JPH0437697A - Thin film of lithium niobate single crystal - Google Patents

Abstract

PURPOSE:To produce the title thin film having a small optical transmission loss and superior optical characteristics by forming a thin film of a lithium niobate single crystal on a lithium tantalate substrate added with an element other than Li and Ta to a part of the surface so that the lattice of the thin film is matched to that of the substrate. CONSTITUTION:An element other than Li and Ta, e.g. Mg or Ti is added to at least a part of the surface of a lithium tantalate substrate to form a pattern having a relatively low refractive index at a waveguide forming part. A thin film of a lithium niobate single crystal is then formed on the substrate so that the lattice of the thin film is matched to that of the substrate and a thin film of a lithium niobate single crystal having <=1.4dB/cm optical transmission loss is produced. Since the thin film formed on the above-mentioned pattern becomes a waveguide, a waveguide forming process can be eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a lithium niobate single crystal thin film having a thickness suitable for various optical materials including thin film waveguide type SHC elements.

(Prior Art) With the recent progress in optical application technology, there is a demand for shorter wavelength laser light sources.

This is because recording density and photosensitivity can be improved by shortening the wavelength, and application to optical equipment fields such as optical discs and laser printers is possible.

For this reason, research has been conducted on second harmonic generation (SHG) elements that can convert the wavelength of incident laser light to 1/2.

As such a second harmonic generation (SHO) element, a bulk single crystal of a nonlinear optical crystal has conventionally been used using a high-output gas laser as a light source. However, there is a strong demand for miniaturization of devices such as optical disk devices and laser printers, and while gas lasers require an external modulator for optical modulation, semiconductor lasers cannot be directly modulated. Semiconductor lasers have come to be mainly used in place of gas lasers due to their availability and low cost. Therefore, since it is necessary to obtain high conversion efficiency with a low light source output of several mW to several tens of mW, a thin film waveguide type SHG element has become necessary.

Conventional nonlinear optical materials for thin-film waveguide-type SHG devices include waveguides made of layers whose refractive index is changed by diffusing Ti or the like into a lithium niobate single crystal bulk, and tantalum. Waveguides are known that use a lithium niobate thin film formed on a lithium oxide substrate by high-frequency sputtering, but in either case, it is difficult to obtain a lithium niobate thin film with excellent crystallinity, and a high conversion rate is required. Couldn't get efficiency.

A liquid phase epitaxial method is considered to be suitable as a method for producing a single crystal thin film with excellent crystallinity, and therefore various liquid phase epitaxial methods have been studied.

Examples of the liquid phase epitaxial method for obtaining a lithium niobate single crystal thin film include: 1) Applied Physics Letters
, Vol. 26, No. IJanuary 1975
, using lithium tantalate as a substrate, Li technology ○, ■
20. An example is described in which a lithium niobate thin film for an optical waveguide is formed by a liquid phase Evita kinial growth method using as a flux to guide light.

2) Japanese Patent Publication No. 51-9720 describes a method of forming a lithium niobate thin film for a leading waveguide by a liquid phase epitaxial growth method using lithium tantalate as a substrate and L1□0, ■20 as a flux. Are listed.

Furthermore, 3) Japanese Patent Publication No. 56-47160 states that L1□
0, Mg-containing lithium diophate was grown on the substrate by epitaxial growth using VzOs as a flux.
A method for forming lithium tantalate solid solution thin film single crystals is described.

However, with the conventional liquid-phase Evita-Kinoyal method, not only is it impossible to obtain a lithium niobate single crystal with excellent crystallinity on a lithium tantalate substrate, but it is also difficult to manufacture SHG devices. It was difficult to obtain a lithium niobate single crystal with the required film thickness.

The film thickness required to manufacture the thin film waveguide type SHO element is the thickness of the fundamental wavelength light of wavelength λ and the wavelength of λ/2 in order to phase match the incident laser light and the second harmonic. This refers to the film thickness that allows the effective refractive index to match that of the second harmonic, and in particular, using a lithium niobate thin film formed on a lithium tantalate substrate, S
When creating an HG element, a lithium niobate thin film with a thickness of 5 μm or more is required in order to match the effective refractive index.

In addition, in order to obtain a high-output thin-film waveguide type SHG device, it is necessary to increase the difference in refractive index between the substrate and the thin-film waveguide layer, and various elements must be added to increase the refractive index of the thin-film waveguide layer. Since the optical properties of the waveguide layer may deteriorate when added, research is being conducted to reduce the refractive index of the substrate. By diffusing vanadium pentoxide into the
A technique has been disclosed in which a diffusion layer with a low refractive index is formed and a tantalum lithium single crystal layer is grown on the diffusion layer in an evita-crystalline manner.

Further, Japanese Patent Publication No. 560-34722 discloses a technique for epitaxially growing a lithium tantalate single crystal layer by simultaneously adding magnesium oxide and vanadium pentoxide to a lithium phosphate substrate.

However, these techniques use lithium tantalate as a thin film waveguide layer, and the linear optical effect is inferior to that of lithium niobate.

Furthermore, since the lattice constant of lithium tantalate, which is the thin film waveguide layer, cannot be changed, it is necessary to add magnesium oxide or vanadium pentoxide within a range that does not change the lattice constant of the substrate, and there is a limit to the amount that can be added. .

Furthermore, in the above technique, the refractive index on the substrate side must be lowered, and the range in which the refractive index can be varied is limited.

As described above, until now, lithium niobate monoliths, which have a large refractive index difference with the substrate and have excellent optical properties, have been used with the film thickness necessary to create optical devices such as SHO elements on lithium tantalate substrates. There was no practical way to produce crystals.

