Isotropic octupolar second harmonic generation response in LaBGeO₅ glass-ceramic with spherulitic precipitation

A spherulitic crystallization of the crystalline phase LaBGeO₅ is generated in the 25La₂O₃-25B₂O₃-50 GeO₂ glass system. Linear and nonlinear optical properties of lanthanum borogermanate glass-ceramic have been investigated at both macroscopic and microscopic scales. Polarized μ-Raman analysis has evidenced a radial distribution of the crystallites along the c-axis inside spherulites, whereas polarized μ-Second Harmonic Generation (SHG) analysis revealed intensity maxima perpendicularly to the c-axis crystallites orientation. Polarized SHG mapping of a spherulite indicate that no dipolar response along the c-axis oriented crystallites occurs despite the individual dipolar symmetry C₃ of the crystallites. At a larger mm scale, the isotropic scattering of spherulites recorded from macroscopic SHG experiment in the forward direction is consistent with an average coherent octupolar response per spherulite. These SHG analyses at different scale are both in accordance with radial antiferroelectric orientation along the c-axis of crystallites inside each spherulite