One of the established measurement techniques in the terahertz spectral range is terahertz time domain spectroscopy (TDS). It is based on the generation of broadband electromagnetic radiation by ultrashort femtosecond (fs) laser pulses and detection using the pump-probe principle. A major advantage of this method is the coherent detection of terahertz waves, which enables high-resolution amplitude and phase measurements of the electrical terahertz field in the time domain. In addition, this technique suppresses incoherent radiation so that neither room temperature nor ambient light interfere with the measurement.
A terahertz time domain spectroscopy system essentially consists of a laser source for ultrashort laser pulses, a terahertz emitter and detector, and a delay line. The antenna structure of a photoconductive antenna is shown in the figure below.
The Pump-Probe Principle in Terahertz Time-Domain Spectroscopy
A femtosecond laser pulse is split into two partial beams using a beam splitter: One is used to generate the terahertz radiation, the other for detection. A delay line is located in front of the detector or emitter to change the time interval between the emitter and detector pulses.