JPH0517295A - Lithium niobate single crystal thin film subjected to domain inversion treatment - Google Patents

Abstract

PURPOSE:To improve nonlinear optical characteristics by subjecting the surface of a lithium tantalate substrate to lattice matching, then to a domain inversion treatment in such a manner that polarization inverts in crystal axis direction at every synchronization. CONSTITUTION:The optically polished lithium tantalate substrate 1 is brought into contact with a melt consisting of Li2O, Nb2O3, V2O5, Na2O, and MgO and the lattice matched Na, Mg-contg. lithium niobate single crystal thin film 2 is grown thereon by an epitaxial growth method. Stripe patterns 3 are formed thereon by photolithography and Ti is deposited by vacuum evaporation over the entire surface thereof, by which a Ti thin film 4 is formed. The stripe patterns 3 are then removed and the substrate is subjected to a heating treatment to diffuse the Ti. The substrate is thereafter rapidly cooled, by which the lithium niobate single crystal thin film having the inverted domain layer 5 is obtd. A part of this crystal is then cut out and one line of pattern is drawn by photolithography method in the direction perpendicular to the Ti pattern. The Ti is deposited by vacuum evaporation and is subjected to ion beam etching, by which a ridge type waveguide 6 is formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium niobate single crystal thin film obtained by subjecting various optical materials including a thin film waveguide type second harmonic generating element to a suitable domain inversion process.

[0002]

2. Description of the Related Art In recent years, there has been a demand for a laser light source having a shorter wavelength with the progress of optical application technology. By shortening the wavelength of the laser light source, the recording density and the photosensitivity are improved, whereby the range of use in the optical equipment field such as optical discs and laser printers is further expanded.

Therefore, the wavelength of the incident laser light is 1 /
Research has been conducted on a second harmonic generation (SHG) element that can be converted to 2. A bulk single crystal of a non-linear optical crystal has been used as such a second harmonic generation (SHG) element, but it is necessary to obtain a high conversion efficiency with a low light source output of several mW to several tens of mW, so that a thin film waveguide type is used. SHG elements have become necessary.

As such a non-linear optical material for a thin film waveguide type SHG element, for example, a layer in which a different element such as Ti is diffused into a bulk lithium niobate single crystal to change the refractive index is used as a waveguide. Also known are those in which a lithium niobate single crystal thin film is used as a waveguide on a lithium tantalate substrate.

When laser light is incident on such a waveguide, SHG is generated. However, due to the interference between the laser light and high frequency, the SHG light causes an interference distance ( Coherence length) The maximum and minimum are repeated for each lc. lc = (2n + 1) λf / 4 (nf-nSH) In the formula, the subscripts f and SH represent the fundamental wave and harmonic components, λ is the wavelength, and n is the refractive index.

Therefore, by alternately inverting the sign of the polarization wave for each lc, the output of the SHG light disappears and the addition occurs on the contrary, and the output of the SHG is added, so that a large conversion efficiency can be obtained. it can. The theory of domain inversion in which polarization is inverted for each lc has been proposed since about 1962, and domain inversion processing has been used since 1988 to increase the output intensity of SHG.

There has been proposed a second high frequency generating element by Cherenkov radiation by providing a domain inversion structure portion on a non-linear optical material substrate and providing an optical waveguide thereon (see, for example, Japanese Patent Application No. 2-93624). However, the beam converted by the Cherenkov radiation type second harmonic generation element is
The beam spot is radiated in the inward direction of the substrate, and the beam spot has a unique shape, for example, a crescent spot. Therefore, when this beam spot is used as a light source for an optical disc or a laser printer, the Cherenkov radiation is diffraction-limited. However, since a lens suitable for this purpose has not been put into practical use at present, the Cherenkov radiation type SHG generating element is not yet practical.

