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α-BBO - Alpha-Barium Borate

Alpha-Barium Borate (α-BaB2O4, α-BBO) is a negative uniaxial crystal which has large birefringence over a broad transparent range of 190 nm-3500 nm. α-BBO is an excellent crystal especially in UV and high power applications. The physical, chemical, thermal and optical properties of α-BBO crystal are similar to those of β-BBO. However, the nonlinear optical properties of α-BBO crystal are nonexistent due to the central symmetry of its crystal structure. α-BBO crystal is not recommended for NLO processes.

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Birefringent Crystals

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Description

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Introduction

Alpha-Barium Borate (α-BaB₂O₄, α-BBO) is a negative uniaxial crystal which has large birefringence over a broad transparent range of 190 nm-3500 nm. α-BBO is an excellent crystal especially in UV and high power applications. The physical, chemical, thermal and optical properties of α-BBO crystal are similar to those of β-BBO. However, the nonlinear optical properties of α-BBO crystal are nonexistent due to the central symmetry of its crystal structure. α-BBO crystal is not recommended for NLO processes.

CASTECH's birefringent crystal α-BBO is featured by

•  High UV transmittance
•  Large birefringence
•  Low bulk absorption suitable for high power applications
•  High damage threshold
•  Stable physical and mechanical properties.

Table 1. Basic Properties
Crystal Structure	Trigonal
Transparency Range	190-3500 nm
Density	3.85 g/cm³
Hygroscopic Susceptibility	Low
Hardness	4.5 Mohs
Thermal Expansion Coefficients	-9.3 × 10^-6 /℃（C） -9.5 × 10^-6 /℃（A）
Damage Threshold	1 GW/cm² @1064 nm, 10 ns, 10 Hz (AR-coated)
Refractive Indices	n_o=1.6776, n_e=1.5534, @532 nm n_o=1.6579, n_e=1.5379, @1064 nm
Sellmeier Equation (λ in μm)	n_o² = 2.7471 + 0.01878 / (λ² - 0.01822) - 0.01354 λ² n_e² = 2.37153 + 0.01224 / (λ² - 0.01667) - 0.01516 λ²

Table 2. Specifications
Size	Aperture up to Φ50 mm and length up to 40 mm
Surface Quality	10/5 to MIL-PRF-13830B
Flatness	λ/4 @633 nm
Optical Axis Orientation	6 arc min
Parallelism	20 arc sec
Clear Aperture	＞90%
Coating	AR-coating or P-coating
Mount	Upon Customer's Specification

Figure 1. Transparency Curve

Keyword:

BIREFRINGENT crystal

alphaα-BBO

high UV transmittance

polarizing optics

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Alpha-Barium Borate (α-BBO) Crystal

CASTECH, Inc. has the largest production scale of α-BBO crystals in the industry, and its technology for growing large-size boule leads the world.

When light propagates through a heterogeneous medium, its speed and refractive index values change with the direction of vibration. There is not only one refractive index value. When a light wave enters a heterogeneous medium, except for special directions, it undergoes double refraction and is decomposed into two polarized light waves with perpendicular vibration directions, different propagation speeds, and unequal refractive indices. This phenomenon is called birefringence, as shown in Figure 1. The crystal that exhibits birefringence is called a birefringent crystal, which functions similarly to two polarizers with perpendicular transmission directions.

Alpha-Barium Borate (α-BBO) crystal is an outstanding birefringent crystal, which possesses exceptional optical properties such as large birefringence and broad transparency range. It has been widely used in optical polarization technologies, high-power laser systems, and ultrafast laser applications.

α-BBO crystal is mainly employed for making:

a. Ultraviolet polarizing prisms such as Glan-Taylor prisms and Wollaston prisms;

b. High-power optical isolators;

c. Group-velocity-dispersion compensators for ultrafast lasers, etc

Characteristics of α-BBO Crystal

There are many types of birefringent crystals, and α-BBO compensates for the shortcomings of traditional birefringent crystals such as YVO₄ and CaCO₃ with its outstanding optical properties, making it an ideal ultraviolet birefringent crystal.

