Picosecond-scale Terahertz pulse characterization with field-effect transistors

Precise real-time detection of terahertz (THz) pulses is a key requirement for characterization of pulsed THz sources and nondestructive testing applications. We experimentally evaluate the speed limits of THz rectification in field-effect transistors (FETs) using the example of pulses from a free-electron laser (FEL). We develop an improved model for the description of these THz pulses and demonstrate its validity experimentally by comparison to spectroscopic data as well as to expectation values calculated from FEL physics. The model in conjunction with the high speed of the detectors permits the detection of an exponential rise time of the pulses as short as 5 ps despite a Gaussian postdetection time constant of 11 and 14 ps for a large area and an antenna-coupled detector, respectively. This proves that FETs are excellent compact, room-temperature THz detectors for applications that require an intermediate frequency bandwidth of several tens of gigahertz.