Compositions – Light transmission modifying compositions – Inorganic crystalline solid
Reexamination Certificate
2000-01-03
2002-05-21
Tucker, Philip (Department: 1712)
Compositions
Light transmission modifying compositions
Inorganic crystalline solid
C252S582000, C423S277000, C423S279000, C372S021000, C372S022000, C117S073000
Reexamination Certificate
active
06391229
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a borate single crystal and a method for growing the same, in which the single crystal has noncentrosymmetry, is transparent in a region from ultraviolet light of 200 nm or less to infrared light of 2,000 nm or more, and is used in fields which require nonlinear optical effects, electrooptical effects, and acoustooptical effects. In particular, the present invention relates to a laser device using the single crystal in the ultraviolet region and a processing apparatus using the laser device.
BACKGROUND ART
Single-crystal materials have been used in a wide range of fields in order to fully utilize functions due to the crystal structure thereof. In particular, single-crystal materials as optical materials can avoid scattering at grain boundaries and thus have a wide range of uses. Among these, materials having crystal structures which do not have centrosymmetry exhibit optical nonlinearity, in addition to piezoelectricity and pyroelectricity, and are important as light wavelength converters.
There is an increasing demand for ultraviolet laser light in microprocessing fields. Excimer lasers are currently used as practical ultraviolet lasers. However, these utilize toxic gases, and it is difficult to increase the pulse repetition rate. Thus, solid-state lasers using wavelength converters have been increasingly in demand.
Many materials are known as light wavelength converters. However, only a few materials, such as LBO (LiB
3
O
5
), BBO (&bgr;-BaB
2
O
4
), and KDP (KH
2
PO
4
) are known for use in consideration of generation of ultraviolet light. Among them, LBO is suitable for the YAG third harmonic wave (355 nm), but is not suitable for shorter wavelengths due to refractive index characteristics. BBO has a high conversion efficiency, but has a significantly small tolerance of the phase-matching angle. Since BBO and KDP show slight absorption of ultraviolet light, these can only output stable powers of at most 500 mW when used for generation of the YAG fourth harmonic wave (266 nm).
In such circumstances, there is a strong demand for new types of crystals. In particular, borate single crystals have attracted attention since these generally have broad transmittance characteristics down to the ultraviolet region and have a considerably large nonlinear optical constant due to conjugated &pgr; electrons (“Optical Crystals”, pp. 321 to 326, published by Baifukan (1995); and Takatomo Sasaki, “Current Status and Future Development in Borate Nonlinear Optical Crystals”, The Review of Laser Engineering, 26(3), p. 215 (1998)). As an example, a cesium lithium borate crystal, CLBO (CsLiB
6
O
10
), is proposed (Japanese Patent Application Laid-Open No. Hei 8-91997). This crystal has a nonlinear optical constant which is substantially equal to that of the LBO crystal. This crystal, however, is strongly hygroscopic and is thus deliquescent causing spontaneous cracking. As a result, it is difficult to produce high-quality crystals at industrial scales, and this crystal is not used in practice.
To solve such problems, crystals represented by a composition in which Rb, K or Tl are partly or entirely substituted for Cs in the CLBO crystal is proposed (Japanese Patent Application Laid-Open No. Hei 8-295507). Among these, RLBO (RbLiB
6
O
10
) in which Rb is entirely substituted for Cs can be readily produced and is very safe.
The present applicant filed a patent application for a light wavelength-converting element composed of an LB4 (Li
2
B
4
O
7
) crystal which has high moisture resistance and which can be used in a high-power range (Japanese Patent Application Laid-Open No. Hei 9-281535).
This RLBO crystal is also hygroscopic, that is, it absorbs moisture, resulting in deterioration of light wavelength conversion efficiency.
On the other hand, the LB4 crystal has problems in that it has a low nonlinear optical constant and a significantly low light wavelength conversion efficiency.
It is an object of the present invention to provide a borate single crystal capable of stable light wavelength conversion with high efficiency down to the ultraviolet region and having high moisture resistance.
It is another object of the present invention to provide a method for growing a borate single crystal.
It is still another object of the present invention to provide a laser device using the borate single crystal and a processing apparatus using the laser device.
DISCLOSURE OF INVENTION
The present inventors have searched a crystal having excellent optical characteristics similar to those of LB4 and having a large nonlinear optical constant; have found that when another alkaline metal is partly substituted for Li in the LB4 crystal, a crystal having a structure which is quite different from that of LB4 and those of known borates is obtainable and that this crystal has a nonlinear optical constant which is superior to that of LB4 and has chemical and mechanical stability comparable to that of LB4; and have completed the present invention.
LB4 is a tetragonal crystal having a P4 mm space group and tetraborates of Na, K, and Rb are triclinic crystals represented by Formula 1, and they are quite different in structure from the orthorhombic crystal of the P2
1
2
1
2
1
space group which is presented in the present invention:
Space Group: P{overscore (1)} Formula 1
The optical nonlinearity of the crystal of the present invention is considered to be a contribution of spiral borate rings of a quite novel structure.
The invention relates to a borate single crystal which is represented by the chemical formula (A
2
O).(B
2
O
3
)
x
, and which is an orthorhombic crystal belonging to the P2
1
2
1
2
1
, space group, wherein A includes two elements selected from the group consisting of Li, Na, K, Rb, and Cs, and 1.5<x<2.5 (hereinafter the same).
The borate single crystal which is represented by the chemical formula (A
2
O).(B
2
O
3
)
x
and which is an orthorhombic crystal belonging to the P2
1
2
1
2
1
space group is chemically and mechanically stable, and has nonlinear optical effects, electrooptical effects, and acoustooptical effects.
Regarding the nonlinear optical effects, this crystal has a nonlinear optical constant which is greater than that of LB4.
The crystal of the present invention substantially has a composition in which x=2. defects and substitution of alkaline ions are easily caused in this crystal structure and thus the crystal has a characteristic borate ring structure over a considerably wide range of 1.5<x<2.5. When x is outside of this range, this structure cannot be maintained.
The invention further relates to a borate single crystal which is a light wavelength-converting crystal represented by the chemical formula Li
1.5−a
Cs
0.5−b
X
a+b
B
4
O
7
wherein X is Rb, K, or Na, 0<a<1.5, and 0<b<0.5.
The light wavelength-converting crystal has a relatively large optical constant and deliquescence resistance. When the subscript a in the crystal is less than 0, Li is abundant and high moisture resistant is achieved. However, the nonlinear optical constant is small and Li
2
B
4
O
7
is formed. Thus, the novel crystal of the invention is not produced. When the subscript b is less than 0, Cs is abundant and a high nonlinear optical constant is achieved. However, moisture resistance is decreased. The other alkaline metal represented by X has an intermediate nature between Li and Cs, in the order of Na, K, and Rb. Thus, a suitable nonlinear optical constant and moisture resistance can be determined by substituting Li or Cs for the alkaline metal.
In addition, the invention relates to a method for growing a borate single crystal comprising weighing and melting predetermined amounts of two borates selected from the group consisting of Li, Na, K, Rb, and Cs, or predetermined amounts of a borate and a carbonate selected from the group together with a predetermined amount of boron oxide; and growing an orthorhombic crystal which is represented by the chemical formula (A
2
O).(B
2
O
3
)
x
and which belongs to the P2
Hanaue Yasuhiro
Kajitani Tsuyoshi
Komatsu Ryuichi
Masada Genta
Ono Yasuhiro
Mitsubishi Materials Corporation
Tucker Philip
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