Handheld THz Instrumentation

http://spie.org/x86630.xml Research is advancing toward THz system miniaturization, once thought nearly impossible to achieve. By X.-C. Zhang and Albert Redo-Sanchez Handheld THz Instrumentation Research is advancing toward THz system miniaturization, once thought nearly impossible to achieve. By X.-C. Zhang and Albert Redo-Sanchez Technological progress in recent years in the area of terahertz (THz) instrumentation and systems has been driven mostly by THz wave spectroscopy applications. THz spectrometers have become more compact and are easier to use for non-expert operators. They also offer improved performance compared to home-made systems built in the laboratory. The trend of accelerated technological progress in THz instrumentation indicates that the gap between application needs and performance and functionality is getting narrower as time advances. The micro-Z, a handheld, battery-operated THz-wave broadband spectrometer, is an example of recent progress in the THz field that realizes devices that were almost unthinkable a few years ago. The trend toward size reduction, higher component integration, and performance improvement is expected to have an impact on high-demanding THz applications such as THz wave sensing, especially in THz pulsed systems for homeland security and nondestructive evaluation (NDE).    Zomega’s Brian Schulkin holding the micro-Z handheld and battery-operated THz TDS spectrometer. Photos courtesy Zomega. THz applications in defense THz wave spectroscopy covers the part of electromagnetic spectrum between 0.1 to 10 THz. Funding agencies have driven much of the recent technological evolution in THz instrumentation and systems, and most of the progress has been toward the development of THz time-domain spectroscopy (TDS) systems. THz TDS has been demonstrated to be an excellent technique for characterizing and identifying many organic compounds, especially explosives and their related compounds.1-4 A TDS system generates and detects a THz pulse (or waveform) in the time domain and computes the spectrum by performing a Fourier Transform on the waveform. Most applications of THz TDS spectroscopy and NDE have been explored intensively in a laboratory environment.5 Industry pushes boundaries In any field, there is always a gap between the demonstration of a new technology in an academic lab and the ability of a wide user base to put the technology into practice for their own particular applications. Usually, it is the role of industry to close that gap by providing user-friendly tools, instruments, and systems to meet practical needs. Companies such as Picometrix (USA), TeraView (UK) and Zomega Terahertz Corp. (USA) have been pushing the boundaries of THz technology to realize promising applications with THz waves. For example, Picometrix was one of the first companies to develop a system targeted to NDE applications. TeraView has focused its efforts on the development of THz spectroscopic systems for the pharmaceutical industry, and Zomega has focused on developing compact and portable THz spectroscopic systems for research and NDE applications.    Terahertz image of moth wings from a Zomega system. THz systems are getting small Most companies offering THz products have been making the technology easier to use for non-expert operators by reducing the size, weight, and power requirements, improving the performance of emitters and receivers, and being sensitive to sample location and shape. For instance, most commercially available THz systems are designed to operate on a laboratory bench or table, and samples have to be prepared and taken to the device. Furthermore, even relatively portable systems require supporting equipment and facilities such as wall-plug power. However, in the last year, a Rensselaer-Zomega team has designed and built a fully integrated, turnkey, handheld, and battery-operated THz wave spectrometer, the micro-Z. The 5-pound device is the size of a cordless drill and was a finalist in the 2011 Prism Awards for Photonics Innovation. Designed to be a lightweight tool for detection and identification of chemicals such as explosives and related compounds in the open field, this compact system is configured to work in normal-incidence reflection in a point-and-shoot fashion. It produces and measures pulsed terahertz waves from 0.1 to 3.0 THz using time-domain techniques in both transmission and reflection geometries, with a waveform acquisition rate of 500 Hz and a time delay >100 ps. Battery life during operation is up to 12 hours. The high waveform acquisition rate allows compensating for the operator hand tremor frequencies, measured to be between 1 to 10 Hz. Although some progress is still necessary for a fully deployable system in a battlefield environment, the headway achieved with micro-Z is just one example of the efforts to address