In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique where a special generation and detection scheme is used to probe material properties with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample material's effect on both the amplitude and the phase of the terahertz radiation. In this respect, the technique can provide more information than conventional Fourier-transform spectroscopy that is only sensitive to the amplitude.

The radiation has several distinct advantages over other forms of spectroscopy: many materials are transparent to THz, THz radiation is safe for biological tissues because it is non-ionizing (unlike for example X-rays), and images formed with terahertz radiation can have relatively good resolution (less than 1 mm). Also, many interesting materials have unique spectral fingerprints in the terahertz range, which means that terahertz radiation can be used to identify them. Examples which have been demonstrated include several different types of explosives, polymorphic forms of many compounds used as Active Pharmaceutical Ingredients (API) in commercial medications as well as several illegal narcotic substances. Again, as many materials are quite transparent to THz radiation, these items of interest can be observed through normally opaque intervening layers (packaging, clothing). Additionally, though not strictly a spectroscopic technique, the ultrashort width of the THz radiation pulses allows for measurements (e.g., thickness, density, defect location) on difficult to probe materials (e.g., foam). The measurement capability shares many similarities to that observed with pulsed ultrasonic systems. Reflections off buried interfaces and defects can be found and precisely imaged. THz measurements are non-contact however.

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.