is formed on the X-cut and Y-cut substrates of the MgO: LN crystal by applying an electric field. As described above, when a domain-inverted portion is formed by applying an electric field to an X-cut or Y-cut substrate, the domain-inverted thickness decreases along the Z-axis direction. This is because the electric field strength due to the electrode pair formed on the substrate surface is smaller in a portion far from the electrode, for example, in the vicinity of the tip of the comb-shaped electrode. Therefore, there is a problem that the domain-inverted thickness is not uniform in the Z direction.

However, by using the polarization inversion forming method of the present invention, the thick polarization inversion portion near the electrode finger can
It can be formed gradually by shifting in the direction, and a domain-inverted portion having a uniform thickness can be formed in a large area.
Accordingly, for example, conventionally, when an optical waveguide device is manufactured by forming an optical waveguide every 30 to 50 μm in a domain-inverted formation region (several 100 μm in the Z direction), there is a problem that characteristics between the waveguides greatly vary. However, there is an effect that this variation can be avoided and a device having uniform characteristics can be manufactured.

As shown in the present embodiment, the domain inversion is formed by applying an electric field, and the electrode pair is re-formed and the electric field is applied to the re-formed electrode pair.
As another effect of forming the domain-inverted portion so that a part thereof partially overlaps with a domain in which domain-inversion has already been formed, there is an advantage that the integration density of the periodic domain-inverted region that can be formed can be improved. there were. This is MgO:
This is made possible by repeatedly forming positive and negative electrodes on the same main surface of the LN crystal substrate. FIG. 6 shows the difference in the degree of integration of the domain inversion formation region between the conventional domain inversion formation method and the domain inversion formation method of the present invention.

FIG. 6A shows an example of an electrode structure in the case of performing domain inversion by applying an electric field, and FIG. 6B shows a state of domain inversion formed by a conventional domain inversion method. c) shows the state of the domain-inverted portion formed in the case of the domain-inverted formation method of the present invention. 6 (a), 6
13 (b) and FIG. 6 (c), 13 is the M of the off cut.
gO: LN crystal substrate. 6A, reference numeral 14a denotes a comb-shaped electrode having a periodic pattern, 14b denotes a striped electrode, and 15 denotes a power supply. Comb-shaped electrode 14a,
The striped electrodes 14b are formed and arranged on the same main surface of the MgO: LN crystal substrate 13. At this time, the comb-shaped electrode 14a is arranged on the + Z-axis side with respect to the electrode 14b.

L1 and L2 indicate the distance between adjacent electrodes, respectively. The stripe electrodes 14b are arranged so as to have a one-to-one correspondence with the comb electrodes 14a. 6 (b) and 6 (c).
Reference numeral 6 denotes the formed domain-inverted portion. When an electric field is applied using the power supply 15 in such a configuration, the formed domain inversion occurs immediately below the tip of the electrode finger of the comb-shaped electrode 14 and grows in the −Z-axis direction.

In the electrode structure shown in FIG. 6A, the distances L1 and L2 between the electrodes need to be 200 μm or more in order to prevent dielectric breakdown when an electric field is applied. More desirably, the intervals L1 and L2 need to be 400 μm or more in order to greatly reduce dielectric breakdown. Therefore, the density of the periodically poled regions (the number of periodically poled structures per unit area) in FIG. 6B is limited depending on the distance. On the other hand, in the domain-inverted formation method of the present invention, since the electrode pair is reformed and the electric field is applied, a domain-inverted structure as shown in FIG. 6C can be formed. That is, it is possible to form a periodic domain-inverted structure having the same period and a high aspect ratio over almost the entire surface of the MgO: LN crystal substrate regardless of the electrode interval.

By setting L1 and L2 to 400 μm and repeating electrode re-formation and electric field application, a uniform periodic domain-inverted structure is formed in the Z direction of the MgO: LN substrate having a length of 10 mm in the Z direction. The periodic polarization reversal was successfully formed uniformly over the region. The amount of displacement of the electrode pair at this time was about 25 μm. As a result, the degree of integration of the periodically poled structure was improved. Therefore, for example, when manufacturing an optical waveguide type wavelength conversion element from a wafer-like MgO: LN crystal substrate, the number of elements that can be manufactured from
There is an advantage that it can be increased up to 15 times or more.
In particular, in an optical waveguide type device using periodic polarization inversion, it is possible to form an optical waveguide in a portion where an optical waveguide could not be formed conventionally (a portion having a small polarization inversion thickness) from the viewpoint of interaction efficiency. Thus, for example, there is an effect that the effective area of the MgO: LN crystal substrate (the area that can be used as an optical waveguide device) can be increased by 10 times or more.

In the present embodiment, MgO: L
Offcut substrate and X-cut of N crystal substrate has been described taking as an example the polarization inversion forming method of Y-cut substrate, a ferroelectric crystal, (hereinafter referred to as LN) LiNbO _3, abbreviated as LiTaO ₃ (hereinafter LT ) Or Zn
Even when LN, LT, or the like doped with In, Sc, or the like is used, domain-inverted formation can be performed by the domain-inverted formation method of the present invention.
Even when a cut substrate and an off-cut substrate are used, a domain-inverted portion is formed by applying an electric field, a pair of electrodes is re-formed, and an electric field is applied through this pair of electrodes. Further, a domain-inverted portion is further formed so that a part of the domain-inverted portion is overlapped, and the effect that the domain-inverted thickness can be increased can be obtained. As a result, the thickness of the domain-inverted layers formed could be increased about 1.3 times.

(Embodiment 2) In the present embodiment, in the step of forming domain-inverted portions in different regions of a ferroelectric crystal, the shape of domain-inverted portions formed by the distance between domain-inverted formation regions is controlled. The method for performing the above will be described in detail. In order to form a periodically poled structure in a ferroelectric crystal by an electric field application method using a contact electrode, an electrode pair is generally formed in a direction substantially parallel to the polarization direction of the ferroelectric crystal surface. In the off-cut substrate, an electrode pair is formed in the same main surface. When a periodic domain-inverted structure is formed, it is necessary that at least the electrode arranged on the + Z-axis side of the crystal has a pattern corresponding to a desired periodic domain-inverted structure. This is because the polarization reversal that is formed
This is because they are generated and grown from the + Z axis side.

A voltage of several kV / mm to several tens of kV / mm is applied between the electrode pair formed in this manner so that the potential of the electrode on the negative side (−Z-axis side) of the spontaneous polarization of the ferroelectric crystal becomes low. By applying a voltage, inversion of spontaneous polarization can be caused. However, in the conventional electric field application method, (1) the width of each domain-inverted portion of the formed periodic domain-inversion is formed larger than the width of each electrode finger (periodic portion) of the periodic pattern electrode ( (For an electrode finger width of 1 μm, the formed polarization inversion width becomes 2 μm or more.) (2) Variation occurs in the width and shape of each polarization inversion portion formed by simultaneous electric field application. (3) Short period In the formation of the domain-inverted structure, there is a problem that it is difficult to increase the domain-inverted thickness in the thickness direction of the substrate.

