JP2002072266A - Optical functionai, element using ferroelectric polarization inversion of lithium tantalate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical functional element in which the controllability of a polarization inversion structure is improved and the resistance against optical damage is further improved. SOLUTION: The following optical elements (1) to (3) or the like can be realized by using a single crystal of lithium tantalate having nearly a stoichiometric composition with excess Li and 0.500 to 0.505 mole fraction of Li2O/(Ta2O5+Li2O) for the substrate. The elements are (1) an optical functional element which converts the wavelength of incident laser light having the wavelength in the visible to near IR region into shorter or longer wavelength by periodically reversing the polarization structure of the lithium tantalite single crystal, (2) an optical memory element or optical circuit element to record various kinds of information in the single crystal by forming polarization reversal in a minute region in the lithium tantalate single crystal in a single polarization state, and (3) an optical element which controls the laser light incident to the single crystal by using the electro-optical effect of the single crystal and which deviates, focuses or switches the light by using the large change in the refractive index in the reversal structure of the ferroelectric polarization of the lithium tantalite single crystal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to lithium tantalate (LiTaO ₃ ) used in fields such as optical information processing using laser light, optical processing technology, photochemical reaction technology, optical measurement control and the like.
More specifically, the present invention relates to an optical functional device made of a single crystal (hereinafter abbreviated as LT), and particularly to a wavelength conversion device that periodically or periodically reverses 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 for deflecting, focusing, and switching light utilizing a large change in the refractive index of a domain-inverted structure, and an optical storage element and an optical circuit element for recording by inverting the polarization of a minute region of an LT single crystal. .

[0002]

2. Description of the Related Art An LT single crystal is a ferroelectric substance mainly used as a substrate of a surface acoustic wave device or an optical modulator, but is transparent in a wide wavelength range from visible to infrared. By applying it, a periodic polarization structure can be created, and to some extent practical optical non-linearity and electro-optical characteristics,
In recent years, a single crystal having a large diameter and high composition homogeneity can be supplied at a relatively low cost, and thus, in recent years, has attracted attention as a substrate for a functional element such as a wavelength conversion element.

In particular, the development of a waveguide type optical second harmonic generation (SHG) element for converting a semiconductor laser having a near-infrared wavelength into a half-wavelength blue light by a nonlinear optical effect is expected.
Above all, as a light source for high-density recording / reproducing of an optical disk, an SHG element using an element in which a polarization structure of an inorganic single crystal such as LN, LT, KTP or the like is periodically inverted is best studied.

This SHG element has a quasi phase matching (Quasi Phas
It is based on the se-matching (QPM) method, in which the difference between the propagation constants of the fundamental wave and the harmonics is compensated by a periodic structure to achieve phase matching. With this method, high conversion efficiency can be obtained,
It has many outstanding features, such as easy parallelization of output light and diffraction-limited focusing, and no limitations on applicable materials and wavelengths.

As a periodic structure for QPM, a structure in which the sign of the SHG coefficient (d coefficient) is periodically inverted is the most effective in obtaining high efficiency. In a ferroelectric crystal, the sign of the d coefficient is strong. Since it corresponds to the polarity of dielectric polarization, a periodic inversion structure of a ferroelectric polarization domain is used. The QPM-SHG scheme, can be wavelength conversion with high efficiency to use also like nonlinear optical constant d ₂₂ and d ₃₃ can not be used in the phase matching method using the birefringence is considered to great advantage.

[0006] Particularly, LT single crystal, a large nonlinear optical constant alongside LN single crystal (d ₃₃ is 26 pm/V) has a strong to light damage compared to LN single crystal, also extends fundamental absorption edge up to 280nm Therefore, it is promising as a short wavelength wavelength conversion material.

For an optical element utilizing the electro-optic effect, see, for example, a document (M. Yamada et al., Appl. Phy.
According to s. Lett., 69, p3659, 1996), an optical element that forms a lens or prism-shaped domain-inverted structure in a ferroelectric crystal and deflects the laser beam passing through it using the electro-optic effect. , Cylindrical lenses, beam scanners, switches, and the like are attracting attention as new optical elements, and LT single crystals are also promising as substrate materials.

[0008] The LT single crystal that has been commercially available is a ferroelectric crystal having a melting point of about 1650 ° C and a Curie temperature of about 600 ° C.
It is usually grown by a Czochralski method from a melt dissolved in an iridium crucible in a reducing atmosphere containing some oxygen. Although detailed phase diagrams of LT single crystals have not been reported, for example, literatures (S. Miyazawa et al. J. Crystal
Growth 10, p276, 1971) shows that LN
It is well known that the stoichiometric composition (stoichiometric composition) does not coincide with the congruent composition (identical melting composition) as in the case of a single crystal.

[0009] Only the congruent composition is a composition in which the melt composition and the crystal composition match, and a crystal having a uniform composition can be grown over the entire crystal. Therefore, the crystals currently manufactured and used for various applications are: All Li ₂ O / (Ta ₂ O ₅ + Li ₂
It is a crystal having a congruent composition in which the molar fraction of O) is from 0.4830 to 0.4853.

