Tunable periodicity in metal nanogratings for optimized plasmon-enhanced upconversion luminescence in Er3+/Yb3+ co-doped PGG glasses

Surface plasmon polaritons provide a powerful tool for manipulating light at the nanoscale, enabling innovative techniques to control the excitation and emission properties of quantum systems. The confinement of electromagnetic fields in extremely small volumes is essential for advancements in nanophotonics, biosensing, biotechnology, and medical imaging. The unique appeal of metallic nanostructures in plasmonics stems from their fascinating linear and nonlinear optical properties, which are significantly influenced by shape, depth, and periodicity. This study investigates the influence of the geometry and periodicity of gold nanostructures on enhancing upconversion emission from Er3+ ions in heavy metal oxide glass co-doped with Er3+/Yb3+ (PGG: Er3+/Yb3+). Focused-ion beam (FIB) lithography was used to fabricate circular and square grating nanostructures on a 170 nm gold film deposited on the glass surface, with periodicities ranging from 300 to 1000 nm. These nanostructures were applied to both undoped and Er3+/Yb3+ co-doped glasses. Upconversion emissions in the green (similar to 550 nm) and red (similar to 655 nm) regions were observed for all co-doped samples when excited at lambda(ex) = 980 nm, with the strongest emissions appearing in samples with nanostructures of smaller periodicities. We propose an energy transfer mechanism involving Yb3+ -> Er3+, followed by resonant coupling between Er3+ and surface plasmon polaritons, which modifies the local field and enhances Er3+ emission intensity. These findings, supported by FDTD simulations, show excellent agreement between theoretical and experimental results, highlighting the potential for developing new photonic platforms that leverage the unique emission properties of rare-earth ions.