TERAHERTZ SPECTROSCOPY: High-resolution terahertz spectrometer sniffs out chemicals

Terahertz imaging systems are well suited for industrial process control and monitoring to improve detection and identification of chemicals.

Goodrich Fig 1ab

Tunable absorption spectrometers using terahertz sources and detectors can detect chemical signatures in cluttered environments from 0.1 to 1.1 THz with line widths and repeatability at the megahertz level.

ALEXANDER MAJEWSKI

The terahertz portion of the spectrum lies between the microwave and optical regions. Many groups have been working to develop terahertz sources and detectors, either by multiplying-up from microwave frequencies, down-converting from optical frequencies, or direct terahertz generation and detection. A wealth of information and references are available about the details of both techniques. Each has its advantages and limitations based upon frequency range, output power, frequency control, detection speed, cryogenic cooling, and cost. When selecting a detector technology that uses a terahertz source, the requirements of the application determine the optimal combination.

Many applications for terahertz systems have evolved—the primary being imaging and spectroscopy. Because terahertz radiation propagates through common clothing and building materials, the imaging systems readily lend themselves to applications such as concealed weapons detection. Studies of terahertz imaging-system design have outlined this potential, and compared the performance of single-pixel versus array imagers with respect to scan time, signal-to-noise ratio, system cost, and complexity. Other phenomena within the terahertz region of the electromagnetic spectrum make them well suited for industrial process control and monitoring.1–5

Terahertz spectroscopy

One of the main spectroscopic applications for terahertz technology is a detection system for toxic industrial chemicals and explosives. Developing a chemical-detection system requires extensive knowledge of the materials to be detected and the background environment in which the system will operate. For example, a chemical-detection system located in the New York City subway system must be able to correctly identify threat chemicals in a cluttered environment consisting of cleaning agents, glues, diesel fumes, perfumes, paint, and a multitude of other chemical species. This scenario posses a unique problem for a terahertz detection system since a complete library of terahertz spectral signatures does not yet exist.

Detecting chemicals and terahertz spectral-library generation are the main applications for these terahertz systems. The phenomenology for terahertz spectral signatures is explained by their rotational-vibrational transition states.6, 7 Two of the widely referenced databases of terahertz spectra are the HITRAN and JPL databases which contain more than 300 molecules; nevertheless, signatures for many of the chemical threat agents are not documented.8, 9 Thus, one of the primary reasons for developing a chemical-detection system is to create a high-resolution spectrometer for augmenting existing spectral libraries. For gas-phase spectroscopy, the location of spectral absorption bands (frequency range of interest) typically spans the 0.1 to 1.5 THz range. The dependence of gas linewidth as a function of pressure is well known (with Doppler-limited linewidths in the megahertz range), therefore instrument resolution, linewidth, and repeatability at least in the megahertz range is desirable.7

High-frequency resolution allows one to take advantage of pressure-broadening effects, which are important for detection of multiple species in a cluttered environment. A common problem in the terahertz region is signature obscuration due to the broad absorption lines of atmospheric water. By reducing the pressure of the sample gas, linewidths are reduced and better spectral separation achieved. Also, standing wave artifacts dominate the terahertz region; poor frequency knowledge leads to inferior background removal and large residual artifacts in processed spectra. Hence, extremely accurate frequency control is required in generating spectral signatures with high confidence.

Spectrometer system

To meet these requirements, we have developed the TMS-700 and TMS-1000 spectrometers to add to the signature library database while concurrently implementing the system as a chemical-detection system. The TMS-1000 is the next-generation system of the TMS-700, with improved performance, reduced size, weight, and cost, with system deliveries planned in the spring of 2008 (see Fig. 1).

Goodrich Fig 1ab
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FIGURE 1. The Goodrich TMS-700 (left) and TMS-1000 (right) terahertz spectrometer systems operate at wavelengths from 0.1 to 1.1 THz with a frequency resolution of 1.4 MHz. (Courtesy of Goodrich)

Both spectrometer systems operate from 0.1 to 1.1 THz with frequency resolution of 1.4 MHz and system repeatability within a 4 MHz range. The TMS-1000 has been redesigned for portability with a volume of 8 ft3 and requiring only 7 A of wall power. The system is designed for measuring gases and vapor-phase spectra of liquids and solids. The architecture is based on continuous-wave photomixer technology. The system operates using a laptop computer with a graphical user interface; remote operation should be possible by the end of 2008.

The system uses an onboard pump through either a direct gas-line connection or the sample in the evaporative chamber to achieve gas transfer into the sample cell. Gas sampling can either be plumbed into a gas-line connection, or for ambient-air monitoring, an in-line filter is placed onto the sample connection port. The sample cell, a White cell design, allows for bake-out up to 200°C. The pump can achieve 70 torr vacuum in approximately 45 seconds. Spectral acquisition is typically on the order of two minutes for a medium-resolution scan (0.1 to 1.1 THz range with 500 MHz step size).