[0010] In particular, a low-cost large-diameter LT from an industrial point of view.
In order to supply crystals, it is important to grow from a precisely controlled congruent composition melt, so the Curie temperature of a composition-sensitive crystal should be controlled within 1 ° C for 601 ° C, for example. Thus, the congruent composition of the LT single crystal is Li ₂ O / (Ta ₂ O ₅ + L
It is precisely determined between i ₂ O) = 0.4830 and 0.4853.

However, the conventional congruent composition LT single crystal has an excess of Ta component, so that Ta ions reaching several percent replace Li ions (anti-site defects).
Li ions sites also have a few percent vacancy defects.
Although this effect is not serious for surface acoustic wave device applications, it cannot be ignored for optical functional device applications.

For this reason, there has been a demand for the development of a crystal having a composition close to the stoichiometric ratio as a substrate for application to optical functional devices.
Literature (S. Miyazawa et al. J. Crystal Growth 10, p27
As can be seen from the phase diagram shown in US Pat. No. 6,1971), crystals having a composition close to the stoichiometric ratio can be precipitated from a melt having a composition in which the Li concentration is higher than the stoichiometric ratio.

[0013] However, when an attempt is made to grow a stoichiometric crystal using the Czochralski method, which has conventionally been used as a means for industrially mass-producing large-diameter LT crystals, the precipitation of crystals is accompanied by the precipitation of crystals. As a result, an excessive amount 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. For this reason, 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 proposed a novel substance different from the conventional commercially available LT crystal having a congruent composition, in which the non-stoichiometric defect concentration of the congruent composition is greatly reduced.
An invention of a single crystal of lithium tantalate having a molar ratio of ₂ O / (Ta ₂ O ₅ + Li ₂ O) of 0.495 to 0.50 and close to a stoichiometric composition of excess Ta was made, and a patent application was filed (Japanese Patent Laid-Open No. 11-35393). ). In addition, the literature was reported on the new crystal as follows.

As a means for reducing the non-stoichiometric defects and developing high quality crystals, the present inventors have proposed a method of growing while continuously supplying raw materials (hereinafter abbreviated as a continuous supply method) ( For example, Y. Furukawa et al. J. Crystal Gr
owth 197, p889, 1999). Specifically, Li ₂ O /
(Ta ₂ O ₅ + Li ₂ O) mole fraction of 0.58
With the crucible having a double structure, the LT crystal having a stoichiometric composition is pulled up from the inner crucible, and the weight of the pulled crystal is measured as needed to obtain a growth rate. In this method, the raw material powder having the same stoichiometric composition is continuously supplied between the outer crucible and the inner crucible.

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

Furthermore, recently, the present inventors have reported that the applied voltage required for polarization reversal can be reduced to about one-tenth of that of a conventional crystal having a Ta excess stoichiometric composition (K. K.
itamura et al. Appl.Phys. Lett. 73, p3073, 1998, Yasunori Furukawa et al., Proceedings of the 43rd Symposium on Artificial Crystals 1A12, 23rd
P. 1998). That is, the existence of several percent of non-stoichiometric defects (anti-site defects and vacancy defects) in the conventional congruent composition crystal depends on the optical characteristics inherent in the LT crystal and the applied voltage necessary to create a periodic polarization structure. Reports that it may be higher.

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

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

Lithium tantalate single crystal close to stoichiometric composition with excess Ta, LT single crystal close to stoichiometric composition (Curie temperature
Quasi-Phase-Matc (680 ~ 685 ℃)
Research on bulk OPO devices in the near-infrared region as hing (QPM) devices has been reported (for example, Takaaki Hatanaka et al., Proceedings of the 60th Annual Meeting of the Japan Society of Applied Physics 2a-k-7, 3rd volume, 932) P., 199
9 years). A periodic electrode is provided on one side of an LT single crystal having a z-coincided molten composition and a uniform electrode is provided on the opposite side, and a pulse voltage of about several KV / mm is applied through this electrode to obtain a thickness of 1 to
A near-infrared 2 mm bulk OPO device can be relatively easily manufactured. However, since it is difficult to make the polarization inversion uniform, the device fabrication is limited to the formation of the domain-inverted structure in a small area, and the domain inversion has not yet been formed over a large area.

Furthermore, the inventors of the present invention have reported on the study of the fabrication of an OPO device having a crystal substrate thickness of 3 mm by using an LT single crystal having a Ta-excess stoichiometric composition close to the stoichiometric composition (Koichiro Nakamura et al. Proceedings of the 47th Annual Conference of the Japan Society of Applied Physics 30p-ZD-
3, 3rd volume, p. 1105, 2000), the control of polarization inversion becomes more difficult, and no bulk OPO device using this as a substrate has been obtained.

[0022]

The most important technique for realizing a wavelength conversion optical function device using a domain-inverted structure on a ferroelectric single crystal substrate is a technique for accurately generating a periodic domain-inverted structure. It is. In the wavelength conversion element using the QPM structure, the tolerance of the QPM condition is very strict, so that if the inversion cycle of the formed element is incomplete, a small and highly efficient element cannot be realized. The voltage application method is well known as a polarization inversion method, and is commonly used.However, it is very difficult to form a complete domain inversion width ratio of 1: 1 or 1: 3. There is also a problem in the reproducibility of.

