Substrate with dielectric thin film, optical waveguide element, and optical modulation element

Abstract

To provide a substrate with a dielectric thin film, which enables production of an optical modulation element that is less susceptible to cracking of a lithium niobate film and suppresses DC drift.SOLUTION: A substrate 1 with a dielectric thin film is provided, comprising a single crystal substrate 2 and a dielectric thin film 3 formed in contact with a main surface 2a of the single crystal substrate 2. The dielectric thin film 3 is made of a lithium niobate film that is a c-axis oriented epitaxial film and includes a twin crystal structure of LiNbO3 comprising first crystals 3a and second crystals 3b obtained by rotating the first crystals 3a by 180° about the c-axis. The first crystals 3a and the second crystals 3b included in an upper region 32 of the dielectric thin film 3, excluding a lower region 31 ranging from the single crystal substrate 2 to halfway in a thickness direction, has maximum domain widths of 80 nm to 300 nm.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a substrate with a dielectric thin film, and an optical waveguide element and an optical modulation element using the same. [Background technology]

[0002] Conventionally, there has been an optical modulation element that uses a lithium niobate film epitaxially grown on a substrate. For example, Patent Document 1 describes an optical modulation element using a substrate with a dielectric thin film. Patent Document 1 describes a substrate with a dielectric thin film, which includes a dielectric thin film made of a c-axis oriented lithium niobate film epitaxially formed on the main surface of a single crystal substrate. Patent Document 1 also describes a dielectric thin film having a twin crystal structure including a first crystal and a second crystal obtained by rotating the first crystal by 180° around the c-axis, and in a pole measurement by an X-ray diffraction method, the ratio of a first diffraction intensity corresponding to the first crystal to a second diffraction intensity corresponding to the second crystal is 0.5 or more and 2.0 or less.

[0003] Furthermore, Patent Document 2 describes an optical modulator having a lithium niobate film which is an epitaxial film formed on the main surface of a single crystal substrate and has a ridge-shaped portion, a buffer layer formed on the ridge-shaped portion, a first electrode formed on the buffer layer, and a second electrode formed in contact with the upper surface and / or step portion of the lithium niobate film.

[0004] Patent Document 3 describes an optical waveguide device that includes an optical waveguide formed in the surface of an electro-optic crystal substrate, first and second electrodes for applying an electric field that changes the refractive index of the optical waveguide, and a buffer layer formed on the substrate between the first and second electrodes. 3 ) or the like, an optical waveguide device is described in which an electric field is applied to an optical waveguide formed in the surface of an electro-optic crystal substrate to change the refractive index. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 016428 [Patent Document 2] JP 2014-142411 A [Patent Document 3] Patent No. 3001027 Summary of the Invention [Problem to be solved by the invention]

[0006] An optical waveguide element and an optical modulation element using a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on the substrate are more productive than an optical waveguide element and an optical modulation element using a lithium niobate single crystal substrate. This is because a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on the substrate can form a lithium niobate film of a desired thickness by adjusting the film formation conditions. In other words, when a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on the substrate is used, it is not necessary to adjust the thickness by cutting or polishing the lithium niobate single crystal substrate as in the case of using a lithium niobate single crystal substrate. Moreover, a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on the substrate is less expensive than a lithium niobate single crystal substrate.

[0007] However, in the optical waveguide element and the optical modulation element using the lithium niobate film epitaxially grown on the substrate, the thickness of the lithium niobate film may be a design limitation of the optical waveguide element and the optical modulation element. More specifically, the lithium niobate film epitaxially grown on the substrate preferably has a sufficient film thickness so that the optical waveguide element and the optical modulation element applicable to a wide range of light from visible light to infrared light can be obtained. However, the lithium niobate film epitaxially grown on the substrate is prone to cracking, and there is a disadvantage that the thicker the film, the more likely the cracking occurs. For this reason, there is a demand for a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on a substrate, the lithium niobate film being less susceptible to cracking.

[0008] In addition, in an optical modulation element using a lithium niobate film, the modulated waveform moves depending on the DC (direct current) voltage applied to the electrodes of the optical modulation element. The modulated waveform in an optical modulation element using a lithium niobate film changes over the time that the DC (direct current) voltage is applied. This change in the modulated waveform over time is called DC drift. Optical modulation elements using a dielectric thin-film substrate having a lithium niobate film epitaxially grown on a conventional substrate have a larger DC drift than optical modulation elements using a lithium niobate single crystal substrate, so there was a demand for reducing the DC drift.

[0009] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a substrate with a dielectric thin film that has a lithium niobate film epitaxially grown on the substrate, which is less likely to crack and can form an optical modulation element with suppressed DC drift.

[0010] Another object of the present invention is to provide an optical waveguide element that is provided with a substrate having a dielectric thin film made of a lithium niobate film, and that can form an optical modulation element in which cracks are unlikely to occur in the lithium niobate film and DC drift is suppressed. Another object of the present invention is to provide an optical modulation element that includes a substrate having a dielectric thin film made of a lithium niobate film, in which cracks are less likely to occur in the lithium niobate film and DC drift is suppressed. [Means for solving the problem]

[0011] A substrate with a dielectric thin film according to one aspect of the present invention includes a single crystal substrate and a dielectric thin film formed on and in contact with a main surface of the single crystal substrate, the dielectric thin film having a thickness of 0.5 μm to 2 μm and made of a lithium niobate film that is an epitaxial film oriented along a c-axis, the LiNbO 3 the dielectric thin film includes a twin crystal structure in which the first crystal and the second crystal contained in an upper region excluding a lower region from the single crystal substrate to halfway in the thickness direction have a maximum domain width of 80 nm to 300 nm, and the maximum domain width in the first crystal and the second crystal is determined by providing a measurement region having a length of 4 μm in a cross-sectional view at an arbitrary location on the interface between the dielectric thin film and the single crystal substrate, measuring crystal domain widths, which are widths in a direction perpendicular to the growth direction of each crystal and are the maximum dimension in the thickness direction of the dielectric thin film, for 10 or more arbitrary first crystals and second crystals present in the measurement region, and taking the median value of the measured values ​​obtained. Effect of the Invention

[0012] The substrate with a dielectric thin film of the present invention is made of a lithium niobate film, which is a c-axis oriented epitaxial film formed on and in contact with the main surface of a single crystal substrate, and includes a first crystal and a second crystal obtained by rotating the first crystal by 180° around the c-axis. 3 Therefore, in the substrate with the dielectric thin film of the present invention, the distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate and the lithium niobate film are eliminated by the LiNbO 3 Therefore, in the substrate with the dielectric thin film of the present invention, cracks are less likely to occur in the lithium niobate film.

[0013] Furthermore, in the substrate with a dielectric thin film of the present invention, the dielectric thin film is made of a lithium niobate film, which is an epitaxial film oriented along the c-axis, and is less susceptible to cracking. Therefore, the thickness of the lithium niobate film can be easily adjusted to a thickness suitable for, for example, the production of optical waveguide elements and optical modulation elements using the substrate with a dielectric thin film. Therefore, the substrate with the dielectric thin film of the present invention can be preferably used when producing an optical waveguide element and an optical modulation element.

[0014] The dielectric thin film of the dielectric thin film-attached substrate of the present invention has a thickness of 0.5 μm to 2 μm, and the upper region of the dielectric thin film, excluding the lower region from the single crystal substrate to halfway in the thickness direction, has a maximum domain width of 80 nm to 300 nm for 10 or more arbitrary first crystals and second crystals present in a measurement region of 4 μm in length in cross section. Therefore, in an optical modulation element using the dielectric thin film-attached substrate, when a DC (direct current) voltage is applied in the in-plane direction from above the dielectric thin film, LiNbO 3 The number of twin boundaries crossed by the DC (direct current) electric field moving through the twin structure of the substrate is appropriate. As a result, the optical modulation element having a signal electrode and a ground electrode provided on the dielectric thin film of the substrate with the dielectric thin film of the present invention has a suppressed DC drift.

[0015] The optical waveguide element of the present invention includes the substrate with the dielectric thin film of the present invention, and therefore the optical waveguide element of the present invention is capable of forming an optical modulation element in which cracks are unlikely to occur in the lithium niobate film forming the dielectric thin film, and in which DC drift is suppressed when a DC (direct current) voltage is applied in the in-plane direction from above the dielectric thin film. Furthermore, since the optical modulation element of the present invention includes a substrate with a dielectric thin film of the present invention, cracks are less likely to occur in the lithium niobate film forming the dielectric thin film, and DC drift is suppressed when a DC (direct current) voltage is applied in the in-plane direction from above the dielectric thin film.

