Wavelength conversion element, optical circuit using same, and light source device

Abstract

Provided is a wavelength conversion element in which high-quality crystal part can be utilized by a simple configuration. This wavelength conversion element comprises a substrate having a main surface, and an optical waveguide disposed on the main surface of the substrate, a core of the optical waveguide including a polar member having a second-order nonlinear optical constant and a first non-polar member disposed adjacent to the polar member in a direction parallel to the main surface of the substrate, and one or both side surfaces of the polar member and the first non-polar member being in contact with each other in a cross section orthogonal to the optical axis of the optical waveguide.

Description

Wavelength conversion element, optical circuit using same, and light source device

[0001] The present disclosure relates to a wavelength conversion element, an optical circuit using the same, and a light source device.

[0002] When strong light is incident on a material with a nonlinear optical effect, a nonlinear polarization proportional to the square or cube of the optical electric field occurs. Second-order nonlinear optical effects include sum frequency generation and difference frequency generation, which are used in second harmonic generation (SHG) and optical parametric amplification. Wavelength conversion is also performed using these nonlinear optical effects. Wavelength conversion waveguides composed of periodically reversed polarizations of ferroelectric materials such as lithium niobate (LN) and lithium tantalate (LT) are commonly known as wavelength converters. However, they are difficult to microfabricate and have a low damage threshold in the visible light band.

[0003] A method has been proposed in which GaN layers are grown in opposing directions from the sidewalls on both sides of a groove formed in a substrate, and a waveguide element for wavelength conversion is fabricated using a first crystalline region and a second crystalline region whose c-axes are in opposite directions (see, for example, Patent Document 1). ２ A quasi-phase-matched waveguide has been proposed in which these are stacked in order on the surface of a substrate (see, for example, Non-Patent Documents 1 and 2).

[0004] Japanese Patent Application Laid-Open No. 2018-194617

[0005] Design of Lateral Quasi-Phase-Matched HfO2 / AlN Waveguide for 230 nm Deep-UV Second-Harmonic Generation, Proceedings of the 82nd Autumn Meeting of the Japan Society of Applied Physics, 2021, Lecture No. 12p-N101-7 Fabrication of Lateral Quasi-Phase-Matched HfO2 / AlN Channel Waveguide for 230 nm Deep-UV Second-Harmonic Generation, Proceedings of the 69th Spring Meeting of the Japan Society of Applied Physics, 2022, Lecture No. 26p-E203-6

[0006] The method of growing a GaN layer with its c-axis oriented in the opposite direction within the grooves is complicated in its fabrication process and has poor crystal quality near the interface with the substrate. ２In the case of a laminated structure, the thickness of the AlN thin film is small, making it difficult to grow a high-quality crystal thin film. One aspect of the present disclosure provides a wavelength conversion element that can utilize a high-quality crystal portion with a simple structure.

[0007] In one embodiment, the wavelength conversion element comprises: a substrate having a main surface; and an optical waveguide disposed on the main surface of the substrate, wherein the core of the optical waveguide includes a polar member having a second-order nonlinear optical constant, and a first nonpolar member disposed adjacent to the polar member in a direction parallel to the main surface of the substrate, and one or both sides of the polar member contact the first nonpolar member in a cross section perpendicular to the optical axis of the optical waveguide.

[0008] A wavelength conversion element that has a simple configuration and can utilize a high-quality crystal portion is realized.

[0009] 4 is a schematic perspective view of a wavelength conversion element of a first embodiment. FIG. 5 is a cross-sectional view perpendicular to the optical axis of FIG. 1. FIG. 6 is a diagram schematically showing a guided mode propagating through an optical waveguide of a wavelength conversion element. FIG. 7 is a schematic diagram of a first modified example of a wavelength conversion element. FIG. 8 is a diagram showing the manufacturing process of the optical waveguide of the wavelength conversion element of FIG. 4. FIG. 9 is a diagram showing the manufacturing process of the optical waveguide of the wavelength conversion element of FIG. 4. FIG. 10 is a diagram showing the manufacturing process of the optical waveguide of the wavelength conversion element of FIG. 4. FIG. 11 is a diagram showing the manufacturing process of the optical waveguide of the wavelength conversion element of FIG. 12. FIG. 12 is a diagram showing the manufacturing process of the optical waveguide of the wavelength conversion element of FIG. 13. ００ 1 is a diagram showing the electromagnetic field distribution of a TM mode of an optical waveguide. ２０ 1 is a diagram showing electromagnetic field distribution of a mode; FIG. 2 is a diagram showing calculation results of a core width for phase matching in a transverse mode; FIG. 3 is a cross-sectional view of a wavelength conversion element of a second embodiment; FIG. 4 is a cross-sectional view of a wavelength conversion element of a third embodiment; FIG. 5 is a schematic diagram of light propagating through a wavelength conversion element of a third embodiment; FIG. 6 is a cross-sectional view of a wavelength conversion element of a fourth embodiment; FIG. 7 is a cross-sectional view of a wavelength conversion element of a modified example of the fourth embodiment; FIG. 8 is a cross-sectional view of a wavelength conversion element of a fifth embodiment; FIG. 9 is a cross-sectional view of a wavelength conversion element of a modified example of the fifth embodiment; FIG. 10 is a schematic diagram of a light source device using a wavelength conversion element of an embodiment; FIG. 11 is a schematic diagram of an optical circuit using a wavelength conversion element of an embodiment.

[0010] Hereinafter, embodiments for carrying out the present disclosure will be described with reference to the drawings. The following description is intended to embody the technical concept of the present disclosure, and unless otherwise specified, the present disclosure is not limited to the following description. In each drawing, components having the same function may be assigned the same reference numerals. For convenience, the embodiments may be shown separately in consideration of ease of explanation or understanding of the main points, but partial substitution or combination of configurations shown in different embodiments or examples is possible. In the embodiments shown later, differences from the previously shown embodiments will be mainly described, and redundant description of matters common to the previously shown embodiments may be omitted. The size and positional relationship of components shown in each drawing may be exaggerated for clarity.

[0011] In the following embodiments, for the sake of simplicity, the angular frequency ω １ Light with angular frequency ω ２ (=2×ω １ However, the wavelength conversion element of the embodiment can also be used for purposes other than SHG. For example, wavelength conversion in the reverse process of SHG (i.e., wavelength conversion with an angular frequency of ω ２ from angular frequency ω １ Furthermore, it can also be used as a wavelength conversion element that performs arbitrary wavelength conversion by sum frequency generation or difference frequency generation within a range that satisfies the phase matching condition described below.

