Li₂B₄O₇ (LB4, Lithium Tetraborate) Laser-Grade Single Crystal

Advantages

• Deep UV transparency: Transmission range extends to 160–170 nm at the short-wavelength cutoff, enabling deep ultraviolet applications.
• Exceptionally high damage threshold: Reported at 40 GW/cm² for 10 ns pulses at 1064 nm, far exceeding LBO and fused quartz.
• Non-hygroscopic and chemically stable: Requires no protective coating in many applications due to its low refractive index and environmental stability.
• Low refractive index: nₒ ≈ 1.598, nₑ ≈ 1.543 at 1064 nm, minimizing Fresnel reflection losses without AR coating.

Typical Specifications

• Crystal structure: Tetragonal, point group 4mm (negative uniaxial)
• Transparency range: 160–3500 nm
• Refractive indices (@1064 nm): nₒ = 1.5980, nₑ = 1.5432
• Birefringence: Δn = 0.057
• Nonlinear coefficients: d₃₁ = 0.12 pm/V, d₃₃ = 0.47 pm/V
• SHG shortest output wavelength: 243.8 nm (noncritical phase matching)
• Optical damage threshold: 40 GW/cm² (1064 nm, 10 ns)
• Mohs hardness: 4–6
• Density: 2.45 g/cm³
• Melting point: 917°C
• Absorption coefficient: α ≤ 10⁻⁴ cm⁻¹ (Top level crystals)

Main Applications

• Deep UV harmonic generation: Fourth and fifth harmonics of Nd:YAG lasers; SHG conversion efficiency of 20% from 532 nm green light.
• UV nonlinear optics: SHG and sum-frequency generation for high-power ultraviolet light sources (245–250 nm).
• THz wave generation: Potential for terahertz radiation generation under phase-matching conditions, though efficiency remains low.

threshold and deep UV transparency are more critical than conversion efficiency.