Spectrometer performance

We demonstrated and verified spectrometer system performance against several gases and library spectra. The gases tested include hydrogen cyanide (HCN), hydrogen chloride (HCl), ammonia (NH3), sulfur dioxide (SO2), water vapor (H2O), ethanol (C2H5OH), and methanol (CH3OH). Background spectra were acquired with the cell filled with either nitrogen (N2) gas or ambient air; background was removed from target gas spectra using customized software (see Fig. 2). We removed the background spectra from target data for 2250 ppm HCN data taken at atmospheric pressures. Hydrogen cyanide has well-known absorption lines spaced at 90 GHz intervals. Comparison of lab data to the HITRAN 2004 library shows excellent correlation with frequency accuracy better than 10 MHz. Departure between measured and modeled data at higher frequencies is a manifestation of instrument saturation rather than a modeling error. We then removed background spectra from target data for 2500 ppm SO2 data taken at atmospheric pressures. Sulfur dioxide also has well-known absorption lines.

Goodrich Fig 2ab
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FIGURE 2. Terahertz spectra of 2250 ppm HCN gas (observed labeled OBS) closely match spectra in the HITRAN library (top). Observed terahertz spectra of 2500 ppm SO2 gas matches that of HITRAN library spectra (bottom). (Courtesy of Goodrich)
(Courtesy of Goodrich)

In addition to detecting the signatures of target chemicals, detection of interferent signatures is also necessary to discriminate between target and interferent; detection of both also enables operation in a cluttered environment in the presence of numerous other materials. The TMS-700 and TMS-1000 terahertz spectrometers coupled with customized software extract an undersampled signature from a "cluttered" environment with the advantages of increased resolution, improved minimum detectable quantity, and rejection of interferent clutter. Such high-resolution, line-narrowed detection schemes can recover a signal buried in clutter and identify it. Combined with the processing algorithms, the flexible hardware/software system can be tailored and optimized to detect a wide variety of gas/interferent scenarios.

In one example, both HCN and HCl at 200 ppm concentrations were introduced into the spectrometer cell in an H2O cluttered environment. The system obtained one measurement at a total cell pressure of 700 torr and the other at 70 torr (see Fig. 3). A comparison of the library spectra to the measured spectra for the gases at atmospheric (700 torr) pressure and a pressure of 70 torr shows similar effects of linewidth broadening due to different pressures. At atmospheric pressures, the HCN and HCl lines overlap. At reduced pressures, the linewidth is narrowed and the lines are well separated. A highly sensitive spectrometer requires excellent frequency knowledge to be able to properly remove this structure and any standing wave artifacts. In addition, algorithmic processing is critical to properly manage the data.

Goodrich Fig 3ab
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FIGURE 3. HITRAN library spectra for HCN and HCl (top) at a pressure of 1013 mbar (dark blue and red lines, respectively) and 100 mbar (light blue and pink lines, respectively) compare favorably with observed data using the Goodrich TMS-700 spectrometer for HCN and HCl at a pressure of 930 mbar (dark blue and red lines, respectively) and 100 mbar (light blue and pink lines, respectively). This demonstrates the robustness of the detection system when two toxic industrial chemicals are simultaneously present. (Courtesy of Goodrich)

Both the TMS-700 and the portable TMS-1000 are capable of high-resolution terahertz spectroscopy at the megahertz level, and can determine absorption signatures of multiple chemicals. Goodrich has completed initial system testing and verified its performance. As a warning system, the terahertz-based system increases the probability of detection and reduced false alarms. A spectrometer system with these capabilities would also find many applications in industrial process control and monitoring.

REFERENCES
1. Z. Jiang and X.-C. Zhang, in D. Mittleman (Ed.), Sensing with Terahertz Radiation, Springer-Verlag, Berlin and Heidelberg, (2003).
2. D.A. Zimdars and J.S. White, Proc. SPIE 5411, 78 (2004).
3. D. Mittleman in D. Mittleman (Ed.) Sensing with Terahertz Radiation, Springer-Verlag, Berlin and Heidelberg (2003).
4. E.R. Brown, Int'l. J. High-Speed Electr. and Syst. 13, 4 (2003).
5. J.F. Federici, et al., Appl. Phys. Lett. 83, 2477 (2003).
6. F. DeLucia in D. Mittleman (Ed.), Sensing with Terahertz Radiation, Springer-Verlag, Berlin and Heidelberg (2003).
7. C.H. Townes and A.L. Schawlow, Microwave Spectroscopy, McGraw-Hill Book Co., New York (1955).
8. L. S. Rothman, et al., Appl. Opt. 26, 19 (1987).
9. H. M. Pickett, J. Molec. Spectroscopy 148, 371 (1991).

ALEXANDER MAJEWSKI is a senior principal physicist at Goodrich ISR Systems, 100 Wooster Heights, Danbury, CT 06810; e-mail: alexander.majewski@goodrich.com; www.goodrich.com.

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