Sensitivity collapse in whispering gallery mode refractometric sensing of porous microspheres: A finite-window mode-misassignment mechanism

Fluorescence-based whispering-gallery-mode (WGM) sensors routinely report refractometric sensitivities far below the intrinsic limit S_intrinsic = λ/n_eff, yet the origin of this discrepancy has not been rigorously quantified. Here we show that a purely geometric mechanism—mode misassignment under finite-bandwidth fluorescence detection—can strongly suppress the apparent sensitivity without any degradation of material quality or fabrication precision. When a sufficiently large refractive-index change Δn drives the tracked resonance beyond the dye emission band, the observable in-window spectral maximum no longer corresponds to the original mode. This defines a window-limited trackability threshold Δn_th, below which intrinsic sensitivity is recovered and above which the apparent sensitivity collapses because of a change in modal identity. We validate this mechanism for hollow mesoporous silica microcapsules (R = 44 μm, ϕ = 0.63, detection window 630–660 nm) using temporal coupled-mode theory (TCMT) and first-principles Mie calculations, which agree within 0.4% on Δn_th. In the trackable regime, both methods recover S_intrinsic ≈ 553 nm/RIU. Beyond threshold, the true resonance exits the observation window and the detected peak is reassigned, yielding an apparent sensitivity of 16.8 nm/RIU—a 33× reduction—while the Q-factor changes by only 2.6%. These results establish finite-window mode misassignment as a quantitative origin of the longstanding gap between intrinsic and reported sensitivities and provide a simple pre-experiment diagnostic criterion for fluorescence-based WGM sensing.