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

In particular, the enlargement of the domain-inverted portions in the width direction has been regarded as a major problem in producing a device with good reproducibility. Also, during the formation of polarization reversal in the z-axis direction on the opposite surface,
Problems such as interruption of the reversal and the difference of the polarization reversal width on both sides of the z-cut crystal also occurred.
QPM devices have not been realized yet. In particular, in the case of a conventional congruent composition LT single crystal, the applied voltage required for polarization reversal requires a high voltage of 20 kV / mm or more.
The thickness of the reversible substrate is also limited to 0.5 mm or less, and application to a high-power laser element having a beam diameter of 1 mm or more has been extremely difficult.

On the other hand, the present inventors have invented earlier,
The use of an LT single crystal with a near-stoichiometric composition with excess Ta for the substrate has been studied to produce a bulk near-infrared region OPO device having a thickness of 1 to 3 mm, but it is difficult to make the polarization inversion uniform. ,
Fabrication of the device is limited to the formation of a domain-inverted structure in a very small area, and it has not yet been possible to form domain-inverted over a large area.

Further, a light modulation element utilizing the electro-optic effect of a ferroelectric single crystal, a lens formed in an LT single crystal, or a prism-shaped domain-inverted structure is manufactured. What is important in realizing a new optical element, such as an optical element that deflects using the effect, a cylindrical lens, a beam scanner, and a switch, is to manufacture a small and highly efficient element. The performance of an optical element that utilizes the electro-optic effect of a single crystal in which the refractive index is inverted by the domain-inverted structure depends on the precision of the design of the lens and prism-shaped domain-inverted structure, the manufacturing process of the domain-inverted structure, and the material It is determined by the magnitude of the electro-optic constant.

However, when a conventional congruent crystal substrate is used, there is still a material property problem that control of reversal of spontaneous polarization is poor as in the case of manufacturing a QPM element. Fabrication of a good lens or a prism-shaped domain-inverted structure has not been realized.

It is said that the LT crystal having the coincident melting composition has a higher light damage resistance than the LN single crystal. However, depending on the wavelength and intensity of the light used, the light damage resistance is still insufficient. There are many. The present inventors have reported that an LT single crystal having a Ta-excess stoichiometric composition has an improved light damage threshold against irradiation with a green light laser having a wavelength of 532 nm as compared with a conventional congruent composition (Furukawa Yasunori Proceedings of the 60th JSAP Academic Lecture Meeting 2p-ZB-1,
(Vol. 3, p. 1001, 2000), however, the light damage threshold for each crystal still varied by more than three orders of magnitude, and the cause was not well understood.

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

Further, the light damage resistance becomes a more severe problem when the light used is in a short wavelength region from blue to ultraviolet.
Conventional LT single crystal photo-damage is more likely to occur when the wavelength of the laser to be irradiated is shortened, and the photo-damage threshold near 400 nm is two orders of magnitude lower than the photo-damage threshold at 532 nm. Was a major problem in the application of optical functional devices to short wavelengths.

From the above, it is expected that a complete LT single crystal containing no non-stoichiometric defects is developed as a means for solving these problems. However, it is difficult to grow a complete crystal without variations in the whole crystal or between crystal lots, the yield is reduced, and industrial production of bulk crystal 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 tool on a congruent crystal substrate.
The method of adding the LPE treatment or the method of adding the Vapor Transport Equilibration treatment to the congruent crystal substrate is known as a method of making the crystal composition more close to the stoichiometric composition, but even in these cases, the composition of the liquid phase, the LPE temperature, Alternatively, the crystal composition fluctuates depending on the VTE processing temperature, and there is still a problem in industrially producing a complete crystal having no defect.

[0033]

Means for Solving the Problems In order to solve the above-mentioned conventional problems, the present inventors have intensively investigated the characteristics of LT crystals having a stoichiometric composition. It has been found that the optical functional device has excellent characteristics of improving the controllability of the domain-inverted structure and further improving the 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 or lengthen the wavelength of an incident laser having a wavelength in the visible to near infrared region. In the optical function device to be used, the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) close to the Li excess stoichiometric composition is 0.500 to 0.505, and the lithium tantalate single crystal is used for the substrate. It is characterized by.

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

Further, the optical functional device of the present invention is an optical device for controlling a laser beam incident on a single crystal by utilizing the electro-optic effect of the single crystal, and comprises a ferroelectric polarization of a lithium tantalate single crystal. In an optical element that deflects, focuses, and switches light using the large refractive index change of the inversion structure, Li
Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) close to excess stoichiometric composition
Has a mole fraction of 0.500 to 0.505, and a single crystal of lithium tantalate is used for the substrate.

In the above-mentioned optical functional device, the Curie temperature of the lithium tantalate single crystal as a substrate is 686 to 6
Preferably it is in the range of 95 ° C.

In the optical functional device, the applied voltage required for reversing the polarization of the lithium tantalate single crystal substrate is 3K.
It is preferably at most V / mm.

In the above-mentioned optical functional device, the light damage threshold of the lithium tantalate single crystal substrate is 407 nm.
Is preferably 10 ³ KW / cm ² or more for the continuous wave laser irradiation.

