Co‐Polarized Second Harmonic Generation Induced by Ferroelectric Domains and Domain Wall Arrays

Abstract Optical second harmonic generation (SHG) is a sensitive and powerful technique to probe various symmetry breaking, especially ferroelectric/magnetic domains, in artificial thin films or heterostructures. In a wide range of ferroelectric crystals, a non‐trivial broken symmetry will emerge at domain boundaries, however, the contribution of such symmetry breaking to SHG is often neglected due to its random orientation. Here, a co‐polarized SHG effect from the well‐aligned domain wall arrays is observed in BiFeO 3 thin films. This SHG component is highly dependent on the local orientation of the wall arrays, and its polarization feature reveals the Néel‐like evolution of electric polar vectors in the wall region. Based on this intriguing effect, mixed‐phase patches with four possible orientations in the quasi‐tetragonal stripes are identified by polarization‐sensitive SHG mapping. These findings not only clarify a critical SHG contribution in this multiferroic material, but also demonstrate an optical approach for exploring phase‐determined function unit.