[0012] In an optical waveguide, the guided mode cannot be strictly separated into a TM mode and a TE mode, and contains both components. This is called a TM-like mode or a TE-like mode. In this specification, for the sake of simplicity, the term TM mode is used, but this refers to a TM-like mode.

[0013] <First embodiment> Fig. 1 is a schematic perspective view of a wavelength conversion element 10 of the first embodiment, and Fig. 2 is a cross-sectional view perpendicular to the optical axis OX of Fig. 1. In the coordinate system of Fig. 1 and Fig. 2, the direction parallel to the optical axis OX of the wavelength conversion element 10 is defined as the Z direction, and the normal direction to the surface of the substrate 5 in the XY plane perpendicular to the optical axis OX is defined as the Y direction.

[0014] The wavelength conversion element 10 includes a substrate 5 having a main surface 501 and an optical waveguide 20 disposed on the main surface 501 of the substrate 5. The core 15 of the optical waveguide 20 includes a polar member 11 having a second-order nonlinear optical constant and a first nonpolar member 12 disposed adjacent to the polar member 11 in the X direction parallel to the main surface 501 of the substrate 5. In the XY plane, which is a cross section perpendicular to the optical axis OX of the optical waveguide 20, one or both side surfaces 111 of the polar member 11 contact the first nonpolar member 12. This allows a region with relatively high crystal quality to serve as a waveguide path, thereby reducing optical loss due to the waveguide itself.

[0015] Here, the "direction parallel to" the main surface 501 does not require that the main surface 501 and the X-axis be strictly parallel in the XY plane, and may include manufacturing errors and process errors, and may include a deviation of, for example, ±2° or less, preferably ±0.1° or less. In the example of FIG. 1 , one side surface 111 of the polar member 11 is disposed in contact with the first non-polar member 12, but as will be described later, the first non-polar member 12 may be disposed in contact with both side surfaces of the polar member 11. The side surface 111 does not need to be perpendicular to the main surface 501 of the substrate 5, and the shape of the polar member 11 in the XY cross section may be a trapezoid or an inverted trapezoid.

[0016] The core 15 of the optical waveguide 20 is configured by the polar member 11 and the first non-polar member 12 that are in contact with each other in the lateral direction (X direction) on the main surface 501 of the substrate 5. The polar member 11 is formed, for example, of a wurtzite crystal having polarization in the c-axis direction. The polar member 11 can receive light with a higher power density than LN or LT, thereby increasing the conversion efficiency of the wavelength conversion element 10. As shown by the upward arrow in Figure 1, the c-axis of the polar member 11 formed on the substrate 5 is perpendicular to the substrate 501, and has polarization in the direction of the arrow. AlN, GaN, ZnO, etc., represented by the general formula AB, have a wurtzite crystal structure. In the embodiment, the polar member 11 is Al ｘ Ga １－ｘ N (0≦x≦1) by adjusting the value of x, the difference in refractive index between the first nonpolar member 12 and the second nonpolar member 13 can be reduced.

[0017] The first non-polar member 12 is a material that does not have polarity. The first non-polar member 12 is preferably formed as an amorphous layer. The first non-polar member 12 is made of Nb ２ O ５ , Ta ２ O ５ , TiO ２ , HfO ２ , ZrO ２ , Si ３ N ４ The composition does not necessarily have to be stoichiometric, and the oxygen or nitrogen content may be higher or lower than the stoichiometric content. Furthermore, the first non-polar member 12 may be a composite material containing two or more of the above materials. Alternatively, the refractive index may be adjusted by doping with an impurity. This can reduce the difference between the refractive index of the first non-polar member 12 and the refractive index of the polar member 11, thereby reducing the angular frequency ω １ of light and angular frequency ω ２ This increases the number of angular frequency combinations that achieve phase matching with light of the same wavelength. By forming the core 15 by arranging the first nonpolar member 12 and the polar member 11 adjacent to each other in the lateral direction (X direction) rather than the stacking direction on the main surface 501 of the substrate 5, the thickness of the polar member 11 can be increased, improving the quality of the crystal. By arranging the non-polar first nonpolar member 12 in contact with the side surface 111 of the polar member 11, spatial modulation is introduced into the core 15 of the optical waveguide 20. This modulation reduces the cancellation of the coupling constant κ, described below, due to the positive and negative antinodes of the amplitude of the higher-order transverse mode generated in the core 15. The spatial modulation here refers to modulation due to the presence or absence of a second-order nonlinear optical constant, and contributes to the efficiency of wavelength conversion.

[0018] The core 15, which is formed by the polar member 11 and the first non-polar member 12, is surrounded by the cladding 16. The cladding 16 preferably covers both the side and top surfaces of the core 15. The cladding 16 surrounding the core 15 may be an air layer, or a layer of a material having a lower refractive index than the core 15 may be provided. Examples of a material having a lower refractive index than the core 15 include silicon oxide (SiO ２ ), silicon oxynitride (SiON), aluminum oxide (Al ２ O３ ), aluminum oxynitride (AlON), etc. The refractive index of the cladding is one of the parameters that determine modal dispersion, and is set appropriately.

[0019] The height h of the core 15 in the XY cross section corresponds to the film thickness of the polar member 11 and the first non-polar member 12. The height h of the core is thick enough to increase the light confinement, and is, for example, 300 nm to 1 μm. The width W of the core 15 is the sum of the width W1 of the polar member 11 and the width W2 of the first non-polar member 12. In the embodiment, the width W of the core 15 relative to the width W1 of the polar member 11 is determined by the TM of the fundamental wave. ００ Mode and second harmonic TM ２０ The mode is set to satisfy a phase matching condition. The phase matching condition will be described later. From the viewpoint of increasing the portion of the polar member 11 with good crystal quality, the height h of the polar member 11 may be equal to or greater than the width W1. The aspect ratio (h / W) of the height h to the width W of the core 15 may be, for example, 1 to 5, preferably 1 to 3. The aspect ratio (h / W1) of the height h to the width W1 of the polar member may be, for example, 2 to 10, preferably 2 to 6. The aspect ratio (h / W2) of the height h to the width W2 of the first non-polar member 12 may be, for example, 2 to 10, preferably 2 to 6. The width W1 of the polar member 11 may be 0.5 to 2 times the width W2 of the first non-polar member 12.

[0020] When the light incident on the optical waveguide 20 is weak, the magnitude of the polarization of the polar member 11 is proportional to the magnitude of the optical electric field. On the other hand, when light of sufficient intensity is incident, second-order and / or third-order nonlinear polarization appears in the polarization of the polar member 11. The effect resulting from such nonlinear polarization is the nonlinear optical effect. SHG is a representative second-order nonlinear optical effect. When two photons with the same angular frequency interact with a nonlinear optical medium, a photon with twice the energy of the original photon is generated. In other words, a photon with twice the angular frequency is generated.

