Self-rolled-up SiN _x microtube with enhanced Q-factor and quasi-single-mode emission for label-free optical sensing

In this study, we propose a self-rolled-up SiN_x microtube with enhanced <i>Q</i>-factor and quasi-single-mode emission characteristics. Two types of microtubes are fabricated using standard CMOS-compatible microfabrication techniques: one directly on a Si substrate, and the other on a SiO₂ thin film. Compared to the microtube formed on SiO₂, the microtube on the Si substrate exhibits stable quasi-single-mode emission due to strong optical coupling between the intrinsic emission peak and the cavity resonance. This enhanced coupling significantly increases the <i>Q</i>-factor, reaching up to 1062. When the microtube is immersed in water, the mismatch between the emission and resonance modes leads to a notable decrease in both emission intensity and <i>Q</i>-factor, indicating a high sensitivity to changes in the surrounding refractive index. Compared to the microtube-based sensing approaches that required mode identification and complex spectral analysis, this method enables label-free detection with clearly observable signal changes. These features make the self-rolled-up SiN_x microtube a promising platform for optical sensing applications, offering high detection sensitivity, compact device integration, and label-free operation.