Multi-resonance transduction near acoustic Brillouin zone in microscale ferroelectric

Abstract

Excitation of the acoustic plate waves (PAW) in ferroelectric wafers with microscale domains and nanoscale interdomain walls is investigated theoretically and experimentally. The periodically poled structures were fabricated in the Z-cut 0.5-mm-thick LiNbO3 wafers. Rf-current applied along the X-axis generates PAW in the fabricated multidomain acoustic superlattices having stop-bands and acoustic Brillouin zones. Two main effects are observed. First, a frequency of maximum acoustic amplitude does not coincide with the domain resonance frequency when a half-wavelength equal to domain length. Second, instead of known single-frequency domain resonance such as in bulk crystals, the series of two or more transduction resonances do exist. The theory, simulation, and experiments allow concluding that the transduction multi-resonances occur due to ultrasound diffraction by the interdomain nano-walls when the acoustic wavelength is close to multidomain period; it happens near acoustic Brillouin zone boundaries. Since different PAW-modes may be excited, the transduction multi-resonances appear at different frequencies. For example, 300-micron multi-domain structure in LiNbO3 demonstrates multiple transduction peaks in the frequency range of 5.6 to 8.3 MHz, just for lowest four symmetric and antisymmetric PAW modes. The experimental results are in agreement with theory. The findings may be applied for designing new MEMS and therapeutic ultrasonic transducers.