DEPARTMENT OF PHYSICS DISSERTATION: Rachel Gruenke-Freudenstein

Ph.D. Candidate:  Rachel Gruenke-Freudenstein

Research Advisor:  Amir Safavi-Naeini

Date: November 15th Time: 9:00 AM

Location: Spilker 232

Zoom Link: https://stanford.zoom.us/j/93661618261

Zoom Password: Email physicsstudentservices [at] stanford.edu (physicsstudentservices[at]stanford[dot]edu) for password

Title: Understanding Mechanical Dissipation at Cryogenic Temperatures in Lithium Niobate using Ultra-Low Loss Acoustic Resonators

Abstract: Lithium niobate is a crystalline material that is strongly piezoelectric, making it an ideal platform for low loss acoustic resonators. These resonators have promising applications in quantum computing as storage cavities or quantum RAM, as well as quantum sensors or transducers. However, any imperfections in the crystal structure introduced in crystal growth or during device fabrication limit the usefulness of these devices by reducing the coherence times of mechanical excitations or phonons. At the cryogenic temperatures where these sensors or storage cavities are operated, the dominating sources of loss are two-level systems (TLS). Efforts to discover the microscopic nature of these losses have been extensive in recent years across many materials, yet a full microscopic model of TLS losses, especially as they arise in LN, has not yet been realized.In this work, we design and measure a series of high quality factor acoustic resonators, namely surface acoustic wave and bulk acoustic wave resonators. By observing the temperature dependent resonance frequencies and power saturable losses in these resonators, we extract the loss tangents of the two-level system losses in resonators with very different strain profiles. We therefore observe the surface and bulk participation of TLS losses in fabricated lithium niobate devices. We also can modify the LN devices with various surface treatments and observe modified TLS losses alongside changes in the material properties. The material is characterized through a series of metrology measurements including atomic force microscopy, x-ray photoelectron spectroscopy, and transmission electron microscopy. From these device and material studies, we present a fuller picture of the sources for TLS in LN and a roadmap of how to investigate and improve the TLS losses in future acoustic devices.