In particular, when forming a periodically poled structure having a short period of about 3 to 4 μm, it is important to solve the above problems. In an optical waveguide device such as an optical waveguide wavelength conversion element, the overlap between the propagation light propagating through the optical waveguide and the domain inversion portion greatly contributes to the conversion efficiency. Means for forming a thicker polarization inversion have been desired. In the off-cut substrate, the shape of the domain-inverted portion has a correlation with the shape of the domain-inverted nucleus generation region (domain-inverted region immediately below the electrode). It was necessary to control the shape of the inversion nucleation region.

Heretofore, the shape of the domain-inverted nucleus generation region has been controlled by the shape of the electrode formed on the surface of the ferroelectric material, the conditions of the applied electric field, and the like. However, it was insufficient to obtain a desired domain-inverted shape (thickness, period, uniformity). Therefore, the method described in the first embodiment has been studied. We have considered in the study that a domain that has already been domain-inverted exists in the -Z direction at a slight distance from the area where the domain-inverted portion is to be formed (i.e., the tip on the -Z side of the electrode). If not, between the regions (the region where the domain-inverted portion is already present and the non-inverted region between the domain-inverted portion newly formed)
It has been discovered that the domain-inverted nucleus generation region is expanded so as to fill the gap, and the domain-inverted portion formed is fused.

A feature of the present invention is that, unlike the method shown in the first embodiment, a method of forming a domain-inverted portion in a different region having an interval, and controlling the interval to form a domain-inverted region between regions. In that the shape of the domain-inverted portion is controlled. By controlling the finally formed domain-inverted shape, it is possible to form domain-inverted portions with a high aspect ratio in domain-inverted formation on a substrate with a small off-cut angle or in short-period periodic domain-inverted formation. became. In the present embodiment, a case will be described in which an off-cut MgO: LN crystal substrate is used as a ferroelectric crystal substrate.

First, the polarization inversion method of the present invention will be described with reference to the drawings. FIG. 7 shows an example of a configuration diagram of a method for forming domain inversion on an off-cut MgO: LN crystal substrate according to the second embodiment of the present invention. FIG. 7C is a cross-sectional view when FIG. 7B is cut along line DD ′ and viewed from the −Z plane. FIG. 7A is a diagram illustrating a configuration in a step of forming a domain-inverted portion in a second region in an MgO: LN crystal in which a first domain-inverted portion has already been formed. In FIGS. 7A to 7C, reference numeral 17 denotes a single-polarized off-cut MgO: LN crystal substrate;
Is a periodically poled portion formed in the first region.

In FIG. 7A, reference numeral 21 denotes a power source, 2
2a is the first main surface of the MgO: LN crystal substrate 17 (+ X
Surface), a comb-shaped electrode having a periodic pattern formed on
2b is a striped electrode paired with the comb electrode 22a, L
Reference numeral 3 denotes a distance between the first domain inversion forming region and the second domain inversion forming region. 7B and FIG. 7C, reference numeral 19 denotes a periodic domain-inverted portion formed in the second region, and reference numeral 20 denotes a region formed between the first region and the second region. This is the polarization reversal part. At this time, the comb-shaped electrode 22a
Has a periodic pattern corresponding to the period of the domain-inverted portions 19 to be formed, and has the same period as the domain-inverted portions 18 in the present embodiment. The power supply 21 is an applied voltage,
It is a pulsed electric field generation source capable of controlling the application time and the rising speed.

In the present embodiment, a 1.5 ° cut X substrate having a thickness of about 500 μm is used as the MgO: LN crystal substrate, and the electrode fingers of the comb-shaped electrode 22 a have a length of 40 to 100 μm.
m, width 1 μm, period 2-3 μm. Comb electrode 2
2a and the material forming the striped electrode 22b are Ta (tantalum), Al (aluminum), Cu
Although conductive materials such as (copper), Au (gold), and Ag (silver) can be used, the resistivity is particularly 1.0 × 1.
By using a material of 0 ^-7 Ωm or less, the electric field intensity distribution immediately below the electrode when an electric field is applied can be made uniform, so that the uniformity of the domain-inverted portions 19 and 20 formed can be improved.

The operation of the polarization inversion method configured as described above will be described below. In this embodiment, a case where a Ta electrode is used will be described. The thickness of the comb-shaped electrode 22a and the stripe-shaped electrode 22b used in the step of forming the domain-inverted portion in the second region is set to several tens.
It was formed between 0 ° and 2000 °. The comb-shaped electrode 22a and the striped electrode 22b can be easily formed by forming a Ta thin film on the surface of the MgO: LN crystal substrate 17 by vapor deposition, sputtering, or the like, and using a photolithography process, an etching process, or a lift-off process. Was completed.

The comb-shaped electrode 22a thus formed
And M using the power source 21 via the stripe-shaped electrode 22b.
By applying an electric field to the gO: LN crystal substrate 17, the domain-inverted portions 19 could be formed. The electric field is applied in an inert insulating liquid, and the
An external electrode was brought into contact with a and the stripe-shaped electrode 22b, and a pulse electric field from the power supply 21 was applied to the MgO: LN crystal substrate 17 through these electrodes. At this time, Mg
O: The potential of the comb-shaped electrode 22a arranged on the + Z-axis side of the LN crystal substrate 17 is higher than the potential of the stripe-shaped electrode 22b arranged on the -Z-axis side. An electric field was applied by applying a pulsed electric field having an application time of 20 ms or less in mm.

When an electric field is applied to the ferroelectric crystal to form a domain inversion portion, a minute current called an inversion current flows. This is because electrons move when an electric field is applied. When an electric field is applied to a single-polarized ferroelectric crystal, the growth direction of polarization reversal (that is, the polarization direction) Flows along. At this time, if there is a region where a domain-inverted portion has already been formed in the vicinity of the domain-inverted formation region, particularly on the −Z side, the reversal current tends to flow toward that region. This is because the polarization direction of the domain-inverted portion is in the forward direction with respect to the direction of the electric field applied to the ferroelectric crystal, and this makes it a region where current flows more easily than the non-inverted region. It is. In this way, a domain-inverted portion is also formed in a non-inverted portion between domain-inverted regions due to an inversion current flowing toward a region in which domain inversion has already been formed.

We have further investigated this phenomenon and found that such a phenomenon occurs when the distance (interval L3) between the already formed domain-inverted region and the region where the domain-inverted portion is newly formed is 10 μm or less. It was found that it was limited to the case. The shape of the domain-inverted portion formed at this time is, as shown in FIG. 7C, the domain-inverted portion 18 formed in the first region and the domain-inverted portion 19 formed in the second region. The width of the domain-inverted portion 20 formed in the region between the first region and the second region is smaller than the width (spread in the periodic direction). This is because the region where the reversal current flows is limited between the domain inversion forming regions. The width of the domain-inverted portions 20 is determined by the interval L3 between the domain-inverted formation regions. The larger the interval L3, the smaller the width of the domain-inverted portions 20. By using the domain-inverted formation method of the present invention, the effect that the domain-inverted thickness can be increased while suppressing the domain-inverted portion from spreading in the periodic direction is obtained.

