JP2007121515A - Optical element substrate and wavelength conversion device using substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element substrate capable of forming a polarization inversion structure of high accuracy even when a ferroelectric crystal wherein current easily flows in a region where the polarization inversion structure is formed is used and to provide a wavelength conversion device using the substrate. <P>SOLUTION: In the optical element substrate 1 comprising a ferroelectric material for manufacturing the wavelength conversion device having a structure (the polarization inversion structure) wherein a spontaneous polarization direction of the ferroelectric crystal having a secondary nonlinear effect is periodically inverted, a charge movement limiting layer 3 for limiting a movement amount of a charge moved along the spontaneous polarization direction when the polarization inversion structure is formed, is provided on one surface orthogonal to spontaneous polarization of a ferroelectric layer 2 of the optical element substrate 1 wherein the polarization inversion structure is formed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical element substrate used in a photo printer, a display, a terahertz laser light source, and the like, and a wavelength conversion device using the substrate.

Ferroelectric crystals such as LiNbO ₃ and LiTiO ₃ have a strong nonlinear optical effect and are used for laser light wavelength conversion elements. In particular, an optical element that periodically inverts the polarization direction and satisfies the phase matching condition (QPM) in a pseudo manner can use a large nonlinear constant, and therefore has high wavelength conversion efficiency. By changing the period of the polarization inversion structure, Various studies have been made since second harmonic generation (SHG) and optical parametric oscillation (OPO) can be performed on laser light in a wide wavelength range.

  For example, as disclosed in Patent Document 1, such a wavelength conversion element can be manufactured by using both surfaces (+ Z plane and −Z plane, or + C plane and −C plane) of a dielectric crystal substrate whose polarization direction is mainly aligned. This is performed by forming a predetermined electrode pattern on the surface and reversing the polarization direction of the portion sandwiched between the electrodes by applying a voltage. However, in order to fabricate an element with high wavelength conversion efficiency, the inversion region must be formed with high accuracy with respect to the design value. In particular, since an incident beam diameter larger than that of the waveguide type is assumed in the bulk type, it is necessary to uniformly reverse the polarization of a thick substrate over a wide range. Various methods have been studied for these problems.

  For example, in Patent Document 2, an electrode on the + Z plane side for applying an electric field is divided into a plurality of segments having a period wider than the target period, and an electric field is individually applied to the domain-inverted structure with a fine period. Is forming. In Patent Document 3, the discharge between the electrodes is prevented by maintaining a vacuum on the + Z plane side on which electrodes having a predetermined pattern are formed, and an electric field is applied by corona charging with the atmospheric pressure on the −Z plane side. Thus, a domain-inverted structure with a fine period is formed. In Patent Document 4, by degrading the crystallinity of the + Z plane surface, polarization reversal of unnecessary portions is suppressed, and a short-period polarization reversal structure is easily formed.

Ferroelectric crystals having a non-linear effect such as LiNbO ₃ and LiTaO ₃ are used for forming the domain-inverted structure by these methods. Among them LiNbO _3, LiTaO ₃ has a high nonlinear effect, but consideration has high stability domain inversion structure is advanced, particularly LiNbO _3, crystal MgO-doped to LiTaO ₃ as an impurity (hereinafter, MgO: LN, MgO : LT) is expected to be a high-power wavelength conversion device for visible light generation without the problem of fundamental absorption due to the interaction between visible light and fundamental wave generated by wavelength conversion (Green Induced IR Absorption: GRIIRA). Yes.
Japanese Patent No. 3277515 JP 2003-307758 A JP-A-7-72521 JP 2000-147484 A

  However, in particular, in MgO: LN, the resistivity of the part where the polarization is reversed decreases and the current flows easily, so that the part where the domain-inverted structure is generated first is heated to cause unevenness of the domain-inverted structure, and other parts. There is a problem that the formation of the domain-inverted structure stops in the middle of the application of the electric field because a predetermined electric field is not applied.

  In view of the above-described problems, the present invention can form a highly accurate domain-inverted structure even when a ferroelectric crystal is used so that a current easily flows in the region where the domain-inverted structure is formed. An object of the present invention is to provide an optical element substrate and a wavelength conversion device using the substrate.

