Side radiation of Rayleigh waves from synchronous leaky SAW resonators

Scanning laser probing of leaky wave synchronous resonators used in "ladder" SAW filters fabricated on lithium tantalate often reveals the presence of distinct acoustic beams radiated outside the resonator from the transducer region. One of the beams is radiated in the direction perpendicular to the resonator, i.e., parallel to the transducer fingers, while two others are directed obliquely, and symmetrically with respect to the first beam, at an angle of about 60 degrees. We suppose that this radiation is generated by the charges accumulating at the tips of the electrodes. Since the velocity of the "leaky SAW" is visibly higher than that of a Rayleigh wave, the synchronism conditions can be met and the periodic system of sources (oscillating charges at the fingertips) generates the beams in the perpendicular direction, as well as the ±1st" diffraction orders resulting in the two obliquely propagating beams. The coupling strength and slowness of the Rayleigh SAW on 36° and 42° cut LiTaO₃ substrates vs. propagation direction is computed using 3D Green\´s function theory and the propagation angles for the Rayleigh wave beams as a function of frequency are predicted. The computed angles compare well with experiments.