Ferroelectrics inorganic crystals

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

Materials. For holographic information storage, materials are required which alter their index of refraction locally by spotwise illumination with light. Suitable are photorefractive inorganic crystals, eg, LiNbO, BaTiO, LiTaO, and Bq2 i02Q. Also suitable are photorefractive ferroelectric polymers like poly(vinyhdene fluoride-i o-trifluorethylene) (PVDF/TFE). Preferably transparent polymers are used which contain approximately 10% of monomeric material (so-called photopolymers, photothermoplasts). These polymers additionally contain different initiators, photoinitiators, and photosensitizers. 

Since the early days of ferroelectric low molar mass LC research, the exploration of the NLO properties of these substances has been a topic of interest. For a recent review on SHG in ferroelectric LCs, see [215]. Unfortunately, the SHG efficiency of most ferroelectric LCs investigated so far [216-219] is orders of magnitude below that of state-of-the-art inorganic crystals, such as lithium niobate. The main reason for the low electronic NLO activity is that the ferroelectric LC compounds and mixtures used in these investigations were optimized for display applications rather than for NLO performance. Quite recently, some research groups started the development of ferroelectric LC compounds specifically devoted to NLO applications, and indeed improved the SHG efficiency by orders of magnitude [220-224]. 

Shelton and Shen, these harmonic generation studies are more novel than useful, in terms of possible practical applications. There has also been recent work on second harmonic generations in ferroelectric liquid crystals [13], but the observed conversion efficiency is still quite small compared to currently commercially available inorganic crystals. 

In the Landolt-Bdmstein data collection, ferroelectric and antiferroelectric substances are classified into 72 families according to their chemical composition and their crystallographic structure. Some substances which are in fact neither ferroelectric nor antiferroelectric but which are important in relation to ferroelectricity or anti-ferroelectricity, for instance as an end material of a solid solution, are also included in these families as related substances. This subsection surveys these 72 families of ferroelectrics presented in Landolt-Bornstein Vol. III/36 (LB III/36). Nineteen of these families concern oxides [5.1,2], 30 of them concern inorganic crystals other than oxides [5.3], and 23 of them concern organic crystals, liquid crystals, and polymers [5.4]. Table 4.5-1 lists these families and gives some information about each family. Substances classified in LB 111/36 as miscellaneous crystals (outside the families) are not included. 

Ferroelectric ceramics and single crystals have found wide applications in many electronic, acoustoptic and piezoelectric devices [1,2], Perovskites represent one of the most important classes of inorganic powders that are of great interest in functional ceramics used for electronic components among them BaTiOj is a typical and most frequently used representative. 

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