KDP, DKDP Nonlinear (NLO) Crystals

KDP (Potassium Dihydrogen Phosphate)、DKDP（Deuterated KDP、KD2PO4), and DKDP (commonly another notation for DKDP) are important materials widely used as Nonlinear Optical (NLO) crystals. These crystals play a particularly important role in laser technology.

Features

KDP (KH2PO4)
Applications: Mainly used for Second Harmonic Generation (SHG), Third Harmonic Generation (THG), and Electro-Optical (EO) purposes.
Characteristics: High transparency (about 200 nm to 1,500 nm), moderate nonlinearity, and high optical homogeneity.
Advantages: Relatively easy to manufacture and capable of growing large-diameter crystals.
DKDP (KD2PO4)
Applications: Mainly used for laser wavelength conversion, electro-optical modulators, and polarization switches.
Characteristics: Suitable for high-power laser systems (especially Nd:YAG lasers) due to its higher laser damage threshold and lower absorption rate compared to KDP.
Advantages: Can be used in the mid-infrared region, especially adopted in laser inertial fusion and high-energy laser systems.

Main Applications

Wavelength Conversion (Second Harmonic Generation, Third Harmonic Generation)
Conversion of the wavelength of Nd:YAG or Nd:Glass lasers to 532 nm (green light) or 355 nm (ultraviolet light).
Mainly utilized in laser processing, scientific research, and the medical field.
Electro-Optical Applications
Used in EO modulators and laser switches (Q switches).
Enables fast and accurate laser control.
High-Power Laser Systems
Widely adopted in Inertial Fusion Energy (IFE) projects and laser systems for high-energy physics.
Spectroscopic Analysis and Communication
Important for laser generation and transmission in the ultraviolet to mid-infrared range.
Precautions
Since KDP and DKDP crystals are hygroscopic, they need to be stored and used in a dry environment.
Advanced techniques are required for the precision processing of large-diameter crystals.

Basic Properties

KDP DKDP
Chemical Formula KH2PO4 KD2PO4
Transparency Range 200-1500 nm 200-1600 nm
Nonlinear Coefficients d36=0.44 pm/V d36=0.40 pm/V
Refractive index (at 1064nm) no=1.4938, ne=1.4599 no=1.4948, ne=1.4554
Electro-optical Coefficients r41=8.8 pm/V
r63=10.3 pm/V		 r41=8.8 pm/V
r63=25 pm/V
Longitudinal half-wave voltage Vπ=7.65 KV(λ=546 nm) Vπ=2.98 KV(λ=546 nm)
Absorptance 0.07 /cm 0.006 /cm
Optical Damage Threshold >5 GW/cm2 >3 GW/cm2
Extinction ratio 30 dB
Sellmeier equations of KDP:(λ in um)
no2=2.259276+0.01008956/(λ2-0.012942625)+13.00522λ22/(λ2-400)
ne2=2.132668+0.008637494/(λ2-0.012281043)+3.2279924λ2/(λ2-400)
Sellmeier equations of DKDP:(λ in um)
no2=1.9575544+0.2901391λ2/(λ2-0.0281399)-0.02824391λ2+0.004977826λ2
ne2=1.5005779+0.6276034λ2/(λ2-0.0131558)-0.01054063λ2+0.002243821λ2