- Lithium niobate
Lithium niobate (LithiumNiobiumOxygen|3) is a compound of
niobium ,lithium , andoxygen . It is a colorless solid that is insoluble in water. Its melting point is 1257 °C and its density is 4.65 g/cm³. ItsCAS number is CASREF|CAS=12031-63-9.Lithium niobate crystals have a
trigonal crystal system and belongs to the 3m (C3v)crystallographic point group . Its crystal structure lacksinversion symmetry and displaysferroelectricity , thePockels effect , thepiezoelectric effect,photoelasticity , and nonlinear optical polarizability.Lithium niobate has negative uniaxial
birefringence which depends slightly on the stoichiometry of the crystal and on temperature. It is transparent for wavelengths between 350 and 5200nanometer s, and has abandgap of around 4 eV.It can be doped by
magnesium oxide , which increases its resistance to optical damage (also known as photorefractive damage) when doped above theoptical damage threshold . Other available dopants are iron, zinc, hafnium, copper, gadolinium, erbium, yttrium, Manganese and boron, creating optical sources that can be modulated by traveling-wave waveguide modulators.Single
monocrystal s of lithium niobate can be grown using theCzochralski process . They are used inlaser frequency doubling,nonlinear optics , Pockels cells,optical parametric oscillator s,Q-switching devices for lasers, other acousto-optic devices,optical switch es for gigahertz frequencies, etc. It is an excellent material for manufacture ofoptical waveguide s.Lithium niobate is used extensively in the telecoms market, e.g. in
mobile telephone s andoptical modulator s. It is the material of choice for the manufacture ofsurface acoustic wave devices.For some uses it can be replaced by
lithium tantalate , lithiumtantalumoxygen|3.Periodically poled lithium niobate (PPLN)
Periodically poled lithium niobate (PPLN) is a domain-engineered lithium niobate crystal, used mainly for achieving
quasi-phase-matching innonlinear optics . Theferroelectric domains point alternatively to the "+c" and the "-c" direction, with a period of typically between 5 and 35 µm. The shorter periods of this range are used forsecond harmonic generation , while the longer ones for optical parametric oscillation.Periodic poling can be achieved by electrical poling with periodically structured electrode. Controlled heating of the crystal can be used to fine-tunephase matching in the medium due to a slight variation of the dispersion with temperature.Periodic poling uses the largest value of lithium niobate's nonlinear tensor, d33= 27 pm/V. Quasi-phase matching gives maximum efficiencies that are 2/π (64%) of the full d33, about 17 pm/V
Other materials used for
periodic poling are wideband gap inorganic crystals like KTP (resulting inperiodically poled KTP ,PPKTP ),lithium tantalate , and some organic materials.ellmeier equations
The Sellmeier equations for the extraordinary index are used to find the poling period and approximate temperature for quasi-phase matching. Jundtcite journal| author=Dieter H. Jundt| journal=Optics Letters | volume=22 |title=Temperature-dependent Sellmeier equation for the index of refraction in congruent lithium niobate| date=1997|pages=1553 ] gives
valid from 20-250 degrees Celsius for wavelengths from 0.4 micrometer to 5 micrometers.
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