Three-dimensional quasi-periodic ferroelectric domain structure and its second harmonic generation effect in potassium tantalate niobate crystal

Nonlinear photonic crystals (NPCs) have garnered significant attention due to their capability to manipulate and enhance optical interactions via quasi-phase-matching. The periodic domain inversion structures inside NPCs, corresponding to periodically modulated second-order nonlinear coefficients χ⁽²⁾, enable the generation of light at new frequencies. However, due to the internal stress and defects, forming a structurally perfect superlattice in naturally grown crystals is challenging, and artificial engineering often entails high costs and complex procedures. In this study, we successfully fabricated a unique three-dimensional (3D) potassium tantalate niobate (KTN) NPC using a direct current poling treatment. Nonlinear Cherenkov diffraction occurs when the fundamental light propagates parallel to the poling direction, whereas nonlinear Bragg diffraction is observed when the light is incident perpendicular to the poling direction. By analyzing both the linear and nonlinear optical responses of the stably poled KTN crystal, we reveal the 3D distribution of ferroelectric domains, which advances the understanding of ferroelectric domain dynamics during the poling process and opens new avenues for research in anisotropic nonlinear optics and optoelectronic applications.