Potassium titanyl phosphate

Last updated
Potassium titanyl phosphate
Other names
Molar mass 197.934 g·mol−1
Appearancecolorless solid
Density 3.026 g/cm3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Potassium titanyl phosphate (KTP) is an inorganic compound with the formula KTiOPO4. It is a white solid. KTP is an important nonlinear optical material that is commonly used for frequency-doubling diode-pumped solid-state lasers such as Nd:YAG and other neodymium-doped lasers. [1]


Synthesis and structure

The compound is prepared by the reaction of titanium dioxide with a mixture of KH2PO4 and K2HPO4 near 1300 K. The potassium salts serve both as reagents and flux. [2]

The material has been characterized by X-ray crystallography. KTP has an orthorhombic crystal structure. It features octahedral Ti(IV) and tetrahedral phosphate sites. Potassium has a high coordination number. All heavy atoms (Ti, P, K) are linked exclusively by oxides, which interconnect these atoms. [2]

Operational aspects

Crystals of KTP are 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 three times higher than KDP. It has a Mohs hardness of about 5. [3]

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).

Structure of KTP viewed down b axis. Color code: red = O, purple = P, bright blu = K, dark blue = Ti). EntryWithCollCode173233downbaxis.png
Structure of KTP viewed down b axis. Color code: red = O, purple = P, bright blu = K, dark blue = Ti).

Some applications

It is used to produce "greenlight" to perform some laser prostate surgery. KTP crystals coupled with Nd:YAG or Nd:YVO4 crystals are commonly found in green laser pointers. [4]

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

Periodically poled potassium titanyl phosphate (PPKTP)

Periodically poled potassium titanyl phosphate (PPKTP) 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.

See also

Other materials used for laser frequency doubling are

Related Research Articles

Nonlinear optics branch of physics

Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typically observed only at very high light intensities (values of atomic electric fields, typically 108 V/m) such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds.

Laser diode semiconductor laser

A laser diode, (LD), injection laser diode (ILD), or diode laser is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction. Laser diodes can directly convert electrical energy into light. Driven by voltage, the doped p-n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generate light with the same phase, coherence and wavelength.

Nd:YAG laser

Nd:YAG (neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12) is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, Nd(III), typically replaces a small fraction (1%) of the yttrium ions in the host crystal structure of the yttrium aluminum garnet (YAG), since the two ions are of similar size. It is the neodymium ion which provides the lasing activity in the crystal, in the same fashion as red chromium ion in ruby lasers.

Ti-sapphire laser

Ti:sapphire lasers (also known as Ti:Al2O3 lasers, titanium-sapphire lasers, or Ti:sapphs) are tunable lasers which emit red and near-infrared light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulses. Lasers based on Ti:sapphire were first constructed and invented in June 1982 by Peter Moulton at the MIT Lincoln Laboratory.

Diode-pumped solid-state lasers (DPSSLs) are solid-state lasers made by pumping a solid gain medium, for example, a ruby or a neodymium-doped YAG crystal, with a laser diode.

An optical parametric amplifier, abbreviated OPA, is a laser light source that emits light of variable wavelengths by an optical parametric amplification process. It is essentially the same as an optical parametric oscillator, but without the optical cavity.

Laser pointer handheld device that emits a laser

A laser pointer or laser pen is a small handheld device with a power source and a laser diode emitting a very narrow coherent low-powered laser beam of visible light, intended to be used to highlight something of interest by illuminating it with a small bright spot of colored light. Power is restricted in most jurisdictions not to exceed 5 mW.

Monopotassium phosphate chemical compound

Monopotassium phosphate, MKP, (also potassium dihydrogenphosphate, KDP, or monobasic potassium phosphate) is the inorganic compound with the formula KH2PO4. Together with dipotassium phosphate (K2HPO4.(H2O)x) it is often used as a fertilizer, food additive, and buffering agent. The salt often cocrystallizes with the dipotassium salt as well as with phosphoric acid.

Optical parametric oscillator

An optical parametric oscillator (OPO) is a parametric oscillator that oscillates at optical frequencies. It converts an input laser wave with frequency into two output waves of lower frequency by means of second-order nonlinear optical interaction. The sum of the output waves' frequencies is equal to the input wave frequency: . For historical reasons, the two output waves are called "signal" and "idler", where the output wave with higher frequency is the "signal". A special case is the degenerate OPO, when the output frequency is one-half the pump frequency, , which can result in half-harmonic generation when signal and idler have the same polarization.

Lithium niobate chemical compound

Lithium niobate (LiNbO3) is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linear and non-linear optical applications. It is a human-made dielectric material that does not exist in nature. Lithium niobate is sometimes referred to by the brand name linobate.

Here, is a list of initialisms and acronyms used in laser physics, applications and technology.

More citations needed An optical frequency multiplier is a nonlinear optical device in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency. Two types of devices are currently common: frequency doublers, often based on lithium niobate (LN), lithium tantalate (LT), potassium titanyl phosphate (KTP) or lithium triborate (LBO), and frequency triplers typically made of potassium dihydrogen phosphate (KDP). Both are widely used in optical experiments that use lasers as a light source.

