  We investigate the electromechanical actuation of a pair of suspended silicon
nitride membranes forming a monolithic optomechanical array. By controlling the
membrane resonators' tensile stress via a piezoelectrically controlled
compressive force applied to the membrane chip we demonstrate noninvasive
tuning of their mechanical mode spectrum, as well as strong intermode
electromechanical coupling. Piezoelectric actuation is also shown to enhance
the nonlinear response of the membranes, which is evidenced either by
parametric amplification of the fundamental mode thermal fluctuations or by
resonant driving of these modes into high amplitude states. Such an
electro-optomechanical membrane array represents an attractive tunable and
versatile platform for sensing, photonics and optomechanics applications.