[0016] The optical waveguide element and the optical modulation element of the present invention are provided with a substrate with a dielectric thin film having a lithium niobate film that is resistant to cracking. Therefore, even if annealing is performed during the manufacturing process of the optical waveguide element and / or the optical modulation element, the lithium niobate film of the substrate with the dielectric thin film is resistant to cracking, resulting in excellent productivity. Furthermore, since the lithium niobate film of the substrate with the dielectric thin film is resistant to cracking, the optical waveguide element and the optical modulation element have excellent durability. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic cross-sectional view showing a substrate 1 with a dielectric thin film according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a plan view showing an example of an optical waveguide element 100 using the substrate 1 with the dielectric thin film shown in FIG. [Diagram 3] FIG. 3 is a cross-sectional view of the optical waveguide element 100 shown in FIG. 2 taken along line AA'. [Figure 4] FIG. 4 is a plan view showing an example of a Mach-Zehnder type optical modulation element 200A using the dielectric thin film-formed substrate 1 shown in FIG. [Diagram 5] FIG. 5 is a cross-sectional view of the light modulation element 200A shown in FIG. 4 taken along the line BB'. [Figure 6] FIG. 6 is a graph showing the relationship between time and DC drift in the modulated waveform of the output light output from the optical modulation elements 200A of Example 5, Example 10, and Comparative Example 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present inventors have conducted extensive research as described below in order to solve the above problems and form an optical modulation element in which cracks are unlikely to occur in a lithium niobate film and DC drift is suppressed in a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on a single crystal substrate.

[0019] That is, in a substrate with a dielectric thin film in which an epitaxially grown lithium niobate film is formed on and in contact with a single crystal substrate, large distortions and stresses are generated at the interface between the single crystal substrate and the lithium niobate film due to differences in lattice constants and linear expansion coefficients between the single crystal substrate and the lithium niobate film.

[0020] For this reason, in the conventional technology, cracks were likely to occur in the lithium niobate film when the lithium niobate film was epitaxially grown in the manufacturing process of the substrate with the dielectric thin film. Furthermore, the above-mentioned strain and stress become larger as the thickness of the lithium niobate film is increased, so that the thicker the lithium niobate film is, the more likely the substrate with the dielectric thin film is to have cracks in the lithium niobate film during and after the manufacturing process. Furthermore, when an optical waveguide element and / or an optical modulation element is manufactured using the substrate with the dielectric thin film, if a process such as annealing is performed, the strain and stress caused by the difference in the linear expansion coefficient between the single crystal substrate and the lithium niobate film are likely to become larger. For this reason, when an optical waveguide element and / or an optical modulation element is manufactured using the substrate with the dielectric thin film, cracks were likely to occur in the lithium niobate film during and after the manufacturing process.

[0021] The present inventors have proposed a method for suppressing the occurrence of cracks in a lithium niobate film by using a LiNbO 3 It has been proposed to use a lithium niobate film consisting of a c-axis oriented epitaxial film having a twin crystal structure of the above. In this case, the twin crystal structure reduces the distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate and the lithium niobate film. It is presumed that this makes the lithium niobate film less susceptible to cracks.

[0022] However, in the optical modulation element using the lithium niobate film made of the epitaxial film having the above-mentioned twin crystal structure, it has been difficult to suppress the DC drift. Therefore, the inventors have conducted extensive research focusing on the relationship between the lithium niobate film consisting of an epitaxial film having the above-mentioned twin crystal structure and the DC drift that occurs when a DC voltage is applied to an optical modulation element using the same.

[0023] As a result, it was found that by forming an optical modulation element using a dielectric thin film having a thickness of 0.5 μm to 2 μm, which is made of a lithium niobate film that is a c-axis oriented epitaxial film formed in contact with the main surface of a single crystal substrate, which includes the above-mentioned twin crystal structure, and in which the first crystal and the second crystal contained in the upper region excluding the lower region from the single crystal substrate to halfway in the thickness direction have a maximum domain width of 80 nm to 300 nm, it is possible to suppress DC drift when a DC (direct current) voltage is applied in the in-plane direction from above the lithium niobate film.The reasons for this are as follows.

[0024] That is, in an optical modulation element using a lithium niobate film made of an epitaxial film having the above-mentioned twin crystal structure, it is more difficult to control DC drift when a DC (direct current) voltage is applied, compared to an optical modulation element using a lithium niobate single crystal substrate. This is presumably because, in an optical modulation element using a lithium niobate film made of an epitaxial film having the above-mentioned twin crystal structure, the modulation waveform that moves by applying a DC (direct current) voltage is affected by crystal defects and / or dislocations present in the twin crystal structure. In a lithium niobate film made of an epitaxial film having the above-mentioned twin crystal structure, crystal defects and / or dislocations may exist at the boundary between the first crystal and the second crystal included in the above-mentioned twin crystal structure (hereinafter, sometimes referred to as the "twin crystal boundary").

[0025] In addition, in an optical modulation element including a substrate with a dielectric thin film having a lithium niobate film made of an epitaxial film having the above-mentioned twin crystal structure, when a DC (direct current) voltage is applied in the in-plane direction from above the lithium niobate film, a DC (direct current) electric field moves near the surface of the lithium niobate film. At this time, if the median value (maximum domain width) of the measured values ​​obtained by measuring the crystal domain width, which is the width in the direction perpendicular to the growth direction of each crystal and the maximum dimension in the thickness direction of the dielectric thin film, for 10 or more arbitrary first crystals and second crystals present in a measurement area with a length of 4 μm in cross section in the upper region of the lithium niobate film is 80 nm to 300 nm, the number of twin boundaries crossed by the DC electric field is suppressed due to the above-mentioned maximum domain width being sufficiently wide. Furthermore, when the first crystals and the second crystals in the upper region of the lithium niobate film have a maximum domain width of 80 nm to 300 nm, the maximum domain width is sufficiently narrow, so that the number of crystal defects and / or dislocations present in the first crystals and the second crystals is suppressed.

[0026] From these facts, it is estimated that when the first crystals and the second crystals in the upper region of the lithium niobate film have a maximum domain width of 80 nm to 300 nm, even if crystal defects and / or dislocations are present at each twin boundary, within the first crystals, and within the second crystals contained in the above-mentioned twin structure, the influence of the crystal defects and / or dislocations on the modulation waveform of the optical modulation element is suppressed.

[0027] Furthermore, since the first crystals and the second crystals in the upper region are within the above-mentioned maximum domain width range and the thickness of the lithium niobate film is 0.5 μm to 2 μm, the influence of the DC electric field applied to the optical modulation element can be prevented from extending to the lower region of the lithium niobate film, and when the lithium niobate film is processed into a ridge shape to form an optical waveguide, the optical waveguide does not become an inappropriate shape.

[0028] As a result, it is estimated that an optical modulation element using a lithium niobate film having a film thickness of 0.5 μm to 2 μm and in which the first crystals and second crystals in the upper region have a maximum domain width of 80 nm to 300 nm will have suppressed DC drift when a DC (direct current) voltage is applied in the in-plane direction from above the lithium niobate film.

[0029] Furthermore, the inventors manufactured an optical modulation element in which a signal electrode and a ground electrode are provided on a lithium niobate film having a film thickness of 0.5 μm to 2 μm and in which the first crystals and second crystals in the upper region have a maximum domain width of 80 nm to 300 nm, and confirmed that cracks are unlikely to occur in the lithium niobate film and that DC drift is suppressed when a DC (direct current) voltage is applied in the in-plane direction from above the lithium niobate film, thereby conceiving the present invention.

[0030] The present invention includes the following aspects. [1] A single crystal substrate; a dielectric thin film formed on and in contact with a main surface of the single crystal substrate; The dielectric thin film has a thickness of 0.5 μm to 2 μm, is made of a lithium niobate film which is an epitaxial film oriented along the c-axis, and includes a first crystal and a second crystal obtained by rotating the first crystal by 180° around the c-axis. 3 The twin structure includes the first crystals and the second crystals included in an upper region of the dielectric thin film excluding a lower region from the single crystal substrate to a half of the thickness direction have a maximum domain width of 80 nm to 300 nm; The maximum domain width in the first crystal and the second crystal is the median value of the measured values ​​obtained by setting a measurement area having a length of 4 μm in a cross-sectional view at an arbitrary location on the interface between the dielectric thin film and the single crystal substrate, measuring crystal domain widths, which are widths in a direction perpendicular to the growth direction of each crystal and are the maximum dimension in the thickness direction of the dielectric thin film, for any of 10 or more of the first crystals and the second crystals present in the measurement area.

[0031] [2] The substrate with a dielectric thin film according to [1], wherein some or all of the first crystals and the second crystals contained in the dielectric thin film have non-uniform domain widths in the thickness direction. [3] The substrate with a dielectric thin film according to [1], wherein the single crystal substrate is a sapphire single crystal substrate, the main surface of which is a c-plane. [4] The substrate with a dielectric thin film described in [1], wherein the twin crystal structure of the dielectric thin film has a ratio of a first diffraction intensity corresponding to the first crystal to a second diffraction intensity corresponding to the second crystal of 0.5 or more and 2.0 or less in a pole measurement by an X-ray diffraction method.

[0032] [5] An optical waveguide element comprising a substrate with a dielectric thin film according to any one of [1] to [4]. [6] The optical waveguide element according to [5], having an optical waveguide made of the dielectric thin film. [7] An optical modulation element comprising a substrate with a dielectric thin film according to any one of [1] to [4]. [8] An optical modulation element according to [7], comprising an optical waveguide made of the dielectric thin film, and a first electrode and a second electrode that apply a voltage in an in-plane direction from above the dielectric thin film to change the refractive index of the optical waveguide.

