Plate mode propagation losses in solidly mounted resonators

This paper investigates the acoustic losses of propagating eigenmodes through the acoustic mirror of a solidly mounted resonator (SMR), aiming to clarify how resonator properties are influenced by reflection coefficients for the thickness shear (TS) wave as well as that for the thickness extensional (TE) wave. To this end, we analyze the effective acoustic admittance for several test structures with different mirror properties. Leaky modes are distinguished from plate-like modes and the propagation losses are quantified by calculating mode quality factors. The dependence of the propagation properties of leaky eigenmodes is compared to the mirror properties in terms of bulk wave transmission coefficients obtained by the one-dimensional Mason's model. It is shown that the TE-like main mode couples with TS-like spurious modes, which then influences the leaky loss of the main mode as well. The coupling strength is strongly frequency dependent and drastically changes with the mirror design. This result explains previous experimental results reported on SMR design.