What Potassium Titanyl Phosphate (KTP) Is Used For
WHAT ARE THE PROPERTIES OF A KTP CRYSTAL AND WHAT IS IT USED FOR?
Before we deep-dive into the applications of Potassium Titanyl Phosphate (KTP), let’s examine the answers to come basic questions such as, “What are KTP crystals?”, “What does KTP do?”, “How are high-quality KTP crystals grown”?, and more.
What are KTP crystals?
What are KTP crystals? KTP is an inorganic crystal most frequently used for second harmonic generation of Nd:YAG. Like many crystal designations, those are both mongrelizations of chemical formulae and symbols and abbreviations. Potassium titanyl phosphate is most correctly KTiOPO4. Reason enough to call it just KTP.
What does KTP do?
KTP crystals and certain other materials have the seemingly magical ability to change the wavelength of light. KTP crystals are most often used to convert 1 micron wavelength invisible infrared light into visible green light. The 1 micron light is typically generated from a neodymium doped crystal of YAG, Yttrium orthovanadate, or YLF. The KTP may be used outside the laser cavity or intracavity where the laser intensity is higher. This frequency doubling effect is also known as second harmonic generation.
What is special about GAMDAN Optics KTP?
GAMDAN Optics uses the highest quality hydrothermally grown KTP. It is characterized by low absorption, and resistance to gray tracking (photochromic damage) in high power applications.
There is a more inexpensive form of KTP which is not suited for high energy lasers. These are flux-grown KTP crystals. If you have a green laser pointer in your pocket, it most likely has a chip of flux grown KTP. However, if you’re looking for high power applications - regardless of the domain, be it medical, laser, defense, research and development, or any other - consider GAMDAN Optics hydrothermally grown KTP crystals. And rest assured that when you opt for GAMDAN Optics KTP crystals, you’re choosing optical crystals of exceptional quality, manufactured in the USA. Plus they boast a wide range of other desirable traits including low absorption, high damage threshold, and durability.
How is high quality KTP grown?
Now that we’ve understood what KTP crystals are and what they do, let’s explore the method for growing high-quality KTP crystals. In the hydrothermal method, a solution of titanium dioxide, KH2PO4, and K2HPO4 (read those last two carefully!) reacts at close to 1300 degree Kelvin in a special high strength steel cylinder containing an autoclave that is lined with unreactive gold. The elevated temperature creates pressure that would burst a lesser vessel, so the whole assembly is typically located underground in a silo to contain any possible explosive force.
KTP SYNTHESIS AND STRUCTURE
A cluster of typically one to three thin plates sliced from previously grown KTP, known as seeds, are suspended in the solution on a system of gold wires and a gold "ladder". The gold lined containment vessel, an autoclave which can withstand the high temperature and pressure without contaminating the growth solution, is welded shut and inserted into the steel cylinder.
Then the temperature is raised to 1300 degrees K to dissolve all the ingredients as a saturated solution. Following that, the whole device is slowly cooled to near room temperature. Supersaturation occurs, and crystals begin to grow as the temperature is slowly reduced. The containment chamber is extracted and cut open. If all has gone well, and with a little bit of luck, oriented KTP crystals have grown outward from both sides of the seeds.
Alternatively, if conditions are not perfect, random nucleation can spontaneously occur. Then no usable large single crystals of KTP will be found. The seeds may have dissolved as well.
KTP Crystal Features
We’ve covered the gist of what KTP crystals are. But what are their features? It’d be worthwhile to know that regardless of how they are grown, KTP crystals are orthorhombic. They have a complicated structure, especially compared to cubic crystals. That structure lends the special properties to KTP. Very pure crystals are optically clear, without measurable absorption or scattering, and transparent from 350 nanometers to 3.5 micrometers. For the physicists reading this, KTP crystals belong to the acentric point group mm2.
Ultra high quality KTP has a combination of desirable properties: high damage threshold, high nonlinear coefficients, large acceptance angle, and high conversion efficiency up to about 60%. Hardness is about 5 on the Mohs scale, and density is 3.026 grams per cubic centimeter. It is not hygroscopic, so it is durable and can be used in lasers outside the laboratory environment.
What are KTP Crystals used for?
Out-of-the-laboratory applications of KTP crystals account for their use in military applications, medical procedures, industrial use including semiconductor manufacturing, and lidar, in addition to scientific endeavors. Given that KTP is a crucial nonlinear optical material, it also has applications in directional couplers, electro-optic modulation, optical waveguide material, and frequency-doubling diode-pumped solid-state lasers. The latter include Nd:YAG as well as other neodymium-doped lasers. (Contact us to request specific data on our KTP to see if it is right for your application.)
The green laser radiation that KTP produces has many applications. Green laser light has good penetration through ocean water, so it is used for communication and bathymetric measurement of the sea floor and riverbed features. It is used in surgical lasers, where selective absorption allows medical ablation while avoiding damage to adjacent tissue. More specifically, the green light produced by KTP is used for performing some types of prostate surgeries.
KTP crystals also enjoy widespread use in cosmetic treatments. While long pulse KTP lasers are used for treating rosacea, poikiloderma, and other skin ailments, short pulse KTP lasers have been found to be effective in pigmentation removal and skin rejuvenation procedures.
Many metals reflect the 1.06 micron radiation of high power Nd:YAG lasers, yet absorb the frequency doubled green light, so cutting and welding can be done efficiently.
KTP may also be used for applications other than frequency doubling. It is often used as an optical parametric oscillator for near IR generation. In a sense this can be considered the "opposite" of harmonic frequency generation, where light is shifted to a shorter wavelength than the input. The physics of this operation, however, are very different from changing infrared light to visible green.
Choose Gamdan KTP
GAMDAN Super Polishing and high Laser Damage Initiation Threshold coatings maximize the special nonlinear properties of KTP. Orientation of the crystals for specific applications are determined by X-ray crystallography. Buy KTP crystals to power your applications!
DENNIS J. GARRITY, AUTHOR
Dennis is an engineer with over 45 years of experience in fabrication, testing, and material evaluation for high precision optics, with extensive hands-on experience. More on the author can be found here.