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Frequently Asked Questions

We have tried to answer some of the more frequent questions that are asked about terahertz radiation and the TeraNova project. Please email Karen Steenson if your question isn' answered below.

What is a “terahertz”?

Frequency is measured in Hertz (abbreviated as: Hz), or reciprocal seconds. The pre-fix “tera” means: one million million (1012). Thus, a frequency of 3 THz is 3,000,000 Hz. It is worth noting that, for electromagnetic radiation, frequency and wavelength are connected via a simple formula. Thus a frequency of 1 THz corresponds to a wavelength of 300 micrometres or, approximately, one-third of a millimetre.

What is “terahertz radiation”?

Each part of the electromagnetic spectrum has a name associated with the type of radiation found there. Thus, the visible part of the spectrum corresponds to wavelengths between (approximately) 450 nanometres (nm) to 650 nm. The generally accepted definition of the terahertz range is normally given, in frequency units, as 300 GHz (i.e. 0.3 THz) up to 10THz. In wavelength terms this corresponds to 1mm to 30 micrometres. Until recently, this range was often known as the “submillimetre” (for the longer wavelengths, or lower frequencies) and “far infrared” (for the shorter wavelengths, or higher frequencies).

Is the Terahertz band licensed by the Regulatory Authorities?

At the moment, there are no specific limitations on the use of the THz frequency range for communications purposes. If a workable THz communication system could be realised, then many advantages would accrue from reduced antenna size and greatly increased bandwidth. Experimental systems, operating at a few hundred GHz are now beginning to appear, but a great deal of work still needs to be done to ensure that they are practicable. Specific problems here relate to the attenuation (reduction in strength) of THz radiation as it passes through the atmosphere; and the lack of certain key components, such as amplifiers and modulators for use at THz frequencies.

Is THz radiation safe?

The real question here is: “safer than what?” At the microscopic level, THz radiation interacts with groups of molecules (e.g. cells in human tissue) but does not have sufficient energy to break up the molecules that make up the tissue. Thus, as far as we can judge, THz radiation is safe. So far, only a handful of safety studies have been reported, exposing cells to THz radiation emitted by equipment that can be bought for use in laboratories or clinics. There is, as yet, no official standard regarding exposure to THz radiation: standards are given for near-infrared and for millimetre-wave radiation (on either side of the THz band) but not, so far, for the THz band itself.

Despite the above comments, it is important to remember that there may be other, more subtle, effects taking place and we are not yet fully aware of these. For example, it may be possible to stimulate “coherent” effects (when cells move together) with THz radiation: this has been predicted theoretically, but not yet observed. If this type of effect did take place, then we would have to revisit our verdict on the safety of THz radiation.

Why is it difficult to make THz radiation?

The THz band lies “between radio and light”. This means that the usual type of source used to make millimetre wave (“radio” – at a stretch!) radiation does not work well as the lower limits of the THz band are approached. This is because these sources rely on the rapid to-and-fro motion of electrons in semiconductors; at the high frequencies needed to generate THz radiation, this becomes less and less efficient. This follows from some quite simple physics and also from some technological limitations relating to how small a semiconductor structure can be made. In practice, even the best “conventional” electronics-based sources begin to fail at around 2-300GHz.

But what about the other side of the THz band, i.e. Infrared or the realm of “photonics”? Could we, for example, make a THz laser? The answer is: yes, of course, and such lasers have been around for years. But, unfortunately, they are bulky and rather complicated and not really useful for practical use in, say, a police car as part of a surveillance system. Fortunately, some hope is now in sight and very small semiconductor lasers operating at THz frequencies have recently been made. In these lasers, a totally new principle is used that relies on quantum mechanics. These lasers can now operate at frequencies of down to about 1 THz, but at the moment they need to be kept very cold to work properly. The TeraNova group has a large programme devoted to building these lasers and to getting them to work at higher temperatures.

Why is THz radiation useful?

THz radiation has many useful properties. It passes easily through many common materials, such as: paper, packaging, wood, ceramics, certain building materials, and plastics. It will also travel a few millimetres in human tissue (rather further in breast tissue) and up to 500 metres in the atmosphere. Its importance, though, stems from the fact that it interacts with group of molecules making them twist, rock, vibrate and jiggle at around this frequency. This leads to the absorption of radiation, giving rise to characteristic “fingerprint” absorption. With the right type of equipment, a THz map, or image, of a specimen (e.g. a skin cancer) can be built up. In addition, and with the right sort of equipment, a map of the refractive index variations across the sample can also be produced. THz radiation can also produce very useful information about the important properties of semiconductor materials and, to some extent, of metals and superconductors. The applications of THz radiation imaging and sensing systems are in: security and surveillance; medical imaging; non-destructive testing and process control; food and drink testing; and research applications in biological and physical science.

Finally, there is a growing interest in developing communications systems at THz frequency as then a very large bandwidth is available and enormous amounts of information can be transported.

Is there THz radiation in space?

Despite the difficulty in generating THz radiation on earth, the entire Universe is pervaded with it! The well-known “cosmic background” stretches well into the THz band; also, there is specific THz radiation emitted from various objects such as stars and galaxies. Thus, it is possible to build up a THz sky map! THz astronomy has been one of the driving forces of the subject over many years and the builders of specialist detection equipment for this field have been very, very ingenious in delivering remarkably sensitive detection systems that can peer further into space (and back in time) than can be easily imagined.

How do you generate THz radiation?

All forms of electromagnetic radiation can be viewed either classically (as electric and magnetic fields) or from a quantum viewpoint (as photons). Normally, sensitive optical detectors will rely on the photon interpretation and sensitive millimetre wave detectors will rely on the classical description in order to explain how they work. The best THz detectors use very sanative detection in the changes of the electrical properties of semiconductors or superconductors at low temperatures and are only found in research laboratories or observatories. Other detectors, for example those used in satellite-borne equipment for use in the lower frequency (submillimetre) part of the band rely on “cat’s whisker” structures where the electrical properties of the junction between the metal wire and the semiconductor are changed by the presence of the THz electric field. In many of the systems used for THz imaging, a radically new technique is employed that has been borrowed from radar. This is known as “coherent detection” and relies on the detector being “woken up” when a THz pulse arrives. Finally, some of the most convenient detectors simply rely on the heating effects of THz radiation to change the volume of a small amount of gas. As the volume is changed, the movement of the wall of the container holding the gas is tracked using a simple optical set-up.

Can you buy THz Equipment?

Yes, you can. There are number of companies who now make off-the-shelf systems for THz applications. In Europe, these companies include AB Millimètre, Teraview and Thruvision Ltd. In addition, several companies provide specialist components needed to build various THz systems. These include Farran Ltd, QMW Instruments, Radiometer-Physik GmbH and Femtolasers.