ABSTRACT
Dielectrics are one of the most common solid and liquid materials on the Earth. A big number of different optical and electrooptical as well as dielectric properties have long been of great fundamental and technological interest. From the solid-state physics point of view, dielectrics are the materials with a bandgap (Egap) more than 3 eV, which means they are optically transparent. On the other hand, in terms of electrodynamics, dielectrics are considered as the materials with a resistivity of more than 108 Ωm (not electrically conductive). However, the last classication is conditional because, for example, dielectrics can exhibit some semiconducting properties depending on the ambient temperature. Most of natural dielectrics are oxides and nitrides of s and p elements (SiO2, Al2O3, MgO, Al2MgO4, AlN, BN, etc.). Basically, the bandgap for such oxides is greater than 6 eV. Dielectrics are also the rare-earth oxides (ZrO2, Gd2O3, Ho2O3, Y2O3, Eu2O3, Tb2O3, etc.) with a bandgap of 4-6 eV.
