JP3487727B2 - Method of forming domain inversion structure of ferroelectric and optical wavelength conversion element - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion element for converting a fundamental wave into a second harmonic wave or the like, and more particularly to an optical wavelength conversion element having a periodic domain inversion structure.

The present invention also relates to a method of forming a domain-inverted structure having a predetermined pattern in a ferroelectric substance having a nonlinear optical effect in order to manufacture such a light wavelength conversion element.

[0003]

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 β (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 methods for forming the above-mentioned periodic domain inversion structure, as shown in Japanese Patent Laid-Open No. 7-72521, a predetermined polarization is formed on one surface of a ferroelectric substance having a nonlinear optical effect which is monopolarized. After forming the periodic electrodes of the pattern, the ferroelectric body is corona-charged by these electrodes and the corona wire arranged on the surface side opposite to the one surface, and an electric field is applied thereto, and the above-mentioned A method is known in which a portion facing the electrode is a local domain inversion portion.

In addition to utilizing this corona charging, as shown in, for example, Japanese Patent Laid-Open No. 4-335620, a full-scale electrode is formed on the surface of the ferroelectric material opposite to the surface on which the periodic electrode having a predetermined pattern is formed. A method is also known in which an electric field is directly applied to a ferroelectric substance by a full-face electrode and a periodic electrode to form a local domain inversion part.

Furthermore, IEICE Technical Report LQE95-94
(1995-11) pp.7-12, when an electric field is applied to the ferroelectric substance via the above-mentioned periodic electrode, the inter-electrode portion of the ferroelectric substance is exchanged with a proton before the electric field is applied. A method of applying a voltage has also been proposed.

[0007]

The method of forming a domain inversion structure of a ferroelectric substance using the periodic electrode described above is superior in productivity as compared with the method of irradiating the ferroelectric substance with an electron beam. On the other hand, domain inversion is likely to occur not only in the portion directly below the ferroelectric periodic electrode but also in the portion between the periodic electrodes. If so, the formed periodic domain inversion structure has poor periodicity, and the optical wavelength conversion element having such a periodic domain inversion structure has low wavelength conversion efficiency.

As described above, it is considered that the domain inversion occurs even in the portion between the electrodes of the predetermined pattern due to the electric field leaking from the electrode portion. If the electrode line width is made narrower, the spread of this seeping-out electric field can be suppressed, but there is a technical limit to making the electrode line width narrower, especially if the period of the periodic domain inversion structure is small. The width must be extremely narrow, which is difficult to handle.

IEICE Technical Report LQE95-94
The method shown in (1995-11) pp.7 to 12 is a method in which the interelectrode portion of the ferroelectric is proton-exchanged in order to suppress domain inversion in the portion between the periodic electrodes. However, here, LiT is used as a ferroelectric substance having a nonlinear optical effect.
Only an example using aO ₃ has been reported, and it has been found that, when LiNbO ₃ doped with MgO is used, for example, the effect of suppressing domain inversion in the portion between the periodic electrodes is extremely low.

That is, in MgO-doped LiNbO ₃ , the surface resistance of the MgO-doped LiNbO ₃ is greatly reduced by the proton exchange, which facilitates the movement of the surface charge and facilitates the domain inversion even between the periodic electrodes. .

The present invention has been made in view of the above circumstances, and an electric field is applied to a ferroelectric substance by using an electrode having a predetermined pattern, and a portion of the ferroelectric substance facing the electrode is localized domain. In the method of forming the inversion portion, it is an object to prevent domain inversion even in a portion between electrodes having a predetermined pattern, and thus to improve the periodicity when forming a periodic domain inversion structure.

Another object of the present invention is to provide an optical wavelength conversion device having a periodic domain inversion structure with excellent periodicity.