In order to solve these problems, the inventors of the present invention conducted various researches, and found that by incorporating various different elements into a notium tantalate substrate and changing the refractive index, the lithium tantalate substrate and lithium niobate substrate were combined. By lattice matching with a single crystal thin film, it has a film thickness necessary and sufficient to create optical devices such as SHG devices, and is free from optical damage (change in the refractive index of the crystal when irradiated with strong light) and The present invention was completed based on the new finding that it is possible to practically obtain a lithium niobate single crystal thin film with extremely excellent optical properties such as extremely low optical propagation loss and a large refractive index difference between the substrate and the thin film waveguide layer. I came to the conclusion.

(Means for Solving the Problems) The present invention provides a lithium niobate single crystal thin film formed on a lithium quantalate substrate, wherein the lithium tantalate substrate has a different element added to at least a part of its surface. The thin film is a lithium niobate single crystal thin film which is lattice matched with the lithium tantalate substrate and has a light propagation loss of 1.4 dB/cm or less.

(Function) The lithium niobate single crystal thin film of the present invention is formed on a lithium tantalate substrate, and the lithium tantalate substrate needs to contain a different element at least in a part of its surface.

The reason for this is that by including a different element, the refractive index of the substrate can be changed, and the difference in refractive index between the substrate and the thin film waveguide layer can be increased.

The above-mentioned different element refers to an element different from the element constituting the substrate.

The different element is preferably a metal element.

The different elements include magnonium, titanium, vanadium,
At least one selected from chromium, iron, nickel, neodymium, etc. is desirable.

In the present invention, a different element is added to a specific location of the lithium tantalate substrate to form a pattern in which the waveguide formation portion has a relatively lower refractive index than the non-formation portion. By simply forming a lithium niobate single crystal thin film in the form of a slab, the lithium niobate single crystal thin film formed in the pattern portion becomes a waveguide, and a processing step for forming the waveguide can be omitted.

The refractive index of the substrate in the waveguide forming portion is relatively lower than that in the non-forming portion. desirable.

The different elements having the effect of increasing the refractive index of the lithium tantalate substrate include Ti, Cr, Nd, Fe, and Ni.
Mg, V, etc. are advantageous as foreign elements having the effect of lowering the refractive index.

These elements can change only the surface refractive index without changing the properties that affect thin film formation on the substrate, such as surface roughness, so three films with the same characteristics as a normal substrate can be It can be manufactured under certain conditions.

Further, the content of the different element in the surface portion is preferably within the composition range shown below.

Ti; 0.2 to 30 mol% Cr; 0.02 to 20 mol% Fe; 0.02 to 20 mol% Ni; 0.02 to 20 mol% Nd; 0.02 to 10 mol% Mg; 0.1 ~20 mol% ■ ; 0.05 ~ 30 mol% The above content is: Different element/(L i Ta 03 + different element) x 10
0. It was calculated as follows.

The reason why the above composition range is preferable is that if the composition ratio is higher than the above range, the crystallinity of the substrate will decrease, and if the composition ratio is lower than the above range, the refractive index will not change.

Furthermore, the content of the different element is preferably within the range shown below.

Ti; 1.0 to 15 mol% Cr; 0.2 to 10 mol% Fe; 0.2 to 10 mol% Ni; 0.2 to 10 mol% Nd; 0.5 to 5 mol% Mg; 2.0 ~10 mol% V; 1.0 to 15 mol% The above content is: different elements/(Li TaOs 10 different elements) x 1
00, is calculated as follows.

Further, the different element can be contained in the lithium tantalate substrate in various forms such as atoms, ions, and oxides.

The lithium niobate single crystal thin film of the present invention needs to be formed on a lithium tantalate substrate containing a different element, and lattice-matched with the lithium tantalate substrate containing the different element. The reason for this is that by lattice-matching the lithium tantalate substrate containing a different element and the lithium niobate single crystal thin film, a lithium niobate single crystal thin film with extremely excellent optical properties can be produced with a thick film thickness that cannot be obtained with conventional techniques. This is because it is formed by

In addition, the reason why the formed lithium niobate single crystal thin film has extremely excellent optical properties is that the lithium niobate single crystal thin film and the lithium tantalate substrate containing different elements are lattice-matched and are integrated with the substrate. This is because the film has extremely low distortion and crystal defects, has excellent crystallinity, and has no micro-crank or the like, resulting in a high-quality film.

In the prior art, it was not possible to produce a thin film that had lattice matching with the substrate, so the crystallinity of the thin film was poor and microcracks were observed in the thin film. Therefore, by smoothing the substrate material surface and the thin film surface by polishing or chemical unoching, or by using high-purity raw materials with a low proportion of impurities, scattering loss at the substrate interface and thin film surface and absorption due to impurity contamination can be reduced. Even if the loss is reduced to some extent, the absorption/scattering loss at grain boundaries or the scattering loss due to microcracks becomes extremely large, resulting in an optical propagation loss of 3 to 5 dB/c.
It was large, about m, and could not be used for optical purposes.

