Terahertz frequency electronics and photonics: materials and devices.

The terahertz frequency region of the electromagnetic spectrum sits at the interface of electronics and optics, lying between the microwave and infrared (IR) spectral regions. Although there are significant challenges to access, understand and exploit this distinctive region of the spectrum, there are immense benefits in its exploration for both discovery- and challenge-led research, from fundamental studies of laser operation through to the development of new spectroscopy instrumentation. The last 25 years has witnessed remarkable efforts to advance the field of terahertz science and engineering, and this is the subject of this article. Advances in the growth of precisely layered semiconductor materials have enabled a number of new terahertz device technologies, including high-performance quantum cascade lasers (QCLs) and quantum well photodetectors. Recent advances have included the use of thin magnetic films for efficient terahertz generation. We also review the increasing interest in contemporary two-dimensional (2D) materials for terahertz optoelectronic devices. New materials including graphene, topological insulators, transition metal dichalcogenides and novel semi-metals have shown promise as highly efficient terahertz radiation detectors and modulators. Finally, we summarize the challenges which still exist in the field of terahertz electronics and photonics, and how new materials and new device technologies might meet these challenges.This article is part of the theme issue 'Science into the next millennium: 25 years on'.