The region between infrared radiation and microwaves in the electromagnetic spectrum is known as the terahertz region. The range of frequencies covered in this region is significant as it encompasses various excitation frequencies in solids. As a result, emissions of electromagnetic waves in the terahertz region can be used to probe the physical properties of solids, including both steady-state and dynamical properties.

In a new review article, researchers from Japan shed light on this front, highlighting various material properties that can be revealed by terahertz radiation and the experimental methods associated with them.

They began by looking at an experiment that used a high-intensity femtosecond laser pulse to excite terahertz emission in a non-centrosymmetric crystal, i.e., a crystal with no spatial inversion symmetry. The incident laser pulse induced a non-linear electric polarization within the crystal and, in turn, terahertz emissions arising from electric dipole radiation.

Next, the researchers took stock of the electronic properties that can be uncovered using this terahertz radiation. They showed that terahertz radiation can be used, for instance to image the domain walls in ferroelectric thin films. This could be done by measuring the amplitude and phase of the terahertz radiation, which corresponded to the magnitude and direction of the ferroelectric polarization, respectively. Thus, it could be used to not only image the domains but also how they moved and switched due to external fields.

Additionally, terahertz radiation was useful for characterizing ferrimagnetic materials. Similar to the electric dipole radiation phenomenon, terahertz emissions resulting from magnetic dipole radiation enabled the visualization of magnetic domains in ferrimagnets.

Finally, the researchers reported new mechanisms for the generation of terahertz radiation. These included processes like impulsive stimulated Raman scattering, circular photogalvanic effect, and a photoinduced ferroelectric phase transition.

Overall, this review summarizes the possibilities laid open by the terahertz radiation process for research in condensed matter physics, which could help provide a richer understanding of solids.