Terahertz radiation occupies a middle ground between microwaves and infrared light waves, and technology for generating and manipulating. It represents the region in the electromagnetic spectrum in which the frequency of electromagnetic radiation becomes too high to be measured by directly counting cycles using electronic counters, and must be measured by the proxy properties of wavelength and energy.

Similarly, in this frequency range the generation and modulation of coherent electromagnetic signals ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, and requires new devices and techniques. The electromagnetic range that is used is very vast. At low frequencies end we have radio waves up to millimeter waves, and at the other end we have optical waves down to the far infrared. Technologies have been developed for both ends of the spectrum which we use in everyday applications. But the terahertz region with wavelength of 30 μm–3 mm has remained largely underdeveloped.

It is possible to produce effectively radiation in the low frequency region (microwaves) with oscillating circuits based on high-speed transistors and at high frequencies (visible spectrum) with semiconductor lasers. But transistors and other electric devices based on electric transport have in principle a limit at about 300 GHz, but are practically limited to about 50 GHz, because devices above this are extremely inefficient and the frequency of semiconductor lasers can only be extended down to about 30 THz. Thus there is a region in between where both technologies do not meet. This region is often referred to as the terahertz gap.