Silica nanospheres for filtering higher-order optical fiber modes

Tapered optical fibers (TOFs) modified by 400 nm-diameter silica nanospheres have been experimentally and theoretically investigated. Responses of the modified TOFs (MTOFs) to methylene blue (MB) solution and surrounding refractive index (RI) have been compared. The higher-order modes are effectively filtered by the silica nanospheres, which is visualized by the interferometric output spectrum of the MTOF. The higher-order mode filtering makes the MTOF respond to MB solution with reduced sensitivity, which endows the MTOF with the potential for distinguishing the RI property from the absorption property of the molecules under detection.