Fabry-Perot optical cavity inspires new surface acoustic wave resonator

Surface acoustic wave (SAW) devices are widely used in analog signal processing, wireless communications, and sensing applications. The creation of a low-loss SAW resonator remains difficult due to insufficient lateral confinement of acoustic waves and subsequent leaking of acoustic energy.

A new study published in Applied Physics Letters borrows principles of design from optical resonators to create a high-quality factor Fabry-Perot SAW resonator. The device uses two tapered phononic crystal mirrors, patterned on a gallium nitride (GaN)-on-sapphire material platform to confine the SAW as it bounces between them. The resulting acoustic resonator has high confinement and a miniaturized footprint.

To mimic a traditional Fabry-Perot cavity from optics, the authors first determined that SAWs could be well-confined by a pair of acoustic mirrors patterned with phononic band gap structures that resemble photonic crystals. They performed a finite element method simulation to find the phononic mirror geometry for optimized acoustic wave reflection and confinement. They then fabricated the devices on a GaN-on-sapphire material.

During characterization experiments, the Fabry-Perot SAW resonator demonstrated further advantages beyond its small on-chip footprint and high quality factor. The authors used an electrical network analyzer and optical vibrometer to measure standing Rayleigh waves inside the cavity that had an intrinsic quality factor exceeding 13,000 — a number comparable to GaN microelectromechanical (MEMS) systems. The Fabry-Perot SAW resonators, however, have the advantage of lower fabrication complexity and no fragile moving parts.

The same approach could feasibly be extended to other frequencies, which may result in an even higher quality factor with the addition of cryogenic temperatures or alternative materials.

Source: “High quality factor surface Fabry-Perot cavity of acoustic waves,” by Yuntao Xu, Wei Fu, Chang-ling Zou, Zhen Shen, and Hong X. Tang, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5013161 .