[0021] 3 is a schematic diagram of a guided mode propagating through the optical waveguide 20. In FIG. 3, the intensity distribution of the pump light and the signal light in the XY cross section is shown as an integral in the Y direction, projected onto the XZ plane.１ TM ００ The pump light of the ω mode is incident on the wavelength conversion element 10 in the Z direction. The incidence of the pump light induces a nonlinear polarization in the polar members 11 of the core 15, and the ω １ and a different angular frequency ω ２ A signal light of ω is generated. ２ For example, ω １ In this example, the angular frequency ω ２ The guided mode of the signal light is TM ２０ It is a mode.

[0022] The polar component 11 of the core 15 has a second-order nonlinear optical constant d. The second-order nonlinear optical constant is a third-order tensor. When the energy of light input to and output from the polar component 11 is sufficiently smaller than the band gap energy of the polar component 11, the second-order nonlinear optical constant d is expressed as a 3×6 matrix element using a contracted expression. ｉｊ The second-order nonlinear optical constant d ｉｊ is the second-order nonlinear susceptibility χ （２） The second-order nonlinear optical constant has a different component that takes a finite value depending on the symmetry of the crystal that constitutes the polar member 11. The polar member 11 has, for example, a second-order nonlinear optical constant d 33 When the c-axis is in a direction perpendicular to the main surface 501 of the substrate 5, d 33 can be used as the maximum component. The angular frequency ω generated by the interaction of the incident pump light with the polar member 11 ２ The signal light of ２０ The light propagates through the optical waveguide 20 in the mode and is output from the wavelength conversion element 10. In this embodiment, the second-order nonlinear optical constant is d 33 Considering the direction of the polarity and the direction of the optical electric field incident on the polar member 11, for example, d 31 Other ingredients may also be used.

[0023] In the wavelength conversion element 10, in order to efficiently generate second harmonic waves by utilizing the second-order nonlinear optical effect, it is necessary to satisfy the phase matching condition between the pump light and the signal light. Generally, since the refractive index depends on the angular frequency, when the angular frequency is ω １ and ω ２It is difficult to achieve phase matching in these two states. To solve this problem, phase matching using birefringence or quasi-phase matching using periodic polarization reversal is used. In this embodiment, phase matching using a transverse mode is used. Light with the wavelength of the pump light (i.e., angular frequency ω １ The refractive index of the material of the core 15 sensed by the light mat (ω １ ), light of the wavelength of the signal light (i.e., angular frequency ω ２ The refractive index of the material of the core 15 sensed by the light mat (ω ２ ), when the band gap energy of the wavelength conversion element 10 is sufficiently larger than the energy of the pump light and the signal light, the refractive index of the core material is larger for light with a short wavelength than for light with a long wavelength, and n mat (ω １ ) <n mat (ω ２ On the other hand, due to the transverse mode dependency (modal dispersion) of the effective refractive index of the core 15, the effective refractive index n ００ is the effective refractive index n in the higher-order transverse mode ｍ０ is greater than n ００ >n ｍ０ Here, m is the mode order. By utilizing the transverse mode dependency of the effective refractive index, the effective refractive index for each mode felt by the pump light and the signal light is made equal, and ００ (ω １ ) = n ｍ０ (ω ２ ) can be satisfied. That is, the pump light and the signal light in the core 15 can satisfy the phase matching condition.

[0024] angular frequency ω １ Modes of and ω ２ The coupling constant κ between the modes is expressed by the following equation:

[0025] The coupling constant κ is the second-order nonlinear susceptibility χ （２） and the electric field E ω１ and the electric field E ω２ The second-order nonlinear susceptibility χ is proportional to the value obtained by multiplying and integrating it over the core cross section. （２）is zero in the region of the first non-polar member 12.

[0026] Therefore, in the example of FIG. 3, for example, the first non-polar member 12 is TM ２０ By designing the electric field distribution of the mode to include one positive and one negative antinode, only the remaining antinode contributes to the coupling constant κ, which can be made larger than when phase matching is achieved with a core consisting only of polar members.

[0027] The coupling constant κ of the core 15 is the TM ００ Mode and TM ２０ For example, the interface between the polar member 11 and the first non-polar member 12 is ２０ By designing the optical waveguide 20 so that it is positioned at a node of the amplitude of the signal light of the mode, the polar member 11 ００ Mode and TM ２０ This reduces the cancellation of positive and negative antinodes in the overlapping modes. This allows the coupling constant κ to be larger than in a configuration in which the core 15 is formed only with polar members 11, in which cancellation of positive and negative antinodes of higher-order modes occurs. Note that in FIG. 3, the transverse mode in the Y direction is preferably the fundamental mode. This increases the coupling constant κ, enabling efficient wavelength conversion.

[0028] (First Modification) Fig. 4 is a schematic diagram of a wavelength conversion element 10A as a first modification of the wavelength conversion element 10. The wavelength conversion element 10A has an optical waveguide 20A on the main surface 501 of the substrate 5. The optical waveguide 20A includes a core 15A and a clad 16. The core 15A has a polar member 11 and a first non-polar member 12 arranged in contact with side surfaces 111 and 112 on both sides of the polar member 11. In an XY cross section perpendicular to the optical axis OX (see Fig. 1), the polar member 11 is sandwiched between the first non-polar members 12. Similar to the wavelength conversion elements 10 of Figs. 1 and 2, the wavelength conversion element 10A has a simple configuration. The polar member 11 and the first non-polar member 12 can be formed thick in the Y direction, which can improve the crystal quality. In addition, the highly symmetrical shape can be used for TM. ００ Mode and TM ２０ Polar materials can be placed in regions where the electric field strength for each mode is high.

[0029] TM ２０ The electric field distribution of the mode has three antinodes. For example, by setting the width of the core 15A so that the polar member 11 includes at least the central antinode and the two first nonpolar members 12 include the vertices of their outer antinodes, the coupling constant κ in Equation 1 can be increased. Therefore, efficient wavelength conversion is possible. The width W1 of the polar member 11 may be, for example, 0.5 to 3 times, preferably 1 to 2 times, the sum (W-W1) of the widths W2 of the two first nonpolar members 12. Furthermore, the aspect ratio (h / W) of the height h and width W of the core 15 may be, for example, 0.5 to 10, preferably 0.5 to 6.

