Terahertz time-domain spectroscopy

In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique in which the properties of a material are probed 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, which is only sensitive to the amplitude.

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 much of the terahertz range, often from 0.05 to 4 THz. For detection, the electric field of the terahertz pulse is sampled and digitized, conceptually similar to the way an audio card transforms the electrical voltage levels of an audio signal into numbers describing the audio waveform. In THz-TDS, the electric 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 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 reconstruct its electric field as a function of time. Subsequently, a Fourier transform is performed to extract the frequency spectrum from the time-domain data.

THz radiation has several distinct advantages over other wavelengths of light used in 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. Since many materials are transparent to THz radiation, these items of interest can be observed through visually opaque intervening layers, such as packaging and clothing. 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 from buried interfaces and defects can be found and precisely imaged. THz measurements are non-contact however.

...
Wikipedia