Lithium niobate
Encyclopedia
Lithium niobate 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.
, 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 , , , , , , , and .
can be produced at 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. 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.
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
, . Other uses are in laser
frequency doubling
, nonlinear optics
, Pockels cells, optical parametric oscillator
s, Q-switching
devices for lasers, other acousto-optic devices, optical switch
es for gigahertz frequencies, etc. It is an excellent material for manufacture of optical waveguides.
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 nanostructure
s.
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.
s for the extraordinary index are used to find the poling period and approximate temperature for quasi-phase matching. Jundt gives
valid from 20-250 °C for wavelengths from 0.4 to 5 micrometer
s, whereas for longer wavelength,
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.
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
, lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
, and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.
Properties
Lithium niobate is a colorless solid insoluble in water. It has trigonal crystal systemCrystal system
In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals...
, which lacks inversion symmetry and displays ferroelectricity
Ferroelectricity
Ferroelectricity is a property of certain materials which possess a spontaneous electric polarization that can be reversed by the application of an external electric field. The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was...
, Pockels effect
Pockels effect
The Pockels effect , or Pockels electro-optic effect, produces birefringence in an optical medium induced by a constant or varying electric field. It is distinguished from the Kerr effect by the fact that the birefringence is proportional to the electric field, whereas in the Kerr effect it is...
, piezoelectric effect, photoelasticity
Photoelasticity
Photoelasticity is an experimental method to determine the stress distribution in a material. The method is mostly used in cases where mathematical methods become quite cumbersome. Unlike the analytical methods of stress determination, photoelasticity gives a fairly accurate picture of stress...
and nonlinear optical
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
polarizability. Lithium niobate has negative uniaxial birefringence
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
which depends slightly on the stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
of the crystal and on temperature. It is transparent for wavelengths between 350 and 5200 nanometers.
Lithium niobate can be doped by magnesium oxide
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
, which increases its resistance to optical damage (also known as photorefractive damage) when doped above the optical damage threshold. Other available dopants are , , , , , , , and .
Nanoparticles
Nanoparticles of lithium niobate and niobium pentoxideNiobium pentoxide
Niobium pentoxide is the inorganic compound with the formula Nb2O5. It is a colourless insoluble solid that is fairly unreactive. It is the main precursor to all materials made of niobium, the dominant application being alloys, but other specialized applications include capacitors, lithium niobate,...
can be produced at 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. 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.
Applications
Lithium niobate is used extensively in the telecoms market, e.g. in mobile telephones and optical modulatorOptical modulator
An optical modulator is a device which is used to modulate a beam of light. The beam may be carried over free space, or propagated through an optical waveguide. Depending on the parameter of a light beam which is manipulated, modulators may be categorized into amplitude modulators, phase...
s. It is the material of choice for the manufacture of surface acoustic wave
Surface acoustic wave
]A surface acoustic wave is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the substrate.-Discovery:...
devices. For some uses it can be replaced by lithium tantalate
Lithium tantalate
Lithium tantalate , is a crystalline solid which possesses unique optical, piezoelectric and pyroelectric properties which make it valuable for nonlinear optics, passive infrared sensors such as motion detectors, terahertz generation and detection, surface acoustic wave applications, cell phones...
, . Other uses are in laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
frequency doubling
Second harmonic generation
An optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency...
, nonlinear optics
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
, Pockels cells, optical parametric oscillator
Optical parametric oscillator
An optical parametric oscillator is a parametric oscillator which oscillates at optical frequencies. It converts an input laser wave into two output waves of lower frequency by means of second order nonlinear optical interaction. The sum of the output waves frequencies is equal to the input wave...
s, Q-switching
Q-switching
Q-switching, sometimes known as giant pulse formation, is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high peak power, much higher than would be produced by the same laser if it were operating in a...
devices for lasers, other acousto-optic devices, optical switch
Optical switch
In telecommunication, an optical switch is a switch that enables signals in optical fibers or integrated optical circuits to be selectively switched from one circuit to another....
es for gigahertz frequencies, etc. It is an excellent material for manufacture of optical waveguides.