[0033] The substrate with dielectric thin film, the optical waveguide element, and the optical modulation element of the present embodiment will be described in detail below with reference to the drawings as appropriate. The drawings used in the following description may show characteristic parts in an enlarged scale for the sake of convenience in order to make the features of the present invention easier to understand. Therefore, the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them, and may be modified as appropriate within the scope of the present invention.

[0034] [Substrate with thin dielectric film] Fig. 1 is a schematic cross-sectional view showing a substrate with a dielectric thin film 1 according to one embodiment of the present invention. As shown in Fig. 1, the substrate with a dielectric thin film 1 of this embodiment has a single crystal substrate 2 and a dielectric thin film 3 formed on and in contact with a main surface 2a of the single crystal substrate 2.

[0035] (Single crystal substrate 2) The single crystal substrate 2 is made of lithium niobate (LiNbO 3 Any known single crystal substrate can be used as long as it is capable of growing an epitaxial film having a c-axis orientation including a twin crystal structure of (1000 nm). The single crystal substrate 2 preferably has a refractive index lower than that of lithium niobate, and for example, a sapphire single crystal substrate, a silicon single crystal substrate, or the like can be used.

[0036] In the substrate 1 with the dielectric thin film of this embodiment, it is particularly preferable to use a sapphire single crystal substrate as the single crystal substrate 2. The sapphire single crystal substrate is LiNbO 3 has a lower refractive index than that of the dielectric thin film 3. For this reason, for example, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical waveguide element and / or an optical modulation element, it can function as a cladding layer. Therefore, when the single crystal substrate 2 is a sapphire single crystal substrate, the dielectric thin film 3 can be suitably used as an optical waveguide layer of an optical waveguide element and / or an optical modulation element without providing a separate cladding layer between the single crystal substrate 2 and the dielectric thin film 3.

[0037] The dielectric thin film 3 formed on the single crystal substrate 2 of the substrate 1 with the dielectric thin film of this embodiment is likely to be formed as an epitaxial film with a c-axis orientation for the single crystal substrate 2 with various crystal orientations. Therefore, in the substrate 1 with the dielectric thin film of this embodiment, the crystal orientation of the single crystal substrate 2 is not particularly limited.

[0038] In the substrate 1 with the dielectric thin film of this embodiment, the dielectric thin film 3 is made of a lithium niobate film, which is an epitaxial film oriented along the c-axis, and LiNbO 3and has a twin crystal structure with three-fold symmetry. For this reason, it is desirable that the crystal orientation of the main surface 2a of the single crystal substrate 2 has the same symmetry as that of the dielectric thin film 3. Therefore, when a sapphire single crystal substrate is used as the single crystal substrate 2, it is preferable that the main surface 2a is a c-plane. Furthermore, when a silicon single crystal substrate is used as the single crystal substrate 2, it is preferable that the main surface 2a is a (111) plane.

[0039] (Dielectric Thin Film 3) The dielectric thin film 3 is made of a lithium niobate film, which is an epitaxial film oriented along the c-axis. The lithium niobate film forming the dielectric thin film 3 is made of lithium niobate (LiNbO 3 Lithium niobate has a large electro-optic constant and is therefore suitable as a material for the optical waveguide layer of optical waveguide elements and / or optical modulation elements.

[0040] The composition of the lithium niobate film forming the dielectric thin film 3 is represented by the general formula LixNbAyOz (wherein A is an element other than Li, Nb and O, x is 0.5 to 1.2, y is 0 to 0.5, and z is 1.5 to 4). In the formula, A represents an element other than Li, Nb, and O. Examples of elements represented by A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce. The element represented by A may be one type only, or two or more types. In the formula, x is an integer of 0.5 to 1.2, and preferably 0.9 to 1.05. In the formula, y is an integer of 0 to 0.5. In the formula, z is from 1.5 to 4, and preferably from 2.5 to 3.5.

[0041] The lithium niobate film forming the dielectric thin film 3 is a LiNbO film including a first crystal 3a and a second crystal 3b obtained by rotating the first crystal 3a by 180° around the c-axis. 3 The twin structure is included.

[0042] The first crystal 3a and the second crystal 3b forming the dielectric thin film 3 are both close to single crystals. However, if the first crystal 3a and the second crystal 3b are too close to a complete single crystal, the effect of alleviating the distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric thin film 3 due to the above-mentioned twin crystal structure is reduced. For this reason, cracks may easily occur in the lithium niobate film. Therefore, it is preferable that the half-width of the rocking curve of the (006) reflection of the first crystal 3a and the second crystal 3b measured by the X-ray diffraction method is in the range of 0.3° or more and 0.6° or less. When the half-width of the rocking curve of the (006) reflection of the first crystal 3a and the second crystal 3b is within this range, the first crystal 3a and the second crystal 3b can be regarded as optically single crystals. Moreover, the first crystals 3a and the second crystals 3b are not too close to being completely single crystals, and therefore the occurrence of cracks in the lithium niobate film can be effectively suppressed.

[0043] The LiNbO of the lithium niobate film forming the dielectric thin film 3 3 In the twin crystal structure, in a pole measurement by an X-ray diffraction method, the ratio of the first diffraction intensity corresponding to the first crystal 3a to the second diffraction intensity corresponding to the second crystal 3b is preferably 0.5 or more and 2.0 or less, more preferably 0.8 or more and 1.25 or less, and the closer to 1.0 the better. In the pole measurement by an X-ray diffraction method, the ratio of the first diffraction intensity corresponding to the first crystal 3a to the second diffraction intensity corresponding to the second crystal 3b corresponds to the ratio of the first crystal 3a to the second crystal 3b.

[0044] Dielectric thin film 3 LiNbO 3 The more equal the ratio of the first crystals 3a and the second crystals 3b in the twin crystal structure, the more effectively the twin crystal structure can alleviate the distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film, and therefore the lithium niobate film is less likely to crack.

[0045] The LiNbO of the lithium niobate film forming the dielectric thin film 3 3 In the twin crystal structure, the first crystal 3a and the second crystal 3b are preferably bonded to each other without a grain boundary. If a grain boundary exists at the boundary between the first crystal 3a and the second crystal 3b, light scattering occurs at the boundary surface. This is because, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical waveguide element and / or an optical modulation element, optical loss increases. In contrast, when the first crystal 3a and the second crystal 3b are bonded to each other and no grain boundary exists at the boundary between them, the refractive indexes of both are the same, so no light scattering occurs. Therefore, the lithium niobate film can have optical properties equivalent to those of a single crystal.

[0046] The lithium niobate film forming the dielectric thin film 3 is LiNbO 3 The lithium niobate film forming the dielectric thin film 3 is preferably a single phase consisting of a twin crystal phase of LiNb 3 O 8 Phase, Li 3 NbO 4 It is preferred that the mixture does not contain any heterophases such as .

[0047] The thickness of the dielectric thin film 3 is 0.5 μm to 2 μm. Since the thickness of the dielectric thin film 3 is 0.5 μm or more, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical modulation element, the influence of the DC electric field applied to the optical modulation element can be prevented from extending not only to the upper region 31 of the dielectric thin film 3 but also to the lower region 31. As a result, DC drift is suppressed. Furthermore, since the thickness of the dielectric thin film 3 is 0.5 μm or more, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical modulation element, it is applicable to a wide range of light from visible light to infrared light.

[0048] Furthermore, since the thickness of the dielectric thin film 3 is 2 μm or less, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is processed into a ridge shape, the shape will not be inappropriate for the thickness of the dielectric thin film 3. Therefore, it is possible to suppress the influence on the DC drift characteristics due to an inappropriate shape of the optical waveguide 10 formed by processing the dielectric thin film 3 into a ridge shape, and the DC drift is suppressed. Furthermore, since the thickness of the dielectric thin film 3 is 2 μm or less, it is possible to effectively suppress the occurrence of cracks in the lithium niobate film forming the dielectric thin film 3.

[0049] Moreover, the dielectric thin film 3 preferably has a "negative (-)" stress value, and is more preferably -80 MPa or less, and even more preferably -200 MPa or less, since this makes it difficult for cracks to occur in the lithium niobate film forming the dielectric thin film 3. Moreover, the stress value of the dielectric thin film 3 is more preferably -400 MPa or more, and even more preferably -300 MPa or more, and most preferably about -250 MPa, since this reduces the amount of warping of the substrate 1 with the dielectric thin film.

[0050] As shown in FIG. 1, the dielectric thin film 3 has a lower region 31 extending from the single crystal substrate 2 to the halfway point in the thickness direction of the dielectric thin film 3, and an upper region 32 of the dielectric thin film 3 excluding the lower region 31.

[0051] In the upper region 32, the median value (maximum domain width) of the measured values ​​obtained by measuring crystal domain widths 3ad, 3bd, which are the widths in the direction perpendicular to the growth direction of each crystal and the maximum dimensions in the thickness direction of the dielectric thin film, for any 10 or more first crystals 3a and second crystals 3b present in a measurement region having a length of 4 μm in cross section, is 80 nm to 300 nm. Therefore, in the optical modulation element including the substrate 1 with the dielectric thin film of this embodiment, even if crystal defects and / or dislocations are present at each twin boundary included in the twin structure of the dielectric thin film 3, it is presumed that the influence of the crystal defects and / or dislocations on the modulated waveform of the optical modulation element is suppressed.