Periodic poling is a formation of layers with alternate orientation in a birefringent material. The domains are regularly spaced, with period in a multiple of the desired wavelength of operation. The structure is designed to achieve quasi-phase-matching (QPM) in the material.

Lithium triborate chemical compound

Lithium triborate (LiB3O5) or LBO is a non-linear optics crystal. It has a wide transparency range, moderately high nonlinear coupling, high damage threshold and desirable chemical and mechanical properties. This crystal is often used for second harmonic generation (SHG, also known as frequency doubling), for example of Nd:YAG lasers (1064 nm → 532 nm). LBO can be both critically and non-critically phase-matched. In the latter case the crystal has to be heated or cooled depending on the wavelength.

Ultrafast laser spectroscopy is a spectroscopic technique that uses ultrashort pulse lasers for the study of dynamics on extremely short time scales. Different methods are used to examine dynamics of charge carriers, atoms and molecules. Many different procedures have been developed spanning different time scales and photon energy ranges; some common methods are listed below.

Second-harmonic generation nonlinear optical process

Second-harmonic generation is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with twice the energy of the initial photons. It is a special case of sum-frequency generation.

Second-harmonic imaging microscopy

Second-harmonic imaging microscopy (SHIM) is based on a nonlinear optical effect known as second-harmonic generation (SHG). SHIM has been established as a viable microscope imaging contrast mechanism for visualization of cell and tissue structure and function. A second-harmonic microscope obtains contrasts from variations in a specimen’s ability to generate second-harmonic light from the incident light while a conventional optical microscope obtains its contrast by detecting variations in optical density, path length, or refractive index of the specimen. SHG requires intense laser light passing through a material with a noncentrosymmetric molecular structure. Second-harmonic light emerging from an SHG material is exactly half the wavelength (frequency doubled) of the light entering the material. While two-photon-excited fluorescence (TPEF) is also a two photon process, TPEF loses some energy during the relaxation of the excited state, while SHG is energy conserving. Typically, an inorganic crystal is used to produce SHG light such as lithium niobate (LiNbO3), potassium titanyl phosphate (KTP = KTiOPO4), and lithium triborate (LBO = LiB3O5). Though SHG requires a material to have specific molecular orientation in order for the incident light to be frequency doubled, some biological materials can be highly polarizable, and assemble into fairly ordered, large noncentrosymmetric structures. Biological materials such as collagen, microtubules, and muscle myosin can produce SHG signals. The SHG pattern is mainly determined by the phase matching condition. A common setup for an SHG imaging system will have a laser scanning microscope with a titanium sapphire mode-locked laser as the excitation source. The SHG signal is propagated in the forward direction. However, some experiments have shown that objects on the order of about a tenth of the wavelength of the SHG produced signal will produce nearly equal forward and backward signals.

Potassium dideuterium phosphate

Deuterated potassium dihydrogen phosphate (KD2PO4) or DKDP single crystals are widely used in non-linear optics as the second, third and fourth harmonic generators for Nd:YAG and Nd:YLF lasers. They are also found in electro-optical applications as Q-switches for Nd:YAG, Nd:YLF, Alexandrite and Ti-sapphire lasers, as well as for Pockels cells.

A parametric process is an optical process in which light interacts with matter in such a way as to leave the quantum state of the material unchanged. As a direct consequence of this there can be no net transfer of energy, momentum, or angular momentum between the optical field and the physical system. In contrast a non-parametric process is a process in which any part of the quantum state of the system changes.

Lightwave Electronics Corporation

Lightwave Electronics Corporation was a developer and manufacturer of diode-pumped solid-state lasers, and was a significant contributor to the creation and maturation of this technology. Lightwave Electronics was a technology-focused company, with diverse markets, including science and micromachining. Inventors employed by Lightwave Electronics received 51 US patents, and Lightwave Electronics products were referenced by non-affiliated inventors in 91 US patents.


  1. Bierlein, John D.; Vanherzeele, Herman (1989). "Potassium Titanyl Phosphate: Properties and New Applications". Journal of the Optical Society of America B. 6: 622–33. doi:10.1364/JOSAB.6.000622.CS1 maint: uses authors parameter (link)
  2. 1 2 3 Norberg, S.T.; Ishizawa, N. (2005). "K-Site Splitting in KTiOPO4 at Room Temperature". Acta Crystallographica Section C. 61: 99–102. doi:10.1107/S0108270105027010.CS1 maint: uses authors parameter (link)
  3. Scheel, Hans J.; Fukuda, Tsuguo (2004). Crystal Growth Technology. John Wiley and Sons. ISBN   978-0-471-49524-6.
  4. Nurmikko, Arto V.; Gosnell, Timothy R. (2003). Compact Blue-green Lasers. Cambridge University Press. ISBN   978-0-521-52103-1.