JP3424125B2 - Optical functional device using ferroelectric polarization reversal of lithium tantalate single crystal - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to lithium tantalate (LiTaO ₃ ) used in the fields of optical information processing using laser light, optical processing technology, photochemical reaction technology, optical measurement control, etc.
Regarding an optical functional element made of a single crystal (hereinafter abbreviated as LT), in particular, a wavelength conversion element for periodically reversing the polarization of the LT single crystal to shorten or lengthen the fundamental wavelength of laser light, and The present invention relates to an element that performs light deflection, focus and switching utilizing a large change in the refractive index of a polarization inversion structure, and an optical storage element and an optical circuit element that are used for recording by inverting the polarization of a minute region of an LT single crystal. .

[0002]

2. Description of the Related Art LT single crystals are ferroelectrics that are mainly used as substrates for surface acoustic wave devices and optical modulators, but they are transparent in a wide wavelength range from visible to infrared and have a high voltage. A periodic polarization structure can be created by applying it, and it has a certain degree of practical optical nonlinearity and electro-optical characteristics.
Since a single crystal having a large diameter and a high compositional homogeneity can be supplied at a relatively low cost, it has recently attracted attention as a substrate for a functional element such as a wavelength conversion element.

In particular, it is expected that a waveguide type optical second harmonic generation (SHG) element for converting a semiconductor laser of near infrared wavelength into blue light of half wavelength by a non-linear optical effect will be developed.
Of these, the SHG element, which uses an element obtained by periodically inverting the polarization structure of an inorganic single crystal such as LN, LT, or KTP, as the light source for high-density recording / reproduction of an optical disc has been most studied.

This SHG element is a quasi phase matching (Quasi Pha
se Matching (QPM) method, which is a method for phase matching by compensating for the difference in the propagation constants of the fundamental wave and harmonics with a periodic structure. With this method, high conversion efficiency can be obtained,
It has many excellent features such as easy collimation of output light and easy diffraction-limited focusing, and no restrictions on applicable materials or wavelengths.

As a periodic structure for QPM, a structure in which the sign of the SHG coefficient (d coefficient) is periodically inverted is most effective in obtaining high efficiency, and in a ferroelectric crystal, the positive and negative values of the d coefficient are strong. Periodic inversion structure of ferroelectric polarization domain is used because it corresponds to the polarity of dielectric polarization. In the QPM-SHG method, the nonlinear optical constants d ₂₂ and d _33, which cannot be used in the phase matching method using birefringence, can also be used, and thus it is considered that high efficiency wavelength conversion can be a great advantage.

In particular, the LT single crystal has a large non-linear optical constant (d ₃₃ is 26 pm / V), which is similar to that of the LN single crystal, is more resistant to optical damage than the LN single crystal, and has a basic absorption edge extending to 280 nm. Therefore, it is promising as a short wavelength wavelength conversion material.

Further, in an optical element utilizing the electro-optical effect, for example, a reference (M. Yamada et al., Appl. Phy.
s.Lett., 69, p3659, 1996), an optical element that forms a lens- or prism-shaped polarization-inverted structure in a ferroelectric crystal and deflects the laser light passing through it by using the electro-optic effect. , Cylindrical lenses, beam scanners, switches, etc. have received attention as new optical elements, and LT single crystals are also promising as substrate materials.

The commercially available LT single crystal has been a ferroelectric crystal having a melting point of about 1650 ° C. and a Curie temperature of about 600 ° C.
Usually, it is grown by the Czochralski method from a melt melted in an iridium crucible in a reducing atmosphere containing a little oxygen. A detailed phase diagram of the LT single crystal has not been reported, but, for example, the literature (S. Miyazawa et al. J. Crystal
Growth 10, p276,1971) shows that LN
It is well known that the stoichiometric composition (stoichiometric composition) and the congruent composition (congruent melting composition) do not match as in the case of a single crystal.

Since only the congruent composition has a composition in which the melt composition and the crystal composition are the same and a crystal having a uniform composition can be grown over the entire crystal, the crystals currently produced and used for various purposes are: All Li ₂ O / (Ta ₂ O ₅ + Li ₂
It is a crystal with a congruent composition in which the molar fraction of O) is 0.4830 to 0.4853.

Particularly, from the industrial viewpoint, the LT having a large diameter and being inexpensive
Since it is important to grow from a precisely controlled congruent composition melt in order to supply the crystal, the Curie temperature of the composition-sensitive crystal is controlled within 1 ° C, for example, 601 ° C. Therefore, the congruent composition of the LT single crystal was Li ₂ O / (Ta ₂ O ₅ + L) during the entire process of growing the crystal.
It is precisely determined between i ₂ O) = 0.4830 and 0.4853.

However, since the conventional Ta single crystal having a congruent composition has an excessive Ta component, Ta ions of up to several percent replace Li ions (anti-site defects).
It also causes a few% of vacancy defects in the Li ion site.
Although this effect is not serious for surface acoustic wave device applications, it cannot be ignored for optical functional device applications.

Therefore, it has been desired to develop a crystal having a composition close to a stoichiometric ratio as a substrate for application to an optical functional element.
Reference (S. Miyazawa et al. J. Crystal Growth 10, p27
As can be seen from the phase diagram shown in (6,1971), crystals having a composition close to the stoichiometry can be precipitated from a melt having a composition having a Li concentration higher than the stoichiometry.

However, when an attempt is made to grow a stoichiometric crystal using the Czochralski method, which has been used as a means for industrially mass-producing large-diameter LT crystals, when crystals are deposited, As a result, an excess of the Li component is left in the crucible, and the composition ratio of Li and Ta in the melt gradually changes, so that the composition ratio of the melt reaches the eutectic point immediately after the start of growth. Therefore, the solidification rate of the crystal is limited to only about 10%, and the quality of the precipitated crystal cannot be used for optical functional device applications.

The present inventors have significantly reduced the nonstoichiometric defect concentration of the congruent composition as a novel substance different from the conventional LT crystal of the congruent composition which is commercially available.
_A lithium tantalate single crystal having a molar ratio of ₂ O / (Ta ₂ O ₅ + Li ₂ O) of 0.495 to 0.50 and a stoichiometric composition close to Ta excess was invented and applied for a patent (JP-A-11-35393). ). In addition, the following literatures were reported regarding this new crystal.