  The invention according to claim 1 is a ferroelectric for producing a wavelength conversion device having a structure (polarization inversion structure) in which the spontaneous polarization direction of a ferroelectric crystal having a second-order nonlinear effect is periodically reversed. An optical element substrate made of a body material, wherein one of the surfaces perpendicular to the spontaneous polarization of the ferroelectric layer of the optical element substrate on which the polarization reversal structure is formed has a spontaneous polarization direction when the polarization reversal structure is formed. The optical element substrate includes a charge transfer limiting layer that limits the amount of movement of charges moving along.

  According to a second aspect of the present invention, the charge transfer limiting layer is formed by adding or removing at least one impurity of Mg, Zn, Se, and In to the optical element substrate. Item 1 is an optical element substrate according to Item 1.

  According to a third aspect of the present invention, the charge transfer limiting layer is made of the ferroelectric material having a different impurity concentration compared to the ferroelectric material forming the optical element substrate. An optical element substrate is provided.

  The invention according to claim 4 is the optical element substrate according to claim 3, wherein the ferroelectric layer and the charge transfer limiting layer are bonded by thermal diffusion bonding.

According to a fifth aspect of the present invention, the inversion coercive field of the ferroelectric layer is E1, the thickness of the ferroelectric layer is d1, the inversion coercive field of the charge transfer limiting layer is E2, and the thickness of the charge transfer limiting layer is If d2 is,
E1 · (d1 + d2) <= E2 · d2... An optical element substrate according to any one of claims 1 to 4 satisfying the formula (1) is provided.

  The invention according to claim 6 is a wavelength conversion device using the optical element substrate according to any one of claims 1 to 5.

  According to the present invention, in the optical element substrate and the wavelength conversion device using the substrate, even when a ferroelectric crystal that makes it easy for current to flow is used in the region where the domain-inverted structure is formed, A highly accurate domain-inverted structure can be formed.

  The best mode for carrying out the present invention is to produce a wavelength conversion device having a structure (polarization inversion structure) in which the spontaneous polarization direction of a ferroelectric crystal having a second-order nonlinear effect is periodically inverted. In the optical element substrate made of a ferroelectric material, along the spontaneous polarization direction at the time of forming the domain-inverted structure with respect to one of the surfaces orthogonal to the spontaneous polarization of the ferroelectric layer of the optical element substrate on which the domain-inverted structure is formed A charge transfer limiting layer is provided for limiting the amount of charge transfer.

  Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. The embodiments described below are preferable specific examples of the present invention, and thus various technically preferable limitations are given. However, the scope of the present invention is particularly limited in the following description. Unless otherwise described, the present invention is not limited to these embodiments.

(Embodiment 1)
The optical element substrate of the present embodiment is characterized in that a charge transfer limiting layer having a structure for limiting charge transfer is provided so that no current flows after the polarization inversion structure is formed at a predetermined position. This charge transfer limiting layer reduces the resistivity of the domain-inverted region, such as MgO: LN, and even in a ferroelectric crystal in which unevenness of the domain-inverted region is likely to occur due to heating by a current, a highly accurate domain-inverted structure is provided. It can be formed. The optical element substrate of this embodiment will be specifically described below.

  FIG. 1 is a cross-sectional view showing the structure of the optical element substrate 1 of the present embodiment. The ferroelectric layer 2 in FIG. 1 is a portion where a domain-inverted structure is formed, and MgO: LN, MgO: LT or the like is used as described in the background art section. The ferroelectric layer 2 shown in FIG. 1 has a uniform polarization direction from the −Z plane side to the + Z plane side, and the charge transfer limiting layer 3 is formed on the −Z plane side.

  As a material used for the charge transfer limiting layer 3, a material having a resistance value lower than that of the ferroelectric layer 2 before polarization inversion and higher than that of the ferroelectric layer 2 after polarization inversion, or a predetermined charge moves. A material (described in Embodiment Mode 2) that stops the movement of the charge after flowing the charge up to can be used.

  For example, the charge transfer limiting layer 3 can be made of the same ferroelectric material as the ferroelectric material forming the ferroelectric layer 2, which are materials having different impurity concentrations. The charge transfer limiting layer 3 can be formed by adding or removing at least one impurity of Mg, Zn, Se, and In to the optical element substrate 1.

  Next, for comparison with the optical element substrate 1 of the present embodiment, the operation (polarization inversion structure forming process) in the case of polarization inversion of the conventional optical element substrate using FIGS. explain. 4A to 4D are cross-sectional views for explaining the operation (the process of forming the polarization inversion structure) in the case of polarization inversion of the conventional optical element substrate 5.

