THIN FILM OF LITHIUM NIOBATE SINGLE CRYSTAL

Abstract: A thin film of a lithium niobate single crystal, formed on a lithium tantalate substrate, which is mainly used in a thin-film waveguide type of a second harmonic generating element which can halve the wavelength of an incident laser beam, is in a state of lattice matching with the lithium tantalate substrate, and exhibits a high electro-optical constant with a reduced propagation loss.

Description: FIELD OF THE INVENTION This invention relates to a thin film of lithium niobate (LiNbO 3 ) single crystal which is suitably applied in various optical materials including thin film wave-guiding second harmonic generating devices. BACKGROUND OF THE INVENTION With the recent development of optics-applied technique, shorter wavelength laser beam sources are increasing in demand. Such shorter wavelength laser beam sources can improve recording density and photosensitivity and can be applied in the field of optical appliances such as optical memory discs and laser printers. Thus, second harmonic generating (SHG) devices which can convert the wavelength of the incident laser beam into a 1/2 level have been studied. A bulk of nonlinear optical single crystal has conventionally been used in such SHG devices utilizing high-output gas lasers as the light sources. However, gas laser is now being replaced mainly by semiconductor laser since there are strong demands for much smaller devices such as optical memory disc apparatus and laser printers, and since the semiconductor laser is less expensive and allows direct light modulation while the gas laser requires an external light modulator. Accordingly, thin film wave-guiding SHG devices are now in demand in order to obtain high conversion efficiency using a low light source output of several mW to several tens of mW. As the nonlinear optical materials for such thin film wave-guiding SHG devices, those having a waveguide layer of a bulk of lithium niobate single crystal on which Ti or the like is diffused so as to modify refractive index, and those having a waveguide made of a thin film of lithium niobate single crystal formed on a lithium tantalate substrate by high-frequency sputtering process are known. However, it is difficult to obtain a thin film of lithium niobate single crystal having excellent crystallinity in either of these methods, and thus they failed to afford high conversion efficiency. By the way, the Liquid Phase Epitaxial Method is supposed to be an ideal way of forming a thin film of single crystal having excellent crystallinity. The Liquid Phase Epitaxial Method for forming a thin film of lithium niobate is described, for example, in: (1) Applied Physics Letters, Vol. 26, No. 1, January 1975, wherein a thin film of lithium niobate is formed as an optical waveguide on a lithium tantalate substrate by liquid phase epitaxial growth using Li 2 O and V 2 O 5 as a flux; (2) Japanese Patent Publication No. 9720/1976, wherein a thin film of lithium niobate is formed as an optical waveguide on a lithium tantalate substrate by liquid phase epitaxial growth using Li 2 O and V 2 O 5 as a flux; and (3) Japanese Patent Publication No. 47160/1981, wherein a solid solution thin film of lithium niobate/lithium tantalate single crystal containing magnesium is formed on a substrate by epitaxial growth using Li 2 O and V 2 O 5 as a flux. However, the known Liquid Phase Epitaxial Method is neither successful in forming a thin film of lithium niobate single crystal having excellent crystallinity on the lithium tantalate substrate nor can form a film of lithium niobate single crystal having a sufficient film thickness required for producing a SHG device, and thus no practical thin film wave-guiding SHG device is so far known. The film thickness required for producing a thin film wave-guiding SHG device is such that the effective index of an incident laser beam having a fundamental wavelength of λ can be made consistent with that of the second harmonic with a wavelength of λ/2 in order to achieve phase-matching between them. Particularly, when a SHG device for semiconductor laser is produced using a thin film of lithium niobate formed on a lithium tantalate substrate, the thin film of lithium niobate single crystal should have a thickness of not less than 5 µm so as to achieve such consistency between these two effective indexes. On the other hand, in order to obtain a high-output thin film wave-guiding SHG device, a greater index difference must be secured between the substrate and the thin film waveguide layer, and studies are made for reducing refractive index of the substrate; for example, as disclosed in: (4) Japanese Patent Publication No. 34722/1985, wherein magnesium oxide and vanadium pentaoxide are simultaneously incorporated to a lithium tantalate single crystal substrate to form a thin film of lithium tantalate single crystal thereon by epitaxial growth method; and (5) Japanese Patent Publication No. 27681/1988, wherein vanadium pentaoxide is diffused on a lithium tantalate single crystal substrate to form a diffusion layer consisting of vanadium pentaoxide and lithium tantalate with a thickness of 5 to 6 µm and a low refractive index, followed by epitaxial growth of a layer of lithium tantalate single crystal thereon to form a triple-layered optical waveguide. These methods disclosed all use lithium tantalate as the thin film waveguide layer and are not the technique for forming a thin film of lithium niobate single crystal having excellent optical properties on the lithium tantalate substrate. In addition to the above methods, the following are also known as disclosed in: (6) Journal of Crystal Growth, Vol. 54 (1981) pp. 572-576, wherein sodium is incorporated into lithium niobate to form a sodium-containing thin film of lithium niobate single crystal having a thickness of 20 µm on a Y-cut lithium niobate substrate by liquid phase epitaxial growth; (7) Journal of Crystal Growth, Vol. 84 (1987) pp. 409-412, wherein sodium is incorporated into lithium niobate to form a sodium-containing thin film of lithium niobate single crystal on a Y-cut lithium tantalate substrate by liquid phase epitaxial growth; (8) U.S. Patent No. 4,093,781, wherein a strain-free lithium ferrite film is formed on a substrate by liquid phase epitaxial growth by partly substituting lithium with sodium to effect matching of lattice constant of the film with that of the substrate; and (9) Japanese Provisional Patent Publication No. 142477/1977, wherein crystal grains are allowed to grow slowly and naturally by allowing crystallization to occur very slowly to obtain a strain-free crystal by liquid phase epitaxial growth. None of the above techniques of (6) to (9) discloses use of the thin film as optical materials. As described above, there has not so far been disclosed a thin film of lithium niobate single crystal, formed on the lithium tantalate substrate, having excellent optical properties and a sufficient film thickness for providing optical devices including SHG devices. The present inventors made various studies with a view to solving these problems to notice that a thin film of lithium niobate single crystal having a film thickness sufficient for producing optical