  This work proposes a double-layer thin-film lithium niobate (LiNbO3)
longitudinally excited shear wave resonator with a theoretical
electromechanical coupling coefficient exceeding 60%, RaR close to 28%, and no
spurious modes. This ultra-large electromechanical coupling coefficient, which
is close to the upper limit of LiNbO3, is much larger than all microwave
acoustic resonators reported so far. Based on X-cut thin-film LiNbO3, when the
film thickness is in the order of hundreds of nanometers, the frequency of the
fundamental mode of the resonator can cover 1GHz to10GHz. The resonator design
is convenient and flexible. The resonant frequency can be modulated
monotonically by changing either the electrode or the thickness of the
thin-film LiNbO3 without introducing additional spurious modes. This ideal
resonator architecture is also applicable to LiTaO3. With the development of
the new generation of mobile communications, this resonator is expected to
become a key solution for future high-performance, ultra-wide-bandwidth
acoustic filters.