[0030] <Fabrication Process of Core 15A> Figures 5A to 5D are diagrams illustrating the fabrication process of the core 15A of the wavelength conversion element 10A. In Figure 5A, a polar material layer 110 is formed on the main surface 501 of the substrate 5. The substrate 5 can be selected from an appropriate substrate, such as a sapphire substrate, a GaN substrate, a quartz substrate, an optical glass substrate, a resin substrate, or a silicon substrate with a silicon oxide film. The substrate 5 is preferably a sapphire substrate or a GaN substrate. This allows the polar material layer 110 to be formed with relatively high quality. As the polar material, a material with a wurtzite crystal structure, such as AlN, GaN, AlGaN, or ZnO, is used. For wavelengths around 800 nm, the refractive index of GaN is 2.35, and the refractive indexes of AlN and ZnO are 2.18 and 2.10, respectively. The polar material layer 110 with good crystal quality is formed by physical vapor deposition (PVD) methods such as sputtering and evaporation, or chemical vapor deposition (CVD). When depositing AlGaN as the polar material layer 110, the Al and Ga compositions may be adjusted to achieve a desired refractive index. The thickness of the polar material layer 110 is, for example, 300 nm to 1000 nm. Since the phase matching condition can be satisfied by controlling the transverse mode in the width direction (X direction) of the core, the polar material layer 110 can be made relatively thick. This improves the crystal quality of the polar material layer 110.

[0031] In FIG. 5B, the layer 110 is processed into a predetermined shape to obtain the polar member 11. The polar member 11 is obtained by processing the layer 110 through a mask patterned into a predetermined shape. As shown in FIG. 1, the predetermined shape is a shape extending in the Z direction with a width W1 and a height h in the XY cross section. To ensure the crystal quality of the polar member 11, the height h may be greater than or equal to the width W1. The aspect ratio (h / W1) of the height h to the width W1 may be 1 or greater and 10 or less, preferably 1.5 or greater and 6 or less. The shape of the XY cross section does not necessarily have to be rectangular; it may be a trapezoid, an inverted trapezoid, or the like. Anisotropic etching of the layer 110 results in side surfaces 111 and 112 that form a predetermined angle with the main surface 501 of the substrate 5. The anisotropic etching may be, for example, a dry process or reactive ion etching. As described above, the polar member 11 is a material suitable for microfabrication and can be processed to a predetermined width.

[0032] In Fig. 5C, a non-polar material layer 120 is formed on the entire surface, covering the polar member 11. The non-polar material layer 120 is an amorphous layer that does not have polarization. As the non-polar material, a material having a refractive index relatively close to that of the polar member 11 is used. The difference between the refractive index of the polar member 11 and the refractive index of the non-polar material layer 120 may be, for example, 0.001 or more and 0.2 or less, or 0.001 or more and 0.1 or less. As an example, the non-polar material is Nb ２ O ５ , Ta ２ O ５ , TiO ２ , HfO ２ , ZrO ２ , Si ３ N ４ The composition does not necessarily have to match the stoichiometry, and the oxygen or nitrogen may be in a state where it is greater or less than the stoichiometry. Furthermore, the non-polar material layer 120 may be a layer of a composite material containing two or more of the above materials. Alternatively, the refractive index may be adjusted by doping with an impurity. This makes it possible to reduce the difference between the refractive index of the non-polar material layer 120 and the refractive index of the polar member 11, so that the angular frequency ω １ of light and angular frequency ω ２For example, when the polar member 11 is made of GaN, the number of combinations of angular frequencies that realize phase matching with light of Nb ２ O ５ or TiO ２ When the polar member 11 is made of AlN, the layer 120 of the non-polar material may be made of Ta. ２ O ５ , HfO ２ , Si ３ N ４ The amorphous non-polar material layer 120 may be formed by a PVD method, a CVD method, an atomic layer deposition (ALD) method, or the like. By using the CVD method or the ALD method, the non-polar material layer 120 with good coverage can be formed on the side surfaces 111 and 112 of the polar member 11.

[0033] In FIG. 5D, the non-polar material layer 120 is processed by anisotropic etching to form a first non-polar member 12 that contacts both side surfaces 111 and 112 of the polar member 11. This results in the core 15A of the wavelength conversion element 10A. By etching the polar member 11 to a sufficient depth, the core 15A with good crystal quality can be obtained in the portion away from the substrate 5. By concentrating light in the portion with good crystal quality, light loss can be reduced and the conversion efficiency can be improved. The steps of FIGS. 5A to 5D can be realized by a wafer process suitable for mass production, and a core 15A with a simple configuration can be manufactured in a simple process. After this, a cladding 16 may be further formed. The cladding 16 may be air.

[0034] (Second Modification) FIG. 6A is a schematic diagram of a wavelength conversion element 10B of a second modification of the wavelength conversion element 10. The optical waveguide 20B of the wavelength conversion element 10B has a core 15B composed of a polar material 11 and a first non-polar material 12B. The first non-polar material 12B contacts both side surfaces 111 and 112 of the polar material 11 and the top surface. In the process of FIG. 5D, the end point of the anisotropic etching is controlled to obtain the core 15B of the shape of FIG. 6A. By ending the anisotropic etching of the non-polar material layer 120 before the top surface of the polar material 11 is exposed, a thin film 121 of the non-polar material remains on the main surface 501 of the substrate 5. The core 15B of the shape of FIG. 6A is advantageous in that it can maintain the film thickness of the polar material 11 without over-removing it. Furthermore, by leaving the thin film 121 of the non-polar material, it is possible to reduce the removal of the first non-polar material 12B that contacts the side surface of the polar material 11. This makes it possible to easily control the effective refractive index.

[0035] (Third Modification) Figure 6B is a schematic diagram of a wavelength conversion element 10C of a third modification of the wavelength conversion element 10. The optical waveguide 20C of the wavelength conversion element 10C has a core 15C composed of a polar material 11C and a first non-polar material 12 on the main surface 501 of the substrate 5. The first non-polar material 12B contacts the side surfaces 111 and 112 on both sides of the polar material 11 and the top surface. In the process of Figure 5B, by controlling the end point of the anisotropic etching of the polar material layer 110, the core 15C of the shape of Figure 6B can be obtained. By ending the anisotropic etching of the polar material layer 110 before the main surface 501 of the substrate 5 is exposed, a thin film 115 of polar material remains on the main surface 501 of the substrate 5, but light can be effectively trapped inside the core 15C. The core 15C of the shape of Figure 6B is also advantageous in that it can ensure a film thickness without excessively removing the polar material 11.