In the optical element substrate 6 shown in FIGS. 4A to 4D, a comb-like electrode (+ electrode 4) is formed on the + Z plane side, and a solid electrode (−electrode 5) is formed on the −Z plane side. An electric field opposite to the polarization direction of the element substrate 6 is applied. The optical element substrate 6 in this description is formed using LiNbO ₃ (MgO: LN) doped with MgO.

  First, when an electric field is applied to the optical element substrate 6 through the + Z plane side electrode (+ electrode 4) and the −Z plane side electrode (−electrode 5), as shown in FIG. A domain-inverted region 7 is generated from the edge portion of the side electrode (+ electrode 4). As the electric field application time advances, the domain-inverted region 7 grows toward the -Z plane as shown in FIGS. 4B and 4C, but the growth rate differs depending on the location.

  During this time, almost no current flows, and a part of the domain-inverted region 7 reaches the -Z plane to form a domain-inverted structure as shown in FIG. This is because the domain-inverted region 7 gradually expands in the horizontal direction, but its speed is very slow compared with the growth in the vertical direction, so that the other domain-inverted regions 7 mainly grow in the vertical direction. .

When the domain-inverted region 7 as shown in FIG. 4C is formed, a current proportional to the area of the domain-inverted region 7 flows along with the domain-inverted region. However, in LiNbO ₃ doped with MgO, current continues to flow through the interface between the domain-inverted region 7 and the non-inverted region. ) Decreases and the domain-inverted region 7 is likely to grow in the lateral direction, so that the penetrated domain-inverted region 7 greatly spreads in the lateral direction as shown in FIG. Is a problem because disappears.

  Next, the operation (polarization inversion structure forming process) when the optical element substrate 1 of this embodiment is inverted in polarization will be described with reference to FIGS. FIGS. 2A to 2D are cross-sectional views for explaining an operation (a process of forming a domain-inverted structure) when the optical element substrate 1 of the present embodiment is domain-inverted. In the optical element substrate 1 shown in FIGS. 2A to 2D, a comb-like electrode (+ electrode 4) is formed on the + Z plane side, and a solid electrode (−electrode 5) is formed on the −Z plane side. An electric field opposite to the polarization direction of the element substrate 1 is applied.

  First, in FIGS. 2A to 2C, the domain-inverted region 7 grows in the same manner as in the conventional case described above with reference to FIGS. However, as shown in FIGS. 2C and 2D, after the domain-inverted region 7 reaches the electric field transfer limiting layer 3, the growth of the domain-inverted region 7 stops, and the current passing through the interface of the domain-inverted region is reduced. Since it is suppressed, heating around the domain-inverted region 7 does not occur. Accordingly, before the domain-inverted region 7 that has reached the charge transfer limiting layer 3 spreads in the lateral direction, the other domain-inverted regions 7 can reach the −Z plane, and a uniform domain-inverted structure compared to the prior art can be obtained. It becomes possible to form.

As described above, according to this embodiment, the charge that moves along the direction of spontaneous polarization when the domain-inverted structure is formed with respect to one of the surfaces orthogonal to the spontaneous polarization of the optical element substrate on which the domain-inverted structure is formed. Since the charge transfer limiting layer for limiting the amount of movement is formed, it is possible to suppress the occurrence of unevenness in the domain-inverted region and improve the uniformity of the domain-inverted structure. <Claim 1>
In addition, by using the optical element substrate of the present embodiment, it is possible to supply a wavelength conversion device that can perform wavelength conversion with high efficiency. <Claim 6>

(Embodiment 2)
The present embodiment is an application example of the first embodiment. Therefore, description of parts (configuration and effects) common to the first embodiment will be omitted, and only characteristic parts will be described below based on the attached drawings. FIG. 3 is a cross-sectional view showing the structure of the optical element substrate 1 in the present embodiment. As shown in FIG. 3, in this embodiment, a LiNbO ₃ substrate doped with 5 mol% of MgO is used as the ferroelectric layer 2 of the optical element substrate 1, and a non-doped LiNbO ₃ substrate (hereinafter referred to as non-doped) is used as the charge transfer limiting layer 3. LN) is used.

The MgO: LN substrate (LiNbO ₃ substrate doped with 5 mol% MgO) decreases the resistance value of the domain-inverted region 7, but the non-doped LiNbO ₃ substrate is more than the amount of charge that moves according to the domain-inverted area. Since no current flows, it can be used as the charge transfer limiting layer 3. The inversion coercive field of 5 mol% MgO: LN is about 5 KV / mm at room temperature, while the inversion coercive field of non-doped LN is about 21 KV / mm.