devices including SHG devices and also excellent optical properties such as low optical damage (change in the refractive index of a crystal to be caused by irradiation of a high intensity light) and very low propagation loss can practically be formed by subjecting the thin film of lithium niobate single crystal and the lithium tantalate substrate to lattice matching, and they previously accomplished an invention (see Japanese Patent Application No. 247179/ 1990). As the result of their further study, they found that if the lithium niobate single crystal has some degree of propagation loss, it can sufficiently be used as an optical device. On the other hand, according to the conventional liquid phase epitaxial growth method, for example, as described in Applied Physics Letter, Vol. 24, No. 9, 1974, pp. 424-426, the thin film of lithium niobate single crystal formed on a lithium tantalate substrate comes to have an electrooptical constant of merely about 1/3 that of the bulk of lithium niobate. Thus, the present inventors found that this is because of the difference between the lattice constant of the thin film of lithium niobate single crystal and that of the lithium tantalate substrate and that a thin film of lithium niobate single crystal having a high electrooptical constant which has never been attained can be formed by achieving lattice matching between them, and they accomplished this invention. DISCLOSURE OF THE INVENTION The gist of the invention is a thin film of lithium niobate single crystal including vanadium which is formed on the (0001) plane of the lithium tantalate substrate having a surface roughness Rmax of at least 170 x 10 -10 m (170 Å) wherein the sodium and magnesium content are 0.1 to 14.3 mol% and 0.8 to 10.8 mol%, respectively, characterized in that lattice matching is achieved by having the a-axis lattice constant of said thin film of lithium niobate in the range of 99.92 to 100.03% of that of the lithium tantalate substrate, the surface roughness Rmax of said thin film of lithium niobate is in the range of 0.001 to 0.1 µm and of which optical propagation loss measured by prism coupling method using 0.83 µm wavelength is more than 1.4 db/cm and smaller than 30 db/cm. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows schematically the (0001) plane of a lithium tantalate substrate which is the plane on which a thin film of lithium niobate single crystal is to be grown (hereinafter simply referred to as growth plane). Fig. 2 is an SEM photograph of a broken section of the thin film of lithium niobate single crystal formed on a lithium tantalate single crystal substrate. Although lattice matching cannot be directly identified electron-microscopically, the stripe pattern observable in the photograph is continuous between the lithium tantalate single crystal substrate and the thin film of lithium niobate single crystal, and if no lattice matching is achieved between them, the stripe pattern will be discontinuous at the interface thereof. Accordingly, this photograph indirectly demonstrates that lattice matching is successfully achieved between the lithium tantalate single crystal substrate and the thin film of lithium niobate single crystal. Fig. 3 shows an X-ray diffraction pattern of the thin film of lithium niobate single crystal of this invention. Fig. 4 shows the relationship between the a-axis lattice constant of the thin film of lithium niobate single crystal and the Na content thereof. Fig. 5 shows the relationship between the a-axis lattice constant of the thin film of lithium niobate single crystal and the Li/Nb molar ratio thereof. Fig. 6 shows the relationship between the a-axis lattice constant of the thin film of lithium niobate single crystal and the Ti content thereof. Fig. 7 shows the relationship between the Na content in the thin film of lithium niobate single crystal and the a-axis lattice constant thereof, when 5 mol % of MgO is incorporated therein, wherein  shows the case where only Na is incorporated, while Δ shows the case where Na and 5 % of MgO are incorporated. Fig. 8 shows the relationship between the a-axis lattice constant of the thin film of lithium niobate single crystal and the MgO content thereof. Fig. 9 shows a triangular map of Li 2 O-V 2 O 5 -Nb 2 O 5 ternary system. Li 2 O / V 2 O 5 / Nb 2 O 5 Li 2 O / V 2 O 5 / Nb 2 O 5 A (88.90, 2.22, 8.88) B (55.00, 43.00, 2.00) C (46.50, 51.50, 2.00) D (11.11, 80.00, 8.89) E (37.50, 5.00, 57.50) F (49.49, 45.46, 5.05) G (11.11, 80.00, 8.89) H (42.81, 22.94, 34.25) I (47.64, 46.12, 6.24) J (27.01, 64.69, 8.30) K (36.71, 37.97, 25.32) L (44.05, 32.97, 22.98) M (45.36, 46.45, 8.19) N (32.89, 57.05, 10.06) O (36.71, 44.30, 18.99) P (44.95, 40.54, 14.51) PREFERRED EMBODIMENT OF THE INVENTION It is necessary that the present thin film of lithium niobate single crystal is formed on a lithium tantalate substrate and that their crystal lattices are matched, because the thin film of lithium niobate single crystal thus formed comes to have extremely excellent optical properties and a sufficient film thickness which has never been attained by the prior art by virtue of the lattice matching achieved therebetween. The reason why the thin film of lithium niobate single crystal comes to have extremely excellent optical properties is that the thin film of lithium niobate single crystal and the lithium tantalate substrate are unified by the lattice matching to provide a high-quality thin film having excellent crystallinity with very little lattice strain or defect, free from microcracks. It has been impossible to form a thin film whose crystal lattice is matched with that of the substrate by the conventional techniques, and thus the resulting thin film suffers poor crystallinity, and microcracks are observed therein. Accordingly, even if the surface of the substrate material and that of the thin film are made smooth by lapping or chemical etching, or if high-purity materials (content of impurity is low) are used, so as to reduce the scattering loss at the interface with the substrate or at the surface of the thin film, as well as, the absorption loss to be caused by the impurity content to some extents, the absorption/scattering loss at the crystal grain boundary or the scattering loss to be caused by microcracks will be great to increase propagation loss, preventing the thin film from being offered for optical applications. While the means for achieving lattice matching between the thin film of lithium niobate single crystal and the lithium tantalate substrate is not particularly limited in this invention, the lattice constant of the thin film of lithium niobate to be formed on the lithium tantalate substrate is made greater than that of the substrate, if the a-axis lattice constant of the lithium tantalate is greater than that of the ordinary lithium niobate (5.148 Å). As a means for increasing the lattice constant of lithium niobate, it is advantageous to incorporate hetero elements, preferably sodium and magnesium into the thin film of lithium niobate single crystal. The reason why the incorporation of sodium and magnesium is advantageous is that sodium and magnesium ions or atoms have the effect of increasing the lattice constant (a-axis) of lithium