If the SH-wave is excited and detected through two IDTs separated by a free surface from each other, then the wave is SSBW . In this case the wave energy is radiated under angle θ with respect to the surface and diffracts into the bulk of the crystal. This results in an increased insertion loss especially if the IDTs are on large distance from each other. However, if a metal strip grating with an equal periodicity as the IDTs is deposited between them, it slows down the wave due to the reflection in each metal strip. Thus, the k-vector becomes parallel to the surface, trapping the acoustic energy and preventing its diffraction into the bulk. Due to the confinement of most of the wave energy adjacent to the surface, these type sensor devices are characterized with higher sensitivity compared to their QCM and SAW counterparts. By changing the metal strip grating with thin solid film, the wave can be transformed from STW to Love wave. The latter occurs only if the wave velocity in the layer is lower than in the substrate.

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