New Opportunities for Terahertz Research

Gregory Flinn

The “terahertz gap” occupies frequencies between 300 GHz and 10 THz and lies between the traditional microwave and optical regions. The term “gap” derives from the historical inaccessibility of this region, a consequence of poor component performance for both generation and detection.

Recent advances, however, have rapidly paved the way to the commercial development of terahertz systems, so much so that the February 2004 issue of Technology Review from Massachusetts Institute of Technology in Cambridge identified terahertz rays in general as one of the 10 most important emerging technologies for that year.

Figure 1. Terahertz emitters and detectors designed for optimal efficiency at specific frequencies offer improvements of up to an order of magnitude in CW imagingperformance compared with the broadband designs used for pulsed systems (spiral). Courtesy of TeraView Ltd.

Besides traditional fields for terahertz technology, such as space and atmospheric sciences, there are related applications in remote sensing and space-based or short-range terrestrial communications. Moreover, many visually opaque materials are partially transparent and/or exhibit molecular resonances at terahertz frequencies, so this technology provides the biomedical and biochemical fields with new diagnostic tools for tomographic imaging and spectroscopic characterization. This ability of terahertz radiation to penetrate an exterior covering also opens potential applications for security screening and process control.

Funding opportunities

Funding for terahertz research is available from a variety of sources worldwide. Numerous opportunities were aired at a recent symposium of the Southeastern Universities Research Association in Washington in March 2004, and a summary is expected at the Optical Society of America’s topical meeting on optical terahertz science and technology in Orlando, Fla., in March. This latter meeting, together with the optoelectronics symposium at Photonics West in San Jose, Calif., this month, will bring the latest research to the fore.

In the US, funding from the Defense Advanced Research Projects Agency and other military bodies generally supports research for security issues, such as the identification of hidden explosives and dangerous chemicals via molecular signatures, concealed weapons recognition through imaging, and short-range communications in battlefield conditions. This last application makes use of the strong absorption of terahertz radiation by atmospheric moisture to prevent transmitted signals from reaching enemy receivers.

Water’s strong absorption gives rise to funding for nonmilitary applications. For example, recent studies show that terahertz imaging can distinguish malignant and healthy tissue, with results comparable to, or better than, histological methods. Recognizing the potential, the National Institutes of Health is encouraging biomedical researchers to propose terahertz-related projects.

In Europe, countries individually and jointly under the European Commission have been funding research into terahertz technologies for some years, much of it in the UK, France, Germany and Austria. The latest pan-European collaboration, TeraNova, partners industry and university research groups to improve the technology for applications including biotechnology, security and health care.

Aside from this mainstream research, the nondestructive and spectroscopic capabilities of terahertz imaging also lend themselves to industrial process-control applications. Quality control within the pharmaceutical and food packaging industries, for example, are two areas of manufacturing where terahertz technology may find a place. Companies in the UK, Japan and the US are targeting these markets, and a small but growing selection of equipment already is available for industrial and scientific research.

Further applications may be realized in the fields of plant physiology and wireless communications. In the former, biologists have proposed studying plant physiology with terahertz radiation on the premise that a plant’s absorption and transport of water is an indication of health. The technology thus may contribute to the nutritional value of farmed crops. In the latter, the higher frequency of terahertz radiation compared with gigahertz waves may provide much-needed bandwidth for the field of short-range communications.

New terahertz tools

Various methods for CW or pulsed terahertz generation abound. One may use electronic and/or multiple harmonic techniques to reach high-gigahertz to low-terahertz frequencies from the millimeter-wave region. Alternatively, frequency mixing/switching or nonlinear processes with visible or near-IR lasers may be performed. Other methods generate coherent terahertz waves directly, using intraband transitions in semiconductor devices or IR-pumped molecular transitions.

Figure 2. Terahertz imaging reveals a concealed ceramic knife. State-of-the-art systems produce 60 dB of dynamic range and diffraction-limited resolution.

Until the past few years, none of these techniques provided the affordability, compactness, efficiency or operating convenience required for commercial application. That is no longer the case. On the macroscopic level, improved efficiency and better beam profiles from electronic sources make them an attractive and reliable option for field-deployable applications. On the microscopic level, advances in quantum cascade laser design have pushed the useful operation regime to temperatures well above 100 K and have suggested the feasibility of high-power sources for niche applications in which thermoelectric cooling is practical.

One technique, the switching of optical frequencies into the terahertz range, has been widely adopted for a fast track to commercialization. In its most effective variant, pulses from a near-IR femtosecond laser are absorbed in the uppermost few microns of a specially engineered GaAs structure, thereby generating photocarriers in the conduction band. The application of a high potential gradient between two contacts on the GaAs surface causes a current transient to flow, and in a process analogous to the rapid discharging of a capacitor, an appropriately designed structure radiates a broadband pulse of terahertz radiation into free space, much like an antenna.

In general, the frequency content of the terahertz pulse is inversely related to the duration of the pulse from the mode-locked laser, and bandwidths in excess of 10 THz have been achieved with ultrashort pulsed lasers. Much work has gone into improving this technology, including optimizing the GaAs growth conditions, the optical absorption and the terahertz emission profile. For pulsed generation, the smaller sizes and falling prices of femtosecond lasers, as well as an easing of the support requirements in some instances, greatly increase the devices’ attractiveness for industrial applications.

An alternative is the use of two CW near-IR sources (or even one source lasing on two lines), with the generated carrier density subject to modulation at the difference in frequency between the two optical fields. Although the conversion efficiency is considerably lower for this approach than for the other, the spectral intensity is higher, and resonant and structural enhancements of absorption, emission and detection parameters provide for an imaging performance that can match that obtained with pulsed terahertz systems. Importantly, the deployment of distributed feedback laser diodes for the optical fields may lead to a rugged, compact, low-linewidth terahertz source.

The current terahertz detectors are based on bolometers, nonlinear optics and semiconductor-absorption technologies, with a recent emphasis on using semiconductor engineering to yield better performance. In addition, topical research focuses on quantum-dot devices, advanced bolometers and superconducting tunnel junctions for detection. Related work is devoted to the development of CCD-like detector arrays for advanced imaging applications. Detector design, however, often is specific to the source, and the choice of technology is complicated by the broadband background terahertz radiation.

Meet the author

Gregory Flinn is an independent technical writer based in Munich, Germany; e-mail: gregory.flinn@gmx.net.