  Optical forces can set tiny objects into states of coherent mechanical
oscillation, also known as mechanical or phonon lasing. We present a novel
pumping mechanism in an opto-mechanical photonic crystal that realizes
mechanical lasing with relaxed requirements for the optical-mechanical modes
and their inter-coupling. It derives from a spontaneously triggered
thermal/free carrier self-pulsing and the anharmonic modulation of the
radiation pressure force that comes as a consequence. Moreover, the feedback of
the mechanics on the self-pulsing frequency-entrains both oscillators, creating
a self-stabilized indecomposable system. A manifold of frequency-entrained
regions with two different mechanical modes (at 54 and 122 MHz) are observed as
a result of the wide tuneability of the natural frequency of the self-pulsing.
The system operates at ambient conditions of pressure and temperature in a
silicon compatible platform, which enables its exploitation in sensing,
intra-chip metrology or time-keeping applications.
