JPH05313219A - Lithium tantalate single crystal substrate formed with polarization inversion grating and optical element - Google Patents

Abstract

PURPOSE:To obtain the tantalum lithium single crystal having an excellent optical damage resistant characteristic and the optical element constituted by using this crystal. CONSTITUTION:A structure periodically inverted with the polarization direction of the crystal is formed on the tantalum lithium single crystal having <=1kW/cm<2> optical damage resistance strength to the incidence of an Ar laser of 0.488mum wavelength, by which the optical damage resistance strength is improved to >=100kW/cm<2>. The polarization inversion region of such tantalum lithium single crystal substrate 21 formed with the polarization inversion grating is formed larger in its depth than the width in the periodic direction. The tantalum lithium single crystal substrate 21 is used for the nonlinear optical element which generates higher harmonic waves by passing the exit light from a laser beam source as a basic wave to the nonlinear optical crystal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium tantalate single crystal substrate used in the field of information processing using laser light or in the field of optical measurement control and communication, and particularly to tantalum having excellent light damage resistance. The present invention relates to a lithium oxide single crystal substrate and an optical element using the same.

[0002]

2. Description of the Related Art A single crystal of lithium tantalate has a melting point of about 16
It is a ferroelectric crystal having a Curie temperature of 50 ° C. and a Curie temperature of about 600 ° C., and is usually grown from the melt by the Czochralski method using an iridium crucible in a reducing atmosphere or a reducing atmosphere containing oxygen. Since the grown single crystal is in a multi-domain state, it is subjected to the single domainization treatment by the electric field application slow cooling method in the air or the oxygen atmosphere while keeping the crystal temperature above the Curie temperature. After that, the crystals are processed into wafers and used in large quantities as substrates for surface acoustic wave devices. Lithium tantalate crystals are relatively inexpensive and can grow large diameter crystals, and have relatively large nonlinear optical constants among inorganic oxide single crystals. It has been conventionally known that the lithium niobate single crystal is superior in light damage resistance, but no quantitative data has been reported. Further, it has been reported that the optical damage resistance of the crystal is improved by an electric field annealing method in which the grown crystal is single-domained near the Curie temperature (Journal of Applied Physics, Vol. 38, page 3109 (1967). )). In recent years, as a compact and lightweight blue light source, a semiconductor laser having a wavelength of 830 nm has been used as a waveguide type SHG element to reduce the wavelength to 415 n.
Attention has been paid to conversion into m blue light. For example
731 of Electronics Letters, 25, 11 (1989)
As discussed on page 732, a method for phase matching using polarization reversal has been proposed. That is, as shown in FIG. 1, a periodic grating was prepared on a LiNbO ₃ substrate by diffusion of Ti, heated to about 1100 ° C. to invert the polarization of only the periodic grating layer, and then an optical waveguide was prepared by a proton exchange method. Waves are incident and SHG light is extracted. Further, as discussed on pages 45 to 60 of Nikkei New Materials (November 4, 1991), since it was recently reported that a low loss optical waveguide can be formed by the proton exchange method. , Lithium tantalate single crystal has been attracting attention as a substrate material for a wavelength conversion element that generates a second harmonic by a quasi-phase matching method because the lithium tantalate crystal has excellent light damage resistance. LiTaO
_When using _three substrates, for example, Appl.Phys.Lett.58 (24)
(1991) As discussed on pages 2732 to 2734, a periodic lattice was prepared by a proton exchange method instead of Ti diffusion, heated to about 600 ° C. to invert the polarization of only the periodic lattice layer, and further the proton exchange method. A method of manufacturing an optical waveguide has also been attempted. This is shown in FIG.
In such an optical element, the width of the optical waveguide is about 4 μm, the depth of the waveguide is about 2 μm, and the SHG light with a wavelength of about 433 nm is about 15 μm.
Since mW is emitted, the optical power density is about 188.
It can be as ^high as KW / cm ² .