Specifically, 1.5 ° cut MgO: L
In the N-crystal substrate, when the interval L3 was set to about 5 μm, a periodic domain-inverted portion having a period of 2 to 3 μm was formed, and a domain-inverted thickness 1.5 times or more as compared with the conventional example could be obtained. At this time, the duty ratio of the domain-inverted portion / non-inverted portion could be made approximately 1: 1. In particular, when a periodically poled structure having a period of 3 μm or less is formed in a ferroelectric crystal having an off-cut angle θ of 0.5 ° ≦ θ ≦ 7 ° by the domain inversion forming method described in the present embodiment. Sets the ratio d / w of the polarization inversion thickness d to the polarization inversion width w in the domain inversion section perpendicular to the polarization direction to 1.
It could be set to 0.

In the 1.5 ° cut substrate, when the distance L3 is larger than 5 μm, the domain-inverted thickness formed in the second region in the domain-inverted cross section becomes small, so that the first region and the region Region 2 and the total thickness of domain inversion formed between the first region and the second region is reduced, or the first region and the second region
It has been found that the domain-inverted portions are formed in a distance from the substrate thickness direction due to insufficient formation of domain-inverted portions between the regions. This indicates that the optimum interval L3 exists in the substrates having any of the off-cut angles.

In this embodiment, as an example of a method of forming a domain-inverted portion in a different region having an interval, a case where a domain-inverted portion is formed in advance in a first region has been described. In the method of simultaneously forming the domain-inverted portions in the region and the second region, a domain-inverted region was induced in the region between the domain-inverted formation regions, and the domain-inverted shape finally obtained could be controlled. This will be described with reference to FIGS.

FIG. 8 shows another example of a method for forming domain inversion on an off-cut MgO: LN crystal substrate according to the second embodiment of the present invention. FIG. 8B shows FIG. 8A as -Y.
It is sectional drawing when it sees from a surface. FIG. 8A and FIG.
In (b), 23 is a single-polarized off-cut M
gO: LN crystal substrate, 24a is MgO: LN crystal substrate 2
3, an island electrode having a periodic pattern formed on the first main surface (+ X plane) (a pattern electrode having an island electrode electrically independently formed on an electrode fingertip of a comb-shaped electrode);
4b is a striped electrode paired with the island electrode 24a, L
4 is a pattern interval of the island-shaped electrodes 24a, and 25 is a power supply. In FIG. 8B, reference numeral 26 denotes a domain-inverted portion formed from a region immediately below the island-shaped electrode 24a, and reference numeral 27 denotes a domain-inverted portion induced in a region between the domain-inverted portions 26.

The operation of the polarization inversion forming method configured as described above will be described below. In this embodiment, a case where a Ta electrode is used will be described. The thickness of the above-mentioned island-like electrode 24a and stripe-like electrode 24b was formed in a range of several hundreds to 2000 degrees. The formation of the island-like electrodes 24a and the stripe-like electrodes 24b is easily performed by forming a Ta thin film on the surface of the MgO: LN crystal substrate 23 by vapor deposition, sputtering, or the like, and using a photolithography process, an etching process, a lift-off, or the like. I was able to. The power supply 25 is connected via the island-like electrode 24a and the stripe-like electrode 24b thus formed.
By applying an electric field to the MgO: LN crystal substrate 23 using the method described above, the domain-inverted portions 26 and 27 could be formed.

The electric field is applied in an inert insulating solution, and an external electrode is brought into contact with the island-like electrode 24a and the stripe-like electrode 24b. Was applied.
At this time, the potential of the island electrode 24a arranged on the + Z axis side of the MgO: LN crystal substrate 17 is higher than the potential of the stripe electrode 24b arranged on the -Z axis side, and the applied voltage is increased. ~ Several tens of kV / mm, application time 20 m
An electric field was applied by applying a pulse electric field of s or less.

As described above, the island-shaped electrode 24a has a structure in which an electrically independent conductor exists at the electrode fingertip of the comb-shaped electrode. However, the distance from the comb electrode is
When it is as small as 00 μm, when an electric field is applied, the electric field acts on the island portion, and the island portion behaves as if it were the tip of the electrode finger of the comb-shaped electrode. Accordingly, the electric field at the island-shaped electrode portion increases, and domain-inverted nuclei are generated immediately below the electrode at the island-shaped portion. At this time, the domain-inverted nucleus generation region becomes a discontinuous region when the interval between the island-shaped portions is large, for example. When a comb-shaped electrode having electrode fingers of a length is used, the length is substantially equal to the length in the Z direction of the domain-inverted nucleus generation region.

In the domain-inverted formation using such an electrode structure, the interval L4 between the island-shaped portions of the island-shaped electrode 24a is set to 10
When the thickness was less than μm, the domain-inverted portions 27 were induced so as to fill the region between the domain-inverted portions 26 generated immediately below the electrodes as described in the present embodiment. The shape of the domain-inverted portion formed at this time is different from the shape of the domain-inverted portion formed by the comb-shaped electrodes, and the domain-inverted width of the domain-inverted portion 27 (the domain-inverted spread in the Y-axis direction) is different from that of the domain-inverted portion 26. It became smaller than the domain inversion width. The domain inversion width of the domain inversion part 27 can be controlled by controlling the interval L4, and it is possible to control the domain inversion shape (particularly domain inversion width) formed by this.

FIG. 9 shows another example of a method of forming domain inversion on an off-cut MgO: LN crystal substrate according to the second embodiment of the present invention. Note that FIG. 9B is obtained by changing FIG.
It is sectional drawing when it sees from a surface. 9 (a) and 9
In (b), 28 is a single-polarized off-cut M
gO: LN crystal substrate, 29a is MgO: LN crystal substrate 2
8, a comb-shaped electrode having a periodic pattern formed on the first main surface (+ X plane), 29b is a stripe-shaped electrode paired with the island-shaped electrode 29a, and 30 is a power supply. FIG.
In FIG. 3B, reference numeral 31 denotes a domain-inverted portion formed from a region immediately below the comb-shaped electrode 29a, and reference numeral 32 denotes a domain-inverted portion induced in a region between the domain-inverted portions 31. In FIG. 9B, reference numeral 33 denotes an insulating material arranged so as to partially prevent the comb-shaped electrode 29a from contacting the surface of the MgO: LN crystal substrate 28.

