ABSTRACT
This chapter reviews the recent advances in terahertz devices and systems for imaging, spectroscopy, and sensing applications, and presents the future outlook of the terahertz technology at the device and system level. Gyrotrons, free electron lasers, and backward wave oscillators are electron beam sources that generate relatively high power terahertz signals. Solid-state sources include resonant tunneling diodes, Gunn or transferred electron devices, and transit time devices such as impact avalanche transit time diodes and tunnel injection transit time diodes. The most promising terahertz semiconductor lasers are quantum cascade lasers. A quantum cascade laser is a unipolar laser, in which the conduction band or the valence band is divided into a few subbands. Optoelectronic techniques exhibit alternative solutions for extracting both amplitude and phase information of a terahertz signal incident on an optically gated terahertz photoconductive antenna or a nonlinear electrooptic crystal.
