  Nanomechanical photonic metamaterials provide a wealth of active switching,
nonlinear and enhanced light-matter interaction functionalities by coupling
optically and mechanically resonant subsystems. Thermal (Brownian) motion of
the nanostructural components of such metamaterials leads to fluctuations in
optical properties, which may manifest as noise, but which also present
opportunity to characterize performance and thereby optimize design at the
level of individual nanomechanical elements. We show that Brownian motion in an
all-dielectric metamaterial ensemble of silicon-on-silicon-nitride nanowires
can be controlled by light at sub-{\mu}W/{\mu}m2 intensities. Induced changes
in nanowire temperature of just a few Kelvin, dependent upon nanowire
dimensions, material composition, and the direction of light propagation, yield
proportional changes of several percent in the few-MHz Eigenfrequencies and
picometric displacement amplitudes of Brownian motion. The tuning mechanism can
provide active control of frequency response in photonic metadevices and may
serve as a basis for bolometric, mass and micro/nanostructural stress sensing.
