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Using a subpicosecond electro-optic sampling technique, we have characterized the photoresponse of current-biased YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) photodetectors, designed as 5-/spl mu/m-wide and 7-/spl mu/m-long microbridges patterned in 100-nm-thick, high-quality epitaxial films grown on LaAlO/sub 3/ substrates by pulsed laser deposition. The bridges were centered in a coplanar waveguide structure, allowing the photogenerated pulses to be measured 20 /spl mu/m from the detector. The experiments were conducted in the temperature range between 20 and 80 K; however, the bridges were biased in the switched (resistive) state, which corresponded to a hot-spot formation at the center of the microbridge. The photoresponse from 100-fs laser pulses (395-nm wavelength) was measured to be in the form of a single spike with the width as short as 1.3 ps. The physical origin of this ultrafast response is attributed to nonequilibrium electron heating, We extracted the intrinsic temporal parameters of the YBCO response-the electron thermalization time equal to 0.56 ps and electron-phonon energy relaxation time equal to 1.1 ps, Our measurements demonstrate that a simple YBCO microbridge can operate as a >100-GHz bandwidth photodetector, e.g., as an optical-to-electrical transducer for optoelectronic interface in YBCO digital electronics.