Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2000-03-09
2004-01-06
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S263000, C423S275000, C117S948000, C359S237000
Reexamination Certificate
active
06673330
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a single crystal of lithium niobate or tantalate for optical use and its production method, an optical element using the single crystal and its production method. Particularly, it relates to a production method for stable growing a single crystal of its stoichiometric composition, which has excellent physical properties and which is used for an element utilizing polarization inversion, from a melt having a high Li concentration.
The present invention further relates to a process and an apparatus for producing an oxide single crystal by using a noble metal crucible of a double crucible structure. Particularly, it relates to a process and an apparatus for producing an oxide single crystal for stably growing a high quality and longitudinal crystal by rotation pulling.
2. Discussion of Background
Functional optical single crystals of which optical characteristics can be controlled by an information signal from outside such as electricity, light of stress, are now essential materials in various optoelectronics fields including optical communication, recording, measurement and optical-optical control. Particularly with respect to a certain single crystal of an oxide, the interaction between the optical characteristics and external factors is significant, and it is thereby used as a frequency-conversion element utilizing the nonlinear optical effect, or as an optical element utilizing the electrooptical effect, such as an electrooptical light modulator, a switch or a reflector.
Such a crystal is used as an element as it is originally grown, in many cases. However, with respect to some ferroelectric crystals, the directions of the dielectric polarization can be inverted by applying voltage thereto without destroying the crystals, and accordingly, their functions have been increased by inverting the polarization periodically.
For example, with respect to a frequency-conversion element, the wavelength can be converted by means of quasi-phase-matching (QPM) by periodically inverting the domain structure of the ferroelectric polarization. This method is effective from the viewpoint that the conversion can be carried out with a high efficiency at a wide wavelength range, and it is thereby expected as a frequency-conversion element to realize a laser light source having a wide range of wavelength covering from the ultraviolet and visible light region to the infrared light region, which is strongly desired in fields including optical communication, recording, measurement and medical care.
Further, with respect to an electrooptical element, according to a known literature (M. Yamada et al., Appl. Phys. Lett., 69, page 3659, 1996), an attention has been drawn to a cylindrical lens, a beam scanner and a switch, and an optical element forming a polarization inversion structure of a lens or a prism in a ferroelectric crystal, and polarizing laser light transmitted therethrough by utilizing the electrooptical effect, as new optical elements.
A single crystal of LiNbO
3
or LiTaO
3
(hereinafter referred to simply as LN single crystal or LT single crystal, respectively) is a ferroelectric which is used mainly as a substrate for a surface elastic wave element or for an electrooptical light modulator. It is transparent at a wide wavelength range of from the visible region to the infrared region, it can form a periodic polarization structure by applying voltage, it has optical nonlinearity and electrooptical characteristics which are practical to some extent, and further, a single crystal having a large diameter and a high composition homogeneity can be provided at a relatively low cost. Accordingly, an attention has been drawn to the LN single crystal or the LT single crystal also as a substrate for a frequency-conversion element by the above-mentioned QPM (hereinafter referred to simply as QPM element) or for an electrooptical element.
Heretofore, the LN single crystal available has been limited to one of the congruent melting composition with a molar fraction of Li
2
O/(Nb
2
O
5
+Li
2
O) of 0.485, containing nonstoichiometric defects of a level of several percent, including the substrate for a surface acoustic wave element, since the phase diagram of the LN single crystal has been known for a long time, and it has been conventionally considered that to produce a LN single crystal having a high composition homogeneity, it is preferred to grow the single crystal by rotation pulling from a melt with a molar fraction of Li
2
O/(Nb
2
O
5
+Li
2
O) of 0.485, which is of the congruent melting composition wherein the crystal and the melt are coexist in equilibrium state with the same composition. Further, as shown in a known literature (D. A. Bryan et al. Appl. Phys. Lett. 44, page 847, 1984), Mg is added in an amount of at least 4.5 mol % to the LN crystal of the congruent melting composition, with a purpose of increasing optical damage resistance. For the LT single crystal available has been limited to one of the congruent melting composition with a molar fraction of Li
2
O/(Ta
2
O
5
+Li
2
O) of 0.483, containing nonstoichiometric defects of a level of several percent, including the substrate for a surface acoustic wave element, since it has been conventionally considered that to produce a LT single crystal having a high composition homogeneity, it is preferred to grow the single crystal by rotation pulling from a melt with a molar fraction of Li
2
O/(Ta
2
O
5
+Li
2
O) of 0.483, which is of the congruent melting composition wherein the crystal and the melt are coexist in equilibrium state with the same composition. Further, as shown in a known literature (F. Nitanda et al. Jpn. J. Appl. Phys. 34, page 1546, 1995), Mg is added in an amount of a level of several mol % to the LT crystal of the congruent melting composition, with a purpose of increasing optical damage resistance or shortening the cut-off wavelength. However, it has been known that the LT single crystal has a relatively high optical damage resistance even without Mg addition, as compared with the LN single crystal, and an adequate effect of improving the optical damage resistance by Mg addition is not always obtained.
In order to realize the QPM element, it is important to prepare a small element having a high efficiency. Downsizing and obtaining high efficiency of the element are significantly dependent on the characteristics of the material to be used, i.e. the material characteristics which the crystal essentially has, although they are greatly dependent on also the structure of the element. For example, the conversion efficiency of the QPM element is in proportion to the square of the nonlinear optical constant and the interaction length, and is in proportion to the fundamental wave power density. The interaction length and the fundamental wave power density are determined by the element design or accuracy of the preparation process, and may be increased by e.g. improvement of techniques. On the other hand, the nonlinear optical constant is a material characteristic which the material essentially has. Since LN is one of the most popular nonlinear optical materials, a large number of measurements of the nonlinear optical constant have been carried out for a long time. Of the LN crystals of the congruent melting composition which have been reported so far, the nonlinear optical constant d
33
has been said to be usually from about 27 to about 34 pm/V at a wavelength of 1.064 &mgr;m. However, the difference among reported values is surprisingly large, and it is twice between the highest and the smallest. These values are obtained by a relative measurement for obtaining the ratio in the nonlinear optical constant with a reference substance. However, the absolute value of the reference substance itself is not determined, and researchers use different values, and the difference is thereby so large. In the conventional measurement methods, the absolute value of the reference substance is based on the value obtained by absolute measurement f
Furukawa Yasunori
Kimura Shigeyuki
Kitamura Kenji
Takekawa Shunji
Bos Steven
National Institute for Research in Inorganic Materials
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