The present application relates to optical fibers having at least one slot. The optical fiber may be used, for example, in various sensing application. In some embodiments, a cross-section of the optical fiber perpendicular to the longitudinal axis has a largest dimension less than or equal to about 4 μm, and the slot has a width of about 5 nm to about 500 nm and a depth of at least about 10 nm. Also disclosed herein are methods of using the optical fibers and apparatuses including the optical fibers.