Imaging and modeling of optical second harmonic generation in complex ferroelectric microstructures

Nonlinear optics is an important tool for probing the structure of materials. In particular, optical second harmonic generation (SHG) is a sensitive probe for broken inversion symmetry, making it a natural choice for understanding the microstructures of polar complex oxides such as ferroelectrics. While physics-based computational methods can predict the formation and evolution of ferroelectric microstructures, a computational method connecting the computed ferroelectric microstructures and the corresponding SHG responses is still lacking. In this work, we develop a theoretical framework that connects complex ferroelectric microstructures with their nonlinear optical properties to predict their spatially resolved SHG responses. We validate our approach by directly comparing computational and experimental microstructures under different electrical and thermal conditions. Our approach provides a crucial connection between computational modeling of ferroelectric materials and SHG-based experimental imaging microscopies. We show that complex domain microstructures that lead to emergent low-symmetry metastable phases with enhanced nonlinear optical coefficients.