JP6308010B2 - Periodic polarization inversion electrode, method of forming periodic polarization inversion structure, and periodic polarization inversion element - Google Patents

Description

  The present invention relates to a periodic polarization reversal electrode using a ferroelectric crystal substrate, a method of forming a periodic polarization reversal structure, and a periodic polarization reversal element.

  For applications such as wavelength conversion and spatial light modulation to obtain laser light of the desired wavelength, a periodically poled device with a periodically poled structure in which the polarization direction is periodically reversed is used inside the ferroelectric crystal substrate. It has been. For example, the periodic polarization reversal element can output laser light having a wavelength that is a second harmonic by quasi-phase matching with incident laser light. For this reason, the periodic polarization reversal element is used, for example, as a quasi phase matching (QPM) type wavelength conversion element.

  In order to form the periodic polarization reversal structure, a voltage application method for applying a voltage to the periodic polarization reversal electrodes arranged at regular intervals on the surface of the ferroelectric crystal substrate is used. For example, a predetermined voltage is applied between a striped electrode periodically arranged on the + Z plane perpendicular to the polarization direction of the ferroelectric crystal substrate and a planar electrode uniformly arranged on the −Z plane of the ferroelectric crystal substrate. Apply. As a result, polarization inversion occurs immediately below the stripe electrode, and a periodic polarization inversion structure is formed inside the ferroelectric crystal substrate (see, for example, Patent Document 1).

JP 2000-147484 A

  However, when the ferroelectric crystal substrate is increased in size and the area of the planar electrode is increased, variation occurs in the in-plane distribution of the electric field generated in the ferroelectric crystal substrate when a voltage is applied to the planar electrode. As a result, there has been a problem that the periodically poled structure becomes non-uniform in the plane.

  In view of the above problems, the present invention provides a periodic polarization reversal electrode capable of forming a polarization reversal structure with a uniform period inside a ferroelectric crystal substrate, a method for forming a periodic polarization reversal structure, and a periodic polarization reversal element. With the goal.

According to one aspect of the present invention, there is provided a periodic polarization reversal electrode for forming a periodic polarization reversal structure in which a polarization direction is periodically reversed inside a ferroelectric crystal substrate, comprising: (a) a ferroelectric crystal A surface electrode having a plurality of stripe-like inversion electrodes arranged on the + Z plane perpendicular to the polarization direction of the substrate, electrically connected to each other, and periodically extending in parallel with each other; A back electrode that is disposed on the -Z plane of the opposing ferroelectric crystal substrate and is divided into a plurality of regions by a divided region that extends obliquely with respect to the extending direction in which the inversion electrode extends as viewed from the + Z surface; The distance along the extending direction between the regions divided by the divided regions is strong in a region where the back electrode is not arranged when a voltage for forming a periodically poled structure is applied between the front electrode and the back electrode. Inside of dielectric crystal substrate Is set to be shorter than the distance coercive field higher than the electric occurs periodically independently voltage is applied to each of the plurality of regions divided in the back electrode with the surface electrodes, periodically poled structure is formed polarized An inversion electrode is provided.

According to another aspect of the present invention, there is provided a method of forming a periodic polarization reversal structure in which a polarization direction is periodically reversed inside a ferroelectric crystal substrate, wherein (a) the electrodes are electrically connected to each other and have a periodicity. A step of disposing a surface electrode having a plurality of stripe-shaped inversion electrodes extending parallel to each other on the + Z plane perpendicular to the polarization direction of the ferroelectric crystal substrate, and (b) inversion as viewed from the + Z plane. Disposing a back electrode divided into a plurality of regions by a divided region extending obliquely with respect to the extending direction of the electrode for extending on the −Z plane of the ferroelectric crystal substrate facing the + Z plane; C) A voltage is independently applied to each of the plurality of divided regions of the back electrode between the surface electrode and a polarization inversion structure is formed inside the ferroelectric crystal substrate below the plurality of inversion electrodes. and a step seen including, minute by the divided region Distance along the extending direction between the regions is set shorter than the internal distance of the coercive field or electric field occurs in the ferroelectric crystal substrate in a region where the back electrode not arranged when a voltage is applied A method for forming a periodically poled structure is provided.

