View source for Lithium niobate



==Overview==
[[Image:Linbo3 Unit Cell.png|thumb|300px|Hexagonal crystal structure of LiNbO<sub>3</sub>]]

'''Lithium niobate''' ({{Lithium}}{{Niobium}}{{Oxygen|3}}) is a compound of [[niobium]], [[lithium]], and [[oxygen]]. It is a colorless solid that is insoluble in water. Its melting point is 1257 °C and its density is 4.65 g/cm³. Its [[CAS number]] is {{CASREF|CAS=12031-63-9}}.

Lithium niobate crystals have a [[trigonal]] [[crystal system]] and belongs to the 3m (C<sub>3v</sub>) [[crystallographic point group]].  Its crystal structure lacks [[inversion symmetry]] and displays [[ferroelectricity]], the [[Pockels effect]], the [[piezoelectric]] effect, [[photoelasticity]], and [[nonlinear optics|nonlinear optical]] polarizability.  

Lithium niobate is has negative uniaxial [[birefringence]] which depends slightly on the stoichiometry of the crystal and on temperature.  It is transparent for wavelengths between 350 and 5200 [[nanometer]]s, and has a [[bandgap]] of around 4 [[electronvolt|eV]].  

It can be doped by [[magnesium oxide]], which increases its resistance to optical damage (also known as photorefractive damage) when doped above the [[optical 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 tube|traveling-wave waveguide modulators]].

Single [[monocrystal]]s of lithium niobate can be grown using the [[Czochralski process]].  They are used in [[laser]] [[second harmonic generation|frequency doubling]], [[nonlinear optics]], [[Pockels effect#Pockels cells|Pockels cell]]s, [[optical parametric oscillator]]s, [[Q-switching]] devices for lasers, other [[acousto-optic effect|acousto-optic]] devices, [[optical switch]]es for gigahertz frequencies, etc. It is an excellent material for manufacture of [[optical waveguide]]s.

Lithium niobate is used extensively in the telecoms market, eg. in the [[mobile telephone]]s and [[optical modulator]]s.  It is the material of choice for the manufacture of [[surface acoustic wave]] devices.

For some uses it can be replaced by [[lithium tantalate]], {{lithium}}{{tantalum}}{{oxygen|3}}.

==Periodically poled lithium niobate (PPLN)==
{{Expand-section|date=June 2008}}
'''Periodically poled lithium niobate''' ('''PPLN''') is a domain-engineered lithium niobate crystal, used mainly for achieving [[quasi-phase-matching]] in [[nonlinear optics]]. The [[ferroelectric]] domains point alternatively to the ''+c'' and the ''-c'' direction, with a period of typically between 5 and 35 [[micrometre|µm]]. The shorter periods of this range are used for [[second harmonic generation]], while the longer ones for [[Optical parametric oscillator|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-tune [[phase 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, d<sub>33</sub>= 27 pm/V.  Quasi-phase matching gives maximum efficiencies that are 2/π (64%) of the full d<sub>33</sub>, about 17 pm/V

Other materials used for [[periodic poling]] are wide [[band gap]] inorganic crystals like [[potassium titanyl phosphate|KTP]] (resulting in [[periodically poled KTP]], [[PPKTP]]), [[lithium tantalate]], and some organic materials.

==Sellmeier equations==
The Sellmeier equations for the extraordinary index are used to find the poling period and approximate temperature for quasi-phase matching.  Jundt<ref name=Jundt>{{cite journal| author=Dieter H. Jundt| journal=Optics Letters | volume=22 |title=Temperature-dependent Sellmeier equation for the index of refraction <math>n_e</math> in congruent lithium niobate| date=1997|pages=1553}} </ref> gives 

<math>n^2_e =
    5.35583 + 4.629 \times 10^{-7}  f
+ {0.100473 + 3.862 \times 10^{-8} f \over \lambda^2 - (0.20692 - 0.89 \times 10^{-8} f)^2 }
+ {     100 + 2.657 \times 10^{-5} f \over \lambda^2 - (11.34927                       )^2 }
- 1.5334 \times 10^{-2} \lambda^2 </math>

valid from 20-250 degrees Celsius for wavelengths from 0.4 [[micrometre|micrometer]] to 5 micrometers.

Deng<ref name=Deng>{{cite journal | author=LH Deng et al. | journal = Optics Communications | volume=268| title=Improvement to Sellmeier equation for periodically poled LiNbO<math>_3</math> crystal using mid-infrared difference-frequency generation|date=2006| pages=110}} </ref>, with measurements at longer wavelengths, gives the following 

<math>n^2_e =
    5.39121 + 4.968 \times 10^{-7}  f
+ {0.100473 + 3.862 \times 10^{-8} f \over \lambda^2 - (0.20692 - 0.89 \times 10^{-8} f)^2 }
+ {     100 + 2.657 \times 10^{-5} f \over \lambda^2 - (11.34927                       )^2 }- (1.544 \times 10^{-2} + 9.62119 \times 10^{-10} \lambda) \lambda^2 </math>

valid for ''T'' = 25 to 180 degrees Celsius, for wavelengths between 2.8 and 4.8 micrometers.
In these equations <math>\lambda</math> is in micrometers and <math>f = (T-24.5)\times(T+570.82)</math> where <math>T</math> is in degrees Celsius.

==References==
{{reflist|2}}

==External links==
*[http://www.inrad.com/pdf/Inrad_datasheet_LNB.pdf Inrad data sheet on lithium niobate]

[[Category:Lithium compounds]]
[[Category:Niobates]]
[[Category:Nonlinear optical materials]]
[[Category:Crystals]]

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