Cutting-edge terahertz technology

Terahertz (THz) radiation, which lies in the frequency gap between the infrared and microwaves, typically referred to as the frequencies from 100 GHz to 30 THz, has long been studied in fields such as astronomy and analytical science. However, recent technological innovation in photonics and nanotechnology is now enabling THz research to be applied in many more sectors. Today, THz technology is finding use in an increasingly wide variety of applications: information and communications technology (ICT); biology and medical sciences; non-destructive evaluation; homeland security; quality control of food and agricultural products; global environmental monitoring; and ultrafast computing among others. Th ere have been numerous recent breakthroughs in the fi eld, which have pushed THz research into the centre stage. Milestones include the development of THz time-domain spectroscopy (THzTDS), THz imaging, and high-power THz generation by means of nonlinear eff ects. Th ese techniques are oft en far superior to conventional tools for analysing a wide variety of materials. Photonics has led the way to the realization of many important THz devices such as the development of the quantum cascade laser (QCL) and the uni-travelling-carrier photodiode (UTC-PD), for example. Th e QCL provides a powerful continuous-wave (c.w.) THz source, and the UTC-PD uses a photomixing technique to produce sub-THz waves for wireless communication. In addition, continual advances in the performance of highfrequency electronics have also benefi ted terahertz technology. State-of-the-art semiconductor and superconductor logic circuits are already operating at frequencies greater than 100 GHz (0.1 THz). At the same time, recently developed solid-state electronic devices such as resonant tunnelling diodes and THz single-photon detectors, as well as conventional electronic devices, such as Schottky barrier diodes (SBDs), continue to move into the THz range. THz technology is now growing rapidly in many independent fi elds. Readers can fi nd the details of the historic achievements and fundamental principles of THz research in many reviews1–7. Here, the focus is on describing the important progress of THz technology — in particular, THz sources, detectors, systems and application-oriented achievements — and a discussion of the future prospects.