Surface and Bulk Two-Level System Losses in Lithium Niobate Acoustic Resonators

TL;DR

This study investigates surface and bulk two-level system losses in lithium niobate acoustic resonators, revealing that bulk TLS limits performance in BAW devices and surface TLS dominates in surface-sensitive devices, guiding future improvements.

Contribution

It provides the first quantitative separation of surface and bulk TLS losses in lithium niobate resonators, establishing bounds and insights for optimizing device performance.

Findings

High-quality resonators with Q up to 6×10^7 achieved.

Bulk TLS limits performance in BAW resonators.

Surface treatments do not affect BAW resonator quality.

Abstract

Lithium niobate (LN) is a promising material for building acoustic resonators used in quantum applications, but its performance is limited by poorly understood material defects called two-level systems (TLS). In this work, we fabricate high-performance acoustic resonators from LN with quality factors up to $6\times10^7$ and use them to separate bulk and surface contributions to TLS loss. By comparing these bulk acoustic wave (BAW) resonators with previous surface acoustic wave and phononic crystal studies, we show that devices with high surface participation ratios are limited by surface TLS, while our BAW devices reveal an intrinsic bulk TLS limit. Through systematic surface treatments and microscopy, we demonstrate that BAW resonator performance remains unchanged despite surface modifications, confirming operation in a bulk-limited regime. Our work establishes quantitative bounds on both surface and bulk TLS losses in LN, within the context of material growth and fabrication approaches we have pursued, and provides guidance for future device engineering and materials development.