As a means for reducing this non-stoichiometric defect and developing a high-quality crystal, the present inventors have proposed a method of growing a raw material while continuously supplying it (hereinafter abbreviated as continuous supply method) ( For example, Y. Furukawa et al. J. Crystal Gr
owth 197, p889, 1999). Specifically, Li ₂ O /
The molar fraction of (Ta ₂ O ₅ + Li ₂ O) is adjusted to 0.58 in excess of the Li component.
The growth rate is determined by setting the crystal growth rate to 00 to 0.5900, pulling up the LT crystal having a stoichiometric composition close to the stoichiometric composition from the inner crucible, and measuring the weight of the pulled crystal from time to time to obtain the growth rate. In this method, raw material powders having the same stoichiometric composition are continuously supplied between the outer crucible and the inner crucible.

By using this method, it is possible to grow a long crystal, and a crystal solidification rate of 100% is realized with respect to the amount of raw material supplied. The crystal grown by this method has a Curie temperature of 675 to 685 ° C., which is much higher than the Curie temperature of 601 of the conventional congruent composition crystal, and a lithium tantalate single crystal close to a stoichiometric composition with Ta excess is obtained. It has been reported that it has been obtained.

Further, recently, the present inventors have reported that in the above crystal having a Ta-excess stoichiometric composition, the applied voltage required for polarization reversal is about one-tenth of the conventional voltage (K. K.
itamura et al. Appl. Phys. Lett. 73, p3073, 1998 and Yasunori Furukawa et al., Abstracts of the 43rd Symposium on Artificial Crystals 1A12, 23
Page, 1998). That is, the existence of a non-stoichiometric defect (antisite defect or vacancy defect) of several percent in a conventional congruent composition crystal causes the optical characteristics originally possessed by the LT crystal and the applied voltage required to create a periodic polarization structure. It reports that it may be high.

Further, it is known that the LT single crystal having a congruent composition varies in the light damage threshold depending on each crystal by several orders of magnitude or more. However, in the lithium tantalate single crystal having a Ta-excess stoichiometric composition close to that of the conventional congruent composition. Compared to the composition, it has been reported that the threshold for photo-damage is improved by irradiation with a green light laser having a wavelength of 532 nm, and the variation between crystals is slightly reduced (for example, Yasunori Furukawa et al., The 60th Annual Meeting of the Applied Physics Society of Japan). Lecture Lecture Proceedings 2p-ZB-1, Volume 3, 1001
Page, 2000).

Further, the LT single crystal having a near stoichiometric composition to which MgO is added, when irradiated with a green light laser having a wavelength of 532 nm, is
It is known to show a threshold value for light damage resistance superior to that of the conventional congruent composition (for example, Akio Miyamoto et al., Proceedings of 4th Symposium on Synthetic Crystals 27A, page 75, 1999). Further, in any composition, optical damage of the LT single crystal with excess Ta easily occurs when the wavelength of the laser to be irradiated becomes short,
It is known that the light damage resistance threshold at a wavelength near 400 nm is more than two orders of magnitude lower than the light damage resistance threshold at a wavelength of 532 nm.

Lithium tantalate single crystal close to stoichiometric composition with excess Ta, LT single crystal close to stoichiometric composition (Curie temperature is
Quasi-Phase-Matc (680-685 ℃)
A study of near-infrared bulk OPO device as a hing (QPM) device has been reported (for example, Takaaki Hatanaka et al., Proc. 2a-k-7, 3rd volume, 932, Proc. Page, 199
9 years). A z-cut LT single crystal with a consistent melting composition is provided with a periodic electrode on one side and a uniform electrode on the other side, and a pulse voltage of several KV / mm is applied through this electrode to obtain a thickness of 1 to
A 2 mm near-infrared bulk OPO device can be made relatively easily. However, since it is difficult to make the polarization inversion uniform, the device fabrication is limited to the formation of the polarization inversion structure in a small area, and the polarization inversion cannot be formed over a large area.

Furthermore, the present inventors have previously studied the production of an OPO element having a crystal substrate thickness of 3 mm by using an LT single crystal close to a stoichiometric composition with excess Ta as a substrate (Koichiro Nakamura et al., No. 1). Proceedings of 47th JSAP Academic Lecture 30p-ZD-
3, Vol. 3, 1105, 2000), it becomes more difficult to control polarization inversion, and bulk OPO devices using this as a substrate have not been obtained.

[0022]

The most important technique for realizing a wavelength conversion optical functional device using a polarization inversion structure on a ferroelectric single crystal substrate is a technique for accurately producing a periodic polarization inversion structure. Is. In the wavelength conversion element using the QPM structure, since the tolerance of the QPM condition is very strict, if the inversion period of the formed element is imperfect, a small size and high efficiency element cannot be realized. The voltage application method is well known as a polarization inversion forming method and is commonly used.However, it is very difficult to form a polarization inversion width ratio of 1: 1 or 1: 3. There is also a problem with the reproducibility of.

For example, in the voltage application method, a periodic electrode is provided on one surface of a z-cut LT single crystal and a uniform electrode is provided on the opposite surface, and a pulse voltage is applied through this electrode so that the portion immediately below the periodic electrode is oriented in the z-axis direction. Although the polarization is inverted toward the opposite direction, the reversed polarization width and the electrode width do not always match, and the manufacturing error is large.

In particular, the enlargement of the domain-inverted portion in the width direction has been a serious problem in manufacturing the device with good reproducibility. Also, during the formation of polarization inversion in the z-axis direction on the opposite surface,
Problems such as the inversion being interrupted and the polarization inversion width being different on both sides of the z-cut crystal have also occurred.
The QPM element has not been realized yet. In particular, in the case of a conventional LT single crystal having a congruent composition, a high voltage of 20 kV / mm or more is required as the applied voltage for polarization reversal,
The substrate thickness that can be inverted is limited to 0.5 mm or less, and it was extremely difficult to apply it to a high-power laser device with a beam diameter of 1 mm or more.

On the other hand, the present inventors previously invented
Although it is considered to make a bulk OPO device with a thickness of 1 to 3 mm in the near-infrared region by using an LT single crystal having a stoichiometric composition with an excess of Ta as a substrate, it is difficult to make polarization reversal uniform. ,
The production of the element is limited to the formation of the domain-inverted structure in a small area, and the domain-inverted structure cannot be formed over a large area.

Further, a light modulation element utilizing the electro-optic effect of the ferroelectric single crystal, a lens or prism-shaped polarization inversion structure formed in the LT single crystal is produced, and the laser light passing through this is electro-optic. What is important in realizing new optical elements such as optical elements that deflect by utilizing the effect, cylindrical lenses, beam scanners, switches, etc. is to produce small and highly efficient elements. The performance of an optical device that uses the electro-optic effect of a single crystal in which the refractive index is inverted by the polarization inversion structure depends on the precision of the design of the lens and prism polarization inversion structure, the manufacturing process of the polarization inversion structure, and the material. It is determined by the magnitude of the electro-optic constant.

