Potassium titanyl phosphate

Potassium titanyl phosphate (KTiOPO4) or KTP  is a nonlinear optical material which is commonly used for frequency doubling diode pumped solid-state lasers such as Nd:YAG and other neodymium-doped lasers. The material has a relatively high optical damage threshold (~15 J/cm²), a large optical nonlinearity and excellent thermal stability. However, it is prone to photochromic damage (called grey tracking) during high power 1064 nm wavelength second harmonic generation which tends to limit its use to low and medium power systems.

It is also frequently used as an optical parametric oscillator for near IR generation up to 4 µm. It is particularly suited to high power operation as an optical parametric oscillator due to its high damage threshold and large crystal aperture. The high degree of birefringent walk-off between the pump signal and idler beams present in this material limit its use as an optical parametric oscillator for very low power applications.

KTP is also used as an electrooptic modulator, optical waveguide material, and in directional couplers.

KTP has orthorhombic crystal structure. It is highly transparent for wavelengths between 350-2700 nm with a reduced transmission out to 4500 nm where the crystal is effectively opaque. Its second harmonic generation (SHG) coefficient is about 3 times higher than KDP. It has a Mohs hardness of about 5.

Crack-free fibers of KTP can be prepared from suitable organometallic compounds (usually ethyl alkoxides of potassium and titanium, and butyl ester of phosphoric acid). 

KTP crystals coupled with YAG or YVO4 crystals are commonly found in green laser pointers.

Other such materials are ErTiOPO4 and KTiOAsO4.

Periodically poled potassium titanyl phosphate (PPKTP)
Periodically poled potassium titanyl phosphate (PPKTP) is a new material for nonlinear optics promising for various photorefractive applications. It can be wavelength tailored for efficient second harmonic generation. The material is based upon quasi-phase-matching, achieved by periodic poling of the crystal, when a structure of regularly spaced ferroelectric domains with alternating orientations is created in the material>.

Other materials used for periodic poling are wide band gap inorganic crystals like lithium niobate (resulting in periodically poled lithium niobate, PPLN), lithium tantalate, and some organic materials.