[0052] In the substrate 1 with dielectric thin film of this embodiment, the median value of the measured values ​​of the crystal domain widths 3ad, 3bd in the upper region 32 is 80 nm or more, so that the crystal domain widths 3ad, 3bd in the upper region 32 are sufficiently wide. Therefore, in an optical modulation element including the substrate 1 with dielectric thin film, when a DC (direct current) voltage is applied in the in-plane direction from above the dielectric thin film 3, the number of twin boundaries crossed by the DC electric field is suppressed. The median value (maximum domain width) of the measured values ​​of the crystal domain widths 3ad, 3bd in the upper region 32 is preferably 100 nm or more.

[0053] In addition, in the substrate 1 with the dielectric thin film of this embodiment, the median value of the measured values ​​of the crystal domain widths 3ad, 3bd in the upper region 32 is 300 nm or less, so that the crystal domain widths 3ad, 3bd in the upper region 32 are sufficiently narrow. Therefore, the number of crystal defects and / or dislocations present in the first crystal 3a and the second crystal 3b is suppressed. The median value (maximum domain width) of the measured values ​​of the crystal domain widths 3ad, 3bd in the upper region 32 is preferably 250 nm or less, and more preferably 200 nm or less.

[0054] Furthermore, in the substrate 1 with the dielectric thin film of this embodiment, the shapes of the first crystals 3a and the second crystals 3b of the dielectric thin film 3 may be such that the median value of the measured values ​​of the crystal domain widths 3ad, 3bd in the upper region 32 is 80 nm to 300 nm, and for example, the crystal domain widths 3ad, 3bd of the first crystals 3a and the second crystals 3b in the lower region 31 are not particularly limited.

[0055] In the optical modulation element including the substrate 1 with the dielectric thin film of this embodiment, even if a DC (direct current) voltage is applied in the in-plane direction from above the dielectric thin film 3, there is little DC electric field across the lower region 31. Therefore, if the median value (maximum domain width) of the measured values ​​of the crystal domain widths 3ad, 3bd in the first crystals 3a and second crystals 3b in the upper region 32 is 80 nm to 300 nm, the influence of crystal defects and / or dislocations in the dielectric thin film 3 on the modulated waveform of the optical modulation element can be suppressed regardless of the maximum domain width in the first crystals 3a and second crystals 3b in the lower region 31.

[0056] In the substrate 1 with a dielectric thin film of this embodiment, all of the first crystals 3a and second crystals 3b contained in the dielectric thin film 3 may have a uniform domain width in the thickness direction, or some or all of them may have a non-uniform domain width in the thickness direction, for example, as shown in Figure 1.

[0057] When all of the first crystals 3a and second crystals 3b in the dielectric thin film 3 have a uniform domain width in the thickness direction, the cross-sectional shapes of the first crystals 3a and second crystals 3b in a cross section obtained by cutting the dielectric thin film 3 in the thickness direction are each a rectangle having a long side dimension equal to the thickness of the dielectric thin film 3. In this case, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical modulation element, the optical loss of the optical modulation element is smaller, which is preferable, compared with a case in which some or all of the first crystals 3a and second crystals 3b have non-uniform domain widths in the thickness direction.

[0058] On the other hand, when some or all of the first crystals 3a and second crystals 3b contained in the dielectric thin film 3 have non-uniform domain widths in the thickness direction, it is easier to control the conditions for growing the dielectric thin film 3 on the single crystal substrate 2, resulting in a substrate 1 with a dielectric thin film that has good productivity, compared to when all of the first crystals 3a and second crystals 3b have uniform domain widths in the thickness direction, which is preferable.

[0059] When some or all of the first crystals 3a and the second crystals 3b contained in the dielectric thin film 3 have non-uniform domain widths in the thickness direction, the cross-sectional shape of the lower region 31 of the dielectric thin film 3 will be different from the cross-sectional shape of the upper region 32, as shown in Figure 1. When some or all of the first crystals 3a and second crystals 3b in the dielectric thin film 3 have non-uniform domain widths in the thickness direction, the shapes of the first crystals 3a and / or second crystals 3b in a cross section of the dielectric thin film 3 cut in the thickness direction are not particularly limited.

[0060] Therefore, the shape of the first crystal 3a and / or the second crystal 3b in a cross section cut in the thickness direction of the dielectric thin film 3 may be, for example, a trapezoid in which the interior angles at both ends of the base are equal or different from each other, or a triangle in which some or all of the three interior angles are different. In addition, the cross-sectional shape of the first crystal 3a and / or the second crystal 3b in a cross section cut in the thickness direction of the dielectric thin film 3 may be symmetrical or asymmetrical with respect to the domain width direction.

[0061] In addition, when the domain width in the thickness direction of the first crystals 3a and the second crystals 3b included in the dielectric thin film 3 is non-uniform in part or all, the first crystals 3a and / or the second crystals 3b may be formed continuously in the thickness direction of the dielectric thin film 3, or may not be formed continuously in the thickness direction of the dielectric thin film 3 in part or all. Therefore, at least a part of the first crystals 3a and / or the second crystals 3b may grow from an arbitrary position in the thickness direction of the dielectric thin film 3, or may stop growing at an arbitrary position in the thickness direction of the dielectric thin film 3. Therefore, at least a part of the first crystals 3a and / or the second crystals 3b may exist only in the lower region 31 or the upper region 32.

[0062] Furthermore, when some or all of the first crystals 3a and second crystals 3b contained in the dielectric thin film 3 have non-uniform domain widths in the thickness direction, at least a portion of the first crystals 3a and / or second crystals 3b may be split or branched at any position in the thickness direction of the dielectric thin film 3. Furthermore, when some or all of the first crystals 3a and second crystals 3b contained in the dielectric thin film 3 have non-uniform domain widths in the thickness direction, the number of first crystals 3a and / or second crystals 3b may be different in the lower region 31 and the upper region 32.

[0063] In the substrate 1 with a dielectric thin film of this embodiment, the dielectric thin film 3 is an epitaxial film formed by epitaxial growth. Therefore, the crystal orientation of the lithium niobate film forming the dielectric thin film 3 is aligned with the crystal orientation of the underlying single crystal substrate 2. More specifically, when the film plane of the lithium niobate film forming the dielectric thin film 3 is defined as the XY plane and the film thickness direction is defined as the Z axis, the crystals of the single crystal substrate 2 and the crystals of the epitaxial film forming the dielectric thin film 3 are aligned with each other in the X-axis, Y-axis, and Z-axis directions.

[0064] The fact that the dielectric thin film 3 is an epitaxial film can be proved, for example, by firstly confirming the peak intensity at the orientation position by 2θ-θ X-ray diffraction, and secondly confirming the pole by pole measurement.

[0065] Specifically, in order to prove that the dielectric thin film 3 is an epitaxial film, the first condition is that when measured by 2θ-θ X-ray diffraction, all peak intensities other than the target plane must be 10% or less, preferably 5% or less, of the maximum peak intensity of the target plane. In the c-axis oriented epitaxial film forming the dielectric thin film 3, the peak intensities other than the (00L) plane must be 10% or less, preferably 5% or less, of the maximum peak intensity of the (00L) plane. (00L) is a general designation for equivalent planes such as (001) and (002).

[0066] The above-mentioned conditions for checking the peak intensity at the orientation position by 2θ-θ X-ray diffraction only show the orientation in one direction. Therefore, even if the above-mentioned first condition is met, if the crystal orientation is not uniform within the plane, the intensity of the X-ray will not increase at a specific angle position and no pole will be observed. Therefore, in order to prove that the dielectric thin film 3 is an epitaxial film, the second condition is that the poles must be visible in the pole measurements.

[0067] LiNbO 3 Since LiNbO has a trigonal crystal structure, the single crystal 3 There are three poles of (014). It is known that when a lithium niobate film is epitaxially grown, it grows epitaxially in a so-called twin state, in which crystals rotated 180° around the c-axis are symmetrically bonded. In this case, two of the three poles are symmetrically bonded, so there are six poles.

[0068] In the substrate 1 with dielectric thin film of this embodiment, when a sapphire single crystal substrate having a c-plane as the main surface 2a is used as the single crystal substrate 2, the c-axis of the lithium niobate film forming the dielectric thin film 3 is preferably misaligned with the single crystal substrate 2 by 5° or less, and more preferably coincides with the dielectric thin film 3 (0°). If the c-axis misalignment between the dielectric thin film 3 and the single crystal substrate 2 is 5° or less, there will be no practical problems in the characteristics of the optical modulation element using the substrate 1 with dielectric thin film, which are caused by the misalignment between the dielectric thin film 3 and the single crystal substrate 2.

[0069] [Method of manufacturing substrate with dielectric thin film] Next, a method for producing the dielectric thin film-coated substrate 1 of this embodiment will be described with an example. When manufacturing the substrate 1 with the dielectric thin film of this embodiment, for example, the dielectric thin film 3 is formed on and in contact with the main surface 2a of the single crystal substrate 2 using the method described below (dielectric thin film forming step).

