  Optomechanical systems combine extreme sensitivity and bandwidth in the
control of mechanical motion, of interest for various applications. Integrated
on a chip, actuated and detected all-optically by a single laser, they could
disrupt sensing technologies. We introduce here a multiphysics model that
describes their operation in the oscillating mode, under sinusoidal modulation
of the laser, when both photothermal forces and radiation pressure are present,
and when nonlinear absorption occurs in the device. The model is validated by
systematic experiments on ultra-high frequency optomechanical disk resonators
and leads to a quantitative assessment of the amplitude and phase of the
demodulated output signal, which carries the sensing information.
