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

TL;DR

This paper explores how polarization-photon coupling influences the ultrafast excitation of ferroelectric thin films by THz pulses, combining analytical theory and dynamical simulations to reveal effects on phonon modes.

Contribution

It introduces a combined analytical and numerical approach to study polarization-photon coupling effects in ferroelectrics under ultrafast THz excitation, highlighting the importance of this coupling.

Findings

Radiation electric field slightly lowers phonon frequencies.

It significantly shortens the relaxational time of polarization oscillations.

Analytical predictions agree with numerical simulations.

Abstract

We investigate the role of polarization-photon coupling (specifically, polarization-oscillation-induced radiation electric field) in the excitation of ferroelectric thin films by an ultrafast terahertz (THz) electric-field pulse. Analytical theory is developed to predict how the frequencies and relaxation time of three-dimensional soft mode phonons (intrinsic polarization oscillation) are modulated by radiation electric field and epitaxial strain. Ultrafast THz-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 relaxational 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 relaxational time. These results demonstrate the necessity of considering polarization-photon coupling in understanding and predicting the response of ferroelectric materials to ultrafast pulses of THz and higher frequencies.