Lithium niobate

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Hexagonal crystal structure of LiNbO₃

Lithium niobate (Li Nb O₃) is a compound of niobium , lithium, and oxygen. It is a colorless solid material with trigonal crystal structure. It is transparent for wavelengths between 350 and 5200 nanometers and has a bandgap of around 4 eV. Its melting point is 1257 °C and its density is 4,65 g/cm³. It is insoluble in water. Its CAS number is [12031-63-9].

It 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.

The lithium niobate crystals lack inversion symmetry and display Pockels effect.

The birefringence of lithium niobate is highly temperature dependent; accurate heating of the crystal can be therefore used to achieve phase matching in the medium.

Czochralski-grown monocrystals have unique electro-optical, piezoelectric, photoelastic and nonlinear optical properties. They are strongly birefringent. They are used 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.

Lithium niobate is used extensively in the telecoms market, eg. in the 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, LiTaO₃.

[edit] Periodically poled lithium niobate (PPLN)

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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 25 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.

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.