We used a surface-plasmon antenna to obtain small photodetectors for LSI on-chip optical interconnection by using near-field light generated by the antenna. Such near-field devices are not constrained by the diffraction limit and they offer an approach to integrated nanoscale photonic devices. A small semiconductor structure is located near the antenna to absorb the near-field light. This structure can be made as small as the Schottky depletion layer, so the separation between electrodes can be reduced to almost the size of the near-field region. We have demonstrated a "Si nano-photodiode" or plasmon photodiode that uses the near-field localized in a subwavelength region, which is usually relatively large in size because of the long absorption length for Si (∼10 μm at a wavelength of ∼800 nm). The Si nano-photodiode has a fast impulse response with a full-width at half-maximum of ∼20 ps even when the bias voltage is small (∼1 V or less). We demonstrated an on-chip optical interconnect chip to operate circuitry in an LSI chip by using waveguide-coupled Si nano-photodiodes.