Terahertz (THz) light is now conveniently generated using a number of methods, such as photoconductive antennas, optical rectification in nonlinear crystals, or via laser-induced air plasma, thanks to major advancements in photonics and the development of turn-key lasers. One of the main applications of THz technology is characterization of materials: THz light can probe processes that occur on picosecond timescales. These include hydrogen bonds such as those in water and proteins as well as carrier dynamics in semiconductor materials and even certain types of glass. This special issue presents six articles that discuss utilization and improvements of well-known techniques such as THz time-domain spectroscopy in both transmission and reflection modes and time-resolved spectroscopy, in addition to other timely topics such as ultrahigh bandwidth spectroscopy, modulation orientation sensitive THz spectroscopy (MOSTS), high-field THz spectroscopy, and dynamic THz emission microscopy (DTEM). Examples to highlight potential applications of these techniques also are presented.

In the first article of the special issue, Enrique Castro-Camus and Mariana Alfaro from Centro de Investigaciones en Optica A.C., Mexico, review the current state of the art of THz photoconductive devices. They clearly and concisely explain the desired characteristics of a THz photoconductive antenna in terms of substrate properties and contact geometries, highlighting differences in requirements for an emitter and a detector. The development of these devices has greatly facilitated exploration of the THz region of the electromagnetic spectrum.

The second article is by Tianwu Wang et al., from the Jepsen Group at the Technical University of Denmark. In this work, a tellurium rich chalcogenide glass is characterized between 1.8 and 18 THz using broadband THz time-domain spectroscopy and time-resolved THz spectroscopy. Chalcogenide glasses are based on the chalcogen elements S, Se, and Te and have a variety of photoinduced effects. They are optically highly nonlinear and are used in optical devices such as waveguides and fiber structures. This thorough investigation includes a theoretical model to support the experimental data and provides great insight into the carrier dynamics upon photoexcitation. These results will facilitate device development based on chalcogenide glasses.

The third article is by Shuting Fan et al. As part of her PhD research at the Chinese University of Hong Kong in the MacPherson Group, Dr. Fan devised a new approach to improve the accuracy of THz reflection spectroscopy when imaging over an area as large as 20mm×20mm. Their algorithm accounts for THz signal fluctuations, fiber drift, optical delay stage jitter, and uneven thickness in the imaging window, and it also reduces the data acquisition time compared with standard THz spectroscopy approaches. The algorithm is simple to implement and useful for any applications requiring high-accuracy THz characterization in reflection geometry.

The fourth article is by Rohit Singh et al., from the Markelz Group at the University of Buffalo, New York. In this work, they report a novel method designated MOSTS to determine the anisotropic THz spectrum of crystalline or aligned samples. This is particularly useful for complex molecules such as proteins that should be measured in their hydrated form because it enables isotropic background contributions to be removed while still preserving the interesting features from the sample. The method works by spinning the sample during measurement and using lock-in detection to measure the time-varying component due to anisotropic absorbance at twice the spinning frequency. This method is simple and fast and is useful for characterizing materials in which the isotropic background masks important sample features.

Article five is by Xiaoguang Zhao from the Zhang Group at Boston University and the Averitt Group at the University of California, San Diego. In this paper, the resonances of solid and flexible nonlinear metamaterial perfect absorber devices are characterized under different THz field strengths using high-field THz time domain spectroscopy. In this way, the device designs and fabrication are evaluated, and it is explained how further structural design could improve the devices for applications such as saturable absorption, optical limiting, and self-focusing.

Finally, in the last article Prof. Hironaru Murakami et al., in the Tonouchi group at Osaka University, reports on how laser THz emission microscopy can effectively be combined with optical pump–probe THz spectroscopy to achieve DTEM. DTEM is able to reveal the spatiotemporal dynamics of photoexcited carriers and thereby detect phenomena that may be missed if optical pump–probe spectroscopy is solely used. Thus, by utilizing this improved characterization technique, it is possible to visualize temporal–spatial dynamics of photoexcited carriers in electronic materials devices such as photoconductive antennas with a spatial resolution of ∼1.5μm and time resolution of ∼100fs.

In summary, this special issue offers great insight into THz characterization methods and how they can be utilized to develop various THz devices as well as reveal interesting and useful material properties.