[0070] (Dielectric thin film deposition process) In the dielectric thin film formation step, the dielectric thin film 3 is formed by epitaxial growth on the main surface 2a of the single crystal substrate 2. The dielectric thin film 3 can be formed by, for example, sputtering, vacuum deposition, pulsed laser ablation (PLD), chemical vapor deposition (CVD), sol-gel method, or the like.

[0071] Of the above methods, sputtering is preferably used as the method for forming the dielectric thin film 3. By forming the dielectric thin film 3 using the sputtering method, a single domain structure can be obtained in the as-formed state without any special treatment after film formation. This is because the heat applied during sputtering and the electric field due to the self-bias also serve as a polarization process. Distribution of polarization is a factor in reducing the electro-optic effect. With a single domain structure, it is possible to obtain an electro-optic coefficient similar to that of a single crystal.

[0072] When sputtering is used as a method for depositing the dielectric thin film 3, a target having a composition in the range of Li / (Li+Nb)=48% to 51%, for example, can be used. The target can be prepared, for example, by the following method. 2 CO 3 and Nb 2 O 5 Next, a sintered body made of ZrO 2 The raw materials are pulverized and mixed into a target powder using a ball mill using balls made of the material. The target powder obtained is sintered using a known method to obtain a target.

[0073] In the target manufacturing process, when the raw material is pulverized using the ball mill, ZrO 2A ball made of Zr is cut off. Therefore, several hundred ppm of Zr is mixed into the target powder obtained after the raw materials are pulverized and mixed. Therefore, Zr is also mixed into the target obtained by sintering the target powder. However, since the amount of Zr contained in the target is small, the dielectric thin film 3 can be epitaxially grown on the main surface 2a of the single crystal substrate 2 by sputtering using the target containing Zr without any problems. Therefore, no problems arise due to Zr being mixed into the target.

[0074] The shape of the target used for depositing the dielectric thin film 3 is not particularly limited. The target is preferably circular with a planar area at least twice that of the single crystal substrate 2, so that a dielectric thin film 3 having a uniform thickness can be obtained. Furthermore, the deposition of the dielectric thin film 3 is preferably performed by arranging a circular target coaxially with the circular single crystal substrate 2, so that a dielectric thin film 3 having a uniform thickness can be obtained.

[0075] In the dielectric thin film forming process in this embodiment, when a sputtering method is used as a film forming method, O 2 The ratio is set to be high, the gas pressure is set to be low, and the temperature of the single crystal substrate 2 is set to be high and the power is set to be low. As a result, a lithium niobate film, which is an epitaxial film with a c-axis orientation, is formed, and LiNbO 3 The domain width in the thickness direction of the first crystals 3a and the second crystals 3b in the upper region 32 of the dielectric thin film 3 including the twin crystal structure described above is controlled.

[0076] Specifically, when a sputtering method is used as a method for forming the dielectric thin film 3, for example, Ar and O are used as sputtering gas. 2 The mixture of O and O in the sputtering gas was used. 2 The ratio can be set to 35% to 60%, the gas pressure to 0.1 Pa to 0.5 Pa, the temperature of the single crystal substrate 2 to 450° C. to 700° C., and a power of 1500 W to 2000 W can be applied so that the film formation rate is 500 nm / h to 600 nm / h.

[0077] As a result, the epitaxial film is made of lithium niobate with a c-axis orientation, and LiNbO 3 A dielectric thin film 3 is obtained which includes a twin crystal structure in which the upper region 32 has a median value (maximum domain width) of 80 nm to 300 nm obtained by measuring crystal domain widths 3ad, 3bd for any of 10 or more first crystals 3a and second crystals 3b present in a measurement region 4 μm long in a cross-sectional view. It is preferable that the dielectric thin film 3 is formed in a so-called single step without changing the film forming conditions midway. Through the above steps, the substrate 1 with the dielectric thin film of this embodiment is obtained.

[0078] The substrate 1 with the dielectric thin film of this embodiment is made of a lithium niobate film, which is an epitaxial film with a c-axis orientation formed on and in contact with the main surface 2a of the single crystal substrate 2, and is LiNbO 3 Therefore, in the substrate 1 with the dielectric thin film of this embodiment, cracks are less likely to occur in the lithium niobate film forming the dielectric thin film 3.

[0079] Moreover, in the substrate 1 with dielectric thin film of this embodiment, the upper region 32 of the dielectric thin film 3 has a median value (maximum domain width) of 80 nm to 300 nm of the measured crystal domain widths 3ad, 3bd of any 10 or more first crystals 3a and second crystals 3b present in a measurement region having a length of 4 μm in cross section, and the thickness of the dielectric thin film 3 is 0.5 μm to 2 μm. Therefore, an optical modulation element having a signal electrode and a ground electrode provided on the dielectric thin film 3 of the substrate 1 with dielectric thin film of this embodiment has suppressed DC drift when a DC (direct current) voltage is applied in the in-plane direction from above the dielectric thin film 3.

[0080] [Optical waveguide element] Fig. 2 is a plan view showing an example of an optical waveguide element 100 using the substrate 1 with the dielectric thin film shown in Fig. 1. Fig. 3 is a cross-sectional view of the optical waveguide element 100 shown in Fig. 2 taken along the line AA'. In the optical waveguide element 100 shown in FIGS. 2 and 3, the same members as those of the substrate 1 with the dielectric thin film shown in FIG. 1 are denoted by the same reference numerals, and the description thereof will be omitted.

[0081] 2 and 3 has an optical waveguide consisting of a ridge portion 4 obtained by processing the dielectric thin film 3 in the dielectric thin film-coated substrate 1 shown in Fig. 1 into a ridge shape (convex shape). The ridge portion 4 of the optical waveguide element 100 is a portion through which the target light propagates in the TM fundamental mode. The optical waveguide element 100 shown in Figures 2 and 3 can be manufactured by processing the dielectric thin film 3 in the dielectric thin film-coated substrate 1 shown in Figure 1 into a ridge shape (convex shape). The dielectric thin film 3 can be processed into a ridge shape by a known method such as an etching method.

[0082] The optical waveguide element 100 shown in Fig. 2 and Fig. 3 includes the substrate 1 with the dielectric thin film shown in Fig. 1. Therefore, cracks are unlikely to occur in the lithium niobate film forming the dielectric thin film 3 of the substrate 1 with the dielectric thin film, and productivity is excellent. Furthermore, cracks are unlikely to occur in the lithium niobate film forming the dielectric thin film 3 of the substrate 1 with the dielectric thin film, and therefore the optical waveguide element 100 has excellent durability. Furthermore, the optical waveguide element 100 of this embodiment can form an optical modulation element in which DC drift is suppressed when a DC (direct current) voltage is applied in the in-plane direction from above the dielectric thin film 3 of the substrate 1 with the dielectric thin film.

[0083] [Light modulation element] Fig. 4 is a plan view showing an example of a Mach-Zehnder type optical modulation element 200A using the substrate 1 with the dielectric thin film shown in Fig. 1. Fig. 5 is a cross-sectional view taken along line BB' of the optical modulation element 200A shown in Fig. 4. The cross-sectional view taken along line AA' of the optical modulation element 200A shown in Fig. 4 is the same as the cross-sectional view of the optical waveguide element 100 shown in Fig. 3. In the light modulation element 200A shown in FIGS. 4 and 5, the same reference numerals are used to designate the members of the dielectric thin film-formed substrate 1 shown in FIG. 1, and the description thereof will be omitted.

[0084] 4 and 5 is a device that applies a voltage to a Mach-Zehnder interferometer formed by the optical waveguide 10 to modulate light propagating through the optical waveguide 10. As shown in Fig. 4, the optical waveguide 10 has a first optical waveguide 10a and a second optical waveguide 10b branched from a single input optical waveguide, and an output optical waveguide 10c in which the first optical waveguide 10a and the second optical waveguide 10b are combined.

[0085] As shown in Fig. 4 and Fig. 5, two first electrodes 7a and 7b are provided on the first optical waveguide 10a and the second optical waveguide 10b. Therefore, the light modulation element 200A has a dual electrode structure. The first electrodes 7a and 7b may be made of, for example, an Au film or a laminated film of a Ti film and an Au film.

[0086] The optical modulation element 200A shown in FIGS. 4 and 5 has a ridge portion 4 formed by processing the dielectric thin film 3 in the dielectric thin film-attached substrate 1 shown in FIG. 1 into a ridge shape (convex shape). In the optical modulation element 200A, the ridge portion 4 forms an optical waveguide 10. As shown in FIG. 5, a first electrode 7a is formed on the ridge portion 4 constituting the first optical waveguide 10a of the optical waveguide 10, with a buffer layer 5 therebetween. Also, a first electrode 7b is formed on the ridge portion 4 constituting the second optical waveguide 10b of the optical waveguide 10, with the buffer layer 5 therebetween. As shown in FIG. 5, the buffer layer 5 is formed so as to cover the upper surface and side surfaces of the ridge portion 4. The buffer layer 5 may be, for example, SiO 2 Made of a film, SiO 2 For example, a thin film containing an oxide of a metal element can be used.

