Role of Polarization-Photon Coupling in Ultrafast Terahertz Excitation of Ferroelectrics

We investigate the role of polarization-photon coupling (specifically, the polarization-oscillation-induced radiation electric field) in the excitation of ferroelectric thin films by an ultrafast terahertz electric field pulse. Analytical formulas are developed to predict how the frequencies and relaxation time of three-dimensional soft mode phonons (intrinsic polarization oscillation) are modulated by the radiation electric field and epitaxial strain. Ultrafast terahertz-pulse-driven excitation of harmonic polarization oscillation in strained single-domain ferroelectric thin film is then simulated using a dynamical phase-field model that considers the coupled strain-polarization-photon dynamics. The frequencies and relaxation time extracted from such numerical simulations agree well with analytical predictions. In relatively thin films, it is predicted that the radiation electric field slightly reduces the frequencies but significantly shortens the relaxation time. These results demonstrate the necessity of considering polarization-photon coupling in understanding and predicting the response of ferroelectric materials to ultrafast pulses of terahertz and higher frequencies.