Multimode Phonon Lasing via Self‐Induced Optical Coherent Pumping

Multimode phonon lasing in cavity optomechanical systems enables the generation of coherent, multi‐frequency mechanical signals with potential applications in RF signal processing, frequency combs, and sensing. However, achieving simultaneous self‐sustained oscillations in multiple mechanical modes is often hindered by strong mode competition. Here, we demonstrate robust multimode phonon lasing in a silicon optomechanical nanobeam, driven by a self‐pulsing (SP) mechanism originating from free‐carrier dispersion and thermo‐optic effects. Unlike previous approaches that rely on external pump modulation or the use of widely spectrally separated modes to mitigate the suppression of self‐oscillations, our system supports lasing of two strongly competing in‐plane flexural modes of the nanobeam under continuous‐wave pumping. The SP cycle dynamically adapts to the mechanical perturbations, providing synchronized optical pumping even in the absence of harmonic frequency relations. This results in stable multimode operation across periodic, quasi‐periodic, and chaotic regimes, all accurately predicted by numerical modeling. These findings establish SP as a passive and self‐organized mechanism for coherent multi‐frequency phonon generation in chip‐scale platforms.