[0036] (Fourth Modification) Figure 6C is a schematic diagram showing yet another modification of the wavelength conversion element 10. The configuration of Figure 6A and the configuration of Figure 6B may be combined to form a wavelength conversion element 10C'. The optical waveguide 20C' has a core 15C' composed of a polar member 11C and a first non-polar member 12B on the main surface 501 of the substrate 5. The first non-polar member 12B is provided to cover the thin film 115 of polar material and the upper surface of the polar member 11C, and a clad 16 is formed to cover the core 15C'. Even with the configuration of Figure 6C, the film thickness can be ensured without excessively removing the polar member 11C.

[0037] <Evaluation of Core Width> Figure 7 is a model diagram for determining the core width that satisfies the phase matching condition. In this example, the core shape of Figure 4 is used as the model. The substrate 5 is made of sapphire, the polar member 11 of the core 15 is made of AlN, and the first non-polar member 12 sandwiching both sides of the polar member 11 is made of Ta. ２ O ５ The cladding 16 covering the core 15 is made of Al. ２ O ３ This allows us to define the width and height of the core, the refractive index of the core, the refractive index of the cladding, and the refractive index of the substrate. From this information, the electromagnetic field distribution and effective refractive index can be obtained using the finite element method. The height h of the core 15 is fixed at 400 nm, the width W of the polar member 11 is fixed at 300 nm, and the width W of the core 15 is changed by changing the width of the first non-polar member 12. Using this model, we can calculate the effective refractive index of light with a wavelength of 810 nm (angular frequency ω １ ) and light with a wavelength of 405 nm (angular frequency ω ２ (=2×ω １ )) the effective refractive index n eff As described above, in the optical waveguide 20, the effective refractive index n eff In order to achieve phase matching by utilizing the transverse mode dependence (modal dispersion) of the fundamental wave TM with a wavelength of 810 nm, ００ mode and the second harmonic TM with a wavelength of 405 nm ２０ Effective refractive index n in the mode eff must be the same or close enough.

[0038] 8A and 8B show the electromagnetic field distributions of two transverse modes in the optical waveguide 20. The TM mode at a wavelength of 810 nm shown in FIG. ００The mode is the guided mode of the pump light, which is the fundamental wave. As shown in the schematic electric field intensity distribution in FIG. 3, one peak exists at the center of the core 15. The TM of the wavelength 405 nm shown in FIG. 8B ２０ The mode is the second harmonic TM ２０ Three electric field loops are located on one polar member 11 and two first non-polar members 12, respectively.

[0039] FIG. 9 shows the TM when the width W of the core 15 is changed while the width W1 of the polar member 11 is fixed at 300 nm. ００ Mode and TM ２０ Effective refractive index n in the mode eff The white circles indicate the angular frequency ω １ The black circles represent the calculated dispersion curves for the transverse mode (TM mode) of ２ Calculation results of the dispersion curve in the transverse mode (TM mode) of ω 1 are shown. １ TM ００ Effective refractive index of the mode and angular frequency ω ２ TM ２０ Therefore, when the width W1 of the polar member 11 is 300 nm, the width W of the core 15 is set to 550 nm, so that the effective refractive index of the fundamental wave TM ００ mode (810 nm) and second harmonic TM ２０ The wavelength conversion efficiency is improved by using the 0th-order and 2nd-order modes in the X direction and the 0th-order modes in the Y direction.

[0040] In FIG. 9, the second harmonic TM ２０ The dispersion curves indicated by black circles other than the dispersion curve for (405 nm) are the TM of the second harmonic. ｍ０ (m is an integer of 0, 1, or 3 or more), and when the width W of the core 15 is in the vicinity of 550 nm, the existence of multiple higher-order modes is permitted. ００The dispersion curves indicated by white circles other than the dispersion curve for 810 nm are higher-order modes (for example, first-order modes) at 810 nm, and the existence of higher-order modes is permitted when the width W of the core 15 is in the vicinity of 550 nm. However, as can be seen from FIG. 9, in the vicinity of this width, the higher-order modes at 810 nm are ２０ When the width W of the core 15 is wider than 800 nm, a higher order mode of 810 nm appears. However, in this width region, the fundamental wave TM ００ Mode and second harmonic TM ２０ The modes are not phase matched.

[0041] From this, in a cross section perpendicular to the optical axis of the optical waveguide 20, the width of the core 15 is a width such that the number of possible modes for a component of a first angular frequency (first frequency component) is three or less, and the number of possible modes for a harmonic component of a second angular frequency (second frequency component) higher than the first angular frequency is two or more. The width W of the core 15 may be 1.5 to 4 times, preferably 1.75 to 3.5 times, and more preferably 2 to 3 times the width W1 of the polar member 11. This improves the degree of freedom in the width of the optical waveguide that allows phase matching.

[0042] The width W1 of the polar member 11 can be processed in the range of 100 nm to 1000 nm, and the fundamental wave TM ００ Mode and second harmonic TM ２０ The width of the core 15 can be appropriately selected within the range of 200 nm to 2000 nm so that the mode satisfies the transverse mode phase matching condition. When the width W1 of the polar member 11 is in the range of 100 nm to 500 nm, the width of the core 15 may be in the range of 200 nm to 1000 nm.

[0043] 9 shows that the core width that satisfies the phase matching condition can be found by determining the angular frequency of the pump light, the angular frequency of the signal light, the refractive index of the core, and the refractive index of the cladding. In this example, the phase matching condition for generating SHG from 810 nm to 405 nm has been described, but the embodiment is not limited to this. The wavelength conversion may be an optical parametric process from 405 nm to 810 nm. Alternatively, conversion to an arbitrary angular frequency may be performed by sum frequency generation or difference frequency generation. Furthermore, since it is only necessary to find a pair of transverse modes that satisfy the phase matching condition between the pump light and the signal light, TM ００ Mode and TM ２０ The phase matching is not limited to the TM mode, but may be a higher order mode. ｐ０ Mode and TM ｑ０ mode (p and q are integers of 1 or more).

[0044] Second Embodiment FIG. 10 is a cross-sectional view of a wavelength conversion element 10D according to a second embodiment. The wavelength conversion element 10D includes an optical waveguide 20D disposed on the main surface 501 of the substrate 5. The core 15D of the optical waveguide 20D includes a polar component 11 having a second-order nonlinear optical constant, a first nonpolar component 12 disposed adjacent to the polar component 11 in a direction parallel to the main surface 501 of the XY cross section (the X direction), and a second nonpolar component 13 disposed outside the first nonpolar component 12 and in contact with the side surface of the first nonpolar component 12. The refractive index of the second nonpolar component 13 is greater than that of the first nonpolar component 12. The "outside" of the first nonpolar component 12 refers to the outside of the polar component 11 when viewed in the X direction of the XY cross section. In this case, the first nonpolar component 12 is located between the polar component 11 and the second nonpolar component 13.

