Theory of nonlinear terahertz susceptibility in ferroelectrics

TL;DR

This paper develops an analytical framework to predict the nonlinear terahertz susceptibility in ferroelectrics, highlighting strain-induced enhancements and phase transition effects for potential THz applications.

Contribution

It introduces a perturbation-based theoretical model for second-order nonlinear susceptibility in ferroelectrics, incorporating frequency, temperature, and strain dependencies.

Findings

Enhanced second-order dielectric susceptibility in strained BaTiO3 and SrTiO3 films.

Low dielectric loss associated with strain-stabilized ferroelectric phases.

Potential for strain-engineering nonlinear THz interactions in ferroelectrics.

Abstract

An analytical theory is developed for predicting the nonlinear susceptibility of ionic polarization to continuous electromagnetic waves in both bulk and strained thin film ferroelectrics. Using a perturbation method for solving the nonlinear equation of motion for ionic polarization within the framework of Landau-Ginzburg-Devonshire theory, the full second-order nonlinear susceptibility tensor is derived as a function of frequency, temperature, and strain. The theory predicts the coexistence of a significantly enhanced second-order dielectric susceptibility and a relatively low dielectric loss in BaTiO3 films with a strain-stabilized monoclinic ferroelectric phase and in a strained SrTiO3 film near its temperature-driven second-order ferroelectric-to-paraelectric phase transition. This work establishes a theoretical framework for predicting and exploiting nonlinear interactions between THz waves and ferroelectric materials, and more generally, suggests exciting opportunities to strain-engineer nonlinear dynamical properties of ferroelectrics beyond the static and quasi-static limits.