JEOS RP ISSN03 | Page 530

J. Eur. Opt. Society-Rapid Publ. 22, 53( 2026) 523

Table 1. Figures of merit for the sensing performance of the uncoated, 100 nm PS-coated, and CsPbI 3-coated microresonators.

Fig. 7. Electric-field intensity of the CsPbI 3-coated microresonator coupled to the optical fiber at the resonance wavelength 886.60 nm. Zoomed views show | E | and E z near the resonator surface. coating is not much higher than that of water, the evanescent tail of the electric field still penetrates strongly into the surrounding medium, and the field intensity at the sensing location is significantly increased( cf. Fig. 7).

It is worth mentioning that the electric field distribution | E | for the CsPbI 3-coated microresonator looks different compared to the usual WGM electric field distributions, observed in both uncoated and PS-coated microresonators. The reason behind this is the presence of the hybrid excitonic-photonic mode in the CsPbI 3-coated microresonator, which, due to the excitonic mode, does not depict the characteristic teardrop shapes typically present in WGM electric field distributions. Such teardrop features are visible in the E z component resulting from the WGM of the microresonator; however, in this case, the E x and E y electric field components associated with the excitonic mode of the CsPbI 3 coating modify the electric field distribution | E |.

In the next step, the coupling distance for the CsPbI 3- coated microresonator is optimized, and an optimal value of d x; CsPbI3 ¼ 350 nm is found, yielding a transmitted power of 51.9 % within the microresonator. Although this transmitted power is substantially lower than in the uncoated and PS-coated cases, the other sensing-related figures of merit are markedly improved. As a result, the CsPbI 3- coated microresonator, due to the hybrid excitonic – photonic coupling, still outperforms the other configurations in terms of sensing functionality. All key sensing figures of merit for the three investigated configurations( uncoated, PS-coated, and CsPbI 3-coated microresonators) are calculated and presented in Table 1. Ascanbeseen, theQ f is about 2.4 times larger in the case of the CsPbI 3-coated microresonator compared to the uncoated case, which is a direct consequence of the hybrid excitonic – photonic mode formation. In addition, the relatively high refractive index of the CsPbI 3 coating reduces the mode volume by about 45 % compared to the uncoated microresonator.

A critical figure of merit for quantifying the performance of our optical biosensor is the electric-field strength at the sensing location | E |, as this parameter determines the excess polarizability of the exosome and, consequently, the magnitude of the resonance wavelength shift. Figure 8 shows the electric-field intensity distribution at the respective resonance wavelengths for all three configurations( uncoated, PS-coated, and CsPbI 3-coated microresonators) along the radial direction, zoomed into a region close to the microresonator surface. It is evident that the addition of the nanocoating shifts the optical mode profile towards the surface, thereby enhancing the field intensity at the sensing location. Moreover, the results show that the perovskite nanocoating both confines the mode and amplifies the electric-field intensity at the sensing location by a factor of 2.11 compared to the PS-coated case and by a factor of 3.68 compared to the uncoated case. This pronounced improvement is directly related to the formation of the hybrid excitonic – photonic mode within our proposed biosensor.

The results summarized in Table 1 further underline the effectiveness of the CsPbI 3 coating compared to both the uncoated and the PS-coated microresonator.

Since the CsPbI 3 coating is lossy, there is a natural concern that absorption in the coating layer could degrade the microresonator performance. To address this, we evaluate the absorption within the nanocoating by integrating the total power dissipation density over the coated region. Because the model is two-dimensional, the integration is carried out over a cross-section, and the resulting absorbed power in the coating region is reported per unit length( W / m). It is found that the absorption losses in the coating are generally low, on the order of 10 �5 W / m. However, as shown in Figure 9, the local absorption losses within the water medium are significantly larger in the presence of the coating, which is a direct consequence of the larger evanescent field tails( i. e. penetration depth) into the water solution.

6 Exosome detection and characterization

The final and decisive figure of merit for evaluating the CsPbI 3-coated microresonator is the resonance-wavelength shift induced by a single exosome. To this end, healthy