Intensity Modulation of Two Weakly Coupled Stimulated Oscillating Mechanical Modes in an Optomechanical Microbubble Resonator

Abstract

We report that when two stimulating mechanical modes in an optomechanical microbubble resonator are weakly coupled to each other, strong oscillation intensity modulation occurs. The modulation was theoretically expected and experimentally observed. We theoretically derived the expressions of the coupling coefficient between the mechanical modes and calculated the region where weak coupling happens. We found that weak coupling exists when the optical quality factor of the microcavity is high and the detuning of the pump laser is close to the beat frequency of the two mechanical modes. Experimentally, we observed that when two mechanical modes are both in stimulated oscillation, they undergo strong intensity modulation as the optical pump power changes, and the coupling coefficient of the two modes is estimated to be 0.962 at the pump power when one mode is in the stimulated region and the other mode is at a stimulating threshold; this proves that the two mechanical modes are weakly coupled. Our results extended the weak coupling conclusion in multimode laser emission to stimulated oscillation in multiple mechanical modes.