In the present invention, the means for lattice matching the lithium niobate single crystal thin film and the lithium tantalate substrate containing different elements is not particularly limited, but it is possible to incorporate sodium and magnesium into the lithium niobate single crystal thin film. It's advantageous. The reason for this is that sodium and magnesium ions or atoms have the effect of increasing the lattice constant (a-axis) of lithium niobate by substitution or doping in the crystal lattice of lithium niobate, so the composition of sodium and magnesium can be adjusted. By doing so, it is possible to easily obtain lattice matching between the lithium tantalate substrate containing dissimilar elements and the lithium niobate single crystal.Furthermore, sodium and magnesium not only do not impair optical properties in any way, but magnesium also This is because it also has the important effect of preventing optical damage. In addition, when sodium and magnesium are contained, their content is preferably 0.1 to 4.8 mol% and 0.8 to 10.8 mol%, respectively, relative to the lithium niobate single crystal. The reason for this is that when the sodium content is less than 0.1 mol%, the lattice constant is high enough to lattice match with the lithium tantalate substrate containing a different element, regardless of the amount of magnonium added. If it does not increase, and if it exceeds 4 or 8 mol%, the lattice constant becomes too large, and in either case, lattice matching between the lithium tantalate substrate containing a different element and the lithium niobate single crystal cannot be obtained. It is.

Furthermore, if the magnesium content is less than 0.8 mol%, the effect of preventing photodamage is insufficient, and 10
．． If it exceeds 8 mol %, it cannot be contained because niobate-based crystals will precipitate.

Regarding the sodium content, Journal
or Crystal Growth 54 (
1981) 572-576, added sodium to lithium niobate and produced Y-
Journal of Crystal Gr is an example of forming a thin film single crystal of sodium-containing lithium niobate with a film thickness of 20 μm on a cut lithium niobate substrate.
owth 84 (1987) 409-412 describes an example of adding sodium to lithium niobate and forming a sodium-containing lithium niobate thin film single crystal on a lithium tantalate substrate using liquid phase epitaxial growth. ing.

However, although these documents describe that the lattice constant of lithium niobate single crystal changes due to sodium content, SAW (Su r race Ac
This is a technology related to (Austic Wave) devices, and there is no mention of optical properties or the fact that a film with excellent optical properties can be obtained by lattice matching with a lithium tantalate substrate.

Furthermore, the lithium niobate single crystal thin films shown in these documents are for SAW devices, and the thin films described in the former document use lithium niobate for the substrate. Although these are formed on a lithium tantalate substrate, none of them can be used as the optical material intended for the purpose of the present application due to the lack of lattice matching between the thin film and the substrate.

Additionally, US Pat. No. 4,093,781 discloses that when forming a lithium ferrite film on a substrate by liquid phase epitaxial growth, lithium is replaced with sodium to match the lattice constant to that of the substrate, thereby forming a strain-free lithium ferrite film. The method is described.

However, this technique is related to lithium ferrite and cannot be used as an optical material aimed at in the present application, and neither technique impedes the novelty or inventive step of the present invention.

The reason why lithium tantalate is used as a substrate in the present invention is that the crystal system of the lithium tantalate substrate is a lithium niobate single crystal [IL] and is easy to epitaxially grow, and the lithium tantalate substrate is commercially available. This is because products of good quality can be stably obtained.

Furthermore, as the lithium tantalate substrate used in the present invention, those containing various elements to change the lattice constant, refractive index, etc., or those whose surfaces have been treated by chemical etching, etc. can be used.

In particular, in the present invention, the (00
01) It is desirable to use a plane.

The (0001) plane of the lithium tantalate substrate refers to a plane perpendicular to the C axis of lithium tantalate.

The reason why it is desirable to use the (0001) plane of the lithium tantalate substrate as the growth plane of the lithium niobate single crystal thin film is that the crystal structure of the lithium tantalate is macrogonal (see Figure 1). This is because the (0001) plane is composed of only the a-axis, so it can be lattice-matched with the lithium niobate single crystal thin film simply by changing the lattice constant of the a-axis.

The lithium niobate single crystal thin film of the present invention needs to have a light propagation loss of 1.4 dB/ci or less.

Optical propagation loss indicates the rate of decrease in luminous intensity per unit length in the traveling direction of light when light is guided through a thin film, and includes scattering loss and absorption loss. The scattering is lost,
It depends on the state of the interface between the substrate and the thin film, the surface state of the thin film, microcracks in the thin film, etc.

On the other hand, absorption loss is related only to the characteristics of the thin film, and depends on the crystallinity of the thin film, the proportion of impurities mixed therein, and the like.

Furthermore, the lattice constant (a
axis) is 9 of the lithium tantalate substrate containing a different element.
It is desirable that it is 9.81 to 100.07%, and 99
．． 92-100.03% is suitable.

For example, the lattice constant of the lithium tantalate substrate is 5.1
If there are 53 people, it is desirable that the number is 5.150 to 5.155.

The reason for this is that when the lattice constant is outside the above range, it is difficult to match the lattice constants of the lithium tantalate substrate containing different elements and the lithium niobate single crystal thin film, which makes it difficult to match the lattice constants of the lithium tantalate substrate containing different elements and the lithium niobate single crystal thin film, which can be used as an optical material. This is because it is not possible to form a sufficiently thick crystal thin film.

Further, the thickness of the lithium niobate single crystal thin film is 5 μm.
The thickness is preferably 10 μm or more, and particularly preferably 10 μm or more.

In particular, the lithium niobate single crystal thin film of the present invention is
When used as an HG element, the ordinary refractive index n0 and extraordinary refractive index n3 of the lithium niobate single crystal thin film are as follows:
For a laser light source (fundamental wavelength) with a wavelength of 0.83 μm, the range of 2.25≦00≦2.40, 2.0
< n * < no o, (range of 11,
Also, the extraordinary refractive index n for the generated second high wavelength (0.415 μm) is the ordinary refractive index n0 for the second harmonic.
A smaller range is desirable.