[0013]

As described above, the method for forming the domain inversion structure of the ferroelectric substance according to the present invention is the + Z plane or the -Z plane of the ferroelectric substance having the monopolarized nonlinear optical effect. After forming an electrode with a predetermined pattern on one surface, an electric field is applied to the ferroelectric substance through this electrode, and the portion of the ferroelectric substance facing the electrode serves as a local domain inversion portion. In the method of forming a domain inversion structure of a dielectric material, a ridge portion having a shape corresponding to the predetermined pattern is formed on one surface of a ferroelectric material, the electrode is formed on the surface of the ridge portion, and then the surface opposite to the one surface is formed . Ferroelectric table
The corona wire placed on the surface side and the
An electric field is applied by Rona charging to form a domain inversion portion deeper than the height of the ridge portion.

In the method of the present invention, the above-mentioned ridge portion forms an electrode of a predetermined pattern on one surface of the ferroelectric substance,
It is formed by etching the one surface using this electrode as a mask.

When the electrode having the predetermined pattern is a periodic electrode which repeats at a predetermined period Λ, the height of the ridge portion is preferably set to 0.1Λ or more.

[0016] On the other hand the optical wavelength conversion element according to the present invention, a ferroelectric having a nonlinear optical effect, Ri Na periodically domain inverted structure is formed by the above method, the ridge portion and Li
Is to shall and characterized in that between the Tsu di portion is a void.

In the optical wavelength conversion device according to the present invention, a ferroelectric substance having a nonlinear optical effect is LiN.
b _x Ta _1-x O ₃ (0 ≦ x ≦ 1) or MgO
Alternatively, a material doped with ZnO is preferably used.

[0018]

FIG. 4A schematically shows the domain inversion structure formed by the conventional method.
In this case, an electrode (for example, a periodic electrode) 32 having a predetermined pattern is formed on one surface 31a of the ferroelectric substance 31, and by using these electrodes 32, the corona charging method or the direct electric field applying method described above is used.
An electric field is applied to the ferroelectric substance 31. The domain inversion portion 33 formed thereby should be originally formed as shown by a solid line, but is formed so as to be spread out as shown by a broken line by the above-mentioned seepage electric field.

On the other hand, in the method of the present invention, a ridge portion 31b having a shape corresponding to the electrode 32 of a predetermined pattern is formed on one surface 31a of the ferroelectric substance 31, as shown in FIG. The electrode 32 is formed on the surface of the ridge portion 31b. By doing so, the strength of the seeping electric field becomes low in the portion between the electrodes of the ferroelectric substance 31 shown by C in the figure, so that the domain inversion portion 33 is formed almost only in the portion directly below the electrode 32 as shown in the figure. Become so.

[0020] In the present invention, after forming the electrodes 32, that form a ridge portion 31b by etching the ferroelectric surface 31a of the electrode 32 as a mask. So
By doing so, the electrode 32 can be effectively used for forming the ridge portion, and thereby the process of forming the domain inversion structure can be simplified.

In the optical wavelength conversion element of the present invention, the periodic domain inversion structure is formed in the ferroelectric substance having the nonlinear optical effect by the above-described method. It ’s good, and
High wavelength conversion efficiency can be realized.

Further, in the above method, the ridge portion is formed so as to remove the influence of the seeping electric field by a so-called physical method. Therefore, as in the case of applying the above-mentioned proton exchange, the ferroelectric substance is used. Surface resistance does not decrease. Therefore, in the optical wavelength conversion element of the present invention, LiNb _x Ta _1-x O ₃ (0 ≦ x
≦ 1), or any of those doped with MgO or ZnO (especially with LiNbO ₃ doped with MgO), the period of the periodic domain inversion structure caused by the surface resistance decrease It does not cause deterioration of sex.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a process of forming a domain inversion structure of a ferroelectric substance according to the first embodiment of the present invention. In FIG. 1, 1 is a ferroelectric having a nonlinear optical effect, MgO-doped LiNbO ₃ (M
gO-LN) substrate. This MgO-LN substrate 1 is monopolarized to have a thickness of 0.5 mm, and is made to have the largest nonlinear optical constant d ₃₃ so that it can be effectively used.
The surface is optically polished.

