  We photoionize laser-cooled atoms with a laser beam possessing spatially
periodic intensity modulations to create ultracold neutral plasmas with
controlled density perturbations. Laser-induced fluorescence imaging reveals
that the density perturbations oscillate in space and time, and the dispersion
relation of the oscillations matches that of ion acoustic waves, which are
long-wavelength, electrostatic, density waves.

Plasma Physics;Atomic Physics