JPH10170966A - Formation of domain inversion structure of ferroelectric substance, and optical wavelength conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of domain inversion even in a part between electrodes of a prescribed pattern in a method for impressing electric field on a ferroelectric substance by using the electrodes of prescribed pattern and forming the part of the ferroelectric substance facing the electrodes described above as local domain inversion part. SOLUTION: The electrodes 2 of the prescribed pattern are formed on one surface 1a of an MgO-LiNbO3 substrate, which is the ferroelectric substance having a monopolarized nonlinear optical effect. The substrate 1 is subjected to corona electrification by these electrodes 2 and a corona wire 4 disposed on the surface 1c side opposite to the one surface 1a and the electric fields are impressed thereon, by which the parts of the substrate 1 facing the electrodes 2 are formed as the local domain inversion parts 6. At this time, ridge parts 1b of the shape corresponding to the electrode patterns are formed on the substrate 1 and the electrodes 2 and formed on the surfaces of these ridge parts 1b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002] The present invention also relates to a method of forming a domain inversion structure of a predetermined pattern on a ferroelectric material having a non-linear optical effect in order to produce such an optical wavelength conversion element.

[0003]

2. Description of the Related Art A method of wavelength-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 material having a nonlinear optical effect is periodically inverted. Has already been proposed by Bleombergen et al. (See Phys. Rev., vol. 127, No. 6, 1918 (1962)).
In this method, the period ド メ イ ン of the domain inversion unit is represented by: c = 2π / {β (2ω) -2β (ω)} (1) where β (2ω) is a propagation constant of the second harmonic β (ω) Is set to be an integral multiple of the coherent length Δc 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 wavelength to be phase-matched is limited to a specific wavelength unique to the crystal. However, according to the above method, the period satisfying (1) for any wavelength By selecting Λ
Phase matching can be efficiently performed.

As one method of forming the above-described periodic domain inversion structure, as disclosed in Japanese Patent Application Laid-Open No. 7-72521, a predetermined surface of a ferroelectric material having a monopolarized nonlinear optical effect is provided. After forming the periodic electrodes of the pattern, these electrodes, and a corona wire disposed on the surface opposite to the one surface, corona-charges the ferroelectric material and applies an electric field thereto, and applies the electric field thereto. There is known a method in which a portion facing an electrode is used as a local domain inversion portion.

In addition to utilizing this corona charging, as shown in, for example, JP-A-4-335620, a full-surface electrode is formed on a ferroelectric surface opposite to a surface on which a periodic electrode having a predetermined pattern is formed. There is also known a method in which an electric field is directly applied to a ferroelectric substance using a full-surface electrode and a periodic electrode to form a local domain inversion portion.

Further, IEICE Technical Report LQE95-94
(1995-11) As shown in pp. 7-12, when applying an electric field to the ferroelectric through the periodic electrode described above, the proton Has been proposed.

[0007]

The above-described method of forming a domain-inverted structure of a ferroelectric using a periodic electrode is superior in productivity as compared with a method of irradiating a ferroelectric with an electron beam. On the other hand, domain inversion is likely to occur not only in the portion immediately 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.

The reason why domain inversion occurs even in a portion between electrodes having a predetermined pattern is considered to be due to an electric field oozing from the electrode portion. If the electrode line width is reduced, the spread of the seeping electric field can be suppressed.However, reducing the electrode line width has technical limitations, especially when the period of the periodic domain inversion structure is small. The width must be extremely narrow, which is difficult to cope with.

The aforementioned IEICE technical report LQE95-94
(1995-11) In the method described in pp. 7 to 12, proton exchange is performed in the interelectrode portion of the ferroelectric to suppress domain inversion in the portion between periodic electrodes. However, here, LiT is used as a ferroelectric material having a nonlinear optical effect.
Only an example using aO ₃ has been reported. For example, when LiNbO ₃ doped with MgO is used, it has been found that the effect of suppressing domain inversion in a portion between periodic electrodes is extremely low.

That is, in the case of MgO-doped LiNbO ₃ , the surface resistance is greatly reduced by proton exchange, so that the surface charge is easily moved, and the domain inversion is likely to occur even in the portion 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 using electrodes of a predetermined pattern, and a portion of the ferroelectric facing the electrode is formed in a local domain. It is an object of the method of forming an inversion portion to prevent domain inversion even in a portion between electrodes of a predetermined pattern, and to improve the periodicity when a periodic domain inversion structure is formed.

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

[0013]

As described above, the method for forming a domain-inverted structure of a ferroelectric according to the present invention has a predetermined pattern on one surface of a ferroelectric having a monopolarized nonlinear optical effect. After forming an electrode, an electric field is applied to the ferroelectric through the electrode, and a method of forming a domain-inverted structure of a ferroelectric, in which a portion of the ferroelectric facing the electrode is a local domain-inverted portion. Wherein a ridge portion having a shape corresponding to the predetermined pattern is formed on one surface of the ferroelectric material, and the electrode is formed on the surface of the ridge portion.

