Source: Advanced Imaging Magazing.

    If it¡¯s in the spectrum somebody will find a use for it. Such is the case with terahertz waves, that occupy the space between microwaves and the infrared optical band (from 30 microns to 1 mm). These ¡°T-rays¡± provide some exciting capabilities, but there are currently some notable challenges.

    T-rays can pass though thin layers of non-metallic substances like clothing, wood, plastic and ceramics and can penetrate up to several centimeters of flesh at some frequencies while reflecting back. Because they are non-ionizing, there is no real risk to living subjects like you find with X-rays. They provide images comparable to backscatter X-rays, and with known transmission/reflection spectra a variety of materials can be easily identified including many common explosives and drugs.

    ¡°There are other techniques like X-rays and millimeter waves where you can image through clothing, but what terahertz allows you to do is when you see something suspect you can then go to that pixel in the image and you can pull out the spectral fingerprints that terahertz spectrum and you can actually use that to identify what the object is,¡± said Dr. Don Arnone, CEO of Teraview (U.K.), a company that has taken an early lead in providing fieldable terahertz technology. ¡°For example, a piece of polyethylene and the leather in a wallet have very different spectral signatures compared to Cemtex or TNT, etc. so it¡¯s a combination of imaging through many common objects allied with the ability that once you actually detect an object in an image you can in some cases identify what it is by comparing its spectrum to a library of spectra.¡±

    Typical applications for this technology include security screening, semiconductor inspection, 2D and 3D imaging, food inspection, pharmaceutical inspection, medical diagnosis in areas like skin cancer and various dental applications.

    However, as noted, there are limitations. T-rays cannot penetrate metal, and are heavily absorbed by water. While they can penetrate fog, clouds and soil the moisture present in all three currently limit the effective range, as is generally the case in the atmosphere. Also, there exists a shortage of engineered, fieldable hardware that will allow terahertz imaging to occur at practical speeds relative to many application goals.

    ¡°The big application at the moment is pharmaceuticals,¡± Williams said. ¡°You look at the uniformity in the morphology of the drug and, say you¡¯re making 10,000 in these pills, you would take one and thoroughly examine it in a couple of hours and hope that the rest of the lot is of the same quality. A higher power source might allow each pill to be tested. If we can do video frame rate imaging, as we walk through an airport a beam would come underneath the carpet and examine our shoes and we would not have to take them off.¡±

    Boosting Illumination

    Research is being conducted in both terahertz illumination and sensor development to facilitate real-time imaging applications, as well as solve problems relative to other applications such as communications. Illumination has typically been provided by infrared lasers, which results in a marginal power output and the requirement to use a laborious scanning process. MIT professor Qing Hu has taken this a step further through the use of a quantum cascade laser that can hit between 100mW and 250mW depending upon the frequency. This allows for some practical real time imaging with distances of a few meters.

    Pushing the wattage further, researchers at Thomas Jefferson National Accelerator Facility (Newport News, Va.), the Brookhaven National Laboratory (Upton, N.Y.) and Lawrence Berkeley National Laboratory (Berkeley, Calif.) sent a beam of electrons at nearly the speed of light through a magnetic field to generate terahertz radiation. ¡°We took a look at all these images that are done by scanning techniques, because the sources are usually pretty low in intensity¡ªhundreds of microwatts or something¡ªand so there are some very promising images of concealed weapons in shoes and on people, but such images typically take 10 minutes to an hour to take,¡± said Gwyn Williams, Jefferson Lab¡¯s basic research program manager, Free Electron Laser Facility. ¡°We have a source that is up to 100W and we typically run it at about 10W. So now that we have 10,000 times more intensity at the source, we¡¯ve been trying to do real-time video frame rate imaging without scanning. The problem with that, is that there really are no cameras built for terahertz with large fields of view, so while we can illuminate targets with several watts of terahertz we¡¯ve been trying to build cameras out of components because there is really nothing that has been built for this.¡±

    For his research, Williams borrowed a thermal imaging camera from the University of Delaware. ¡°I had made some calculations about how much power you would need to get an image and this camera seemed to satisfy the criteria because you have to look at an image in the 60th of a second and in fact we can get images and a 60th of a second. The problem is, it did not work initially because many of the lens components and windows inside it were not made of a material that transmitted terahertz very well. So this is an area were actually working on,¡± he said. ¡°People know how to detect it, it¡¯s just that you have to build an array of these detectors and put them on a chip with a read out and everything which is straightforward but you¡¯re talking hundreds of thousands of dollars to a million for the first one but then $5,000 each after that.¡±

    Metamaterials

    Research is also being conducted into materials that are more conducive to developing terahertz hardware. ¡°The major problem with terahertz imaging is that we do not have enough hardware to use to detect and manipulate the terahertz radiation,¡± said Hou-tong Chen, a post doctorate research associate at Los Alamos National Laboratory (Los Alamos, N.M.). ¡°This is primarily a materials issues, since many materials do not have terahertz response. A terahertz modulator and phase shifter are not present currently. This is being actively pursued internationally, for example, in developing terahertz quantum cascade lasers to provide a powerful source of generating terahertz radiation. And on the terahertz detection aspect it is also challenging with electro-optic and bolometric methods being pursued. The goal is to have as many useful devices, from lamps to modulators and switches to manipulate terahertz radiation in amplitude, intensity, phase and polarization manipulation.¡±

    Chen¡¯s research, in collaboration with his former supervisor former supervisor Richard Averitt, now at Boston University and Willie Padilla, has centered on effectively modulate the intensity of terahertz radiation using metamaterial-based devices. Metamaterials are artificially constructed materials that derived their properties from the structure rather than from the materials they are composed of, which results in some unusual phenomenon not occurring with natural materials.

    ¡°Naturally occurring materials rarely have the terahertz response we are looking for and we can manipulate metamaterials to operate at terahertz frequency,¡± said Chen. ¡°We use the resonance responses of metamaterials to enhance the terahertz wave and material interaction with the goal of controlling the terahertz response with actively applied voltage or photon excitation. Currently these devices are still at the laboratory phase, but the technology is successful and solid. I suspect this will lead to practical applications in the near future, perhaps just a couple of years.¡±

    Williams agrees that the future is bright for terahertz imaging, and sees the problem as being more financial and engineering that research or science. ¡°I think we have a really good handle on the physics now, how to make the terahertz and how to detect it,¡± he said. ¡°Now, we¡¯re into an engineering/physics situation which involves amounts of money in the millions which is above a lot of thresholds. Getting prototypes built is going to be expensive, but after that it gets cheap again.¡±