Lithium niobate

Lithium niobate

Identifiers
CAS Number	12031-63-9
ChemSpider	10605804
Jmol interactive 3D	Image
PubChem	159404
InChI InChI=1S/Li.Nb.3O/q+1;;;;-1 Key: GQYHUHYESMUTHG-UHFFFAOYSA-N InChI=1/Li.Nb.3O/q+1;;;;-1/rLi.NbO3/c;2-1(3)4/q+1;-1 Key: GQYHUHYESMUTHG-YHKBGIKBAK
SMILES [Li+].[O-][Nb](=O)=O
Properties
Chemical formula	LiNbO3
Molar mass	147.846 g/mol
Appearance	colorless solid
Density	4.65 g/cm^3 [1]
Melting point	1,257 °C (2,295 °F; 1,530 K)[1]
Solubility in water	None
Band gap	4 eV
Refractive index (nD)	no 2.30, ne 2.21[2]
Structure
Crystal structure	trigonal
Space group	R3c
Point group	3m (C3v)
Hazards
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	8000 mg/kg (oral, rat)[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Lithium niobate (LiNbO₃) is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linear and non-linear optical applications.

Properties

Lithium niobate is a colorless solid insoluble in water. It has a 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.

Growth

Single crystals of lithium niobate can be grown using the Czochralski process.^[4]

A Z-cut, single crystal Lithium Niobate wafer

After a crystal is grown, it is sliced into wafers of different orientation. Common orientations are Z-cut, X-cut, Y-cut, and cuts with rotated angles of the previous axes.^[5]

Nanoparticles

Nanoparticles of lithium niobate and niobium pentoxide can be produced at low temperature.^[6] 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.^[7] Spherical nanoparticles of lithium niobate with a diameter of approximately 10 nm can be prepared by impregnating a mesoporous silica matrix with a mixture of an aqueous solution of LiNO₃ and NH₄NbO(C₂O₄)₂ followed by 10 min heating in an IR furnace.^[8]

Applications

Lithium niobate is used extensively in the telecoms market, e.g. in mobile telephones and optical modulators.^[9] It is the material of choice for the manufacture of surface acoustic wave devices. For some uses it can be replaced by lithium tantalate, LiTaO₃. 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.

It's also used in the making of optical spatial low-pass (anti-aliasing) filters.

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.^[10][11]

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

Sellmeier equations

The Sellmeier equations for the extraordinary index are used to find the poling period and approximate temperature for quasi-phase matching. Jundt^[12] 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 to 250 °C for wavelengths from 0.4 to 5 micrometers, whereas for longer wavelength,^[13]

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

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.

More generally for ordinary and extraordinary index for MgO doped LiNbO3:

$n^2 = a_1 + b_1 f + {a_2 + b_2 f \over \lambda^2 - (a_3 + b_3 f)^2 } + {a_4 + b_4 f \over \lambda^2 - a_5^2 } - a_6 \lambda^2$,

with:

Parameters	5% MgO doped CLN		1% MgO doped SLN
	ne	no	ne
a1	5.756	5.653	5.078
a2	0.0983	0.1185	0.0964
a3	0.2020	0.2091	0.2065
a4	189.32	89.61	61.16
a5	12.52	10.85	10.55
a6	1.32×10^−2	1.97×10^−2	1.59×10^−2
b1	2.860×10^−6	7.941×10^−7	4.677×10^−7
b2	4.700×10^−8	3.134×10^−8	7.822×10^−8
b3	6.113×10^−8	-4.641×10^−9	-2.653×10^−8
b4	1.516×10^−4	-2.188×10^−6	1.096×10^−4

for congruent LiNbO3 (CLN) and stochiometric LiNbO3 (SLN).^[14]

See also

• Crystal
• Crystal structure
• Crystallite
• Crystallization and engineering aspects
• Seed crystal
• Single crystal
• Laser-heated pedestal growth
• Micro-Pulling-Down

References

1. Spec sheet of Crystal Technology, Inc.
2. "Luxpop". Retrieved June 18, 2010. (Value at n_D=589.2 nm, 25 °C.)
3. http://chem.sis.nlm.nih.gov/chemidplus/rn/12031-63-9
4. 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 978-3-540-70765-3.
5. Wong, K. K. (2002). Properties of Lithium Niobate. London, United Kingdom: INSPEC. p. 8. ISBN 0 85296 799 3.
6. Grange, R.; Choi, J.W.; Hsieh, C.L.; Pu, Y.; Magrez, A.; Smajda, R.; Forro, L.; Psaltis, D. (2009). "Lithium niobate nanowires: synthesis, optical properties and manipulation". Applied Physics Letters 95: 143105. doi:10.1063/1.3236777.
7. 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.
8. Grigas, A; Kaskel, S (2011). "Synthesis of LiNbO₃ nanoparticles in a mesoporous matrix". Beilstein Journal of Nanotechnology 2: 28–33. doi:10.3762/bjnano.2.3.
9. Toney, James (2015). Lithium Niobate Photonics. Artech House. ISBN 978-1-60807-923-0.
10. S. Grilli; P. Ferraro; P. De Natale; B. Tiribilli; M. Vassalli (2005). "Surface nanoscale periodic structures in congruent lithium niobate by domain reversal patterning and differential etching". Applied Physics Letters 87 (23): 233106. doi:10.1063/1.2137877.
11. 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 (13): 133111. doi:10.1063/1.2357928.
12. Dieter H. Jundt (1997). "Temperature-dependent Sellmeier equation for the index of refraction $n_e$ in congruent lithium niobate". Optics Letters 22 (20): 1553–5. doi:10.1364/OL.22.001553. PMID 18188296.
13. LH Deng; et al. (2006). "Improvement to Sellmeier equation for periodically poled LiNbO$_3$ crystal using mid-infrared difference-frequency generation". Optics Communications 268 (1): 110. doi:10.1016/j.optcom.2006.06.082.
14. O.Gayer; et al. (2008). "Temperature and wavelength dependent refractive index equations for MgO-doped congruent and stoichiometric LiNbO3". Appl. Phys. B 91: 343–348. doi:10.1007/s00340-008-2998-2.

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.

External links

• Inrad data sheet on lithium niobate