The above-mentioned ridge portion can be easily formed by forming an electrode of a predetermined pattern on one surface of the ferroelectric and etching the one surface using the electrode as a mask.

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

On the other hand, an optical wavelength conversion element according to the present invention is one in which a periodic domain inversion structure is formed on a ferroelectric having a nonlinear optical effect by the above-described method.

In the optical wavelength conversion device according to the present invention, the ferroelectric material 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 a domain inversion structure formed by a 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 material 31. Using these electrodes 32, the above-described corona charging method or direct electric field application method is used.
An electric field is applied to the ferroelectric 31. The domain inversion portion 33 formed by this should be originally formed as shown by a solid line, but is spread and formed as shown by a broken line due to the seeping electric field described above.

On the other hand, in the method of the present invention, as shown in FIG. 1B, a ridge portion 31b having a shape corresponding to the electrode 32 having a predetermined pattern is formed on one surface 31a of the ferroelectric material 31. The electrode 32 is formed on the surface of the ridge 31b. In this manner, the intensity of the seeping electric field is reduced in the interelectrode portion of the ferroelectric material 31 shown in FIG. 2C, and therefore, the domain inversion portion 33 is formed almost only immediately below the electrode 32 as shown in the figure. Become so.

After the electrode 32 is formed, this electrode 32
If the ridge portion 31b is formed by etching the ferroelectric surface 31a using the mask 32 as a mask, the electrode 32 can be effectively used for forming the ridge portion, thereby simplifying the process of forming the domain inversion structure. .

In the optical wavelength conversion element of the present invention, a periodic domain inversion structure is formed on a ferroelectric material having a nonlinear optical effect by the above-described method. And it ’s good,
High wavelength conversion efficiency can be realized.

Further, in the above method, the influence of the seeping electric field is removed by a so-called physical method of forming a ridge portion. Does not lower the surface resistance. Therefore, in the optical wavelength conversion element of the present invention, LiNb _x Ta _{1 -x} O ₃ (0 ≦ x
Period of ≦ 1) and, we are used LiNbO ₃ which is also (especially MgO doped with either within, such as those MgO or ZnO doped), periodic domain reversals due to the surface resistance drop structure It does not cause a decrease in sex.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a step of forming a ferroelectric domain inversion structure according to the first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes MgO-doped LiNbO ₃ (M
gO-LN). The MgO-LN substrate 1 is a single polarization processing is formed to made be a thickness of 0.5 mm, as the largest nonlinear optical constant d ₃₃ can be effectively utilized Z
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, for example, 50 nm, and then, as shown in FIG. A periodic electrode 2 made of a Cr thin film is formed. The periodic electrode 2 repeats at a constant period Λ in the X-axis direction of the substrate 1, for example. Although not shown in the drawing, the periodic electrodes 2 are all extended from a common base, and are in a state of being electrically connected to each other.

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

In this manner, the ridge portion 1b having a shape corresponding to the shape of the periodic electrode 2 is formed on the surface 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 a ground wire 3 and dropped to ground,
An electric field is applied to the substrate 1 by corona charging using the corona wire 4 disposed above the Z plane 1c and the high voltage power supply 5 connected thereto. At this time, the temperature of the substrate 1 is from room temperature.
Set the temperature between 300 ° C, corona wire 4 and substrate 1
Is set to 10 mm, and a voltage of, for example, 7 kV is applied from the high-voltage power supply 5 through the corona wire 4 for 2 seconds.

By this voltage application, the -Z surface 1a of the substrate 1
Domain inversion portion 6 penetrating from the position where periodic electrode 2 was formed to + Z surface 1c. These domain inversion units 6 repeat at the same cycle as the cycle Λ of the periodic electrode 2. And these domain inversion units 6
The electrode 2 does not spread toward the portion between the periodic electrodes 2 and
Is formed only in the portion immediately below the surface of the substrate to have excellent periodicity. The reason is as described in detail above.

Next, a second embodiment of the method for forming a domain inversion structure of a ferroelectric according to 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 reference numerals, and overlapping descriptions thereof will be omitted.

In the second embodiment, FIG.
As shown in FIG. 1, a Cr periodic electrode 2 is formed on the + Z plane 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 the MgO-L
Proton exchange is performed between the electrodes of the N substrate 1. In FIG. 2B, the portion where the proton exchange has been performed is shown as 1d. At this time, if the pyrophosphoric acid is excessively heated, the Cr periodic electrode 2 is melted, so that the temperature condition needs to be set 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 at a ratio of 1: 2. Then, the periodic electrode 2 made of Cr having resistance to hydrofluoric-nitric acid is not etched, and the proton exchange portion 1d has a high etching rate. Therefore, only the proton exchange portion 1d, that is, only the portion between the electrodes is favorably etched. .

