  Radio frequency (RF) acoustic resonators have long been used for signal
processing and sensing. Devices that integrate acoustic resonators benefit from
their slow phase velocity (vp), in the order of 3 to 10 km/s, which allows
miniaturization of the device. Regarding the subject of small form factor,
acoustic resonators that operate at the so-called fundamental antisymmetric
mode (A0), feature even slower vp (1 to 3 km/s), which allows for smaller
devices. This work reports the design and fabrication of A0 mode resonators
leveraging the advantages of periodically poled piezoelectricity (P3F) lithium
niobate, which includes a pair of piezoelectric layers with opposite
polarizations to mitigate the charge cancellation arising from opposite stress
of A0 in the top and bottom piezoelectric layers. The fabricated device shows a
quality factor (Q) of 800 and an electromechanical coupling (k2) of 3.29,
resulting in a high figure of merit (FoM, Q times k2) of 26.3 at the resonant
frequency of 294 MHz, demonstrating the first efficient A0 device in P3F
platforms. The proposed A0 platform could enable miniature signal processing,
sensing, and ultrasound transducer applications upon optimization.