[0045] The refractive index of the polar member 11 is n １ , the refractive index of the first nonpolar member 12 is n ２ , the refractive index of the second nonpolar member 13 is n ３ Then, n １ >n ３ >n ２ For example, when GaN is used as the polar member 11 and SiO is used as the first non-polar member 12, ２ The second non-polar member 13 is made of Ta. ２ O５ When considering the wavelength and the guided mode separately, the refractive index n of the material that the light of the pump light wavelength and the light of the signal light wavelength experience is mat is n mat (ω １ ) <n mat (ω ２ ) and the refractive index experienced by the short wavelength signal light is high. ００ Mode and TM ２０ The magnitude relationship of the effective refractive index in the mode is n ００ >n ２０ and TM ００ The refractive index felt by the mode is TM ２０ Here, by utilizing the transverse mode dependency of the effective refractive index, n ００ (ω １ ) = n ｍ０ (ω ２ ) to satisfy the phase matching condition.

[0046] By adopting the configuration of the core 15D, it is possible to concentrate the optical density in the polar member 11 and to achieve phase matching by utilizing the dispersion of the transverse mode. ２０ The signal light, which is the second harmonic of the mode, is confined in the first non-polar member 12 and the polar member 11 sandwiched between the first non-polar members 12, so that leakage out of the core 15D is reduced. ２０ Since one of the antinodes of the mode can be concentrated on the polar member 11, the coupling constant κ can be increased according to Equation 1, and wavelength conversion by nonlinear polarization can be performed efficiently. ００ The pump light of the mode can see the refractive index of the second nonpolar member 13, and the refractive index experienced when propagating through the core 15 becomes higher.

[0047] In the configuration of the second embodiment, by selecting the refractive index of the material, the degree of freedom in designing phase matching that utilizes dispersion of the transverse mode is increased.

[0048] Third Embodiment FIG. 11 is an XY cross-sectional view of a wavelength conversion element 10E according to a third embodiment. The wavelength conversion element 10E includes an optical waveguide 20E disposed on the main surface 501 of the substrate 5. The core 15E of the optical waveguide 20E further includes a third non-polar member 14 disposed on the upper surface of the polar member 11 and the upper surface of the first non-polar member 12. The refractive index of the third non-polar member 14 is equal to or greater than that of the polar member 11. In this configuration, the third non-polar member 14, having a refractive index equal to or greater than that of the polar member 11, allows light to be guided closer to the third non-polar member 14 than to the substrate 5. This reduces loss by concentrating light on the upper side of the core 15, where the crystal quality is better, and improves conversion efficiency. This is because the crystal quality of the polar member 11 on the upper side of the core 15 is better than that at the interface between the substrate 5 and the core 15. For example, AlN may be used for the polar member 11 and Ta for the first non-polar member 12. ２ O ５ and the third non-polar member 14 is Nb ２ O ５ When the polar material 11 and the first non-polar material 12 are used, the refractive index of the polar material 11 and the first non-polar material 12 becomes approximately the same, and the refractive index of the third non-polar material 14 becomes equal to or greater than the refractive index of the polar material 11 and the first non-polar material 12 .

[0049] 12 is a schematic diagram of light propagating through the wavelength conversion element 10E. The light L including the pump light and the signal light is concentrated in the upper part of the core 15E, which has a large difference in refractive index from the cladding 16. In this state, the TM of the pump light ００ Effective refractive index in the mode and TM of the signal light ２０ The effective refractive indices in the modes become equivalent, and the phase matching condition is satisfied. In the configuration of the third embodiment, by adjusting the refractive index distribution in the core 15, it is possible to reduce loss and increase conversion efficiency.

[0050] <Fourth Embodiment> Figure 13 is a cross-sectional view of a wavelength conversion element 10F of the fourth embodiment. The wavelength conversion element 10F has an optical waveguide 20F arranged on the main surface 501 of the substrate 5. The core 15F of the optical waveguide 20F has a polar member 11F and a first non-polar member 12 that contacts both side surfaces of the polar member 11F. The polar member 11F has a first polar member 116 and a second polar member 117 and / or a third polar member 118 in the normal direction of the main surface 501 of the substrate 5. The first polar member 116 is arranged between the substrate 5 and the second polar member 117 or the third polar member 118. The refractive indexes of the second polar member 117 and the third polar member 118 are each greater than the refractive index of the first polar member 116. Only one of the second polar member 117 and the third polar member 118 may be used.

[0051] When the polar member 11F is made of nitride, Al ｘ Ga １－ｘ The refractive indexes of the first polar member 116, the second polar member 117, and the third polar member 118 may be adjusted by adjusting the composition x of N (0≦x≦1). For example, the first polar member 116 may be formed of AlGaN, and the second polar member 117 or the third polar member 118 may be formed of GaN. InGaN may also be used for the polar member 11F. The configuration of the fourth embodiment also concentrates light on the upper side of the core 15F, which has a good crystalline state, thereby reducing loss and improving conversion efficiency. Furthermore, when both the second polar member 117 and the third polar member 118 are present, the refractive index of the third polar member 118 may be greater than the refractive indexes of the first polar member 116 and the second polar member 117. This allows light to be guided closer to the third polar member 118 than to the substrate 5. The crystalline quality of the third polarity member 118 is higher than that of the first polarity member 116, so that light can be concentrated closer to the third polarity member 118 than to the first polarity member 116, reducing light loss.

[0052] FIG. 14 is a cross-sectional view of a wavelength conversion element 10G, a modified example of the fourth embodiment. The wavelength conversion element 10G has an optical waveguide 20G disposed on the main surface 501 of the substrate 5. The core 15G of the optical waveguide 20G has a polar member 11G and a first non-polar member 12 contacting both side surfaces of the polar member 11G. The polar member 11G has a superlattice, which is a periodic superposition of multiple types of crystal lattices. The piezoelectric field generated at the heterointerface of the superlattice increases the coupling constant κ of the polar member 11G, thereby improving the conversion efficiency. For example, a GaN / AlGaN superlattice is formed by stacking an AlGaN thin film 114 and a GaN thin film 113, each having a predetermined thickness, on the main surface 501 of the substrate 5 at a predetermined repetition rate.