In the region where the MgO: LN portion is inverted in polarization, the resistance value of MgO: LN decreases, and the electric field corresponding to the applied voltage is temporarily concentrated on the non-doped LN portion. Therefore, in order to prevent the non-doped LN portion from being destroyed by excessive electrolysis and reducing the resistance value, the MgO: LN thickness is d1, the inversion coercive electric field is E1, and the non-doped LN thickness is d2. The coercive electric field is set to E2, and the following formula (1) may be satisfied.
E1 · (d1 + d2) <= E2 · d2 (1)

  By the way, the optical element substrate 1 of the present embodiment is preferably manufactured using thermal diffusion bonding because the ferroelectric layer 2 and the charge transfer limiting layer 3 have no difference other than the presence or absence of MgO as a dopant. This is because this thermal diffusion bonding does not require the use of an adhesive, so the uniformity of the bonding interface is high, and adverse effects on the formation of the domain-inverted structure due to unevenness of the bonding interface state can be avoided.

  As described above, according to the present embodiment, the charge transfer limiting layer 3 is different in that impurities such as Mg, Zn, Se, and In are added or added to the ferroelectric layer 2 at different concentrations. The only difference is that there is little difference in material properties, so that the optical element substrate can be easily formed by thermal diffusion bonding. In addition, according to the present embodiment, since the charge transfer limiting layer and the ferroelectric layer are bonded by thermal diffusion bonding, the uniformity of the bonding interface is high, and the adverse effect on the polarization inversion structure formation due to uneven bonding interface state Can be avoided. <Claim 4>

Furthermore, according to this embodiment, the inversion coercive electric field of the ferroelectric layer is E1, the thickness of the ferroelectric layer is d1, the inversion coercive field of the charge transfer limiting layer is E2, and the thickness of the charge transfer limiting layer is d2. Then,
E1 · (d1 + d2) <= E2 · d2 (1)
Since the above formula (1) is satisfied, it can be avoided that the charge transfer limiting layer is destroyed by application of an excessive electric field and the resistance value decreases. <Claim 5>

It is sectional drawing which showed the structure of the optical element substrate in 1st Embodiment. (A)-(d) is sectional drawing for demonstrating the operation | movement (formation process of a polarization inversion structure) in the case of carrying out polarization inversion of the optical element substrate of this embodiment. It is sectional drawing which showed the structure of the optical element substrate in 2nd Embodiment. (A)-(d) is sectional drawing for demonstrating the operation | movement (formation process of a polarization inversion structure) in the case of carrying out polarization inversion of the conventional optical element substrate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical element board | substrate 2 Ferroelectric layer 3 Charge transfer limiting layer 4 + Electrode 5-Electrode 6 Optical element board | substrate (conventional)
7 Polarization inversion region

Claims (6)

An optical element substrate made of a ferroelectric material for producing a wavelength conversion device having a structure (polarization inversion structure) in which the spontaneous polarization direction of a ferroelectric crystal having a second-order nonlinear effect is periodically reversed. And
Limiting the amount of movement of charges that move along the direction of spontaneous polarization when the domain-inverted structure is formed with respect to one of the surfaces orthogonal to the spontaneous polarization of the ferroelectric layer of the optical element substrate on which the domain-inverted structure is formed An optical element substrate comprising a charge transfer limiting layer.   2. The charge transfer limiting layer is formed by adding or removing at least one impurity of Mg, Zn, Se, and In to the optical element substrate. Optical element substrate.   2. The optical element substrate according to claim 1, wherein the charge transfer limiting layer is made of the ferroelectric material having a different impurity concentration as compared with the ferroelectric material forming the optical element substrate.   4. The optical element substrate according to claim 3, wherein the ferroelectric layer and the charge transfer limiting layer are bonded by thermal diffusion bonding. When the inversion coercive field of the ferroelectric layer is E1, the thickness of the ferroelectric layer is d1, the inversion coercive field of the charge transfer limiting layer is E2, and the thickness of the charge transfer limiting layer is d2.
E1 * (d1 + d2) <= E2 * d2 ... Formula (1) is satisfy | filled, The optical element substrate of any one of Claim 1 to 4 characterized by the above-mentioned.   A wavelength conversion device using the optical element substrate according to claim 1.

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