According to yet another aspect of the present invention, a ferroelectric crystal substrate is provided, and the ferroelectric crystal substrate is periodically parallel to each other as viewed from the + Z plane perpendicular to the polarization direction of the ferroelectric crystal substrate. A plurality of striped domain-inverted regions are formed to extend, each of the plurality of domain-inverted regions is formed continuously in the extending direction in the vicinity of the + Z plane, and the −Z plane of the ferroelectric crystal substrate facing the + Z plane , A period in which at least some of the plurality of domain-inverted regions are divided by regions that are not domain-inverted in a region facing the region formed continuously in the stretching direction in the vicinity of the + Z plane . A polarization reversal element is provided.

  According to the present invention, it is possible to provide a periodic polarization reversal electrode, a method of forming a periodic polarization reversal structure, and a periodic polarization reversal element that can form a polarization reversal structure having a uniform period inside a ferroelectric crystal substrate.

It is a typical perspective view which shows the structure of the electrode for periodic polarization inversion which concerns on embodiment of this invention, Fig.1 (a) shows the whole and FIG.1 (b) shows a back surface electrode. It is a schematic diagram which shows the structure of the electrode for periodic polarization inversion concerning embodiment of this invention, Fig.2 (a) is a top view, FIG.2 (b) is the cross section along the II-II direction of Fig.2 (a). FIG. It is a schematic diagram which shows the area | region where a polarization inversion structure is formed, FIG. 3 (a) is a top view, FIG.3 (b) is sectional drawing along the III-III direction of Fig.3 (a). It is a photograph which shows the state after polarization reversal using the electrode for periodic polarization reversal which concerns on embodiment of this invention. It is a typical top view which shows the structure of the electrode for periodic polarization inversion which concerns on other embodiment of this invention.

  Embodiments of the present invention will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationship between the thickness and the planar dimensions, the ratio of the thickness of each layer, and the like are different from the actual ones. Therefore, specific thicknesses and dimensions should be determined in consideration of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

  Further, the embodiments described below exemplify apparatuses and methods for embodying the technical idea of the present invention, and the embodiments of the present invention include the material, shape, structure, arrangement, etc. of components. Is not specified as follows. The embodiment of the present invention can be variously modified within the scope of the claims.

  The periodic polarization reversal electrode 10 according to the embodiment of the present invention is a periodic polarization reversal electrode for forming a periodic polarization reversal structure in which the polarization direction is periodically reversed inside the ferroelectric crystal substrate 20. As shown in FIG. 1A, the periodic polarization reversal electrode 10 includes a surface electrode 11 disposed on a + Z plane 21 perpendicular to the polarization direction of the ferroelectric crystal substrate 20, and a strong electrode facing the + Z plane 21. And the back electrode 12 disposed on the −Z surface 22 of the dielectric crystal substrate 20.

  The surface electrode 11 has a plurality of stripe-shaped inversion electrodes 111 that are electrically connected to each other and periodically extend in parallel with each other. The extending direction of the reversal electrode 111 is hereinafter referred to as “stretching direction”. The surface electrode 11 shown in FIG. 1A has a ladder shape, and power feeding portions 112 are disposed at both ends of the inversion electrode 111. The plurality of inversion electrodes 111 are electrically connected by the power feeding unit 112.

  The region immediately below the inversion electrode 111 of the ferroelectric crystal substrate 20 undergoes polarization inversion. For this reason, the width w of the inversion electrode 111 and the interval d between the adjacent inversion electrodes 111 are set according to the region of the ferroelectric crystal substrate 20 where the polarization is inverted. For example, when a periodically poled element is used as a quasi-phase matching (QPM) type wavelength conversion element, a ferroelectric material is used depending on the wavelength of the laser light incident on the wavelength conversion element and the wavelength of the output laser light. The width w and interval d of the region of the crystal substrate 20 where the polarization is inverted are set as appropriate.

  The back electrode 12 is divided into a plurality of regions 121 to 122 as shown in FIG. 1B by divided regions 120 extending obliquely with respect to the extending direction of the inversion electrode 111 when viewed from the + Z surface 21. . As shown in FIG. 2A, the width of the divided region 120 perpendicular to the extending direction of the divided region 120 is “width A”. In addition, an angle formed between the extending direction of the inversion electrode 111 and the extending direction of the divided region 120 is defined as “angle θ”.

  Here, the distance along the extending direction of the inversion electrode 111 between the region 121 and the region 122 divided by the divided region 120 is defined as “distance Dy”. The distance Dy is expressed as Dy = A / sin θ.