On the other hand, a second harmonic generating element has been proposed in which a domain inversion process is applied to the waveguide to obtain a circular or elliptical beam spot-shaped output with high efficiency. There is. However, as a means for performing domain inversion processing on the waveguide, for example, when Ti is diffused into a lithium niobate single crystal bulk or a proton exchange is performed to form the waveguide, the crystal structure of lithium niobate is changed to change the niobate. The lithium crystal structure is no longer present, so that the nonlinear optical characteristics inevitably change, the value of the nonlinear polarization tensor becomes small, and the nonlinear optical characteristics deteriorate.

[0009]

DISCLOSURE OF THE INVENTION As a result of various studies to improve the above-mentioned points and to provide a second harmonic generating element having a good harmonic beam spot shape and a large output, The inventors have found that a lithium niobate single crystal thin film in which a waveguide made of a lattice-matched lithium niobate single crystal thin film on a lithium tantalate substrate is polarization-inverted achieves the above object, and completed the present invention. The purpose of
Another object of the present invention is to provide a lithium niobate single crystal thin film obtained by subjecting various optical materials including a thin film waveguide type second harmonic generating element to a suitable domain inversion process.

[0010]

The gist of the present invention is to provide a lithium niobate single crystal which is lattice-matched on a lithium tantalate substrate and which is domain-inverted so that the polarization is inverted in the crystal axis direction at every period. It is a crystal thin film. That is, in the lithium niobate single crystal thin film lattice-matched on the lithium tantalate substrate, the polarization is inverted in the crystal axis direction at every period so as to satisfy the condition of the following general formula (I). (2m + 1) λf / 4 (nf-nSHG) (I) In the formula, nf is an effective refractive index in the fundamental wavelength light, nSHG
Is the effective refractive index of the second harmonic light, and m is an integer.

In the present invention, since the lithium niobate single crystal thin film is formed on the lithium tantalate substrate, it is Ti diffused on the waveguide obtained by proton diffusion on the conventional lithium niobate substrate or on the lithium niobate substrate. It is superior in nonlinear optical characteristics to the obtained waveguide, and since the lithium niobate single crystal thin film is lattice-matched with lithium tantalate, it is expected to reduce the crystallinity and propagation loss of the lithium niobate film. Further, a good quality film is formed with a large film thickness. Further, since the value of the non-linear polarization tensor of lithium niobate is the same as that of the lithium niobate bulk, improvement in SHG conversion efficiency can be expected.

Since the value of the nonlinear polarization tensor of the lithium niobate single crystal thin film on the lithium tantalate substrate is smaller than that of the lithium niobate bulk, lattice matching is indispensable for obtaining a practical SHG element. Is.
Lattice matching is performed by changing the a-axis lattice constant of the lithium niobate single crystal thin film to the a-axis lattice constant of the lithium tantalate substrate.
9.81-100.07%, more preferably 99.92
It is to be set to 100.03%.

In the present invention, the means for lattice-matching the lithium niobate single crystal thin film and the lithium tantalate substrate is not particularly limited, but international application number PCT /
The method described in JP90 / 01207 is preferable, 1) sodium and magnesium are contained in a lithium niobate single crystal thin film, and 2) the Li / Nb ratio is 41/5.
There is a method of changing between 9 and 56/44, 3) a method of adding an element such as Ti that reduces the a-axis lattice constant of the lithium tantalate substrate, and the method of 1) is advantageous.

The reason for this is that the ions or atoms of sodium and magnesium have the effect of increasing the lattice constant (a axis) of lithium niobate by substitution or doping with respect to the crystal lattice of lithium niobate. By adjusting the composition, it is possible to easily obtain lattice matching between the lithium tantalate substrate and the lithium niobate single crystal, and sodium and magnesium do not impair the optical characteristics at all, and Has an important effect of preventing optical damage.

When sodium and magnesium are contained, the content of sodium is 0.1 to 14.3 with respect to lithium niobate single crystal.
The amounts of mol% and magnesium are preferably 0.8 to 10.8 mol%. The reason is that, when the content of sodium is less than 0.1 mol%, the lattice constant does not become large enough to be lattice-matched with the lithium tantalate substrate, regardless of the amount of magnesium added. .3
On the other hand, if it exceeds mol%, the lattice constant becomes too large, and in any case, the lattice matching between the lithium tantalate substrate and the lithium niobate single crystal cannot be obtained.