[0003]

However, the above-mentioned prior art has the following problems. It has been said that the lithium tantalate single crystal manufactured by the above-mentioned conventional technique has a higher light damage resistance strength than that of the lithium niobate single crystal, but it is rather weaker than the lithium niobate single crystal against light damage and has a wavelength conversion element. The present inventors have found that the optical damage occurs during the practical use of the above-mentioned optical application, which is a big problem that prevents the practical use. The optical damage referred to here is a phenomenon in which the refractive index of the crystal locally changes due to the incidence of laser light, and is called light-induced refractive index change.
The cause of this optical damage is said to be due to the transition metal impurities contained in the crystal, and the phenomenon is explained especially by the change in the valence of Fe ions in the crystal. That is, when light is incident in a direction that is not parallel to the Z axis (optical axis) of the crystal, F existing in a portion where the light intensity is strong in the light irradiation portion.
The e ²⁺ ion is excited and emits an electron to be converted into Fe ³⁺ .
The electrons thus generated are generally trapped by other Fe ³⁺ in the non-irradiated part of the crystal or in the weakly irradiated region, and such ions are converted to Fe ²⁺ . The overall effect due to such a phenomenon appears as a change in the distribution of Fe ²⁺ ions, and as a result, it appears as a non-uniformity in the local refractive index distribution due to the electro-optic effect of the crystal itself. When a crystal is used as a substrate for optical applications such as a wavelength conversion element, it is extremely difficult that the element does not operate stably due to such a change in the refractive index of the light irradiation part and that the characteristics originally possessed by the crystal cannot be fully utilized. Causes big problems. This light damage becomes more remarkable as the wavelength of light used is shorter. Therefore, the problem of light damage becomes more serious in device applications using light of short wavelength, and is a particularly large problem in blue SHG light. It is said that this photo-damage occurs remarkably in the lithium niobate single crystal, and the light damage resistance strength of the lithium tantalate single crystal is stronger than that of the lithium niobate single crystal. So far, lithium tantalate single crystals have been used for surface acoustic wave device applications, and in these applications, a lot of subgrain boundaries and transition metal impurities contained in the crystal are included Did not pose a big problem. However, in optical applications, there are problems such as unstable operation of element characteristics due to occurrence of optical damage and light scattering at the subgrain boundary. Aiming at the application of lithium tantalate single crystal as an optical element, we studied the improvement of quality by reducing impurities and subgrain boundaries. As a result, the results shown in FIG. 3 were obtained, and regarding the light damage resistance of lithium tantalate, unlike the conventional theory that lithium tantalate is strong against light damage, it is different from the conventional theory that lithium tantalate single crystal has a high resistance to light damage. We found that it was less than light damage resistance. Furthermore, reducing the amount of transition metal impurities such as iron,
Although the optical damage resistance is also improved by the oxidation treatment of the grown crystal by the heat treatment, it is about 1 KW / cm ² at the incident intensity of Ar laser having a wavelength of 488 nm, which is insufficient for optical element application. It has been found that the light intensity is far below the cm ² . As described above, it is said that the main cause of the occurrence of optical damage is the concentration of the selected metal impurities contained in the crystal, especially the Fe impurity concentration.
If this is reduced to, for example, 1 ppm or less, the effect of improving the light damage resistance is certainly improved, but it is difficult to completely remove it. The reason for this is that in the growth of oxide single crystals, the purity of the raw material that can be purchased is about 4N to 5N, and impurities are incorporated into the grown crystals from the crucible material and the refractory heat insulating material in the furnace. This is because it is impossible to highly purify and there is a limit to the reduction of impurities.
Further, even by the electric field annealing method which has been conventionally said to be effective in improving the light damage resistance strength, optical damage occurs when the power density of the element used is increased, which is not sufficient.
Therefore, there has been a problem that a crystal that sufficiently satisfies the light damage resistance strength cannot be obtained in the device application using short-wavelength light. Therefore, it is very difficult to grow a single crystal substrate excellent in light damage resistance as discussed in Nikkei New Materials (November 4, 1991, p. 52). It is very expensive to manufacture a crystal substrate that satisfies the light damage resistance strength. In addition, the wavelength of SHG light that can be generated due to large light absorption in the visible light region of the lithium tantalate single crystal substrate or the basic absorption edge on the long wavelength side in the small and light blue light source that has been recently developed. At most about 400
There is a problem that it is limited to about nm. Therefore, in order to develop a light source with a shorter wavelength, a single crystal substrate material having excellent light transmissivity in a short wavelength region is required. However, light absorption and oxygen by impurities contained in a conventional lithium tantalate single crystal are required. There are problems such as light absorption due to coloring due to defects.
In terms of SHG element efficiency, in the method of performing phase matching using polarization inversion reported so far, as shown in FIG. 1, the cross-sectional shape of the polarization inversion grating is formed by the Ti diffusion method. Is a triangle, and is a semicircle as shown in FIG. 2 in the proton exchange method. Therefore, the SHG device having an ideal polarization reversal grating with a rectangular cross section has the original efficiency of the SHG device.
Not able to generate light. The present invention has been made to solve the problems of light damage and light transmittance of conventional lithium tantalate single crystals as described above, and lithium tantalate having improved light damage resistance strength and light transmittance. Providing a single crystal substrate at low cost, and using this single crystal substrate for the SHG element that generates the second harmonic by passing the light emitted from the laser light source as the fundamental wave to the nonlinear optical crystal, the optical element is stably manufactured. , Is intended to work.

