  The momentum and radiation pressure of light in negative-index metamaterials
(NIMs) are commonly expected to reverse their direction from what is observed
for normal materials. The negative refraction and inverse Doppler effect of
light in NIMs have been experimentally observed, but the equally surprising
phenomenon, the negative radiation pressure of light, still lacks experimental
verification. We show by simulating the exact position- and time-dependent
field-material dynamics in NIMs that the momentum and radiation pressure of
light in NIMs can be either positive or negative depending on their
subwavelength structure. In NIMs exhibiting negative radiation pressure, the
negative total momentum of light is caused by the sum of the positive momentum
of the electromagnetic field and the negative momentum of the material. The
negative momentum of the material results from the optical force density, which
drives atoms backward and reduces the local density of atoms at the site of the
light field. In contrast to earlier works, light in NIMs exhibiting negative
radiation pressure has both negative total momentum and energy. For the
experimental discovery of the negative radiation pressure, one must carefully
design the NIM structure and record the joint total pressure of the field and
material momentum components.