If the magnesium content is less than 0.8 mol%, the effect of preventing optical damage is insufficient, and if it exceeds 10.8 mol%, magnesium niobate-based crystals are formed. Since it precipitates, it cannot be contained.

Next, a method of manufacturing the lithium niobate single crystal thin film according to the present invention will be described, but the lithium niobate single crystal thin film of the present invention is not limited to the manufacturing method.

First, in the present invention, a lithium niobate single crystal thin film is lattice-matched to a lithium tantalate substrate to form the single crystal thin film. As a method for producing the lithium niobate single crystal thin film, a liquid phase epitaxial growth method, a sputtering method, a vapor deposition method or the like is desirable, but a liquid phase epitaxial growth method is particularly preferable.

The reason for this is that a homogeneous film having excellent crystallinity can be easily obtained, and as a result, light propagation loss is small, and the nonlinear optical effect and acousto-optical effect of lithium niobate suitable as an optical waveguide can be fully utilized. This is because a lithium niobate single crystal thin film having the above characteristics can be obtained and the productivity is also excellent.

As the liquid phase epitaxial growth method, Li
_The lithium tantalate substrate was brought into contact with a melt composed of ₂ O, Nb ₂ O ₅ , V ₂ O ₅ , Na ₂ O, MgO, etc., and the a-axis lattice constant of the lithium niobate single crystal thin film was measured by liquid phase epitaxial growth. It is preferable to use a method of matching the lattice constant of the a-axis of the lithium oxide substrate because a high quality crystal can be obtained.

In the present invention, Li ₂ O, Nb ₂ O ₅ , V ₂ O ₅ and Na ₂
The reason for using the melt composed of O, MgO, etc. is that Li ₂ O and V ₂ O ₅ act as a flux to realize liquid phase epitaxial growth of a lithium niobate single crystal.

Further, by including Na and Mg in the lithium niobate single crystal thin film, the a-axis lattice constant of the lithium niobate single crystal thin film can be increased, so that the formed lithium niobate single crystal thin film is formed. The lattice constant of the a-axis can be matched with the lattice constant of the a-axis of the lithium tantalate substrate, whereby a lithium niobate single crystal thin film having a large film thickness can be obtained.

The lithium niobate single crystal thin film thus obtained is subjected to polarization reversal treatment. There are the following two types of polarization inversion processing. A. A method of diffusing Ti into a lithium niobate single crystal waveguide, lowering the Curie point, and then heating and cooling at a temperature slightly lower than the Curie point to invert the polarization. B. A lithium sputtered resist was formed on the lithium niobate single crystal waveguide with a photoresist, SiO ₂ was sputtered, and then this was heated so that Li was changed to SiO 2.
Spread to ₂ . Then, SiO ₂ is removed, and the polarization is inverted by heating at a temperature slightly lower than the Curie point and cooling.

These methods will be described with reference to the drawings. A
The method will be described with reference to FIG. A lattice-matched lithium niobate single crystal thin film 2 is grown on a lithium tantalate substrate 1 by an epitaxy method, and a sprite pattern 3 is formed on the lithium niobate single crystal thin film by photolithography (Fig. C). .. Then, a Ti thin film 4 is formed by vacuum deposition of Ti on the entire surface having the sprite pattern (Fig. D), and after removing the sprite pattern (Fig. E), the Ti thin film 4 is heat treated to niobium Ti. It was diffused in the acid single crystal thin film (Fig. F) and then rapidly cooled to obtain a lithium niobate single crystal thin film having the domain inversion layer 5.

A part of the niobate single crystal thin film having the domain inversion layer 5 is cut out (FIG. 3G), and a single-line pattern 6 is drawn in the direction perpendicular to the Ti pattern by photolithography to show Ti. It is vacuum-deposited (Fig. H), and then a ridge type waveguide 6 is formed by ion beam etching.
(Fig. i).

