Until recently the terahertz region has been the orphan child of the spectrum. Research in terahertz wave technology, however, has gathered momentum as evidenced by an increase in patent filings in recent years. The literature is unclear exactly what the spectral range is, but the Photonics Dictionary identifies it as between 0.3 THz and 10 THz. Other papers extend the range from 0.1 THz to 30 THz. Generically, it is known as the far-infrared region.
What has scientists interested in this portion of the spectrum is its ability to penetrate materials that are opaque to both visible and infrared radiations. It can penetrate wood, plastics, clothing, fog, and so forth, and images can be formed without contact. Also attractive is that terahertz radiation is non-ionizing, unlike biologically harmful x-ray or gamma radiation.
There are limitations, however. Terahertz waves do not penetrate metals and they are largely absorbed by water. Though T-rays can penetrate fog, clouds, and the ground, moisture levels affect the range or depth. Another limitation is that hardware used to detect or generate terahertz is difficult to develop because many materials do not function or are insensitive at these wavelengths.
Commercial applications for terahertz technology continue to expand. They include homeland security, military, aviation, and biological imaging, and new applications are growing. Better security screening to view hidden materials that x-rays would pass through, the ability to pierce bad weather, on-line pharmaceutical or microelectronics inspection, predicting tooth decay, spectroscopy, analyzing cell dynamics, and early detection screening for various cancers are just a few of the practical applications envisioned or in experimentation. People are beginning to explore methods to generate images in real time as the increase in intellectual property (IP) demonstrates.
One example of where terahertz imaging would be a great benefit is shown in patent application 2007050598/WO-A2 “Terahertz imaging non-destructive inspection systems and methods,” which describes an inspection method very similar to dye penetrant inspection. The dyes used in traditional inspection are usually toxic and require special environmental handling to remove and dispose. With terahertz imaging, common water could be used as the penetrant.
The greatest interest in terahertz imaging is in security. Beyond identifying hidden metal weapons, terahertz technology also can characterize hidden materials such as plastic explosives or biological agents. Each material has a unique signature that would appear as a different color that then can be compared to a library of known chemicals. This application is being aggressively pursued for the postal and package shipping industry. The ability to rapidly inspect thousands of letters and packages for a biological agent, such as anthrax, without any delay in delivery or damage to materials is attractive.
Just a few months ago the U.S. Homeland Security Department started testing a system at the international airport in Phoenix, AZ. The system uses backscatter technology that looks through clothing but penetrates only a couple of millimeters of a human body surface. The image is much more revealing than most people are comfortable with, and the American Civil Liberties Union has called this technology a “tremendous invasion of privacy.” New patents addressed this issue by developing terahertz thermal imaging that allows the sensor to map a profile of a body, similar to an infrared image, without the details.
Patent 07202808 “Surveilled subject privacy imaging,” assigned to SafeView Inc. (Santa Clara, CA), has offered another solution to imaging people subjected to surveillance. The thought being people might be more willing to allow surveillance if it is performed in a manner that protects their privacy. The technology uses a subtractive method of two images to highlight only the object of interest. SafeView, which was acquired by L-3 Communications in 2006, is one of the companies identified in the heat map (Fig. 2) as a leader in this field.
Figure 2. A heat map identifies the major players in terahertz based on numbers of patents and applications.
Because this range of wavelengths is largely absorbed by water, success in whole body imaging for therapeutic analysis has been problematic. Development of more sensitive detectors and more powerful sources are on the rise. The advantages over more conventional methods are non-ionization of tissue, ability to identify most chemicals and disease tissues, and high spatial resolution.
Pharmaceutical chemical inspection is a more near-term reality. The ability to conduct a non-destructive inspection of every pill for proper fill, contamination, or correct mixture of compounds is possible. Moreover, it will be easy to identify counterfeit or incorrect dosages. Clinical researchers hope to be able to examine interactions between drugs and enzymes, too.
Still others hope to discover new information about biological molecules with this new spectrum analysis. An example is Patent 06957099 “Method and apparatus for terahertz imaging,” assigned to TeraView/Terraprobe (Cambridge, UK), another leader identified in the heat map, which describes taking advantage of the “windows” in the terahertz spectra absorbance rate for water to identify cancer tumors.
