Level crossing and anti-crossing of optomechanical modes in high- Q microbubble optical resonators

Level crossing and anti-crossing are fundamental phenomena when two resonances are coupled. However, research on crossing and anti-crossing of two mechanical resonances in an optomechanical system is rarely reported. Here we report experimental demonstration of crossing and anti-crossing of mechanical resonances in high-quality-factor optical microbubble resonators (MBRs). The optomechanical resonances are manipulated by changing the density and acoustic velocity of aqueous silver nitrate solutions flowing inside the bubble of the MBR. Mechanical mode crossing was observed between the azimuthal order M =5 wineglass mode and the azimuthal order M =0 breathing mode in the microcavity, while significant mode anti-crossing was observed between the M =1 string-like mechanical mode and multiple M =0 breathing modes. Numerical simulations confirm that anti-crossing occurs when mechanical modes are strongly coupled.