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

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

[0034]

(Equation 1)

After approximating the Laplace equation, the electric field intensity 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 intensity was highest at the crystal surface, that is, at the electrode interface, and rapidly decreased toward the inside of the crystal. Then, it was confirmed that the electric field intensity was reduced to about 30% of the maximum value at a depth of 0.1 ° inside the crystal just below the electrode edge portion at the time of the optimum electrode shape. If the electric field strength is reduced to at least this extent, domain inversion is less likely to occur in the portion between the electrodes. Therefore, it is desirable to set the height of the ridge to a value of 0.1 ° or more.

In the two embodiments described above,
Since the ridge portion 1b is formed 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 for example, a method of physically etching the ferroelectric surface between the electrode patterns by dry etching, ion beam milling, or the like. Etc. can also be applied.

On the other hand, a method of applying an electric field to the ferroelectric
The method is not limited to the corona charging method applied in the above-described embodiment, and for example, the above-described direct electric field application method or the like can also be used.

Next, an optical wavelength conversion device according to the present invention will be described. For example, as in the first embodiment, M
By forming a periodic domain inversion portion 6 arranged in the X-axis direction of the gO-LN substrate 1, polishing the X surface and the -X surface, and applying an anti-reflection coating to form light passing surfaces 20a and 20b, respectively. Bulk crystal type optical wavelength conversion device as shown in 3
20 is obtained.

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

The semiconductor laser pumped YAG laser includes a semiconductor laser 11 for emitting a laser beam 10 as pumping light having a wavelength of 809 nm, a condensing lens 12 for converging 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 a convergence position of the laser beam 10;
A resonator mirror 14 is provided on the front side (right side in the figure) of the YAG crystal 13. The optical 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 applied to the end face 13a of the YAG crystal on which a predetermined coating is applied.
And the mirror surface 14a of the resonator mirror 14
A 946 nm solid state laser beam 15 is obtained.

The solid-state laser beam 15 is incident on the optical wavelength conversion element 20, and the wavelength is １ ／, ie, the second laser beam having a wavelength of 473 nm.
Converted to harmonic 16. The solid-state laser beam 15 and the second harmonic 16 as fundamental waves are phase-matched (so-called quasi-phase-matching) in the period domain inversion region.
Only the harmonic 16 exits from the resonator mirror 14. As described above, since the domain inversion unit 6 has excellent periodicity, good phase matching is achieved, and a high-intensity second harmonic 16 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 case will be described with reference to specific data. In the present example, when the output of the semiconductor laser 11 was 200 mW, the yield of the crystal from which the second harmonic 16 of 10 mW was obtained was greatly improved to about 50% or more. Also,
A maximum output of 20 mW was obtained. On the other hand, when the method of the present invention was not applied, the maximum output was 10 mW, and the yield of the above crystal was 10% or less.

In the above description, MgO-doped LiN
Although the embodiment in which the domain inversion structure is formed in bO ₃ has been described, the present invention relates to another ferroelectric material such as ZnO.
Doped with 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 the drawings]

FIG. 1 is a schematic diagram showing a state of forming a periodic domain inversion structure by a method according to a first embodiment of the present invention.

FIG. 2 is a schematic view showing a state in which a periodic domain inversion structure is formed by a method according to a second embodiment of the present invention.

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

FIG. 4 is an explanatory diagram illustrating the operation 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 unit 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 light wavelength conversion element

Claims (5)

[Claims] An electrode having a predetermined pattern is formed on one surface of a monopolarized ferroelectric material having a non-linear optical effect, and an electric field is applied to the ferroelectric material via the electrode to thereby form the ferroelectric material. In the method of forming a domain inversion structure of a ferroelectric material, wherein a portion facing the electrode is a local domain inversion portion, a ridge portion having a shape corresponding to the predetermined pattern is formed on one surface of the ferroelectric material. A method for forming a domain-inverted structure of a ferroelectric, comprising forming the electrode on a surface of a ridge portion. 2. The ridge portion according to claim 1, wherein an electrode of the predetermined pattern is formed on one surface of the ferroelectric, and the one surface is etched using the electrode as a mask. A method for forming a domain inversion structure of a ferroelectric. 3. The method according to claim 1, wherein the height of the ridge portion is set to 0.1 ° or more when the electrode of the predetermined pattern is a periodic electrode that repeats at a predetermined period Λ. A method of forming a domain inversion structure of a ferroelectric. 4. An optical wavelength conversion element in which a periodic domain inversion structure is formed on a ferroelectric having a nonlinear optical effect by the method according to claim 1. 5. The method according to claim 1, wherein the ferroelectric is LiNb _x Ta ₁ -xO.
₃ (0 ≦ x ≦ 1) or MgO or ZnO
5. The optical wavelength conversion device according to claim 4, wherein is doped.