Chapter 18 THz Spectroscopy of Advanced Materials Can Koral, Gianpaolo Papari, and Antonello Andreone Abstract Materials research is an essential component of modern terahertz systems: novel, broadband, low loss and tunable devices all heavily rely on new materials, ranging from graphene to transition metal dichalcogenides and doped semiconduc- tors. At the same time, terahertz spectroscopy provide a powerful tool for the characterisation of a broad range of materials, including composite polymers and biomolecules. In this review, we will present our recent progress on the use of time domain spectroscopy to extract microscopic information in an otherwise inaccessible portion of the electromagnetic spectrum. 18.1 Introduction Due to its unique properties, the THz spectral regime is still out of reach of optical laser wavelengths, while at the same time standard high frequency electronics starts to fail. There is therefore an ever-expanding need for devices and systems operating at the THz frequencies that are efficient, tunable and broadband. Development of new and advanced materials can drive the rapid evolution of critical technologies like revolutionized information and telecommunication systems, highly efficient photovoltaic technologies and high colour contrast light emitting devices, among others. It is not farfetched that this demand would continually rise in the foreseeable future. When used as a probe, the THz radiation can provide simultaneous and straight- forward measure of the materials most fundamental properties, i.e. charge population C. Koral National Institute for Nuclear Physics, Naples Unit, Naples, Italy e-mail: can.koral@na.infn.it G. Papari · A. Andreone (*) Department of Physics, University of Naples “Federico II”, Naples, Italy e-mail: gianpaolo.papari@unina.it; andreone@unina.it © Springer Nature B.V. 2021 M. F. Pereira, A. Apostolakis (eds.), Terahertz (THz), Mid Infrared (MIR) and Near Infrared (NIR) Technologies for Protection of Critical Infrastructures Against Explosives and CBRN, NATO Science for Peace and Security Series B: Physics and Biophysics, https://doi.org/10.1007/978-94-024-2082-1_18 253