1.Introduction of α-BBO Crystal

Barium metaborate (BaB₂O₄) has two polymorphs: high-temperature phase (α-BBO) and low-temperature phase (β-BBO). Its melting point is 1095℃, and its phase transition temperature is around 925℃^[1]. It is well known that β-BBO crystal is an excellent nonlinear optical crystal, widely used in laser SHG(second harmonic generation), mixing, and OPO(optical parametric oscillation). CASTECH is the world leader in BBO crystal, with a market share of more than 70%. In the development process of more than 30 years, it has played a huge role in promoting the development of ultrafast lasers and quantum science.

α-BBO crystal belongs to the trigonal crystal system, with a single-cell parameter of a=b=0.7235nm, and c=3.9192 nm. α-BBO crystal is a negative uniaxial crystal with a large birefringence and a transmission range of 189~3500nm. It has stable physical and chemical properties and excellent mechanical processing properties. However, due to its center symmetry structure, it does not have second-order nonlinear effects like β-BBO crystal.

2.Propagation of light in α-BBO crystal

α-BBO crystal is a negative uniaxial crystal. According to the characteristics of o-ray and e-ray, the propagation direction of o-ray and e-ray in the crystal can be determined by using Huygens' principle^[2]. We will discuss the following cases:

（1） Oblique incidence:

①Since the optical axis is in the incident plane (in this case, the plane of the paper), the two principal planes coincide, so both rays are in the incident plane. The polarization direction of o-ray is perpendicular to the incident plane, and the polarization direction of e-ray is parallel to the incident plane, as shown in Figure 2.

②If the optical axis is not in the incident plane, the two principal planes no longer coincide, and point E is not in the incident plane, so e-ray is also not in the incident plane.

③When the incident direction is parallel to the optical axis, e-ray will be on the same side of the normal as the incident line, as shown in Figure 3.

（2）Perpendicular incidence:

①The optical axis is perpendicular to the surface of the crystal and parallel to the incident plane. The o-ray and e-ray have the same speed and direction, and no birefringence occurs, as shown in Figure 4.

②When the optical axis is parallel to the crystal surface and perpendicular to the incident surface, the directions of o-ray and e-ray are the same, but their velocities are different. They undergo birefringence and cannot be distinguished by visual inspection. This is shown in Figure 5.

③When the optical axis is parallel to the crystal surface and parallel to the incident plane, the directions of o-ray and e-ray are the same, but their velocities are different. They do not separate visually but undergo birefringence, as shown in Figure 6.

3

Optical Properties of α-BBO crystal

α-BBO crystal has excellent optical properties and good transparency from ultraviolet to mid-infrared range. It has the following characteristics :

①It has a large birefringence;

②It has a transmission range of 189~3500nm;

③It has stable physicochemical properties;

④It has excellent mechanical processing properties;

⑤Particularly, it has a high transmittance in the ultraviolet region (95%), and its performance is better than YVO₄ and CaCO₃ crystals. Table 1 compares the properties of α-BBO crystal with those of YVO₄ and CaCO₃.

Table 1 compares the optical properties of α-BBO crystal with other birefringent crystals^[3]

2

Development of α-BBO crystals at CASTECH

CASTECH has been engaged in the R&D of α-BBO crystals for many years, and has accumulated a wealth of research and experimental results and experience in raw material formulation design, raw material synthesis and processing, and temperature field design optimization. It has the largest production scale of α-BBO crystals in the industry, and achieved an important breakthrough in the field of large-size α-BBO crystal boule growth technology in 2022, realizing stable growth of high-quality blank of 1 kilogram level. This has advanced the world's level of α-BBO crystal growth reported so far, as shown in Figure 7.

