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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.<ref name="Carrascosa M 2015">{{cite journal | last1=Carrascosa | first1=M. | last2=García-Cabañes | first2=A. | last3=Jubera | first3=M. | last4=Ramiro | first4=J. B. | last5=Agulló-López | first5=F. | title=LiNbO<sub>3</sub>: A photovoltaic substrate for massive parallel manipulation and patterning of nano-objects | journal=Applied Physics Reviews | publisher=AIP Publishing | volume=2 | issue=4 | year=2015 | issn=1931-9401 | doi=10.1063/1.4929374 | page=040605| bibcode=2015ApPRv...2d0605C | hdl=10486/669584 | hdl-access=free }}</ref><ref name="García-Cabañes A 2018">{{cite journal | last1=García-Cabañes | first1=Angel | last2=Blázquez-Castro | first2=Alfonso | last3=Arizmendi | first3=Luis | last4=Agulló-López | first4=Fernando | last5=Carrascosa | first5=Mercedes | title=Recent Achievements on Photovoltaic Optoelectronic Tweezers Based on Lithium Niobate | journal=Crystals | publisher=MDPI AG | volume=8 | issue=2 | date=2018-01-30 | issn=2073-4352 | doi=10.3390/cryst8020065 | page=65| doi-access=free }}</ref> 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.<ref name="Blázquez-Castro A 2018">{{cite journal | last1=Blázquez-Castro | first1=A. | last2=García-Cabañes | first2=A. | last3=Carrascosa | first3=M. | title=Biological applications of ferroelectric materials | journal=Applied Physics Reviews | publisher=AIP Publishing | volume=5 | issue=4 | year=2018 | issn=1931-9401 | doi=10.1063/1.5044472 | page=041101| arxiv=2109.00429 | bibcode=2018ApPRv...5d1101B | s2cid=139511670 }}</ref> For example, iron-doped lithium niobate excited with visible light has been shown to produce cell death in tumoral cell cultures.<ref name="Blázquez-Castro A 2011">{{cite journal | last1=Blázquez-Castro | first1=Alfonso | last2=Stockert | first2=Juan C. | last3=López-Arias | first3=Begoña | last4=Juarranz | first4=Angeles | last5=Agulló-López | first5=Fernando | last6=García-Cabañes | first6=Angel | last7=Carrascosa | first7=Mercedes | title=Tumour cell death induced by the bulk photovoltaic effect of LiNbO<sub>3</sub>:Fe under visible light irradiation | journal=Photochemical & Photobiological Sciences | publisher=Springer Science and Business Media LLC | volume=10 | issue=6 | year=2011 | pages=956–963 | issn=1474-905X | doi=10.1039/c0pp00336k | pmid=21336376 | doi-access=free }}</ref>
 
==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 &nbsp;[[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 &nbsp;pm/V. Quasi-phase -matching gives maximum efficiencies that are 2/π (64%) of the full ''d''<sub>33</sub>, about 17 &nbsp;pm/V.<ref>{{cite journal |doi=10.1007/s003400100623 |title=Fabrication of periodically poled lithium tantalate for UV generation with diode lasers |journal=Applied Physics B |volume=73 |issue=2 |pages=111–114 |year=2001 |last1=Meyn |first1=J.-P. |last2=Laue |first2=C. |last3=Knappe |first3=R. |last4=Wallenstein |first4=R.|last5=Fejer |first5=M. M. |bibcode=2001ApPhB..73..111M |s2cid=119763435}}</ref>
 
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.
 
