  Optical microresonators have proven powerful in a wide range of applications,
including cavity quantum electrodynamics, biosensing, microfludics, and cavity
optomechanics. Their performance depends critically on the exact distribution
of optical energy, confined and shaped by the nanoscale device geometry.
Near-field optical probes can image this distribution, but the physical probe
necessarily perturbs the near field, which is particularly problematic for
sensitive high quality factor resonances. We present a new approach to mapping
nanophotonic modes that uses a controllably small and local optomechanical
perturbation introduced by a focused lithium ion beam. An ion beam (radius
about 50 nm) induces a picometer-scale dynamic deformation of the resonator
surface, which we detect through a shift in the optical resonance wavelength.
We map five modes of a silicon microdisk resonator (Q > 20,000) with both high
spatial and spectral resolution. Our technique also enables in-situ observation
of ion implantation damage and relaxation dynamics in a silicon lattice.
