TEP Seminar - Hyeongyu Bae

Photon upconversion (UC) is a nonlinear optical process in which higher-energy photons are emitted through the absorption of multiple lower-energy photons. The generation of upconverted photons in lanthanide systems involves complex photophysical pathways, which are influenced by the local crystal field environment. This presentation investigates the synthesis, photophysics, and site-symmetry-dependent UC behavior of erbium-incorporated perovskites. Two major perspectives are addressed: (1) elucidation of the photophysical pathways responsible for UC, and (2) examination of how local site symmetry affects UC efficiency. Er3+ ions, with their characteristic 4f energy levels, emit green and red photons in the visible region, and their radiative probabilities are sensitive to symmetry-induced perturbations in the host lattice. To probe this relationship, erbium was selectively doped into the A- and B-sites of BaTiO3, which undergoes a tetragonal-to-cubic phase transition near 125 °C. Temperature-dependent UC studies revealed a drop in luminescence intensity for B-site-doped samples at the transition temperature, demonstrating that centrosymmetric environments suppress 4f–4f transitions, while non-centrosymmetric fields enhance them.
Additionally, excitation-power-dependent experiments under 1532 nm (NIR-II) irradiation revealed three distinct excitation-power-regimes governed by photon absorption and cross-relaxation dynamics. These results elucidate the population mechanisms of intermediate 4 I13/2 states and the transition to saturation. Complementary studies on CsBiNb2O7:Er3+ and oxygen-deficient Er2O3 further confirmed that non-centrosymmetric coordination promotes efficient UC emission. Overall, this presentation establishes a clear connection between site symmetry and upconversion photophysics in Er3+ -incorporated crystals, providing fundamental insights for the rational design of high-efficiency UC materials for photonic and optoelectronic applications.