A self-stabilized coherent phonon source driven by optical forces

[EN] We report a novel injection scheme that allows for phonon lasing in a one-dimensional optomechanical photonic crystal, in a sideband unresolved regime and with cooperativity values as low as 10-2. It extracts energy from a cw infrared laser source and is based on the triggering of a thermooptical/free-carrier-dispersion self-pulsing limit-cycle, which anharmonically modulates the radiation pressure force. The large amplitude of the coherent mechanical motion acts as a feedback that stabilizes and entrains the self-pulsing oscillations to simple fractions of the mechanical frequency. 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 platform, which enables its exploitation in sensing, intra-chip metrology or time-keeping applications.