JPH07261212A - Formation of domain inversion structure of ferroelectric substance - Google Patents

Abstract

PURPOSE:To form a domain inversion part having high uniformity within a plane parallel with electrodes by using electrodes of which the outer ends exist on the side inner than the outer end of an electrode of the side to impart a charge as the electrodes to be grounded. CONSTITUTION:The grounding electrodes lining up at a prescribed pitch are mounted at the central part of the +z face of an MgO-LiNbO3 substrate 1 and the flat planar minus electrode 3 is mounted at the -z face 1b. The vertical x horizontal size of the minus electrode 3 to be used as the electrode of the side to impart the charges is the same as the substrate size and the vertical x horizontal size of the grounding electrodes 2 is set smaller than this size. The grounding electrodes 2 are set at zero potential. On the other hand, the minus electrode 3 is connected to an electrode 4 and the charge is impressed to the MgO-LiNbO3 substrate 1 from this minus electrode 3. As a result, the domain inversion part 5 having the patterns repeated at the prescribed period lambdain correspondence to the grounding electrodes 2 is formed by inversion of the polarization of the part held by both electrodes 2, 3 of the substrate 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a domain inversion structure having a predetermined pattern in a ferroelectric material having a nonlinear optical effect, for example, for producing an optical wavelength conversion element having a periodic domain inversion structure. It is a thing.

[0002]

2. Description of the Related Art A method of converting a fundamental wave into a second harmonic using an optical wavelength conversion element provided with a region in which spontaneous polarization (domain) of a ferroelectric substance having a nonlinear optical effect is periodically inverted. Have already been proposed by Bleombergen et al. (See Phys. Rev., vol. 127, No. 6, 1918 (1962)).
In this method, the period Λ of the domain inversion part is represented by Λc = 2π / {β (2ω) -2β (ω)} (1) where β (2ω) is the propagation constant 2β (ω) of the second harmonic. Is set to be an integral multiple of the coherent length Λc given by the propagation constant of the fundamental wave, so that the fundamental wave and the second harmonic can be phase-matched. When wavelength conversion is performed using a bulk crystal of a nonlinear optical material, the phase-matching wavelength is limited to a specific wavelength peculiar to the crystal, but according to the above method, a period that satisfies (1) for any wavelength By choosing Λ,
It is possible to achieve phase matching efficiently.

As one of the methods for forming the above-mentioned periodic domain inversion structure, there is a conventional method, for example, Japanese Patent Laid-Open No. 4-33.
As disclosed in Japanese Patent No. 5620 and Japanese Patent Laid-Open No. 5-210132, an electrode having a predetermined pattern is formed on one surface of a ferroelectric substance having a monopolarized nonlinear optical effect.
A method of forming a counter electrode on the other surface facing the one surface, applying a charge to the ferroelectric substance via these two electrodes, and forming a domain inversion portion corresponding to the predetermined pattern thereat Are known.

Further, in the method of forming an inversion domain by applying an electric field to a ferroelectric substance through a pair of electrodes in this way, one of these two electrodes is grounded to keep its potential at zero. It is conceivable to apply + or-charge to the ferroelectric substance from the other electrode.

As described above, when an electric field is applied to a ferroelectric substance while keeping the potential on one electrode side at zero, conventionally, two electrodes have substantially the same size and their outer ends are aligned with each other. , Or the outer end of the grounded electrode,
The electrodes having different sizes are used so that they are located outside the outer end of the electrode that gives the electric charge.

[0006]

However, such a problem
In the method using one electrode, it has been recognized that the uniformity of domain inversion in a plane parallel to the electrode is not good. In other words, originally, domain inversion should be performed over the entire area in the area sandwiched by the electrodes in the above plane, and domain inversion should be completely suppressed in other areas. In some cases, inversion may not occur, or conversely, domain inversion may occur in some of the other regions.

Therefore, an object of the present invention is to provide a method for forming a domain inversion structure of a ferroelectric substance, which can form a domain inversion portion having high uniformity in a plane parallel to an electrode.

[0008]

As described above, the method for forming a domain inversion structure of a ferroelectric substance according to the present invention comprises forming a predetermined pattern of electrodes on one surface of a ferroelectric substance having a non-linear optical effect monopolarized. Along with the formation, an opposite electrode is formed on the other surface facing the one surface, and one of these two electrodes is grounded to keep its potential at zero, while the other electrode gives a charge to the ferroelectric substance, In the method of forming a domain inversion structure of a ferroelectric in which a domain inversion portion corresponding to the predetermined pattern is formed in the ferroelectric, an outer end portion of the electrode that gives a charge is used as the grounded electrode. It is characterized in that one located inside the outer end is used.

