Direct measurement of the out-of-plane (optical) phonon spectrum in a 2D complex plasma crystal

The out-of-plane phonon spectrum of a 2D complex plasma crystal in the sheath of a RF discharge was measured using a novel imaging technique that allowed us to resolve the particle motion in the 3 dimensions. The out-of-plane mode was proven to have negative optical dispersion, as predicted a long time ago. Comparison with theory showed a remarkably good agreement. The effect of the plasma wakes on the dispersion relation is discussed.