The millimeter-wave (MMW) and terahertz (THz)-regime portions of the electromagnetic spectrum have enormous application potential, including high-data-rate communications, remote sensing and threat detection, high resolution radar, biomedical imaging, and spectroscopic analysis. To exploit this potential, new sources of coherent electromagnetic radiation are needed in the frequency range of 100 - 1000 GHz. The ideal sources would provide high power with high efficiency in a compact, lightweight, and low-cost package. Many ...