JP2677205B2 - Method for growing β-BaB2O4 single crystal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for growing a β-BaB ₂ O ₄ single crystal, and more specifically, it grows a long high-quality crystal by using a seed crystal processed so as to be pulled up to a phase matching orientation. On how to do.

[0002]

_2. Description of the Related Art β-BaB ₂ applied to a wavelength conversion element
O ₄ (beta-barium borate, hereinafter abbreviated as BBO) single crystal is conventionally formed by using a platinum rod instead of a seed crystal, and is made of BaB.
_It was grown by the pulling method from the ₂ O ₄ composition melt or by the pulling method using a seed crystal in the c-axis direction which is the crystal axis.

The BBO single crystal grown in the c-axis direction has a method in which the harmonics generated in the crystal and the incident fundamental wave have the same refractive index in order to obtain higher harmonics as a wavelength conversion element (phase matching method). It is necessary to process into. Therefore, a phase matching azimuth for wavelength conversion of this crystal is determined using X-rays and a laser beam, and a step of processing into a wavelength conversion element has been taken.

The phase matching azimuth of the BBO wavelength conversion element changes depending on the wavelength and polarization direction of the fundamental laser light,
In the example of obtaining the harmonics of the extraordinary ray from the fundamental wave of the ordinary ray, θ is represented by Equation 1.

[0005]

(Equation 1)

(N ₀ is the refractive index of ordinary light, n _e is the refractive index of extraordinary light, ω is the fundamental wave, and 2ω is the harmonic wave.)

By the way, if the long BBO single crystal can be pulled in the phase matching azimuth necessary for the wavelength conversion element, merits such as reduction of the processing steps and improvement of the yield can be expected. However, in the case of BBO single crystal,
When a crystal is grown in an orientation with an angle from the c-axis, a crystal habit develops in an orientation close to the c-plane, which develops in a direction in which the diameter decreases with respect to the pulling direction. Get lost. In the past, in order to avoid this, the melt temperature was controlled at the point where crystal habit started to occur, and attempts were made to suppress diameter fluctuations.However, since grain boundaries and inclusions are generated from the part where the melt temperature is fluctuated, it is high. It was not possible to grow a long crystal with high quality.

An object of the present invention is to study the growth conditions for growing a crystal in an orientation with an angle from the c-axis,
This is to enable high quality and long crystal growth without generation of grain boundaries and inclusions.

[0008]

The present invention is characterized in that a BBO single crystal is used as a seed crystal having an orientation tilted from the c-axis, and the temperature of the melt after seeding is lowered at a constant rate. It is a method for growing a long BBO crystal by the method.

The crystal habit of BBO, which is generated in the direction in which the diameter of the grown crystal decreases, in the direction close to the c-plane, receives the heat radiation from the melt to increase the temperature of the crystal, so that the crystal diameter gradually decreases. . If the decrease in crystal size is suppressed by forcibly controlling the temperature of the melt, the heat flow conditions at the growth interface change abruptly, and grain boundaries and inclusions occur.
However, by lowering the temperature of the melt at a constant rate irrespective of the variation in the diameter of the crystal grown before the crystal habit is generated after the growth is started, the crystal habit generation interval is set as shown in FIG. It is possible to carry out pulling and growing while maintaining a constant period without rapidly changing the heat flow conditions at the growing interface. Therefore, no grain boundary or inclusion is generated at the growth interface, and it becomes possible to grow a long and high-quality BBO single crystal having a phase matching orientation in the pulling direction without cutting the crystal from the melt. .

[0010]

When a crystal is grown from the melt, a heat flow always exists at the growth interface. The heat flow is an important parameter that determines the solidification rate from the melt to the crystal, the growth rate, and the shape of the interface. This fluctuation in heat flow causes grain boundaries and inclusions that cause deterioration of crystal quality.
That is, the quality of the grown crystal depends on how to properly control the heat flow. In the case of BBO, since the β phase, which is a low temperature phase, is grown directly from the BaB ₂ O ₄ composition melt by using a large amount of supercooling, the degree of deterioration of quality due to changes in heat flow is higher than that of other crystals. large. When BBO is pulled up and grown with a seed crystal having a phase matching orientation tilted from the c-axis, a crystal habit of an orientation close to the c-plane appears on the side surface of the grown crystal, and this surface develops in a direction in which the diameter of the grown crystal decreases. If the temperature of the melt is kept constant and pulling is continued, the crystal will separate from the melt. Therefore, when the temperature of the melt is raised or lowered in order to grow a crystal with a small diameter variation, the heat flow at the growth interface also changes accordingly. Therefore, if the diameter of the grown crystal is controlled by the melt temperature, the quality of the crystal deteriorates due to the occurrence of grain boundaries. So in the case of BBO,
It can be seen that in order to grow a high quality and long crystal in the phase matching direction, the method of controlling the temperature of the melt after the crystal diameter fluctuates is already slow.

Therefore, it was considered to grow a long crystal by suppressing the change of the heat flow at the growth interface as much as possible while changing the diameter, instead of suppressing the change of the diameter of the grown crystal by the melt temperature. Even if the crystal habit grows and the diameter decreases, by selecting the temperature drop rate of the melt at an appropriate constant rate before the crystal habit grows and the diameter fluctuates Can be changed in an increasing direction. In this case, the heat flow at the growth interface does not change significantly even if the diameter changes, so grain boundaries and inclusions do not occur in the crystal, and a long, high-quality crystal with constrictions at regular intervals as shown in FIG. Can be trained.

