Lithium niobate

Lithium niobate (LiNbO₃) is a non-naturally-occurring salt consisting 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.[4] Lithium niobate is sometimes referred to by the brand name linobate.[5]

Lithium niobate

__ Li^+     __ Nb^5+     __ O^2−
Names
Other names Lithium niobium oxide, lithium niobium trioxide
Identifiers
CAS Number	12031-63-9
3D model (JSmol)	Interactive image
ChemSpider	10605804
ECHA InfoCard	100.031.583
PubChem CID	159404
CompTox Dashboard (EPA)	DTXSID00894183
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

Properties

Lithium niobate is a colorless solid, and it is insoluble in water. It has a trigonal crystal system, which lacks inversion symmetry and displays ferroelectricity, the Pockels effect, the 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.[6]

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.[7]

Nanoparticles

Nanoparticles of lithium niobate and niobium pentoxide can be produced at low temperature.[8] 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.[9] 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 infrared furnace.[10]

Applications

Lithium niobate is used extensively in the telecommunications market, e.g. in mobile telephones and optical modulators.[11] 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.

In the past few years lithium niobate is finding applications as a kind of electrostatic tweezers, an approach known as optoelectronic tweezers as the effect requires light excitation to take place.[12][13] This effect allows for fine manipulation of micrometer-scale particles with high flexibility since the tweezing action is constrained to the illuminated area. The effect is based on the very high electric fields generated during light exposure (1–100 kV/cm) within the illuminated spot. These intense fields are also finding applications in biophysics and biotechnology, as they can influence living organisms in a variety of ways.[14] For example, iron-doped lithium niobate excited with visible light has been shown to produce cell death in tumoral cell cultures.[15]

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.[16]

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.[17][18]

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[19] gives

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

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

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

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:

${\displaystyle {n^{2}\approx a_{1}+b_{1}f+{a_{2}+b_{2}f \over \lambda ^{2}-(a_{3}+b_{3}f)^{2}}+{a_{4}+b_{4}f \over \lambda ^{2}-a_{5}^{2}}-a_{6}\lambda ^{2}}}$,

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).[21]

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. "ChemIDplus - 12031-63-9 - PSVBHJWAIYBPRO-UHFFFAOYSA-N - Lithium niobate - Similar structures search, synonyms, formulas, resource links, and other chemical information".
4. Weis, R. S.; Gaylord, T. K. (1985). "Lithium Niobate: Summary of Physical Properties and Crystal Structure". Applied Physics A: Materials Science & Processing. 37 (4): 191–203. doi:10.1007/BF00614817.
5. Staebler, D.L.; Amodei, J.J. (1972). "Thermally fixed holograms in LiNbO₃". Ferroelectrics. 3: 107–113. doi:10.1080/00150197208235297., seen in Yeh, Pochi; Gu, Claire, eds. (1995). Landmark Papers On Photorefractive Nonlinear Optics. World Scientific. p. 182. ISBN 9789814502979.
6. 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.
7. Wong, K. K. (2002). Properties of Lithium Niobate. London, United Kingdom: INSPEC. p. 8. ISBN 0 85296 799 3.
8. 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 (14): 143105. Bibcode:2009ApPhL..95n3105G. doi:10.1063/1.3236777. Archived from the original on 2016-05-14.
9. 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. CiteSeerX 10.1.1.465.1919. doi:10.1166/jnn.2009.1087.
10. 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. PMC 3045940. PMID 21977412.
11. Toney, James (2015). Lithium Niobate Photonics. Artech House. ISBN 978-1-60807-923-0.
12. Carrascosa M, García-Cabañes A, Jubera M, Ramiro JB, and Agulló-López F. LiNbO3: A photovoltaic substrate for massive parallel manipulation and patterning of nano-objects. Applied Physics Reviews 2: 040605 0(2015).doi:10.1063/1.4929374
13. García-Cabañes A, Blázquez-Castro A, Arizmendi L, Agulló-López F, and Carrascosa M. Recent Achievements on Photovoltaic Optoelectronic Tweezers Based on Lithium Niobate. Crystals 8: 65 (2018).doi:10.3390/cryst8020065
14. Blázquez-Castro A, García-Cabañes A, and Carrascosa M. Biological applications of ferroelectric materials. Applied Physics Reviews 5: 041101 (2018).doi:10.1063/1.5044472
15. Blázquez-Castro A, Stockert JC, López-Arias B, Juarranz A, Agulló-López F, García-Cabañes A, and Carrascosa M. Tumour cell death induced by the bulk photovoltaic effect of LiNbO3:Fe under visible light irradiation. Photochemcial & Photobiological Sciences 10: 956-963 (2011).doi:10.1039/c0pp00336k
16. Meyn, J.-P.; Laue, C.; Knappe, R.; Wallenstein, R.; Fejer, M.M. (2001). "Fabrication of periodically poled lithium tantalate for UV generation with diode lasers". Applied Physics B. 73 (2): 111–114. doi:10.1007/s003400100623.
17. 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. Bibcode:2005ApPhL..87w3106G. doi:10.1063/1.2137877.
18. 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. Bibcode:2006ApPhL..89m3111F. doi:10.1063/1.2357928.
19. Dieter H. Jundt (1997). "Temperature-dependent Sellmeier equation for the index of refraction ${\displaystyle n_{e}}$ in congruent lithium niobate". Optics Letters. 22 (20): 1553–5. Bibcode:1997OptL...22.1553J. doi:10.1364/OL.22.001553. PMID 18188296.
20. LH Deng; et al. (2006). "Improvement to Sellmeier equation for periodically poled LiNbO₃ crystal using mid-infrared difference-frequency generation". Optics Communications. 268 (1): 110–114. Bibcode:2006OptCo.268..110D. doi:10.1016/j.optcom.2006.06.082.
21. O.Gayer; et al. (2008). "Temperature and wavelength dependent refractive index equations for MgO-doped congruent and stoichiometric LiNbO3". Appl. Phys. B. 91 (2): 343–348. Bibcode:2008ApPhB..91..343G. doi:10.1007/s00340-008-2998-2.

Further reading

• Ferraro, Pietro; Grilli, Simonetta; De Natale, Paolo, eds. (2009). 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. ISBN 978-3-540-77963-6.

External links

• Inrad data sheet on lithium niobate