①CASTECH has achieved world-leading levels in the growth and innovation of α-BBO crystals after years of research and development.Through special ion doping technology, the crystal structure is ensured to be stable and internal stress is improved, reducing cracking during crystal processing ^[3][4]. Barium borate (BaB₂O₄) has two polymorphs: high-temperature phase (α-BBO) and low-temperature phase (β-BBO), with a melting point of 1095°C and a phase transition temperature of around 925°C ^[1]. Typically, α-BBO crystals are grown from the BBO melt using the Czochralski (CZ) method. However, during the cooling process, as the crystal passes through the phase transition temperature (925°C), it transforms from the α phase to the β phase, causing the crystal to become opaque and destructively fragmented. Moreover, α-BBO crystals grown by the CZ method are easily cleaved along the (001) plane. Therefore, it is extremely difficult to grow large, complete, and high-quality α-BBO crystals using the CZ method. The project team specially ion-doped the α-BBO raw material, enabling the crystal to remain in the α phase structure when cooled from high to low temperatures, thus avoiding severe cracking caused by structural changes. On the other hand, as the crystal is grown at high temperature, thermal stress inevitably exists internally, which can easily cause cracking during subsequent processing, such as cutting and polishing. The addition of doping ions effectively improves the internal thermal stress and greatly reduces cracking during subsequent processing, which greatly contributes to improving product yield and reducing costs.

②Through precise automatic pulling control and optimization of various process parameters, the internal defects of the crystal are minimized, and the optical uniformity is high, making the crystal characterized by small bulk absorption and small wavefront distortion. There are few manufacturers in China can provide α-BBO crystals and devices in batches, but CASTECH has dozens of automated growth furnaces that produce stable and high-quality α-BBO crystals and components, as shown in Figure 8.

The product specifications of α-BBO crystals from CASTECH include stable and large-scale supply of high-precision α-BBO crystal components, with basic processing parameters as shown in Table 2.

The α-BBO crystals grown by CASTECH have excellent internal quality, low absorption (Figure 9), and good UV transmission (Figure 10).

reference:

[1]Levin EM，Mccurdie HF.j.Research of the National Bureau of Standards，

1949，42(2):131-138.

[2]许婕, 陈理想, 郑国梁等. 双折射晶体中旋光效应的耦合波理论[J]. 物理学报, 2007, 56(8):4615-4621.

[3]吴少凡,大尺寸双折射晶体YVO4和α-BBO生长研究.中国科学院福建物质结构研究所学位论文-2002.

[4]Wu shaofanWang，Guofu，Xie JianLing et al.Jorunal of Crystal Growth，

2002，(245):84-86.

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α-BBO - Alpha-Barium Borate
Alpha-Barium Borate (α-BaB2O4, α-BBO) is a negative uniaxial crystal which has large birefringence over a broad transparent range of 190 nm-3500 nm. α-BBO is an excellent crystal especially in UV and high power applications. The physical, chemical, thermal and optical properties of α-BBO crystal are similar to those of β-BBO. However, the nonlinear optical properties of α-BBO crystal are nonexistent due to the central symmetry of its crystal structure. α-BBO crystal is not recommended for NLO processes.

Description

Cas'Tech Class

Parameters

Download

Introduction

Alpha-Barium Borate (α-BaB₂O₄, α-BBO) is a negative uniaxial crystal which has large birefringence over a broad transparent range of 190 nm-3500 nm. α-BBO is an excellent crystal especially in UV and high power applications. The physical, chemical, thermal and optical properties of α-BBO crystal are similar to those of β-BBO. However, the nonlinear optical properties of α-BBO crystal are nonexistent due to the central symmetry of its crystal structure. α-BBO crystal is not recommended for NLO processes.

CASTECH's birefringent crystal α-BBO is featured by

•  High UV transmittance
•  Large birefringence
•  Low bulk absorption suitable for high power applications
•  High damage threshold
•  Stable physical and mechanical properties.

Table 1. Basic Properties
Crystal Structure	Trigonal
Transparency Range	190-3500 nm
Density	3.85 g/cm³
Hygroscopic Susceptibility	Low
Hardness	4.5 Mohs
Thermal Expansion Coefficients	-9.3 × 10^-6 /℃（C） -9.5 × 10^-6 /℃（A）
Damage Threshold	1 GW/cm² @1064 nm, 10 ns, 10 Hz (AR-coated)
Refractive Indices	n_o=1.6776, n_e=1.5534, @532 nm n_o=1.6579, n_e=1.5379, @1064 nm
Sellmeier Equation (λ in μm)	n_o² = 2.7471 + 0.01878 / (λ² - 0.01822) - 0.01354 λ² n_e² = 2.37153 + 0.01224 / (λ² - 0.01667) - 0.01516 λ²