The patent reference portfolio that was analyzed for trending was based specifically on terahertz imaging. To represent the innovation rate taking place within this area, the portfolio was filtered to account for filings of a particular patent within multiple patenting jurisdictions.
It is encouraging that the pace of recent patent filings indicates many of the applications envisioned are closer to reality. The following graph (Fig. 1) shows that only two patents were filed in 1995 on terahertz imaging, with a continuing growth curve to today. Note the significantly higher difference in the average rates of patent filing from 2000 through 2002 and 2003 through 2005. The misleadingly smaller total in 2006 is due to the delay between patent publication and filing, and based on the expanding number of application areas, the overall publication pace is expected to be on par with the most recent activity.
Fig. 1. Number of terahertz patents by year and application areas.
The graph also illustrates the increasing breadth of topic areas being addressed by terahertz imaging, as demonstrated by expansions into multiple International Patent Classification (IPC) code areas. While the graph is broken down by the major classification areas, further analysis into IPC sub-classifications shows emphasis on investigating or analyzing materials by “determining their chemical or physical properties” (G01N), and also applications such as “diagnosis, surgery, identification” (A61B).
The heat map (Fig. 2, above) identifies the major players in the area based on numbers of patents and applications. Each small rectangle represents one patent reference. Green indicates very recent references, and as the colors migrate to red, they indicate older references.
As can be seen, TeraView/Terraprobe has a large number of fairly recent applications and is fourth in number of granted patents. Lucent has been an acknowledged leader in the field, although it appears their patenting activity in this area has slowed.
Possible license opportunities may exist with Rensselaer Polytechnic Institute (Troy, NY) and New Jersey Institute of Technology (Newark, NJ), as they both have active research programs. The U.S. Government through the Department of the Army has already licensed the technology behind U.S. Patent 7,087,902, which was issued in August of 2006 to Rensselaer Polytechnic Institute. This patent is titled “Fresnel lens tomographic imaging” and describes systems and methods for reconstructing a plurality of images of an object.
Nikon (Melville, NY) has a U.S. application 20010029436 titled “Semiconductor electrical characteristics evaluation apparatus and semiconductor electrical characteristics evaluation method” that could be used to measure the electrical material quantities (such as the carrier density, the mobility, the resistivity, and the electrical conductivity) without contaminating or damaging the semiconductor material.
While this report focuses on the companies that are researching imaging using terahertz technology, others are looking into basic research on the hardware to either detect or generate terahertz waves. Patent history goes back to the early 1990s, but the real growth has been only in the past four years. People can envision the benefits of this technology, although significant hurdles remain to achieve many of the benefits.
Homeland security and the package shipping industry have immediate needs for portable imagers to detect biological signatures not possible with current non-contact equipment. Pharmaceutical companies also view the potential with great interest. The development of meta-materials, materials that are artificially constructed, is another area that can be researched.
An example of how far afield terahertz applications are being investigated can be seen in patent WO2005060644, titled “Gravitational Wave Propulsion and Telescope,” assigned to Gravwave LLC (Playa del Rey, CA). It describes a gravitational wave generator that uses terahertz to excite a mass of nuclei in a controlled fashion to propel an object. This device produces waves in the space-time-universe continuum and can be used to explore cosmological theories.
The terahertz portion of the spectrum opens an area of science only dreamed of previously, and will be worth watching as the technology matures.
Nerac Inc. (Tolland, CT) is a business, technology, and intellectual property research firm providing customized research services for scientists, engineers, and IP professionals. Nerac analysts work with clients to develop or refine a technology, explore market opportunities, and evaluate IP strategies. Find out more at www.nerac.com.
Nerac analyst and SPIE member Jerry Burke's primary focus is medical devices and optics. With 45 years of experience in engineering and R&D, he has been involved with developing fiber optic devices used in imaging and illumination.
Nerac analyst Patrick Terry's expertise is the advanced analytics of IP portfolios. An information technology and engineering professional, he has developed and executed information management strategies as well as application technical strategies.