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Chemical compound
Potassium titanyl phosphate (KTP) is an inorganic compound with the formula K+[TiO]2+PO3−4. 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[edit]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[edit]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.[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 = K, light blue = Ti, pink = P).[2]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)[edit]This section
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(June 2008)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.
Potassium Titanyl Phosphate (KTP): A Versatile Nonlinear Optical Material[edit]Potassium titanyl phosphate (KTiOPO₄ or KTP) is an inorganic crystal widely recognized for its excellent nonlinear optical (NLO) properties, making it a cornerstone in various photonics and laser applications. Its unique attributes, including a high nonlinear optical coefficient, broad transparency range, and robust physical characteristics, have established KTP as a material of choice for frequency conversion and other advanced optical processes.
Chemical and Structural Properties[edit]KTP is a crystalline solid with the chemical formula KTiOPO₄. It possesses an orthorhombic crystal structure, belonging to the point group mm2 and space group Pna21. The crystal structure is characterized by a rigid three-dimensional framework of titanium-oxygen octahedra (TiO₆) and phosphorus-oxygen tetrahedra (PO₄). These octahedra and tetrahedra are interconnected, forming infinite chains along specific crystallographic directions, with potassium cations occupying channels within this framework. This unique arrangement contributes to KTP's ferroelectric and electro-optic properties.
Key physical properties of KTP include:
The commercial manufacturing of KTP crystals primarily employs two methods:
KTP is renowned for its excellent nonlinear optical properties, particularly its large second-harmonic generation (SHG) coefficient, which is about three times higher than that of potassium dihydrogen phosphate (KDP). Its broad temperature and spectral bandwidths, wide angular acceptance, and small walk-off angle further enhance its performance in frequency conversion applications.
The electro-optic properties of KTP are significant, characterized by a high electro-optic coefficient and low dielectric constant. These features allow KTP to operate efficiently at high frequencies, making it suitable for electro-optic modulators and optical switches. The dielectric constant of KTP is approximately 13.
For optimal performance in high-power or high-power-density SHG of Nd:lasers, heating KTP crystals to a certain temperature (e.g., 80 °C) can enhance their damage threshold.
KTP's versatile properties lead to its widespread use across various fields:
Periodically poled KTP (PPKTP) is an engineered variant of KTP designed to overcome the phase-matching limitations of bulk KTP for certain applications. PPKTP consists of KTP with switched ferroelectric domains created by periodic poling of the crystal. This technique enables quasi-phase-matching (QPM), where the nonlinear interaction is periodically reversed to compensate for phase velocity mismatch between the interacting waves.
Advantages of PPKTP include:
PPKTP is particularly significant in quantum optics for its ability to generate heralded single photons and entangled photon pairs, supporting cutting-edge quantum research and technologies.
Challenges and Advancements[edit]Historically, KTP has been susceptible to "grey tracking" or photochromic damage when exposed to high-power 1064 nm second-harmonic generation, which could limit its use. However, advancements in crystal growth techniques, such as the low-temperature hydrothermal method, have led to the development of High Gray Track Resistance (HGTR) KTP crystals. These improved crystals exhibit significantly reduced absorption and higher damage thresholds, allowing for consistent and reliable conversion efficiency even at higher operating temperatures and power densities.
The continuous optimization of KTP crystal growth and processing, including specialized coating processes for enhanced damage thresholds, ensures its ongoing relevance and expanded application in modern optical and quantum technologies.
References:
Other materials used for laser frequency doubling are
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