In the fictional Star Trek universe, a tricorder is a handheld device used for scanning an area, interpreting and displaying data from scans to the user. Now similar imaging systems that can detect naturally occurring terahertz radiation with unprecedented sensitivity and resolution may soon be able to detect early tumors.
Researchers at the National Institute of Standards and Technology (NIST) have demonstrated the new imaging system that may become a new tool for chemical and biochemical analyses ranging from early tumor detection to rapid and precise identification of chemical hazards for homeland security instruments.
Terahertz radiation falls between microwaves and infrared radiation on the electromagnetic spectrum, with frequencies from about 300 billion cycles per second to about 3 trillion cycles per second.
All materials such as rocks, plants, animals and people emit terahertz waves each having its own frequency pattern as a kind of "fingerprint." They can pass through smoke, clouds and many solid materials like clothing, and in some cases, even walls. Detecting and measuring them is a unique challenge because the signals are weak and absorbed rapidly by the atmosphere.
The NIST prototype imager uses a sensitive superconducting detector combined with microelectronics and optics technologies to operate in the terahertz range. The system has its best resolution centered around a frequency of 850 gigahertz, a "transmission window" where terahertz signals can pass through the atmosphere. The system can detect temperature differences smaller than half a degree Celsius, which helps to differentiate between, for example, tumors and healthy tissue.
The system has a tiny device that measures incoming terahertz radiation by mixing it with a stable internal terahertz signal. This mixing occurs in a thin-film superconductor, which changes temperature with even a minute amount of radiation energy. The slight frequency difference between the two original terahertz signals produces a more easily detected microwave frequency signal.
The device and antenna, combined with an amplifier on a chip smaller than a penny, was developed by NIST in collaboration with the University of Massachusetts. Called a hot electon bolometer (HEB), the technology is sensitive enough to detect the weak terahertz signals naturally emitted by samples, eliminating the need to generate terahertz radiation to actively illuminate the samples. This greatly reduces complexity and minimizes safety concerns. In addition, the NIST "mixer" system delivers more information by detecting both the magnitude and phase (the point where each individual wave begins) of the radiation.
Because passively emitted signals are so weak, the current system takes about 20 minutes to make a single 40 x 40 pixel image. NIST researchers are working on an improved version that will scan faster and operate at two frequencies at once. Future systems also should be able to achieve better spatial resolution.
Other organizations have been working on imaging techniques using terahertz radiation.
ThruVision Ltd, a spin out company of the Science and Technology Facility Council, Rutherford Appleton Laboratory (RAL), UK unveiled earlier this year security imaging technology that can "see" explosives, liquids, narcotics, weapons, plastics and ceramics hidden under clothing from 25 metres.
TeraView Ltd, Cambridge UK a spin-off of Toshiba Research Europe recently announced a number of contracts to supply terahertz systems to partners for evaluation in the detection of explosives at different 'stand-off' distances. This laboratory system will be used to evaluate future applications in suicide bomber identification and screening at airports and other installations.
A team of researchers, which includes scientists at the Louvre Museum France, Picometrix LLC USA and University of Michigan USA, used terahertz imaging to detect colored paints and graphite drawing of a butterfly through 4 mm of plaster. A paper was written last year Terahertz imaging for non-destructive evaluation of mural paintings.
Flexographically printed metamaterials are also promising for terahertz devices.
Also read an earlier article "Metamaterials: Printing the cloak of invisibility".
References: NIST, ThruVision, TeraView, University of Michigan