The lithium niobate single crystal thin film of the present invention also contains Nd in order to change optical properties such as refractive index as necessary.
, Rh, Zn, Ni, Co, Ti, and the like.

When creating an SHG element using the lithium niobate single crystal thin film of the present invention, the transmittance of the second harmonic light is close to 100% or 100%, and the fundamental laser light is hardly transmitted or not transmitted at all. Wavelength selective thin film (
It is desirable to form a filter) behind the light output end face or directly on the light output end face.

The reason for this is that unnecessary fundamental laser light can be removed from the emitted light and only the necessary second harmonic light can be extracted efficiently.

In addition, by forming the wavelength-selective thin film directly on the emission end face and adjusting it so as to satisfy the anti-reflection condition for second harmonic light, there is a large difference in the refractive index between the lithium niobate single crystal thin film layer and air. Because of this, it is possible to reduce loss due to reflection of second harmonic light generated at the output end face, and SHC,
Output can be improved.

The wavelength-selective thin film may be formed at a position remote from the output end face behind the output end face, or may be fixed on the output end face using a suitable adhesive.

When fixing on the output end face using the adhesive, the refractive index and thickness of the adhesive layer may be adjusted to meet the anti-reflection conditions for the second harmonic light to improve the G output. desirable.

Furthermore, in the lithium niobate single crystal thin film of the present invention, SHG
When producing the device, it is desirable to integrate the SHG device and the hair chip of the semiconductor laser.

The reason for this is that it is better to integrate the SHG element and the semiconductor laser hair chip because it is more compact, mass-producible, cost efficient,
This is because it is advantageous in terms of stability.

Further, the semiconductor laser device in which the SHO device and the semiconductor laser light source are integrated is desirably sealed in a sealed package in order to protect the hair chip of the semiconductor laser and extend its life.

It is desirable that an active gas is sealed in the pan cage.

Advantageously, said inert gas is nitrogen.

Further, it is necessary that the hermetically sealed package is provided with a window through which the second harmonic light is emitted.

The window for emitting the second harmonic light refers to a hole that is large enough to emit the second harmonic light, and a translucent plate is fitted into the hole to maintain airtightness.

It is desirable that the light-transmitting plate has wavelength selectivity.

The reason for this is that by forming a window for first harmonic light emission using a wavelength-selective light-transmitting plate, the chip can be hermetically sealed.
It can serve all purposes such as removing the fundamental wavelength light, improving the transmittance of the second harmonic light, and protecting the semiconductor laser hair chip and SHG element, and the window for the second harmonic light emission is made of ordinary glass. However, this is because the manufacturing process can be simplified, costs can be reduced, and the transmittance of second harmonic light can be improved compared to when a wavelength-selective film is provided on the inside or outside of the window glass.

As the wavelength selective thin film or transparent plate,
A colored glass filter, a glass substrate coated with a wavelength-selective interference film, etc. can be used.

As the material of the wavelength selective thin film or the light-transmitting plate, oxides such as 5iOz, MgO1ZnO1An20, L i N b O, l, L i T a Oz
, 'l 3 C; a 50 +z, Gdi Gas
Composite oxides such as 012, organic substances such as PMMA and MNA, etc. can be used, and multilayer thin films made by stacking these can also be used.

Methods for producing the wavelength-selective thin film include sputtering method, liquid phase epitaxial method, vapor deposition method, and MBE (Molecular Beam Epitaxial method).
M Epitaxial) method, MOCVD (Meta
l Organic Chemical Vap
or Deposit.

It is preferable to use a method such as n) method, ion blating method, LB method, spin code method, dip method, etc.

Next, the manufacturing method of the present invention will be explained.

In the present invention, it is necessary to add a different element to at least a portion of the surface of the lithium tantalate substrate, and then form a lithium niobate single crystal thin film that is lattice-matched to the lithium tantalate substrate.

Methods for adding the different elements include thermal diffusion, ion exchange, ion implantation, etc., as well as liquid phase epitaxial growth, a method of mixing different elements in advance into the raw material of the lithium tantalate bulk single crystal, etc. Can be used.

Further, when the thermal diffusion, ion exchange, or ion implantation method is used, a diffusion layer of a different element is formed, and the diffusion layer preferably has a thickness of 1 μm or more.

The reason for this is that if the thickness of the diffusion layer is less than 1 μm, a large proportion of the guided light will spread to the parts of the substrate where different elements are not diffused, making it impossible to satisfy the refractive index required for the substrate. First, sufficient characteristics as a leading wavepath cannot be obtained.

Desirable methods for producing the lithium niobate single crystal thin film of the present invention include liquid phase epitaxial growth, sputtering, and vapor deposition, with liquid phase epitaxial growth being particularly preferred.

The reason for this is that it is easy to obtain a homogeneous film with excellent crystallinity, and as a result, it has low optical propagation loss and is suitable as an optical waveguide.In addition, the lithium niobate single crystal has nonlinear optical effects, electro-optic effects, and acousto-optic effects. This is because a lithium niobate single-crystal thin film with excellent properties that can take full advantage of the properties can be obtained, and it also has excellent productivity.

The liquid phase epitaxial growth method includes Liz O,
A lithium tantalate substrate containing a different element is brought into contact with a melt consisting of V203, Nb2O5, Na, O, MgO, etc., and the lattice constant of the a-axis of the lithium niobate single crystal thin film is changed to that of the lithium tantalate substrate containing a different element by epitaxial growth. It is preferable to use a method of matching the lattice constant of the axis, as this method yields high quality crystals.

