Reminiscences on selected millennium highlights in the quest for tunable terahertz-submillimeter wave oscillators

The generation of coherent electromagnetic radiation across the spectrum has been one of man's major achievements. With the invention of the laser, the microwave and optical spectral range has seen intense activity in both physical and quantum electronics research and engineering. However, the submillimeter-terahertz region has been somewhat left behind, partly due to the unique challenges it presents in developing sources, detectors, and components. The paper traces selected highlights from the work of Hertz at 50 GHz to current research at terahertz frequencies. There is always the aim that a review might trigger some new ideas to apply to terahertz oscillators.