[0009]

[Operation and effect of the invention] According to the study by the present inventors,
In the conventional method, the uniformity of domain inversion is impaired because a part of the electrode to be grounded exists at a position facing the outer end of the electrode to give the charge. It is considered that this is because the domain inversion proceeds deviated from the ferroelectric portion in the vicinity of the outer end of the electrode to be applied. This phenomenon can also be understood from the fact that the charges in the conductor generally have the property of gathering at the ends of the conductor.

Therefore, as in the method of the present invention, if a ground-side electrode whose outer end portion is located inside the outer end portion of the electrode that gives the electric charge, the electrode that gives the electric charge is used. Since the ground side electrode does not exist at a position facing the outer end of the electrode, it is possible to prevent the domain inversion from being biased from the ferroelectric portion in the vicinity of the outer end of the electrode giving the electric charge. It becomes possible to form a domain inversion part having high uniformity in a plane parallel to the electrodes.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention will be described in detail below with reference to the embodiments shown in the drawings. FIG. 1 shows the formation of a domain inversion structure according to the first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a MgO-doped LiNbO ₃ substrate which is a ferroelectric substance having a nonlinear optical effect. As the MgO-LiNbO ₃ substrate 1, so that the largest nonlinear optical material constant d ₃₃ can be effectively utilized, z plate which is optically polished in z-plane it is used. Further, this MgO-LiNbO ₃ substrate 1 has been previously monopolarized by a known method, and has a thickness of, for example, 0.
The size is 3 mm, and the size of length x width is 4 x 4 mm.

A ground electrode 2 having comb-shaped portions arranged at a predetermined pitch is attached to the center of the + z surface 1a of the MgO-LiNbO ₃ substrate 1, and a flat negative electrode 3 is attached to the -z surface 1b. Install. As will be described later, the size of the minus electrode 3 which becomes an electrode for giving an electric charge is 4 × 4 mm, which is the same as the substrate size, and the length × width of the ground electrode 2 is 2 × which is smaller than that. 1 mm
It is said that.

The ground electrode 2 is grounded as shown in the figure to keep its potential at zero, while the negative electrode 3 is connected to the power source 4 so that the negative electrode 3 is connected to MgO-LiNbO.
₃ Negative charge is applied to the substrate 1. In this embodiment −
A current of 800 μA was applied for 3 minutes.

When a charge is applied to the MgO--LiNbO ₃ substrate 1 as described above, the polarization of the portion sandwiched between the electrodes 2 and 3 of the substrate 1 is inverted, and the shape of the comb-teeth portion of the ground electrode 2 is formed. Correspondingly, domain inversion unit 5 having a pattern repeating with a predetermined period Λ is formed. Where the period Λ
In consideration of the wavelength dispersion of the refractive index of MgO-LiNbO _3, the thickness was set to 4.6 μm along the x direction of the substrate 1 so as to have a first-order period near 946 nm.

When the state of the domain inversion portion 5 in a plane parallel to the + z plane 1a is observed with a microscope, the domain sandwiched between the electrodes 2 and 3 in this plane is domain-reversed over the entire area, and It was confirmed that domain inversion was completely suppressed in the region, and the uniformity of domain inversion in this plane was extremely high.

Next, a second embodiment of the present invention will be described with reference to FIG. In FIG. 2, elements that are the same as the elements in FIG. 1 are given the same numbers, and duplicated description thereof is omitted (the same applies hereinafter).

MgO used in this second embodiment
The —LiNbO ₃ substrate 1 is also a z-plate that has been monopolarized in advance by a known method, and has a thickness of 0.3 mm and a length × width of 2 × 4 mm.

A flat plate-shaped ground electrode 12 is attached to the center of the -z surface 1b of the MgO-LiNbO ₃ substrate 1, and a plus electrode 13 having comb-teeth-shaped portions arranged at a predetermined pitch on the + z surface 1a. Install. As will be described later, the size of this plus electrode 13, which serves as an electrode for giving an electric charge, is 2 × 4 mm, which is the same as the substrate size.
The size of 12 length x width is 0.2 x 1 mm, which is smaller than that.

The earth electrode 12 is grounded as shown in the figure to keep its potential at zero, while the positive electrode 13 is connected to a power source 14 to give a positive charge to the MgO-LiNbO ₃ substrate 1. Apply. In this embodiment, + 1200μ
The current of A was applied for 5 seconds.