niobate by incorporating them by substitution into the crystal lattice of lithium niobate or by doping. Lattice matching between the lithium tantalate substrate and the lithium niobate single crystal can be facilitated by adjusting the amounts of sodium and magnesium. Sodium is particularly useful to greatly increase the lattice constant of lithium niobate. Although magnesium can also increase the lattice constant of lithium niobate, its effect is not so great as sodium but has an important effect of preventing optical damage. When sodium and magnesium are incorporated in the thin film of lithium niobate single crystal, desirably the content of sodium is 0.1 to 14.3 mol %, preferably 0.3 to 4.8 mol %, and that of magnesium is 0.8 to 10.8 mol %, preferably 3.5 to 8.6 mol %, relative to the lithium niobate single crystal. The reason is that if the content of sodium is less than 0.1 mol %, the lattice constant of the thin film may not be increased sufficiently so as to allow lattice matching with the lithium tantalate substrate regardless of the amount of the magnesium to be incorporated; whereas if the content of sodium is more than 14.3 mol %, the lattice constant thereof will be too great, and thus lattice matching cannot be achieved in both cases. On the other hand, if the content of magnesium is less than 0.8 mol %, the effect of preventing optical damage may not sufficiently be exhibited; whereas if it is more than 10.8 mol %, magnesium niobate system crystals precipitate undesirably. The relationship between the Na content and the a-axis lattice constant of lithium niobate is as shown in Fig. 4. Incorporation of sodium is as described in Journal of Crystal Growth Vol. 54 (1981), pp. 572-576, where sodium is incorporated to lithium niobate to form a sodium-containing thin film of lithium niobate single crystal having a film thickness of 20 µm on a Y-cut lithium niobate substrate by liquid phase epitaxial growth method; and in Journal of Crystal Growth, Vol. 84 (1987) pp. 409-412, where sodium is incorporated into lithium niobate to form a sodium-containing thin film of lithium niobate single crystal on a Y-cut lithium tantalate substrate by liquid phase epitaxial growth method. It is true, however, that these literatures describe incorporation of sodium into lithium niobate single crystal can modify its lattice constant, but the techniques are both for SAW (Surface Acoustic Wave) devices, and they refer neither to optical properties nor to the lattice matching of lithium niobate single crystal with the lithium tantalate substrate whereby to form a film having excellent optical properties. The thin films of lithium niobate single crystal described in these literatures are for SAW devices, and further the former method uses lithium niobate as the substrate, whereas in the latter method no lattice matching is achieved between the lithium tantalate substrate and the thin film formed thereon, although the thin film is formed on a lithium tantalate substrate. Therefore, both of them cannot be used as the desired optical material according to this invention. The lattice constant of lithium niobate can otherwise be increased by modifying the Li/Nb molar ratio of the lithium niobate single crystal, as shown in Fig. 5. It can be seen from Fig. 5 that the molar ratio of Li/Nb is preferably in the range of 41/59 to 56/44. Alternatively, the a-axis lattice constant of lithium tantalate substrate may be reduced. In this case, titanium is incorporated as the hetero atom to the lithium tantalate. Titanium is added in an amount of 0.2 to 30 mol %, because the lattice matching of the substrate with the thin film of lithium niobate single crystal will be difficult if the titanium content is out of the specified range. Titanium may be diffused on the lithium tantalate substrate or added to the raw material composition to carry out crystal pulling by Czochralski method and molded into a form of substrate. Otherwise, when a thin film of lithium niobate single crystal is subjected to lattice matching with a single crystal substrate having an a-axis lattice constant of greater than 5,148 Å, a hetero element is incorporated into the lithium niobate to reduce the lattice constant thereof. When Ti is incorporated as the hetero atom, the a-axis lattice constant of lithium niobate changes as shown in Fig. 6. Titanium is incorporated suitably in an amount of 0.2 to 30 moles. In this invention, lithium tantalate is used as the substrate, and the substrate may not particularly be limited so long as it has a hexagonal crystal structure and an a-axis lattice constant of 5.128 to 5.173 Å. The form of substrate is not limited only to the flat plate. The substrate material may typically be exemplified by ZnO, Gd 3 Ga 5 O 12 , MgO and alumina, but lithium tantalate is particularly advantageously used. The reason why lithium tantalate is used is that the crystal system of lithium tantalate substrate is very similar to that of lithium niobate single crystal to facilitate epitaxial growth of the lithium niobate single crystal, and that high quality lithium tantalate substrates are always commercially available. The lithium tantalate having an a-axis lattice constant in the range of 5.128 to 5.173 Å can be used as the substrate, and the shape thereof may be any of fiber and bulk. The roughness of the surface of the lithium tantalate substrate on which a thin film of lithium niobate single crystal is to be formed is desirably R max = 10 to 1,000 Å according to JIS B0601, because it is extremely difficult to reduce the R max value below 10 Å, while the crystallinity of the thin film of lithium niobate single crystal will be lowered if the R max value exceeds 1,000 Å. The thin film of lithium niobate single crystal according to this invention is formed on a lithium tantalate substrate, and the lithium tantalate substrate may contain a hetero element at least partly on the surface. The reason why such hetero element is incorporated partly on the surface of lithium tantalate substrate is that it can greatly modify the refractive index of the substrate and thus the index difference between the substrate and the thin film waveguide layer can be increased. At least one hetero element selected from Mg, Ti, V, Cr, Fe, Ni and Nd is preferably used. As the way of increasing the difference between the refractive index of the thin-film waveguide layer and that of substrate, the refractive index of the lithium tantalate substrate may be reduced or that of the thin film of lithium niobate may be increased. For such purposes, Mg or V is incorporated to the lithium tantalate substrate, while Ti, Cr, Fe, Ni or Nd can be incorporated to the thin film of lithium niobate, as the hetero atoms, respectively. Particularly for SHG oscillation, the difference between the refractive index of lithium tantalate substrate and that of the thin film of lithium niobate must be increased. When the hetero element is incorporated into the lithium tantalate substrate, the hetero element may not uniformly be present throughout the substrate. In this invention, if a hetero element is incorporated into the lithium tantalate substrate at a specific portion so as to form a pattern of waveguide forming portion having a