[0087] As shown in Fig. 4 and Fig. 5, the second electrodes 8a, 8b, 8c are provided spaced apart from each other via the first electrodes 7a, 7b. The second electrodes 8a, 8b, 8c are formed in contact with the upper surface of the slab portion made of the dielectric thin film 3. The second electrodes 8a, 8b, 8c may be made of, for example, an Au film or a laminated film of a Ti film and an Au film. The first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c apply a voltage that changes the refractive index of the first optical waveguide 10a and the second optical waveguide 10b of the optical waveguide 10 in the in-plane direction from above the dielectric thin film 3. The slab portion made of the dielectric thin film 3 is formed by thinning a part of the upper surface of the dielectric thin film 3 in the substrate 1 with the dielectric thin film shown in Fig. 1 by etching or the like. As shown in Fig. 4, the first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c are connected by a termination resistor 9.

[0088] [Method of manufacturing optical modulation element] The light modulation element 200A shown in FIGS. 4 and 5 can be manufactured, for example, by the manufacturing method described below. First, the dielectric thin film 3 in the substrate 1 with a dielectric thin film shown in FIG. 1 is processed into a ridge shape (convex shape) using a known method such as an etching method to form an optical waveguide 10 consisting of a ridge portion 4 and a slab portion consisting of the dielectric thin film 3.

[0089] Next, a buffer layer 5 is formed so as to cover the top and side surfaces of the ridge portion 4 using a known method such as sputtering, vacuum deposition, pulsed laser ablation (PLD) or chemical vapor deposition (CVD). Thereafter, using a known method such as sputtering or vacuum deposition, second electrodes 8a, 8b, and 8c are formed in contact with the upper surface of the slab portion made of the dielectric thin film 3, and first electrodes 7a and 7b are formed on the buffer layer 5. Through the above steps, the light modulation element 200A shown in FIGS. 4 and 5 is obtained.

[0090] [Operation principle of light modulation element] Next, the operating principle of the light modulation element 200A will be described. 4, the two first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c are connected by a termination resistor 9 to function as traveling wave electrodes. The first electrodes 7a, 7b are used as signal electrodes, and the second electrodes 8a, 8b, 8c are used as ground electrodes. So-called complementary signals, which have the same absolute value, different positive and negative phases, and are not shifted, are input to the two first electrodes 7a, 7b from the input sides 15a, 15b of the first electrodes 7a, 7b of the optical modulation element 200A.

[0091] In this embodiment, when a signal is input from the input side 15a, 15b, a DC (direct current) voltage is applied in a superimposed manner in the in-plane direction of the dielectric thin film 3 from the first electrodes 7a, 7b toward the second electrodes 8a, 8b, 8c. This causes the refractive indexes of the first optical waveguide 10a and the second optical waveguide 10b of the optical waveguide 10 to change in proportion to the DC voltage, and output light having a modulated waveform is output from the output optical waveguide 10c. The modulated waveform of the output light output from the optical modulation element 200A changes with the application time of the DC (direct current) voltage. This change in the modulated waveform over time is called DC drift.

[0092] The lithium niobate film forming the dielectric thin film 3 in the substrate 1 with the dielectric thin film has an electro-optic effect. Therefore, the refractive indexes of the first optical waveguide 10a and the second optical waveguide 10b change to +Δn and -Δn, respectively, depending on a DC (direct current) voltage applied to the first optical waveguide 10a and the second optical waveguide 10b. As a result, the phase difference between the first optical waveguide 10a and the second optical waveguide 10b changes. Signal light having a modulated waveform that is intensity-modulated by this change in phase difference is output to the output side 12 from the output optical waveguide 10c in which the first optical waveguide 10a and the second optical waveguide 10b are multiplexed.

[0093] The light modulation element 200A of this embodiment includes a substrate 1 with a dielectric thin film shown in Fig. 1. Therefore, the light modulation element 200A of this embodiment is less susceptible to cracks in the lithium niobate film forming the dielectric thin film 3 of the substrate 1 with a dielectric thin film, and is excellent in productivity and durability. The light modulation element 200A of this embodiment includes a substrate 1 with a dielectric thin film as shown in Fig. 1. Therefore, the first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c suppress DC drift when a DC (direct current) voltage is applied in an in-plane direction from above the dielectric thin film 3. Therefore, the light modulation element 200A of this embodiment is highly reliable and can be suitably used as, for example, an optical communication device.

[0094] The optical modulation element 200A of this embodiment is preferably one in which, when heated to 120°C and a signal is input from the input sides 15a and 15b of the first electrodes 7a and 7b, a DC (direct current) voltage is applied from the first electrodes 7a and 7b toward the second electrodes 8a, 8b and 8c in the in-plane direction of the dielectric thin film 3 in a superimposed manner, the DC drift does not exceed 50% within one hour. The DC drift is a numerical value calculated using the following formula (I).

[0095] DC drift (%) = (shift voltage (V) / applied DC voltage (V)) x 100 (I) (In formula (I), the shift voltage (V) is a DC (direct current) voltage that indicates the amount of phase shift (amount of drift) of the modulated waveform relative to the modulated waveform at the time when the DC (direct current) voltage is applied.)

[0096] In the light modulation element 200A of this embodiment, if the DC drift does not exceed 50% before one hour when heated to 120° C., the DC drift can be easily suppressed. This is because the light modulation element 200A is provided with a feedback driver (control circuit) that controls the DC (direct current) voltage applied from the first electrodes 7a, 7b to the second electrodes 8a, 8b, 8c, and the feedback driver applies a DC voltage corresponding to the shift voltage (V), thereby easily compensating for the shift amount of the phase of the modulation waveform.

[0097] Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. The present invention can be modified in various ways without departing from the spirit of the present invention, and it goes without saying that such modifications are also included within the scope of the present invention. EXAMPLES

[0098] "Examples 1 to 13, Comparative Examples 1 to 6" The substrate 1 with the dielectric thin film shown in FIG. 1 was produced by the method described below. As the single crystal substrate 2, a 4-inch sapphire single crystal substrate having a c-plane as the main surface 2a was prepared.

[0099] (Dielectric thin film deposition process) In the dielectric thin film formation step, the dielectric thin film 3 was formed on the main surface 2a of the single crystal substrate 2 by epitaxial growth using a sputtering method. The target used was a circle having a diameter of 8 inches and a composition of Li / (Li+Nb)=50%.

[0100] The target was prepared using the following method. The raw material was Li with a purity of 3N or higher. 2 CO 3 and Nb 2 O 5 Next, we prepared a sintered body made mainly of ZrO 2 The raw materials were pulverized and mixed using a ball mill using balls made of the material, and the target powder was obtained by sintering the target.

[0101] The dielectric thin film 3 was formed by arranging the target thus obtained coaxially with the single crystal substrate 2 so that the distance from the main surface 2a of the single crystal substrate 2 was 110 mm. The dielectric thin film 3 was formed using Ar and O as sputtering gas. 2 The mixture of O and O in the sputtering gas was used. 2The ratio was set to 35% to 60%, the gas pressure to 0.1 Pa to 0.5 Pa, the temperature of the single crystal substrate 2 to 450° C. to 700° C., and a power of 1500 W to 2000 W was applied so that the film formation rate was 500 nm / h to 600 nm / h. The dielectric thin film 3 was formed in a so-called single step without changing the film formation conditions midway. By the above steps, substrates 1 with dielectric thin films of Examples 1 to 13 and Comparative Examples 1 to 6 were obtained.

[0102] "Example 14" A substrate with a dielectric thin film was produced in the same manner as in Example 8, except that the deposition time of the dielectric thin film 3 was shortened. "Example 15" A substrate with a dielectric thin film was produced in the same manner as in Example 8, except that the deposition time of the dielectric thin film 3 was increased.

[0103] "Comparative Example 7" A substrate with a dielectric thin film was produced in the same manner as in Example 8, except that the deposition time of the dielectric thin film 3 was shortened. "Comparative Example 8 and Comparative Example 9" A substrate with a dielectric thin film was produced in the same manner as in Example 8, except that the deposition time of the dielectric thin film 3 was increased.

[0104] For the substrates with dielectric thin film thus obtained in Examples 1 to 15 and Comparative Examples 1 to 9, the dielectric thin film 3 was confirmed to be an epitaxial film by the above-mentioned methods, firstly, by confirming the peak intensity at the orientation position by 2θ-θ X-ray diffraction, and secondly, by confirming the poles by pole measurement.

[0105] In addition, for the substrates with dielectric thin film of Examples 1 to 15 and Comparative Examples 1 to 9, the dielectric thin film 3 was converted into LiNbO 3 As a result, the dielectric thin film 3 of the substrates with the dielectric thin film in Examples 1 to 15 and Comparative Examples 1 to 9 was all LiNbO 3 It was confirmed that the material had a twin crystal structure.

[0106] "LiNbO 3 "Method for confirming that a material has a twin crystal structure" The cross section of the substrate with the dielectric thin film cut in the thickness direction was observed using a transmission electron microscope (TEM) (manufactured by FEI) to obtain a dark field (DF) image. 3 The beam incidence conditions were adjusted so that the image of either the first crystal 3a or the second crystal 3b contained in the twin crystal structure was in a high contrast (bright) state.

[0107] In the dark-field image obtained by adjusting the beam incidence conditions as described above, the dielectric thin film 3 is LiNbO 3 When the dielectric thin film 3 has a twin crystal structure, one of the first crystal 3a and the second crystal 3b has a high contrast (bright) image and the other has a low contrast (dark) image, so that the first crystal 3a and the second crystal 3b can be clearly distinguished from each other. 3 It was confirmed that the material had a twin crystal structure.