  When a voltage is applied between the front electrode 11 and the back electrode 12 in order to form a periodically poled structure inside the ferroelectric crystal substrate 20, the larger the area of the back electrode 12, the more in the ferroelectric crystal substrate. The in-plane distribution of the electric field varies greatly. As a result, the periodically poled structure is not uniform in the plane, and a periodically poled element having desired characteristics cannot be obtained.

  However, in the periodic polarization inversion electrode 10 shown in FIGS. 1A and 1B, the back electrode 12 is divided into a plurality of regions 121 to 122. A voltage is applied independently to each of the divided regions 121 to 122 of the back electrode 12 between the front electrode 11 and the front electrode 11. That is, the area to which the voltage is applied is small. For this reason, variation in the in-plane distribution of the electric field is suppressed. Therefore, even when the ferroelectric crystal substrate 20 is large, the periodically poled structure can be made uniform in the plane.

  In order to obtain the above effect, the width A of the divided region 120 is such that when the applied voltage V is applied between any one of the region 121 and the region 122 and the surface electrode 11, the region 121 and the region 122 are electrically connected. It is set to a distance that can be insulated. That is, the width A of the divided region 120 depends on the magnitude of the applied voltage V. According to the study by the present inventors, for example, when the applied voltage V is several kV, the width A of the divided region 120 may be 80 μm or more.

  On the other hand, the distance Dy is set to be equal to or less than a certain distance so that the region where the polarization is reversed continues in the extending direction. That is, a phenomenon is used in which a domain-inverted structure is formed in a region where the back electrode 12 is not disposed beyond the region sandwiched between the front electrode 11 and the back electrode 12 when the applied voltage V is applied. This phenomenon is caused by an electric field higher than the coercive electric field that reverses the direction of polarization, even in a region where the back electrode 12 is not disposed, as in the region E shown in gray in FIGS. 3 (a) and 3 (b). It happens to occur. Hereinafter, the distance along the extending direction of the domain-inverted structure formed outside the end portion of the inversion electrode 111 in the extending direction is referred to as “distance Ly”. At this time, by setting the distance Dy to satisfy Dy ≦ 2 × Ly, the domain-inverted structure continues in the stretching direction. That is, the distance Dy between the region 121 and the region 122 is set to be shorter than the distance at which an electric field equal to or higher than the coercive electric field is generated in the extending direction in the ferroelectric crystal substrate 20 in the region where the back electrode 12 is not disposed.

  As shown in FIG. 3A, the distance Ly is long in the ferroelectric crystal substrate 20 particularly in the Y-axis direction. Therefore, as shown in FIG. 2A, the extending direction of the inversion electrode 111 is preferably the Y-axis direction. As a result, the domain-inverted structure can be more reliably continued in the stretching direction. According to the investigation by the present inventors, the distance Ly in the Y-axis direction is several tens of μm to several hundreds of μm, depending on the material of the ferroelectric crystal substrate 20 and the applied voltage V.

  However, in the vicinity of the −Z plane 21, the extending distance of the domain-inverted structure is small in a region not sandwiched between the front electrode 11 and the back electrode 12. For this reason, as shown in FIG. 3B, the domain-inverted structure is formed only directly above the back electrode 12. That is, a region B that is not polarization-inverted remains in the vicinity of the −Z plane 21, and the domain-inverted region is divided by this region B. However, in the domain-inverted region that is not arranged above the divided region 120, the domain-inverted region continues even in the vicinity of the -Z plane 21.

  From the relationship of Dy = A / sin θ, the angle θ is preferably as large as possible in order to reduce the distance Dy. However, if θ is 90 degrees, the areas of the regions 121 to 122 become large, and the effect of suppressing variation in the in-plane distribution of the electric field cannot be obtained. On the other hand, when the angle θ is 0 degree, since the width A of the divided region 120 is generally longer than the period of the domain-inverted structure, the region where the domain is not inverted in the region where the divided region 120 is formed becomes wider. . That is, the periodic polarization inversion structure is not continuously formed.

  For this reason, the angle θ is set to, for example, 10 degrees to 80 degrees. More preferably, the angle θ is 30 degrees or more, for example, about 30 degrees to 70 degrees. Further, according to the study by the present inventors, in order to electrically insulate the divided regions of the back electrode 12 and make the domain-inverted structure continuous, the width A of the divided region 120 is 80 μm to 600 μm. It is preferable that it is a grade.

