KD₂PO₄ (DKDP, Potassium Dideuterium Phosphate) Single Crystal

Advantages

• Broader transparency range 200–2150 nm than KDP (200–1500 nm).
• Much larger electro-optic coefficient: r₆₃ = 25 pm/V (versus 10.3 pm/V for KDP), enabling significantly lower half-wave voltage (2.98 kV @ 546 nm vs. 7.65 kV for KDP).
• High laser damage threshold: >3 GW/cm² at 1064 nm (ns pulses), suitable for high-power laser systems .
• Large crystal growth capability: Can be grown to very large apertures, making it the material of choice for ICF and high-power laser facilities.
• Tunable deuterium content: Properties can be engineered by adjusting deuteration level (70% or >98%), enabling optimization for specific phase-matching and electro-optic applications.
• Good UV transmission and high birefringence: Well-suited for harmonic generation and electro-optic modulation.

Typical Specifications

• Chemical formula: KD₂PO₄ (deuteration level 70%, up to 99%)
• Crystal structure: Tetragonal, space group I4̄2d (point group 4̄2m)
• Transparency range: 200–2150 nm
• Refractive indices @1064 nm: n₀ = 1.4948, nₑ = 1.4554
• Nonlinear coefficient: d₃₆ = 0.40 pm/V
• Electro-optic coefficients: r₄₁ = 8.8 pm/V; r₆₃ = 25 pm/V
• Longitudinal half-wave voltage: Vπ = 2.98 kV @ 546 nm
• Absorption coefficient: 0.005–0.006 cm⁻¹
• Laser damage threshold: >3 GW/cm² @1064 nm (ns pulses); >5 J/cm² @1064 nm, 10 ns
• Mohs hardness: 2.5
• Density: 2.355 g/cm³
• Curie temperature: ~222 K (tetragonal-to-monoclinic transition)
• Extinction ratio (electro-optic devices): 30 dB (typical)
• Hygroscopic susceptibility: High; requires protective coating or controlled environment

Sellmeier equations (λ in μm) :
n₀² = 1.9575544 + 0.2901391λ²/(λ² − 0.0281399) − 0.02824391λ² + 0.004977826λ⁴
nₑ² = 1.5005779 + 0.6276034λ²/(λ² − 0.0131558) − 0.01054063λ² + 0.002243821λ⁴

Main Applications

• DKDP (typically 70% deuterated) is the primary THG crystal, >98%-deuterated DKDP is used for Pockels cells in laser fusion energy programs.
• High-power harmonic generation Second (532 nm), third (355 nm), and fourth (266 nm) of Nd:YAG and Nd:glass lasers.
• Electro-optic Q-switches and Pockels cells: The large r₆₃ coefficient (25 pm/V) and low half-wave voltage make DKDP the material of choice for high-power electro-optic modulators, Q-switches, and Pockels cells .
• Optical parametric chirped-pulse amplification (OPCPA): DKDP crystals with engineered deuteration levels (70-98%) enable broadband OPCPA for ultra-intense femtosecond laser systems, including petawatt-class lasers .
• Gradient-deuterium THG and broadband frequency conversion: DKDP crystals with spatially varying deuterium content enable group-velocity matching and rapid adiabatic passage for broadband THG, achieving efficiencies up to 36% .
• Terahertz wave generation: Demonstrated as a potential terahertz source.