However, when the conventional congruent crystal substrate is used, the problem of material characteristics that the control of the reversal of the spontaneous polarization is poor, which is the same as in the case of manufacturing the QPM element, is still left, so that the accuracy of The production of good lenses and prism-shaped polarization inversion structures has not been realized.

Further, the LT melt crystal of the congruent composition is said to have greater light damage resistance than the LN single crystal, but depending on the wavelength and intensity of the light used, the light damage resistance is still not sufficient. There are many. The present inventors have reported that the LT single crystal close to the stoichiometric composition with excess Ta has an improved photo-damage threshold against irradiation with a green light laser having a wavelength of 532 nm as compared with the conventional congruent composition (Yasunori Furukawa. Proceedings of the 60th JSAP Academic Lecture Meeting 2p-ZB-1,
(3rd volume, 1001 pages, 2000), the threshold value of light damage resistance still varies by three digits or more for each crystal, and the cause has not been clarified.

For this reason, crystals having a slight Ta excess component side (Li ₂ O / (Ta ₂ O ₅ + Li ₂ O mole fraction 0.495 to 0.50) so far are stable and resistant to optical damage. In order to provide crystals, it was necessary to add additives such as Mg, but in the production of Mg-containing LT single crystals, the Mg element was uniformly distributed in the crystals to improve the optical quality. In order to grow crystals without deterioration, the crystal growth rate had to be slower than in the case of non-added crystals, and there was a problem that productivity deteriorated. The LT single crystal having a composition close to that of the composition has excellent optical damage resistance, but the controllability of polarization reversal depends on the MgO concentration. There is also a new problem that it is difficult to create with good reproducibility.

Furthermore, the light damage resistance becomes a more severe problem when the light used is in the short wavelength light range from blue to ultraviolet.
The optical damage of the conventional LT single crystal is more likely to occur when the wavelength of the laser to be irradiated becomes shorter, and the optical damage resistance threshold near the wavelength of 400 nm is more than two digits lower than the optical damage resistance threshold at the wavelength of 532 nm. Has been a major problem in the application of optical functional devices to short wavelengths.

From the above, it is expected that a perfect LT single crystal containing no non-stoichiometric defects will be developed as a means for solving these problems. However, it is difficult to grow a perfect crystal without variation in the whole crystal or between crystal lots, the yield is reduced, and industrial production of bulk crystals has not been achieved.

On the other hand, a thin film or a stoichiometric crystal having a thickness of about 0.5 mm is used as a development means on a congruent crystal substrate
The method of adding LPE treatment, or the method of adding Vapor Transport Equilibration treatment to the congruent crystal substrate is known as a method that makes it easier to bring the crystal composition closer to the stoichiometric composition, but even in these cases, the composition of the liquid phase and the LPE temperature, Alternatively, the crystal composition changes depending on the VTE treatment temperature, and again there is a problem in industrially producing a perfect crystal having no defects.

[0033]

In order to solve the above conventional problems, the inventor of the present invention has been earnestly continuing to investigate the characteristics of LT crystals having a stoichiometric composition, and found that LT crystals having a stoichiometric composition in excess of Li It has been found that the controllability of the domain-inverted structure is improved, and further, it has excellent characteristics as an optical functional element that is improved in light damage resistance.

That is, in the optical functional device of the present invention, the polarization structure of the lithium tantalate single crystal is periodically inverted to shorten the wavelength of the incident laser having a wavelength in the visible to near infrared region or to increase the wavelength. In the optical functional element, the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O), which is close to the stoichiometric composition of Li, is 0.500 to 0.505, and lithium tantalate single crystal was used as the substrate. Is characterized by.

Further, in the optical functional device of the present invention, various information is recorded in the single crystal by forming polarization reversal in a minute region in the lithium tantalate single crystal in the single domain state and reversing the polarization. In an optical storage element or optical circuit element,
Li ₂ O / (Ta ₂ O ₅ + Li ₂₎ close to the stoichiometric composition of excess Li
The molar fraction of O) is 0.500 to 0.505, and a lithium tantalate single crystal is used for the substrate.

The optical functional element of the present invention is an optical element that controls the laser light incident on the single crystal by utilizing the electro-optical effect of the single crystal, and is a ferroelectric polarization of the lithium tantalate single crystal. In an optical device that uses the large change in the refractive index of the inversion structure to deflect, focus, and switch light,
Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) close to excess stoichiometry composition
The mole fraction of is 0.500 to 0.505, and lithium tantalate single crystal is used for the substrate.

In the optical functional element, the Curie temperature of the lithium tantalate single crystal serving as the substrate is 686-6.
It is preferably in the range of 95 ° C.

In the optical functional element, the applied voltage required for polarization reversal of the lithium tantalate single crystal substrate is 3K.
It is preferably V / mm or less.

Further, in the above-mentioned optical functional device, the threshold value of light damage resistance of the lithium tantalate single crystal substrate is 407 nm.
It is preferably 10 ³ KW / cm ² or more with respect to continuous wave laser irradiation.

The present inventors have found out that the problem of polarization reversal and optical damage control in the optical functional device using the LT single crystal lies in the composition of the single crystal substrate. The present invention is focused on an LT crystal single crystal substrate in a certain composition range as an optical functional device application utilizing the polarization inversion structure of the LT crystal. The molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is greater than 0.50 and Li
The present inventors have for the first time found that an LT single crystal having an excessive stoichiometric ratio of components has excellent polarization reversal controllability unlike conventional characteristics.

Further, the composition of the LT crystal was changed to Li ₂ O / (Ta ₂ O ₅
By making the mole fraction of + Li ₂ O) larger than 0.50 and making the Li component excess, it became possible to significantly improve the optical damage resistance of the optical functional device. It was clarified that the characteristics of the optical functional element applied to the short wavelength light are dramatically improved by utilizing this.

The polarization inversion characteristics and the light damage resistance characteristics found this time are also the effects peculiar to the LT single crystal having this mole fraction. The LT single crystal, which is close to the stoichiometric composition, is a crystal that has finally made it possible to produce an optically homogeneous substrate by the double feed crucible method in which the raw materials are continuously fed. Not revealed. In particular, the present inventors have clarified for the first time the optical characteristics of the LT crystal substrate in which the Li component is in excess of the stoichiometric ratio. Further, it is an undeveloped field to make a great improvement of the characteristics of the optical functional device by utilizing this characteristic.

