  We show that a slab of meta-material (with $\epsilon=\mu=-1+i\Delta$)
possesses a vortex-like surface wave with no ability to transport energy, whose
nature is completely different from a localized mode or a standing wave.
Through computations based on a rigorous time-dependent Green's function
approach, we demonstrate that such a mode inevitably generates characteristic
image oscillations in two dimensional focusing with even a monochromatic
source, which were observed in many numerical simulations, but such
oscillations are weak in three dimensional focusing.
