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. Xi-Cheng Zhang, founding director of the Center for Terahertz Research, is looking at the potential of using THz radiation to warn of chemical and biological hazards. His group is developing tools to prescreen sealed envelopes, cardboard and plastic containers, and other objects to discover if there are dangerous materials within. The research team has demonstrated that 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.) In other work, Zhang’s group is exploring the potential of T-rays for detecting TNT, C4, and other explosives, and the researchers have obtained promising preliminary results. In another security-related project, about 16 researchers on Zhang’s team are working on a T-ray imaging system that 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, Zhang says, it will be possible to build a portable system that would be of great use to the military or to law enforcement officers. Zhang’s group has also proposed a THz method for long-distance sensing of objects buried in soil. This technology could be used to discover buried landmines, both for homeland defense and for humanitarian projects.

Mastering the THz frequency range opens up promising new routes to such scientific advances as medical imaging without harmful radiation, superfast computing, and new instruments for observing quantum phenomena. THz science took a major leap forward a half dozen years ago, when Prof. Hyun-Shik Kang of the Chonbuk National University in Korea came to Rensselaer for a year’s sabbatical. At the time, Zhang was searching for a more efficient crystal that could be used in an emitter for THz imaging and sensing. Kang grew a unique form of ZnTe (zinc-tellurium) crystals, and he suggested Zhang give them a try. ZnTe crystals are now the basis for THz emitters and detectors in more than 100 laboratories around the world, a development that has made THz imaging possible.

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. They have taken pictures of fresh and drying leaves, bacon, insects, and computer chips.

Now Zhang is using more advanced systems for two promising biotechnology projects. He is working with doctors at the Boston Medical Center to explore the possibility of an improved method to detect breast cancer and with the Australia Biomedical Engineering Center to study T-ray imaging of bone marrow cancer samples.

One problem with T-ray microscopes has been their 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. Once again, the images do not subject patients to 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.

Rensselaer’s researchers use the THz radiation that lies in the gap between the infrared and microwave bands to obtain 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.

Working since 1996 with Rensselaer’s Center for Advanced Interconnect Systems Technologies, Zhang has invented ways to use THz spectroscopy to measure the dielectric response of semiconductor materials, information about conductivity that is vital to the electronics industry.

Wilke also is interested in ultrafast and THz time-domain spectroscopy, using these techniques to measure dielectric and superconducting properties of thin films and bulk materials.

Her expertise in femtosecond (one millionth of a nanosecond) optical lasers has drawn her into another research area, the use of laser pulses to punch holes in biological cells. This new procedure, first used in Germany, has enormous potential for genetic technologies such as targeted gene therapy or delivery of drugs into living cells. Unlike other methods in which large numbers of cells die, 100 percent of the cells survive in this approach.

In her work at the University of Hamburg, Wilke created another innovative use of THz technology, using THz detectors to measure the length of very short electron bunches. This information is important in the operation of the newest generation of linear accelerators. Her method is now in use at DESY in Hamburg, at Fermi Lab, at the Stanford Linear Accelerator Center, and other accelerator facilities are picking it up.

She plans to continue this work at Rensselaer, using THz technology for further experimental and theoretical studies of the principles behind these measurements as well as for studies of radiation damage in electro-optic materials.