Terahertz spectroscopy refers to spectroscopic techniques that probe materials using electromagnetic radiation in the terahertz (THz) region, typically from ~0.1 to 10 THz (3 mm–30 μm). THz spectroscopy accesses low-energy excitations such as rotational and vibrational transitions in molecules, phonons and collective modes in solids, charge transport in semiconductors and superconductors, and intermolecular interactions in liquids and biomaterials. Two major approaches are used: time-domain terahertz spectroscopy (THz-TDS), which employs pulsed THz radiation and waveform sampling, and frequency-domain terahertz spectroscopy (THz-FDS), which uses continuous-wave radiation and sweeps frequency. THz-TDS provides broadband access to amplitude and phase information and is widely used for materials characterization, imaging, and non-destructive evaluation. THz-FDS offers extremely high spectral resolution, often below 1 MHz, enabling Doppler-limited gas-phase spectroscopy, rotational spectroscopy, and precision metrology. THz spectroscopy has applications in condensed-matter physics, chemistry, atmospheric and environmental sensing, security screening, pharmaceuticals, telecommunications, cultural-heritage analysis, biomedical imaging, and industrial quality control. THz systems can operate in free-space or waveguide geometries and may employ photoconductive emitters and detectors, nonlinear optical generation, photomixing, electronic sources, or quantum cascade lasers.
Background There are a great variety of techniques to generate THz radiation and to detect THz fields. One can, e.g., use an antenna, a quantum-cascade laser, a free-electron laser, or optical rectification to produce well-defined THz sources. The resulting THz field can be characterized via its electric field ETHz(t). Present-day experiments can already output ETHz(t) that has a peak value in the range of MV/cm (megavolts per centimeter). To estimate how strong such fields are, one can compute the level of energy change such fields induce to an electron over microscopic distance of one nanometer (nm), i.e., L = 1 nm. One simply multiplies the peak ETHz(t) with elementary charge e and L to obtain e ETHz(t) L = 100 meV. In other words, such fields have a major effect on electronic systems because the mere field strength of ETHz(t) can induce electronic transitions over microscopic scales. One possibility is to use such THz fields to study Bloch oscillations where semiconductor electrons move through the Brillouin zone, just to return to where they started, giving rise to the Bloch oscillations. The THz sources can be also extremely short, down to single cycle of THz field's oscillation. For one THz, that means duration in the range of one picosecond (ps). Consequently, one can use THz fields to monitor and control ultrafast processes in semiconductors or to produce ultrafast switching in semiconductor components. Obviously, the combination of ultrafast duration and strong peak ETHz(t) provides vast new possibilities to systematic studies in semiconductors. Besides the strength and duration of ETHz(t), the THz field's photon energy plays a vital role in semiconductor investigations because it can be made resonant with several intriguing many-body transitions. For example, electrons in conduction band and holes, i.e., electronic vacancies, in valence band attract each other via the Coulomb interaction. Under suitable conditions, electrons and holes can be bound to excitons that are hydrogen-like states of matter. At the same time, the exciton binding energy is few to hundreds of meV that can be matched energetically with a THz photon. Therefore, the presence of excitons can be uniquely detected based on the absorption spectrum of a weak THz field. Also simple states, such as plasma and correlated electron–hole plasma can be monitored or modified by THz fields.
Terahertz time-domain spectroscopy
In optical spectroscopy, the detectors typically measure the intensity of the light field rather than the electric field because there are no detectors that can directly measure electromagnetic fields in the optical range. However, there are multiple techniques, such as antennas and electro-optical sampling, that can be applied to measure the time evolution of ETHz(t) directly. For example, one can propagate a THz pulse through a semiconductor sample and measure the transmitted and reflected fields as function of time. Therefore, one collects information of semiconductor excitation dynamics completely in time domain, which is the general principle of the terahertz time-domain spectroscopy.
By using short THz pulses, a great variety of physical phenomena have already been studied. For unexcited, intrinsic semiconductors one can determine the complex permittivity or THz-absorption coefficient and refractive index, respectively. The frequency of transversal-optical phonons, to which THz photons can couple, lies for most semiconductors at several THz. Free carriers in doped semiconductors or optically excited semiconductors lead to a considerable absorption of THz photons. Since THz pulses passes through non-metallic materials, they can be used for inspection and transmission of packaged items.
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