Layer guided shear horizontal acoustic plate mode sensors

A dispersion equation is derived for shear horizontally polarized acoustic waves propagating on a finite thickness substrate covered by a finite thickness guiding layer with a shear acoustic speed less than that of the substrate. It is shown that the equation has two types of solution corresponding to traditional Love waves and to shear horizontally polarized acoustic plate modes (SH-APMs). Numerical calculations show that each Love wave mode has an associated spectrum of layer-guided acoustic plate modes and that higher order Love wave modes are continuations of the layer-guided SH-APMs. Further, it is shown that the mass sensitivity is related to the slope of the dispersion equation and that layer-guided SH-APMs have far higher mass sensitivity than non-layer-guided SH-APMs. Sensors based on these layer-guided SH-APM modes may have advantages for liquid phase operation because the mode can be excited using the opposite face to that used for sensing. Preliminary experimental results are presented for the dispersion curve of layer-guided SH-APMs. The effect of guiding layer thickness, high and multiple frequency operation and dispersion is discussed for Love waves on a (semi-) infinite substrate.