This paper theoretically investigates the optical-rectification (OR) effect in metallic structures with periodic modulation. The nonlinear surface and bulk currents responsible for the OR effect are formulated based on the Drude model with electron-gas pressure. The new formulation clarifies the generation mechanism of the OR effect and reveals that the OR effect is strongly enhanced by a combination of spatial variation of the metallic structure and asymmetry of electric-field strength. In order to consider the validity of our theory, the OR effect in the metallic structure is numerically calculated. The calculation results show fairly well agreement with the experiments.
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