  Nanocrystalline materials exhibit properties that can differ substantially
from those of their single crystal counterparts. As such, they provide ways to
enhance and optimise their functionality for devices and applications. Here we
report on the optical, mechanical and thermal properties of nanocrystalline
silicon probed by means of optomechanical nanobeams to extract information of
the dynamics of optical absorption, mechanical losses, heat generation and
dissipation. The optomechanical nanobeams are fabricated using nanocrystalline
films prepared by annealing amorphous silicon layers at different temperatures.
The resulting crystallite sizes and the stress in the films can be controlled
by the annealing temperature and time and, consequently, the properties of the
films can be tuned relatively freely, as demonstrated here by means of electron
microscopy and Raman scattering. We show that the nanocrystallite size and the
volume fraction of the grain boundaries play a key role in the dissipation
rates through non-linear optical and thermal processes. Promising optical
(13000) and mechanical (1700) quality factors were found in the optomechanical
cavity realised in the nanocrystalline Si resulting from annealing at 950 C.
The enhanced absorption and recombination rates via the intra-gap states and
the reduced thermal conductivity boost the potential to exploit these
non-linear effects in applications, including NEMS, phonon lasing and
chaos-based devices.
