Resonant sensors and molecule detection methods utilizing split frequency. Optical energy is introduced into a microcavity, such as a toroid-shaped or spherical microcavity. A portion of the optical energy is backscattered and interacts with the introduced optical energy to form first and second modes of optical energy at respective first and second frequencies, also referred to as split frequency or mode doublets. One or more molecules bind to an outer surface of the microcavity and interact with an evanescent field of optical energy resonating within the microcavity. Binding of one or more molecules to the outer surface is detected based at least in part upon a change of the split frequency relative to a baseline split frequency.