[0012]

【Example】

(Example 1) The phase matching azimuth of Type I for obtaining the FHG light (fourth harmonic) of the YAG laser is θ = 47.6 °.
It is. Length 20mm with this direction in the longitudinal direction 4mm in diameter
After seeding the seed crystal processed into 2 and starting pulling up at 2 mm / hr, the temperature gradient of the melt was adjusted to 0.5-3.0 ° C.
1.5 ± 0.2 when descending at a constant rate of / hr
High-quality BBO that does not include grain boundaries and inclusions in a long phase-matching azimuth while the diameter fluctuates as shown in Fig. 1 when the temperature is lowered at a rate of ° C / hr.
It was found that a single crystal can be grown.

(Embodiment 2) THG of the YAG laser (third embodiment)
The same experiment was conducted with a seed crystal having a direction of generation of harmonics of θ = 31.1 °, and the temperature drop rate was 1.6 ± 0.2 ° C.
In the case of / hr, it was possible to grow a long crystal similar to that shown in FIG. 1, although there was a diameter variation, but no inclusion or grain boundary.

(Embodiment 3) YAG laser SHG (second
When a crystal was grown in the same manner as in Example 1 with a seed crystal having θ = 22.8 °, which is the direction of generation of higher harmonics, when the temperature drop rate was 1.7 ± 0.2 ° C./hr, Similar to the above, it was possible to grow a long crystal without inclusion and grain boundary, although there was a diameter variation.

Table 1 shows the correlation between the angle θ from the c-axis and the temperature drop rate.

[0016]

[Table 1]

As shown above, the angle θ from the c-axis is 0.
Even if the temperature is changed by 90 °, high-quality BBO single crystal can be grown by selecting an appropriate temperature drop rate in that direction. In the present example, an experiment was conducted using a crucible having a diameter of 40 mm.

(Comparative Example 1) Same as in Example 1 θ = 47.6
Seed crystal seeded with a length of 20 mm and a diameter of 4 mm with the orientation of ° in the longitudinal direction was seeded and started to pull up at 2 mm / hr, and the temperature of the melt was kept constant to pull up the crystal. The crystal habit near the c-plane grew and the diameter decreased, and the crystal separated from the melt.

(Comparative Example 2) Same as in Example 1 θ = 47.6
Seed crystal processed to have a length of 20 mm and a diameter of 4 mm with the azimuth direction in the longitudinal direction is seeded and the pulling is started at 2 mm / hr, and the temperature of the melt is controlled so that the diameter of the crystal is maintained at about 10 mm. When a single crystal was grown, a crystal containing inclusions and having a large grain boundary was grown as shown in FIG.

The crystal growth method of the present invention is not limited to the YAG laser crystal shown in this embodiment, but may be a YLF laser or Y crystal.
It goes without saying that it can also be applied to a crystal for a VO ₄ laser. At this time, each SHG, THG, FHG direction is YA
Since each direction of the G laser exists within a range of ± 0.5 °, an optimum crystal can be grown by adjusting the direction within this range to a laser used appropriately.

[0021]

According to the present invention, when a BBO single crystal is grown by a pulling method using a seed crystal having a phase matching orientation, a high quality long single crystal without grain boundary and inclusion is grown. be able to.

[Brief description of the drawings]

FIG. 1 is a schematic view of a BBO single crystal grown with a seed crystal having an orientation of θ = 47.6 ° and a melt temperature lowered at 1.5 ° C./hr.

FIG. 2 is a schematic diagram (Comparative Example 1) of a BBO single crystal grown with the same kind of crystal as in FIG. 1 while keeping the melt temperature constant.

FIG. 3 is a schematic diagram of a BBO single crystal grown by controlling the melt temperature with the same kind of crystal as in FIG. 1 to suppress the diameter variation (Comparative Example 2).
It is.

[Explanation of symbols]

 1 Seed crystal 2 Crystal habit 3 Boundary 4 Inclusion

Claims (4)

(57) [Claims] 1. A long crystal is obtained by pulling up a β-BaB ₂ O ₄ single crystal by using a seed crystal having an orientation tilted from the c-axis and decreasing the temperature of the melt after seeding at a constant rate. A method for growing a β-BaB ₂ O ₄ single crystal, which comprises growing the single crystal. 2. A seed crystal processed to have an orientation of θ = 22.8 ± 0.5 ° in the longitudinal direction is seeded to start pulling, and then the temperature of the melt is 1.7 ± 0. A method for growing a β-BaB ₂ O ₄ single crystal, which is characterized in that pulling is further performed while lowering at a rate of 2 ° C./hr. 3. A seed crystal processed to have an orientation of θ = 31.1 ± 0.5 ° in the longitudinal direction is seeded to start pulling, and then the temperature of the melt is set to 1.6 ± 0. A method for growing a β-BaB ₂ O ₄ single crystal, which is characterized by further pulling while lowering at a rate of 2 ° / hr. 4. A seed crystal processed so as to have an orientation of θ = 47.6 ± 0.5 ° in the longitudinal direction is seeded to start pulling, and then the temperature of the melt is set to 1.5 ± 0. A method for growing a β-BaB ₂ O ₄ single crystal, which is characterized by further pulling while lowering at a rate of 2 ° / hr.