The operation of the polarization inversion forming method configured as described above will be described below. In this embodiment, a case where a Ta electrode is used will be described. First,
As described above, in order to prevent the comb-shaped electrode 29a from partially contacting the surface of the MgO: LN crystal substrate 28, an insulating material (for example, SiO ₂ film) is sputtered with MgO: LN.
It was formed on the surface of the crystal substrate 28 and patterned by a photolithography process and an etching process. At this time, the thickness of the insulating material is set to 200 so that the portion of the comb-shaped electrode 29a that does not contact the surface of the MgO: LN crystal substrate 28 is sufficiently separated from the surface of the MgO: LN crystal substrate 28.
0 ° or more. Further, the length of the insulating material 33 in the Z direction (that is, the MgO: LN crystal substrate 28 of the comb-shaped electrode 29a)
The length of the portion not in contact with the surface in the Z direction) was 2 to 15 μm.

Next, the thickness of the comb-shaped electrode 29a and the stripe electrode 29b was formed in a range of several hundreds to two thousand degrees. The formation of the comb-shaped electrode 29a and the striped electrode 29b could be easily performed by forming a Ta thin film on the surface of the MgO: LN crystal substrate 28 by vapor deposition or sputtering, and using a photolithography process and an etching process. At this time, the positioning of the pattern of the insulating material 33 and the comb-shaped electrode 29a could be performed with an accuracy of 0.1 μm in the Z direction. The power supply 3 is connected via the comb-shaped electrode 29a and the striped electrode 29b thus formed.
By using 0 to apply an electric field to the MgO: LN crystal substrate 28, the domain-inverted portions 31 and 32 could be formed.

As shown in this embodiment, the insulating material 33
As shown in FIG. 9B, the domain-inverted portions 31 generated immediately below the comb-shaped electrodes 29a are formed in discrete regions in the Z direction. At this time, as in the case of using the above-mentioned island-shaped electrode, when the length of the insulating material 33 in the Z direction is 10 μm or less, the domain inversion is performed so as to fill the region between the domain inversion portions 31 generated immediately below the electrode. Section 32 was induced. The shape of the domain-inverted portion formed at this time is different from the shape of the domain-inverted portion formed by the comb-shaped electrode formed without disposing the insulating material 33, and the domain-inverted width of the domain-inverted portion 32 (in the Y-axis direction). (Polarization reversal spread) is smaller than the polarization reversal width of the domain reversal part 31.

The width of the domain inversion of the domain inversion section 32 can be controlled by controlling the length of the insulating material 33 in the Z direction, so that the domain inversion shape (particularly the domain inversion width) formed thereby can be controlled. there were. Also, particularly, the off-cut angle θ
When a periodically poled structure having a period of 3 μm or less is formed in a ferroelectric crystal in which 0.5 ° ≦ θ ≦ 7 ° by the domain inversion forming method described in this embodiment, the polarization direction and In the domain-inverted cross section in the vertical direction, the ratio d / w of the domain-inverted thickness d to the domain-inverted width w could be set to 1.0.

(Embodiment 3) In this embodiment, in a method of forming periodic domain inversion in a ferroelectric crystal substrate, domain inversion is performed by using an electrode structure having a period that is an integral multiple of a desired period. A method of controlling the domain-inverted shape by forming a domain inversion at a different position for each process will be described. When performing periodic domain inversion in a ferroelectric crystal substrate,
For example, when a very fine periodic structure of 3 μm or less is formed, for example, the space between the electrode fingers of the comb-shaped electrode becomes very small, and the electric fields of adjacent electrode fingers act on each other. There is a problem that the polarization reversal width increases.

For example, there is a crystal substrate called a stoichiometric crystal, which has attracted attention as an optical device material because of its large nonlinearity and high light damage resistance in LiNbO ₃ and LiTaO ₃ crystals, for example. These substrates have the merit that the threshold voltage of domain inversion formation (applied voltage at which domain inversion can be formed) can be reduced, but compared with a crystal substrate having a congruent composition widely used,
When applied with an electric field, it is susceptible to the electric field of adjacent electrode fingers, so that it has a characteristic that the domain inversion in the periodic direction is large, and it is difficult to form a domain inversion with a period of 4 μm or less. .

On the other hand, a feature of the present invention is that in the method of forming a periodic domain inversion cycle in a ferroelectric crystal, domain inversion is performed using an electrode having a cycle that is an integral multiple of a desired cycle Λ. In this method, a polarization inversion forming step is repeated a plurality of times, and a desired periodic polarization inversion is obtained by shifting an electrode formation position in each step. According to the present invention, it becomes possible to perform the domain-inverted formation while controlling the shape in the short-period periodic domain-inverted formation or the domain-inverted formation in the above-described ferroelectric crystal substrate having a fast spreading speed in the periodic direction. In this embodiment, a case where an MgO: LN crystal substrate is used as a ferroelectric crystal substrate will be described.

First, the polarization inversion forming method of the present invention will be described with reference to the drawings. FIG. 10 shows an example of a configuration diagram of a method for forming domain inversion on an X-cut MgO: LN crystal substrate according to the third embodiment of the present invention. In the present embodiment, a case will be described where a desired period in the formation of the periodic domain-inverted structure is Λ, and a period of the periodic pattern of electrodes used for domain-inverted formation is 2 Λ. FIG.
FIG. 10B is a view of the ferroelectric crystal substrate after the first polarization inversion forming step is viewed from the Z plane, and FIG. 10C is the ferroelectric crystal substrate after the second polarization inversion forming step. The figure which looked at the dielectric crystal from the Z plane is shown.

In FIGS. 10 (a), 10 (b) and 10 (c), reference numeral 34 denotes an X-cut MgO: LN crystal substrate. 10A, reference numeral 35 denotes a comb-shaped electrode having a periodic pattern used in the first polarization inversion forming step, 36 denotes a comb-shaped electrode having a periodic pattern used in the second polarization inversion forming step, and 37 denotes a comb-shaped electrode. A stripe-shaped electrode 38 paired with the electrode 35 and the comb-shaped electrode 36 is a power source.
In FIGS. 10B and 10C, reference numeral 39 denotes a domain-inverted portion formed in the first domain-inverted formation step. In FIG. 10C, reference numeral 40 denotes a domain-inverted portion formed in the second domain-inverted formation step. As described above, in the present embodiment, the period of the periodic pattern of the comb electrodes 35 and 36 is twice (= 2) the period の of the finally obtained periodically poled structure. Power supply 3
Reference numeral 8 denotes a pulse electric field generation source capable of controlling an applied voltage, an application time, and a rising speed.

In this embodiment, an X-cut substrate having a thickness of about 500 μm is used as a MgO: LN crystal substrate.
The electrode fingers of the comb-shaped electrodes 35 and 36 are 40 to 100 μm in length.
m, width 1 μm, period 2-3 μm. Comb electrode 3
5, 36 and the material forming the striped electrode 37 are Ta (tantalum), Al (aluminum), C
Although conductive materials such as u (copper), Au (gold), and Ag (silver) can be used, in particular, the resistivity is 1.0 ×
By using a material having a resistivity of 10 ⁻⁷ Ωm or less, the electric field intensity distribution immediately below the electrode when an electric field is applied can be made uniform, so that the uniformity of the domain-inverted portions 39 and 40 formed can be improved.

