:Terahertz:

Terahertz-camera  

The European Space Agency StarTiger project has taken the first terahertz picture of a human hand. Terahertz radiation is on the boundary between radio and light waves and is far more difficult to detect and analyse than either, but is of interest for medical, security, environmental and short-range communication uses; the technology could, for instance, theoretically carry wireless data at terabit speeds over short distances. It is absorbed by the atmosphere ruling out long distance communication.

ESA originally investigated the radiation for sensing atmospheric and ground phenomena from satellites, but it is now examining terrestrial applications of the new frequencies. "We have recognised the huge potential in non-space applications, and in parallel to exploiting the use of terahertz waves and the StarTiger technology in space, we have kicked-off a commercialisation study to identify the best way of transferring it into terrestrial systems," said Pierre Brisson, head of ESA's Technology Transfer and Promotion Office.

One terahertz is 1,000 gigahertz, and most current radio technology stops at around 100 GHz: 0.1THz. Everything gives off terahertz radiation naturally, and like radio waves -- but unlike heat or light -- the waves can pass through some solid objects. Like light, it is possible to focus the radiation and create images as if the intervening material were translucent, and by analysing the frequencies given off the chemical and physical characteristics of the object can be worked out.

Terahertz radiation has wavelengths too short for normal radio antennae to pick up but too long for normal optical techniques and thus the band has been closed to experimenters and scientists in the solid-state era. Until now, the only known user of the frequencies has been a species of moth. By using nano-engineering techniques to create micro-machined arrays of minute antennae, the StarTiger team has created a sensor array that can image objects at 0.2THz and 0.3THz.

A terahertz imager can show details of features under the skin, confirming the potential of this technique. The project has released images of a hand taken through 15mm of paper, while related work by UK technology company Qinetiq shows pictures of the human body imaged through clothing.

Because the field is so new and unexplored, many applications are still to be tested. Detecting explosives or biological agents in parcels, cancers beneath the skin, the state of wounds beneath dressings, and seeing through fog: all have been suggested by researchers.

Focusing on Breast Cancer

Faculty members in Rensselaer's Center for Terahertz Research believe terahertz radiation, or T-rays, could provide sharper, safer, more informative, and less expensive images for breast cancer detection than current X-ray techniques.

Screening mammography is now the only practical method to detect breast cancer lesions of 1 cm or less, when they are 95 percent curable. But mammography subjects large numbers of women to a sometimes painful test that exposes them to potentially harmful radiation.

Mammography also is less effective when imaging radiographically dense breasts, and it creates the need for many biopsies in women who do not have cancer.

T-rays are non-ionizing and do not require heavy lead shielding, and they can be focused, creating much sharper pictures. They can give spectroscopic information about the chemical composition as well as the shape and location of the tissue being imaged. T-ray techniques provide high-contrast information about tissues with different water concentrations and about the different bond states of water within the cells and tissues. This ability to identify both the physical and the biochemical nature of the cancer would be particularly valuable in diagnosis and choice of treatment in breast cancer.

T-Rays: A New Technology to Detect Previously Hidden Dangers

Terahertz (THz or T-rays) radiation has shown promise in developing new tools for national security, such as detection of chemical and biological hazards.

THz imaging can identify the presence of a powder and/or bacterial spores and can provide coarse specificity. In one experiment, Dr. Carmen Mannella from the Wadsworth Medical Center in Albany prepared three envelopes: one empty, one containing approximately 500 milligrams of Bacilus thurengiensis spores , and one containing approximately 500 milligrams of aggregated flakes of spores. The THz system was able to pick out the envelope containing spores and images of that envelope showed the location of the spores. These non-virulent spores were chosen because they are very similar to anthrax spores.

Researchers are exploring the potential of T-rays for detecting TNT, C4, and other explosives, with promising preliminary results.

A T-ray imaging system can look through walls, doors, and window curtains to locate people and weapons within a building. At present, T-rays can produce images for objects 15 meters away. If this distance can be increased to 100 meters or more, it will be possible to build a portable system that would be of great use to the military and law enforcement officers, while greatly reducing privacy and restricting personal freedoms.

THz could also detect objects buried in soil, such as landmines, both for homeland defense and for humanitarian projects.

A New Way of Looking at the World

THz frequency range opens up medical imaging without harmful radiation, superfast computing, and new instruments for observing quantum phenomena.

ZnTe crystals are now the basis for THz emitters and detectors.

Just as X-ray radiation can create images, THz radiation can form pictures, but T-rays have substantial advantages. Their lower frequency means they have lower levels of photon energy, which allows the imaging of biological tissue without the harmful radiation found in X-rays. They also offer a second type of information not available from X-rays, spectrographic data that provides a “fingerprint” of the molecular structure of the material being imaged. They can see through cloth, paper and other materials, which could help security personnel identify hidden weapons, and they can produce non-invasive images of moving objects, turbulent flows, or explosions.

In developing THz imaging systems, Zhang’s group has produced images so accurate they could reveal the number of pages in a book or the amount of money in a pile.

Doctors at the Boston Medical Center are exploring detection of breast cancer while the Australia Biomedical Engineering Center is studying T-ray imaging of bone marrow cancer samples.

One problem with T-ray microscopes has been resolution. Since one THz is 300 microns long, conventional T-ray spectroscopy could not be resolved under 300 microns, making it useless for imaging cells with diameters of 10 microns or less, or for inspecting individual semiconductor devices. Recently, Kersting’s group took a different approach, borrowing a technique from optical microscopy and achieved a resolution of less than one micron (0.8 microns). This makes it possible for the first time to take T-ray pictures of objects on micrometer scale, for instance, of biological cells.

X-rays are widely used in computed tomographic (CT) scans, and Zhang is working on a system in which T-rays would be used to create similar computerized 3-D pictures. As in X-ray CT scans, a THz tomographic system takes repeated images of sections of a 3-D object and then reconstructs them. The images do not subject patients to known harmful radiation, and they provide additional information about the chemical composition of the objects being scanned.

Last year the team made two major breakthroughs toward their goal of developing a THz wave tomographic imaging system that will provide the first-ever THz capability to produce real-time, large-scale, long-distance 3-D images. They improved the spatial resolution of THz CT images and demonstrated an improved type of THz CT, using a Fresnel lens, a special type of silicon lens. Recently, they fabricated a 10-cm plastic Fresnel lens, with which they are able to image a 3-D target from 1 meter away. Zhang hopes to greatly increase this distance for a variety of purposes, including security systems that could see gunmen and hostages through walls or locate buried land mines.

THz Spectroscopy: A New Test for Material Properties

T-rays do not always need to form pictures to provide valuable information. In conventional infrared spectroscopy, infrared radiation passes through a substance and the radiation is absorbed or emitted in a readable pattern. The spectrum formed as radiation passes through a material is individual to that substance and can be used to identify the material. Microwave spectroscopy can obtain other information about materials by analyzing the patterns formed when microwave radiation passes through.

THz radiation, in the gap between infrared and microwave, gives new spectroscopic information. Ultrafast lasers generate short pulses of broadband THz radiation for a process known as time-domain spectroscopy. This technique can obtain information about material properties that is not easy to acquire by conventional means.

Ultrafast and THz time-domain spectroscopy can measure dielectric and superconducting properties of materials.