‘‘Terahertz (THz) radiation’’ defines the electromagnetic waves in 0.1 and 10 THz frequency range. Terahertz signals were until recently an almost unexplored area of research due to the difficulties in generation and detection of electromagnetic fields at these wavelengths.

Neither optical nor microwave techniques are directly applicable in the terahertz range since optical wavelengths are too short and microwave wavelengths are too long compared to terahertz field wavelengths.

The potential of THz radiation is impressive in many fields, such as space, security, medical, biology, microscopy. In particular, in medical applications the THz radiation will allow in vivo early disease diagnostic, tissue identification of anomalies, cancer diagnosis. Difference in absorption of THz radiation of tissue can be used to non-invasively probe the skin of humans, up to a depth of ~ 1 cm. This can e.g. be used for diagnosing skin cancer (Teraview) or for dental cavities analysis.

In the security field, the capability of THz radiation to penetrate materials such as plastic, cardboard, paper and to reveal concealed weapons and dangerous substances like drugs or explosive, without the risk for the health typical of X-ray, could be used in crowded areas such as airports to detect potential risks.
THz radiation can also be used to distinguish between materials with varying water content —for example, fat versus lean meat, as it is readily absorbed by water. These properties lend themselves to applications in process and quality control as well as biomedical imaging.

In this context, there are four different well established approaches to the THz generation: optical downconversion (such as photomixing),electronic upconversion, direct generation of THz radiation using lasers such as QCLs and traditional vacuum tubes such as backward-wave oscillators (BWO).
However, the lack of compact, cheap, powerful and efficient THz sources has strongly limited the realization of THz based systems, and therefore this region of the electromagnetic spectrum has remained unexploited (THz gap).

The previous examples give only a small fraction of the potential applications of THz radiation, explaining why such a big effort is going on both in research and industrial fields in order to develop this technology.

For these reasons, the device proposed in the OPTHER project, a compact and powerful THz amplifier, represents an appealing goal both for the scientific and the industry community.