The operation of the polarization inversion forming method configured as described above will be described below. In this embodiment, a case where a Ta electrode is used will be described. The above-mentioned comb-shaped electrodes 35 and 36 and the stripe-shaped electrode 37 were formed with a thickness of several hundreds to two thousand. First, Ta is deposited on the surface of the MgO: LN crystal substrate 34 by vapor deposition or sputtering.
A thin film was formed, and a comb-shaped electrode 35 and a striped electrode 37 were formed by using a photolithography process and an etching process. By applying an electric field to the MgO: LN crystal substrate 34 using the power supply 38 via the comb-shaped electrode 35 and the striped electrode 37 thus formed, the domain-inverted portion 39 can be formed. Was.

The application of an electric field is performed in an inert insulating solution, and an external electrode is brought into contact with the comb-shaped electrode 35 and the striped electrode 37. Applied. At this time, the potential of the comb electrode 35 arranged on the + Z-axis side of the MgO: LN crystal substrate 34 is higher than the potential of the stripe-shaped electrode 37 arranged on the -Z-axis side, and the applied voltage is An electric field was applied by applying a pulse electric field of several tens of kV / mm and an application time of 20 ms or less. Next, after removing the comb-shaped electrode 35 by wet etching or the like, a Ta thin film is again formed on the surface of the MgO: LN crystal substrate 34, and the comb-shaped electrode 36 and the striped electrode 37 are formed by using a photolithography process and an etching process. did.

At this time, the comb-shaped electrode 36 was formed at a position shifted by Λ in the Y direction with respect to the formation position of the comb-shaped electrode 35 by positioning the photomask. Positioning could be performed by photolithography with an accuracy of ± 0.05 μm or less. The comb-shaped electrode 36 thus formed
And Mg using a power source 38 through a stripe electrode 37.
By applying an electric field to the O: LN crystal substrate 34, the domain-inverted portions 40 could be formed. As a result, a structure having a uniform period Λ could be obtained as a periodically formed domain-inverted structure.

In the present embodiment, the desired domain inversion period is set to 2.5 to 3.8 μm. Therefore, the period of the electrode fingers (periodic pattern portions) of the comb-shaped electrodes 35 and 36 is 5.
0 to 7.6 μm. Conventionally, it has been considered that the electrode finger period and the extension of the domain-inverted portion in the period direction are hardly related. However, when the period of the domain-inverted portions to be formed is 3.0 μm or less (that is, the electrode finger interval is 1.5 μm or less), the direction in which the domain-inverted portions spread in the periodic direction is greatly affected by the electric field from the adjacent electrode fingers. The phenomenon of increase was seen. Therefore, we formed a comb-shaped electrode with a period that can sufficiently reduce the influence of adjacent electrode fingers and an integer multiple of the desired period, and shifted the electrode formation position in the period direction to obtain the desired periodic polarization inversion. The method of forming the structure was studied.

First, a comb-shaped electrode 35 having an electrode finger period of 2.0 to 8.0 μm was formed, and the extension of the domain-inverted portion formed in one domain-inversion forming step in the periodic direction was measured. As a result, it was found that when the electrode finger period was 3 μm or less, the domain-inverted portions significantly expanded in the period direction. In addition, when it is desired to obtain a polarization inversion of 2 μm or more as a polarization inversion thickness, in order to completely remove the influence of an electric field from an adjacent electrode finger, the electrode finger spacing is 2 μm or more (the electrode finger width is 1 μm or less). The electrode finger cycle is 3 μm or more)
It was found that it is desirable to be. At this time, the spread in the periodic direction of the domain-inverted portions can be set to about 1.5 μm, and the aspect ratio of the domain-inverted portions (the ratio d / w of the domain-inverted thickness d to the domain-wide spread w of domain inversion) is 1 or more. We were able to.

Then, as described above, 2.5 to 3.
In order to obtain a periodic polarization inversion period of 8 μm,
And 36 electrode fingers (periodic pattern portions) were formed with a period of 5.0 to 7.6 μm, and an attempt was made to obtain a desired periodic domain-inverted structure using two domain-inversion forming steps. As shown in FIGS. 10B and 10C, a domain-inverted portion 39 having a period of 2 ° is formed in the first domain-inverted formation process, and a domain-inverted portion 40 having a period of 2 ° is formed in the second domain-inverted formation process. did. According to this method, the periodic domain-inverted structure formed by the conventional method has a large width in the periodic direction (about 1.8 μm or more), whereas the periodic domain-inverted structure has a period of 2.5 to 3.8 μm. It is possible to form a uniform periodic domain-inverted structure having a spread in the periodic direction of about 1.5 μm.

Although the present embodiment has been described using an X-cut MgO: LN crystal substrate, a Y-cut
The same effect can be obtained in the off-cut and Z-cut MgO: LN crystal substrates. In addition, even when LN, LT, or LN, LT, or the like doped with Zn, In, Sc, or the like is used as the ferroelectric crystal, the domain inversion can be formed by the domain inversion method of the present invention. Even when an X-cut substrate, a Y-cut substrate, and an off-cut substrate of each crystal are used, in the method of forming a periodic domain-inverted period, domain-inverted After the formation, the domain-inverted formation process was repeated a plurality of times, and by shifting the electrode formation position in each process, a desired periodic domain-inverted structure could be obtained while suppressing the spread of the domain-inverted portion in the periodic direction. Further, a periodic domain-inverted structure with a period of 3 μm or less was formed in a ferroelectric crystal having an off-cut angle θ of 0.5 ° ≦ θ ≦ 7 ° by the domain-inverting forming method described in this embodiment. In such a case, in the domain-inverted cross section perpendicular to the domain direction, the ratio d /
w could be set to 1.0.

(Embodiment 4) In this embodiment, by including a periodically poled structure and an optical waveguide formed by using the method described in the first embodiment, wavelength conversion with high conversion efficiency is achieved. The point at which the element can be manufactured will be described in detail. Wavelength conversion elements that perform wavelength conversion using a periodically poled structure are roughly classified into an optical waveguide type wavelength conversion element and a bulk type wavelength conversion element.
In the case of an optical waveguide type wavelength conversion element, an optical waveguide is formed in an MgO: LN crystal, and laser light serving as a fundamental wave is incident on the optical waveguide to perform wavelength conversion. At this time, in order to perform wavelength conversion with high efficiency, it is necessary that the guided light propagating in the waveguide and the periodic polarization inversion have a sufficient overlap.

Further, in the optical waveguide type wavelength conversion element,
Ratio of the domain-inverted part and the non-polarized part (duty ratio)
Is closer to 1: 1, the better the quasi-phase matching state is formed, which also became a factor of high conversion efficiency. Further, for example, when this harmonic is used as a light source for an optical disk, it is desired that the wavelength spread is small, and since a large wavelength spread may cause a decrease in conversion efficiency, the harmonic obtained by wavelength conversion may be used. In order to reduce the wavelength spread, it is important that the formed domain inversion period is uniform.

