  High-frequency atomic force microscopy has enabled extraordinary new science
through large bandwidth, high speed measurements of atomic and molecular
structures. However, traditional optical detection schemes restrict the
dimensions, and therefore the frequency, of the cantilever - ultimately setting
a limit to the time resolution of experiments. Here we demonstrate
optomechanical detection of low-mass, high-frequency nanomechanical cantilevers
(up to 20 MHz) that surpass these limits, anticipating their use for
single-molecule force measurements. These cantilevers achieve 2 fm / sqrt(Hz)
displacement noise floors, and force sensitivity down to 132 aN / sqrt(Hz).
Furthermore, the ability to resolve both in-plane and out-of-plane motion of
our cantilevers opens the door for ultrasensitive multidimensional force
spectroscopy, and optomechanical interactions, such as tuning of the cantilever
frequency in situ, provide new opportunities in high-speed, high-resolution
experiments.
