  A whole class of two-color experiments involves intense, short Terahertz
radiation pulses. A fast detector that is sensitive and able to resolve both
near-infrared and Terahertz pulses at the same time is highly desirable. Here
we present the first detector of this kind. The detector element is a
GaAs-based field effect transistor operated at room temperature. THz detection
is successfully demonstrated at frequencies up to 4.9 THz. The THz detection
time constant is shorter than 30 ps, the optical time constant is 150 ps. This
detector is ideally suited for precise, simultaneous resolution of optical and
THz pulses and for pulse characterization of high-power THz pulses up to tens
of kW peak power levels. The dynamic range of the detector was as large as 65
$\pm$ 3 dB/$\sqrt{Hz}$, enabling applications in a large variety of experiments
and setups, also including table-top systems.