The present inventors have found that the problem of polarization inversion and optical damage control in an optical functional device using an LT single crystal lies in the composition of the single crystal substrate. The present invention focuses on an LT crystal single crystal substrate within a certain composition range as an optical functional device application utilizing a domain-inverted structure of an LT crystal. Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) mole fraction greater than 0.50
The present inventors have found for the first time that the LT single crystal in which the components are excessively close to the stoichiometric ratio has excellent polarization reversal controllability different from the conventional characteristics.

Further, the composition of the LT crystal was changed to Li ₂ O / (Ta ₂ O ₅
By making the molar fraction of + Li ₂ O) larger than 0.50 and making the Li component excessive, it was possible to greatly improve the light damage resistance characteristics of the optical functional device. It has been clarified that by using this, the characteristics of an optical functional element applied to short-wavelength light can be dramatically improved.

The polarization reversal characteristics and light damage resistance characteristics found this time are also effects peculiar to the LT single crystal having this mole fraction. The LT single crystal having a near stoichiometric composition is a crystal that has finally been able to produce an optically homogeneous substrate by a continuous raw material supply double crucible method. Not disclosed. 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 excessive relative to the stoichiometric ratio. In addition, the use of this characteristic to significantly improve the characteristics of an optical functional device has been an unexplored field.

[0043]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Originally, the ideal composition of an LT single crystal is
Although the Li: Ta ratio is 1: 1, conventionally commercially available LT single crystal substrates having a congruent composition have a large excess of the Ta component due to the restriction of the single crystal growth technology (Li ₂ O / (Ta ₂ O ₅ + L
The molar fraction of i ₂ O) is about 0.485). On the other hand, by the raw material continuous supply double crucible method, Li component excess (for example, Li ₂ O / (Ta ₂ O ₅ + L
The LT crystal having a composition close to the stoichiometric composition developed from a melt having a molar fraction of (i ₂ O) of 0.56 to 0.60) still has a slightly excessive Ta component (Li ₂ O / (Ta ₂ O ₅ + Li). The molar fraction of ₂ O is between 0.495 and 0.50).

The inventors of the present invention intend to grow crystals from a melt in which the composition of the melt is remarkably excessive in the Li component (the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is 0.60 to 0.67). In addition, an LT single crystal in which the Li component is close to an excess stoichiometric composition (the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is larger than 0.50) can be grown, and the non-stoichiometric defect concentration due to the excess Ta component can be reduced. It has been clarified for the first time that the suppressed single crystal exhibits excellent characteristics as an optical functional element substrate.

That is, it has been found that a large number of defects formed by excess Ta in the conventional crystal cause a great problem for optical functional device applications. Due to the existence of this defect, the hysteresis curve showing the relationship between the applied voltage required for polarization reversal and spontaneous polarization became asymmetric, and it was found that a high voltage of several tens of kV / mm was required for polarization reversal. Was. Furthermore, it is technically difficult to accurately perform polarization inversion even when a voltage application method is used because defects are unevenly distributed inside the crystal and polarization inversion is likely to be pinned in a portion where the defect concentration is high. It became clear that there was.

In the LT single crystal, in the paraelectric phase higher than the Curie temperature, Li and Ta ions are arranged in an electrically neutral position. In the ferroelectric phase lower than the Curie temperature, Li and Ta ions are + A slight shift in the z or -z direction. The direction of positive and negative polarization of the domain is determined by the direction of this ion shift. In an optical function device having a domain-inverted structure,
By applying a high electric field, it is necessary to force the ions to move at a low temperature.

When there are many nonstoichiometric defects of the coincident molten composition
Means that Li ions can easily diffuse and move through vacancies, but Li
It is not easy to move excess Ta entering the site
Therefore, a large applied voltage is required for polarization reversal. This
From the Ta excess component side (Li _TwoO / (Ta_TwoO_Five+ Li_TwoO mole
Fraction with a Ta component excess of 0.495 to 0.50).
Li component completely eliminating stoichiometric defects is close to excessive stoichiometric composition
LT single crystal (Li_TwoO / (Ta _TwoO_Five+ Li_TwoO) molar fraction of 0.500
~ 0.505) is excellent in polarization inversion controllability.

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

It was also found that an excessive component of Ta caused a problem with respect to optical damage. LT single crystal (Li ₂ O / (Ta ₂ O ₅ + Li ₂
A 532 nm continuous-wave green light laser with a light intensity of 10 ³ KW / cm ² or more on various LT single crystals having a molar fraction of O) of 0.500 to 0.505) or a Curie temperature in the range of 686 to 695 ° C. Irradiation does not cause any optical damage.

Further, the threshold value against light damage is improved even when a laser beam having a shorter wavelength than before has been solved.
Quite light damage to the continuous wave laser radiation of a wavelength 407nm light intensity 10 ³ KW / cm ² was not observed, it can be also stably operate an optical device using the crystal substrate revealed.

