Piston mode generation in thin plate lamb wave device

An acoustic wave resonator comprising a plurality of interdigital transducer (IDT) electrodes disposed on upper and lower sides of a piezoelectric film, the IDT electrodes on the upper side of the piezoelectric film being offset from the IDT electrodes on the lower side of the piezoelectric film by λ/4, λ being a wavelength of a main acoustic wave generated by the acoustic wave resonator to enable the acoustic wave resonator to generate piston mode acoustic waves responsive to electrical excitation of the plurality of IDT electrodes with an alternating current, the plurality of IDT electrodes being electrically coupled to a frequency generator and phase shifter that causes a phase of an excitation voltage applied to the IDT electrodes on the upper side of the piezoelectric film to be phase shifted from an excitation voltage applied to the IDT electrodes on the lower side of the piezoelectric film by greater than zero degrees and up to 90°.