For most part of history, light has been the only natural radiation that human beings have been aware of. It was only in the 19th and 20th centuries that it became known, through a series of discoveries, that there was nothing very special about light, that a number of electromagnetic waves exist in nature and that “light”, as we normally know it, is part of them. What humans see is in fact a tiny part of this very broad electromagnetic spectrum that comprises waves whose wavelengths range from a few trillionths of a metre at one end to thousands of metres at the other.
Depending on their wavelength, these radiations exhibit signature properties that could be very useful in certain situations. For example, X-rays, because of the fact that they could penetrate human skin but not the bones, came to be used extensively in medical imaging. In the last nearly 100 years, scientists have mastered the exploitation of nearly every bit of the electromagnetic spectrum for different kinds of purposes — from gamma rays and X-rays, to microwaves and radio waves.
A very small part of this spectrum, tucked between the microwave and optical infra-red region, and corresponding to frequencies in the range of 0.1 to about 3 terahertz (1 THz = 1 trillion Hz), however, has remained largely unutilised till now. Radiations in the terahertz frequency range are known to have a number of properties that can be put to good use. For example, it too can penetrate human skin and could be used for body imaging, but unlike X-rays, these are not very harmful to human tissues, making them ideal for use in the medical field and security scanners like those at the airports. Terahertz radiation is also great for communication. Besides, it can be used in a number of research activities.
The problem is that scientists have so far not been able to develop a reliable source of terahertz radiations that can generate these waves as and when required, like they can with X-rays or other radiations. The terahertz region in the electromagnetic spectrum, in fact, is often referred to as the terahertz gap.