Next, the optical functional device of the present invention is used.
The production method and physical properties of the LT single crystal are shown below. Commercially available high purity Li
_TwoCO_Three, Ta_TwoO_FivePrepare raw material powder of
Li _TwoCO_Three: Ta_TwoO_FiveOf 0.60: 0.40, 0.62: 0.38, 0.64:
They were mixed at a ratio of 0.36, 0.66: 0.34. Also, separate stoichiometry
Li as a stoichiometric composition raw material_TwoCO_Three: Ta_TwoO_Five= 0.50: 0.50 ratio
And mixed. Next, 1ton / cm^TwoRubber press with hydrostatic pressure
The raw materials of each composition ratio are molded in oxygen at about 1050 ° C.
The raw material rod was sintered. In addition, powder material for continuous supply
About 13% of the stoichiometric composition
Sintering in the air at 50 ° C also creates a stoichiometric composition raw material
Was.

Next, a Li-excess LT single crystal close to a stoichiometric composition was grown by using the double crucible method of continuously supplying raw materials. Melt with excess Li component in double crucible (eg, Li ₂ O / (Ta
_A seed crystal is placed at 0.60, 0.62, 0.64, 0.66) with a molar fraction of ₂ O ₅ + Li ₂ O), with a pulling rate of 0.5 mm / h and a crystal rotation speed of 20 rpm, which is close to the stoichiometric composition. Was obtained as much as possible. In order to precisely control the density and structure of nonstoichiometric defects, the amount of Li ₂ O / (Ta
_The crystal was grown while a raw material for continuous supply having a stoichiometric composition ratio of 0.50 (2O ₅ + Li ₂ O) was automatically supplied to the outer crucible.

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

In the growth process, as in the growth of a congruent LT single crystal for optical use, transition metal impurities such as iron and chromium, which are considered to be one factor that induces optical damage, should be minimized. Paid attention.
By growing for about one week, a colorless and transparent LT single crystal having a diameter of about 55 mm and a length of about 70 mm was obtained without cracks.
The domain state inside the obtained as-grown crystal was a multi-domain state.

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

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

Further, the Curie temperature of the sample cut out from one of the obtained crystals was constant within the measurement error regardless of the cut-out position of the sample, and it was confirmed that the homogeneity of the crystal composition was extremely good. . In addition, the Curie temperature of the grown crystal did not become much higher than the Curie temperature of the standard sintered sample of 694 ° C.

The composition of the melt in the double crucible was Li ₂ O / (Ta ₂ O ₅ + L
As the molar fraction of i ₂ O) was gradually increased to an excess of Li less 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 the maximum value in a perfect stoichiometric crystal. The reason that the maximum value of the Curie temperature of the obtained crystal, 695 ° C, is higher than the Curie temperature of the stoichiometric standard sintered sample, 694 ° C, is thought to be due to the measurement error of the Curie temperature determined by the suggested thermal analysis method. Can be from these things,
It is conceivable that the grown crystal has a perfect stoichiometric composition or that the Li component is in excess of the stoichiometric composition.

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

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

As described above, the Li single component in which the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is in the range of 0.500 to 0.505 is excessively close to the stoichiometric composition of the LT single crystal or the aforementioned Curie temperature. Is 686 ~ 6
LT single crystal substrates in the range of 95 ° C. have excellent optical homogeneity.

The present inventors have found that the above-mentioned single crystal in which the non-stoichiometric defect concentration due to the excess of the Ta component is suppressed has a large amount of defects formed by excess Ta in the conventional crystal, and the single crystal required for polarization reversal is applied. The hysteresis curve showing the relationship between voltage and spontaneous polarization becomes asymmetrical.Moreover, high voltage of several tens of kV / mm is required for polarization reversal, and the defect concentration which is unevenly distributed inside the crystal increases. High-precision polarization inversion of optical functional devices is solved by solving the problem that polarization inversion is easily pinned in high places, and it is technically difficult to accurately perform polarization inversion even by using a voltage application method. It was clarified that the formation of was possible.

Next, 30 mm ×
A z-cut sample having a thickness of 30 mm and a thickness of 0.5 to 3.0 mm was cut out. After electrodes were formed on both z-planes, a voltage was applied, and a polarization reversal voltage was measured from a change in current value. As a result, as shown in FIG. 1, the applied voltage required for the polarization inversion is about 25 kV / mm (21 kV of the inversion voltage of 21 kV / mm) in the conventional matched melt composition LT single crystal.
It is necessary to add a value several kV / mm higher than / kV / mm), whereas it is confirmed that the polarization is reversed at an applied voltage of about 2 to 4 kV / mm as the composition approaches the stoichiometric composition.

This result is consistent with the result previously reported by the present inventors. The present inventors have further found that Li ₂ O / (Ta ₂ O
_{In the case of an} LT single crystal sample in which the molar fraction of ( ₅ + Li ₂ O) is in the range of 0.500 to 0.505 and the Li component is close to the stoichiometric composition, the applied voltage required for polarization reversal is further reduced, and the applied voltage is 0. .
A sample capable of forming polarization inversion even at about 5 to 1 kV / mm was obtained. Furthermore, the hysteresis of spontaneous polarization-applied voltage showed perfect symmetry, and the internal electric field was 0 kV / mm within the measurement error. For this reason, it became clear that the domain inversion process was extremely reversible, and the controllability of the domain inversion process was excellent.