Table 2. Specifications
Size	Aperture up to Φ50 mm and length up to 40 mm
Surface Quality	10/5 to MIL-PRF-13830B
Flatness	λ/4 @633 nm
Optical Axis Orientation	6 arc min
Parallelism	20 arc sec
Clear Aperture	＞90%
Coating	AR-coating or P-coating
Mount	Upon Customer's Specification

Figure 1. Transparency Curve

Alpha-Barium Borate (α-BBO) Crystal

CASTECH, Inc. has the largest production scale of α-BBO crystals in the industry, and its technology for growing large-size boule leads the world.

When light propagates through a heterogeneous medium, its speed and refractive index values change with the direction of vibration. There is not only one refractive index value. When a light wave enters a heterogeneous medium, except for special directions, it undergoes double refraction and is decomposed into two polarized light waves with perpendicular vibration directions, different propagation speeds, and unequal refractive indices. This phenomenon is called birefringence, as shown in Figure 1. The crystal that exhibits birefringence is called a birefringent crystal, which functions similarly to two polarizers with perpendicular transmission directions.

Alpha-Barium Borate (α-BBO) crystal is an outstanding birefringent crystal, which possesses exceptional optical properties such as large birefringence and broad transparency range. It has been widely used in optical polarization technologies, high-power laser systems, and ultrafast laser applications.

α-BBO crystal is mainly employed for making:

a. Ultraviolet polarizing prisms such as Glan-Taylor prisms and Wollaston prisms;

b. High-power optical isolators;

c. Group-velocity-dispersion compensators for ultrafast lasers, etc

Characteristics of α-BBO Crystal

There are many types of birefringent crystals, and α-BBO compensates for the shortcomings of traditional birefringent crystals such as YVO₄ and CaCO₃ with its outstanding optical properties, making it an ideal ultraviolet birefringent crystal.

1.Introduction of α-BBO Crystal

Barium metaborate (BaB₂O₄) has two polymorphs: high-temperature phase (α-BBO) and low-temperature phase (β-BBO). Its melting point is 1095℃, and its phase transition temperature is around 925℃^[1]. It is well known that β-BBO crystal is an excellent nonlinear optical crystal, widely used in laser SHG(second harmonic generation), mixing, and OPO(optical parametric oscillation). CASTECH is the world leader in BBO crystal, with a market share of more than 70%. In the development process of more than 30 years, it has played a huge role in promoting the development of ultrafast lasers and quantum science.

α-BBO crystal belongs to the trigonal crystal system, with a single-cell parameter of a=b=0.7235nm, and c=3.9192 nm. α-BBO crystal is a negative uniaxial crystal with a large birefringence and a transmission range of 189~3500nm. It has stable physical and chemical properties and excellent mechanical processing properties. However, due to its center symmetry structure, it does not have second-order nonlinear effects like β-BBO crystal.

2.Propagation of light in α-BBO crystal

α-BBO crystal is a negative uniaxial crystal. According to the characteristics of o-ray and e-ray, the propagation direction of o-ray and e-ray in the crystal can be determined by using Huygens' principle^[2]. We will discuss the following cases:

（1） Oblique incidence:

①Since the optical axis is in the incident plane (in this case, the plane of the paper), the two principal planes coincide, so both rays are in the incident plane. The polarization direction of o-ray is perpendicular to the incident plane, and the polarization direction of e-ray is parallel to the incident plane, as shown in Figure 2.

②If the optical axis is not in the incident plane, the two principal planes no longer coincide, and point E is not in the incident plane, so e-ray is also not in the incident plane.

③When the incident direction is parallel to the optical axis, e-ray will be on the same side of the normal as the incident line, as shown in Figure 3.

（2）Perpendicular incidence:

①The optical axis is perpendicular to the surface of the crystal and parallel to the incident plane. The o-ray and e-ray have the same speed and direction, and no birefringence occurs, as shown in Figure 4.

