2. From Heinrich Rubens to the 21st century: a brief history of THz sources

The visible part of the electromagnetic spectrum has been studied and exploited for millennia, but visible light sources and detectors have made considerable progress with the advent of electrical technology. Infrared was discovered by Herschel in the early 19th century (although Giambattista della Porta, an Italian intellectual, had already pointed out the existence of invisible radiation carrying heat in his book Magia naturalis, published in 1558). Following Maxwell's electromagnetic theory, interest in the high-frequency domain began with the famous work of Hertz, who emitted and detected signals at frequencies of ~ 50 MHz in his first experiment to generate, propagate and detect electromagnetic waves.

Following on from Hertz's work, the far infrared was soon investigated. Hertz used the spark produced between two spheres (dipoles) under a high potential difference,...