relatively low refractive index compared with that of non-forming portion, the thin film of lithium niobate single crystal formed on the pattern merely by depositing it in the form of slab on the substrate constitutes a waveguide. Accordingly, the step of securing a waveguide portion can be omitted. As a way of reducing the refractive index of the waveguide forming portion of the substrate relative to the non-forming portion thereof, the refractive index of the waveguide forming portion of the substrate may be reduced or that of the non-forming portion thereof may be increased, desirably. If the hetero element is distributed nonuniformly on the substrate, a waveguide is formed only on the portion which conforms to the requirements for the waveguide. The content of hetero element on the surface of the lithium tantalate substrate is desirably as follows: 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 to 20 mol % V 0.05 to 30 mol % The above values were calculated according to the following formula: Hetero element/(LiTaO 3 + hetero element) x 100. The respective contents of the hetero elements are more preferably as follows: 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 to 10 mol % V 1.0 to 15 mol % The reason why the above formulation range are preferred is because the crystallinity of the substrate is lowered if the content of the hetero element is above the specified range, whereas if it is less than the specified range, the refractive index of the substrate cannot be modified. Of the above hetero elements, those having an action of increasing the refractive index of the lithium tantalate substrate are Ti, Cr, Nd, Fe and Ni, while those having an action of reducing the refractive index thereof are Mg and V. These elements can be incorporated into the lithium tantalate substrate in various forms such as atom, ion and oxide. Since these elements can modify the refractive index only at the surface of the substrate without substantially affecting the characteristics participating in the film formation on the substrate, e.g. surface roughness, thin films having properties equivalent to those of the ordinary substrates containing no hetero element can be formed under the same conditions. It is particularly preferred in this invention to use the (0001) plane of the lithium tantalate substrate as the growth plane. The (0001) plane of the lithium tantalate substrate means the plane which is perpendicular to the c-axis of lithium tantalate. The reason why it is preferred to use the (0001) plane of the lithium tantalate substrate as the growth plane is because lithium tantalate has a hexagonal crystal structure (see Fig. 1) and the (0001) plane consists only of a-axis, so that the lattice matching of the substrate with the thin film of lithium niobate single crystal can be achieved merely by modifying the a-axis lattice constant. The thin film of lithium niobate single crystal according to this invention shows a propagation loss in the range of more than 1.4 dB/cm and less than 30 dB/cm, more preferably less than 15 dB/cm. The propagation loss is measured by prism coupling method using an incident semiconductor laser beam having a wavelength of 0.83 µm. The present inventors previously disclosed (see Japanese Patent Application No. 247179/1990) that the thin film of lithium niobate single crystal is required to show a propagation loss of not more than 1.4 dB/cm when it is used as various optical materials. However, as the result of their further study, they found that a propagation loss of less than 30 dB/cm is useful as an optical material. But if the propagation loss exceeds 30 dB/cm, the intended object of this invention cannot be achieved. Propagation loss means the percentage of photometric reduction of light per unit length of the light traveling direction when the light is guided through the thin film, and it includes scattering loss and absorption loss. The scattering loss depends on the state of the interface between the substrate and the thin film, the surface state of the thin film and the microcracks present in the thin film. On the other hand, the absorption loss is subject only to the properties of the thin film and depends on the crystallinity of the thin film and the content of impurities therein. The lattice constant (a-axis) of the thin film of lithium niobate single crystal is 99.92 to 100.03 % of that of the lithium tantalate substrate. For example, when the lattice constant of the lithium tantalate substrate is 5.153 Å, the lattice constant of thin film of lithium niobate single crystal is desirably 5.150 to 5.155 Å. The reason is that if the lattice constant is out of the specified range, lattice matching between the lithium tantalate substrate and the thin film of lithium niobate single crystal will be difficult and therefore it will not be feasible to form a thin film of lithium niobate single crystal having a sufficient thickness as well as excellent optical properties so as to be used as optical materials. The thickness of the thin film of lithium niobate single crystal is preferably at least 5 µm, particularly preferably 10 µm or more. The thin film of lithium niobate single crystal according to this invention desirably contains at least one element selected from chromium (Cr), neodymium (Nd), rhodium (Rh), zinc (Zn), nickel (Ni), cobalt (Co), titanium (Ti) and vanadium (V) so as to modify the optical properties thereof including refractive index, as necessary. The content of each element is as follows: Rh 0.05 to 20 mol % Zn 0.02 to 30 mol % Ni 0.1 to 20 mol % Co 0.05 to 20 mol % Cr 0.02 to 20 mol % Ti 0.2 to 30 mol % Nd 0.02 to 10 mol % V 0.05 to 30 mol %. The content of each element is more preferably as follows: Rh 0.1 to 10 mol % Zn 0.5 to 15 mol % Ni 1.0 to 10 mol % Co 0.1 to 10 mol % Cr 0.2 to 10 mol % Ti 1.0 to 15 mol % Nd 0.5 to 5 mol % V 1.0 to 15 mol %. While the content of each hetero element is shown in terms of mol % relative to lithium niobate single crystal, addition of such hetero element in an amount of below its lower limit is meaningless since it causes small or almost no change in the refractive index; whereas addition of such hetero element in an amount of above the upper limit causes to lower not only the crystallinity of the lithium niobate single crystal but also the optical properties of the thin film of lithium niobate single crystal. By the way, when a hetero element such as Cr, Nd, Ti, V, Rh, Zn, Ni and Co is incorporated in the thin film of lithium niobate single crystal, it is desirable to adjust the content thereof, as necessary, since the lattice constant and refractive index of the thin film of lithium niobate single crystal simultaneously shift. Determination of lattice constant is carried out by means of ordinary X-ray powder diffractometry in this invention. The lattice constant is calculated using 2 values of 15 peaks in lithium niobate detected at Cu - 2 = 45 to 90° and the plane index thereof according to least squares method. In this measurement, Si is used as an internal standard. When the thin film of lithium niobate single crystal of this invention is used in a SHG device, it is desired that the refractive index of ordinary ray n o and the refractive index of