As described above, even if the LT single crystal has 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) in which the Ta component excess non-stoichiometric defects are completely eliminated, compared to a crystal in which the molar fraction of ₂ O is in the range of 0.495 to 0.50).
In the LT single crystal in which the molar fraction of is in the range of 0.500 to 0.505 and the Li component is excessively close to the stoichiometric composition, a significant improvement in the polarization inversion controllability was confirmed.

The present inventors have also paid attention to the fact that an excessive component Ta causes a problem with respect to optical damage. An LT single crystal in which the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is in the range of 0.500 to 0.505 and the Li component is excessively close to the stoichiometric composition, or the Curie temperature is in the range of 686 to 695 ° C. Some L
Continuous oscillation green light of 532 nm wavelength (Verdi, manufactured by Coherent) and continuous oscillation blue-violet light of 407 nm (Krypton laser, Innova, manufactured by Coherent) were incident on the T single crystal, and the presence or absence of optical damage was examined.

From the crystals having various compositions, cubes of about 5 mm ³ were cut out, and both sides x, y and z were optically polished to prepare evaluation samples. The above laser light is narrowed down with a lens from the y-axis and x-axis directions of the crystal, and the light intensity is changed little by little in the range of 10 ^-3 to 10 ³ KW / cm ² , and the laser light is transmitted through the crystal. Was observed with a beam profiler (Hamamatsu Photonics) 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 one minute, the incident light intensity was defined as an optical damage threshold.
As shown in FIG. 2, a slight excess of Ta (Li ₂ O / (Ta ₂ O ₅
In a crystal in which the molar fraction of + Li ₂ O is 0.495 to 0.50), the light damage threshold greatly fluctuated by more than two orders depending on the sample. This light damage threshold greatly depends on the heat treatment state of the crystal.

That is, it was considered that the light absorption caused by the polaron induced in a state where the excess Ta ion replacing the Li ion site was reduced was one of the causes of lowering the light damage threshold. On the other hand, in the case of the LT single crystal in which the Li component in which the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is in the range of 0.500 to 0.505 is close to an excess stoichiometric composition, any of the crystals Even when the incident light intensity was 10 ³ KW / cm ² , no damage was observed, and it was clarified that stable operation of an optical device using this crystal as a substrate could 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 for substituting the Li ion site.

It should be noted that there has been some Ta excess component side (Li
_In a crystal having a molar fraction of ₂ O / (Ta ₂ O ₅ + Li ₂ O of 0.495 to 0.50), in order to stably provide a crystal resistant to photodamage, Mg
It was necessary to add additives such as On the other hand, in the case of lithium tantalate single crystal having an excess of Li component, the fact that the threshold value against light damage is high without adding an additive such as Mg is a great advantage in practical use.

The reason for this is that, in the production of an LT single crystal containing Mg, in order to grow the crystal without distributing the Mg element uniformly in the crystal and deteriorating the optical quality, it is necessary to use an additive-free crystal. In comparison, there was a problem that the crystal growth rate had to be slowed down, and the productivity was deteriorated. Furthermore, since the polarization reversal characteristics of the Mg-containing crystal are different from those of the non-added crystal,
Although there was a problem that controllability deteriorated, the present invention has made it possible to solve these problems.

[0072]

The present invention will be described more specifically with reference to the following examples. Example 1 A description will be given of characteristics when applied to an optical wavelength conversion element as one of the optical functional elements using the LT single crystal prepared by the above method. FIG. 3 shows an LT single crystal having a composition of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) in which the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is in the range of 0.500 to 0.505 and the Li component is excessively close to the stoichiometric composition. Crystals or various L with Curie temperature in the range of 686-695 ° C
1 is a schematic configuration diagram of a QPM device in which a T domain is used as a substrate and a periodically poled structure is formed on the substrate.

A comb-shaped electrode and a parallel electrode were patterned on the + z plane of a substrate 1 having a thickness of 0.5 mm to 3.0 mm which had been optically polished on both sides. The period was about 3.2 μm, and it was designed to be quasi-phase matched to a wavelength of 850 nm. On the -z plane, an electrode was deposited on the entire surface. 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 periodic domain-inverted regions 2.

The used LT crystal has a very uniform polarization state in advance. When a 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 in the range is excessively close to the stoichiometric composition, or 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. It becomes possible to form a domain-inverted structure. In an LT crystal having a normal congruent composition, a large number of small domain-inverted regions (microdomains) exist in the crystal. Therefore, when a fine domain-inverted structure is formed, nonuniformity of the domain-inverted shape due to the microdomain occurs. Further, the LT single crystal substrate of the present invention is characterized in that the period of the periodically poled structure can be easily shortened.

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

On the other hand, the Li ₂ O /
L having a molar fraction of (Ta ₂ O ₅ + Li ₂ O) in the range of 0.500 to 0.505
In the case of LT single crystal with i component that is close to the stoichiometric composition, the applied electric field required for reversal is about 0.5 to 2 kV / mm, which is 1/12 to 1/25 or less of the conventional, and the substrate thickness as a bulk element can be expanded. 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 very few microdomains, it is possible to prevent the polarization reversal from spreading to the periphery of the electrode. Conventionally, the single crystal has a thickness exceeding about 3 μm. It has been 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, and the y-plane of the side face crystal was polished and etched with a mixed solution of hydrofluoric acid and nitric acid to examine the state of domain inversion. By optimizing the pulse width and current of the applied voltage, the periodic polarization inversion width ratio and the shape of the polarization can be accurately set to the ideal 1: 1 polarization inversion width ratio of the periodic polarization over the entire sample. It was confirmed that.