  By the way, the ferroelectric crystal substrate 20 is made of, for example, a lithium tantalate (LT) single crystal, a lithium niobate (LN) single crystal, or the like. The thickness of the ferroelectric crystal substrate 20 is, for example, about 0.4 to 1 mm. The magnitude of the applied voltage V is set according to the material and thickness of the ferroelectric crystal substrate 20. Therefore, the width A and the angle θ of the divided region 120 depend on the material and thickness of the ferroelectric crystal substrate 20.

The lithium tantalate (LiTaO ₃ ) single crystal and the lithium niobate (LiNbO ₃ ) single crystal used for the ferroelectric crystal substrate 20 have a congruent composition (coincidence melting composition) or stoichiometric composition (stoichiometric composition). Things are used. For example, in the case of lithium tantalate, the coercive electric field is reduced to about 1/10 by using the stoichiometric composition. That is, the applied voltage V can be reduced to 1/10.

  Further, magnesium (Mg), zinc (Zn), scandium (Sc), indium (In), or the like may be added to the ferroelectric crystal substrate 20 made of lithium tantalate single crystal or lithium niobate single crystal. . Thereby, light damage resistance can be improved. In the case of lithium niobate, the coercive electric field can be reduced to about a quarter by adding about 5 mol% of Mg. Thereby, the applied voltage V can be reduced to about a quarter.

  For example, a tantalum (Ta) film or an aluminum (Al) film can be used for the periodic polarization reversal electrode 10. Besides, gold (Au) film, silver (Ag) film, chromium (Cr) film, copper (Cu) film, nickel (Ni) film, nickel chromium alloy (Ni-Cr) film, palladium (Pd) film, A molybdenum (Mo) film, a tungsten (W) film, or the like can also be used.

  The surface electrode 11 is formed, for example, by patterning a Ta film formed on the + Z surface 21 of the ferroelectric crystal substrate 20 by using a photolithography technique and an etching technique.

  For the back electrode 12, for example, a metal film such as a Ta film or an Al film can be employed. For example, after a metal film is uniformly formed on the −Z surface 22 of the ferroelectric crystal substrate 20, the divided regions 120 are formed using a photolithography technique and an etching technique. Further, the divided region 120 may be formed by a lift-off method. Alternatively, the divided region 120 may be formed by a dicer. In this case, a groove cut by the dicer is easily formed on the −Z surface 22 of the ferroelectric crystal substrate 20.

  Hereinafter, formation of a domain-inverted structure using the periodic domain-inverted electrode 10 will be described. Here, a lithium niobate substrate doped with MgO was used as the ferroelectric crystal substrate 20. The extending direction of the inversion electrode 111 was set to the Y-axis direction of the ferroelectric crystal substrate 20. That is, the inversion electrodes 111 are periodically arranged in the X-axis direction. Further, the width A of the divided region 120 was 80 μm, and the angle formed by the Y-axis direction and the extending direction of the divided region 120 was set to 45 degrees.

  First, the ladder-shaped surface electrode 11 having the inversion electrode 111 and the power feeding portion 112 as shown in FIG. 1A is disposed on the + Z plane 21 of the ferroelectric crystal substrate 20. Further, the back electrode 12 divided into a plurality of regions by the divided region 120 is disposed on the −Z surface 22 of the ferroelectric crystal substrate 20.

  Thereafter, an applied voltage V is independently applied to each of the plurality of divided regions of the back electrode 12 between the surface electrode 11 and a polarization inversion structure is formed inside the ferroelectric crystal substrate 20. For example, the applied voltage V is sequentially applied to a plurality of divided regions of the back electrode 12.

  That is, first, an applied voltage V is applied between the power feeding portion 112 of the front surface electrode 11 disposed on the + Z surface 21 and the region 121 of the back surface electrode 12 disposed on the −Z surface 22. Thereby, a domain-inverted structure is formed between the inversion electrode 111 and the region 121. At this time, a domain-inverted structure (hereinafter referred to as “first stretched region”) having a distance Dy is formed below the outer end of the inversion electrode 111.

