J. Eur. Opt. Society-Rapid Publ. 22, 53( 2026) 525
Fig. 10. Electric-field intensity distribution when a single healthy exosome( H) is placed at the immediate vicinity of( a) the uncoated and the PS-coated microresonators and( b) the CsPbI 3-coated microresonator, plotted together with the polarization magnitude | P | inside the exosome. The vertical colorbars denote | E |. Note that the color scale for the CsPbI 3-coated microresonator is different and the maximum value is twice as high as in the other cases. The horizontal colorbars indicate | P | inside the healthy exosome. Note the different magnitude ranges for each plot. involving multiple simultaneously adsorbed exosomes are beyond the scope of this study, as the sensing concept is based on controlled delivery and positioning of individual exosomes, for example through microfluidic-assisted transport and ligand-based surface functionalization.
7 Conclusion
Fig
. 11. Polarization in the healthy, borderline and cancerous exosomes next to the CsPbI 3-coated microresonator.
sensitivity, assuming ideal binding and precise placement at the sensing interface. To assess the impact of non-ideal placement, a complementary study on the same biosensor platform was conducted, in which the exosome position was systematically varied in both radial and angular directions( see [ 54 ]), assuminga50nmexosomewithn eff = 1.3847 [ 36 ]. The results show that, while radial displacement leads to a pronounced reduction in the sensing response for all configurations due to the rapid evanescent-field decay, the CsPbI 3-coated microresonator exhibits a markedly reduced dependence on small angular position variations compared to the uncoated and PS-coated cases, with an almost unchanged local electric field( variation 0.3 %), as well as nearly invariant resonance wavelength shift and induced exosome polarization [ 54 ], as can be seen in Figure 12. This behavior is attributed to the more uniform near-surface field distribution associated with the hybrid photonic – excitonicmodesupportedbytheperovskitecoating.
From a practical perspective, the reduced angular dependence of the proposed CsPbI 3-coated microresonator, specifically designed for single-exosome detection, relaxes the requirement for precise exosome positioning at a single optimal location and supports more robust sensing under realistic conditions. At the same time, the results confirm that maintaining the exosome in close proximity to the resonator surface remains essential for maximizing sensitivity, highlighting the importance of surface-functionalization strategies for effective exosome trapping [ 54 ]. Scenarios
In this work, we have proposed and numerically demonstrated an excitonic – photonic hybrid whispering-gallerymode microresonator as an ultra-sensitive optical biosensor for single-exosome detection and health-state classification. A30lm silica microdisk with a 100 nm nanocoating was modeled in an aqueous environment and excited via a single-mode fiber using full-wave driven-mode computational electromagnetics simulations in Comsol Multiphysics. To the best of our knowledge, this represents the first full-field driven-mode study of coated WGM microresonators with explicit fiber coupling for biosensing applications. In parallel, we developed a physics-based framework to determine the effective refractive index of individual exosomes as a function of their protein and nucleic-acid content, combining the Barer relation for the lumen, a core – shell exosome geometry, and volume-averaged D / E retrieval from fullwave simulations. This yields dispersive effective refractive indices n ðH; B; CÞ eff ðkÞ for healthy, borderline, and cancerous exosomes and provides, to the best of our knowledge, the first ERI-based description of exosomes explicitly parameterized by health state.
On the photonic side, we systematically compared uncoated, PS-coated, and CsPbI 3-coated microresonators. The PS nanocoating was shown to enhance the sensing performance by increasing the quality factor Q f by 15 %, reducing the mode volume V mode by 25.8 %, and amplifying the electric field at the sensing location by a factor of 1.75 relative to the uncoated case. Building on our previous eigenmode screening [ 36, 41 ], we then identified a configuration in which a CsPbI 3 perovskite nanocoating exhibits strong excitonic – photonic coupling with the silica WGM