Ll, ○, Vz O in the epitaxial growth method
Desirable ranges for the three components of s and N b t Os are shown below.

(Litu, NtgOs, v, o, ) Composition range (
1)) Li, 05VzOs, NbzOs
(7) The composition range is L it 0-Vz 0s-Nb
, Os (In the phase diagram of the D3 component system, A(49,4
9,45,46,5,05), B (42,81,22
,94°34, 25), C(11,11,80,00
. /Liz 0 is 20/98
．． 0-93.5/6.5, molar ratio MgO/NbtO
5 is preferably within a composition range satisfying 0.2/99.8 to 40.0/60.0.

The melt mainly contains Li, 0, V 20 s, Nb
The reason why it is necessary to consist of z Os , Naz 01Mg○ will be explained below.

Li, O and ■20. can act as a flux to realize liquid phase epitaxial growth of lithium niobate single crystals.

Furthermore, by incorporating Na and Mg into the lithium niobate single crystal thin film, the a-axis lattice constant of the lithium niobate single crystal thin film can be increased. Since the constant can be adjusted to the a-axis lattice constant of the lithium tantalate substrate containing a different element, a lithium niobate single crystal thin film having a large thickness can be obtained.

Further, in the present invention, LizOlVzOs, Nbzo
The composition range of s is LizO-V20sNb, ○, 3
In the triangular diagram of the component system, A(49, 49, 45, 46
, 5, 05), B (42, 81, 22, 94, 34,
25) The reason why the composition ratio is required is within the composition region surrounded by the three composition points C(1111, 80, 00, 8, 89) and does not include the composition line connecting the composition points A and B. This is because the optical properties of the lithium niobate thin film obtained within this range are excellent, especially the light propagation loss is low, and a high quality lithium niobate single crystal thin film can be obtained.

The composition range of L iz OVz Os N bz Os is D(47, 64,
46,12,6,24), E(27,0164,69,
8,30), F (36,71,3797,25,32
), C(44,05,32,97,22,98), and H(45,36,46
,458,19),! (32,89,57,0510
06), J (36,71,44,30,18,99)
, K (44, 95, 40, 54, 14, 51).

In addition, as the composition ratio of Nag O, the molar ratio is N
a 20/L + z O is 2.0/98.0-9
The reason why it is desirable that the range satisfies 3.5/6.5 is that when the proportion of NazO is lower than the above molar ratio range,
Even if MgO is contained in a limited amount, the a-axis lattice constant of the lithium niobate single crystal thin film cannot be made large enough to achieve lattice matching with a lithium tantalate single crystal substrate containing a different element, and the molar ratio This is because if the ratio of Na, 0 is higher than the range, the a-axis lattice constant becomes too large, making it difficult to lattice match the lithium tantalate single crystal substrate containing a different element.

As the composition ratio of the Nat O, the molar ratio is Na
20/L 1z O is 7.4/92.6~80
．． It is preferable that the range satisfies 0/20.0, and in particular, the molar ratio of Na t O/L i t O is 16
．． A range of less than 7/83.3 to 48.4151.6 is preferred.

In addition, as the composition ratio of MgO, the molar ratio is MgO/b
20. The reason why it is desirable that the composition range is from 0.2/99.8 to 40.0/60.0 is that if the proportion of MgO is lower than the above range, the optical damage prevention effect of Mg is insufficient. If the MgO content is higher than the above range, magnenium niobate crystals will precipitate, making it impossible to obtain a monocrystalline thin film of sulfur niobate.

As the composition ratio of MgO, the molar ratio is MgO/N.
bzOs is 0. 7150. 0-9. 0150.
A composition range that satisfies 0 is preferable, and the molar ratio is Mgo/Nb
,0. However, a composition range satisfying 3.5150.0 to 6.0150.0 is suitable.

(Li2O, NbJs, v, o, (7) Composition range (2
)) The melt used in the liquid phase epitaxial growth method mainly includes LiA0, ■AOs, Nb205, Na
2O, MgO, and the above LizO120s, N
The composition range of bz Os is LixOVz Os N
In the triangular diagram of the three-component system of bz Os, A' (8
B,90,2.22.8.88),B'(55,0
0,43,00,2,00), C' (46,50,
5], 50,2.00), D" (37,505,00
.

Said Li2O, V2O5, Nb2O5, Na2O, M
The action of gO is similar to that explained in composition range (1).

Furthermore, the Li, O1V! O5, Nby Os (D
The composition range is Liz OVz 05 Nbz Os
In the phase diagram of the three-component system, E" (69, 85, 2
1,33,8,82), F” (49,95,45゜0
2.5.03), G' (44, 13, 16, 76, 3
9, 11), H' (54, 72, 11, 12, 34,
16) The composition ratio is preferably shown in the region surrounded by the four composition points, L1□0Vz Os Nb10
In the triangular diagram of the three-component system of 5, I' (57, 43
, 35, 05, 7, 52), J' (49,9542
, 53, 7, 52), K' (47, 36, 26, 3
2,26°32), L' (56,38,17,91
, 25, 71) is preferably the composition ratio indicated by the region surrounded by the four composition points.

In addition, as for the composition ratio of Na and O, the molar ratio is Naz O/
L z○ is 2.0/98.0~93°5/6
．． It is desirable that the range satisfies 5.

The reason for this is that if the proportion of NatO deviates from the above molar ratio range, it is impossible to lattice match the lithium niobate single crystal thin film and the lithium tantalate substrate containing a different element.