[0004]

To achieve the above object, the present inventor periodically reverses the polarization direction of the crystal in order to improve the light damage resistance of the lithium tantalate single crystal. The inversion region has a light damage resistance strength of 1 by creating a structure whose depth is larger than the width in the periodic direction.
00KW / cm ² or more to improve, using the single crystal substrate in a non-linear optical element for generating harmonic by passage through the nonlinear optical crystal light emitted from the laser light source as the fundamental wave. Furthermore, in the lithium tantalate single crystal, 280
M in order to improve the light transmittance in the 400 nm band
The content of gO was 1 mol% or more. With the above structure, the light damage resistance strength of the crystal can be significantly improved, and further, the obtained crystal has improved light transmission characteristics, so that it is possible to stabilize optical elements such as wavelength conversion elements using short wavelength light. Can be operated. In the production of the lithium tantalate single crystal of the present invention, there is no limitation on the means for growing the single crystal, it is generally by the Czochralski method, and in some cases, by the Bridgman method, the floating zone method or the fiber pedestal method. It is also possible to do so. Further, the compounding ratio of Li ₂ CO ₃ and Ta ₂ O ₅ as raw materials is desirable from the viewpoint of single crystal growth because it is easy to obtain a high quality single crystal with an ordinary congruent composition. A different refractive index may be needed. In such a case, a desired single crystal substrate can be obtained by changing the compounding ratio of Li ₂ CO ₃ and Ta ₂ O ₅ .

[0005]