Therefore, a wavelength conversion element was manufactured using the periodically poled structure formed by using the polarization inversion forming method of the present invention, and its characteristics were evaluated. FIG. 11 shows an example of a configuration diagram of an optical waveguide type wavelength conversion element according to Embodiment 4 of the present invention. In FIG. 11, reference numeral 41 denotes an optical waveguide type wavelength conversion element manufactured on an off-cut MgO: LN crystal substrate, reference numeral 42 denotes a periodic polarization inversion, and reference numeral 43 denotes an optical waveguide. At this time, the periodic direction of the periodic domain inversion 42 and the optical waveguide 43
Are arranged so as to be substantially parallel to each other, and wavelength conversion is performed by nonlinear interaction between the guided light propagating in the waveguide and the MgO: LN crystal. The end face of the optical waveguide 43 is polished so as to be substantially perpendicular to the periodic direction of the periodic domain inversion 42. In this embodiment, the working length of the optical waveguide type wavelength conversion element is 10 mm.
And

The operation of the wavelength conversion element configured as described above will be described below. The optical waveguide 43 is formed on the surface of the MgO: LN crystal substrate by evaporation or sputtering.
aA thin film is formed, and an optical waveguide pattern is formed by a photolithography process and an etching process.
O: LN crystal substrate could be formed by immersing the substrate in high-temperature benzoic acid, pyrophosphoric acid, or the like, followed by annealing. At this time, the optical waveguide width was 5 μm, and the depth in the substrate thickness direction was 2.5 μm. At this time, the thickness of the formed periodic domain inversion 42 is largest in the domain in which the domain inversion occurs at the tip of the electrode finger, and gradually decreases in the growth direction of the domain inversion. In order to sufficiently overlap, it is preferable that the distance between the tip of the electrode finger of the pattern electrode used to form the periodic polarization inversion 42 and the optical waveguide pattern is small.

Further, since the substrate surface becomes non-uniform due to an etching process or the like in the electrode formation portion, the waveguide loss increases. Therefore, specifically, the distance between the tip of the electrode finger and the optical waveguide 43 is desirably 0 to 10 μm. Further, when the periodic domain inversion 42 is formed by the domain inversion forming method described in the first embodiment (a method in which after the formation of the electrode pair and the application of the electric field, the re-formation of the electrode pair and the application of the electric field are repeated), In a 1.5 ° cut X plate, a domain inversion period of 2.2 to 3 μm, a domain inversion width of about 1.5 to 1.7 μm,
It was a uniform periodic domain inversion with a domain inversion thickness of about 1.8 μm.

After the periodic polarization inversion 42 and the optical waveguide 43 are formed, the MgO:
The end face of the LN crystal substrate is polished, and A
The optical waveguide type wavelength conversion element 41 was produced by applying an R coat. In the present embodiment, a SiO ₂ film was used as the AR film. In the optical waveguide type wavelength conversion element 41, it is possible to improve the overlap between the guided light guided through the optical waveguide having a thickness of about 2.5 μm and the periodic polarization inversion,
The conversion efficiency of the wavelength conversion element was significantly improved.

Using this optical waveguide type wavelength conversion element, FIG.
A harmonic generation light source as shown in FIG. 12, reference numeral 44 denotes the optical waveguide type wavelength conversion element shown in FIG. 11, reference numeral 45 denotes periodic polarization inversion, and reference numeral 46 denotes an optical waveguide. Further, 47 is a laser light source, 48 is a laser beam from the laser light source 47, and 49 is a harmonic of the racer light 48 obtained by the optical waveguide type wavelength conversion element 44. Also, 50
Is a coupling lens for causing the laser light 48 to enter the optical waveguide 46.

In this system, the optical waveguide type wavelength conversion element 4
When the characteristics of No. 4 were evaluated, the normalized conversion efficiency, which was about 500% / W in the conventional optical waveguide type wavelength conversion element, could be improved to 1000% / W or more. In particular, it was confirmed that the conversion efficiency was significantly improved by realizing a high aspect ratio in the short-period domain inversion formation. As a result, in the generation of the second harmonic in the wavelength band of 380 nm to 420 nm, an output power improvement of 2 to 5 times as compared with the related art was achieved. In addition, the output harmonics 49
Was suppressed to 1.0 nm or less in full width at half maximum.

When an MgO: LN crystal substrate is used, an optical waveguide type wavelength conversion element having high light damage resistance can be manufactured, and a wavelength conversion element having high output and high conversion efficiency can be manufactured. It had the advantage of being possible.

In the present embodiment, the case where the periodic domain-inverted structure formed by the method shown in the first embodiment is used has been described as an example. Even when the periodic domain-inverted structure formed by the method shown is used, the same uniform domain-inverted pattern can be formed with a similar short period and the above-mentioned normalized conversion efficiency is 1.8 to 2 times the conventional conversion efficiency. Could be improved.

Further, the periodic domain-inverted structure formed by the method described in the second embodiment has a characteristic shape. The shape and characteristics will be described with reference to FIG.
FIG. 13A shows a periodic domain-inverted portion formed by the conventional domain-inverted forming method, and FIG. 13B shows a periodic domain-inverted portion formed by the domain-inverted forming method according to the second embodiment of the present invention. 3 shows a polarization inversion section. 13A and 13B are views of the off-cut substrate viewed from the Z plane.
13A and 13B, reference numeral 51 denotes an off-cut MgO: LN crystal, and reference numeral 52 denotes a domain-inverted portion. In FIG. 13B, W1 is the MgO: LN crystal 51.
, W2 is the polarization inversion width at point B inside the MgO: LN crystal 51 above point A, and W3 is MgO: LN than point B above the point B.
This is the width of polarization reversal at point C inside the crystal 51.

As shown in FIG. 13A, the shape of the domain-inverted portion 51 formed by the conventional domain-inverted formation method is such that the domain-inverted width decreases from near the surface of the MgO: LN crystal 51 toward the inside. Becomes Therefore, for example, when used as an optical waveguide device, the overlap between the optical waveguide and the domain-inverted structure becomes non-uniform with respect to the cross section of the optical waveguide. That is, the duty ratio of the periodic domain-inverted structure (the ratio between the domain-inverted portion and the non-inverted portion) changes in the depth direction of the waveguide from the substrate surface. Cause. on the other hand,
The shape of the domain-inverted portion 52 formed by the domain-inverted formation method shown in the second embodiment of the present invention can satisfy W1 ≧ W3 ≧ W2 as shown in FIG. Therefore, there is an effect that it is possible to form a periodically poled structure while maintaining substantially the same duty ratio in the thickness direction of the substrate.