The composition ratio of Na and O is Na z
o/Lit○ is 7.4/92.6~80°0
It is preferable to satisfy /20.0, and 16.7/83.
It is preferable that the range satisfies 3 to 48.4151.6.

In addition, as the composition ratio of MgO, the molar ratio is MgO/
Lithium niobate is 0.1/99.9 to 25.0/7
It is desirable that the range satisfies 5.0.

The reason for this is that when the proportion of MgO is lower than the above range,
The effect of preventing Mg from optical damage is insufficient, and MgO
This is because if the ratio is high, magnonium niobate crystals will precipitate, making it impossible to obtain a lithium niobate single crystal thin film.

The above-mentioned lithium niobate refers to the theoretical amount of lithium niobate that can be precipitated from the melt composition.

Further, as for the composition ratio of MgO, it is preferable that the molar ratio of MgO/lithium niobate is in the range of 0.7/100 to less than 9°0/100, and 3.5/100 to 6.
It is preferable that it is less than 0/100.

In this case, the raw material component is preferably an oxide or a compound that changes into an oxide upon heating, such as NatCOs, NbzOs, LizCOx,
Examples include compositions of VzOsMgO.

The raw material components are preferably heated and melted at 600 to 1300'C in an air atmosphere or an oxidizing atmosphere.

After the melt is supercooled, it is preferable to bring it into contact with a lithium tantalate substrate containing a different element and grow it.

The cooling rate for bringing the melt into a supercooled state is 0.
The rate is preferably 5 to 300°C/hour.

Further, the temperature for the growth is preferably 600 to 1250°C, because the melting point of lithium niobate is 1250°C, and crystals do not crystallize at temperatures higher than 600°C. 'C is the melting point of the melting agent, so
This is because the raw material cannot be made into a molten liquid at a temperature lower than this.

During the growth, it is desirable to rotate the substrate.
This is because crystals with uniform properties and film thickness can be formed by rotating the substrate.

Further, it is preferable that at least the surface of the lithium tantalate substrate containing a different element is optically polished and then chemically etched. The thickness of the lithium single crystal thin film is determined by the contact time between the substrate and the melt,
The temperature of the melt can be controlled by appropriately selecting the time. In the present invention, the melt composition includes Nb2O2, V2O3, LizO, NazO, MgO.
In addition to NdR11+ Zn, N+, Co, T
Oxides of elements selected from +, etc. can be used.

Next, examples of the present invention will be described.

Example 1 (1) NazCOs 20 mol χ, Li, C0,3
0 mol χ, v, o, 40 mol χ, NbzOs 1
A mixture containing 0 mol χ and 2 mol χ added to 1 NbOz was placed in a platinum crucible and heated to 1100°C in an air atmosphere in an epitaxial growth growth apparatus to dissolve the contents of the crucible.

(2) The melt was cooled at a cooling rate of 60°C per hour.
It was slowly cooled down to °C. Lithium tantalate single crystal (00
01) After optically polishing the surface, a V film with a thickness of 500 mm was formed by RF sputtering, thermally diffused at 1000°C, and chemically etched to obtain a substrate material. The ordinary light refractive index of this substrate material was reduced by I Xl0-j compared to a substrate material that does not diffuse (1). This substrate material was immersed in the melt for 13 minutes while rotating at 100 rpm+s.

(3) Pull up the substrate material from the solution and rotate at 1000 rotations.
"After shaking off the melt on the melt for 30 seconds at p-, it was slowly cooled to room temperature to obtain a single crystalline lithium niobate film containing sodium and magnosium with a thickness of about 11 μm on the substrate material. .

(4) The amounts of sodium and mag7sium in the obtained lithium niobate single crystal thin film were 3 mol χ and 2 mol χ, respectively.
Met. The lattice constant (a-axis) of the thin film is 5°156, and the refractive index measured at an incident light wavelength of 1.15 μm is 2.23.
It was 0.001 on May 5th.

Example 2 (II LizCOs 50 mol χ, Shi!O 540 mol χ, Nb2O2 10 mol χ, NazCOs to LiN
b0. 45 mol χ addition, MgO to LiNbO
A mixture in which 7 mol χ was added to x was put in a platinum crucible, and 1
The contents of the crucible were dissolved by heating to 100°C.

(2) Cool the melt at a cooling rate of 60°C per hour to 918°C.
It was slowly cooled down to °C. Lithium tantalate single crystal (00
01) After optically polishing the surface, a film with a thickness of 1,000 mm and a width of 5 μm was formed by photolithography and RF sputtering.
After forming the gO film, it was thermally diffused at 1000°C, and a material having an MgO diffusion channel with a width of 5 μm was used as a substrate material. The ordinary refractive index of this channel part was 15xlO-' [slightly lower than that of the part where MgO was not diffused.

This substrate material was immersed in the melt for 9 minutes while rotating at 1100 rpm.

(3) Pull up the substrate material from the melt, shake it off on the melt for 30 seconds at a rotation speed of 100° rpm,
The mixture was slowly cooled to room temperature, and a lithium niobate single crystal thin film containing sodium and magnesium with a thickness of about 37 μm was obtained on the substrate material.

(4) The amounts of sodium and magnesium in the obtained lithium niobate single crystal thin film were 2 mol χ and 6 mol χ, respectively.
Met. The lattice constant (a-axis) is 5.155, and the refractive index measured at an incident light wavelength of 1.15 μm is 2.231 ± 0.
．． It was 001.

(5) The obtained lithium niobate single crystal thin film was
When the end face was polished perpendicularly to the -go diffusion channel of m, the laser beam was incident on the end face, and the near field pattern of the emitted light was observed.
It was confirmed that the gO was well confined on the diffusion channel.

