This paper describes the recent trend of the surface-plasmon technology for photo-detectors. Surface plasmons create strong near-field on a metal surface thereby reducing the size of the photodetectors. One of the merits for such small detectors is their extremely small electric capacitance. A possible effect of the small capacitance is to bring about the high efficiency of the photo detector circuit at very high frequency. Small photodiodes for on-chip optical interconnection and data communication are reviewed as an example of such applications. A near-field probe for sub-wavelength THz imaging is also discussed.