Potassium titanyl phosphate

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]

Potassium titanyl phosphate

Names
Other names KTP
Identifiers
CAS Number	12690-20-9
3D model (JSmol)	Interactive image
ChemSpider	50645143
PubChem CID	102601599
CompTox Dashboard (EPA)	DTXSID50925734
InChI InChI=1S/K.H3O4P.O.Ti/c;1-5(2,3)4;;/h;(H3,1,2,3,4);;/q+1;;;+2/p-3 Key: SALUEWWYAZZYMC-UHFFFAOYSA-K
SMILES [O-]P(=O)([O-])[O-].O=[Ti+2].[K+]
Properties
Chemical formula	K[TiO]PO4
Molar mass	197.934 g·mol−1
Appearance	colorless 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

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 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]

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.

Potassium Titanyl Phosphate (KTP): A Versatile Nonlinear Optical Material

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

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:

• Molecular Weight: 197.936 g/mol
• Appearance: Colorless crystalline solid
• Melting Point: Approximately 1172 °C (incongruent melting), with partial decomposition around 1150 °C
• Curie Point: 936 °C
• Density: ~3.01–3.03 g/cm³
• Mohs Hardness: Approximately 5
• Hygroscopic Susceptibility: Non-hygroscopic, contributing to its durability in various environments.
• Transparency Range: Highly transparent from 350 nm to 2700 nm, with reduced transmission up to 4500 nm.
• Thermal Conductivity: Ranges from 2.0 to 3.3 x 10⁻² W/cm/°C depending on the axis.
• Specific Heat: 0.1643 – 0.1737 cal/g°C.

Synthesis Methods

The commercial manufacturing of KTP crystals primarily employs two methods:

• Flux Growth Method: This is a more traditional and generally cheaper method. However, crystals grown by this method are more susceptible to "grey tracking" (photochromic damage) at higher power densities due to metal contamination and differences in electronic structure, limiting their use to low- and medium-power laser systems.
• Hydrothermal Method: This method involves growing crystals at much lower temperatures (typically between 350°C and 600°C) in closed autoclaves often lined with inert precious metals like silver, platinum, or gold. This low-temperature, closed-system approach minimizes thermally induced defects, oxygen vacancies, and chemical impurities. Hydrothermally grown KTP exhibits very low absorption and a higher damage threshold, making it suitable for high-power applications and demonstrating superior resistance to grey tracking. Low-temperature hydrothermal growth offers even greater advantages in terms of low absorption and higher damage threshold compared to high-temperature hydrothermal or flux-grown KTP.
• Co-precipitation Method: This method is also used for synthesizing KTP nanocrystals, often employing capping agents like oxalic acid to control particle size and morphology.

Operational Aspects and Electro-Optical Properties

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.

Applications

KTP's versatile properties lead to its widespread use across various fields:

• Frequency Doubling (Second Harmonic Generation - SHG): KTP is the most commonly used material for frequency doubling of Nd:YAG lasers and other Nd-doped lasers. It efficiently converts invisible infrared light (e.g., 1064 nm from Nd:YAG) into visible green light (532 nm), which has numerous applications. It is used in green laser pointers, medical lasers for procedures like prostate surgery, dermatology (rosacea, poikiloderma, pigmentation removal, skin rejuvenation), and industrial applications such as cutting and welding.
• Optical Parametric Oscillators (OPOs): KTP serves as an efficient OPO crystal, tunable from visible (0.6 μm) to mid-IR (4.5 μm) wavelengths. It is particularly suited for high-power OPO operations due to its high damage threshold and large crystal aperture, enabling the generation of tunable outputs.
• Electro-Optic Modulation and Switching: Due to its high electro-optic coefficient and low dielectric constant, KTP is employed in electro-optic modulators, optical switches, and directional couplers, which are crucial for telecommunications and optical communication systems for efficient data transmission.
• Quantum Optics: Periodically poled KTP (PPKTP) is a leading material for quantum applications, including the generation of single and entangled photon pairs through spontaneous parametric down-conversion (SPDC). It is widely used in quantum key distribution and quantum computation research.
• Waveguide Applications: KTP's large figure of merit makes it a valuable material for optical waveguides, enabling integrated nonlinear optical and electro-optic devices.
• Other Frequency Conversion Processes: KTP is also effective for sum-frequency generation (SFG) and difference-frequency generation (DFG) of lasers with wavelengths ranging from approximately 1 μm to 3.4 μm. It can be used for frequency mixing of Nd:YAG lasers and diode lasers to produce blue output.

Periodically Poled Potassium Titanyl Phosphate (PPKTP)

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:

• Enhanced Nonlinearity: PPKTP typically has a larger effective nonlinear coefficient than bulk KTP.
• Broadened Phase-Matching Range: QPM allows for efficient frequency conversion across a wider range of wavelengths (e.g., 400–3500 nm) without the constraints of birefringent phase matching.
• High Damage Threshold: Engineered poling can enhance resistance to photorefraction and offer high damage thresholds, especially in high-power applications.
• Versatility: PPKTP is highly versatile for various nonlinear optical processes, including spontaneous parametric down-conversion (SPDC), optical parametric oscillation (OPO), difference frequency generation (DFG), and sum-frequency generation (SFG).

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

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:

• Potassium Titanyl Phosphate (KTP) | AMERICAN ELEMENTS ®
• KTP Crystal Properties and What They're Used For - Gamdan Optics
• or KTP - Laser Components
• Potassium titanyl phosphate - Wikipedia
• Understanding the Role of Potassium Titanyl Phosphate Crystals in Electronics
• Exploring the Benefits of Potassium Titanyl Phosphate Crystals: Unleashing the Power of KTP_CASTECH INC.
• Synthesis and Characterization of Potassium Titanyl Phosphate (KTiOPO4) Nanocrystals: The Impact of Hydrothermal and Co-Precipitation Methods with Oxalic Acid Capping¹ Agent - Nanochemistry Research²
• Potassium Titanyl Phosphate (KTP) - Roditi International
• KTP - Crystech
• HGTR KTP Crystals Nonlinear Crystal
• Crystal structure of the KTP in the projection onto the ac plane.... | Download Scientific Diagram - ResearchGate
• KTP Properties - United Crystals
• KTP(Electro-optic Crystal) - CryLink
• KTP - Potassium Titanyl Phosphate - CASTECH Inc.
• Periodically Poled KTP (PPKTP) - Stanford Advanced Materials
• PPKTP - Periodically poled potassium titanyl - 3photon.com
• PPKTP Crystal for quantum applications - Raicol Crystals

See also

Other materials used for laser frequency doubling are

• Lithium triborate (LBO), used for high output power green or blue DPSS lasers
• Beta barium borate (BBO), used for high output power DPSS blue lasers