Optical detection of symmetry breaking on a nanoscale in SrTiO3:Ca

Spontaneous and field-induced linear birefringence is measured at low temperatures on single-domained single crystals of weakly doped SrTiO3:Ca (x = 0.002 and 0.007) in order to investigate polar ordering processes near to T-c approximate to 4 and 18 K, respectively. In agreement with direct polarization measurements the size of mesoscopic polar nanodomains is determined within the framework of a nonlinear polarization optical response model. Quasi-first-order Raman scattering of soft and hard polar modes, TO1, TO2 and TO4, is observed on local precursor nanodomains above T-c, Below T-c the change of the global symmetry from D-4h to C-2v is reflected by selection rules imposed on first-order Raman lines in various scattering geometries.