Finite plasmonic loading in WGM-assisted TiO₂/Au photocatalysts: a resonant optical design framework

Whispering-gallery-mode (WGM) microresonators enhance light-matter interaction through strong optical confinement and long photon lifetimes, enabling applications in sensing, optofluidics, and resonant photochemistry. A cavity-level optical design framework is developed for Auloaded mesoporous TiO₂ WGM microspheres to describe the competition between plasmonic absorption and absorption-induced degradation of resonant field buildup. This competition leads to a finite, wavelength-dependent optimal nanoparticle loading. Increasing loading enhances cavity-weighted absorption but eventually reduces resonant enhancement due to finite WGM interaction depth and absorption-induced cavity loss, so that the loading maximizing total Beer-Lambert absorption does not coincide with that maximizing the resonant optical design metric. An optimum is predicted near λ ≈ 592 nm at $f$ ≈ 1.1 × 10^-3, with an optimal range of $f$ * ≃ (1.8-3.9) × 10^-3 in the 560-600 nm window. Comparison with a recent monodisperse TiO₂/Au WGM-localized surface plasmon resonance microsphere platform indicates that the optically active loading fraction is significantly lower than the nominal chemical loading, indicating that mode overlap governs the effective plasmonic response. Finite nanoparticle loading emerges as a general optical design principle for WGM-assisted plasmonic photocatalytic systems