  On the basis of angular spectrum representation, a formalism describing
paraxial beams propagating through an isotropic left-handed material (LHM) slab
is presented. The treatment allows us to introduce the ideas of beam focusing
and phase compensation by LHM slab. Because of the negative refractive index of
LHM slab, the inverse Gouy phase shift and the negative Rayleigh length of
paraxial Gaussian beam are proposed. It is shown that the phase difference
caused by the Gouy phase shift in right-handed material (RHM) can be
compensated by that caused by the inverse Gouy phase shift in LHM. If certain
matching conditions are satisfied, the intensity and phase distributions at
object plane can be completely reconstructed at the image plane.
