  We present an ultrafast graphene-based detector, working in the THz range at
room temperature. A logarithmic-periodic antenna is coupled to a graphene flake
that is produced by exfoliation on SiO2. The detector was characterized with
the free-electron laser FELBE for wavelengths from 8 um to 220 um. The detector
rise time is 50 ps in the wavelength range from 30 um to 220 um.
Autocorrelation measurements exploiting the nonlinear photocurrent response at
high intensities reveal an intrinsic response time below 10 ps. This detector
has a high potential for characterizing temporal overlaps, e. g. in two-color
pump-probe experiments.