[0119]

As described above, according to the present invention, in the method for forming domain inversion by applying an electric field, an electrode pair is formed on the main surface of an off-cut ferroelectric crystal substrate, and an electric field is applied through this electrode. After the domain-inverted region is formed, the electrode pair is reformed, and an electric field is applied through the re-formed electrode, thereby forming a periodic domain-inverted material having a high aspect ratio about 1.3 times that of the related art. Was completed. This makes it possible to form a thick domain inversion while preventing the width of the domain inversion from being widened, especially in short-period domain inversion. In particular, when an optical waveguide device is used on the premise of optical coupling with a semiconductor laser, a ferroelectric crystal substrate with a small off-cut angle θ capable of low-loss coupling (off-cut with 0 ° <θ ≦ 3 °) It is possible to form a periodically poled structure having a high aspect ratio on the substrate). This has made it possible to realize an optical waveguide device capable of performing highly efficient nonlinear interaction.

Further, it is possible to form a domain-inverted region having a high aspect ratio in a wide area in the Z direction by using the above-mentioned domain-inverted formation method. In the case of manufacturing an element, there is an advantage that the number of elements that can be manufactured from the same crystal substrate can be ten or more times that in the related art.

In addition, the above-mentioned polarization inversion forming method is X-cut,
By applying the invention to a Y-cut substrate, it is possible to suppress the non-uniformity of the domain-inverted portion thickness and the domain-inverted portion aspect ratio, which have conventionally occurred in the Z direction, and to provide a uniform domain-inverted formation region over substantially the entirety. It has become possible to manufacture an optical waveguide device having characteristics. Further, in the domain-inverted formation region, an optical waveguide device having uniform characteristics at intervals of 30 to 50 μm can be manufactured, and there is an effect that high integration can be achieved.

Further, by manufacturing an optical waveguide type wavelength conversion element by using the polarization inversion forming method shown in the present invention,
Thick periodic polarization inversion with a high aspect ratio can be formed, and as a result, an extremely high normalized conversion efficiency can be obtained as compared with the related art. This allows
For example, as a small wavelength conversion type short wavelength light source, an output of 100
It has become possible to obtain a harmonic output light of about 20 mW, which is a fundamental wave of an infrared semiconductor laser of mW. In particular, the achievement of a high aspect ratio in the formation of a short-period domain-inverted structure has led to a 400 nm wavelength short-wavelength light source that has been delayed in practical use because of its low conversion efficiency and low output. Was peeled off. Further, as a ferroelectric crystal substrate, M
Even when a gO-doped LiNbO ₃ crystal substrate is used, the polarization inversion forming method of the present invention can be applied, which has an effect that an optical waveguide type wavelength conversion element having high light damage resistance can be manufactured.

[Brief description of the drawings]

FIG. 1 (a) is a diagram showing an example of a configuration for performing a first domain-inversion formation in the domain-inversion forming method according to the first embodiment of the present invention. (B) FIG. (C) A diagram showing an example of the configuration when performing the second polarization inversion forming in the polarization inversion forming method according to the first embodiment of the present invention. (D) FIG. 1 (c) Sectional view when viewed from the -Y plane

FIG. 2A is a diagram illustrating an example of a domain-inverted portion formed in an off-cut substrate by a first domain-inverted formation step according to the first embodiment of the present invention; FIG. Sectional view when cut along the -A 'section and viewed from the -Z plane. (C) The polarization inversion portion formed in the off-cut substrate by the second polarization inversion forming step in the first embodiment of the present invention. FIG. 2D is a cross-sectional view of FIG. 2C cut along a line BB ′ and viewed from a −Z plane.

FIG. 3A is a diagram showing an example of a domain-inverted portion formed in an X-cut substrate by a first domain-inverted formation step according to the first embodiment of the present invention; (C) A cross-sectional view when cut along the -L 'section and viewed from the -Z plane. (C) A cross-sectional view when cut along the MM' cross section of FIG. 3A and viewed from the -Z plane. FIG. 3E is a view showing an example of a domain-inverted portion formed in the X-cut substrate by the second domain-inverted formation step in the first embodiment of the present invention. (E) FIG. Sectional view when viewed from the -Z plane. (F) Sectional view when cut along the OO 'section of FIG. 3D and viewed from the -Z plane.

FIG. 4A is a diagram showing a relationship between an off-cut angle θ of an off-cut substrate and a domain-inverted thickness formed by applying an electric field according to the first embodiment of the present invention; Showing the relationship between the off-cut angle θ of the off-cut substrate and the aspect ratio of the domain-inverted portion formed by applying an electric field

FIG. 5 shows the relationship between the amount of displacement of the electrodes used in the first domain inversion forming step and the second domain inversion forming step and the thickness of the formed domain inversion portion in the first embodiment of the present invention. Figure

6A is a diagram showing an example of a configuration for forming a periodic domain inversion according to the first embodiment of the present invention. FIG. 6B is a diagram showing a conventional domain inversion forming method using the configuration shown in FIG. FIG. 6C is a diagram illustrating an example of a domain-inverted portion formed. FIG. 6C is a diagram illustrating an example of a domain-inverted portion formed by the domain-inverted forming method of the present invention using the configuration of FIG.

FIG. 7A is a diagram showing an example of a configuration when forming domain-inverted portions in different regions according to the second embodiment of the present invention; FIG. 7B is a diagram showing a configuration in which domain-inverted portions are formed in different regions; FIG. 7C shows an example of a domain-inverted shape when a domain-inverted region is induced. FIG. 7C is a cross-sectional view taken along the line DD ′ of FIG. 7B and viewed from the −Z plane.

8A is a diagram showing an example of a configuration when domain-inverted formation is performed using island-shaped electrodes according to the second embodiment of the present invention. FIG. 8B is a view of FIG. Cross section of case

9A is a diagram illustrating an example of a configuration when performing polarization inversion using a comb-shaped electrode partly insulated from a substrate by an insulating material according to the second embodiment of the present invention; FIG. Sectional view when (a) is viewed from the −Y plane.

FIG. 10 (a) is a diagram illustrating a polarization by performing a first polarization inversion forming step and a second polarization inversion forming step using an electrode having a period that is an integral multiple of a desired period in the third embodiment of the present invention. FIG. 4B is a diagram illustrating an example of a configuration when an inversion portion is formed. FIG. 6C is a diagram illustrating an example of a polarization inversion portion formed in a substrate by a first polarization inversion forming step in the third embodiment of the present invention. FIG. 7 is a diagram showing an example of a domain-inverted portion formed in a substrate by a first domain-inverted formation step and a second domain-inverted formation step in Embodiment 3 of the present invention.

FIG. 11 is a diagram illustrating an example of a configuration of an optical waveguide type wavelength conversion element according to a fourth embodiment of the present invention.

FIG. 12 is a diagram illustrating an example of a configuration of a harmonic generation light source using an optical waveguide type wavelength conversion element according to a fourth embodiment of the present invention.

