Why are terahertz frequencies so useful?

                                                                                                                                                                  
Non-damaging

Electromagnetic radiation at terahertz frequencies has far less energy than electromagnetic radiation at higher frequencies (for example, x-rays). 

The energy of electromagnetic radiation is described by the following equation:

                                                                                     E = hf 
                           
where E  is the energy of the photon, f is its frequency and h is Planck's constant

The energy of terahertz waves is too low to knock the electrons from atoms, i.e. they do not have the potential to ionise materials, and therefore won't damage living tissue. This makes them very attractive for medical uses, but also for security application, such as scanning airline passengers.


Higher resolution than other safe wavelengths

Electromagnetic radiation across the spectrum can be used to produce images. X-rays are a familiar example. The level of detail of an image will depend on the wavelength of the electromagnetic radiation being used. The smaller the wavelength, the better the resolution. Terahertz waves may not have the same level of resolving power as x-rays or visible light, but they are better than using microwaves or radio waves. It isn't always desirable to use ultra-violet or x-rays because they are hazardous to living cells. Visible light cannot be used to image in all circumstances (for example if the item being imaged is covered in something that is opaque to visible light). Which leads on to another important aspect of terahertz radiation...

Many common materials are transparent to terahertz radiation

Many materials that are opaque to visible light are transparent to terahertz radiation, including many textiles, paper and cardboard.  This opens up a potential for security applications, for example, as terahertz radiation could scan people for concealed weapons without using damaging x-rays. This technology is already in use in some airports including Schipol in the Netherlands.

                                                                              
Terahertz wavelengths correspond with the behaviour of many organic and biological molecules

Terahertz waves span a section of the electromagnetic spectrum at which many molecules emit and absorb radiation. Molecules have a distinct chemical "fingerprint" as a result of the particular wavelengths of electromagnetic radiation that they absorb and emit. This can be used in a technique called spectroscopy to identify substances. Concealed plastic explosives could be identified by this technique, or a patient's breath analysed.

Band-width and data transfer

The terahertz segment of the electromagnetic spectrum offers huge potential for high data transmission rates.  Data rates are constrained by the available bandwidth, and the terahertz frequencies offer wide bandwidths in an uncrowded part of the electromagnetic spectrum.