[0108] Further, for the thus obtained substrates with dielectric thin films of Examples 1 to 15 and Comparative Examples 1 to 9, the dielectric thin film 3 was subjected to the following test. 3 We investigated whether the dielectric thin film 3 is a single layer, whether the domain width of the dielectric thin film 3 is uniform, the maximum domain width of the first crystals 3a and the second crystals 3b of the dielectric thin film 3, the film thickness of the dielectric thin film 3, and the ratio of the first diffraction intensity corresponding to the first crystals 3a to the second diffraction intensity corresponding to the second crystals 3b. The results are shown in Tables 1 and 2. Tables 1 and 2 show the types of single crystal substrates 2 used in the substrates with dielectric thin films in Examples 1 to 15 and Comparative Examples 1 to 9.

[0109] [Table 1]

[0110] [Table 2]

[0111] "Dielectric thin film 3 is LiNbO 3 Single layer or not?” The refractive index of the lithium niobate film, which is the dielectric thin film 3 of the substrate 1 with the dielectric thin film, was measured using a prism coupler (manufactured by Metricon), and the Li content in the lithium niobate film was measured. 2 When the O content is within the range of 47.5 mol% to 50.0 mol%, the lithium niobate film is LiNbO 3 The refractive index of the lithium niobate film measured using a prism coupler can be used to determine the Li 2 By using the method for measuring the O content, the Li content in the lithium niobate film, which is the dielectric thin film 3 of the substrate 1 with a dielectric thin film, can be measured nondestructively and with high accuracy.

[0112] First, a lithium niobate single crystal substrate with a Li content of 47.5 mol% and a lithium niobate single crystal substrate with a Li content of 50.0 mol% were prepared, and the refractive index of each was measured using a prism coupler. The refractive index measured using a prism coupler changes almost in proportion to the Li content. For this reason, a calibration curve was created using the refractive indexes measured using a prism coupler for a lithium niobate single crystal substrate with a Li content of 47.5 mol% and a lithium niobate single crystal substrate with a Li content of 50.0 mol%, and the Li content (content rate) contained in the lithium niobate film that is the dielectric thin film 3 of each of the dielectric thin film-attached substrates 1 of Examples 1 to 15 and Comparative Examples 1 to 9 was calculated using this calibration curve.

[0113] As a result, the lithium niobate films of the dielectric thin film-attached substrates 1 in Examples 1 to 15 and Comparative Examples 1 to 9 all contained 47.5% by mass to 49.8% by mass of Li. From this, it can be seen that the lithium niobate films of the dielectric thin film-attached substrates 1 in Examples 1 to 15 and Comparative Examples 1 to 9 all contained LiNbO3 It was confirmed that the temperature was single phase.

[0114] "Whether the domain width of the dielectric thin film 3 is uniform" "Maximum domain width of the first crystal 3a and the second crystal 3b of the dielectric thin film 3" A cross section of the dielectric thin film 3 of the substrate 1 with the dielectric thin film cut in the thickness direction was observed using a transmission electron microscope (TEM) (manufactured by FEI) to obtain a dark field (DF) image. 3 The beam incidence conditions were adjusted so that the image of one of the first crystal 3a and the second crystal 3b contained in the twin crystal structure was in a high-contrast (bright) state. As a result, a dark-field image was obtained in which the image of one of the first crystal 3a and the second crystal 3b was in a high-contrast (bright) state and the image of the other was in a low-contrast (dark) state. Therefore, the obtained dark-field image clearly distinguished the first crystal 3a and the second crystal 3b.

[0115] The dark-field images thus obtained were subjected to image analysis to check whether the domain widths of the first crystals 3a and the second crystals 3b present on the interface with the 4 μm long single crystal substrate 2 were uniform. If the domain shape was a rectangular, so-called columnar shape, the domain width was evaluated as uniform, and if other domain shapes were also present, the domain width was evaluated as non-uniform. As a result, it was confirmed that the lithium niobate films of the substrates 1 with dielectric thin films of Examples 1 to 15 and Comparative Examples 1 to 9 had non-uniform domain widths.

[0116] Moreover, the above dark field image was subjected to image analysis to determine the maximum domain width of the first crystals 3a and the second crystals 3b contained in the upper region 32 of the dielectric thin film 3 by the method described below. That is, a measurement area having a length of 4 μm in cross-sectional view was set at an arbitrary location on the interface between the dielectric thin film 3 and the single crystal substrate 2 in the above dark-field image, and the crystal domain widths of 10 or more arbitrary first crystals 3a and second crystals 3b present in the measurement area were measured, and the median value of the obtained measurements was calculated as the maximum domain width.

[0117] Here, the crystal domain width of each of the first crystals 3a and each of the second crystals 3b refers to the domain width of each of the first crystals 3a and each of the second crystals 3b measured in a direction perpendicular to the growth direction of each crystal, and is the length dimension of the domain width at the thickest (in other words, the widest) point in the thickness direction of the dielectric thin film 3. In addition, there is no need to distinguish between the first crystals 3a and the second crystals 3b among the 10 or more arbitrary first crystals 3a and second crystals 3b present in the measurement area for measuring the crystal domain width to examine the maximum domain width. Therefore, the ratio between the number of the first crystals 3a and the number of the second crystals 3b for measuring the crystal domain width is not particularly limited, and it is sufficient that the total number of the first crystals 3a and the number of the second crystals 3b for measuring the crystal domain width is 10 or more.

[0118] "Thickness of dielectric thin film 3" The thickness of the dielectric thin film 3 in the substrate 1 with the dielectric thin film was determined by performing high-resolution analysis using a scanning transmission electron microscope (STEM) (manufactured by FEI) to measure the thickness of the dielectric thin film 3 at 10 or more points within a rectangular field of view of 1.3 μm in length and 4 μm in width, and calculating the median value.

[0119] "Ratio of the first diffraction intensity corresponding to the first crystals 3a of the dielectric thin film 3 to the second diffraction intensity corresponding to the second crystals 3b" For the dielectric thin film 3 of the substrate 1 with the dielectric thin film, pole measurement was performed by X-ray diffraction using an X-ray diffractometer (manufactured by Rigaku Corporation) to measure a first diffraction intensity corresponding to the first crystals 3a and a second diffraction intensity corresponding to the second crystals 3b of the dielectric thin film 3. Using the results, the ratio of the first diffraction intensity to the second diffraction intensity was calculated.

[0120] Furthermore, for the substrates with dielectric thin film of Examples 1 to 15 and Comparative Examples 1 to 9, "stress in the dielectric thin film," "presence or absence of cracks in the dielectric thin film 3," and "presence or absence of cracks caused by annealing" were examined by the methods described below. The results are shown in Tables 3 and 4.

[0121] [Table 3]

[0122] [Table 4]

[0123] "Stress in thin dielectric films" The amount of warping of the dielectric thin film 3 in the substrate 1 with the dielectric thin film was measured using a needle-type step gauge (manufactured by KLA-Tenchore), and the stress of the dielectric thin film was calculated by the Stoney formula. When the stress value of the obtained dielectric thin film was "plus (+)", it was evaluated as tensile stress. When the stress value of the dielectric thin film was "minus (-)", it was evaluated as compressive stress. When the stress value of the dielectric thin film was "-400 MPa to -80 MPa", no cracks occurred and the amount of warping of the substrate with the dielectric thin film was small, so the stress was evaluated to be within a suitable range.

[0124] "Presence or absence of cracks in dielectric thin film 3" The substrate with the dielectric thin film was observed using an optical microscope (manufactured by Olympus) with an objective lens magnification of 20x and a field of view diameter of approximately 0.5 mm, and an objective lens magnification of 100x and a field of view diameter of approximately 0.1 mm, to check for the presence or absence of cracks in the dielectric thin film 3. When no cracks were found, the specimen was rated as "absent." When even one crack was found, the specimen was rated as "present."

[0125] "Whether or not cracks occur in the dielectric thin film 3 due to annealing" The substrate 1 with the dielectric thin film was annealed in an oxygen atmosphere at a pressure of 1 atm at 600° C. for 1 hour. Thereafter, the presence or absence of cracks in the dielectric thin film 3 after annealing was examined and evaluated in the same manner as in the above-mentioned "Presence or absence of cracks in the dielectric thin film 3".

[0126] [Manufacturing of light modulation elements] The optical modulation element 200A shown in FIGS. 4 and 5 was manufactured by the manufacturing method described below using the substrate 1 with the dielectric thin film of each of Examples 1 to 15 and Comparative Examples 1 to 9. That is, the dielectric thin film 3 in the substrate 1 with the dielectric thin film was processed into a ridge shape (convex shape) by etching to form an optical waveguide 10 consisting of a ridge portion 4, and a slab portion consisting of the dielectric thin film 3.

