Cryogenic Characterization of Low-Loss Thin-Film Lithium Niobate on Sapphire Shear Horizontal Surface Acoustic Wave Devices

This article reports a cryogenic study on wideband shear horizontal surface acoustic wave (SH-SAW) devices based on an emerging Y-cut $\mathrm{LiNbO}_3 / \mathrm{SiO}_2 /$ sapphire lithium niobate on sapphire (LNOS) platform. To perform a comprehensive study, low-loss acoustic delay lines (ADLs) equipped with unidirectional transducers were designed with a wavelength ( $\lambda$ ) of $4 \mu \mathrm{~m}(900 \mathrm{MHz}$ ) and a wide fractional bandwidth (FBW) of around $7 \%$ , featuring various physical delays ranging from $5 \lambda$ to $200 \lambda$ as testing structures. By cooling the temperature down to 5 K, the insertion loss (IL) of the longest ADL and the extracted propagation loss (PL) were characterized as $4.1 \mathrm{~dB} / \mathrm{mm}$ and $3.5 \mathrm{~dB} / \mathrm{mm}$ , respectively. Compared with an IL of 5.78 dB and a PL of $4.37 \mathrm{~dB} / \mathrm{mm}$ at 275 K, the temperature-dependent acoustic losses diminish at low temperatures, with the overall PL dominated by the acoustic waveguide formed by the acoustic velocity mismatch between layers. Furthermore, a one-port resonator $(\lambda=2.8 \mu \mathrm{~m})$ with a large perceived effective electromechanical coupling greater than $40 \%$ was also characterized using the same technique, showing a $2 x$ boost in the maximum Bode- $Q$ at cryogenic temperatures. This study not only characterized the acoustic properties of wideband LNOS SH-SAW devices but also validated their excellent performance across a wide temperature range, suggesting their potential applications in cryogenic phononic circuits.