Sensitive transduction of the motion of a microscale cantilever is central to many applications in mass, force, magnetic resonance, and displacement sensing. Reducing cantilever size to nanoscale dimensions can improve the bandwidth and sensitivity o
f techniques like atomic force microscopy, but current optical transduction methods suffer when the cantilever is small compared to the achievable spot size. Here, we demonstrate sensitive optical transduction in a monolithic cavity-optomechanical system in which a sub-picogram silicon cantilever with a sharp probe tip is separated from a microdisk optical resonator by a nanoscale gap. High quality factor (Q ~ 10^5) microdisk optical modes transduce the cantilevers MHz frequency thermally-driven vibrations with a displacement sensitivity of ~ 4.4x10^-16 msqrt[2]{Hz} and bandwidth > 1 GHz, and a dynamic range > 10^6 is estimated for a 1 s measurement. Optically-induced stiffening due to the strong optomechanical interaction is observed, and engineering of probe dynamics through cantilever design and electrostatic actuation is illustrated.
