Enhanced Upconversion Photoluminescence Assisted by Flexoelectric Field in Oxide Nanomembranes

Flexoelectricity refers to the linear coupling between electric polarization and strain gradients, and exhibits in all materials with arbitrary crystal symmetries. Recent breakthroughs on synthesizing high-quality freestanding perovskite oxides have provided new opportunities to couple this universal effect to various functionalities. In particular, the interplay between flexoelectricity and upconversion emission in lanthanide doped freestanding perovskite oxide SrTiO₃:Er³⁺ nanomembranes is experimentally demonstrated. The tunable flexoelectricity leads to an over fourfold enhancement in upconversion photoluminescence (PL) through strain gradient engineering. The observed significant PL enhancement can be ascribed to the strain gradient induced polarization, or more fundamentally, inversion symmetry breaking. Furthermore, this behavior is reversible and exhibits excellent antifatigue characteristics even after 10⁴ bending cycles. The showcased strong coupling between flexoelectricity and photoluminescence in nonpolar materials offers dramatically greater design freedom for various strain-tunable optoelectronic devices, regardless of the lattice symmetry of the constituent materials.