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Lithium niobate
Identifiers
CAS number	12031-63-9
PubChem	23665650
Properties
Molecular formula	LiNbO3
Molar mass	147.846 g/mol
Appearance	colorless solid
Density	4.65 g/cm3
Melting point	1257 °C
Solubility in water	None
Band gap	4 eV
Refractive index (nD)	2.007
Structure
Crystal structure	trigonal
Space group	3m (C3v)
Hazards
EU Index	Not listed
	(what is this?) (verify); Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
	Infobox references

Lithium niobate (Li Nb O₃) is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.

[edit] Properties

Lithium niobate is a colorless solid insoluble in water. It has trigonal crystal system, which lacks inversion symmetry and displays ferroelectricity, Pockels effect, piezoelectric 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 5200 nanometers.

Lithium niobate 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 Fe, Zn, Hf, Cu, Gd, Er, Y, Mn and B, creating optical sources that can be modulated by traveling-wave waveguide modulators.

[edit] Growth

Single crystals of lithium niobate can be grown using the Czochralski process.^[2]

Nanoparticles of lithium niobate and niobium pentoxide can be produced in a low-temperature. The complete protocol implies a LiH induced reduction of NbCl₅ followed by in situ spontaneous oxidation into low-valence niobium nano-oxides. These niobium oxides are exposed to air atmosphere resulting in pure Nb₂O₅. Finally, the stable Nb₂O₅ is converted into lithium niobate LiNbO₃ nanoparticles during the controlled hydrolysis of the LiH excess.^[3]

[edit] Applications

Lithium niobate is used extensively in the telecoms market, e.g. in mobile telephones and optical modulators. It is the material of choice for the manufacture of surface acoustic wave devices. For some uses it can be replaced by lithium tantalate, Li Ta O₃. Other uses are in laser frequency doubling, nonlinear optics, Pockels cells, optical parametric oscillators, Q-switching devices for lasers, other acousto-optic devices, optical switches for gigahertz frequencies, etc. It is an excellent material for manufacture of optical waveguides.

[edit] Periodically poled lithium niobate (PPLN)

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 µm. The shorter periods of this range are used for second 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-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, 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 wide band gap inorganic crystals like KTP (resulting in periodically poled KTP, PPKTP), lithium tantalate, and some organic materials.

The periodic poling technique can also be used to form surface nanostructures.^[4]^[5]

However, due to its low photorefractive damage threshold, PPLN only find limited applications in the very low power level. MgO doped lithium niobate is fabricated by periodically poled method. Periodically poled MgO doped lithium niobat (PPMgOLN) therefore expands the application to medium power level.

[edit] Sellmeier equations

The Sellmeier equations for the extraordinary index are used to find the poling period and approximate temperature for quasi-phase matching. Jundt^[6] gives

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

valid from 20-250 °C for wavelengths from 0.4 to 5 micrometers, whereas for longer wavelength,^[7]

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

which is valid for T = 25 to 180 °C, for wavelengths λ between 2.8 and 4.8 micrometers.
In these equations f = (T-24.5)(T+570.82), λ is in micrometers, and T is in °C.

[edit] See also

[edit] References

^ ^a ^b Spec sheet of Crystal Technology, Inc.
^ Volk, Tatyana; Wohlecke, Manfred (2008). Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching. Springer. pp. 1-9. doi:10.1007/978-3-540-70766-0. ISBN 9783540707653.
^ Aufray M, Menuel S, Fort Y, Eschbach J, Rouxel D, Vincent B (2009). "New Synthesis of Nanosized Niobium Oxides and Lithium Niobate Particles and Their Characterization by XPS Analysis". Journal of nanoscience and nanotechnology 9 (8): 4780-4789. doi:10.1166/jnn.2009.1087.
^ S. Grilli; P. Ferraro, P. De Natale, B. Tiribilli, and M. Vassalli (2005). "Surface nanoscale periodic structures in congruent lithium niobate by domain reversal patterning and differential etching". Applied Physics Letters 87: 233106. doi:10.1063/1.2137877.
^ P. Ferraro; S. Grilli (2006). "Modulating the thickness of the resist pattern for controlling size and depth of submicron reversed domains in lithium niobate". Applied Physics Letters 89: 133111. doi:10.1063/1.2357928.
^ Dieter H. Jundt (1997). "Temperature-dependent Sellmeier equation for the index of refraction $n e$ in congruent lithium niobate". Optics Letters 22: 1553. doi:10.1364/OL.22.001553.
^ LH Deng et al. (2006). "Improvement to Sellmeier equation for periodically poled LiNbO $3$ crystal using mid-infrared difference-frequency generation". Optics Communications 268: 110. doi:10.1016/j.optcom.2006.06.082.

[edit] Further reading

Ferraro, Pietro; Grilli, Simonetta; De Natale, Paolo, eds. Ferroelectric Crystals for Photonic Applications Including Nanoscale Fabrication and Characterization Techniques. Springer Series in Materials Science. 91. doi:10.1007/978-3-540-77965-0. http://www.springer.com/materials/book/978-3-540-77963-6.

[edit] External links

Inrad data sheet on lithium niobate

[CT-0] Spec sheet of Crystal Technology, Inc.

[1] Volk, Tatyana; Wohlecke, Manfred (2008). Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching. Springer. pp. 1-9. doi:10.1007/978-3-540-70766-0. ISBN 9783540707653.

[2] Aufray M, Menuel S, Fort Y, Eschbach J, Rouxel D, Vincent B (2009). "New Synthesis of Nanosized Niobium Oxides and Lithium Niobate Particles and Their Characterization by XPS Analysis". Journal of nanoscience and nanotechnology 9 (8): 4780-4789. doi:10.1166/jnn.2009.1087.

[3] S. Grilli; P. Ferraro, P. De Natale, B. Tiribilli, and M. Vassalli (2005). "Surface nanoscale periodic structures in congruent lithium niobate by domain reversal patterning and differential etching". Applied Physics Letters 87: 233106. doi:10.1063/1.2137877.

[4] P. Ferraro; S. Grilli (2006). "Modulating the thickness of the resist pattern for controlling size and depth of submicron reversed domains in lithium niobate". Applied Physics Letters 89: 133111. doi:10.1063/1.2357928.

[Jundt-5] Dieter H. Jundt (1997). "Temperature-dependent Sellmeier equation for the index of refraction $n e$ in congruent lithium niobate". Optics Letters 22: 1553. doi:10.1364/OL.22.001553.

[Deng-6] LH Deng et al. (2006). "Improvement to Sellmeier equation for periodically poled LiNbO $3$ crystal using mid-infrared difference-frequency generation". Optics Communications 268: 110. doi:10.1016/j.optcom.2006.06.082.

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