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J. Eur. Opt. Society-Rapid Publ. 22, 53( 2026)
mode( WGM) microresonators, namely the quality factor Q f, the mode volume V mode, and the local electric field intensity at the sensing location | E( r)| 2. Rather than modifying the resonator geometry, we add a thin nanocoating with refractive index n coat > n SiO2 around the silica microresonator. An extensive material study among dielectrics, polymers, quantum dots, and perovskites, based on their dispersive refractive indices and, in the cases of quantum dots and perovskites, their bandgaps, showed that appropriate candidates for our purpose are silicon nitride Si 3 N 4 and titanium dioxide TiO 2( dielectrics), polystyrene( PS; polymer), cadmium selenide( CdSe) and cadmium zinc( CdZn) quantum dots, as well as cesium lead triiodide( CsPbI 3) perovskite. The corresponding dispersive refractive indices and extinction coefficients of these materials, together with the refractive index of silica as the baseline, obtained from Ref. [ 39, 40 ], are illustrated in Figure 1. Among the investigated materials, the PS polymer and the CsPbI 3 perovskite are chosen as the nanocoating materialsinthiswork.
2.1 Polymer coating( polystyrene)
In our earlier eigenmode analysis of coated silica microspheres, we systematically explored several high-refractive-index dielectric and polymer materials as candidate nanocoatings, namely TiO 2, Si 3 N 4, andPS, withvarious thicknesses [ 36 ]. For each coating, the quality factor Q f, the mode volume V mode, and the normalized electric field at the sensing interface j E norm j¼ jEj surf
= jEj ðuncoatedÞ surf were obtained from our eigenmode models. All three nanocoatings led to an increase in Q f and a reduction in V mode compared to the uncoated microsphere, confirming the general effectiveness of the nanocoating concept for improving the sensitivity. However, the different materials exhibited distinct trade-offs between radiation-loss suppression and evanescent-field penetration into the surrounding sensing medium, i. e., the water in which the exosomes are suspended.
The highest-index TiO 2 nanocoating provided a strong suppression of radiative leakage and thus a substantial Q f enhancement, together with a reduced mode volume, but the WGM was largely confined inside the nanocoating, which resulted in a diminished electric field at the outer interface. The Si 3 N 4 nanocoating offered the best purely optical figures of merit within the eigenmode framework: it yielded the largest increase in Q f, the smallest V mode and an enhancement of the surface field by approximately a factor of two relative to the uncoated case. PS, on the other hand, produced a somewhat smaller Q f enhancement, yet still reduced the mode volume and increased the surface field by a factor of 2.25, while its refractive index, being closer to that of water( n water = 1.3264 + i4.29 10 �6 in the corresponding wavelength range), allowed the evanescent tail of the mode to penetrate more efficiently into the external sensing region [ 36 ]. This behaviour was also reflected in the radial field profiles, where the PS nanocoating shifted the mode towards the surface without overly confining it within the coating.
Figure 1. a) Dispersive refractive indices of SiO 2, Si 3 N 4, TiO 2, PS, CdSe, CdZn, and CsPbI 3 versus wavelength, and b) the extinction coefficients for CdSe, CdZn, and CsPbI 3.
Beyond these optical metrics, practical fabrication considerations strongly favour polymer coatings for implementing nanocoatings on curved microresonators. High-index dielectric layers such as TiO 2 or Si 3 N 4 typically require vacuum-based deposition techniques( e. g. CVD, ALD, sputtering) that can introduce surface roughness and residual stress, which in realistic devices reintroduce scattering losses and may compromise the high Q f values predicted by ideal eigenmode simulations. In contrast, PS can be deposited by standard spin-coating or dip-coating from solution on silica substrates, yielding conformal nanocoatings with low surface roughness and good thickness control, and thus preserving the high intrinsic Q f of the underlying resonator. Taking into account both optical performance and fabrication feasibility, we therefore selected PS as the polymer nanocoating material in the present work and use it as the reference system for comparison with higher-index perovskite coatings.
It is important to note that the initial material screening in Ref. [ 36 ] was performed in an eigenmode setting, which is ideal for extracting intrinsic Q f and V mode butdoesnot explicitly include the excitation and out-coupling configuration. In an eigenmode formulation, the resonator is treated as a source-free open cavity, yielding complex eigenfrequencies x and hence an intrinsic quality factor