Periodically poled lithium niobate (PPLN)
Periodically poled lithium niobate (PPLN) is a domain-engineered lithium niobate crystal, used mainly for achieving quasi-phase-matchingQuasi-phase-matching
Quasi-phase-matching is a technique in nonlinear optics which allows a positive net flow of energy from the pump frequency to the signal and idler frequencies by creating a periodic structure in the nonlinear medium. Momentum is conserved, as is necessary for phase-matching, through an additional...
in nonlinear optics
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
. The ferroelectric domains point alternatively to the +c and the -c direction, with a period of typically between 5 and 35 µm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
. The shorter periods of this range are used for second harmonic generation
Second harmonic generation
An optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency...
, while the longer ones for optical parametric oscillation
Optical parametric oscillator
An optical parametric oscillator is a parametric oscillator which oscillates at optical frequencies. It converts an input laser wave into two output waves of lower frequency by means of second order nonlinear optical interaction. The sum of the output waves frequencies is equal to the input wave...
. Periodic poling
Periodic poling
Periodic poling is a formation of layers with alternate orientation in a birefringent material. The domains are regularly spaced, with period in a multiple of the desired wavelength of operation. The structure is desired to achieve quasi-phase-matching in the material.Periodically poled crystals...
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
Periodic poling
Periodic poling is a formation of layers with alternate orientation in a birefringent material. The domains are regularly spaced, with period in a multiple of the desired wavelength of operation. The structure is desired to achieve quasi-phase-matching in the material.Periodically poled crystals...
are wide band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
inorganic crystals like KTP
Potassium titanyl phosphate
Potassium titanyl phosphate or KTP is a nonlinear optical material which is commonly used for frequency doubling diode pumped solid-state lasers such as Nd:YAG and other neodymium-doped lasers. The material has a relatively high optical damage threshold , a great optical nonlinearity and excellent...
(resulting in periodically poled KTP, PPKTP), lithium tantalate
Lithium tantalate
Lithium tantalate , is a crystalline solid which possesses unique optical, piezoelectric and pyroelectric properties which make it valuable for nonlinear optics, passive infrared sensors such as motion detectors, terahertz generation and detection, surface acoustic wave applications, cell phones...
, and some organic materials.
The periodic poling technique can also be used to form surface nanostructure
Nanostructure
A nanostructure is an object of intermediate size between molecular and microscopic structures.In describing nanostructures it is necessary to differentiate between the number of dimensions on the nanoscale. Nanotextured surfaces have one dimension on the nanoscale, i.e., only the thickness of the...
s.
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 equationSellmeier equation
The Sellmeier equation is an empirical relationship between refractive index and wavelength for a particular transparent medium. The equation is used to determine the dispersion of light in the medium....
s for the extraordinary index are used to find the poling period and approximate temperature for quasi-phase matching. Jundt gives
valid from 20-250 °C for wavelengths from 0.4 to 5 micrometer
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s, whereas for longer wavelength,
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.
See also
- CrystalCrystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
- Crystal structureCrystal structureIn mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
- CrystalliteCrystalliteCrystallites are small, often microscopic crystals that, held together through highly defective boundaries, constitute a polycrystalline solid. Metallurgists often refer to crystallites as grains.- Details :...
- CrystallizationCrystallizationCrystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
and engineering aspects - Seed crystalSeed crystalA seed crystal is a small piece of single crystal/polycrystal material from which a large crystal of the same material typically is to be grown...
- Single crystalSingle crystalA single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries...
- Laser-heated pedestal growthLaser-heated pedestal growthLaser-heated pedestal growth is a crystal growth technique. The technique can be viewed as a miniature floating zone, where the heat source is replaced by a powerful CO2 or YAG laser...
- Micro-Pulling-DownMicro-pulling-down-Basics:The micro-pulling-down method is a crystal growth technique based on continuous transport of the melted substance through micro-channel made in a crucible bottom. Continuous solidification of the melt is progressed on a liquid/solid interface positioned under the crucible...