  A classical dusty plasma experiment was performed using two different dust
grain sizes to form a strongly coupled asymmetric bilayer (two closely spaced
interacting monolayers) of two species of charged dust particles. The
observation and analysis of the thermally excited particle oscillations
revealed the collective mode structure and wave dispersion in this system; in
particular the existence of the theoretically predicted $k=0$ energy
(frequency) gap was verified. Equilibrium molecular dynamics simulations were
performed to emulate the experiment, assuming Yukawa type inter-particle
interaction. The simulations and analytic calculations based both on lattice
summation and on the QLCA approach are in good agreement with the experimental
findings and help identifying and characterizing the observed phenomena.
