Explanation
Typically, the terahertz pulses are generated by an ultrashort pulsed laser and last only a few picoseconds. A single pulse can contain frequency components covering the whole terahertz range from 0.05 to 4 THz. For detection, the electrical field of the terahertz pulse is sampled and digitized, conceptually similar to the way an audio card transforms electrical voltage levels in an audio signal into numbers that describe the audio waveform. In THz-TDS, the electrical field of the THz pulse interacts in the detector with a much-shorter laser pulse (e.g. 0.1 picoseconds) in a way that produces an electrical signal that is proportional to the electric field of the THz pulse at the time the laser pulse gates the detector on. By repeating this procedure and varying the timing of the gating laser pulse, it is possible to scan the THz pulse and construct its electric field as a function of time. Subsequently, a Fourier transform is used to extract the frequency spectrum from the time-domain data.
Read more about this topic: Terahertz Time-domain Spectroscopy
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