In the QPM-SHG element shown in FIG. 3, the polarization width, which has been a problem in the related art, was suppressed from expanding in the horizontal direction. Also, the formation of this periodically poled structure is 0.5 m thick.
Not only the m sample but also other thicker samples are similarly formed with high precision, and these thick samples are considered to be optimal as, for example, an internal resonator type wavelength conversion element. Next, a sample was cut out of the wafer and the end face was polished.

For high-efficiency wavelength conversion, the QPM-SHG element employs a method of inserting a laser serving as a fundamental wave and a resonator, or forming an optical waveguide to confine a fundamental-wave semiconductor laser. , Approximately 50% for a 10 mm long sample
It was confirmed that the SHG output was stable with the conversion efficiency. QPM-S
In the evaluation of the characteristics of the HG device, a tunable high-output Ti sapphire laser (wavelength 850 nm) 4 was used as a fundamental wave. Optical coupling was performed using the lens 5. The stoichiometric LT crystal has a nonlinear optical constant 1.2 times or more that of the congruent composition LT crystal, and the nonlinear optical constant of the substrate is improved, so that a highly efficient light wavelength conversion element can be formed.

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

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

The first reason is that, in the LT crystal having the composition of the present invention, the non-stoichiometric defect concentration is much smaller than that of the conventional matched-melting composition crystal, so that the photocarrier is hardly scattered and the mobility is low. Is large, so that photoconductivity is high. It is considered that if photoconductivity is high, the localization of photocarriers that causes optical damage is canceled out, and optical damage hardly occurs.

The second is Li ₂ O / (Ta ₂
In the lithium tantalate single crystal in which the molar fraction of O ₅ + Li ₂ O) is larger than 0.50 and the Li component is excessive, there is no excess Ta ion to replace the Li ion site. Polaron is considered to be not induced.

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

Fourth, since the stoichiometric crystal has a low non-stoichiometric defect concentration, it contains almost no macroscopic crystal defects such as light scattering factors and striations, and the crystal has very low light absorption.

In particular, in a high-power SHG element, there is a possibility that optical damage due to a thermal lens effect may occur due to an increase in light absorption due to a fundamental wave or a harmonic, but the crystal integrity is high and a constant light absorption is small. It is understood that these problems can be solved by the specific composition LT single crystal. In addition, here, the embodiment in which the QPM-SHG element that generates blue light with respect to the fundamental wave of near infrared light of 850 nm is described in detail. The present invention is not limited to one wavelength, and can be applied to a wavelength range where the LT single crystal is transparent and phase matching is possible.

Further, the optical functional device of the present invention for periodically inverting the polarization structure of the LT single crystal to shorten or lengthen the wavelength of an incident laser having a wavelength in the visible to near-infrared region is described below. The present invention can be applied not only to the second harmonic generation element but also to various application fields such as remote sensing and gas detection such as an optical parametric oscillator element.

Example 2 Next, the Li component having a molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) in the range of 0.500 to 0.505 produced by the above-described method is close to an excessive stoichiometric composition. A lens and a prism-shaped domain-inverted structure were formed on an LT single crystal, and a deflection element using an electro-optic effect and optical elements such as a cylindrical lens, a beam scanner, and a switch were manufactured.

FIG. 4 shows a wavelength conversion element 8 having periodic polarization inversion, a polarization inversion structure 12 of lenses 9 and 10 and a prism 11.
FIG. 3 is a configuration diagram of an optical element using a Li component excess LT single crystal substrate 6 on which is integrated. 2 inch diameter, 0.2-2.0m thickness
m, prepare a z-cut Li component excess LT single crystal polished on both sides, form an Al electrode with a thickness of about 0.2 micron on both z faces by sputtering, and use a lithograph to create a lens or prismatic pattern. Was formed. Thereafter, a pulsed voltage of about 0.5 to 2.0 kV / mm was applied to the + z plane to invert the polarization.

Further, heat treatment was performed to eliminate the non-uniformity of the refractive index, which is said to be introduced upon polarization inversion. Further, the end face of the crystal was mirror-polished to provide a laser light input / output surface. The performance of the optical element utilizing the electro-optic effect of the LT single crystal in which the refractive index is inverted by the prototyped domain-inverted structure is determined by 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. The lens and the prism-inverted structure in which the prototype was fabricated here were notable, and it was notable that good element characteristics were obtained because the polarization inversion electric field was low and the control of the polarization inversion was very easy. That is.

In a conventional LT crystal having a congruent melting composition, it is difficult to control a domain-inverted structure because a large applied voltage is required for domain inversion. In addition, LT with a Ta component excess near the stoichiometric composition
When a single crystal in which a small amount of MgO is added to a crystal has a short reversal period and a complicated reversal structure, it is difficult to produce a lens or a prism-like polarization reversal structure with high accuracy.