the needs of users. Once thought very difficult or impossible to achieve, we expect that this trend of miniaturization, compact design, and ease of use of THz systems will continue in the future. THz-wave imaging has potential Looking ahead, we anticipate that a major driver for the technological development will be imaging applications. THz-wave imaging has shown its potential in NDE and security applications6-12 because THz waves can penetrate dry, non-metallic, and non-polar materials such as cloth, paper, cardboard, and ceramic, which enables the interrogation of optically non-transparent targets. Moreover, because the wavelength is on the order of millimeters or less, the resolution of the images is also a few millimeters, which can provide great detail in images of macroscopic objects. The use of THz for imaging has also been driven by security applications, such as full-body scanners now being tested at many airports. In this application, companies such as Thruvision (UK) and Brijot (USA), acquired by Microsemi in 2011, have been leading the technical effort. Advantages of CW system The THz technology in such a system is continuous wave (CW) as opposed to the pulsed approach of spectroscopic systems. Because most such imaging systems are single-pixel and therefore require raster scanning to form an image, CW had several advantages over pulsed systems, including faster data acquisition speeds and higher output powers. However, pulsed systems offered unique features for NDE imaging applications, such as spectroscopic imaging. As recently as a few years ago, acquiring a waveform required minutes to complete. This made imaging impractical, as it could take tens of minutes to acquire an image. With recent progress in fast waveform acquisition rates (up to 500 waveforms per second) and smaller footprint sizes, pulsed systems have suddenly become more feasible for imaging applications because system operation is no longer limited by the speed of acquisition but by the scanning speed. We expect technological development targeted at increasing the imaging speed and performance of these THz imaging systems for a variety of applications, including in biomedical testing and clinical trials. SPIE Member X.-C. Zhang is director of the University of Rochester’s Institute of Optics and chairman and president of Zomega Terahertz Corp. He is also distinguished professor at Wuhan National Laboratory for Optoelectronics and the former director of the Center for Terahertz Research at Rensselaer Polytechnic Institute. He has a PhD in physics from Brown University and frequently teaches courses for SPIE. Albert Redo-Sanchez is director of business development at Zomega Terahertz Corp. Formerly a research assistant professor at the Rensselaer Center for Terahertz Research, he holds a PhD in physics from University of Barcelona. References 1. J. Chen, Y. Chen, H. Zhao, G.J. Bastiaans, and X.-C. Zhang. “Absorption coefficients of selected explosives and related compounds in the range of 0.1-2.8 THz,” Optics Express 15(19), 12060–12067 (2007). 2. Y. Shen, T. Lo, P.F. Taday, B.E. Cole, W. Tribe, and M. Kemp. “Detection and identification of explosives using terahertz pulsed spectroscopic imaging,” Applied Physics Letters 86, 241116 (2005). 3. C. Baker, T. Lo, W. Tribe, B. Cole, M. Hogbin, and M. Kemp. “Detection of Concealed Explosives at a Distance Using Terahertz Technology,” Proceedings of IEEE 95(8), 1559–1565 (2007). 4. Existing and Potential Standoff Explosives Detection Techniques, National Research Council of the National Academies Press. (2004). (ISBN: 10: 0-309-09130-6; ISBN-13: 978-0-309-09130-5) 5. M. Tonouchi. “Cutting-edge terahertz technology,” Nature Photonics 1, 97 (2007). 6. C. Baker, T. Lo, W.R. Tribe, B.E. Cole, M. Hogbin, and M.C. Kemp. “Detection of Concealed Explosives at a Distance Using Terahertz Technology,” Proceedings of the IEEE 95, 1559-1565 (2007). 7. K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue. “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Optics Express 11, 20, 2549-2554 (2003). 8. A. Dobroiu, M. Yamashita, Y.N. Ohshima, Y. Morita, C. Otani, and K. Kawase. “Terahertz imaging system based on a backward-wave oscillator,” Applied Optics 43, 30, 5637-5646 (2004). 9. J.P. Dougherty, G.D. Jubic, and W.L. Kiser. “Terahertz imaging of burned tissue,” Proceedings of SPIE 6472, 64720N (2007). 10. N. Karpowicz, D. Dawes, M.J. Perry, and X.-C. Zhang. “Fire damage on carbon fiber materials characterized by THz waves,” Proceedings of SPIE 6212, 62120G (2006). 11. A.J.L. Adam, P.C.M. Planken, S. Meloni, and J. Dik. “Terahertz imaging of hidden paint layers on canvas,” Optics Express 17, 5, 3407-3416 (2009). 12. N. Karpowicz, A. Redo-Sanchez, H. Zhong, X. Li, J. Xu, and X.-C. Zhang. “Continuous-wave terahertz imaging for non-destructive testing applications,” IRMMW-THz, 1, 329-330 (2005).