KNbO3

KNbO3, or potassium niobate, is an inorganic crystalline solid with a perovskite-derived structure. It belongs to the family of ABO3 oxides in which potassium occupies the A-site and niobium the B-site. The material is well known for its nonlinear optical and piezoelectric properties and is used in various photonics applications.

Temperature-dependent phase transitions: At high temperatures KNbO3 adopts a paraelectric cubic structure; as it is cooled, it passes through a sequence of ferroelectric phases, typically tetragonal and orthorhombic, before reaching a low-temperature phase. The exact transition temperatures depend on sample quality and dopants. These phase changes are associated with changes in spontaneous polarization and optical properties.

Optical properties and uses: It has a wide transparency range from the near-UV to the infrared and a large second-order nonlinear susceptibility, making it suitable for second-harmonic generation and other frequency conversion processes. It is also used for electro-optic modulators and as a nonlinear optical crystal in lasers. KNbO3 crystals can be grown by the Czochralski method or flux growth and can be doped to modify properties. The material is more temperature-sensitive and prone to photorefractive effects than some alternatives such as LiNbO3 or KTP, which can limit certain applications.

Compared with related materials, KNbO3 offers high nonlinearity and relatively simple fabrication but requires careful thermal and optical management.