Gas phase approaches to the near millimeter wave source problem

The lack of appropriate sources of radiation in the spectral region between 100 GHz and 1000 GHz is a major impediment to the development of many otherwise advantageous systems and applications. Because the NMMW spectral region lies between the microwave region where classical electronic techniques are optimum, and the optical where laser and thermal sources are favorable, many approaches to the generation of NMMW radiation are viable. These include: 1) gas phase devices, 2) bulk solid-state devices, 3) junction solid-state devices, and 4) electron beam devices. This paper addresses the first of these categories, gas phase devices. These are complex and archetypically interdisciplinary. While this may make detailed understanding difficult, it also holds the promise of unexplored avenues and fruitful discoveries. Thus, we attempt in this paper to provide a fundamental basis for the understanding of these devices with the anticipation that such a basis will stimulate new ideas and further progress in the field. We discuss atomic and molecular energy levels, excitation and inversion mechanisms, chemical processes, existing devices, fundamental considerations, and avenues of inquiry that seem especially attractive.