  Vibrating nano- and micromechanical resonators have been the subject of
research aiming at ultrasensitive mass sensors for mass spectrometry, chemical
analysis and biomedical diagnosis. Unfortunately, their merits diminish
dramatically in liquids due to dissipative mechanisms like viscosity and
acoustic losses. A push towards faster and lighter miniaturized nanodevices
would enable improved performances, provided dissipation was controlled and
novel techniques were available to efficiently drive and read-out their minute
displacement. Here we report on a nano-optomechanical approach to this problem
using miniature semiconductor disks. These devices combine mechanical motion at
high frequency above the GHz, ultra-low mass of a few picograms, and moderate
dissipation in liquids. We show that high-sensitivity optical measurements
allow to direct resolve their thermally driven Brownian vibrations, even in the
most dissipative liquids. Thanks to this novel technique, we experimentally,
numerically and analytically investigate the interaction of these resonators
with arbitrary liquids. Nano-optomechanical disks emerge as probes of
rheological information of unprecedented sensitivity and speed, opening
applications in sensing and fundamental science.