Cr is sputtered on the -Z surface 1a of the MgO-LN substrate 1 to form a Cr thin film having a thickness of 50 nm, for example, and then photolithography and dry etching are performed, as shown in FIG. The periodic electrode 2 made of a Cr thin film is formed. The periodic electrode 2 repeats, for example, in the X-axis direction of the substrate 1 at a constant period Λ. Although not shown in the figure, the periodic electrodes 2 are all extended from a common base, and are in electrical connection with each other.

Next, the MgO-LN substrate 1 is dipped in an etching solution in which HF (hydrofluoric acid) and HNO ₃ (nitric acid) are mixed in a ratio of 1: 2. Then, since the periodic electrode 2 made of Cr having the resistance to hydrofluoric acid is not etched, the periodic electrode 2 acts as a mask, and the inter-electrode portion of the MgO-LN substrate 1 is removed as shown in (2) of FIG. Is etched.

Thus, the ridge portion 1b having a shape corresponding to the shape of the periodic electrode 2 is formed on the surface portion of the MgO-LN substrate 1, and the periodic electrode 2 is placed on the surface of each ridge portion 1b. .

Next, as shown in (3) of FIG. 1, the periodic electrode 2 is connected to the ground wire 3 and dropped to the ground, and the +
An electric field is applied to the substrate 1 by corona charging using a corona wire 4 arranged above the Z plane 1c and a high-voltage power supply 5 connected thereto. At this time, the temperature of the substrate 1 is from room temperature
Set in the range between 300 ℃, corona wire 4 and substrate 1
The distance between and is set to 10 mm, and a voltage of, for example, 7 kV is applied from the high voltage power source 5 through the corona wire 4 for 2 seconds.

By applying this voltage, the -Z surface 1a of the substrate 1 is
The domain inversion part 6 is formed so as to penetrate from the place where the periodic electrode 2 was formed to the + Z plane 1c. These domain inversion parts 6 are repeated in the same cycle as the cycle Λ of the periodic electrode 2. And these domain inversion parts 6
It does not spread to the part between the periodic electrodes 2 and is almost
It is formed only in the portion immediately below, and has excellent periodicity. The reason is as described in detail above.

Next, with reference to FIG. 2, a second embodiment of the method of forming the domain inversion structure of the ferroelectric substance of the present invention will be described. 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.

In the second embodiment, (1) in FIG.
As shown in, the Cr periodic electrode 2 is formed on the + Z surface 1c of the MgO-LN substrate 1. Then this MgO-LN
When the substrate 1 is immersed in, for example, pyrophosphoric acid kept at 150 ° C., the periodic electrode 2 acts as a mask, and MgO-L
Proton exchange is performed between the electrodes of the N substrate 1. In addition, in (2) of the figure, the portion where the proton exchange is performed is shown as 1d. At this time, if the pyrophosphoric acid is set to an excessively high temperature, the Cr periodic electrode 2 will be melted, so it is necessary to set the temperature condition appropriately.

Next, the MgO-LN substrate 1 is immersed in an etching solution in which HF (hydrofluoric acid) and HNO ₃ (nitric acid) are mixed in a ratio of 1: 2. Then, since the periodic electrode 2 made of Cr having a resistance to hydrofluoric acid is not etched and the proton exchange portion 1d has a high etching rate, only the proton exchange portion 1d, that is, only the interelectrode portion is etched well. .

Therefore, also in this case, the ridge portion 1b having a shape corresponding to the shape of the periodic electrode 2 is formed on the surface portion of the MgO-LN substrate 1, and the periodic electrode 2 is placed on the surface of each ridge portion 1b. It will be in a state of being. Then, an electric field is applied to the substrate 1 by corona charging in the same manner as in the first embodiment, and in this case also, the domain inversion portion is formed almost only in the portion directly below the electrode 2.

