  This work reports a 50.74 GHz lithium niobate (LiNbO3) acoustic resonator
with a high quality factor (Q) of 237 and an electromechanical coupling (k2) of
5.17% resulting in a figure of merit (FoM, Q x k2) of 12.2. The LiNbO3
resonator employs a novel bilayer periodically poled piezoelectric film (P3F)
128 Y-cut LiNbO3 on amorphous silicon (a-Si) on sapphire stack to achieve low
losses and high coupling at millimeter wave (mm-wave). The device also shows a
Q of 159, k2 of 65.06%, and FoM of 103.4 for the 16.99 GHz tone. This result
shows promising prospects of P3F LiNbO3 towards mm-wave front-end filters.