Example 3 (]) NaZCO312 mol χ, VzOs 4O-
ID, NbzOs 10 mol χ, LizCCh 38 mol χ, ? IgO to LiNb0. A mixture to which 5 mol χ was added was placed in a platinum crucible and heated to 1100° C. in an air atmosphere in an epitaxial growth and growth apparatus to melt the contents of the crucible.

(2)? g melt at a cooling rate of 60 °C per hour
It was slowly cooled to 38'C. Lithium tantalate single crystal (
0001) After optically polishing the surface, a window with a radius of 5 μm was formed using photolithography and the RF Svanta method.
After forming an iJ film of 400 layers, it was thermally diffused at 1000'C, and a material having Ti undiffused channels with a radius of 5 μm was used as a substrate material. The Ti diffused portion had an ordinary refractive index increased by 2×10−′ compared to the channel portion.

This substrate material was immersed in the melt for 15 minutes while rotating at 100 rpm+m.

(3) Pull up the basic material from the melt and rotate at 1000 rotations.
After shaking off the melt at rp+w for 30 seconds, the melt was slowly cooled to room temperature to obtain a single crystal thin film of lithium niobate containing sodium and magnesium with a thickness of about 17 μm on the substrate material.

(4) The amounts of sodium and magnesium in the obtained single crystal thin film of lithium niobate were 1 mol χ 6 mol χ
Met. The lattice constant (a-axis) is 5.153, and the refractive index measured at an incident light wavelength of 1.15 μm is 2.231 ± 0.
．． It was 001.

(5) The obtained lithium niobate single crystal thin film was
When the end face was polished perpendicularly to the Ti undiffused channel with a width of 5 μm, the laser beam was incident on the end face, and the near field pattern of the emitted light was observed.
It was confirmed that it was well confined on the i-undiffused channel.

Example 4 (1) NazCOi 12.8 mol χ, LitC (
h 37.2 mol χ, V, O.

40.0 mol χ, NbzOs 10.0 mol χ, N
A mixture in which 0.8 mol χ of d, co, and LiNb were added to 0° was placed in a platinum crucible and heated to 1100° C. in an air atmosphere in an epitaxial growth growth apparatus to melt the contents of the crucible.

(2) Cool the melt at a cooling rate of 60°C per hour to 927°C.
After slow cooling to °C, lithium tantalate single crystal (0
001) After optically polishing the surface, by RF spa treatment,
After forming a Ni1l film with a thickness of 400 nm, it was thermally diffused at 1000'C, and then chemically etched to form a substrate material. This substrate material had an increased ordinary refractive index lXl0-' compared to a substrate material that does not diffuse Ni. The substrate material was immersed in the melt for 7 minutes while rotating at 100 rpm.

(3) 78 Pull up the basic material from the melt and rotate at 10
After shaking off the melt at 00 rpm for 30 seconds, the melt was slowly cooled to room temperature to obtain a single crystal thin film of lithium niobate containing sodium and neodymium with a thickness of about 11 μm on the substrate material.

(4) The amounts of sodium and odium oxide in the obtained lithium niobate single crystal thin film were 1.2 mol χ and 0.4 mol χ, respectively. The lattice constant (a-axis) is 5.153, and the refractive index measured at an incident light wavelength of 1.15 μm is 2.232.
It was ±0.001.

Example 5 (+) NazCO: + 27.2 mol χ, LizC
Oz 22.8 mol χ, VzO 140.0 mol χ,
NbzOs 10.0 mol χ+ T, o, Li
A mixture in which 12.0 mol χ was added to NbJ was placed in a platinum crucible, and the contents of the crucible were melted by heating to 1100'C in an air atmosphere in an epitaxial growth growth apparatus.

(2) The melt was cooled to 896°C at a cooling rate of 60°C per hour.
It was slowly cooled down to °C. Lithium tantalate single crystal (00
01) After optically polishing the surface, the M
A film thickness of 4 is applied to the parts other than the gO film and this 5 μm wide MgO film.
After forming 00 Cu1Il, it was thermally diffused at 1000°C and chemically etched to have an MgO diffusion channel with a width of 5 μm, thereby obtaining a substrate material. The ordinary refractive index of the MgO-diffused channel part and the Cu-diffused part other than the channel part is reduced by 10 x 103 and I, respectively, compared to the substrate material without any diffusion.
Xl0-' was increasing. This substrate material was immersed in the melt for 11 minutes while rotating at 1100 rpm.

(3) Pull up the substrate material from the melt and rotate at 1000 rotations.
After shaking off the melt on the melt for 30 seconds with RP
The mixture was slowly cooled to room temperature, and a lithium niobate single crystal thin film containing sodium and titanium with a thickness of about 7 μm was obtained on the substrate material.

(4) The amounts of sodium and titanium in the obtained single crystal thin film of lithium niobate were 4.6 mol χ and 5.0 mol χ, respectively. Also, the lattice constant (a-axis) is 5.153 people,
The refractive index measured at an incident light wavelength of 1.15 μm is 2.24
It was 1±0.001.

(5) The obtained lithium niobate single crystal thin film was
When the end face was polished perpendicularly to the MgO diffusion channel with a width of 5 μm, the laser beam was incident on the end face, and the near field pattern of the emitted light was observed.
It was confirmed that the gO was well confined on the diffusion channel.

Example 6 (1) The surface of the LiNb○3 single crystal thin film obtained in Example 1 was mirror-polished to create a slab-type waveguide using this LiNbO3 thin film as a waveguide layer.

