Buckling-mediated phase transitions in nano-electromechanical phononic waveguides

Acoustic signals in the megahertz to gigahertz regimes enable applications in phononic, electric and photonic circuitry. Hence, advancement in these multi-physics circuitries requires studying the transmission of elastic waves in micro/nano-electromechanical systems. Therefore, our work investigated the transmission of flexural waves in unidirectional phononic waveguides of coupled nano-electromechanical drumhead resonators. In particular, we studied the manipulation and tailoring of the transmission by thermal control of buckling within the waveguide. We showed—via experimental and computational evidence—that buckling tunes the frequency of transmission within the waveguide as determined by previous studies in literature. However, in our work, we demonstrated that excessive buckling stops the transmission in the fundamental (lowest frequency) passband of the waveguide, thus, inducing a phase transition in the waveguide leading to the absence of transmission in the respective frequency range. We computationally proved that this buckling-mediated phase transition necessitates the presence of weak inhomogeneities (∼5% variations) within the waveguide, which are inevitable in any fabrication process. In fact, these inhomogeneities are amplified due to buckling-residual stresses leading to structural aperiodicity that only affects the fundamental passband by converting the extended shapes of the transmission modes into localized shapes that prevent signal propagating throughout the waveguide.