The sub-millimeter spectrum covers frequencies from 300 GHz and 3 THz. As shown in figure 1, it is sandwiched between the microwave band and the infrared band.
Terahertz has long been an unexploited frequency band. The reason is that the operation of electronic components deteriorates as the frequency increases and is very poor above 100 GHz. In fact, the lack of electronic components at terahertz frequencies has a name: it is called the terahertz gap (figure 2). As one can see from figure 2, frequency multipliers (dark circle) dominate other electronic devices (triangle) above 150 GHz up to 1 THz. Cryogenic sources are shown as hollow circles. In the far infrared region, lasers dominate but do not emit below 1.2 THz. They also tend to be bulky (gas laser) or unstable (QCL).
Due to high atmospheric absorption, terahertz waves cannot be used for long range communications. However, they offer many advantages. First and foremost, they are essential for the astronomy community. In fact, sub-millimeter radiation contains about half of the luminosity of the Universe and 98% of the photons emitted since the big bang. Furthermore, they are used more and more in military and civil imaging, since they have a low photonic energy (meV) , are non ionizing and can penetrate many materials (see figure 3). Finally, many molecular rotational and vibrational modes correspond to terahertz frequencies which makes them useful for spectroscopy.