Microfiber Knot Resonators: Structure, Spectral Properties, and Sensing Applications

In recent years, compact micro-/nanofiber sensors with fast response and high resolution have become a hot field in sensing and nanotechnology. In addition to the unique properties of microfibers such as strong evanescent field, low optical loss, and easy integration, microfiber knot resonators further improve the sensing capability due to their resonance properties. The potential high-quality factor allows higher sensing sensitivity and resolution, which is very attractive for optical sensing at the micro-/nano scale. More application potential can be effectively stimulated by improving the structure shape or principle or combining with functional sensitive materials. This paper first introduces the preparation process and spectral characteristics of conventional microfiber knots and then focuses on three new types of devices for enhancing the sensing performance, namely material-coated, shape-improved, and principle-extended, and briefly analyzes their spectral properties. In addition, the research progress and applications of these sensors are summarized. Finally, the existing problems in this field are analyzed, and the directions for further development are discussed and prospected. The preparation process and spectral characteristics of microfiber knot resonators (MKR) are introduced, and three new types of structures for enhancing sensing performance are described in detail. Then, the sensing applications of MKR sensors are summarized. Finally, the existing problems and the directions for further development are discussed.image