Sub-GHz Optical Pulsing using a Thermally Generated Heterostructure with Strong Optomechanical Coupling

Thermal engineering can be used to exploit absorption in a silicon optical cavity. In this work, the steady state profile of the heat generated by absorption is shaped and used to generate a dynamic heterostructure in a weakly confined silicon optical cavity. This is demonstrated in an edge defect photonic crystal optomechanical cavity to produce phonon lasing and sub-GHz optical pulsing with photon-phonon cooperativity of 0.088. It is typically challenging to meet the conditions for phonon lasing. The cooperativity must be at least unity, and the cavity operated in the optomechanical sideband resolved regime. Here, our thermal design uses absorption-generated heat to modify the refractive index of the cavity and dynamically form a heterostructure, compressing the optical mode volume and relaxing the constraints on the optical quality factor, mechanical quality factor, and threshold power. The compressed mode then couples to a thermo-optical/free-carrier-dispersion limit cycle, resonantly exciting the optomechanical cavity. While edge defects have been shown to have high optomechanical sensitivity, the cavity lacks sufficient mode confinement to generate the limit cycle without the thermal heterostructure. The formation of the heterostructure results in phonon lasing and sharp optical pulsing at 30 MHz. These results demonstrate the novel use of thermal engineering to initiate phonon lasing with further improvements leading to a fully integrated, sub-GHz optical frequency comb.