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Phononic Combs in Lithium Niobate Acoustic Resonators

Ian Anderson, Jack Kramer, Tzu-Hsuan Hsu, Yinan Wang, Vakhtang Chulukhadze, Ruochen Lu

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.

Phononic Combs in Lithium Niobate Acoustic Resonators

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 () 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.
Paper Structure (5 sections, 1 equation, 4 figures)

This paper contains 5 sections, 1 equation, 4 figures.

Table of Contents

  1. Author Declarations
  2. Data Availability Statement
  3. Section

Figures (4)

  • Figure 1: Device overview detailing design and operation. (a) 3D model and optical image of device highlighting the resonance conditions and directions. (b) COMSOL FEA simulations highlighting the high Q modes that are used for our devices. (c) Measured admittance response of device, with the specific modes again highlighted. (d) schematic of different regimes of driving for comb generation, consisting of linear, parametric down-conversion, and then comb regime.
  • Figure 2: (a) Chosen device driven with high power, showing only duffing nonlinearity. (b) Same device that exhibits complex behavior of resonance bending due to mode coupling for driven comb resonance. (c) Wide spectrum analysis of device driven into parametric down-conversion with $f_d$ = 256.435 MHz and $P_d$ = 12.5 dBm. (d-f) Zoom in images of each tone when driven into comb regime, showing (d) lower, (e) middle, and (f) upper tones with $f_d$ = 257.32 MHz and $P_d$ = 16.5 dBm.
  • Figure 3: Different comb regimes for high power driving, with spectrum centered at $f_0$ = 171.25 MHz and $P_d$ = 16.5 dBm. Featured regimes are no mixing, parametric down-conversion, comb, and "sech" regimes.
  • Figure 4: (top) Heatmap featuring the spectrum at different drive powers, paired with the comb spacing at each of those powers, highlighting different regimes based on different spacings. (bottom) Heatmap featuring spectrum against drive frequency, highlighting how the behavior change will typically depend on changes in the admittance plot.