X-shaped phononic crystals for side lobe reduction in ultra high frequency slanted finger inter-digitated aluminum nitride lamb wave transducers

This paper reports, for the first time, an approach to suppress the side lobe magnitude of aluminum nitride (AlN) Lamb wave slanted finger interdigital transducer (SFIT) delay lines by engineering and integrating an X-shaped phononic crystal (X-PnC) in the structure. The X-PnC generates two frequency bandgaps that sandwich the delay line pass-band around 780 MHz. The acoustic isolation provided by these two frequency bandgaps for longitudinal modes results in a side lobe reduction of 10 dB. The combined result of the X-PnC and the delay line displays an unaltered bandwidth similar to the standalone delay line with a minimum 23 dB of insertion loss and a smooth (almost flat) out-of-band response. These preliminary results demonstrate the feasibility of integrating PnCs with radio frequency electroacoustic devices, and especially show a practical utilization of micromachined PnCs.