It is desirable that the height of the ridge portion 1b should be 0.1Λ or more with respect to the period Λ of the periodic electrode 2. The reason is as follows. The potential φ (X, Z) induced just below the periodic electrode satisfies the following two-dimensional Laplace equation.

[0034]

[Equation 1]

After the Laplace equation was approximately calculated, the electric field strength component in the electric field Z direction, that is, the crystal depth direction was calculated. As a result, it was found that the electric field strength was the highest on the crystal surface, that is, the electrode interface, and decreased sharply toward the inside of the crystal. It was confirmed that the electric field strength was reduced to about 30% of the maximum value at the depth of 0.1Λ inside the crystal just below the electrode edge portion when the optimum electrode shape was adopted. If the electric field strength decreases to at least this extent, domain inversion does not occur easily between the electrodes, so it is desirable to set the height of the ridge portion to a value not less than 0.1Λ.

In the two embodiments described above,
Since the ridge portion 1b is formed by using the periodic electrode 2 as a mask for etching or proton exchange, the process up to the formation of the ridge portion is simplified, which is preferable.

However, the method of forming the ridge portion is not limited to such a method, and other methods such as a method of physically etching the ferroelectric surface between the electrode patterns by dry etching, ion beam milling or the like. Etc. are also applicable.

[0038]

Next, the optical wavelength conversion device according to the present invention will be described. For example, as in the first embodiment, M
By forming the periodic domain reversals 6 arranged in the X-axis direction of the gO-LN substrate 1, polishing the X-plane and the -X-plane, and applying antireflection coating to the light-passing planes 20a and 20b, respectively, Bulk crystal type optical wavelength conversion device as shown in FIG.
You get 20.

The bulk crystal type optical wavelength conversion device 20 having the periodic domain inversion structure is arranged in the resonator of the semiconductor laser pumped YAG laser shown in FIG. in this case,
In order to achieve phase matching with the fundamental wavelength of 946 nm, Mg
Considering the wavelength dispersion of the refractive index of O-LN, the period Λ of the electrode 2 is set to 4.7 μm.

This semiconductor laser-excited YAG laser includes a semiconductor laser 11 which emits a laser beam 10 as pumping light having a wavelength of 809 nm, a condenser lens 12 which converges the laser beam 10 in a divergent light state, and Nd (neodymium). A YAG crystal 13 which is a doped laser medium and is arranged at the converging position of the laser beam 10;
It is composed of a resonator mirror 14 arranged on the front side (right side in the figure) of this YAG crystal 13. The light wavelength conversion element 20 is arranged between the resonator mirror 14 and the YAG crystal 13.

The YAG crystal 13 is excited by the laser beam 10 having a wavelength of 809 nm and emits light having a wavelength of 946 nm.
This light is emitted from the YAG crystal end face 13a with a predetermined coating.
Between the mirror surface 14a of the resonator mirror 14 and the
A 946 nm solid-state laser beam 15 is obtained.

This solid-state laser beam 15 is incident on the light wavelength conversion element 20 and has a wavelength of 1/2, that is, a second wavelength of 473 nm.
Converted to harmonics 16. The solid-state laser beam 15 as the fundamental wave and the second harmonic 16 are phase-matched (so-called quasi-phase matching) in the periodic domain inversion region, and almost the second
Only harmonics 16 exit the resonator mirror 14. As described above, since the domain inversion unit 6 has excellent periodicity, good phase matching is achieved, and the second harmonic wave 16 of high intensity can be obtained with high wavelength conversion efficiency. .

Hereinafter, the point that the periodicity of the domain inversion unit 6 is improved as compared with the conventional one will be described with reference to specific data. In this example, when the output of the semiconductor laser 11 is 200 mW, the yield of the crystal that can obtain the second harmonic wave 16 of 10 mW is significantly improved to about 50% or more. Also,
A maximum output of 20 mW was obtained. On the other hand, the maximum output when the method of the present invention was not applied was 10 mW, and the yield of the above crystals was 10% or less.

