LGX pure shear horizontal SAW for liquid sensor applications

Reports theoretical and experimental properties of the shear horizontal (SH) mode for the LGX family of crystals, which includes langasite (LGS), langanite (LGN), and langatate (LGT). These crystals are. of the trigonal class 32 group, as quartz, and they exhibit the shear horizontal (SH) symmetry type uncoupling for the Euler angles [0° θ 90°]. This surface acoustic mode, also know as surface transverse wave (STW) is especially attractive for liquid sensing, because the pure horizontal particle polarization characteristic of this wave, with the material particle motion parallel to the surface, is not as severely damped as observed for the general polarized SAW or pure sagittal Rayleigh waves, which have a significant polarization component normal to the surface. The results focus on one of the materials from the LGX family, LGT. The experimental data uncovers a zero temperature coefficient of delay (TCD) propagation direction around 140°C on the LGT crystal. Liquid loading experiments with both water and photoresist at the surface of SH wave resonators and delay lines along that orientation have been carried out, showing the expected moderate attenuation of the SH mode.