Observation of harmonic generation and nonlinear coupling in the collective dynamics of a Bose-Einstein condensate

We report the observation of harmonic generation and strong nonlinear coupl ing of two collective modes of a condensed gas of rubidium atoms. Using a m odified time averaged orbiting potential trap we changed the trap anisotrop y to a value where the frequency of the m = 0 high-lying mode corresponds t o twice the frequency of the m = 0 low-lying mode, thus leading to strong n onlinear coupling between these modes. By changing the anisotropy of the tr ap and exciting the low-lying mode we observed significant frequency shifts of this fundamental mode and also the generation of its second harmonic.