Comparative analysis of single-mode silicon nitride microring resonators at telecom wavelengths and TE and TM polarization for refractive index sensors

We evaluate the refractive index sensing performance of moderate-confinement silicon nitride ( Si 3 N 4 ) microring resonators operating at 1310 and 1550 nm under both TE and TM polarizations. The Si 3 N 4 devices are fabricated through a commercial rapid prototyping process using direct-write electron beam lithography in a silicon photonics foundry. Using 0.4 µm thick Si 3 N 4 strip microring resonators to sense sodium chloride (NaCl) solutions, we demonstrate bulk sensitivities of 127.77 and 221.36 nm/RIU for 1310 and 1550 nm wavelengths, respectively, under TE polarization, as well as 155.77 and 266.63 nm/RIU for 1310 and 1550 nm wavelengths, respectively, under TM polarization. The intrinsic quality factors with deionized water (DI water) on top of the microring resonator are 1.3×10 5 and 1.2×10 4 at 1310 and 1550 nm wavelengths, respectively, for the TE-polarized mode device, as well as 5.4×10 4 and 7.3×10 3 at 1310 and 1550 nm wavelengths for the TM-polarized mode device. Our Si 3 N 4 microring resonator sensor maintains superior quality factors even upon exposure to NaCl salt solutions compared to silicon designs. This corresponds to intrinsic limits of detection of 7.8×10 −5 and 5.8×10 −4 RIU at 1310 and 1550 nm, respectively, under TE polarization, and 1.5×10 −4 and 7.9×10 −4 RIU at 1310 and 1550 nm, respectively, under TM polarization, corresponding to NaCl concentration detection limits in the low millimolar range. Overall, our devices exhibit comparable sensitivity and limit of detection compared with previously reported Si 3 N 4 strip microring sensors, demonstrating the effectiveness of the 0.4 µm thick platform. These results highlight the potential of moderate-confinement Si 3 N 4 microring resonators for biosensing, emphasizing their versatility and compatibility with existing photonic technologies at key telecom wavelengths.