New optical tools used for characterization of phase transitions in nonlinear nano-crystals. Example of Yb3+-doped BaTiO3

The main objective of this paper is to focus on the use of new techniques to specifically characterize BaTiO3 nonlinear nano-crystals as a test case. We use the second harmonic generation (SHG) of an IR YAG:Nd pump laser and detection of the Yb3+ ion pairs to investigate effect of doping and synthesis route (sol-gel between 700 and 1200 deg. C) on both crystallite sizes (25-60 nm) and different phase symmetries (tetragonal, cubic, hexagonal) in Yb3+-doped BaTiO3. In nano-crystals, a higher intensity of SHG was observed in the acentric tetragonal phase, whereas the isotropic cubic phase was detected by the absence of a SHG signal. Cooperative emission, indicating the presence of Yb3+ ion pairs, was easily detected in the cubic phase and not seen in the ferroelectric tetragonal structure