  Next, an applied voltage V is applied between the power feeding portion 112 of the front electrode 11 and the region 122 of the back electrode 12. As a result, a domain-inverted structure is formed between the inversion electrode 111 and the region 122. At this time, a domain-inverted structure (hereinafter referred to as “second stretched region”) having a distance Dy is formed below the outer end of the inversion electrode 111.

  As a result of forming the domain-inverted structure using the region 121 and the region 122, a first stretched region formed when a voltage is applied to the region 121 and a second stretched region formed when a voltage is applied to the region 122 Are connected in the divided region 120. As a result, each of the domain-inverted regions is continuously formed in the extending direction at least in the vicinity of the + Z plane 21 inside the ferroelectric crystal substrate 20.

  On the other hand, the domain-inverted region formed above the divided region 120 includes a region B that is not domain-inverted in the vicinity of the −Z plane 22 as shown in FIG. The imaginary line that connects the non-polarized regions of the adjacent domain-inverted regions extends along the divided region 120. Since the angle formed between the extending direction of the inversion electrode 111 and the extending direction of the divided region 120 is set to 45 degrees, the angle formed between the extending direction of the imaginary line and the extending direction of the domain-inverted region is 45 degrees. is there.

  After the domain-inverted region is formed, the front electrode 11 and the back electrode 12 are peeled from the ferroelectric crystal substrate 20 to complete the periodic domain-inverted element. The periodic polarization reversal element may be formed so that only the region where the periodic polarization reversal structure is formed remains by cutting and removing unnecessary portions of the ferroelectric crystal substrate 20.

  As described above, in the method for forming a periodic polarization reversal structure according to the embodiment of the present invention, a region where the polarization is not reversed remains in the vicinity of the −Z plane 22. For this reason, the periodic polarization inversion element formed by the above method is preferably used as a waveguide type wavelength conversion element or a waveguide type spatial light modulator that uses only the vicinity of the + Z plane 21 of the ferroelectric crystal substrate 20.

  FIG. 4 shows an example of a periodically poled device manufactured by the above-described method of forming a periodically poled structure. The domain-inverted structure is formed with a constant period in the X-axis direction. The black portion in the vicinity of the center in FIG. 4 is due to the formation of the groove on the −Z surface 22 of the ferroelectric crystal substrate 20 by forming the divided region 120 with a dicer. It can be seen that the domain-inverted structure is also continuously formed above the divided region 120.

  As described above, in the method for forming a periodically poled structure using the periodically poled electrode 10 according to the embodiment of the present invention, the back surface electrode 12 disposed on the −Z plane 22 is divided to have a small area. The domain-inverted structure is sequentially formed in each region. For this reason, variation in the in-plane distribution of the electric field is suppressed, and a domain-inverted structure can be formed in the large ferroelectric crystal substrate 20 with a uniform period. For example, the periodic polarization reversal electrode 10 can be suitably used to form a polarization reversal structure used for a multi-channel spatial light modulator, a large wavelength conversion element that requires a large number of periodic patterns, and the like.

  Further, the distance Dy between the regions obtained by dividing the back electrode 12 is set to be shorter than the distance at which an electric field higher than the coercive electric field is generated in the extending direction of the inversion electrode 111 in the region not sandwiched between the front electrode 11 and the back electrode 12. Has been. For this reason, a domain-inverted structure is continuously formed from one end of the inversion electrode 111 to the other end.

(Other embodiments)
As mentioned above, although this invention was described by embodiment, it should not be understood that the description and drawing which form a part of this indication limit this invention. From this disclosure, various alternative embodiments, examples and operational techniques will be apparent to those skilled in the art.

  In the above description, the case where the back electrode 12 is divided into two has been exemplarily described. However, the number of divisions of the back electrode 12 can be arbitrarily set according to the size of the periodically poled structure to be formed, and is divided into three or more. Of course, it may be divided. For example, as shown in FIG. 5, the back electrode 12 may be divided into four regions 121 to 124 by the divided region 120.

  As described above, the present invention naturally includes various embodiments not described herein. Therefore, the technical scope of the present invention is defined only by the invention specifying matters according to the scope of claims reasonable from the above description.