When electric charges are applied to the MgO-LiNbO ₃ substrate 1 as described above, the polarization of the portion sandwiched between the electrodes 12 and 13 of the substrate 1 is inverted, and the shape of the comb-teeth portion of the plus electrode 13 is formed. Correspondingly, domain inversion portion 5 having a pattern repeating at a predetermined cycle is formed.

The domain inversion portion 5 formed by this method was observed with a microscope, and it was confirmed that the uniformity of domain inversion in the plane parallel to the electrodes 12 and 13 was very high.

Next, a comparative example for confirming the effect of the method of the present invention will be described. FIG. 3 shows how the domain inversion structure is formed by the method of the first comparative example to which the conventional method is applied. M used in this first comparative example
The gO-LiNbO ₃ substrate 1 is also a z-plate that has been subjected to a monopolarization treatment in advance by a known method, and has a thickness of 0.3 m as an example.
m, and the size of length × width is 2 × 4 mm.

A ground electrode 22 having comb-shaped portions arranged at a predetermined pitch is attached to the + z surface 1a of the MgO-LiNbO ₃ substrate 1, and a flat plate-shaped negative electrode 23 is provided at the center of the -z surface 1b. Install. Ground electrode 22 vertical ×
The horizontal size is 2 × 4 mm, which is the same as the substrate size, and the negative electrode 23 that will be the electrode for giving an electric charge as described later.
The vertical x horizontal size is 0.2 x smaller than the ground electrode 22.
It is set to 1 mm.

Similar to the first embodiment, the ground electrode 22
Is grounded to keep its potential at zero, while the negative electrode 23
Is connected to the power source 4 and the negative electrode 23
A negative charge is applied to the LiNbO ₃ substrate 1. In this example, a current of -1200 μA was passed for 5 seconds.

When electric charges are applied to the MgO-LiNbO ₃ substrate 1 as described above, the polarization of the portion sandwiched between the electrodes 22 and 23 of the substrate 1 is inverted, and the shape of the comb-teeth portion of the ground electrode 22 is formed. Correspondingly, domain inversion unit 5 having a pattern repeating with a predetermined period Λ is formed. Where the period Λ
Was set to 4.6 μm as in the first embodiment.

When the domain inversion portion 5 formed by this method is observed with a microscope, the periodic domain inversion portion 5 is formed in the substrate portion near the outer end portion of the smaller minus electrode 23.
Electrodes 2 showed a tendency to connect adjacent
It was confirmed that the uniformity of domain inversion in the plane parallel to 2 and 23 was not good.

Next, a second comparative example will be described with reference to FIG. MgO − used in this second comparative example
The LiNbO ₃ substrate 1 is also a z-plate that has been monopolarized in advance by a known method, and as an example, has a thickness of 0.3 mm and a length x
The lateral size is 4 × 4 mm.

A flat plate-shaped ground electrode 32 is attached to the -z surface 1b of the MgO-LiNbO ₃ substrate 1,
A plus electrode 33 having comb-shaped portions arranged at a predetermined pitch is attached to the center of the z-plane 1a. The vertical × horizontal size of the ground electrode 32 is 4 × 4 mm, which is the same as the substrate size, and the vertical × horizontal size of the plus electrode 33, which is the electrode for giving an electric charge, is smaller than the ground electrode 32, as will be described later. × 1 mm
It is said that.

As with the second embodiment described above, the ground electrode 32
Is grounded to keep its potential at zero, while the positive electrode 33 is connected to the power source 14 so that the MgO--Li
A positive charge is applied to the NbO ₃ substrate 1. In this example, a current of +2000 μA was applied for 1 minute.

When electric charges are applied to the MgO-LiNbO ₃ substrate 1 as described above, the polarization of the portion sandwiched between the electrodes 32 and 33 of this substrate 1 is inverted, and the shape of the comb-teeth portion of the plus electrode 33 is formed. Correspondingly, domain inversion portion 5 having a pattern repeating at a predetermined cycle is formed.

When observing the domain inversion portion 5 formed by this method with a microscope, in the substrate portion near the outer end portion of the smaller plus electrode 33, adjacent portions of the periodic domain inversion portion 5 tend to be connected. Recognized, electrode 32,
It was confirmed that the uniformity of domain inversion in the plane parallel to 33 was not good.

