BBO (β-BaB₂O₄) Single Crystal

Advantages

• Wide transparency range: 189–3500 nm.
• Broad phase-matching range: SHG phase matching achievable from 409.6 to 3500 nm (Type-I) and 525 to 3500 nm (Type-II).
• Large effective nonlinear coefficient: Approximately 6 times that of KDP crystal, enabling high conversion efficiency.
• High damage threshold: 5 GW/cm² at 1064 nm and 1 GW/cm² at 532 nm (10 ns pulses).
• Wide temperature acceptance bandwidth: ~55°C·cm (Type-I SHG at 1064 nm).
• Large electro-optic coefficient (γ₁₁ = 2.7 pm/V) suitable for Q-switching and Pockels cells.

Typical Specifications

• Chemical formula: β-BaB₂O₄ (low-temperature phase barium metaborate)
• Crystal structure: Trigonal, space group R3c; negative uniaxial
• Transparency range: 189–3500 nm (at “0” transmittance level)
• Refractive indices:
  • @ 1064 nm: nₒ = 1.6551, nₑ = 1.5426
  • @ 532 nm: nₒ = 1.6750, nₑ = 1.5555
  • @ 355 nm: nₒ = 1.7055, nₑ = 1.5775
  • @ 266 nm: nₒ = 1.7571, nₑ = 1.6139
  • @ 213 nm: nₒ = 1.8465, nₑ = 1.6742
• SHG phase-matching range: 409.6–3500 nm (Type-I); 525–3500 nm (Type-II)
• Nonlinear coefficients: d₁₁ = 5.8 × d₃₆ (KDP); d₃₁ = 0.16 pm/V; d₂₂ = 2.2 pm/V
• Electro-optic coefficients: γ₁₁ = 2.7 pm/V; γ₂₂, γ₃₁ < 0.1 γ₁₁
• Damage threshold: 5 GW/cm² (1064 nm, 10 ns); 10 GW/cm² (1064 nm, 1.3 ns); 1 GW/cm² (532 nm, 10 ns); 7 GW/cm² (532 nm, 250 ps)
• Mohs hardness: 4–5
• Density: 3.85 g/cm³
• Melting point: ~1095°C
• Thermal expansion coefficients: α₁₁ = 4 × 10⁻⁶/K; α₃₃ = 36 × 10⁻⁶/K
• Thermo-optic coefficients: dnₒ/dT = −9.3 × 10⁻⁶/°C; dnₑ/dT = −16.6 × 10⁻⁶/°C
• Absorption coefficient: α < 0.1%/cm @ 1064 nm
• Resistivity: >10¹¹ Ω·cm

Sellmeier equations (λ in μm):
nₒ² = 2.7359 + 0.01878/(λ² − 0.01822) − 0.01354λ²
nₑ² = 2.3753 + 0.01224/(λ² − 0.01667) − 0.01516λ²

Main Applications

• Harmonic generation of Nd:YAG and Nd:YLF lasers: Second (532 nm), third (355 nm), fourth (266 nm), and fifth (213 nm) harmonic generation; conversion efficiencies exceeding 70% for SHG, 60% for THG, and 50% for 4HG have been demonstrated.
• Tunable and ultrafast laser systems: SHG, THG, and SFM of dye, Ti:Sapphire, and Alexandrite lasers; femtosecond and picosecond pulse frequency conversion.
• Optical parametric oscillators and amplifiers (OPO/OPA/OPCPA): Tunable output from the UV to NIR (400–3000 nm) when pumped by SHG or THG of YAG or Ti:Sapphire lasers.
• Electro-optic Q-switching and modulation: High-power Q-switches, Pockels cells, pulse pickers, and modulators for UV and high-power laser systems.
• Deep UV generation: Sum-frequency mixing to produce output down to 189 nm; BBO is one of the few practical crystals for SHG and SFM below 500 nm.
• Intracavity SHG: Frequency doubling of Argon ion, copper vapor, and high-power Nd:YAG lasers.