The present invention will be described in more detail based on the following examples. A sample was prepared by the following manufacturing method. First, by the Czochralski method, about 5 kg of LiTaO ₃ was placed in a crucible made of iridium having a diameter of 100 mm and a depth of 120 mm.
Raw material powder (The raw material used for growing is Li ₂ O, Ta _{2 with a} purity of 4N.
This is a mixture of O ₅ powders. ) Was melted by high frequency heating to form a melt, and then seeding was performed, and a 2-inch single crystal was grown in a predetermined orientation for about 3 days. The growing speed at this time is 1 to 3 mm / h, and the rotation speed is 10 to 30 rpm. Next, the crystal grown by the above method was subjected to a single domainization treatment. For example, a Pt electrode plate is provided so as to face the crystal in the Z-axis direction through a conductive powder that is non-reactive with the crystal, and the crystal is inserted into an electric furnace to perform a single domainization process. After that, each edge from each crystal is parallel to the x-axis direction, the y-axis direction, and the z-axis direction.
A square block of 0 × 10 × 10 mm ³ was cut out and each surface thereof was mirror-polished. Alternatively, a 2-inch wafer was prepared from each crystal. Thus, a lithium tantalate single crystal was prepared and examined for improvement in light damage resistance. The light damage resistance is measured in a crystal with a wavelength of 0.4.
A powerful argon laser with an output of 88 μm of 300 mW and a beam diameter of 1.4 mm was incident, and the optical damage, that is, the refractive index change induced by the laser was detected by a 1 mW helium neon laser with a weak output. An example of this result is shown in FIG. SAW grade lithium tantalate single crystal containing a lot of transition metal impurities such as iron has a power density of 20 W / c.
The incidence of m ² argon laser causes a great deal of optical damage, and the refractive index changes greatly. The lithium tantalate single crystal (referred to as optical grade), in which the impurities were reduced to 1/5 and the small tilt grain boundaries in the crystal were reduced, caused less optical damage than the iron-rich crystal. 3
Quantitative results were obtained that optical damage was more than twice as likely to occur as compared with SAW grade lithium niobate single crystal containing ppm. This result is completely different from the previous report that the lithium tantalate single crystal is more resistant to optical damage than the lithium niobate single crystal. Further, as shown in FIG. 4 (a), 1 KW of Ar laser light having a wavelength of 488 nm was narrowed down by a lens and was made incident on the Y-axis direction from the X plane of the lithium tantalate single crystal, and the change in the output beam shape was examined. When the incident power intensity is very small and optical damage does not occur, the output beam shape is circularly symmetric as shown in FIG.
However, any of the lithium tantalate single crystals suffers optical damage at a power with an incident power density of 1 KW / cm ² or less, so the outgoing beam changes from a spot shape and spreads in the Z-axis direction as shown in FIG. 4 (c). It was seen. The present inventors have heated the lithium tantalate single crystal above the Curie temperature of the crystal to bring the crystal into a multi-domain state, and then again examined the change of the output beam shape of the Ar laser beam by the above method, and caused optical damage. As a result of the evaluation, it was found that, as shown in FIG. 4 (d), the beam spreads slightly in a circular shape and the beam deformation in the Z-axis direction as seen in (c) is not observed. As a result of a more detailed evaluation, the polarization in the + Z and −Z directions in the multi-domain state of the beam irradiation part is almost symmetrical, and the beam is deformed as if the polarization is alternately inverted with respect to the beam traveling direction. Not seen,
It was found that no light damage occurred. The reason for this is not clear, but is considered as follows. The optical damage referred to here is a phenomenon in which the refractive index of the crystal locally changes due to the incidence of laser light, and is called light-induced refractive index change.
That is, when light is incident in a direction that is not parallel to the Z axis (optical axis) of the crystal, the electrons generated in the high light intensity portion in the light irradiation portion drift and are captured in the non-irradiation portion in the crystal,
It appears as non-uniformity of the local refractive index distribution through the electro-optic effect of the crystal itself, which forms an electric field. In the multi-domain state, the direction of the electro-optic effect is different between the + Z direction and the −Z direction, and therefore the changing direction of the refractive index is different, so that the refractive index changes are alternately canceled with respect to the traveling direction of the incident beam. Therefore, it seems that optical damage does not occur. Based on such an idea, a multi-domain state is controlled to form a lattice in which polarization is accurately inverted with respect to the traveling direction of light, and the problem of optical damage occurs as the polarization inversion interval is reduced to the μm order. I thought it could be solved. Therefore, in order to confirm that the light damage resistance of a lithium tantalate single crystal in which light damage occurs at a power of an incident power density of Ar laser light having a wavelength of 488 nm of 1 KW / cm ² or less is improved, a single crystal is used as a substrate. A polarization inversion grating was formed by the method shown in FIG. Figure 5
As shown in (a), a substrate 11 is prepared by polishing the -Z (c) surface of a LiTaO ₃ substrate. (B) A Ta film 51 is formed on the −Z plane of 11 by sputtering with a thickness of 30 nm.
(C) A photoresist 52 is spin-coated on the 51 film,
The photoresist 5 is formed by a normal photolithography technique using a photomask in which a portion where the polarization inversion 12 is performed is opened.
2 was patterned. The pattern cycle of the photomask is adjusted to the cycle of SHG light generated at 1 to 10 μm. (D) Using the patterned photoresist 52 as a mask, the Ta film 51 is patterned by dry etching by RIE using CF ₃ Cl gas.
(E) The photoresist 52 is removed with acetone, and proton exchange is performed using pyrophosphoric acid at 260 ° C. for 30 to 60 minutes, whereby 16 proton exchange layers are formed.
(F) The Ta film 51 is etched with an aqueous solution of NaOH. (G) The polarization inversion layer 12 is obtained by inserting the substrate on which the proton exchange layer of 16 is formed into an electric furnace and performing heat treatment.
To form. The depth of the polarization inversion grating was larger than the depth of the optical waveguide formed on the substrate surface and smaller than the substrate thickness.
The width was almost equal to the width of the proton exchange pattern. After the polarization inversion grating 12 was formed, the proton exchange layer on the substrate surface was removed by polishing, and an optical waveguide was formed on the substrate surface by the usual proton exchange method. Finally, the SHG element is manufactured by optically polishing the end face of the waveguide. Depending on the heat treatment time, when the cross section of the produced polarization inversion lattice is observed, it may become rectangular. When the depth of the domain-inverted region such as the rectangular shape is larger than the width in the periodic direction, the shape ratio of the domain-inverted portion to the non-inverted portion with respect to the incident direction of light becomes 1: 1. Become. On the other hand, when the method of creating the polarization inversion, such as the heat treatment time, is changed, the cross section of the created polarization inversion lattice may become semicircular when observed. When the depth of the domain-inverted region such as a semi-circular shape is smaller than the width in the periodic direction, the shape ratio of the domain-inverted portion and the non-inverted portion with respect to the incident direction of light deviates from perfect 1: 1. It will be. An optical waveguide was formed on the above-mentioned polarization inversion grating, and an SHG element having an element length of 1 cm was prepared. When a titanium-sapphire laser was used as a light source of the fundamental wave and a fundamental wave having a wavelength of 830 nm was incident on the manufactured SHG element, blue SHG light of 415 nm was obtained. At this time,
When the domain-inverted grating has a rectangular cross section and the domain-inverted region has a depth larger than the width in the periodic direction, the SHG light output is 15 mW, and high-power SHG light is obtained with a power density of 1
A stable output was obtained at 65 KW / cm ² , and no optical damage was observed. That is, Ar laser and blue S
Despite the difference in wavelength from HG light, the light damage resistance is 1K
Even with a crystal of W / cm ² or less, the light damage resistance was improved, and the SHG element could be sufficiently used. On the other hand, when the cross section of the domain-inverted grating is semicircular and the depth of the domain-inverted region is smaller than the width in the periodic direction, SHG light of 9 mW was obtained and an output of power density 105 KW / cm ² was obtained. There is light damage. However, even in this case, the light damage resistance strength is 1
Even with a crystal having a KW / cm ² or less, the light damage resistance was improved, so that it could be sufficiently used for SHG elements. By this,
A polarization inversion lattice is formed in a lithium tantalate crystal, and preferably, the cross section has a rectangular shape, and when the depth of the polarization inversion region is larger than the width in the periodic direction, the polarization inversion portion with respect to the light incident direction is formed. It was found that the shape ratio of the non-inverted portion became 1: 1 and the light damage resistance was significantly improved, which was useful for a highly efficient SHG element.

