Trade-off and performance of optoplasmonic mesoporous WGM sensors

Optoplasmonic whispering-gallery-mode (WGM) sensors are often presented as a direct route to enhanced detection limits because localized surface plasmon resonances can create extreme near-field intensities. However, for passive WGM refractometric sensing in porous resonators, plasmonic inclusions also introduce absorption that can strongly reduce the cavity quality factor and may negate any sensitivity gain. Here, we develop a quantitative framework that builds on the Foreman–Vollmer T-matrix approach using a continuous-density approximation for volumetrically distributed nanoparticles (a sum-to-integral replacement). We combine this with a two-step effective-medium model (Bruggeman + Maxwell–Garnett) to obtain the complex effective refractive index of the nanoparticle-doped mesoporous shell. This enables a direct evaluation of the trade-off between refractive-index sensitivity and absorption-induced Q degradation through the figure of merit FOM= S × Q total . For passive, visible-wavelength operation at 645 nm, we find that any nonzero volumetric loading degrades the refractometric FOM relative to the bare mesoporous resonator: for gold nanoparticles, the best-performing doped case in our sweep occurs at the smallest nonzero fraction considered ( f =10 −3 ) and still reduces the FOM by 40.8%, while silver yields a smaller but still negative change (12.9%). Beyond equilibrium performance, we couple the optical model to hindered diffusion in the porous network and show that nanoparticle loading can imprint temporal signatures that encode transport and binding kinetics. These results establish a practical design rule for hollow mesoporous WGM refractometric sensors in the visible range: plasmonic doping does not improve the passive-cavity FOM, and the bare sensor remains optimal unless time-resolved kinetic fingerprinting (or an active/gain-compensated architecture) is the primary goal.