Observation of mechanical bound states in the continuum in an optomechanical microresonator.

Bound states in the continuum (BICs) are a type of waves that are perfectly confined in the continuous spectrum of radiating waves without interaction with them. Here, we fabricated, with CMOS-compatible processes on a silicon chip, a wheel-shaped optomechanical microresonator, in which we experimentally observed the BIC in the micromechanical domain. The BIC results from destructive interference between two dissipative mechanical modes of the microresonator under broken azimuthal symmetry. Such BICs can be obtained from devices with large and robust supporting structures with variable sizes, which substantially reduces fabrication difficulty and allows for versatile application environments. Our results open a new way of phonon trapping in micromechanical structures with dissipation channels, and produce long phonon lifetimes that are desired in many mechanical applications such as mechanical oscillators, sensors, and quantum information processors.