Phononic Combs in Lithium Niobate Acoustic Resonators

TL;DR

This paper demonstrates microwave frequency comb generation using thermal nonlinearity in high-Q lithium niobate acoustic resonators, highlighting potential for compact, efficient comb sources in microwave applications.

Contribution

It introduces a novel method of generating microwave frequency combs via thermal nonlinearity in lithium niobate acoustic resonators, expanding the scope of comb technology.

Findings

Comb spacing varies with drive frequency and power

Comb generation relies on nonlinear mixing regimes

Thermal nonlinearity enables efficient comb generation

Abstract

Frequency combs consist of a spectrum of evenly spaced spectral lines. Optical frequency combs enable technologies ranging from timing, LiDAR, and ultra-stable signal sources. Microwave frequency combs are analogous to optical frequency combs, but often leverage electronic nonlinearity for comb generation. Generating microwave frequency combs using piezoelectric mechanical resonators would enable this behavior in a more compact form factor, thanks to the shorter acoustic wavelengths. In this work, we demonstrate a microwave frequency comb leveraging thermal nonlinearity in high quality factor ($Q$) overmoded acoustic resonators in thin film lithium niobate. By providing input power at 257 MHz, which is the sum frequency of two acoustic modes at 86 MHz and 171 MHz, we generate parametric down conversion and comb generation. We explore the nonlinear mixing regimes and the associated conditions for comb generation. Comb spacing is observed to vary significantly with drive frequency and power, and its general behavior is found to rely heavily on initial conditions. This demonstration showcases the potential for further improvement in compact and efficient microwave frequency combs, leveraging nonlinear acoustic resonators.