[0053] Fifth Embodiment In the first to fourth embodiments or their modified examples, the height of the polar members and the height of the non-polar members do not necessarily have to be the same. For example, the height of the polar members may be higher or lower than the height of the non-polar members. １ and angular frequency ω ２ The height of the polar member and the height of the non-polar member can be set appropriately depending on the value of , the combination of the materials of the polar member and the non-polar member, the width of the polar member and the non-polar member, etc. An example is shown below.

[0054] FIG. 15 is a cross-sectional view of a wavelength conversion element 10H according to a fifth embodiment. The wavelength conversion element 10H includes an optical waveguide 20H disposed on the main surface 501 of the substrate 5. The core 15H of the optical waveguide 20H includes a polar member 11 and a first non-polar member 12 that contacts both side surfaces of the polar member 11. The core 15H formed by the polar member 11 and the first non-polar member 12 is surrounded by a cladding 16. In the fifth embodiment, the height of the polar member 11 is lower than the height of the first non-polar member 12. For example, even if the corner 12x of the first non-polar member 12 is deformed during the etching process, the height of the polar member 11 is lower than the height of the first non-polar member 12, so that the light propagating through the core 15H is less affected by the deformed corner 12x. Therefore, the deviation in the phase matching condition due to variations in the difference in height between the polar member 11 and the first non-polar member 12 is smaller than the design value, enabling desired wavelength conversion.

[0055] 16 is a cross-sectional view of a wavelength conversion element 10I which is a modified example of the fifth embodiment. Except for the points described below, the wavelength conversion element 10I is the same as the wavelength conversion element 10H shown in FIG. 15. That is, the substrate 5 has a convex portion 5x, and a polar member 11 is disposed on a main surface 502 of the convex portion 5x. The first non-polar member 12 is disposed on a main surface 501 of the substrate 5 on which the convex portion 5x is not provided, and the first non-polar member 12 contacts the polar member 11 on both side surfaces of the polar member 11.

[0056] The first non-polar member 12 constituting a part of the core 15I and a part of the cladding 16 are disposed on the main surface 501 of the substrate 5, and the polar member 11 constituting a part of the core 15I is disposed on the main surface 502 of the convex portion 5x of the substrate 5. As a result, in the X direction parallel to the main surface 501 of the substrate 5, the effective refractive index of the portion including the convex portion 5x below the polar member 11 is lower than the effective refractive index in the X direction of the portion including the polar member 11 due to the inclusion of the convex portion 5x. Therefore, light propagating through the optical waveguide 20I is efficiently confined in the portion including the polar member 11, which has a relatively high effective refractive index, thereby reducing loss due to the influence of the substrate 5. Furthermore, because the height of the polar member 11 is lower than the height of the first non-polar member 12, light propagating through the core 15I is less affected by the deformed corners 12x, as with the wavelength conversion element 10H shown in FIG. 15. Therefore, the deviation of the phase matching condition due to the variation in the difference in height between the polar member 11 and the first non-polar member 12 is small relative to the design value, and the desired wavelength conversion can be achieved.

[0057] <Application example of wavelength conversion element> Fig. 17 is a schematic diagram of a light source device 100 using the wavelength conversion element 10 of the embodiment. The light source device 100 includes a light source 101 such as a laser diode, and a wavelength conversion element 10 optically coupled to the light source 101. The wavelength conversion element 10 may have any of the configurations of the wavelength conversion elements 10A to 10I of the above-mentioned examples and modified examples. The light source 101 emits light having an angular frequency ω ０ The laser light incident on the wavelength conversion element 10 interacts with the core 15, and the wavelength conversion element 10 emits light having an angular frequency of 2×ω. ０is generated and output from the wavelength conversion element 10. For example, when the light source 101 that emits light in the blue wavelength band is used, ultraviolet light can be emitted from the light source device 100 due to SHG in the wavelength conversion element 10. The light source device 100 is used as a highly efficient ultraviolet light source.

[0058] 18 is a schematic diagram of an optical circuit 200 using the wavelength conversion element 10 of the embodiment. The optical circuit 200 includes a light source 101, a mode converter 103 optically coupled to the light source 101, and a wavelength conversion element 10 optically coupled to the mode converter 103. ω emitted from the light source 101 １ The fundamental mode light is converted into a higher mode light by the mode converter 103. The mode converter 103 may be a Y-shaped waveguide formed by an optical waveguide, a directional coupler, a multimode interferometer, or the like. The mode converter 103 may be connected continuously to the wavelength conversion element 10, or a part of the wavelength conversion element 10 may be used as the mode converter 103.

[0059] Mode-converted ω １ The light is incident on the wavelength conversion element 10 as pump light. An optical parametric process occurs due to the interaction between the pump light and the optical waveguide 20 of the wavelength conversion element 10, and ω ２ photons are generated. The signal light generated in the optical parametric process is input to a circuit element 105 that constitutes a sensor circuit or the like. Alternatively, signal light of a different wavelength may be input to the wavelength conversion element 10 together with the pump light output from the mode converter 103 to generate difference frequency generation or sum frequency generation. The optical circuit 200 realizes a highly sensitive sensor that utilizes the optical parametric process. The wavelength conversion element 10 is a waveguide wavelength converter that utilizes phase matching in a transverse mode, and can be integrated with the mode converter 103 and the light source 101. This realizes an optical circuit 200 with a simple configuration and high conversion efficiency.

[0060] Although the above description has been based on specific configuration examples, the present disclosure is not limited to the above configuration examples. Each embodiment or modified example can be combined with another. For example, the configuration of the polar member 11F or 11G of the fourth embodiment shown in FIGS. 13 and 14 may be applied to the polar member 11 of FIGS. 1, 10, and 11. The configuration of the third embodiment shown in FIG. 11 may be combined with the configuration of the second embodiment shown in FIG. 10. The wavelength conversion element of the embodiment can be used as an optical amplifier for an optical sensor, and can also be applied to a light source device that outputs ultraviolet light or mid-infrared light that is difficult to generate by current injection.