[0043]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Originally, the ideal composition of LT single crystal is
Although the Li: Ta ratio is 1: 1, the congruent composition LT single crystal substrate that has been commercially available in the past has a large amount of Ta component excess due to the limitation of the single crystal growth technique (Li ₂ O / (Ta ₂ O ₅ + L
i ₂ O) mole fraction is about 0.485). On the other hand, the Li component excess (for example, Li ₂ O / (Ta ₂ O ₅ + L
The LT crystal with a composition close to the stoichiometric composition developed from a melt having a mole fraction of (i ₂ O) of 0.56 to 0.60) also has a side with a slight excess of Ta component (Li ₂ O / (Ta ₂ O ₅ + Li The mole fraction of ₂ O is 0.495 to 0.50).

The present inventors found that when a crystal was grown from a melt in which the composition of the melt was remarkably Li component excess (the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) was 0.60 to 0.67). , An Li component close to an excessive stoichiometric composition (the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is greater than 0.50) can grow an LT single crystal, and the non-stoichiometric defect concentration due to the Ta component excess can be increased. This is the first clarification that the suppressed single crystal exhibits excellent characteristics as an optical functional device substrate.

That is, it has been found that a large amount of defects formed by excessive Ta in the conventional crystal causes a great problem for the application of the optical functional device. It is clear that the presence of this defect makes the hysteresis curve that shows the relationship between the applied voltage required for polarization reversal and spontaneous polarization asymmetrical, and requires a high voltage of several tens of kV / mm for polarization reversal. It was Further, since polarization reversal is likely to be pinned at locations where defects are unevenly distributed inside the crystal and the defect concentration is high, it is technically difficult to accurately perform polarization reversal even when using the voltage application method. It became clear that there is.

In the LT single crystal, Li and Ta ions are arranged at an electrically neutral position in the paraelectric phase above the Curie temperature, but in the ferroelectric phase below the Curie temperature, Li and Ta ions are +. It is slightly shifted in the z or -z direction. The positive and negative polarization directions of the domain are determined by the direction of this ion shift. In the optical functional device with the polarization inversion structure,
It is necessary to forcibly move these ions at a low temperature by applying a high electric field.

When there are many non-stoichiometric defects of coincident melting composition
Is Li ion, though it tends to diffuse and move through the vacancy.
It is not easy to move excess Ta that has entered the site
Therefore, a large applied voltage is required for polarization reversal. This
Therefore, the Ta excess component side (Li ₂O / (Ta₂O_Five＋ Li₂O mole
A crystal with a fraction of 0.495 to 0.50) has a Ta component in excess.
Li component that completely eliminated stoichiometric defects is close to an excessive stoichiometric composition.
LT single crystal (Li₂O / (Ta ₂O_Five＋ Li₂O) mole fraction 0.500
~ 0.505) has excellent controllability of polarization inversion.

The present invention has revealed that, as an LT single crystal having a Li component close to a stoichiometric composition, a crystal having a Curie temperature in the range of 686 to 695 ° C. exhibits the same characteristics. This requires a chemical analysis by a highly skilled technique to accurately evaluate the crystal composition, and requires a long measurement time. On the other hand, for example, Curie temperature measurement by the suggestive thermal analysis method whose properties largely depend on the crystal composition is useful as a method for easily managing and evaluating the crystal composition.

Also, it was revealed that Ta, which is an excessive component, caused a problem with respect to optical damage. LT single crystal (Li ₂ O / (Ta ₂ O ₅ + Li ₂
O) mole fraction of 0.500 to 0.505), or various LT single crystals with Curie temperature in the range of 686 to 695 ℃, 532 nm continuous wave green laser with light intensity of 10 ³ KW / cm ² or more. No light damage was observed when irradiated with.

Further, the threshold value for light damage resistance is improved even with respect to the incidence of laser light of a shorter wavelength, which has not been solved in the past.
No optical damage was observed when irradiated with a continuous wave laser having a wavelength of 407 nm and a light intensity of 10 ³ KW / cm ² , and it was revealed that an optical element using this crystal as a substrate can also operate stably.

Next, it is used as an optical functional device of the present invention.
2 shows the manufacturing method and physical properties of the LT single crystal. Commercial high-purity Li
₂CO₃, Ta₂O_FivePrepare the raw material powder of
Li ₂CO₃: Ta₂O_FiveThe ratio of 0.60: 0.40, 0.62: 0.38, 0.64:
It was mixed in a ratio of 0.36, 0.66: 0.34. Also, the stoichiometry
Li as a raw material₂CO₃: Ta₂O_Five= 0.50: 0.50 ratio
Mixed in. Next, 1ton / cm²Rubber press with hydrostatic pressure
Molded, and the raw material of each composition ratio in oxygen at about 1050 ° C
A raw material rod was prepared by sintering. In addition, with powder material for continuous supply
And mixed about 13
Sintering in the air at 50 ° C also creates a stoichiometric composition raw material.
It was

Next, an LT single crystal in which Li was excessive and had a nearly stoichiometric composition was grown by using the double crucible method of continuously supplying the raw materials. A melt with an excessive Li component composition in the double crucible (for example, Li ₂ O / (Ta
With a seed crystal to 0.60,0.62,0.64,0.66) with ₂ O ₅ + Li ₂ O) molar fraction of pulling rate 0.5 mm / h, close to the stoichiometric composition by crystal rotation speed 20 rpm, i.e., stoichiometric defect concentration A single crystal in which the In order to precisely control the density and structure of nonstoichiometric defects, the amount of Li ₂ O / (Ta
Crystals were grown while automatically supplying a continuous feed material having a stoichiometric composition ratio of ₂ O ₅ + Li ₂ O) of 0.50 to the outer crucible.

The crucible used for the growth was made of iridium, the outer crucible had a diameter of 125 mm and a height of 70 mm, and the inner crucible had a diameter of 85 mm and a height of 90 mm. Also in this case, in order to make the melt composition uniform, the crucible was rotated at a speed of 4 rpm in the direction opposite to the seed crystal during the growth. The growth conditions were a crystal rotation speed of 20 rpm and a pulling speed of 0.5 mm / h, which was constant.
The growing atmosphere was nitrogen containing 0.05% oxygen.

In the growing process, as in the case of growing a normal optical congruent LT single crystal, the transition metal impurities such as iron and chromium, which are considered to be one factor that induces optical damage, should be prevented from entering as much as possible. I paid attention.
After growing for about 1 week, a colorless and transparent LT single crystal having a diameter of about 55 mm and a length of about 70 mm and having no crack was obtained.
The domain state inside the obtained as-grown crystal was a multi-domain state.

Therefore, the obtained L is obtained before the polling.
The Curie temperature of the T single crystal was determined by the suggestive thermal analysis method.
A standard sintered sample having a stoichiometric composition prepared in advance and sintered at 1500 ° C. was prepared in advance, and its Curie temperature was confirmed to be 694 ° C.