extraordinary ray n e of the thin film of lithium niobate single crystal are respectively in the range of 2.25 ≤ n o ≤ 2.40 and 2.0 ≤ n e ≤ n o - 0.01 against a laser beam source having a wavelength of 0.83 µm (fundamental wavelength), and that the refractive index of extraordinary ray n e against the second harmonic (0.415 µm) generated is smaller than the refractive index of ordinary ray n o against the second harmonic. Next, the process for forming the thin film of lithium niobate single crystal according to this invention will be described. As the process for forming the thin film of lithium niobate single crystal according to this invention, liquid phase epitaxial growth, sputtering, vapor deposition, MOCVD and CVD are desirably used, and particularly preferably liquid phase epitaxial growth is used. The reason why liquid phase epitaxial growth method is preferred is that it allows easy formation of uniform films having excellent crystallinity, and thus thin films of lithium niobate single crystal with small propagation loss which can suitably be used as optical waveguides can be formed without impairing the excellent properties of the lithium niobate single crystal such as nonlinear optical effect, electrooptical effect and acoustic optical effect. The film can be formed at high productivity. In order to give a high-quality crystals, the liquid phase epitaxial growth is desirably carried out by bringing a lithium tantalate substrate into contact with a melt containing Li 2 O, Nb 2 O 5 , V 2 O 5 , Na 2 O, MgO, etc. to effect liquid phase epitaxial growth of a thin film of lithium niobate single crystal while the a-axis constant of the lithium tantalate substrate is matched with that of the thin film of lithium niobate single crystal. Na and Mg have the effect of increasing the lattice constant of the thin film of lithium niobate single crystal, and further Mg has an effect of preventing optical damage. To describe now the composition range of Li 2 O, V 2 O 5 and Nb 2 O 5 excluding Na 2 O and MgO referring to the triangular map of the Li 2 O-V 2 O 5 -Nb 2 O 5 ternary system, it is advantageously in the region surrounded by the five points of composition A (88.90, 2.22, 8.88), B (55.00, 43.00, 2.00), C (46.50, 51.50, 2.00), D (11.11, 80.00, 8.89) and E (37.50, 5.00, 57.50). The parenthesized values of the respective points of composition are shown in terms of mol % in the order of Li 2 O, V 2 O 5 and Nb 2 O 5 . The advantage of such composition range is that the lattice matching between the thin film of lithium niobate single crystal and the lithium tantalate substrate can be facilitated by the magnesium and sodium incorporated, providing a high-quality thin film of lithium niobate single crystal having excellent optical properties with particularly low propagation loss. Further the composition range of Li 2 O, V 2 O 5 and Nb 2 O 5 excluding Na 2 O and MgO referring to the triangular map of the (Li 2 O-V 2 O 5 -Nb 2 O 5 ) ternary system , it is advantageously in the region surrounded by the three points of composition F (49.49, 45.46, 5.05), G (11.11, 80.00, 8.89) and H (42.81, 22.94, 34.25), more preferably by the four points I (47.64, 46.12, 6.24), J (27.01, 64.69, 8.30), K (36.71, 37.97, 25.32) and L (44.05, 32.97, 22.98), most preferably by the four points M (45.36, 46.45, 8.19), N (32.89, 57.05, 10.06), O (36.71, 44.30, 18.99) and P (44.95, 40.54, 14.51). The composition ratio of Na 2 O/Li 2 O in this invention is desirably 2.0/98.0 to 93.5/6.5 in terms of molar ratio. The reason is because if the amount of Na 2 O incorporated is out of the specified range, it is difficult to achieve lattice matching between the lithium tantalate substrate and the thin film of lithium niobate single crystal. The composition ratio of Na 2 O/Li 2 O in this invention is more desirably 7.4/92.6 to 80.0/20.0, most suitably 16.7/83.3 to 48.4/51.6 in terms of molar ratio. The composition ratio of MgO/LiNbO 3 (theoretical value of lithium niobate precipitatable from the melt composition) is desirably 0.1/99.9 to 25.0/75.0 in terms of molar ratio. The reason is because if the proportion of MgO is less than the specified range, the optical damage preventive effect of Mg will be exhibited insufficiently; whereas if it is more than the specified range, magnesium niobate system crystals precipitate to prevent formation of thin film of lithium niobate single crystal. The composition ratio of MgO/LiNbO 3 in this invention is more desirably 0.7/100 to 9.0/100, most suitably 3.5/100 to 6.0/100 in terms of molar ratio. Particularly in the composition range where Li 2 O/Nb 2 O 5 > 1, the molar ratio of MgO/Nb 2 O 5 is desirably 0.2/99.8 to 40.0/60.0. The composition ratio of MgO/Nb 2 O 5 is more desirably 0.7/50.0 to 9.0/50.0, particularly suitably 3.5/50.0 to 6.0/50.0 in terms of molar ratio. It is also preferred to incorporate a hetero element to the substrate at least partially on the surface layer, prior to formation of a thin film of lithium niobate single crystal having its lattice constant matched with that of the lithium tantalate substrate. The way of incorporating such hetero element includes liquid phase epitaxial growth or incorporation of hetero element -in advance into the raw material of lithium tantalate bulk single crystal, as well as, heat diffusion method, ion exchange method and ion implantation method. In the heat diffusion method, a compound containing a hetero element (usually an oxide) is sputtered onto the surface of a substrate and the hetero element is diffused by heating from the surface of the substrate. In the ion exchange method, a substrate is impregnated with a melt of a compound containing a hetero element to replace the Li ion around the surface of the substrate with the hetero element. In the ion implantation method, a compound containing a hetero element is volatilized and accelerated under an electric field to be allowed to impinge directly onto the substrate surface. In the raw material incorporation method, a hetero element is added to the raw materials to carry out crystal pulling by Czochralski method, wherein the seed crystal is preferably rotated in contact with the melt of raw materials. While a hetero element diffusion layer is formed when any of the heat diffusion, ion exchange and ion implantation methods is used, the diffusion layer desirably has a thickness of 0.005 to 20 µm. If the diffusion layer has a thickness of less than 0.005 µm, the proportion of the light guided therethrough and diffused to the portion of the substrate with no hetero element diffused therein will be too great to satisfy the refractive index to be required for the substrate; whereas if it has a thickness of more than 20 µm, the crystallinity thereof will be reduced. Particularly when MgO is diffused, the thickness of the diffusion layer is preferably 250 to 1,000 Å. The substrate is desirably heated to effect heat diffusion of the hetero element and then brought into contact with a melt for liquid phase epitaxial growth with continuous heating. The reason why such continuous heating is necessary is because the crystallinity of the substrate is reduced if the substrate is once cooled after completion of heat diffusion and then heated again for carrying