  We conducted theoretical and numerical investigations of the optical
rectification (OR) effect in metallic structures with periodic modulation. A
new formulation of the OR effect is presented, and the mechanism by which the
OR effect is generated, which has been a controversial issue in previous
studies, is clarified. We reveal that the OR effect is strongly enhanced by a
combination of spatial variation of the metallic structure and local electric
field enhancement. Our theory was numerically evaluated and agreed fairly well
with experiment.
