KH₂PO₄ (KDP, Potassium Dihydrogen Phosphate) Single Crystal

Advantages

• High laser damage threshold: >5 GW/cm² at 1064 nm (ns pulses), enabling use in high-power laser systems including ICF facilities .
• Broad transparency range: 180–1550 nm.
• Large electro-optic coefficients (r₄₁ = 8.8 pm/V, r₆₃ = 10.3 pm/V) suitable for Q-switches, Pockels cells, and electro-optic modulators .
• Large crystal growth capability: Can be grown to very large sizes.
• Good environmental tolerance: Operates at room temperature with high optical homogeneity and good mechanical properties for device fabrication.

Typical Specifications

• Crystal structure: Tetragonal, space group I4̄2d (point group 4̄2m); ferroelectric below ~123 K
• Lattice parameters: a = b = 7.4529 Å, c = 6.9751 Å
• Transparency range: 180–1550 nm
• Refractive indices @1064 nm: n₀ = 1.4938, nₑ = 1.4599
• Nonlinear coefficient: d₃₆ = 0.44 pm/V
• Electro-optic coefficients: r₄₁ = 8.8 pm/V; r₆₃ = 10.3 pm/V
• Longitudinal half-wave voltage: Vπ = 7.65 kV @ 546 nm
• Absorption coefficient: 0.04 cm⁻¹ at 1.06 μm
• Laser damage threshold: >5 GW/cm² @1064 nm (ns pulses); up to 100 GW/cm² for 10 ps pulses
• Mohs hardness: 2.5
• Density: 2.332 g/cm³
• Thermal expansion coefficients: α₁₁ = 2.5 × 10⁻⁵ K⁻¹; α₃₃ = 4.4 × 10⁻⁵ K⁻¹
• Thermal conductivity: k₁₁ = 1.9 × 10⁻² W/(cm·K)
• Hygroscopic susceptibility: High; requires protective coating or controlled environment
• Extinction ratio (for electro-optic devices): 30 dB
• Sellmeier equations (λ in μm) : n₀² = 2.259276 + 0.01008956/(λ² − 0.012942625) + 13.00522λ²/(λ² − 400) nₑ² = 2.132668 + 0.008637494/(λ² − 0.012281043) + 3.2279924λ²/(λ² − 400)

Main Applications

• Inertial confinement fusion (ICF): KDP and its deuterated analog DKDP are the primary frequency-conversion crystals for second, third, and fourth harmonic generation of Nd:glass laser output at 1053 nm.
• High-power harmonic generation: Second (532 nm), third (355 nm), and fourth (266 nm) harmonic generation of Nd:YAG and Nd:glass lasers; KDP is particularly preferred for fourth harmonic generation .
• Electro-optic modulators and Q-switches: Pockels cells, Q-switches, and electro-optic modulators for laser systems, exploiting the large r₆₃ coefficient .
• Optical parametric oscillators and amplifiers: Type-II angle-tuned OPOs for tunable visible and near-IR output (450–650 nm and 800–1600 nm) when pumped at 355 nm .
• X-ray monochromators: Used as monochromating crystals in X-ray spectroscopy and synchrotron beamlines due to high crystal perfection.