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

Further, since the light damage resistance does not matter, the optical functional element using blue-violet to green short-wavelength light is
Even when a lens or a prism-shaped domain-inverted structure is manufactured and optical elements such as a deflection element, a cylindrical lens, a beam scanner, and a switch utilizing the electro-optic effect are manufactured, there is no problem of beam deformation due to optical damage.

[0095] In addition, since the crystals have a large electro-optical constant r ₃₃ than crystals of congruent, more superior device performance with a small operation voltage were obtained. 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 operating at about 100 V / mm and a switching operation at about 500 V / mm were obtained. Example 3 Next, an LT single crystal having a molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) in the range of 0.500 to 0.505 and an Li component excessively close to a stoichiometric composition prepared by the above method. FIG. 5 is a conceptual diagram showing a method of manufacturing an optical storage element using a small domain-inverted portion of an LT single crystal using the method described above.

As described above, the LT single crystal with excess Li component according to the present invention has a feature that the 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 about several microns, which is smaller than the size of the domain of the LN single crystal of several mm to several cm. It is much smaller. 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 a single crystal.

Therefore, as shown in FIG.
m, a 10 mm × 20 mm square double-side polished z-mucat Li component-excess LT single crystal 14 was prepared, and as shown in FIG.
From the surface, a voltage was applied to the minute region using a scanning microscope utilizing Maxwell stress 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 the alternating electric field applied between the conductive chip 15 and the conductive chip 16, a polarization inversion is formed in the crystal by an electric field induced by minute vibration. At the same time as the formation of the polarization inversion, the state of the sample surface, the oscillation 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, it was confirmed that an optical device using ferroelectric polarization, which can easily write and store two-dimensional information with a width of about 1 micron in a Li-rich single crystal close to the stoichiometric LT composition, could be obtained. did.

[0099]

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

[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
By using a lithium tantalate single crystal having a wavelength of 、 0.505, it has excellent light damage resistance and a wavelength of 407 with a light intensity of 10 ³ KW / cm ² or more.
Since the device can be operated stably with irradiation of a continuous wave laser of nm, an optical functional device having excellent performance can be provided. Accordingly, the present invention has a great effect of promoting the practical use of optical functional devices in fields such as optical information processing using laser light, optical processing technology, photochemical reaction technology, and optical measurement control.

[Brief description of the drawings]

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

FIG. 2 is a graph showing the relationship between the composition of an LT single crystal and an optical damage threshold.

FIG. 3 is a conceptual diagram showing an optical wavelength conversion element according to one 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 illustrating a method for manufacturing an optical storage element using a fine domain-inverted portion of an LT single crystal.

[Explanation of symbols]

 1 Li component excess LT single crystal substrate 2 Polarization 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 polarization inversion region 9 Convex lens 10 Concave lens 11 Prism 12 Polarization inversion Region 13 Emission laser 14 Li component excess LT single crystal substrate 15 Electrode 16 Chip 17 Micro domain reversal region 18 Laser 19 Photodetector 20 Lock-in amplifier

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) G02F 1/39 G02F 1/39 (72) Inventor Yasunori Furukawa 1-1-1, Namiki, Tsukuba, Ibaraki Pref. In-house (72) Inventor Kenji Kitamura 1-1-1, Namiki, Tsukuba, Ibaraki Pref., Japan Science and Technology Agency Inorganic Materials Research Laboratory (72) Inventor Shunji Takekawa 1-1, Namiki, Tsukuba, Ibaraki Pref. Reference) 2H079 AA02 CA05 DA03 HA12 KA20 2K002 AB06 AB12 BA01 CA03 EA13 FA27 GA04 GA05 GA07 HA02 HA20

Claims (6)

[Claims] 1. An optical functional device for periodically or 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 stoichiometric composition of
An optical functional element 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 for a substrate. 2. An optical storage element or an optical circuit element in which polarization inversion is formed in a minute region in a lithium tantalate single crystal in a single domain state and various information is recorded in the single crystal by inverting polarization. The molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) close to the Li excess stoichiometric composition is 0.500 to 0.505
An optical functional element, wherein a lithium single crystal of lithium tantalate is used for a substrate. 3. An optical element for controlling a laser beam incident on a single crystal by utilizing an electro-optic effect of the single crystal, wherein a large refractive index change of a reversal structure of ferroelectric polarization of a lithium tantalate single crystal is provided. In an optical element that performs light deflection, focus, and switching by using, the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) close to the Li-excess stoichiometric composition is 0.500 to 0.505. An optical functional element, wherein a lithium oxide single crystal is used for a substrate. 4. The optical functional element using a lithium tantalate single crystal having a composition close to the stoichiometric composition has a Curie temperature of 686 to 695 of the lithium tantalate single crystal serving as a substrate.
4. The optical functional device according to claim 1, wherein the temperature is in the range of ° C. 5. The lithium tantalate single crystal substrate having a composition close to the stoichiometric composition, wherein an applied voltage required for polarization inversion is 2 kV / mm or less.
The optical functional device according to any one of the above. 6. The lithium tantalate single crystal substrate having a stoichiometric composition close to the stoichiometric composition has a light damage threshold of 10 ³ KW / cm ² or more with continuous wave laser irradiation at a wavelength of 407 nm. Item 5. The optical functional element according to any one of Items 1 to 4.