Simulation study of photo-induced phase transitions and coherent optical phonon generation in BaTiO₃

The effects of above band gap ultrafast optical excitation on the structural properties and lattice dynamics of the prototypical perovskite multiferroic, BaTiO₃, are investigated using a constrained density functional method (É. D. Murray et. al. PRB 72, 060301 (2005), P. Tangney and S. Fahy, PRB 65, 054302 (2002)). We calculated the dependences of phonon frequencies and structural stability on excited carrier density. Our work shows a reduction in ionicity due to electrons being returned by O anions to Ti cations. At moderate excitation levels, this causes a decrease in polarization, a softening of several long wavelength optical phonons, and a lowering of local barriers to ferroelectric domain reversal. This indicates that a transient photo-induced lowering of both the coercive field and the phase transition temperature is possible. We suggest that pump probe spectroscopy could be used to induce a purely-displacive transition to a higher symmetry phase at low temperature and/or to generate and study the decay of coherent optical phonons. Our findings may also provide guidance to the design of optically controlled devices.