J. Eur. Opt. Society-Rapid Publ. 22, 53( 2026) 519 n o R x
Q int ¼ n o; 2 I x ð1Þ
together with the corresponding modal field profiles and V mode for the isolated structure. This framework does not capture coupling-induced loading, mode selectivity under a specific excitation, nor the transmitted-spectrum line shape. By contrast, our full-wave driven-mode simulations explicitly launch the single-mode optical fiber( SMF) fundamental mode and compute the transmission T of the coupled fiber – microdisk system. The extracted linewidth therefore corresponds to the loaded quality factor Q L,
1
¼ 1 þ 1; Q L Q int Q ext ð2Þ
where Q ext accounts for external coupling. This driven formulation enables optimizing the coupling gap d x( under- / critical- / over-coupling) based on T, and it provides power-normalized field amplitudes at the sensing region under realistic excitation conditions, which is essential for quantitatively predicting exosome-induced resonance shifts. Accordingly, in the present study, these eigenmode-based design guidelines are validated and extended using full driven-mode simulations with explicit fiber-optic coupling. The driven-mode results confirm the qualitative trends found previously and demonstrate that PS nanocoatings indeed yield the expected enhancement of the exosomeinduced resonance wavelength shifts under realistic excitation conditions.
2.2 Perovskite nanocoating and exciton – photon coupling
To further enhance the sensing performance in our approach beyond what is achievable with purely dielectric or polymer nanocoatings, we investigated whether an excitonic – photonic coupling between the microresonator whispering-gallery mode( WGM) and an excitonic transition in the nanocoating can be exploited. The basic idea is to choose a coating material that supports a pronounced excitonic resonance in the same spectral range as the fundamental WGM of the silica microresonator, such that the photonic mode hybridizes with the excitonic mode and forms mixed exciton – photon states with enhanced field confinement and a dispersive response at the sensor surface.
As a first step, we carried out a material screening of potential candidates for excitonic material systems, focusing on commercially available quantum-dot materials( CdSe, CdZn-based quantum dots) and metal-halide perovskites. Using reported dispersion data and bandgap energies( cf. Fig. 1b), we identified materials whose bandgaps lie in the vicinity of 700 nm, i. e. close to the WGM resonance of our 30 lm silica microresonator in water. This analysis confirmed that CdSe and CdZn quantum dots as well as the perovskite CsPbI 3 exhibit excitonic transitions in the targeted spectral range. In a subsequent eigenmode study, each of these materials was introduced as a nanocoating with thicknesses in the range of 100 – 200 nm around the silica microresonator, and the resulting resonator figures of merit – the quality factor Q f, the mode volume V mode and the surface electric field | E | – were systematically evaluated.
The quantum-dot coatings, due to their relatively high extinction coefficients( cf. Fig. 1b), not only failed to achieve any clear photonic – excitonic coupling but also substantially degraded the photonic mode of the microresonator and, in several cases, prevented the formation of well-defined resonances. In contrast, the CsPbI 3 perovskite nanocoating led to a distinct photonic – excitonic coupling and accordingly provided the most pronounced simultaneous improvement of all sensing figures of merit. In the eigenmode simulations, the perovskite-coated resonator exhibited a substantial enhancement of Q f, a compression of V mode and a strong increase of the evanescent field at the outer interface compared to the uncoated resonator and the resonators with the best-performing polymer( PS) and dielectric( TiO 2, Si 3 N 4) coatings [ 41 ]. The spectral dependence of the eigenfrequencies and field distributions indicated that the WGM of the silica core hybridizes with the excitonic resonance of the CsPbI 3 layer: close to the perovskite band edge, the mode is pulled into the coating, the effective index exhibits a pronounced dispersive feature and the field intensity at the coating – water interface is strongly amplified. These observations are consistent with an excitonic – photonic coupling mechanism rather than a purely geometric or index-contrast effect.
A natural concern when employing excitonic materials as microresonator coatings is their intrinsic loss: the strong oscillator strength that gives rise to the excitonic resonance is accompanied by a non-negligible imaginary part of the refractive index, which could in principle degrade the microresonator’ s Q f and offset any sensitivity gain. For this reason, we explicitly compared all sensing criteria of the CsPbI 3-coated resonator against those of the PS and dielectric( TiO 2, Si 3 N 4) nanocoatings. Despite its higher material loss, the CsPbI 3 coating was found to outperform all other options: in the spectral region where the WGM and excitonic resonance overlap, the perovskite nanocoating yields the highest Q f, thesmallestV mode and the strongest surface field enhancement, which together translate into the largest resonance wavelength shift induced by a single exosome. These eigenmode-based findings, first reported in our earlier perovskite-coating study [ 41 ], are in the present work corroborated and extended by full driven-mode simulations with explicit fiber coupling. The results obtained within this fully numerical framework should be interpreted as a predictive assessment of the sensing performance under realistic excitation conditions, indicating that the excitonic CsPbI 3 nanocoating could significantly enhance the sensing functionality of silica microresonators.
The analysis presented here focuses on the idealized optical response of an intact CsPbI 3 nanocoating within a numerical design framework. Long-term chemical stability of bare CsPbI 3 in aqueous or electrolyte-containing biological environments is not assumed and remains a separate materials-engineering challenge. Practical implementation in biofluids will therefore require appropriate stabilization and encapsulation strategies for the perovskite shell, along with careful control of the local chemical environment.