DESCRIPTION OF SYMBOLS 10 ... Periodic polarization reversal electrode 11 ... Front surface electrode 12 ... Back surface electrode 20 ... Ferroelectric crystal substrate 21 ... Back surface electrode 21 ... + Z surface 22 ... -Z surface 111 ... Reversal electrode 112 ... Feed part 120 ... Divided area

Claims (11)

A periodic polarization reversal electrode for forming a periodic polarization reversal structure in which a polarization direction is periodically reversed inside a ferroelectric crystal substrate,
A surface electrode having a plurality of stripe-like inversion electrodes arranged on a + Z plane perpendicular to the polarization direction of the ferroelectric crystal substrate, electrically connected to each other and periodically extending in parallel with each other;
A plurality of regions are formed by divided regions arranged on the −Z surface of the ferroelectric crystal substrate facing the + Z surface and extending obliquely with respect to the extending direction in which the inversion electrode extends as viewed from the + Z surface. A divided back electrode, and
The distance between the regions divided by the divided regions along the extending direction is such that the back electrode is disposed when a voltage for forming the periodically poled structure is applied between the front electrode and the back electrode. Is set shorter than the distance at which an electric field equal to or higher than the coercive electric field is generated inside the ferroelectric crystal substrate in the non-region,
Said surface electrode independently the voltage to each divided plurality of regions of the back surface electrode is applied between the periodically poled electrode, characterized in that the periodically poled structure is formed.   The periodic polarization reversal electrode according to claim 1, wherein the reversal electrode extends in the Y-axis direction of the ferroelectric crystal substrate.   3. The periodic polarization inversion electrode according to claim 1, wherein an angle formed between the extending direction and the extending direction of the divided region is 30 degrees or more. A method of forming a periodically poled structure in which a polarization direction is periodically reversed inside a ferroelectric crystal substrate,
Disposing on the + Z plane perpendicular to the polarization direction of the ferroelectric crystal substrate, a surface electrode having a plurality of stripe-like inversion electrodes electrically connected to each other and periodically extending parallel to each other; ,
A back electrode divided into a plurality of regions by a divided region extending obliquely with respect to the extending direction in which the inversion electrode extends as viewed from the + Z plane is a negative electrode of the ferroelectric crystal substrate facing the + Z plane. Placing on the Z plane;
A voltage inversion structure is formed inside the ferroelectric crystal substrate below the plurality of inversion electrodes by applying a voltage independently to each of the plurality of divided regions of the back electrode between the front surface electrodes. look including a step of forming,
The distance along the stretching direction between the regions divided by the divided regions is such that an electric field equal to or greater than the coercive electric field is generated inside the ferroelectric crystal substrate in the region where the back electrode is not disposed when the voltage is applied. A method for forming a periodic domain-inverted structure, characterized in that the period is set to be shorter than a generated distance . 5. The method for forming a periodically poled structure according to claim 4 , wherein the inversion electrode extends in the Y-axis direction of the ferroelectric crystal substrate. 6. The method for forming a periodically poled structure according to claim 4 , wherein an angle formed between the extending direction and the extending direction of the divided regions is 30 degrees or more. The method for forming a periodically poled structure according to any one of claims 4 to 6 , wherein the voltage is sequentially applied to the divided regions of the back electrode. A ferroelectric crystal substrate;
A plurality of striped domain-inverted regions extending in parallel with each other when viewed from the + Z plane perpendicular to the polarization direction of the ferroelectric crystal substrate are formed inside the ferroelectric crystal substrate,
In the vicinity of the + Z plane, each of the plurality of domain-inverted regions is formed continuously in the stretching direction ,
In the vicinity of the −Z plane of the ferroelectric crystal substrate facing the + Z plane, at least some of the plurality of domain-inverted regions are continuously formed in the stretching direction in the vicinity of the + Z plane. A periodic polarization reversal element characterized in that a region opposite to the region is divided by a region that is not polarization-reversed. The periodic polarization reversal element according to claim 8 , wherein the domain-inverted region extends in a Y-axis direction of the ferroelectric crystal substrate. When viewed from the + Z plane, the angle formed by the extending direction of the imaginary line connecting the positions divided by the non-polarized regions of each of the adjacent domain-inverted regions and the extending direction of the domain-inverted regions is 30. The periodic polarization reversal element according to claim 8 or 9 , wherein the periodic polarization reversal element is at least. The width of the non-polarized region of the ferroelectric crystal substrate is gradually narrowed from the −Z plane toward the + Z plane, and the domain-inverted region is continuous in the stretching direction in the vicinity of the + Z plane. The periodic polarization reversal element according to claim 8, wherein the periodic polarization reversal element is provided.