Optical microscopy is limited by the wavelength of light. But today's nano devices have features under 10 nanometers, orders of magnitude smaller than the wavelength of visible light! Scanning probe microscopy breaks this limit by using specially fabricated nanoprobes which use mechanical forces to map out feature topography or other properties.

Near-field scanning optical microscopy (NSOM) combines an optical probe with a mechanical probe by using a sharp metal tip as an "optical antenna," which focuses a far-field laser source to a sub-wavelength hotspot.

We use this optical hotspot as a dynamic probe to measure mechanical vibrations. We are able to measure vibrations less than one picometer with a spatial resolution of just a few nanometers. This sensitive tool can be used to probe graphene devices, carbon nanotube resonators, or complex MEMS devices.

I analyzed the sensitivity of our probe with a computational Monte-Carlo simulation to account for different sources of noise. The model matched closely with our experimental predictions: we achieved a noise-limited sensitivity of 0.45 $pm/Hz^{1/2}$, as opposed to our theoretical prediction of 0.4 $pm/Hz^{1/2}$. Based on our simulation, there is a fundamental sensitivity limit of 0.05 $pm/Hz^{1/2}$ due to thermal vibrations of the cantilever at room temperature, so the technique is currently limited by noise in the photodetector used to record the light signal.

## Publications

Ahn, X. Chen, Z. Zhang, M. Ford, D. Rosenmann, I. W. Jung, C. Sun, and O. Balogun. Dynamic near-field optical interaction between oscillating nanomechanical structures, Scientific Reports 5, 10058 (2015)