[0061] Embodiments of the present disclosure may include, for example, the following configurations: (Item 1) A wavelength conversion element comprising: a substrate having a main surface; and an optical waveguide disposed on the main surface of the substrate, wherein a core of the optical waveguide includes a polar member having a second-order nonlinear optical constant and a first nonpolar member disposed adjacent to the polar member in a direction parallel to the main surface of the substrate, and one or both side surfaces of the polar member and the first nonpolar member are in contact with each other in a cross section perpendicular to the optical axis of the optical waveguide. (Item 2) The wavelength conversion element according to Item 1, wherein the first nonpolar member is in contact with both side surfaces of the polar member in the cross section perpendicular to the optical axis of the optical waveguide. (Item 3) The wavelength conversion element according to Item 2, wherein the core further includes a second nonpolar member provided in contact with a side surface of the first nonpolar member on the outer side of the first nonpolar member as viewed from the polar member in the cross section orthogonal to the optical axis, wherein the refractive index of the second nonpolar member is greater than that of the first nonpolar member. (Item 4) The wavelength conversion element according to any one of Items 1 to 3, wherein the core further includes a third nonpolar member disposed on an upper surface of the polar member and an upper surface of the first nonpolar member, wherein the refractive index of the third nonpolar member is equal to or greater than that of the polar member. (Item 5) The wavelength conversion element according to any one of Items 1 to 4, wherein the polar member includes a first polar member and a second polar member in a normal direction of the main surface of the substrate, wherein the first polar member is disposed between the substrate and the second polar member, and wherein the refractive index of the second polar member is greater than that of the first polar member. (Item 6) The wavelength conversion element according to any one of Items 1 to 5, wherein, in the cross section orthogonal to the optical axis of the optical waveguide, the core width is a width such that the number of modes that can exist for a first frequency component of a first angular frequency is 3 or less, and a width such that the number of modes that can exist for a second frequency component of a second angular frequency higher than the first angular frequency is 2 or more. (Item 7) The wavelength conversion element according to any one of Items 1 to 6, wherein, in the cross section orthogonal to the optical axis, the core width is 200 nm or more and 2000 nm or less. (Item 8) The wavelength conversion element according to any one of Items 1 to 7, wherein, in the cross section orthogonal to the optical axis, the core width is 1.5 times or more and 4 times or less the width of the polar member.(Item 9) The wavelength conversion element according to any one of Items 1 to 8, wherein the polar member is formed of a wurtzite crystal having polarization in a c-axis direction. (Item 10) The wavelength conversion element according to Item 9, wherein the c-axis of the polar member is perpendicular to the main surface of the substrate. (Item 11) The polar member is Al. ｘ Ga １－ｘ 12. The wavelength conversion element according to claim 1, wherein the first non-polar member is an amorphous layer. 13. The wavelength conversion element according to claim 1, wherein the first non-polar member is an amorphous layer. ２ O ５ , Ta ２ O ５ , TiO ２ , HfO ２ , ZrO ２ , Si ３ N ４ The wavelength conversion element according to any one of Items 1 to 12, which is one selected from the group consisting of: (Item 14) An optical circuit comprising: a laser element; a mode converter optically coupled to the laser element; and the wavelength conversion element according to any one of Items 1 to 13, which is optically coupled to the mode converter. (Item 15) A light source device comprising: a laser element; and the wavelength conversion element according to any one of Items 1 to 13, which is optically coupled to the laser element.

[0062] This application claims priority based on Japanese Patent Application No. 2023-105432 filed on June 27, 2023, and includes the entire contents of this Japanese patent application.

[0063] 5 Substrate 501 Main surface 10, 10A to 10I Wavelength conversion element 11, 11F, 11G Polar member 111, 112 Side surface 116 First polar member 117 Second polar member 118 Third polar member 12 First non-polar member 13 Second non-polar member 14 Third non-polar member 15, 15A to 15I Core 16 Cladding 20, 20A to 20I Optical waveguide 100 Light source device 101 Light source 103 Mode converter 105 Circuit element 200 Optical circuit

Claims

1. A wavelength conversion element comprising: a substrate having a principal surface; and an optical waveguide disposed on the principal surface of the substrate, wherein a core of the optical waveguide includes a polar member having a second-order nonlinear optical constant, and a first nonpolar member disposed adjacent to the polar member in a direction parallel to the principal surface of the substrate, and one or both sides of the polar member are in contact with the first nonpolar member in a cross section perpendicular to the optical axis of the optical waveguide.

2. The wavelength conversion element according to claim 1, wherein the first non-polar member contacts both side surfaces of the polar member in the cross section perpendicular to the optical axis of the optical waveguide.

3. The wavelength conversion element described in claim 2, wherein the core further has a second non-polar member arranged outside the first non-polar member as viewed from the polar member in the cross section perpendicular to the optical axis, and in contact with a side surface of the first non-polar member, and the refractive index of the second non-polar member is greater than the refractive index of the first non-polar member.

4. The wavelength conversion element described in claim 1, wherein the core further has a third non-polar member arranged on an upper surface of the polar member and an upper surface of the first non-polar member, and the refractive index of the third non-polar member is greater than or equal to the refractive index of the polar member.

5. A wavelength conversion element according to any one of claims 1 to 4, wherein the polar member has a first polar member and a second polar member in a normal direction of the main surface of the substrate, the first polar member is disposed between the substrate and the second polar member, and the refractive index of the second polar member is greater than the refractive index of the first polar member.

6. A wavelength conversion element according to any one of claims 1 to 4, wherein in the cross section perpendicular to the optical axis of the optical waveguide, the core width is such that the number of modes that can exist for a first frequency component of a first angular frequency is three or less, and is such that the number of modes that can exist for a second frequency component of a second angular frequency higher than the first angular frequency is two or more.

7. A wavelength conversion element according to claim 1, wherein the width of the core is 200 nm or more and 2000 nm or less in the cross section perpendicular to the optical axis.

8. A wavelength conversion element according to any one of claims 1 to 4, wherein the width of said core is 1.5 to 4 times the width of said polar member in said cross section perpendicular to said optical axis.

9. The wavelength conversion element according to any one of claims 1 to 4, wherein the polar member is formed of a wurtzite crystal having polarization in the c-axis direction.

10. The wavelength conversion element according to claim 9, wherein the c-axis of the polar member is perpendicular to the main surface of the substrate.

11. The polar member is Al ｘ G １－ｘ The wavelength conversion element according to claim 9 , wherein the wavelength conversion element is formed of N (where 0≦x≦1).

12. The wavelength conversion element according to any one of claims 1 to 4, wherein the first non-polar member is an amorphous layer.

13. The first non-polar member is Nb ２ O ５ , Ta ２ O ５ , TiO ２ , HfO ２ , ZrO ２ , Si ３ N ４ The wavelength conversion element according to claim 1 , wherein the wavelength conversion element is one selected from the group consisting of:

14. An optical circuit comprising: a laser element; a mode converter optically coupled to the laser element; and a wavelength conversion element according to any one of claims 1 to 4 optically coupled to the mode converter.

15. A light source device comprising: a laser element; and a wavelength conversion element according to any one of claims 1 to 4, optically coupled to the laser element.