The above is the LiN doped with MgO.
Although the embodiment in which the domain inversion structure is formed in bO ₃ has been described, the present invention is not limited to other ferroelectric materials such as ZnO.
Doped LiNbO ₃ , MgO or ZnO
There doped LiTaO _3, further is applicable also for the formation of domain inversion structure in LiNbO ₃ or LiTaO ₃ or the like of the non-doped, and in which the same effect.

[Brief description of drawings]

FIG. 1 is a schematic view showing how a periodic domain inversion structure is formed by the method of the first embodiment of the present invention.

FIG. 2 is a schematic view showing how a periodic domain inversion structure is formed by the method of the second embodiment of the present invention.

FIG. 3 is a schematic side view showing a usage state of a bulk crystal type optical wavelength conversion device having a periodic domain inversion structure produced by the method of the present invention.

FIG. 4 is an explanatory diagram for explaining the action and effect of the method of the present invention.

[Explanation of symbols]

1 MgO-LiNbO ₃ substrate (Z plate) 1a MgO-LiNbO ₃ -Z face 1b MgO-LiNbO ₃ ridge portion 1c MgO-LiNbO ₃ + Z face 1d proton exchange unit 2 period of MgO-LiNbO ₃ substrate substrate substrate substrate Electrode 3 Ground wire 4 Corona wire 5 High-voltage power supply 6 Domain inversion section 10 Laser beam (pumping light) 11 Semiconductor laser 12 Condensing lens 13 YAG crystal 14 Resonator mirror 15 Solid-state laser beam (fundamental wave) 16 Second harmonic 20 Bulk Crystal type optical wavelength conversion element

─────────────────────────────────────────────────── ───Continuation of front page (56) Reference JP-A-8-220578 (JP, A) JP-A-6-194706 (JP, A) JP-A-7-72521 (JP, A) JP-A-6- 242478 (JP, A) JP 8-5854 (JP, A) JP 7-159637 (JP, A) JP 2-150807 (JP, A) JP 5-313031 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/35-1/39 JISST file (JOIS) WPI (DIALOG) INSPEC (DIALOG)

Claims (4)

(57) [Claims] 1. An electrode having a predetermined pattern is formed on one surface, which is the + Z plane or the −Z plane of a ferroelectric substance having a non-linear optical effect monopolarized, and an electric field is applied to the ferroelectric substance through the electrode. Is applied to form a domain inversion structure of a ferroelectric substance in which a portion of the ferroelectric substance facing the electrode is a local domain inversion part, and the electrode having the predetermined pattern is formed on one surface of the ferroelectric substance. After that, the one surface is etched by using this electrode as a mask to form a ridge portion having a shape corresponding to the predetermined pattern, and then a roller arranged on the side of the ferroelectric surface opposite to the one surface.
Forming a domain inversion structure of a ferroelectric characterized in that a domain inversion portion is formed deeper than the height of the ridge portion by applying an electric field described above by corona charging using a nawire and the electrode. Method. 2. The ferroelectric according to claim 1, wherein when the electrode having the predetermined pattern is a periodic electrode which repeats at a predetermined period Λ, the height of the ridge portion is set to 0.1Λ or more. Method for forming domain inversion structure of body. 3. A ferroelectric having a non-linear optical effect, wherein a periodic domain inversion structure is formed by the method according to claim 1 or 2, wherein a gap is provided between the ridge portions. An optical wavelength conversion element characterized by. 4. The ferroelectric material is LiNb _x Ta.
The optical wavelength conversion element according to claim 3, wherein the optical wavelength conversion element is _{1-xO 3} (0 ≦ x ≦ 1) or is doped with MgO or ZnO.