Photo-induced Effect in Quantum Paraelectric Materials Studied by Transient Birefringence Measurement

Strontium titanate SrTiO3 is known as a quantum paraelectric material, and its lattice dynamics and unusual dielectric character have been studied extensively. The cubic (Oh) structure above the structural phase transition temperature (TC = 105 K) changes into the tetragonal (D4h) structure below TC. At low temperatures, dielectric constant increases up to about 3x104, where the paraelectric phase is stabilized by quantum fluctuations even below the classical Curie temperature 37 K (Muller & Burkard, 1979). Photo-induced effect in dielectric materials is an attractive topic. Some kind of ferroelectric materials such as SbSI (Ueda et al., 1967) and BaTiO3 (Volk et al., 1973; Godefroy et al., 1976) are known to show photo-induced effects. In this decade, much interest has been paid on the giant enhancement in dielectric constants under ultraviolet (UV) illumination and DC electric field in quantum paraelectrics, strontium titanate SrTiO3 and potassium tantalate KTaO3 (Takesada et al., 2003; Hasegawa et al., 2003; Katayama et al., 2003), because weak light illumination gives rise to an intense response in dielectricity. The two models shown Fig. 1, the ferroelectric cluster model (Takesada et al., 2003; Hasegawa et al., 2003; Katayama et al., 2003) and the conductive-region model (Homes et al., 2001; Katayama et al., 2003), have been proposed to explain the origin of the giant dielectric constants. At present, however, it is still not clear which model is better. In the ferroelectric cluster model, the photo-induced ferroelectric region has a huge dipole moment, where it is expected that a photo-induced polar domain generates spatial lattice distortion. In the conductive-region model, on the other hand, the superposition of insulative and photoinduced conductive regions, which is characterized by the boundaries between the two regions, makes the apparent dielectric constants to be enormous. Giant dielectric response has been observed in some types of nonferroelectric materials (Homes et al., 2001; Wu et al., 2002; Dwivedi et al., 2010). The enormous increase in dielectric constants is attributed to the formation of barrier layer capacitors and the resultant Maxwell-Wagner polarization or interfacial polarization. This giant dielectric response often occurs in materials with grains surrounded by the insulating grain boundary and is explained by the conductive-region model. According to the measurement of dielectric constants, a doped crystal Sr1-xCaxTiO3 undergoes a ferroelectric transition above the critical Ca concentration xc = 0.0018 (Bednorz