Rolled-up SiO_x/SiN_x microtubes with an enhanced quality factor for sensitive solvent sensing

The microtubes made through rolling-up of strain-engineered nanomembranes have received growing research attention after their first invention due to the technology's high flexibility, integrability, and versatility. These rolled-up microtubes have been used for a variety of device applications including sensors, batteries and transistors, among others. This paper reports the development of highly sensitive whispering-gallery mode (WGM) chemical sensors based on rolled-up microtube optical microcavities (RUM-OCs). For the first time, such microcavities were batch fabricated through rolling-up of plasma-enhanced chemical vapor deposition (PECVD)-synthesized SiO_x/SiN_x bilayer nanomembranes, which have better optical properties than the conventional electron-beam-deposited SiO/SiO₂ bilayers. Benefiting from the high refractive index (RI) of PECVD-deposited SiN_x, our RUM-OC shows an enhanced quality factor of 880 that is much higher than that (50) of a SiO/SiO₂ RUM-OC with the same dimensions. The developed RUM-OC is used for sensitive WGM solvent sensing, and demonstrate a limit of detection of 10^-4 refractive index unit (RIU), which is 10 times lower than that (10^-3 RIU) of a SiO/SiO₂ RUM-OC.