13A is a diagram showing an example of a periodic domain inversion formed by a conventional domain inversion forming method. FIG. 13B is a diagram showing an example of a periodic domain inversion formed by the method according to the second or third embodiment of the present invention. Diagram showing an example of the shape of periodic polarization inversion

[Explanation of symbols]

1,7,10,13,17,23,28,34,51
MgO: LN crystal substrate 2a, 4a, 14a, 22a, 29a, 35, 36 Comb electrodes 2b, 4b, 14b, 22b, 24b, 29b, 37
Stripe electrodes 6,15,21,25,30,38 Power supply 3,5,8,9,11,12,16,18,19,2
0, 26, 27, 31, 32, 39, 40, 52 Polarization inversion portion 24a Island electrode 33 Insulating material 41, 44 Optical waveguide type wavelength conversion element 42, 45 Periodic polarization inversion 43, 46 Optical waveguide 47 Laser light source 48 Laser beam 49 Harmonics 50 Coupling lens L1, L2, L3, L4 Interval W1, W2, W3 Polarization inversion width

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuhisa Yamamoto 1006 Kadoma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. F term (reference) 2K002 AB12 BA01 CA03 FA24 FA27 GA07 HA20

Claims (24)

[Claims] 1. A step of forming a single-polarized ferroelectric crystal and a domain-inverted portion in a first region of the ferroelectric crystal, and a domain-inverted portion in a second region of the ferroelectric crystal. Forming a portion, wherein at least a part of the first region and the second region overlap each other. A step of forming a domain-inverted portion in a single-polarized ferroelectric crystal and a different region of the ferroelectric crystal, wherein the step causes a domain-inverted region between the different regions. A method of forming a domain-inverted structure, wherein a portion is formed and a shape of domain-inverted formed between the regions is controlled by a distance between the regions. 3. The method according to claim 2, wherein the step of forming domain inversion in the different regions is performed simultaneously. 4. The method according to claim 1, wherein the step of forming a domain inversion in the different region is divided into a step of forming a domain inversion of a first region and a step of forming a domain inversion of a second region. The method of forming a domain inversion according to claim 2. 5. A semiconductor device comprising a plurality of steps of forming a single-polarized ferroelectric crystal and forming a periodically poled portion having a period of an integral multiple of a desired length 内 in the ferroelectric crystal a plurality of times. Wherein the positions at which the periodic domain-inverted portions are formed are different from each other. 6. The method according to claim 5, wherein the formed domain-inverted portions have the period of Λ. 7. A position for forming said periodic domain-inverted portion,
7. The method according to claim 5, wherein each of the steps is different from each other in a direction perpendicular to a periodic direction of the periodic polarization inversion. 8. The polarization inversion forming method according to claim 1, wherein a polarization direction of said ferroelectric crystal is substantially parallel to a surface of said ferroelectric crystal. 9. An off-cut substrate, wherein said ferroelectric crystal is cut on a plane forming an angle θ (0 ° <θ <90 °) with respect to the direction of spontaneous polarization of said ferroelectric crystal. The angle (off-cut angle) θ between the polarization direction of the substrate and the surface of the ferroelectric crystal is in a range of 0.5 ° ≦ θ ≦ 7 °. 3. The polarization inversion forming method according to 1. above. 10. In each step of forming said domain-inverted portion or said periodic domain-inverted portion, an electrode pair is formed on a first surface of said ferroelectric crystal, and said ferroelectric substance is interposed via said electrode pair. The polarization inversion forming method according to any one of claims 1 to 9, wherein an electric field is applied to the crystal. 11. A back electrode is formed on a second surface of the ferroelectric crystal opposite to the first surface in addition to the electrode pair,
The method of claim 1, wherein an electric field is applied through the pair of electrodes and the back electrode. 12. The electrode pair comprises an electrode a having at least a part of a spire portion, and a stripe-shaped electrode b,
The polarization inversion forming method according to claim 10 or 11, wherein the polarization inversion portion or the periodic inversion portion is formed from the spire portion. 13. The method according to claim 1, wherein in each of the steps of forming the domain-inverted portions or the periodic domain-inverted portions, positions of the electrode pairs are different from each other in a direction parallel to a polarization direction of the ferroelectric crystal. The polarization inversion forming method according to claim 10. 14. The polarization inversion forming method according to claim 13, wherein the off-cut angle θ and the shift amount L between the positions where the electrode pairs are formed have a relationship of L × tan θ ≦ 1.5. . 15. An electrode interval of the electrode pair used in the step of forming the domain-inverted portions or the periodic domain-inverted portions is equal in each step or smaller than in the previous step. 3. The polarization inversion forming method according to 1. above. 16. The step of forming a domain inversion in the different region includes a step of applying an electric field to the ferroelectric crystal via a comb-shaped electrode formed on a first surface of the ferroelectric crystal, 4. The polarization inversion forming method according to claim 3, wherein at least a part of a comb-shaped portion of the comb-shaped electrode is in contact with the first surface via an insulating film. 17. A part that does not include a tip of said comb-like part,
17. The polarization inversion forming method according to claim 16, wherein the first surface is in contact with the first surface via the insulating film. 18. The domain-inverted formation according to claim 17, wherein a length of said comb-shaped portion in contact with said first surface via said insulating film is 5 μm or less. Method. 19. The method according to claim 19, wherein the ferroelectric crystal is made of MgO, Zn, S
c is either one of doped LiNbO ₃ crystal or stoichiometric LiNbO ₃ crystals of doping amount x is from claim 1, characterized in that a 0.5mol% ≦ x ≦ 6.0mol% 19. The polarization inversion forming method according to claim 18. 20. A domain-inverted section or a periodic domain-inverted section formed by the domain-inverted formation method according to any one of claims 1 to 19, wherein the domain-inverted section is perpendicular to the polarization direction. A domain inversion width w1 at an arbitrary point A in the thickness direction, a domain inversion width w2 at a point B inside the crystal from the point A, and a domain inversion at a point C inside the crystal from the point B. When the width w3 is w1 ≧ w3 ≧ w2 or w3 ≧
An optical wavelength conversion element having a domain-inverted shape satisfying a relationship of w1 ≧ w2. 21. An off-cut angle θ of 0.5 ° ≦ θ ≦ 7 °
20. A period 3 formed by the polarization inversion forming method according to claim 1 in a ferroelectric crystal having the following structure.
It has a periodic domain-inverted structure of μm or less, and the ratio between the domain-inverted thickness d and the domain-inverted width w is d / w ≧ 1.0 in a domain-inverted cross section perpendicular to the polarization direction. An optical wavelength conversion element characterized by the above-mentioned. 22. An optical waveguide, wherein the optical wavelength conversion element comprises the periodic domain-inverted portion and an optical waveguide formed in the ferroelectric crystal so as to cross the domain-inverted portion substantially vertically. 22. The optical wavelength conversion device according to claim 20, wherein the optical wavelength conversion device is a type optical wavelength conversion device. 23. The device according to claim 22, wherein the optical waveguide is formed in a portion other than a region where the electrode pair used in forming the periodic polarization inversion portion is formed. Light wavelength conversion element. 24. The distance between the optical waveguide and the tip of the electrode pair used for forming the periodic polarization inversion portion is 10 or more.
25. The optical wavelength conversion device according to claim 23, wherein the wavelength is not more than μm.