Next, a melt having a Li component excess composition (for example, Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) in the double crucible has a molar fraction of 0.60,0.
The Curie temperature of each LT single crystal obtained from the composition 62, 0.64, 0.66) was measured. The Curie temperature of each crystal is in the range of 686 to 695 ° C, which is higher than the Curie temperature of 675 to 685 ° C of the LT single crystal having a composition close to the stoichiometric composition reported so far. Moreover, it was found that it was close to the Curie temperature of the standard sintered sample of stoichiometric composition.

Further, it was also confirmed that the Curie temperature of the sample cut out from one crystal obtained here is constant within the measurement error regardless of the cutting position of the sample, and the homogeneity of the crystal composition is extremely good. . Moreover, the Curie temperature of the grown crystal was not significantly higher than the Curie temperature of the standard sintered sample, 694 ° C.

The melt composition in the double crucible was changed to Li ₂ O / (Ta ₂ O ₅ + L
As the molar fraction of (i ₂ O) was gradually increased to more than 0.60, the Curie temperature of the obtained crystals tended to gradually decrease. This means that the Curie temperature of the LT single crystal has a maximum value in a perfect stoichiometric crystal. The maximum Curie temperature of 695 ° C of the obtained crystals is higher than the Curie temperature of 694 ° C of the standard sintered sample of stoichiometric composition, which is considered to be due to the measurement error of the Curie temperature obtained by the suggestive thermal analysis method. To be from these things,
It is considered that the grown crystal may have a perfect stoichiometric composition or the Li component may be in excess of the stoichiometric composition.

Therefore, the composition was directly analyzed by the chemical analysis method. In chemical analysis, it is difficult to accurately determine the absolute value of the composition ratio, and in the case of LT crystal, the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) contains an error of about 0.001 to 0.005. Therefore, the LT crystal having a composition close to the stoichiometry was analyzed very carefully. The same sample was evaluated using several different analyzers, and the average value was calculated. as a result,
Melt with excess composition of Li component in double crucible (eg, Li ₂ O /
(Ta ₂ O ₅ + Li ₂ O) mole fraction 0.60, 0.62, 0.64, 0.66)
In the case of each LT single crystal obtained from the composition, Li ₂ O /
The value of the mole fraction of (Ta ₂ O ₅ + Li ₂ O) was in the range of 0.500 to 0.505, and it was found that the LT single crystal was extremely close to the stoichiometric composition in which the Li component was excessive.

Next, each of the obtained crystals is heated to about 750 ° C., which is higher than the Curie temperature, and then the crystals are heated from the Z-axis direction to about 750 ° C.
By applying a voltage of 5-10 V / cm and cooling to room temperature, a single domain was created. A block-like sample having a size of 35 mm × 35 mm × 50 mm was cut out from the LT single crystal having a single domain, and the surface was polished by mechanochemical polishing. When the optical homogeneity of the sample was evaluated by the Mach-Zehnder interferometry, no macroscopic defects or optically non-uniform parts were found,
The change in refractive index in the sample was 1 × 10 ⁻⁵ or less, which confirmed that the optical homogeneity was excellent.

Thus, the LT single crystal having a Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) mole fraction in the range of 0.500 to 0.505 and having an excessive Li component close to the stoichiometric composition, or the above-mentioned Curie temperature Is 686-6
The LT single crystal substrate in the 95 ° C. range has excellent optical homogeneity.

The inventors of the present invention have described that the above-mentioned single crystal in which the nonstoichiometric defect concentration due to the Ta component excess is suppressed has a large amount of defects formed by the excessive Ta in the conventional crystal, and thus is required to be applied for polarization reversal. The hysteresis curve showing the relationship between the voltage and the spontaneous polarization becomes asymmetric, and the problem that a high voltage of several tens of kV / mm is required for polarization reversal and the defect concentration unevenly distributed inside the crystal Since the polarization reversal is likely to be pinned at a high place, it is technically difficult to accurately perform the polarization reversal even if the voltage application method is used. It was clarified that the formation of

Next, 30 mm × from various single crystals grown.
A z-cut sample having a thickness of 30 mm and a thickness of 0.5 to 3.0 mm was cut out. After forming electrodes on both z surfaces, a voltage was applied and the polarization reversal voltage was measured from the change in current value. As a result, as shown in FIG. 1, in the conventional congruent melting composition LT single crystal, the applied voltage required for polarization reversal was about 25 kV / mm (reversal voltage of 21 kV
It is necessary to add a value of a few kV / mm higher than / mm), whereas it was confirmed that the polarization is inverted at an applied voltage of about 2 to 4 kV / mm when approaching the stoichiometric composition.

This result is consistent with the result previously reported by the present inventors. The present inventors have further added that Li ₂ O / (Ta ₂ O
_{In the} LT single crystal sample having a molar fraction of ( ₅ + Li ₂ O) in the range of 0.500 to 0.505, which is close to the stoichiometric composition with an excessive Li component, the applied voltage required for polarization reversal was even smaller, and the applied voltage was 0. .
A sample that can form polarization inversion even at about 5 to 1 kV / mm was also obtained. Moreover, the spontaneous polarization-applied voltage hysteresis showed perfect symmetry, and the internal electric field was 0 kV / mm within the measurement error. Therefore, it was also clarified that the polarization inversion process is extremely reversible and the controllability of the polarization inversion process is excellent.

As described above, even in the LT single crystal close to the same stoichiometric composition, a slight Ta excess component side (Li ₂ O / (Ta ₂ O ₅ + Li
Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) that completely eliminates non-stoichiometric defects with excess Ta component than crystals with a mole fraction of ₂ O of 0.495 to 0.50)
It was confirmed that the polarization reversal controllability was significantly improved in the LT single crystal having a molar fraction in the range of 0.500 to 0.505 and having a Li component in excess of the stoichiometric composition.

Further, the present inventors have noted that Ta, which is an excessive component, causes a problem with respect to optical damage. Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) mole fraction in the range of 0.500 to 0.505, LT single crystal close to stoichiometric composition with excess Li component, or Curie temperature in the range of 686 to 695 ℃ A variety of L
The T single crystal was irradiated with continuous wave green light having a wavelength of 532 nm (Verdi manufactured by Coherent Co., Ltd.) and continuous wave blue-violet light having a wavelength of 407 nm (Krypton laser Inova manufactured by Coherent Co., Ltd.) to examine the presence or absence of optical damage.

Cube samples of about 5 mm ³ square were cut out from crystals of various compositions, and both x, y and z surfaces were optically polished to prepare samples for evaluation. Laser light that has passed through the crystal by squeezing the laser light from the y-axis and x-axis directions of the crystal with a lens and gradually changing the light intensity in the range of 10 ^-3 to 10 ³ KW / cm ² Was observed with a beam profiler (Hamamatsu Photonics KK) through a filter. The polarization direction of the laser was parallel to the z-axis direction of the crystal.