In the method B, as in the method A, a lattice-matched lithium niobate single crystal thin film 2 is provided on the lithium tantalate substrate 1 (FIG. 2B), and the sprite pattern 3 is formed thereon. After forming (Fig. C), SiO ₂
Was evaporated to form a SiO ₂ film 7 (Fig. D), and the sprite pattern was removed (Fig. E). After that, by subjecting this to a heat treatment, Li is diffused into SiO ₂ to form a diffusion portion 7 ′.
After forming, quench rapidly. (Figure f). Further, SiO ₂ was removed by polishing, whereby a lithium niobate single crystal thin film having a domain inversion layer 5 was obtained (FIG. 7G). After that, the Ridge type waveguide 6 is formed in the same manner as the method A (i.
(Figure). Next, the present invention will be described more specifically with reference to Examples.

[0027]

【Example】

Example 1 (1) Na ₂ CO ₃ 21 mol%, Li ₂ CO ₃ 29 mol%, V ₂ O ₅ 40 mol%, Nb ₂ O ₅ 10 mol%, Mg
A mixture in which 2 mol% of O was added to the theoretical amount of LiNbO _{3 that} can be precipitated from the melt composition was placed in a platinum crucible, and was placed in an epitaxial growth and growth apparatus under an air atmosphere,
The contents of the crucible were melted by heating to 1100 ° C. (2) Melt at 912 ° C at a cooling rate of 60 ° C per hour
After slowly cooling down to (000) of lithium tantalate single crystal
1) After optically polishing the surface, the chemically etched product was used as a substrate material and immersed in a melt for 12 minutes while rotating at 100 rpm.

(3) After pulling up the substrate material from the solution body, shaking off the melted material on the melted material at a rotation speed of 1000 rpm for 30 seconds, and then gradually cooling it to room temperature, sodium and magnesium having a thickness of about 12 μm on the substrate material. A contained lithium niobate single crystal thin film was obtained. (4) The obtained lithium niobate single crystal thin film (hereinafter, LN
The amounts of sodium and magnesium contained in the film) were 3 mol% and 2 mol%, respectively. or,
The lattice constant (a-axis) of the thin film is 5.156Å, the incident light wavelength is 0.8
The refractive index measured at 3 μm is 2.173 ± 0.001, 0.00
The refractive index measured at 415 μm was 2.314.
This film was polished to a thickness of 2 μm. At this time, 830
The refractive index of the laser beam of nm in the waveguide mode was 2.165. The refractive index of the 415 nm laser light in the waveguide mode was 2.295.

A sprite pattern with a spacing of 1.60 μm was formed on this by photolithography. Here, Ti was vacuum-deposited to form a 500 Å film. The sprite pattern was removed, and a Ti pattern was formed in the LN film. This is treated in a tubular furnace at 1036 ° C.
By diffusing i into the LN film and then quenching,
An LN film having a domain inversion layer was obtained. A part of this film was cut out, subjected to HF etching, and observed under a microscope. As a result, it was confirmed that only Ti-diffused portions had polarization inversion.

A width of 5 μm is perpendicular to the Ti pattern.
A single line pattern is formed by photolithography, and Ti is vacuum-deposited and lifted off to obtain L
A Ti pattern was formed on the N film, and the LN film was shaved by ion beam etching to form a ridge type waveguide.
(Refer to FIG. 1) The end face of this waveguide is polished to a wavelength of 830 nm.
Then, when the light of the semiconductor laser of 50 mW was made incident, the laser light having the wavelength of 415 nm was emitted from the end face on the opposite side. The conversion efficiency at this time was 7.0%. From this, it was confirmed that this element is an extremely excellent SHG element. The domain period of this device is as follows. (2m + 1) λ / 4 (np-nsh) = 1.60 μm

Example 2 (1) Na ₂ CO ₃ 13 mol%, V ₂ O ₅ 39 mol%, N
b ₂ O ₅ 10 mol%, Li ₂ CO ₃ 38 mol%, MgO
5 mol% was added to the theoretical amount of LiNbO _{3 which} can be precipitated from the melt composition, and the mixture was put into a platinum crucible and placed in an epitaxial growth and growth apparatus under an air atmosphere at 1
The contents of the crucible were dissolved by heating to 100 ° C. (2) The melt was gradually cooled to 938 ° C. at a cooling rate of 60 ° C. per hour, and then a lithium tantalate single crystal (0001) was formed.
Optically polished surface is used as substrate material in melt
It was immersed for 20 minutes while rotating at 0 rpm.