②When the optical axis is parallel to the crystal surface and perpendicular to the incident surface, the directions of o-ray and e-ray are the same, but their velocities are different. They undergo birefringence and cannot be distinguished by visual inspection. This is shown in Figure 5.

③When the optical axis is parallel to the crystal surface and parallel to the incident plane, the directions of o-ray and e-ray are the same, but their velocities are different. They do not separate visually but undergo birefringence, as shown in Figure 6.

3

Optical Properties of α-BBO crystal

α-BBO crystal has excellent optical properties and good transparency from ultraviolet to mid-infrared range. It has the following characteristics :

①It has a large birefringence;

②It has a transmission range of 189~3500nm;

③It has stable physicochemical properties;

④It has excellent mechanical processing properties;

⑤Particularly, it has a high transmittance in the ultraviolet region (95%), and its performance is better than YVO₄ and CaCO₃ crystals. Table 1 compares the properties of α-BBO crystal with those of YVO₄ and CaCO₃.

Table 1 compares the optical properties of α-BBO crystal with other birefringent crystals^[3]

2

Development of α-BBO crystals at CASTECH

CASTECH has been engaged in the R&D of α-BBO crystals for many years, and has accumulated a wealth of research and experimental results and experience in raw material formulation design, raw material synthesis and processing, and temperature field design optimization. It has the largest production scale of α-BBO crystals in the industry, and achieved an important breakthrough in the field of large-size α-BBO crystal boule growth technology in 2022, realizing stable growth of high-quality blank of 1 kilogram level. This has advanced the world's level of α-BBO crystal growth reported so far, as shown in Figure 7.

①CASTECH has achieved world-leading levels in the growth and innovation of α-BBO crystals after years of research and development.Through special ion doping technology, the crystal structure is ensured to be stable and internal stress is improved, reducing cracking during crystal processing ^[3][4]. Barium borate (BaB₂O₄) has two polymorphs: high-temperature phase (α-BBO) and low-temperature phase (β-BBO), with a melting point of 1095°C and a phase transition temperature of around 925°C ^[1]. Typically, α-BBO crystals are grown from the BBO melt using the Czochralski (CZ) method. However, during the cooling process, as the crystal passes through the phase transition temperature (925°C), it transforms from the α phase to the β phase, causing the crystal to become opaque and destructively fragmented. Moreover, α-BBO crystals grown by the CZ method are easily cleaved along the (001) plane. Therefore, it is extremely difficult to grow large, complete, and high-quality α-BBO crystals using the CZ method. The project team specially ion-doped the α-BBO raw material, enabling the crystal to remain in the α phase structure when cooled from high to low temperatures, thus avoiding severe cracking caused by structural changes. On the other hand, as the crystal is grown at high temperature, thermal stress inevitably exists internally, which can easily cause cracking during subsequent processing, such as cutting and polishing. The addition of doping ions effectively improves the internal thermal stress and greatly reduces cracking during subsequent processing, which greatly contributes to improving product yield and reducing costs.

②Through precise automatic pulling control and optimization of various process parameters, the internal defects of the crystal are minimized, and the optical uniformity is high, making the crystal characterized by small bulk absorption and small wavefront distortion. There are few manufacturers in China can provide α-BBO crystals and devices in batches, but CASTECH has dozens of automated growth furnaces that produce stable and high-quality α-BBO crystals and components, as shown in Figure 8.

The product specifications of α-BBO crystals from CASTECH include stable and large-scale supply of high-precision α-BBO crystal components, with basic processing parameters as shown in Table 2.

The α-BBO crystals grown by CASTECH have excellent internal quality, low absorption (Figure 9), and good UV transmission (Figure 10).

reference:

[1]Levin EM，Mccurdie HF.j.Research of the National Bureau of Standards，

1949，42(2):131-138.

[2]许婕, 陈理想, 郑国梁等. 双折射晶体中旋光效应的耦合波理论[J]. 物理学报, 2007, 56(8):4615-4621.

[3]吴少凡,大尺寸双折射晶体YVO4和α-BBO生长研究.中国科学院福建物质结构研究所学位论文-2002.

[4]Wu shaofanWang，Guofu，Xie JianLing et al.Jorunal of Crystal Growth，

2002，(245):84-86.

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