  The diffraction of electromagnetic plane waves by a rectangular grating
formed by discrete steps in the interface of a homogeneous, isotropic, linear,
negative phase--velocity (negative index) material with free space is studied
using the semi--analytic C method. When a nonspecular diffracted order is of
the propagating type, coupling to that order is significantly larger for a
negative index material than for conventional material. The computed coupling
strengths reported here are in agreement with recent experiments, and
illustrate the role of evanescent fields localized at the grating interface in
producing this enhanced coupling.