[0127] Next, a SiO 2 A buffer layer 5 made of a film was formed. Thereafter, a sputtering method was used to form a Ti film that would become the second electrodes 8a, 8b, and 8c in contact with the upper surface of the slab portion made of the dielectric thin film 3, and a Ti film that would become the first electrodes 7a and 7b was formed on the buffer layer 5. Using the obtained Ti film as a seed layer, an Au film was formed on the Ti film by plating, thereby forming the second electrodes 8a, 8b, and 8c and the first electrodes 7a and 7b made of a laminated film of the Ti film and the Au film. By the above steps, optical modulation elements 200A each including a substrate 1 with a dielectric thin film of Examples 1 to 15 and Comparative Examples 1 to 9 were obtained.

[0128] "DC drift evaluation" For each of the optical modulation elements 200A thus obtained, a DC drift evaluation was performed by the method described below. That is, the optical modulation element 200A was heated to 120°C, and a signal was input from the input sides 15a, 15b of the first electrodes 7a, 7b. At that time, a DC (direct current) voltage of 8 V was superimposed and applied from the first electrodes 7a, 7b to the second electrodes 8a, 8b, 8c in the in-plane direction of the dielectric thin film 3. Then, for one hour from the time when the DC (direct current) voltage was applied in the in-plane direction of the dielectric thin film 3, the modulated waveform of the output light output from the output optical waveguide 10c of the optical modulation element 200A to the output side 12 was observed by an oscilloscope.

[0129] As a result, the DC drift calculated by the above formula (I) from the phase shift of the modulated waveform relative to the modulated waveform at the time of applying the DC voltage exceeded 50% before the application time of the DC voltage exceeded 1 hour was evaluated as failing, and the DC drift that did not exceed 50% before the application time of the DC voltage exceeded 1 hour was evaluated as passing. The results are shown in Tables 3 and 4.

[0130] FIG. 6 shows the results of observing, by an oscilloscope, the modulated waveform of the output light output from the output optical waveguide 10c of the optical modulation element 200A to the output side 12 for one hour from the time when a DC (direct current) voltage was applied in the in-plane direction of the dielectric thin film 3 for the optical modulation elements 200A of Examples 5, 10, and Comparative Example 3.

[0131] FIG. 6 is a graph showing the relationship between time and DC drift in the modulated waveform of the output light output from the optical modulation elements 200A of Example 5, Example 10, and Comparative Example 3. As shown in FIG. 6, in the light modulation elements 200A of Examples 5 and 10, the DC drift did not exceed 50% before the application time of the DC voltage exceeded one hour. In contrast, in the light modulation element 200A of Comparative Example 3, the DC drift exceeded 50% before the application time of the DC voltage exceeded one hour.

[0132] As shown in Table 3, in the substrates with dielectric thin film of Examples 1 to 15, the "stress in the dielectric thin film" was compressive stress, and there were no cracks in the dielectric thin film 3 or cracks caused by annealing. Furthermore, in all of the optical modulation elements of Examples 1 to 15, the shift amount (DC drift) of the modulated waveform did not exceed 50% before the application time of the DC (direct current) voltage exceeded one hour, and the DC drift evaluation was passed.

[0133] In contrast, as shown in Table 4, in the optical modulation elements using the substrates with dielectric thin films of Comparative Example 1 in which the maximum domain width of the first crystals 3a and the second crystals 3b of the dielectric thin film 3 is 68 nm, and Comparative Example 2 in which the maximum domain width of the first crystals 3a and the second crystals 3b of the dielectric thin film 3 is 70 nm, cracks in the dielectric thin film 3 and cracks caused by annealing were [present], and DC drift could not be evaluated.

[0134] In addition, as shown in Table 4 and Fig. 6, in the optical modulation element using the substrate with the dielectric thin film of Comparative Example 3 in which the maximum domain width of the first crystal 3a and the second crystal 3b of the dielectric thin film 3 is 75 nm, the shift amount (DC drift) of the modulation waveform exceeded 50% before the application time of the DC (direct current) voltage exceeded 1 hour, and the DC drift evaluation was unsuccessful. This is presumably because the maximum domain width of the first crystal 3a and the second crystal 3b of the dielectric thin film 3 is narrow, and therefore the number of twin boundaries crossed by the DC electric field is not sufficiently suppressed when a DC (direct current) voltage is applied in the in-plane direction from above the dielectric thin film 3. In addition, in the substrate with dielectric thin film of Comparative Example 3, the stress value of the dielectric thin film 3 was −80 MPa or less, and there were no cracks in the dielectric thin film 3 or cracks caused by annealing.

[0135] Furthermore, as shown in Table 4, in the optical modulation elements using the substrates with dielectric thin film of Comparative Example 4 to Comparative Example 6 in which the maximum domain width of the first crystals 3a and the second crystals 3b of the dielectric thin film 3 exceeds 300 nm, the shift amount (DC drift) of the modulation waveform exceeded 50% before the application time of the DC (direct current) voltage exceeded 1 hour, and the DC drift evaluation was unsuccessful, as in Comparative Example 3. The cause of this is unknown, but it may be due to the large number of crystal defects and / or dislocations present in the first crystals 3a and the second crystals 3b of the dielectric thin film 3.

[0136] From these findings, it was confirmed that when the maximum domain width of the first crystals 3a and the second crystals 3b of the dielectric thin film 3 is 80 nm to 300 nm, cracks are less likely to occur in the dielectric thin film 3, and DC drift can be suppressed in an optical modulation element using this.

[0137] Moreover, as shown in Table 4, in Comparative Example 7 using a substrate with a dielectric thin film 3 having a thickness of 0.3 μm, the shift amount (DC drift) of the modulated waveform exceeded 50% before the application time of the DC (direct current) voltage exceeded 1 hour, and the DC drift evaluation was unsuccessful. This is presumably because the thickness of the dielectric thin film 3 was insufficient, and the influence of the DC electric field applied to the optical modulation element extended not only to the upper region 31 of the dielectric thin film 3 but also to the lower region 31.

[0138] Also, in Comparative Example 8 and Comparative Example 9, which used a substrate with a dielectric thin film 3 having a thickness exceeding 2.0 μm, the shift amount (DC drift) of the modulated waveform exceeded 50% before the application time of the DC (direct current) voltage exceeded 1 hour, and the DC drift evaluation was unsuccessful. This may be because the thickness of the dielectric thin film 3 was too thick, causing the shape of the optical waveguide 10 consisting of the ridge portion 4 of the optical modulation element formed by processing the dielectric thin film 3 into a ridge shape to become inappropriate.

[0139] From these, it was confirmed that by setting the thickness of the dielectric thin film 3 to 0.5 μm to 2 μm, DC drift can be suppressed in the optical modulation element using this. [Explanation of symbols]

[0140] 1 substrate with dielectric thin film, 2 single crystal substrate, 2a main surface, 3 dielectric thin film, 3a first crystal, 3b second crystal, 4 ridge portion, 5 buffer layer, 7a, 7b first electrode, 8a, 8b, 8c second electrode, 9 termination resistor, 10 optical waveguide, 10a first optical waveguide, 10b second optical waveguide, 10c output optical waveguide, 12 output side, 15a, 15b input side, 31 lower region, 32 upper region, 100 optical waveguide element, 200A optical modulation element.

Claims

1. A single crystal substrate; a dielectric thin film formed on and in contact with a main surface of the single crystal substrate; The dielectric thin film has a thickness of 0.5 μm to 2 μm, is made of a lithium niobate film which is an epitaxial film oriented along the c-axis, and includes a first crystal and a second crystal obtained by rotating the first crystal by 180° around the c-axis. ３ The twin structure includes the first crystals and the second crystals included in an upper region of the dielectric thin film excluding a lower region extending from the single crystal substrate to a half of the thickness direction have a maximum domain width of 80 nm to 300 nm; The maximum domain width in the first crystal and the second crystal is the median value of the measured values ​​obtained by setting a measurement area having a length of 4 μm in a cross-sectional view at an arbitrary location on the interface between the dielectric thin film and the single crystal substrate, measuring crystal domain widths, which are widths in a direction perpendicular to the growth direction of each crystal and are the maximum dimension in the thickness direction of the dielectric thin film, for any of 10 or more of the first crystals and the second crystals present in the measurement area.

2. 2. The substrate with a dielectric thin film according to claim 1, wherein a domain width in a thickness direction of the first crystals and the second crystals included in the dielectric thin film is non-uniform in part or all of the first crystals and the second crystals.

3. 2. The substrate with a dielectric thin film according to claim 1, wherein the single crystal substrate is a sapphire single crystal substrate, the main surface of which is a c-plane.

4. 2. The substrate with the dielectric thin film according to claim 1, wherein the twin crystal structure of the dielectric thin film has a ratio of a first diffraction intensity corresponding to the first crystal to a second diffraction intensity corresponding to the second crystal of 0.5 or more and 2.0 or less in a pole measurement by an X-ray diffraction method.

5. An optical waveguide element comprising a substrate with a dielectric thin film according to any one of claims 1 to 4.

6. 6. The optical waveguide element according to claim 5, comprising an optical waveguide made of the dielectric thin film.

7. 5. An optical modulation element comprising a substrate with a dielectric thin film according to claim 1.

8. 8. The optical modulation element according to claim 7, comprising: an optical waveguide made of the dielectric thin film; and a first electrode and a second electrode for applying a voltage in an in-plane direction from above the dielectric thin film to change the refractive index of the optical waveguide.

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