Abstract
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.