  We report the observation of harmonic generation and strong nonlinear
coupling of two collective modes of a condensed gas of rubidium atoms. Using a
modified TOP trap we changed the trap anisotropy to a value where the frequency
of the m=0 high-lying mode corresponds to twice the frequency of the m=0
low-lying mode, thus leading to strong nonlinear coupling between these modes.
By changing the anisotropy of the trap and exciting the low-lying mode we
observed significant frequency shifts of this fundamental mode and also the
generation of its second harmonic.