[0006]

[Effects of the Invention] A lithium tantalate single crystal having a light damage resistance against an Ar laser of a wavelength of 0.488 μm and having a light damage resistance of 1 kW / cm ² or less is formed to have a structure in which the polarization direction of the crystal is periodically inverted to obtain 100 kW / Since the light damage resistance is improved to more than 1 cm ^{2, the} lithium tantalate single crystal can be used for the substrate for the optical device using the short wavelength light, and the SHG device utilizing the large nonlinear optical constant of the lithium tantalate single crystal can be used. The stability and high output characteristics can be improved.

[Brief description of drawings]

FIG. 1 is a conventional SHG using a triangular polarization inversion grating.
It is a figure which shows an element.

FIG. 2 is a diagram showing a conventional SHG element using a semi-circular polarization inversion grating.

FIG. 3 is a diagram showing incident time dependence of optical damage induced by argon laser incidence on various lithium tantalate single crystals and lithium niobate single crystals.

FIG. 4 is a diagram showing how Ar laser light having a wavelength of 488 nm is narrowed down by a lens, is incident in the Y-axis direction from the X plane of a lithium tantalate single crystal, and the shape of the emitted beam is changed.

5 (a) to 5 (h) are diagrams showing a method for manufacturing a spike-shaped polarization inversion grating according to the present invention.

[Explanation of symbols]

11 substrate (LiTaO ₃ ) 12 polarization inversion region 13 channel type optical waveguide 14 fundamental wave incident light 15 SHG output light 16 proton exchange layer 21 substrate (LiNbO ₃ ) 31 triangular polarization inversion region 41 semicircular polarization inversion region 51 Ta film 52 photoresist

Front page continued (72) Inventor Masazumi Sato 5200 Mikkaji, Kumagaya, Saitama, Hitachi Metals Co., Ltd. Magnetic Materials Research Laboratory (72) Kohei Ito 5200 Mikkaji, Kumagaya, Saitama Hitachi Metals Co., Ltd.

Claims (3)

[Claims] 1. A light-damage-resistant material is prepared by forming a structure in which the polarization direction of the crystal is periodically inverted in a lithium tantalate single crystal having a light-damage strength of 1 kW / cm ² or less against the incidence of an Ar laser having a wavelength of 0.488 μm. A polarization inversion lattice lithium tantalate single crystal substrate having improved strength and a light damage resistance of 100 kW / cm ² or more. 2. The domain-inverted lattice lithium tantalate single crystal substrate according to claim 1, wherein the domain-inverted region of the domain-inverted lattice lithium tantalate single-crystal substrate has a depth larger than a width in the periodic direction. .. 3. A lithium tantalate single crystal according to claim 1, wherein the nonlinear optical crystal is a nonlinear optical element that generates a harmonic by passing light emitted from a laser light source as a fundamental wave to the nonlinear optical crystal. A non-linear optical element characterized by using.