(3) The substrate material was pulled up from the melt, the melt was shaken off on the melt at a rotation speed of 1000 rpm for 30 seconds, then gradually cooled to room temperature, and sodium having a thickness of about 23 μm was deposited on the substrate material. A magnesium-containing lithium niobate single crystal thin film was obtained. (4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were 1 mol% and 6 mol%, respectively. The lattice constant (a-axis) is 5.1
The refractive index measured at 53Å and the incident light wavelength 0.83 μm is
The refractive index measured at 2.231 ± 0.001 and the incident light wavelength of 0.415 μm was 2.372 ± 0.001.

(5) This was polished to form a 1.9 μm film. At this time, the refractive index of the 830 nm laser light in the waveguide mode was 2.162. The refractive index of the 415 nm laser light in the waveguide mode was 2.294. Spacing 4.72 μm on this by photolithography
Sprite pattern was formed. SiO ₂ was sputtered here to form a 0.5 μm film. A SiO ₂ pattern was formed on the LN film by lifting off the sprite pattern. This was processed in a tube furnace at 1000 ° C. When a part of this film was cut out and subjected to HF etching and observed with a microscope, it was observed that the polarization was inverted only where SiO ₂ was on. Surface S
iO ₂ was removed by reactive ion etching to obtain a domain-inverted lithium niobate crystal thin film. (See Figure 2)

A waveguide was cut out from this film, its end face was polished, and when light from a semiconductor laser having a wavelength of 830 nm and a wavelength of 50 mW was incident, the end face on the opposite side had a laser beam of 415 nm wavelength.
The light came out. The conversion efficiency at this time was 4.8%. From this, it was confirmed that this element is an extremely excellent SHG element. The domain period of this device was as follows. (2m + 1) λ / 4 (np-nsh) = 4.72 μm

Example 3 (1) Na ₂ CO ₃ 12.8 mol%, Li ₂ CO ₃ 37.2
LiN capable of precipitating mol%, V ₂ O ₅ 39.0 mol%, Nb ₂ O ₅ 11.0 mol% and Nb ₂ O ₃ from the melt composition.
A mixture in which 0.8 mol% was added to the theoretical amount of bO ₃ was put into a platinum crucible and heated to 1100 ° C. under an air atmosphere in an epitaxial growth and growth apparatus to melt the contents of the crucible. (2) The melt was gradually cooled to 925 ° C. at a cooling rate of 60 ° C. per hour, and then (0001) of lithium tantalate single crystal was obtained.
After the surface was optically polished, the chemically etched product was used as a substrate material and immersed in the melt for 5 minutes while rotating at 100 rpm. (3) Withdrawing the basic material from the melt and rotating at 1000 r
The melt was shaken off at pm for 30 seconds and then gradually cooled to room temperature to obtain a sodium-neodymium-containing lithium niobate single crystal thin film having a thickness of about 8 μm on the substrate material.

(4) The amounts of sodium and neodymium contained in the obtained lithium niobate single crystal thin film were 1.2 mol% and 0.4 mol%, respectively. The lattice constant (a-axis) is 5.153Å, the incident light wavelength is 0.83 μm, 50
The refractive index measured at mW is 2.232 ± 0.001, and the refractive index measured at incident light wavelength 0.415 μm is 2.373 ±.
It was 0.001. (5) This was polished to form a film having a thickness of 2.1 μm. At this time, the refractive index of the 830 nm laser beam in the waveguide mode was 2.166. The refractive index of the 415 nm laser light in the waveguide mode was 2.297. Then, a 4.75 μm sprite pattern was formed by photolithography. SiO ₂ was sputtered here to form a 0.5 μm film. By lifting off the SiO ₂ film, a pattern of SiO ₂ was formed on the LN film. This was processed in a tube furnace at 1000 ° C. in air. When a part of this film was cut out and subjected to HF etching and observed with a microscope, it was observed that the polarization was inverted only where SiO ₂ was on (see FIG. 3).

