fig2

Figure 2. Stably poled KTN. (A) SHG patterns when the FW is incident along the x, y, and z directions. The left part shows three QPM processes of nonlinear Bragg diffraction, and the right part shows the phase-matching condition of nonlinear Cherenkov diffraction. l = 45 mm. Scale bars = 4 mm. (B) Self-assembled domain structure. The left part shows ferroelectric domains flipped along the y-axis, producing in-plane G_Bragg parallel to the z-axis, and the right part shows ferroelectric domains flipped along the x- and y-axes, with 180° domain walls along the x- and y-axes. White arrows represent the orientation of P_s and the tetrahedra in the insert are the building blocks of this structure. (C) Real-space imaging. The black dashed lines are in-plane domain walls in (B and C).