Terahertz time-domain spectroscopy

The terahertz band of the electromagnetic spectrum was the least explored region of the spectrum prior to the introduction of the technique known as time-domain spectroscopy (TDS) in the late 1980s. Since its introduction, terahertz TDS has enabled the study of a plethora of physical, chemical and biological phenomena; from excitons and Cooper pairs in solids to the hydration dynamics of biomolecules. Terahertz techniques can be used to non-destructively analyse samples from diverse fields, such as art conservation and industrial quality control, whereas terahertz imaging can act as a sensitive hydration probe in biological tissue and other materials. This article focuses on TDS, a unique hybrid between microwave and optical technologies. By measuring the time-dependent electric field waveform, rather than the intensity of the electromagnetic wave, one directly accesses the spectral amplitude and phase of the electric field. As a result, both the refractive index and absorption coefficient (or the complex dielectric function) of a sample can be measured simultaneously. The technique is based on the generation and detection of single-cycle pulses of radiation, enabling measurements with sub-picosecond time resolution. This Primer summarizes the basics of such systems and gives a few illustrative application examples.