When a waveguide was cut out from this film and the end face was polished and light of a semiconductor laser having a wavelength of 830 nm was incident, laser light having a wavelength of 415 nm was emitted from the opposite end face. The conversion efficiency at this time was 4.8%. From this, it was confirmed that this element is an extremely excellent SHG element.
The domain period of this device was as follows. (2m + 1) λ / 4 (np-nsh) = 4.75 μm

Example 4 (1) Na ₂ CO ₃ 21 mol%, Li ₂ CO ₃ 29 mol%, V ₂ O ₅ 40 mol%, Nb ₂ O ₅ 10 mol%, Mg
A mixture in which 2 mol% of O was added to the theoretical amount of LiNbO ₃ capable of precipitating from the melt composition was placed in a platinum crucible, and the mixture was placed in an epitaxial growth / growing apparatus under an air atmosphere at 1
The contents of the crucible were dissolved by heating to 100 ° C. (2) The melt was gradually cooled to 914 ° C at a cooling rate of 60 ° C per hour. After optically polishing the (0001) plane of the lithium tantalate single crystal, a film thickness of 500 is obtained by the RF sputtering method.
A substrate material was formed by forming a Mg film of Å and then forming a diffusion layer of 250 Å which was thermally diffused at 1000 ° C. This substrate material had an ordinary refractive index reduced by 1 × 10 ⁻³ as compared with the substrate material in which Mg was not diffused. This substrate material was immersed in the melt for 13 minutes while rotating at 100 rpm.

(3) The substrate material was pulled up from the melt, the melt was shaken off on the melt for 30 seconds at a rotation speed of 1000 rpm, then gradually cooled to room temperature, and sodium and magnesium having a thickness of about 11 μm were deposited on the substrate material. A contained lithium niobate single crystal thin film was obtained. (4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were 3 mol% and 2 mol%, respectively. The lattice constant (a-axis) of the thin film is 5.156Å, the refractive index measured at an incident light wavelength of 0.83 μm is 2.235 ± 0.001, the incident light wavelength is 0.415 μm.
The refractive index measured by 1. was 2.376 ± 0.001.
This film was processed in the same manner as in Example 1 to obtain a lithium niobate single crystal thin film in which a domain inversion layer was formed.

A waveguide was cut out from this film, its end face was polished, and light from a semiconductor laser having a wavelength of 830 nm and 50 mW was incident on it.
The light came out. The conversion efficiency at this time was 5.8%. From this, it was confirmed that this element is an extremely excellent SHG element. The domain period of this device was as follows. (2m + 1) λ / 4 (np-nsh) = 4.44 μm

Example 5 (1) Li ₂ CO ₃ 27.7 mol%, Nb ₂ O ₅ 29.3 mol%, K ₂ CO ₃ 21.5 mol%, V ₂ O ₅ 21.5 mol%, MgO A mixture of 6 mol% of lithium niobate added to the theoretical amount was placed in a platinum crucible and heated to 1100 ° C. under an air atmosphere in an epitaxial growth and growth apparatus to dissolve the contents of the crucible. (2) The melt was gradually cooled to 896 ° C at a cooling rate of 60 ° C per hour. After optically polishing the (0001) plane of the lithium tantalate single crystal, a MgO film having a film thickness of 800 μm and a width of 5 μm and a film thickness of 400 μm was formed on a portion other than the MgO film having a width of 5 μm by photolithography and RF sputtering.
After the Cr film was formed, the film was thermally diffused at 1000 ° C., and one having a MgO diffusion channel with a width of 5 μm was chemically etched to obtain a substrate material. In the channel portion in which MgO was diffused and the portion other than the channel portion in which Cr was diffused, the ordinary refractive index was decreased by 10 × 10 ⁻³ and increased by 1 × 10 ^−3, respectively, as compared with the substrate material in which nothing was diffused. It was This substrate material was immersed in the melt for 11 minutes while rotating at 100 rpm.