Next, an optical wavelength conversion device prepared from the MgO-LiNbO ₃ substrate 1 having the periodic domain inversion structure formed in the first embodiment will be described. By polishing the x-plane and the -x-plane of the substrate 1 to form light-passing surfaces 40a and 40b, respectively, as shown in FIG.
A bulk crystal type optical wavelength conversion element 40 having
The bulk crystal type optical wavelength conversion device 40 was arranged in the resonator of the laser diode pumped YAG laser shown in the same figure.

This laser diode pumped YAG laser has a laser diode 44 which emits a laser beam 43 as pumping light having a wavelength of 809 nm, a condenser lens 45 which converges the laser beam 43 in a divergent light state, and Nd.
A YAG crystal 46, which is a laser medium doped with (neodymium) and is arranged at the converging position of the laser beam 43, and a resonator mirror 47 arranged on the front side (right side in the drawing) of the YAG crystal 46. Consists of. The light wavelength conversion element 40 has a crystal length of 1 mm and is arranged between the resonator mirror 47 and the YAG crystal 46.

The YAG crystal 46 is excited by the laser beam 43 having a wavelength of 809 nm and emits a laser beam 48 having a wavelength of 946 nm. The solid-state laser beam 48 resonates between the YAG crystal end face 46a having a predetermined coating and the mirror surface 47a of the resonator mirror 47, and the optical wavelength conversion element 40
Second harmonic of wavelength ½ or 473 nm
Converted to 49. Solid-state laser beam as fundamental wave 48
And the second harmonic wave 49 are phase-matched (so-called pseudo-phase matching) in the periodic domain inversion region, and almost only the second harmonic wave 49 is emitted from the resonator mirror 47.

In this example, when the output of the laser diode 44 is 200 mW, the second harmonic wave 49 of high output of 10 mW is obtained.
was gotten. On the other hand, the MgO—LiNbO ₃ substrate 1 in which the periodic domain inversion structure is formed according to the first comparative example.
In the case where the bulk crystal type optical wavelength conversion element prepared in the same manner as described above is placed in the resonator of the laser diode pumped YAG laser of FIG.
Output of 200 mW, the output of the second harmonic 49 is 1 mW
Met.

As described above, since the optical wavelength conversion element formed by applying the method of the present invention has realized extremely high wavelength conversion efficiency, the domain inversion part 5 is uniformly formed according to the method of the present invention. Proven to get.

Although the method of the above-mentioned embodiment is applied for producing a bulk crystal type optical wavelength conversion element, the method of the present invention is also applicable to the production of an optical waveguide type optical wavelength conversion element. It is also possible to apply for.

Further, the method of the present invention comprises LiNbO ₃ and MgO-L in addition to MgO-LiNbO ₃ in the above-mentioned embodiment.
The same can be applied when forming a domain inversion structure in a ferroelectric substance such as iTaO ₃ , LiTaO ₃ , KNbO ₃ , or KTP.

[Brief description of drawings]

FIG. 1 is an explanatory diagram illustrating a method of forming a domain inversion structure according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating a method of forming a domain inversion structure according to a second embodiment of the present invention.

FIG. 3 is an explanatory diagram illustrating a method of forming a domain inversion structure by a conventional method.

FIG. 4 is an explanatory view illustrating a method of forming a domain inversion structure by another conventional method.

FIG. 5 is a side view of a solid-state laser provided with an optical wavelength conversion element having a domain inversion structure formed according to the present invention.

[Explanation of symbols]

1 MgO-LiNbO ₃ substrate 1a -z surface 2,12,22,32 earth electrode 3 and 23 negative electrodes of the MgO-LiNbO ₃ substrate + z face 1b MgO-LiNbO ₃ substrate 4,14 Power 5 domain reversals 13, 33 Positive electrode 40 Optical wavelength conversion element 43 Laser beam (pumping light) 44 Laser diode 45 Condenser lens 46 YAG crystal 47 Resonator mirror 48 Solid-state laser beam (fundamental wave) 49 Second harmonic

Claims (1)

[Claims] 1. A single-polarized ferroelectric substance having a nonlinear optical effect is provided with an electrode having a predetermined pattern on one surface, and an opposite electrode is formed on the other surface facing the one surface. One of the electrodes is grounded to keep its potential at zero, while the other electrode gives an electric charge to the ferroelectric substance,
In the method of forming a domain inversion structure of a ferroelectric in which a domain inversion portion corresponding to the predetermined pattern is formed in the ferroelectric, an electrode of which an outer end portion gives the electric charge is an electrode of the one to be grounded. A method of forming a domain inversion structure of a ferroelectric material, characterized in that it is located inside the outer end portion of.