Envision being able to read historic documents a few layers deep that may disintegrate if touched, without touching them. Or having that airport scanner as a hand held device to seek out individuals with nefarious intentions with weapons. Terahertz (THz) imaging has the capability to provide that.

Terahertz imaging takes advantage of a specific span of the electromagnetic spectrum that is transparent to a lot of electrically nonconductive materials. Radiation with a wavelength between 0.1 and 1 millimeter corresponds to a frequency of approximately one terahertz (a trillion hertz) and is denoted as the THz region (see figure below to get a sense of THz in relation to other commercial radiation sources).

Materials like paper, plastics, leather, fabrics and other composite materials are transparent for THz radiation. THz imaging can be employed for quality control and analysis of internal structure or boundary surfaces of a wide range of technologically and historically important materials.

Although there are numerous applications for THz technology, the high costs of obtaining the necessary sources had impeded the advancement of research until recently. Novel innovations such as optical parametric oscillator (OPO) technology has provided cost effective compact versions of THZ sources in recent years. These new innovations have allowed for new and improved research and applications.

Since THz radiation is less energetic as compared to ionizing radiation like X-rays. they do not cause any changes in the chemical structure of the investigated substances. Hence THz-radiation is harmless to the health which is a significant advantage over other imaging methods. Further more, owing to the intrinsically improved spatial resolution that can be achieved with the THz systems as compared to longer wavelength millimeter wave systems, THz sources are ideal for many applications.

Some interesting applications of THz Sources are:

• Imaging of hidden objects in clothing materials and determining chemical and bio-agent composition to detect explosives and prohibited substances

• Image analysis of ancient artifacts

• Packing inspection in food industry

• Non-destructive analysis of non-metallic composites

• Layer thickness measurement of electronic materials

• Transition frequencies in molecules and spectroscopy

Example of imaging hidden objects are seen below.

Analysis from the Department of Photonics and engineering at the Danish Center for Laser infrastructure shows how THz imaging can probe in to layers of a material.

Another educational application of THz imaging is used to the study the internal structure of historic paintings to understand the applied execution techniques. Technical University of Denmark use THz sources to image the fourteenth century iconic painting of the Virgin with the Child and a Saint from the public Library of Taormina in Sicily, Italy.

All the examples provided above used sophisticated lab equipment for their imaging. The advent of compact and cost effective THz sources is paving the way for extensive research and interesting applications.