The periodic -poling technique can also be used to form surface [[nanostructure]]s.<ref>{{cite journal |title=Surface nanoscale periodic structures in congruent lithium niobate by domain reversal patterning and differential etching |journal=Applied Physics Letters |volume=87 |issue=23 |pages=233106 |year=2005 |doi=10.1063/1.2137877|bibcode=2005ApPhL..87w3106G |last1=Grilli |first1=Simonetta |last2=Ferraro |first2=Pietro |last3=De Natale |first3=Paolo |last4=Tiribilli |first4=Bruno |last5=Vassalli |first5=Massimo |doi-access=free }}</ref><ref>{{cite journal |title=Modulating the thickness of the resist pattern for controlling size and depth of submicron reversed domains in lithium niobate |journal=Applied Physics Letters |volume=89 |issue=13 |pages=133111 |year=2006 |doi=10.1063/1.2357928| bibcode =2006ApPhL..89m3111F |last1=Ferraro |first1=P. |last2=Grilli |first2=S. }}</ref>
 
However, due to its low photorefractive damage threshold, PPLN only finds limited applications:, namely, 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 equation]]s 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=Jundt, Dieter H. | journal=Optics Letters |volume=22 |title=Temperature-dependent Sellmeier equation for the index of refraction <math>n_e</math> in congruent lithium niobate| |year=1997 |pages=1553–51553–1555 |doi=10.1364/OL.22.001553| |pmid=18188296| |issue=20| |bibcode=1997OptL...22.1553J}}</ref> gives
 
: <math>{
n^2_e \approx 5.35583 + 4.629 \times 10^{-7} f
+ \frac{0.100473 + 3.862 \times 10^{-8} f \over }{\lambda^2 - (0.20692 - 0.89 \times 10^{-8} f)^2}
+ \frac{100 + 2.657 \times 10^{-5} f \over }{\lambda^2 - 11.34927^2}
- 1.5334 \times 10^{-2} \lambda^2,
}</math>
 
valid from 20 to 250&nbsp;°C for wavelengths from 0.4 to 5 &nbsp;[[micrometre|micrometer]]s, whereas for longer wavelengthwavelengths,<ref name=Deng>{{cite journal |journal = Optics Communications |volume=268| |title=Improvement to Sellmeier equation for periodically poled LiNbO<sub>3</sub> crystal using mid-infrared difference-frequency generation |issue=1 |year=2006| |pages=110–114 |doi=10.1016/j.optcom.2006.06.082 |bibcode = 2006OptCo.268..110D |last1=Deng |first1=L. H. |last2=Gao |first2=X. M. |last3=Cao |first3=Z. S. |last4=Chen |first4=W. D. |last5=Yuan |first5=Y.Q. |last6=Zhang |first6=W. J. |last7=Gong |first7=Z. B. }}</ref>
 
: <math>{
n^2_e \approx 5.39121 + 4.968 \times 10^{-7} f
+ \frac{0.100473 + 3.862 \times 10^{-8} f \over }{\lambda^2 - (0.20692 - 0.89 \times 10^{-8} f)^2}
+ \frac{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>
 
which is valid for ''T'' = 25 to 180&nbsp;°C, for wavelengths λ between 2.8 and 4.8 micrometers.
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More generally for ordinary and extraordinary index for MgO-doped {{chem2|LiNbO3}}:
 
: <math>{
n^2 \approx a_1 + b_1 f
+ \frac{a_2 + b_2 f \over }{\lambda^2 - (a_3 + b_3 f)^2}
+ \frac{a_4 + b_4 f \over }{\lambda^2 - a_5^2}
- a_6 \lambda^2,
}</math>,
 
with:
Line 165 ⟶ 166:
| ''b''<sub>4</sub> || 1.516×10<sup>−4</sup> || −2.188×10<sup>−6</sup> || 1.096×10<sup>−4</sup>
|}
for congruent {{chem2|LiNbO3}} (CLN) and stochiometric {{chem2|LiNbO3}} (SLN).<ref name=gayer>{{cite journal |journal = Appl. Phys. B |volume=91 |issue = 2 | title=Temperature and wavelength dependent refractive index equations for MgO-doped congruent and stoichiometric LiNbO<sub>3</sub> |year=2008| |pages=343–348 |doi=10.1007/s00340-008-2998-2 |bibcode = 2008ApPhB..91..343G |s2cid = 195290628 |last1=Gayer |first1=O. |last2=Sacks |first2=Z. |last3=Galun |first3=E. |last4=Arie |first4=A. }}</ref>
 
==See also==
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