(2) The film thickness of the slab type waveguide was changed to a phase matching film thickness of 2.50 μm ± 0.05 μm by ion beam etching.
Adjusted to.

(3) The slab waveguide obtained in (1) and (2) above was photolithographically coated with a thickness of 10 μm and a film JI2.
．． A glue channel type waveguide with a thickness of 50 μm±0.05 μm and a step difference of 1 μm was fabricated.

(4) Both end faces of the channel-type waveguide obtained in (3) were mirror-polished by puff polishing to enable light to enter and exit from the end faces, thereby forming a second high-frequency wave generating (SHG) element.

(5) After precisely aligning the SHG element made of the channel-type waveguide created in (4) above with the light emitting region of the semiconductor laser facing one end face of the channel-type waveguide, place it on the silicon block. Next, the semiconductor laser chip and SHG element were fixed using ultraviolet curing resin.

Furthermore, wires were bonded to the electrodes on the upper and lower surfaces of the semiconductor laser, making it possible to supply driving power.

(6) After integrating the semiconductor laser and the SHG element in this way, place it in a metal hermetically sealed pancage as shown in Figure 1, electrically connect the external bottle and wire, and operate it using an external pin. In addition to making it possible to supply electricity, the cap was covered with a wavelength-selective glass window, and the inside was hermetically sealed with a high-purity nitrogen gas atmosphere.

When an operating voltage such that the output from the semiconductor laser is 480 mW is applied to the hermetically sealed packaged device fabricated using the SHG device made of the LiNb03ffl film of the present invention in this way, the second harmonic is emitted from the glass window. The output of the laser is 2.0mW, and the output of the semiconductor laser is 0.1mW.
Therefore, the harmonics could be extracted efficiently.

The optical propagation loss of the lithium niobate single-crystalline thin products of the present invention obtained in Examples 1 to 5 was measured by prism coupling with respect to a semiconductor laser beam having a wavelength of 0.83 μm, and the results are shown in Table 1.

Table 1: Loss of light propagation (dB/cm) 1.1 1.0 1.2 These values indicate extremely excellent characteristics that could not be obtained with the prior art.

(Effects of the Invention) According to the present invention, a lithium tantalate substrate containing different elements has extremely excellent optical properties that could not be obtained conventionally, and at the same time has a sufficiently thick lithium niobate monolayer that is necessary for use in optical devices. Because crystalline thin films can be practically formed,
It is not only optimal as a constituent material for thin-film waveguide type SHG elements, but also useful as a constituent material for optical modulators and multimode optical devices.

It is.

[Brief explanation of the drawing]

Figures 1 and 2 show Liz○-■20. ~Nbz
It is a triangular diagram of a three-component system of Os. Each composition point is (mol% of Li20, ■mol% of 20.,
(mol% of NbzOs). A (49,49,45,46,5,05)B (42
,8122,9434,25)C(11,11,80,
00,8,89)D(47,64,46,12,6,2
4)E (27,01,64,69,8,30)F (
36,71,37,9725,32)G (44,05
,32,97,22,98)H(45,36,46,4
58,19)+ (32,89,57,05,10,0
6) J (36,71,44,3018,99)K(4
4,95,40,5414,51)V2O5 A' (88,90,2,22□ 8.88)B'
(55,00,43,00,2,00)C' (46
,505],,50. 2. 00)D' (3
7,50,5,00,57,50)E' (69,8
5,21,33,8,82)F" (49,95,45
,02,5,03)C' (44,13,16,76
, 39, 11) H' (54, 72, 11, 12, 3
4,16) Bi(57,43,35,057,52)J
(49,95,42,53,7,52)K' (
47,36,26,32,26,32)L (56
, 38, 17, 91, 25, 71) FIG. 3 is a schematic diagram showing the (0001) plane of a lithium tantalate single crystal substrate. FIG. 4 is a schematic diagram of a pancage of the semiconductor laser and SHG element obtained in Example 6. 1-----1 Wavelength selection filter 2~--m--S HG device 3-----1 Semiconductor filter 4-----Hermetically sealed pan cage Figure 1 i2O b20s External bottle nitrogen gas that's all

Claims (1)

[Scope of Claims] 1. A lithium niobate single crystal thin film formed on a lithium tantalate substrate, wherein the lithium tantalate substrate has a different element added to at least a part of its surface; A lithium niobate single crystal thin film, characterized in that the thin film is lattice matched with the lithium tantalate substrate, and has a light propagation loss of 1.4 dB/cm or less. 2. The lithium niobate single crystal thin film according to claim 1, wherein the different element is at least one selected from magnesium, titanium, vanadium, chromium, iron, nickel, and neodymium. 3. The lithium niobate single crystal thin film according to claim 1 or 2, wherein the lithium niobate single crystal thin film contains sodium and magnesium. 4. The content of sodium and magnesium is 0.1 to 4.8 mol% and 0.8 to 10.8 mol%, respectively.
The lithium niobate single crystal thin film according to any one of claims 1 to 3, wherein the lithium niobate single crystal thin film is within the range. 5. The lithium niobate single crystal thin film according to any one of claims 1 to 4, wherein the lithium niobate single crystal thin film is formed on the (0001) surface of the lithium tantalate substrate. 6. The a-axis lattice constant of the lithium niobate single crystal thin film is 99.81 to the a-axis lattice constant of the lithium tantalate substrate.
The lithium niobate single crystal thin film according to any one of claims 1 to 5, wherein the lithium niobate single crystal thin film is lattice matched by being within the range of 100.07%.