  We study the flow map associated to the cubic Schrodinger equation in space
dimension at least three. We consider initial data of arbitrary size in $H^s$,
where $0<s<s_c$, $s_c$ the critical index, and perturbations in $H^\si$, where
$\si<s_c$ is independent of $s$. We show an instability mechanism in some
Sobolev spaces of order smaller than $s$. The analysis relies on two features
of super-critical geometric optics: creation of oscillation, and ghost effect.