When optical damage occurs, the beam spreads in the z-axis direction of the crystal and the shape is distorted (beam fanning).
When beam fanning occurred and was observed during irradiation for 1 minute, the incident light intensity was defined as the optical damage threshold.
As shown in Fig. 2, some Ta-rich component side (Li ₂ O / (Ta ₂ O ₅
In the crystals with a + Li ₂ O mole fraction of 0.495 to 0.50), the light damage resistance threshold value fluctuated by two or more digits depending on the sample. This light damage resistance threshold value was largely dependent on the heat treatment state of the crystal.

That is, it was considered that the optical absorption caused by polaron induced in a state where the excessive Ta ions substituting the Li ion sites were reduced was one of the causes for lowering the optical damage threshold. On the other hand, in the case of LT single crystal having a molar ratio of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) in the range of 0.500 to 0.505, which is close to the stoichiometric composition of Li component, Moreover, no damage was observed even when the incident light intensity was 10 ³ KW / cm ² , and it was revealed that stable operation of an optical element using this crystal as a substrate can be expected. In this case, it is considered that polaron is not induced even in the reduced state because there is no excess Ta ion that replaces the Li ion site.

It should be noted that, up to now, some Ta excess component side (Li
₂ O / In _{(Ta 2 O 5 + Li 2} O mole fraction of from 0.495 to 0.50) in a crystalline, in order to provide a stable and strong crystal optical damage Mg
It was necessary to add additives such as. On the other hand, a lithium tantalate single crystal having an excessive Li component has a high optical damage threshold without adding an additive such as Mg, which is a great practical advantage.

The reason for this is that in the production of LT single crystal containing Mg, in order to grow the crystal without degrading the optical quality by uniformly distributing the Mg element in the crystal, the case of the undoped crystal was used. Compared with this, the crystal growth rate must be slowed down, and there is a problem that productivity is deteriorated. Furthermore, since the crystal containing Mg has a polarization reversal characteristic different from that of the undoped crystal,
Although there is a problem that the controllability deteriorates, the present invention makes it possible to solve these problems.

[0072]

EXAMPLES The present invention will be described in more detail with reference to the following examples. Example 1 As one of the optical functional elements using the LT single crystal produced by the above method, the characteristics when applied to an optical wavelength conversion element will be described. Fig. 3 shows an LT single crystal having a composition Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) with a molar fraction in the range of 0.500 to 0.505 and having an excessive Li component, which is close to a stoichiometric composition. Crystals or various L with Curie temperature in the range of 686-695 ℃
It is a schematic block diagram of the QPM device which formed the periodic polarization inversion structure on the board | substrate using T single crystal as a board | substrate.

A comb-shaped electrode and a parallel electrode were patterned on the + z surface of the substrate 1 having a thickness of 0.5 mm to 3.0 mm which was optically polished on both sides. The period was about 3.2 μm and it was designed to be quasi-phase matched to the wavelength of 850 nm. An electrode was vapor-deposited on the entire surface of the −z plane. Between the comb electrode and the parallel electrode, and between the comb electrode and -z
A voltage was applied to each of the back electrodes of the surfaces to form the periodically poled regions 2.

The LT crystal used has a very uniform polarization state in advance. Even when periodic polarization inversion is formed in the crystal, the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is 0.500 to 0.505.
In the LT single crystal in which the Li component is close to the stoichiometric composition with an excessive amount, or in the various LT single crystals in which the Curie temperature is in the range of 686 to 695 ° C, the uniformity of the crystal is excellent, A polarization inversion structure can be formed. In an LT crystal having a normal congruent composition, a large number of small domain-inverted regions (microdomains) are present in the crystal, so that when a fine domain-inverted structure is formed, nonuniformity of domain-inverted shape due to microdomains occurs. Furthermore, the LT single crystal substrate of the present invention is characterized in that the periodic domain-inverted structure can be easily shortened.

In the conventional congruent composition LN crystal,
The applied voltage required for polarization reversal is about 25 kV / mm (
The value is very large, which is several kV / mm higher than 21 kV / mm), so it was necessary to reduce the electrode interval to 0.5 mm or less in order to avoid dielectric breakdown. Further, the polarization inversion portion tends to expand to the electrode periphery through the micro domain due to the generation of the electric field in the electrode periphery, and it is difficult to form a short-period polarization inversion structure. It was difficult to form an inverted structure of about 3 μm even in a short cycle.

On the other hand, Li ₂ O / produced by the above method
The molar fraction of (Ta ₂ O ₅ + Li ₂ O) is in the range of 0.500 to 0.505 L
The LT single crystal with an i component close to the stoichiometric composition has an applied electric field of about 0.5 to 2 kV / mm, which is 1/12 to 1/25 or less of the conventional one, and the substrate thickness as a bulk element can be expanded. It was

As described above, the LT created by the above method
Since the single crystal has a very uniform polarization structure in the crystal and has very few microdomains, it is possible to prevent the extension of polarization inversion to the electrode peripheral portion, and in the conventional case, it exceeds about 3 μm. It was found that a short-period domain-inverted structure of 2 μm or less can be easily formed.

After forming the domain-inverted lattice, the crystal was removed, the y-plane of the crystal, which became the side surface, was polished and etched with a mixed solution of hydrofluoric acid / nitric acid to examine the state of domain inversion. By optimizing the pulse width and current of the applied voltage, the periodic polarization reversal width ratio and its polarization shape can be created accurately and ideally at the ideal polarization reversal width ratio of 1: 1 over the entire sample. Was confirmed.

In the QPM-SHG element shown in FIG. 3, the polarization width, which has been a problem in the past, has been suppressed from widening in the lateral direction. The formation of this periodic domain-inverted structure is 0.5m thick.
Not only the sample of m but also other thicker samples are formed with high accuracy, and these thick samples are considered to be optimal as, for example, an internal resonator type wavelength conversion element. Next, a wafer was cut out and a sample whose end face was polished was prepared.

For highly efficient wavelength conversion, the QPM-SHG element is inserted into the laser and the resonator serving as the fundamental wave, or an optical waveguide is formed to confine the semiconductor laser of the fundamental wave. , About 50% for a sample with a device length of 10 mm
The generation of stable SHG output was confirmed with the conversion efficiency of. QPM-S
A tunable high-power Ti sapphire laser (wavelength 850 nm) 4 was used as the fundamental wave for the evaluation of the characteristics of the HG device. Optical coupling was performed using the lens 5. The stoichiometric LT crystal has a non-linear optical constant 1.2 times or more that of the congruent composition LT crystal, and the non-linear optical constant of the substrate was improved, so that a highly efficient optical wavelength conversion element could be formed.

