Crystals of KTP are highly transparent for wavelengths between 350 and 2700 nm with a reduced transmission out to 4500 nm where the crystal is effectively opaque. Its
second-harmonic generation (SHG) coefficient is about three times higher than
KDP. It has a
Mohs hardness of about 5. KTP is also 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. The material has a relatively high threshold to optical damage (~15 J/cm2), an excellent optical nonlinearity and excellent thermal stability in theory. In practice, KTP crystals need to have stable temperature to operate if they are pumped with 1064 nm (
infrared, to output 532 nm green). However, it is prone to
photochromic damage (called grey tracking) during high-power 1064 nm second-harmonic generation which tends to limit its use to low- and mid-power systems. Other such materials include potassium titanyl arsenate (KTiOAsO4). ==Some applications==