out liquid phase epitaxial growth. The heat diffusion is preferably carried out when the substrate is preheated in the same oven in which the melt is heated. The heat diffusion is preferably carried out at 850 to 1,050°C. If the heat diffusion temperature is lower than 850°C, the hetero elements may not be diffused; whereas if it is higher than 1,050°C, not only the crystallinity of the substrate will be lowered but also external diffusion of lithium will occur. The time required for the heat diffusion is desirably 0.25 to 20 hours. In the process of this invention, the substrate is desirably preheated before it is brought into contact with the melt, since lithium tantalate substrate is very susceptible to thermal shock. The preheating time is desirably 5 to 120 minutes. The preheating is desirably carried out at the height of 5 to 15 mm from the surface of the melt. The raw material components of the melt according to this invention are desirably of oxides or compounds which can be converted into oxides by heating, for example, compositions of Na 2 CO 3 , Nb 2 O 5 , Li 2 CO 3 , V 2 O 5 , MgO, NaNbO 3 , NaVO 3 , LiNbO 3 and LiVO 3 , all of which are preferably of optical grade. The above raw material components are desirably heat-melted at 600 to 1,300°C in an aerobic or oxidative condition. In the liquid phase epitaxial method, the melt is supercooled first and then brought into contact with the lithium tantalate substrate to effect crystal growth. The cooling rate for supercooling the melt is desirably 0.5 to 300°C/hour. In this invention, the melt is desirably cooled at the rate of 0.5 to 1,200°C/hour after completion of the liquid phase epitaxial growth. The above cooling is preferably carried out exponentially from 400°C. At the Curie point of the lithium tantalate substrate, the melt is maintained at this temperature for a predetermined time or cooled at a rate of 0.1 to 5°C/min. The reason is because cracking which may occur concomitantly with the phase transition of crystal at the Curie point can be prevented. While the Curie point of the lithium tantalate substrate can be shifted by incorporating hetero elements thereto, it is generally 650°C. The temperature for the liquid phase epitaxial growth is desirably in the range of 600 to 1,250°C. If the temperature is higher than the specified range, no crystal will precipitate since the melting point of lithium niobate is 1,250°C; whereas if it is lower than the specified range, the raw materials cannot be melted since the flux has a melting point of 600°C. The lithium tantalate substrate is desirably rotated when the liquid phase epitaxial growth is carried out, since such rotation of lithium tantalate substrate allows growth of crystal having uniform properties and film thickness. The rotation of substrate is desirably carried out on a horizontal plane and advantageously at a rate of 50 to 150 rpm. The lithium tantalate substrate is desirably subjected to optical lapping at least on the growth surface, followed by chemical etching. Further, the edges of the substrate are desirably chamfered. If the edges are not chamfered, fine flaws may be formed at the edges and cracking readily occurs from these flaws when exposed to thermal shock. The chamfered edges may be by rounding or beveling. The lithium tantalate substrate desirably has a thickness of 0.5 to 2.0 mm. If the substrate has a thickness of less than 0.5 mm, cracking easily occurs; whereas if it has a thickness of more than 2.0 mm, it will suffer pyroelectric effect (discharge effect caused by heating) since it is electrified by heating or lapping to attract lapping waste which easily forms scratches on the surface of the substrate. The thickness of the thin film of lithium niobate single crystal to be precipitated on the lithium tantalate substrate according to this invention can be controlled by suitably selecting the time for contacting the lithium tantalate substrate with the melt and the temperature of the melt. The rate of growing the thin film of lithium niobate single crystal according to this invention is desirably 0.01 to 1.0 µm/min. If the growing rate is higher than the specified range, the thin film of lithium niobate single crystal comes to have waviness; whereas if it is lower than the specified range, growth of the thin film takes too much time. It is desired in this invention to remove the flux from the surface of the lithium tantalate substrate and that of the thin film of lithium hiobate single crystal after completion of the liquid phase epitaxial growth. Any residual flux on the surface causes the film to have nonuniform thickness. The flux can advantageously be removed by spinning the thin film of lithium niobate single crystal formed on the lithium tantalate substrate at a rate of 100 to 10,000 rpm. The rotation time is preferably 5 to 60 minutes. The melt is desirably stirred for 6 to 48 hours using an appropriate stirring procedure. If the melt is not sufficiently stirred, crystal nuclei remain undissolved in the melt and crystals grow around these nuclei to form an irregular surface on the thin film of lithium niobate single crystal. The surface roughness of the thin film of lithium niobate single crystal is R max = 0.001 to 0.1 µm according to JIS B0601. In this invention, oxides of elements selected from Nd, Rh, Zn, Ni, Co and Ti can be used as the melt components in addition to Nb 2 O 5 , V 2 O 5 , Li 2 O, Na 2 O and MgO. As a way for achieving lattice matching of the lithium tantalate substrate with the thin film of lithium niobate single crystal by modifying the molar ratio of Li/Nb, the liquid phase epitaxial growth method using a melt composition containing at least K 2 O, V 2 O 5 , Li 2 O and Nb 2 O 5 is advantageous. The reason for the above will now be described. K 2 O and V 2 O 5 serve as a flux. The use of K 2 O and V 2 O 5 as the flux can prevent supply of Li from the flux, so that the ratio of Li/Nb in the thin film of lithium niobate single crystal to be precipitated can be changed by modifying the composition ratio of Li 2 O to Nb 2 O 5 in the raw material composition. Since the a-axis lattice constant changes if the Li/Nb ratio is changed, the a-axis lattice constant of thin film of lithium niobate single crystal can be controlled by controlling the composition ratio of Li 2 O/Nb 2 O 5 in the raw material composition, whereby the lattice matching between the thin film of lithium niobate single crystal and lithium tantalate substrate can be achieved. Further, Na 2 O and MgO may be added to the melt consisting of K 2 O, V 2 O 5 , Li 2 O and Nb 2 O 5 . The reason for the addition of MgO and Na 2 O is that it can increase the a-axis lattice constant of the thin film of lithium niobate single crystal, and MgO can further prevent optical damage. The molar composition ratio of Li 2 O/Nb 2 O 5 in the melt is desirably 43/57 to 56/44, more preferably 43/57 to 50/50. The reason is because the crystal structure of the lithium niobate single crystal to be precipitated will be changed to deteriorate the optical properties thereof if the molar ratio is not within the above range. It is further desired that MgO is added in such an amount that the theoretical value of the lithium niobate single