Loss investigations of high frequency lithium niobate Lamb wave resonators at ultralow temperatures

Abstract

Lamb wave resonators (LWRs) operating at ultralow temperatures serve as promising acoustic platforms for implementing microwave-optical transduction and radio frequency (RF) front-ends in aerospace communications because of the exceptional electromechanical coupling (k^2) and frequency scalability. However, the properties of LWRs at cryogenic temperatures have not been well understood yet. Herein, we experimentally investigate the temperature dependence of the quality factor and resonant frequency in higher order antisymmetric LWRs down to millikelvin temperatures. The high-frequency A1 and A3 mode resonators with spurious-free responses are comprehensively designed, fabricated, and characterized. The quality factors of A1 modes gradually increase upon cryogenic cooling and shows 4 times higher than the room temperature value, while A3 mode resonators exhibit a non-monotonic temperature dependence. Our findings provide new insights into loss mechanisms of cryogenic LWRs, paving the way to strong-coupling quantum acoustodynamics and next-generation satellite wireless communications.

Figures (5)

• Figure 1: (a) The simulated resonant frequency of A1 and A3 modes as a function of LN film thickness, respectively. The solid and dash lines represent the resonant frequency for electrically open and short boundaries, respectively. The insets illustrate the cross-sectional view of the displacement mode shapes of A1 and A3 modes. (b) The simulated $k^2$ with respect to $\lambda\textsubscript{L}$ for A1 and A3 modes.
• Figure 2: (a) The simulated frequency dependence of admittance for the conventional and recessed electrode design. (b) Displacement profiles at the resonant frequency for three designs. (c) Fabrication error analyses of the etching depth for recessed electrodes. (d) The admittance spectrum of the A3 mode for various electrode metals. The displacement profiles for A1 and A3 modes with Al electrodes are shown in the inset, respectively.
• Figure 3: Fabrication process and characterization of LWRs. (a) Schematic of the six-step fabrication process: (i) Initial LN-on-Si wafer thinning via IBE; (ii) Boundary definition through photoresist patterning and IBE etching; (iii) Recessed area patterning with photoresist; (iv) Shallow etching (70 nm depth) using IBE; (v) Al electrode deposition via sputtering and lift-off; (vi) Final Si substrate release using XeF$_2$ etching. (b) SEM image of the fabricated resonator structure. Electrical characterization of $S_{11}$ parameters and $Y_{11}$ admittance for (c) A1 mode and (d) A3 mode resonators at room temperature using GSG probes.
• Figure 4: (a) Schematic diagram of low-temperature measurement setup for LWRs. (b) Measured magnitude of $S_{11}$ versus frequency for the A1 resonator at 300 K and 16 mK, respectively. (c) Extracted $Q\textsubscript{3dB}$ and resonant frequency $f\textsubscript{r}$ as a function of temperature. The inset plots $Q\textsubscript{3dB}$ as a function of on-chip input power at 16 mK. The error bars denote the variations from multiple measurements.
• Figure 5: Magnitude (a) and phase (b) responses of the A3 resonator at 12 mK. The experimental and fitted reflection data $S_{11}$ are represented as symbols and solid lines, respectively. (c) The normalized $\mid$$S_{11}$$\mid$ at various temperatures across $T\textsubscript{c}$ of Al electrodes, along with the corresponding Lorentz fittings represented by the solid lines. (d) The temperature revolution of $Q\textsubscript{i}$ and $f\textsubscript{r}$ of the A3 resonator. The error bars denote the variations from multiple measurements and fitting errors.