Band-Selective LDOS Engineering of Yb/Er Upconversion: An Electromagnetic–Kinetic Diagnostic Framework

Abstract A persistent challenge in plasmonic upconversion is decoupling pump-field enhancement from emission-side local-density-of-optical-states (LDOS) engineering to achieve selective band manipulation. Here, we show that a corrugated SU-8/Au/Al2O3 grating coated with a NaYF4:Yb/Er upconversion nanoparticle (UCNP) monolayer realizes a truly band-selective platform. A broad plasmonic resonance near 670 nm modulates the red Er3+ decay rate by ±15% as a function of the Al2O3 spacer thickness, while leaving the green transition experimentally invariant (<1% change). Simultaneously, the 980 nm pump field is monotonically suppressed below free-space levels, ensuring that steady-state and time-resolved observables cleanly probe the emission-side LDOS without pump interference. We analyze this system using a coupled electromagnetic–kinetic framework that integrates finite-difference time-domain (FDTD) calculations of Purcell factors and pump fields with a six-level Yb/Er rate-equation model. The framework quantitatively reproduces the 670 nm plasmonic resonance, the red-band decay-rate modulation, and the monotonic decrease of the green/red intensity ratio. Crucially, the model serves as a powerful diagnostic tool: it overpredicts a green-band rate reduction, but systematic parametric testing rules out geometric (apex smoothing) and material (grain-boundary damping, interband loss) imperfections as the cause. Instead, it traces the residual discrepancy to measurement-versus-model factors (finite-aperture angular averaging) and missing nonradiative kinetic channels, establishing a clear roadmap for the rational design and validation of future plasmonic-UCNP architectures.