Potassium titanyl phosphate
) 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
. The material has a relatively high optical damage threshold
(~15 J/cm²), a great optical nonlinearity and excellent thermal stability. However, it is prone to photochromic
damage (called grey tracking
) during high-power 1064nm second-harmonic generation which tends to limit its use to low- and mid-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 walkoff 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 threfold 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 laser or Nd:YVO4 crystals are commonly found in green laser pointers.
Other such materials include KTiOAsO4.
Periodically poled potassium titanyl phosphate (PPKTP)
Periodically poled potassium titanyl phosphate
) consists of KTP with switched domain regions within the crystal for various nonlinear optic
applications and frequency conversion. It can be wavelength tailored for efficient second harmonic generation
, sum frequency generation
, and difference frequency generation. The interactions in PPKTP are based upon quasi-phase-matching
, achieved by periodic poling
of the crystal, whereby a structure of regularly spaced ferroelectric domains
with alternating orientations are created in the material.
PPKTP is commonly used for Type 1 & 2 frequency conversions for pump wavelengths of 730-3500 nm.
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.
Other materials used for laser frequency doubling are