Bridging the terahertz gap using solid-state devices

Terahertz (THz) frequencies nestled between the microwave and infrared ranges in the electromagnetic spectrum radiation remain one of the most attractive research topics. A particular attention is given to the issues related to the development of solid-state-based room-temperature high-power, stable and portable terahertz emitters and detectors as well as user-friendly THz imaging and spectroscopy. At the dawn of this research, four decades ago,academician Juras Požela [J. Požela and V. Jucienė, Physics of High-Speed Transistors (Vilnius, Mokslas, 1985)] considered possible physical mechanisms – hot electrons, plasma effects, Josephson junctions, masers, etc. – that can successfully be employed to cover the THz frequencies using solid-state physics approaches. In this work, we briefly overview the recent achievements and advances illustrating an incredibly high precision of the scientific predictions given by Acad. Juras Požela based on his wide erudition, deeply sensitive intuition and great insights, gifted feeling of scientific trends and evolution. The paper presents a structured snapshot of the modern devices with highlights in their physics behind the operation and main parameters and includes contemporary topics in THz science and technology related to electrically pumped GaN-based sources and quantum semiconductor structures such as resonant tunnelling diodes, quantum cascade lasers, and quantum semiconductor superlattices. Possible challenges in further development of the described approaches and devices are illuminated.