Delineating complex ferroelectric domain structures via second harmonic generation spectral imaging

TL;DR

This paper introduces a novel second harmonic generation spectral imaging technique using a custom laser-scanning microscope to analyze complex ferroelectric domain structures, revealing detailed polarization and phase information.

Contribution

The study develops and demonstrates a new SHG spectral imaging method with supervised and unsupervised analysis for detailed ferroelectric domain characterization.

Findings

Revealed persistent orthorhombic phase at low temperatures.

Enabled comprehensive polarization vector mapping.

Identified phase competition phenomena.

Abstract

Understanding the mechanisms and spatial correlations of crystallographic symmetry breaking in ferroelectric materials is essential to tuning their functional properties. While optical second harmonic generation (SHG) has long been utilized in ferroelectric studies, its capability for probing complex polar materials has yet to be fully realized. Here, we develop a SHG spectral imaging method implemented on a home-designed laser-scanning SHG microscope, and demonstrate its application for a model system of (K,Na)NbO3 single crystals. Supervised model fitting analysis produces comprehensive information about the polarization vector orientations and relative fractions of constituent domain variants as well as their thermal evolution across the polymorphic phase transitions. We observe an unexpected persistence of the orthorhombic phase at low temperatures, pointing to the phase competitions. Besides, we show that unsupervised matrix decomposition analysis can quickly and faithfully reveal domain configurations without a priori knowledge about specific material systems. The SHG spectral imaging method can be readily extended to other ferroelectric materials with potentials to be further enhanced.