Ultrafast Electric Field-Induced Phase Transition in Bulk Bi_0.5Na_0.5TiO₃ under High-Intensity Terahertz Irradiation

Ultrafast polarization switching is being considered for the next generation of ferroelectric-based devices. Recently, the dynamics of the field-induced transitions associated with this switching have been difficult to explore, due to technological limitations. The advent of terahertz (THz) technology has now allowed for the study of these dynamic processes on the picosecond (ps) scale. In this paper, intense THz pulses were used as a high-frequency electric field to investigate ultrafast switching in the relaxor ferroelectric, Bi_0.5Na_0.5TiO₃. Transient atomic-scale responses, which were evident as changes in reflectivity, were captured by THz probing. The high-energy THz pulses induce an increase in reflectivity, associated with an ultrafast field-induced phase transition from a weakly polar phase (Cc) to a strongly polar phase (R3c) within 20 ps at 200 K. This phase transition was confirmed using X-ray powder diffraction and by electrical measurements, which showed a decrease in the frequency dispersion of relative permittivity at low frequencies.