Technically, terahertz frequencies (THz) are high-frequency microwaves, also known as sub-millimeter waves, or long-wavelength light. Their frequency range is between 100 GHz and 10 THz and their wavelengths between 3 mm and 30 µm.
The wavelengths from 30 µm to 3 mm are also attributed to far- infrared light( FIR), whose wavelength range is between 15 µm and 1 mm. We are thus concerned with the wavelength range between microwaves and infrared light, with waves emitted by the sun and by warm objects. Since this wavelength range has been of limited use until now, it is also referred to as the terahertz gap.
Because terahertz radiation penetrates many materials and biological tissues, this technique is of particular interest for nondestructive materials research and can be used to shine through bodies. The advantage of THz radiation over X-rays is that, due to the low energy of the photons, they are non-ionizing. As with microwave irradiation, various materials absorb terahertz oscillations and convert them into heat.
In terms of applications, THz technology is suitable, among other things, for detector systems to check airline passengers, for non-destructive material testing, for military and private control systems such as the screening of passengers, parcels or letters, and also for medical technology.