(3) The basic material was pulled up from the melt, the melt was shaken off on the melt at a rotation speed of 1000 rpm for 30 seconds, and then gradually cooled to room temperature to contain MgO having a thickness of about 7 μm on the substrate material. A lithium niobate single crystal thin film was obtained. (4) In the same manner as in Example 2, a lithium niobate single crystal thin film having a domain inversion layer was formed. Wavelength 830
LN has a refractive index of 2.173 and a wavelength of 415 with respect to nm.
It was 2.314 with respect to nm. A waveguide is cut out from this film, and the end face of the waveguide is polished to obtain a wavelength of 830 nm.
When light from a semiconductor laser of 0 mW was incident, laser light with a wavelength of 415 nm was emitted from the end face on the opposite side. The conversion efficiency at this time was 6.8%. From this, it was confirmed that this element is an extremely excellent SHG element. The domain period of this device was as follows. (2m + 1) λ / 4 (np-nsh) = 4.44 μm

Example 6 This example is basically the same as Example 1, except that a different element is contained in the substrate and the thin film, and the substrate and the thin film are lattice-matched, and then an SHG element is produced. did. Table 1 shows the conversion efficiency of the SHG element.

[0044]

[Table 1]

Comparative Example In FIG. 4, a 1.47 μm sprite pattern 3 was formed on the LN substrate 1 by photolithography (b).
Figure). Here, Ti was vacuum-deposited to form a 550 Å film (Fig. C). A Ti pattern was formed on the LN substrate by lifting off the Ti film (FIG. D). This was treated at 1035 ° C. in a tubular furnace to diffuse Ti on the LN substrate (Fig. E). When a part of this film was cut out and subjected to HF etching and observed with a microscope, it was observed that the polarization was inverted only where Ti was diffused. This T
A single line pattern having a width of 7 μm was formed perpendicular to the i pattern by photolithography, and Ti was vacuum-deposited and lifted off to form a Ti pattern on the LN film. Only the pattern was treated with benzoic acid at 250 ° C. In this way, a proton exchange waveguide was prepared (Fig. F). The end face of this waveguide is polished to a wavelength of 830n.
When light from a semiconductor laser of m or 50 mW was incident, laser light with a wavelength of 415 nm was emitted from the end face on the opposite side. The conversion efficiency at this time was 0.78. The refractive index of LN was 2.169 for a wavelength of 830 nm and 2.310 for a wavelength of 415 nm.

[0046]

As described above, in the present invention, since the lithium niobate single crystal thin film is lattice-matched with the substrate lithium tantalate, it can be formed with a large film thickness, and in the present invention. Is heated to a temperature slightly higher than the Curie point without performing proton exchange as in the conventional case, and is then subjected to domain inversion treatment by rapid cooling, so that high conversion can be achieved without changing the crystal structure of the lithium niobate single crystal thin film. An efficient SHG can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a manufacturing process of Examples 1, 4, and 6 of a lithium niobate single crystal thin film according to the present invention.

FIG. 2 is a diagram showing a manufacturing process of Examples 2 and 5 of a lithium niobate single crystal thin film according to the present invention.

FIG. 3 is a diagram showing a manufacturing process of Example 3 of the lithium niobate single crystal thin film according to the invention.

FIG. 4 is a diagram showing a manufacturing process of a conventional lithium niobate single crystal thin film.

[Explanation of symbols]

1 Lithium tantalate substrate 2 Lithium niobate single crystal thin film (LN film) 3 Sprite pattern 4 Ti thin film 4'Ti diffusion part 5 Domain inversion layer 6 Waveguide 7 SiO ₂ film 7 'SiO ₂ diffusion part 8 Proton exchange conduction Waveguide

Claims (1)

Claim: What is claimed is: 1. A lithium niobate single crystal thin film which is lattice-matched on a lithium tantalate substrate and which has been domain-inverted so that the polarization is inverted in the crystal axis direction at every period.