Further, it was confirmed that the light damage resistance was significantly improved and the stability and reproducibility were improved. When the conventional LT single crystal with excess Ta component is used, blue light (wavelength: 400 nm) of several tens mW or more is used.
When the band) was output, the output became unstable due to optical damage. That is, in the conventional QPM-SHG device using the single crystal of the congruent melting composition as a substrate, when the high output was generated due to this optical damage, the phenomenon that the SHG optical output decreased with time was occasionally observed.

On the other hand, Li ₂ O / produced by the above method
The molar fraction of (Ta ₂ O ₅ + Li ₂ O) is in the range of 0.500 to 0.505 L
By using an LT single crystal having an i-component close to a stoichiometric composition, a stable output can be obtained even for blue light of 50 mW or more. In particular, with respect to SHG light having a wavelength of 415 nm or less, the improvement of the light damage resistance strength was remarkable.

The reason for this is as follows. First, in the LT crystal having the composition of the present invention, the non-stoichiometric defect concentration is much smaller than that of the conventional congruent melting composition crystal. Therefore, the photoconductivity is high. It is considered that if the photoconductivity is high, the localization of the photocarriers that causes the photodamage is canceled and the photodamage is less likely to occur.

Secondly, Li ₂ O / (Ta ₂
In a lithium tantalate single crystal having a molar fraction of O ₅ + Li ₂ O) of more than 0.50 and having an excess of Li component, there is no excess Ta ion that replaces the Li ion site, which may cause optical damage even in the reduced state. It is thought that this is because the polaron is not induced.

Thirdly, in the domain-inverted element of the present invention, the domain-inverted width is as small as several microns and the ratio is perfect.
Since it is formed 1: 1, even if there is some optical damage to the material, the optical damage having anisotropy in the z-axis direction is canceled between the adjacent polarizations.

Fourth, since the non-stoichiometric defect concentration is low in the stoichiometric composition crystal, macroscopic crystal defects such as light scattering factors and striations are scarcely contained, and the light absorption of the crystal is very small.

In particular, in a high-power SHG element, optical damage due to the thermal lens effect may occur due to an increase in optical absorption due to the fundamental wave and higher harmonics, but the crystal perfection is high and the optical absorption is small. It is understood that the specific composition LT single crystal solves these problems. In addition, here, it is described in detail about an example in which a QPM-SHG element that generates blue light with respect to a fundamental wave of near infrared light of 850 nm is prepared, but according to the present invention, the fundamental wave is The present invention is not limited to one wavelength, but can be applied to a wavelength range in which the LT single crystal is transparent and phase matching is possible.

Furthermore, the optical functional element for periodically reversing the polarization structure of the LT single crystal of the present invention to shorten or lengthen the wavelength of the incident laser having a wavelength in the visible to near infrared region is Not limited to the second harmonic generation element, it can be applied to various application fields including remote sensing and gas detection such as an optical parametric oscillator element.

Example 2 Next, the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) prepared by the above method is in the range of 0.500 to 0.505, and the Li component is close to an excessive stoichiometric composition. A lens or prism-shaped polarization inversion structure was formed on the LT single crystal to produce a deflection element utilizing the electro-optical effect, and an optical element such as a cylindrical lens, a beam scanner, or a switch.

FIG. 4 shows a wavelength conversion element 8 in which periodic polarization inversion is formed, and a polarization inversion structure 12 of lenses 9, 10 and a prism shape 11.
FIG. 3 is a configuration diagram of an optical element using a Li component-excess LT single crystal substrate 6 in which is integrated. Diameter 2 inches, thickness 0.2-2.0m
Prepare z-cut Li-rich excess LT single crystal polished on both sides, and form Al electrodes with a thickness of about 0.2 micron on both z-planes by sputtering. Was formed. After that, a pulsed voltage of about 0.5 to 2.0 kV / mm was applied to the + z plane to reverse the polarization.

Further, heat treatment was applied to eliminate the nonuniformity of the refractive index which is said to be introduced upon polarization reversal. Further, the end face of the crystal was mirror-polished to form a laser beam input / output face. The performance of the optical element utilizing the electro-optic effect of the LT single crystal in which the refractive index reversal is formed by the prototyped domain-inverted structure depends on the accuracy of the design of the lens- or prism-shaped domain-inverted structure and the manufacturing process of the domain-inverted structure. It was determined by the magnitude of the electro-optic constant of the material. In the prototyped lens and the polarization reversal structure of the prism pattern, it is noteworthy that good element characteristics were obtained because the polarization reversal electric field was low and the control of the polarization reversal property was very easy. That is.

In the conventional LT crystal having the congruent melting composition, it was difficult to control the domain inversion structure because a large applied voltage is required for domain inversion. In addition, LT with an excess of Ta component is close to the stoichiometric composition
In a single crystal in which a small amount of MgO is added to the crystal, the inversion period becomes short and the inversion structure becomes complicated, so that it is difficult to manufacture a lens or a prismatic polarization inversion structure with high accuracy.

On the other hand, the Li ₂ O /
The molar fraction of (Ta ₂ O ₅ + Li ₂ O) is in the range of 0.500 to 0.505 L
By using an LT single crystal having an i component close to an excessive stoichiometric composition, or a lithium tantalate single crystal substrate having a Curie temperature in the range of 686 to 695 ° C as an optical functional device using a polarization inversion structure, It was possible to form highly accurate polarization inversion of the device.

Furthermore, since the light damage resistance characteristic does not matter, an optical functional element using short wavelength light of blue-violet to green is
There was no problem of beam deformation due to optical damage when a lens or prism-shaped polarization-inverted structure was manufactured, and optical elements such as a deflection element, a cylindrical lens, a beam scanner, and a switch, which used the electro-optical effect, were manufactured.

Furthermore, since the present crystal has a larger electro-optic constant r ₃₃ than the crystal of the congruent melting composition, superior device performance was obtained at a smaller operating voltage. For example, in the case of a deflection element, a large deflection angle of about 6 ° C. was obtained at a voltage of about 600 V / mm. In addition, a lens that operates near 100V / mm and a switching operation at about 500V / mm were obtained. Example 3 Next, an LT single crystal having a Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) molar fraction in the range of 0.500 to 0.505 produced by the above-mentioned method and having a Li component in excess of a stoichiometric composition A conceptual diagram of FIG. 5 shows a method of manufacturing an optical storage element using the minute polarization inversion part of the LT single crystal by using.

