Dynamical back-action using optical gradient forces

Optical gradient fields have been successfully used in various areas, e.g. as optical tweezers. Recently, evanescent fields were employed to sense nanomechanical motion. Enhancing the optomechanical coupling via a cavity offers significantly higher sensitivities. More importantly, cavity retardation effects can render the back-action which any linear position measurement must entail dynamical. We show that the evanescent field of ultra-high finesse whispering gallery mode(WGM) microcavities can be efficiently used to sense the motion of nanomechanical objects and report the first observation of dynamical back-action using optical gradient fields.