Tunable Periodicity in Metal Nanogratings for Optimized Plasmon-Enhanced Upconversion Luminescence in Er³⁺/Yb³⁺ Co-Doped GPG Glasses

Surface plasmonics provide a powerful means to manipulate light at the nanoscale, offering innovative techniques to control the excitation and emission properties of quantum systems. Confining electromagnetic fields in extremely small volumes is essential for advancements in fields such as nanophotonics, biosensing, biotechnology, medical imaging, and more. The unique appeal of metallic nanostructures in plasmonics stems from their fascinating linear and nonlinear optical properties, which are significantly affected by parameters like shape, depth, and periodicity. Understanding and leveraging these factors enable precise tuning of optical responses, enhancing the performance and functionality of nanophotonic devices. This study analyzes the influence of the shape and periodicity of Au-plasmonic nanostructures on the enhancement of upconversion emission from Er³⁺ (1.0 wt%) and Yb³⁺ (3.0 wt%) ions. Circular and square grating nanostructures with periodicities ranging from 200 to 1000 nm were fabricated on GeO₂-PbO-Ga₂O₃ glasses (GPG: Er³⁺/Yb³⁺) using focused ion beam (FIB) lithography on a gold film. Plasmon-enhanced upconversion was studied using a combination of a confocal microscope and a motorized microscope setup. Upconversion emissions in the green (~550 nm) and red (~655 nm) spectra were observed for all samples when excited with a λ_ex= 980 nm in the near-infrared. The strongest upconversion emissions were noted in nanostructures with smaller periodicities. The excitation of Er³⁺ ions leads to emissions facilitated by plasmonic nanostructures, attributed to extraordinary optical transmission from the periodic nanostructures. To explain this behavior, we propose an energy transfer mechanism Yb³⁺→ Er³⁺ followed by a resonant coupling between Er³⁺ and surface plasmon polariton. This coupling modifies the local field, improving the emission intensity of Er³⁺. These findings can be very useful for nanophotonic device applications employing a transparent medium with optical gain.<br/><br/><br/><b>Acknowledgments:</b> This work was supported by FAPESP grants 2013/07276–1 and 2023/11979-0.<br/><br/>[1] Lozano C., G.; Silva, O. B.; Ferri, F. A.; Rivera, V. A. G.; Marega, E.. Demonstration of multiple quantum interference and Fano resonance realization in far-field from plasmonic nanostructure in Er³⁺-doped tellurite glass. <i>Scientific Reports</i>, v. 12, p. 5015, 2022.<br/>[2] Luo W., Xu F., Li A., Sun Z. Resonant Control and Enhancement of Upconversion Luminescence of NaYF₄:Yb,Er Nanoparticles on Metal Gratings. <i>Adv. Optical Mater</i>, v. 10, p. 2102668, 2022