Second Order Optical Nonlinearity Originated from Ferroelectric Spherulite with Vortex Domain

So far, second harmonic generation (SHG) is usually obtained through ferroelectric materials with spontaneous polarization because of the broken spatial inversion symmetry. In recent years, ferroelectric systems with topological defects such as vortex domains have aroused great research interest. The SHG response from such ferroelectric topological structures is intriguing toward SHG applications and topological characterizations. In this work, a spherulite material with a single ferroelectric vortex domain is synthesized and studied on its ferroelectric properties and optical nonlinearity. The spherulite material is in a sphere-cap shape, composed of polar molecules RM734, with both the average molecular orientation and the polarity arranged azimuthally, forming a single vortex domain, which is demonstrated by the ferroelectric hysteresis loop and piezoelectric force microscopy. Such a large single vortex domain can produce second-order nonlinearity with local polarity features, which are, in turn, used to characterize the topological structure of the ferroelectric vortex domain. The topological similarity between such a single vortex domain material and the optical vortex beam may shed light on promising ways of optical vortex generation.