Membrane phononic crystals for high-Qm mechanical defect modes at MHz frequencies in piezoelectric aluminum nitride

Abstract

Nanomechanical resonators with exceptionally low dissipation are advancing mechanics-based sensors and quantum technologies. The key for these advances is the engineering of localized phononic modes that are well-isolated from the environment, i.e., that exhibit a high mechanical quality factor, Qm. Membrane phononic crystals fabricated from strained thin films can realize high-Qm single or multiple localized phononic defect modes at MHz frequencies. These defect modes can be efficiently interfaced with out-of-plane light or coupled to a microwave quantum circuit, enabling readout and control of their motion. When membrane phononic crystals are fabricated from a crystalline film, they could offer built-in functionality. We demonstrate a membrane phononic crystal realized in a strained 90 nm thin film of aluminum nitride (AlN), which is a crystalline piezoelectric material. We engineer a high-Qm localized phononic defect mode at 1.8 MHz with a Qm×fm-product of 1.5×1013 Hz at room temperature. In future devices, the built-in piezoelectricity of AlN can be utilized for direct coupling to qubits or in situ tuning of mechanical mode frequencies, defect mode couplings, or acoustic bandgaps, which can be used as building blocks of tunable phononic circuits or low-noise sensors.