Surface Transverse Wave (STW) resonators on langasite

The Surface Transverse Waves (STW) cut of langasite (LGS), Euler angles (0°, 22°, 90°), provides temperature stable delay of STW with coupling coefficient a few times larger than that of 36° rotated Y-cut of quartz which also supports STW. It was expected that, as in the case of quartz, one can get excellent Q-factor resonators based on low loss shear surface acoustic waves (STW). Such resonators are in great demand for high temperature sensors and for this application Platinum (Pt) electrodes are necessary. However, recent attempts to develop such resonators resulted in mediocre Q factors of the order of 1000 rather than the expected Q>10000 in the 434 MHz ISM frequency range. In this paper we show that for a thickness of Pt (h/λ close to 2%) on this cut of LGS there is no wave-guiding, which means that the STW energy is radiated into the busbars. This effect can be explained by the concave shape of the slowness curve for SAW in the bus-bar area due to a large increase in the wave velocity with deviation of the propagation direction from 0Z axis. The use of thicker Pt layers, (yielding a relative thickness h/λ in excess of 3%) makes the phase velocity of STW in the periodic grating at resonance drop below the wave velocity of all types of waves propagating under the bus-bars due to the high reflectivity of the electrodes, establishing the usual wave-guiding conditions.