  We report an optomechanical system with two dielectric membranes inside a
Fabry-Perot cavity. The cavity resonant frequencies are measured in such a
two-membrane-in-the-middle system, which show an interesting
band-structure-like diagram. This system exhibits great controllability on the
parameters of the system. The positions and angles of each membrane can be
manipulated on demand by placing two membranes inside the cavity separately.
The eigenfrequencies of the vibrational modes of the membranes can also be
tuned individually with piezoelectricity. This scheme could be
straightforwardly extended to multiple-membrane-in-the-middle systems, where
more than two membranes are involved. Such a well controllable multiple
membrane optomechanical system provides a promising platform for studying
nonlinear and quantum dynamical phenomena in multimode optomechanics with
distinct mechanical oscillators.