As described above, the LT single crystal with excess Li component of the present invention is characterized in that polarization inversion can be easily formed as compared with the conventional LT single crystal with excess Ta component. Further, the LT single crystal itself is in a multi-domain state in the as-grown state, but the size of each domain is as small as several microns, which is smaller than the domain size of the LN single crystal of several mm to several cm. Remarkably small. This means that the LT single crystal itself is LN
This suggests that it is easier to form minute domains of several microns or less than single crystals.

Therefore, as shown in FIG. 5, the thickness is 0.2 to 1 m.
An LT single crystal 14 having a size of m and a size of 10 mm × 20 mm square and having both sides polished z mucut Li component was prepared, and as shown in FIG. 5, + z
From the surface, using a scanning microscope utilizing Maxwell stress, a voltage was applied to the minute region to reverse the polarization direction of the portion 17.

In this method, the electrode attached to the -z plane of the crystal
In addition to an alternating electric field applied between the conductive tip 15 and the conductive tip 16, a polarization inversion is formed in the crystal by an electric field induced by a slight vibration. Simultaneously with the formation of the polarization inversion, the state of the sample surface, the vibration amplitude and phase of the induced electric field, and the polarization state of the crystal were observed using the laser 18 and the light receiving element 19. Using this method, we confirmed that it is possible to obtain an optical device that utilizes ferroelectric polarization to easily write and store two-dimensional information with a width of about 1 micron in a single crystal of Li-rich near a stoichiometric LT composition. did.

[0099]

As described above in detail, according to the present invention, a Li substrate having a Li-excess stoichiometric composition close to that of Li is used.
By using a lithium tantalate single crystal with a molar fraction of ₂ O / (Ta ₂ O ₅ + Li ₂ O) of 0.500 to 0.505, an element with excellent polarization reversal controllability can be realized, and You can expect a significant improvement.

[0100] Further, near the Li excess stoichiometric composition crystal substrate _{L i2 O / (Ta 2 O} 5 + Li 2 O) molar fraction of 0.500
~ 0.505 lithium tantalate single crystal, excellent light damage resistance, light intensity 10 ³ KW / cm ² or more wavelength 407
Since it can be stably operated with respect to continuous wave laser irradiation of nm, it is possible to provide an optical functional element with excellent performance. As a result, the present invention brings about a great effect of promoting the practical application of the optical functional device in the fields of optical information processing using laser light, optical processing technology, photochemical reaction technology, optical measurement control and the like.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the composition of LT single crystal and polarization reversal characteristics.

FIG. 2 is a graph showing the relationship between the composition of LT single crystal and the photodamage threshold value.

FIG. 3 is a conceptual diagram showing a light wavelength conversion element according to an embodiment of the present invention.

FIG. 4 is a conceptual diagram showing an optical element in which a polarization-inverted wavelength conversion element, a lens, and a prism are integrated.

FIG. 5 is a conceptual diagram showing a method for manufacturing an optical storage element using a minute polarization inversion portion of LT single crystal.

[Explanation of symbols]

1 Li component excess LT single crystal substrate 2 domain inversion region 3 Periodic polarization inversion width 4 Tunable laser 5 lens 6 Li component excess LT single crystal substrate 7 Semiconductor laser 8 Periodic domain inversion region 9 convex lens 10 concave lens 11 prism 12 domain inversion region 13 Emitting laser 14 Li component excess LT single crystal substrate 15 electrodes 16 chips 17 Micro polarization reversal region 18 laser 19 Light receiving element 20 Lock-in amplifier

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI G02F 1/39 G02F 1/39 (72) Inventor Yasunori Furukawa 1-1-1, Namiki, Tsukuba, Ibaraki Prefectural Institute of Inorganic Materials (72) Kenji Kitamura 1-1, Namiki, Tsukuba-shi, Ibaraki, Institute for Inorganic Materials, Science and Technology Agency (72) Inventor Shunji Takekawa 1-1-1, Namiki, Tsukuba, Ibaraki, Institute for Inorganic Materials (56) ) References JP-A-11-35393 (JP, A) JP-A-5-310500 (JP, A) JP-A-10-215131 (JP, A) JP-A-61-68397 (JP, A) JP-A 63-265896 (JP, A) Furukawa et al., Growth and optical characterization of stoichiometric LiTaO3 single crystals, Proc. 43rd Annual Meeting of the Artificial Crystal Discussion Group, 1998, pp. 23-24 Kitamura et al., LT・ Morphology of LN ferroelectric polarization wall Non-stoichiometric composition dependence, Journal of the Crystal Growth Society of Japan, July 22, 1999, Vol. 26, No. 2, page 7 Kitamura et al. , Appl. Phys. Let t. , November 23, 1998, Vol. 73, No. 21, pp. 3073-3075 V. Gopalan et al. Appl. Phys. Lett. , April 20, 1998, Vol. 72, No. 16, pp. 1981-1983 (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/29-7/00 C30B 1/00-35/00 JISST file (JOIS)

Claims (6)

(57) [Claims] 1. An optical functional device for periodically reversing the polarization structure of a lithium tantalate single crystal to shorten or lengthen the wavelength of an incident laser having a wavelength in the visible to near-infrared region. Close to the stoichiometry composition of
An optical functional element characterized by using a lithium tantalate single crystal having a molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) of 0.500 to 0.505 as a substrate. 2. An optical storage element or an optical circuit element for recording various information in a single crystal by forming polarization reversal in a minute region in a single domain lithium tantalate single crystal and reversing the polarization. , The molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) close to the stoichiometric composition of excess Li is 0.500 to 0.505
An optical functional element characterized by using the above-mentioned lithium tantalate single crystal as a substrate. 3. An optical element for controlling a laser beam incident into a single crystal by utilizing the electro-optical effect of the single crystal, which has a large refractive index change of a ferroelectric polarization inversion structure of lithium tantalate single crystal. In an optical device that uses light to deflect, focus, and switch light, the tantalum with a molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) close to that of Li-excess stoichiometry is 0.500 to 0.505. An optical functional element characterized by using a lithium oxide single crystal as a substrate. 4. An optical functional device using a lithium tantalate single crystal having a stoichiometric composition close to the Curie temperature of 686 to 695 as a substrate.
4. The optical functional device according to claim 1, wherein the optical functional device has a temperature range of ° C. 5. The lithium tantalate single crystal substrate having a stoichiometric composition close to that described above is characterized in that an applied voltage required for polarization reversal is 2 kV / mm or less.
The optical functional element according to any one of 1. 6. The lithium tantalate single crystal substrate having a stoichiometric composition having a light damage threshold of 10 ³ KW / cm ² or more when irradiated with a continuous wave laser having a wavelength of 407 nm. Item 5. The optical functional device according to any one of items 1 to 4.