Experimental elastic wave control in a piezoelectric phononic crystal with spatio-temporal modulation of electrical conditions

This work concerns the propagation of elastic waves in a piezoelectric phononic crystal made of several identical piezoelectric elements separated by electrodes. In such a structure, electrical conditions prescribed to the electrodes can be modulated in space and in time and therefore can strongly modify the dispersion curves of the phononic crystal. For example, a periodic grounding of the electrodes introduces a Bragg band gap which does not exist when the electrodes have a floating potential condition [Degraeve et al., J. Appl. Phys. 115, (2014)]. When the electrical conditions of the electrodes are modulated in space and in time, dispersion curves are tilted with respect to the configuration in the reference medium and nonreciprocal wave propagation may occur at some modulation speeds in specific frequency bands [Croënne et al., Appl. Phys. Lett. 110 , (2017)]. The experimental set-up is based on a stack of 113 piezoelectric rings and 114 electrodes. The spatio-temporal modulation of the electrical conditions is performed using a digital signal processor (DSP) in order to reach high modulation speeds. This modulation is performed by shifting the position of the periodic ground conditions of the electrodes versus time. The experimental dispersion curves are obtained using measurements of the normal displacement of the piezoelectric rings with a scanning laser vibrometer. The evolution of dispersion curves as a function of modulation speed is analyzed and compared to finite element simulation results.