crystal precipitatable from the composition of MgO/raw materials may satisfy 30/100 in terms of molar ratio. The reason is because magnesium niobate system crystals precipitate if the amount of MgO exceeds the above value. The composition of K 2 O and V 2 O 5 is desirably in such a range that the theoretical value of the lithium niobate single crystal precipitatable from the composition of K 2 O/V 2 O 5 flux (in terms of KVO 3 )/raw materials satisfies a molar ratio of 25/75 to 75/25. The reason is because the crystal structure of the lithium niobate single crystal to be precipitated will be changed to deteriorate the optical properties thereof if a molar ratio of the flux is not within the above range. The molar composition ratio of K 2 O/V 2 O 5 is advantageously 1 : 1. In this invention, lattice matching can be achieved by adjusting the a-axis lattice constant of the lithium tantalate substrate by adding a hetero element thereto so as to match it with that of the lithium niobate single crystal. The lithium tantalate substrate is desirably manufactured by CZ (Czochralski) method. Lithium carbonate, tantalum pentaoxide, titanium oxide and vanadium pentaoxide can be mentioned as the raw materials. These raw materials are advantageously heat-melted in an iridium crucible or a platinum-rhodium crucible to pull lithium tantalate single crystals. Use of iridium crucible is more preferred since it scarcely allows migration of impurities into the crystal. The a-axis lattice constant of the lithium tantalate substrate can be increased by incorporating a hetero element such as sodium. It is further preferred to modify the a-axis lattice constant of thin film of lithium niobate single crystal and that of lithium tantalate substrate simultaneously to effect lattice matching. It is also desired in this invention to incorporate titanium in the thin film of lithium niobate single crystal when the film is subjected to lattice matching with a hexagonal single crystal substrate having an a-axis lattice constant of 5.128 to 5.150 Å. In order to achieve such matching between the a-axis lattice constant of the thin film of lithium niobate single crystal and that of the lithium tantalate substrate, a composition mainly containing Li 2 O, V 2 O 5 , Nb 2 O 5 and TiO 2 is desirably used as the melt, and a substrate is contacted with the melt to effect liquid phase epitaxial growth of crystals and matching of their a-axis constants. To describe now the composition ratio of Li 2 O/V 2 O 5 /Nb 2 O 5 referring to the triangular map of the Li 2 O-V 2 O 5 -Nb 2 O 5 ternary system, it is desirably in the region surrounded by the five points of composition A (88.90, 2.22, 8.88), B (55.11, 43.00, 2.00), C (46.50, 51.50, 2.00), D (11.11, 80.00, 8.89) and E (37.50, 5.00, 57.50). The amount of TiO 2 is advantageously in the range of 0.2 to 60 mol % relative to the theoretical amount of lithium niobate single crystal precipitatable from the melt composition. By the way, in order to use the lithium niobate single crystal as a constituent of optical devices including SHG devices, it is required to have optically useful properties such as electrooptical effect and nonlinear optical effect. In order that lithium niobate or lithium tantalate single crystal may have optically useful properties such as electrooptical effect and nonlinear optical effect, the crystal must be subjected to poling by applying an electric field with heating to a temperature not lower than the Curie point during the manufacturing process. On the other hand, it is known that single crystals of lithium niobate and lithium tantalate containing hetero elements cannot easily be subjected to poling. However, the present thin film of lithium niobate single crystal is always polarized no matter how the lithium tantalate substrate is polarized or electrically neutralized by polaritic inversion and can show various properties such as excellent electrooptical effect and nonlinear optical effect. Accordingly, it is an advantage of this invention that the production process can be simplified because the thin film of lithium niobate single crystal and the lithium tantalate substrate need no poling process, and therefore those lithium tantalate substrates containing hetero elements which have been used with difficulty in the prior art can be employed. This invention will be described more specifically by way of Examples. Example 1 (1) Into a platinum crucible was introduced a mixture of 50 mol % of Li 2 CO 3 , 40 mol % of V 2 O 5 , 10 mol % of Nb 2 O 5 , Na 2 CO 3 in an amount of 47 mol % added per theoretical amount of LiNbO 3 precipitatable from the melt composition and MgO in an amount of 7 mol % added per theoretical amount of LiNbO 3 precipitatable from the melt composition, and the mixture in the crucible was melted by heating to 1,100°C in an epitaxial growth apparatus under aerobic conditions. The melt was further stirred using a propeller at a speed of 100 rpm for 6 hours. (2) A 1.5 mm thick lithium tantalate single crystal was subjected to optical lapping and then chemical etching, on the (0001) plane, followed by chamfering (beveling, C = 0.01 mm, JIS B0701). The thus obtained substrate material had a surface roughness of R max = 170 Å as measured according to JIS B0601.After the melt was gradually cooled to 922°C at a cooling rate of 60°C/hour, the substrate was preheated at 922°C for 10 minutes and then immersed in the melt under rotation at 100 rpm for 23 minutes.A thin film of lithium niobate was formed at a growing rate of 1.65 µm/min. (3) The substrate material was drawn up from the melt and further rotated at 1,000 rpm above the melt for 30 seconds to fling off the unnecessary portion of melt therefrom. The substrate material was then gradually cooled to the Curie point of the lithium tantalate single crystal at a cooling rate of 300°C/hour, and after the substrate was maintained at this temperature for 1 hour, it was further cooled to room temperature at a cooling rate of 60°C/hour to form a Na/Mg containing thin film of lithium niobate single crystal having a thickness of about 38 µm on the substrate material. (4) The amounts of sodium and magnesium contained in the thin film of lithium niobate single crystal were 2 mol % and 6 mol %, respectively. The thus obtained thin film had a lattice constant (a-axis) of 5.155 Å and a refractive index of 2.231 ± 0.001 as measured at an incident light wavelength of 1.15 µm. The thin film of lithium niobate had a surface roughness of R max = 1,000 Å as measured according to JIS B0601. Example 2 (1) Into a platinum crucible was introduced a mixture of 19 mol % of Na 2 CO 3 , 31 mol % of Li 2 CO 3 , 40 mol % of V 2 O 5 , 10 mol % of Nb 2 O 5 and MgO in an amount of 2 mol % added per theoretical amount of LiNbO 3 precipitatable from the melt composition, and the mixture in the crucible was melted by heating to 1,100°C in an epitaxial growth apparatus under aerobic conditions. (2) The melt was gradually cooled to 916°C at a cooling rate of 60°C/hour. A lithium tantalate single crystal was subjected to optical lapping on the (0001) plane and chamfering (beveling, C = 0.7 mm, JIS B0701), and then a vanadium film having a thickness of 500 Å was formed by RF sputtering, followed