Enhancing Mechanical Quality Factors of Micro-Toroidal Optomechanical Resonators Using Phononic Crystals

Fabrication of efficient optomechanical resonators with high mechanical Q-factors without degrading their optical Q -factors or reducing effective mass is challenging. One of the limiting sources of mechanical loss for silica micro-toroids is the clamping loss. Previous efforts on reducing this loss were focused on reducing the connectivity between the toroid and the pillar, which results in reduced effective mass and degraded phase noise. We explore a new approach for reducing the mechanical loss that relies on phononic crystals (PnCs). Since it is based on the geometrical modification of substrate, it does not affect the optical Q-factor or effective mass. First, we experimentally verify the performance of the proposed PnC patterns without a micro-toroid by measuring their acoustic transmission properties. This experiment is used to verify the effectiveness of the PnCs for blocking outgoing acoustic waves and our numerical model. Then, using the same numerical model, we study the effects of PnCs on the mechanical Q-factors of a micro-toroid previously investigated. Our results show that the PnCs improve the mechanical Q-factor about 150%. This technique can be used in conjunction with other previously demonstrated methods for the enhancement of the Q-factor for even greater improvements.