by heat diffusion at 970°C to form a 500 Å diffusion layer and then chemical etching to provide a substrate material. The substrate had a surface roughness of R max = 300 Å as measured according to JIS B0601. The refractive index of ordinary ray of the substrate material dropped by 1 x 10 -3 compared with that of the vanadium-undiffused substrate. The substrate material was immersed in the melt under rotation at 100 rpm for 15 minutes. A thin film of lithium niobate was formed at a growing rate of 0.60 µm/min. (3) The substrate material was drawn up from the melt and further rotated at 1,000 rpm above the melt for 30 seconds to fling off the unnecessary portion of melt therefrom. The substrate material was then gradually cooled to room temperature at a cooling rate of 1°C/min to form a Na/Mg containing thin film of lithium niobate single crystal having a thickness of about 9 µm on the substrate material. (4) The amounts of sodium and magnesium contained in the thin film of lithium niobate single crystal were 3 mol % and 2 mol %, respectively. The thus obtained thin film had a lattice constant (a-axis) of 5.156 Å, and a refractive index of 2.235 ± 0.001 as measured at an incident light wavelength of 1.15 µm. The thin film of lithium niobate had a surface roughness of R max = 1,000 Å as measured according to JIS B0601. Example 3 (1) Into an iridium crucible was introduced a mixture of 50 mol % of Li 2 CO 3 , 40 mol % of V 2 O 5 , 10 mol % of Nb 2 O 5 , Na 2 CO 3 in an amount of 43 mol % added per theoretical amount of LiNbO 3 precipitatable from the melt composition and MgO in an amount of 7 mol % added per theoretical amount of LiNbO 3 precipitatable from the melt composition, and the mixture in the crucible was melted by heating to 1,100°C in an epitaxial growth apparatus under aerobic conditions. The melt was further stirred using a propeller at a speed of 200 rpm for 12 hours. (2) The melt was gradually cooled to 915°C at a cooling rate of 60°C/hour. A lithium tantalate single crystal was subjected to optical lapping on the (0001) plane to provide a thickness of 1.8 mm, followed by chamfering (beveling). After an MgO film having a thickness of 1,000 Å and a width of 5 µm was formed by photolithography and RF sputtering, followed by heat diffusion at 920°C to provide a substrate material having a MgO diffusion channel with a thickness of 1,000 Å and a width of 5 µm. The diffusion layer had a thickness of 1,000 Å.The refractive index of ordinary ray of the channel dropped by 15 x 10 -3 compared with that of the MgO-undiffused portion. The substrate had a surface roughness of R max = 300 Å as measured according to JIS B0601. After the substrate material was preheated at 915°C for 30 minutes, it was immersed in the melt under.rotation at 100 rpm for 17 minutes. A thin film of lithium niobate was formed at a growing rate of 1.94 µm/min. (3) The substrate material was drawn up from the melt and further rotated at 1,000 rpm above the melt for 30 seconds to fling off the unnecessary portion of melt therefrom. The substrate material was then gradually cooled to the Curie point of the lithium tantalate single crystal at a cooling rate of 300°C/hour, and after it was maintained at this temperature, it was gradually cooled to room temperature at a cooling rate of 60°C/hour to form a Na/Mg-containing thin film of lithium niobate single crystal having a thickness of about 33 µm on the substrate material. (4) The amounts of sodium and magnesium contained in the thin film of lithium niobate single crystal were 2 mol % and 6 mol %, respectively. The thus obtained thin film had a lattice constant (a-axis) of 5.155 Å and a refractive index of 2.231 ± 0.001 as measured at an incident light wavelength of 1.15 µm. (5) The thin film of lithium niobate single crystal was subjected to end face lapping perpendicular to the 5 µm wide MgO diffusion channel. When a laser beam was allowed to impinge into the end face to observe the near field pattern of the outgoing beam, it was identified that the laser beam was successfully confined in the 5 µm wide MgO diffusion channel. The thin film of lithium niobate single crystal had a surface roughness of R max = 1,000 Å as measured according to JIS B0601.The thin films of lithium niobate single crystal obtained in the Examples were measured for propagation loss against a 0.83 µm semiconductor laser beam and Pockels constant (an electrooptical constant) by prism coupling method in a Mach-Zehnder interferometer, and the results are summarized in Table 1. An example of X-ray diffraction of the present thin films of lithium niobate single crystal is shown in Fig. 3, and comparison between 2 values of 15 peaks (1 ○to ) specific to the present lithium niobate and those of the conventional lithium niobate is shown in Table 2. The control sample was prepared according to the method described in Applied Physics Letters, vol. 24, No. 9, 1974, p 424. Plane index 2 value of prior art LiNbO 3 2 value of present LiNbO 3 Peak No. (h, k, l) (deg) (deg) 1 0 2 4 48.515 48.467 2 1 1 6 53.243 53.213 3 0 1 8 56.993 56.933 4 2 1 4 61.091 61.043 5 3 0 0 62.435 62.351 6 2 0 8 68.549 68.495 7 1 0 10 71.207 71.147 8 2 2 0 73.523 73.427 9 3 0 6 76.091 75.983 10 3 1 2 78.509 78.419 11 1 2 8 79.247 79.151 12 0 2 10 81.779 81.683 13 1 3 4 82.811 82.727 14 2 2 6 86.489 86.381 15 0 4 2 88.847 88.739 Note: a-axis lattice constant of prior art LiNbO 3 = 5.1483 Å a-axis lattice constant of present LiNbO 3 = 5.1535 Å The 2 values are the angles of diffraction against the CuK α 1 ray. APPLICABILITY IN INDUSTRIES According to this invention, a thin film of lithium niobate single crystal having very excellent optical properties which have never been attained in the prior art and also having a sufficient thickness to be used as optical devices can practically be formed on a lithium tantalate substrate, so that the present thin film-of lithium niobate single crystal is not only best suited as a constituent of thin film wave-guiding SHG devices but also useful as a constituent of light modulators and multi-mode system optical devices.

Claims: A thin film of lithium niobate single crystal including vanadium which is formed on the (0001) plane of the lithium tantalate substrate having a surface roughness Rmax of at least 170 x 10 -10 m (170 Å) wherein the sodium and magnesium content are 0.1 to 14.3 mol% and 0.8 to 10.8 mol%, respectively, characterized in that lattice matching is achieved by having the a-axis lattice constant of said thin film of lithium niobate in the range of 99.92 to 100.03% of that of the lithium tantalate substrate, the surface roughness Rmax of said thin film of lithium niobate is in the range of 0.001 to 0.1 µm and of which optical propagation loss measured by prism coupling method using 0.83 µm wavelength is more than 1.4 db/cm and smaller than 30 db/cm. The thin film of lithium niobate single crystal according to claim 1, wherein the lithium tantalate substrate has a MgO diffusion layer having a thickness of 250 x 10 -10 to 1,000 x 10 -10 m (250 to 1,000 Å).

Inventors: OONO SATOSHI, OONO, SATOSHI
Applicants: IBIDEN CO LTD, IBIDEN CO., LTD.
Application year: 1991
Main classification: G02F1
IPC codes: C30B19, G02F1, G02B6, C30B29