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Analytical Chemistry

Spreading the Word on Terahertz Light

Advances in far-infrared probe methods drive applications in spectroscopy and imaging

by Mitch Jacoby
October 3, 2005 | A version of this story appeared in Volume 83, Issue 40

ACS MEETING NEWS

SEE-THROUGH
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Credit: COURTESY OF RPI
With its ability to penetrate various types of materials, such as the plastic used in laptop computer casings, light in the far-IR (or terahertz) region is being studied for potential use in airport security and other types of imaging applications.
Credit: COURTESY OF RPI
With its ability to penetrate various types of materials, such as the plastic used in laptop computer casings, light in the far-IR (or terahertz) region is being studied for potential use in airport security and other types of imaging applications.

Consult your favorite instrumental analysis book for analytical methods that make use of light sandwiched between the infrared and microwave regions of the electromagnetic spectrum. Chances are you'll find very little.

Using far-infrared light to probe chemical systems isn't a new idea. For decades, some scientists have used light in the far-IR and nearby microwave regions to interrogate molecules for studies in spectroscopy, astronomy, and other areas. Yet the number of researchers active in the field has been relatively small.

In just the past few years, however, a handful of developments--mainly in instrumentation--has made it possible to conduct new types of chemical analyses with far-IR light. The advances are beginning to bolster the field with new interest and new researchers and have even led to a new name--terahertz spectroscopy.

"It's just a frequency unit," said Charles A. Schmuttenmaer, explaining the origin of the name. Strictly speaking, the terms "terahertz" (1 THz = 1012 Hz) and "far-IR" can be used interchangeably, the Yale University chemistry professor added. But nowadays, terahertz tends to refer to the fast laser-based methods for generating and detecting THz light--as opposed to the continuous light sources and techniques employed in earlier studies. In addition, the new name is associated with applications and procedures that weren't possible or weren't being used in traditional far-IR studies. Among other topics, the list includes time-resolved THz spectroscopy; THz imaging in medical, security, and other applications that capitalize on the light's ability to penetrate plastics, paper, and textiles; and development of instrumentation for THz spectroscopy.

Despite the recent advances, THz spectroscopy remains largely unfamiliar to chemists. "We're only just beginning to make inroads in physical and analytical chemistry," Schmuttenmaer said. So to help spread the word about the technique's potential, Schmuttenmaer and Xi-Cheng Zhang, a professor of physics and electrical engineering and director of the Center for Terahertz Research at Rensselaer Polytechnic Institute, organized a symposium devoted to analytical applications of THz spectroscopy. The symposium, which included basic tutorials as well as presentations on the latest research, was convened at the American Chemical Society's recent national meeting in Washington, D.C. The event was cosponsored by the Analytical and Physical Chemistry Divisions.

WORKING DEFINITIONS in THz spectroscopy vary somewhat from researcher to researcher. In general, though, the term encompasses spectral measurements based on radiation with submillimeter wavelengths and a frequency range of roughly 0.1 to 10 THz or 3 to 300 cm-1. For comparison, conventional benchtop Fourier transform IR spectrometers scan in the range of 400 to 4,000 cm-1, but commercial IR and Raman instruments can also be configured to probe the far-IR region.

Schmuttenmaer explained that, compared with light in the mid-IR range (thousands of cm-1), THz light stimulates low-frequency intermolecular modes of vibration, rotation, and other types of motions. These modes are distinct from the more familiar intramolecular modes in classic IR spectroscopy.

SPREADING THE NEWS
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Credit: PHOTO BY MITCH JACOBY
Zhang (left) and Schmuttenmaer organized a symposium to educate chemists about the potential of terahertz spectroscopy.
Credit: PHOTO BY MITCH JACOBY
Zhang (left) and Schmuttenmaer organized a symposium to educate chemists about the potential of terahertz spectroscopy.

Using water as an example, the Yale chemistry professor pointed out that light in the 3,400- and 3,600-cm-1 regions excites O-H stretching motions (within individual molecules). At 1,700 cm-1, water exhibits a bending mode. In contrast, low-frequency THz light (below 600 cm-1) causes groups of water molecules to undergo collective twisting motions and can cause neighboring molecules to approach and then pull back from one another. In very large molecules such as proteins, the low-frequency radiation can induce intramolecular motions. And in crystals, THz light can excite phonon modes (collective lattice vibrations or waves).

To educate and draw interested researchers toward THz spectroscopy, part of the symposium addressed the nuts and bolts of generating and detecting THz light. "If you're equipped to do ultrafast laser spectroscopy, you're 90% of the way toward setting up a THz system," Schmuttenmaer asserted.

One technique for generating far-IR pulses, which was customized for THz frequencies by Zhang and coworkers, is based on a process known as optical rectification. In that method, femtosecond pulses of 800-nm (near-IR) light from a titanium-sapphire laser irradiate a nonlinear crystal such as ZnTe. The method produces short pulses of far-IR light with a frequency range of about 2­100 cm-1.

As a portion of the laser pulse is converted to THz light and used to probe a sample, another portion of the light is directed to a detector system, which includes a second ZnTe crystal or other nonlinear medium. While the THz light that probed the sample propagates through the ZnTe detector crystal, the crystal becomes birefringent--meaning its index of refraction varies with crystal lattice direction. According to Schmuttenmaer, that property causes the polarization of the 800-nm light to be rotated slightly--but only while the THz light impinges on the crystal. The upshot is that by using routine laser spectroscopy procedures to adjust the arrival time--at the detector--of the near-IR and THz light, researchers can detect and process the THz signal.

R. Alan Cheville, an associate professor of electrical and computer engineering at Oklahoma State University, Stillwater, described a device known as a photoconductive antenna, which provides an alternative method for generating and detecting THz light. The device consists of an undoped wafer of GaAs that has been patterned lithographically with a pair of long, parallel electrodes separated by some 80 mm. Applying a voltage across the electrodes and focusing pulses from a Ti-sapphire laser in the micrometer-sized gap causes electrons that are liberated by the laser light to be accelerated across the gap. The process generates short bursts of THz light, Cheville explained.

Korter
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Credit: PHOTO BY MITCH JACOBY
Credit: PHOTO BY MITCH JACOBY

In a similar way, a second antenna device can be used as a detector. The principle is that shining the laser pulses on a semiconductor with a fast response time stirs up photoelectrons--yet there's no flow of current in the absence of an applied voltage or electric field, Cheville pointed out. By adjusting the relative arrival times of the THz and laser pulses, however, researchers can use the electric field associated with the THz light to switch on a short-lived flow of current, which is measured by a current meter to "read" the THz pulse.

Yamamoto
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Credit: PHOTO BY MITCH JACOBY
Credit: PHOTO BY MITCH JACOBY

After a discussion of other aspects of THz spectroscopy, Cheville displayed a THz generator that was built by undergraduate student Lesley A. Hess. "If you want to build one of these," Cheville told attendees, "you can go to our website (http://THz_spectrometer.okstate.edu), where you'll find an illustrated tutorial with parts lists and step-by-step instructions featuring all kinds of tips and tricks."

With access to THz spectrometers and methodology on the rise, researchers are beginning to compile libraries of THz spectra. But interpreting the data can be challenging because, as Timothy M. Korter noted, "even simple compounds lead to pretty complex spectra." So to make headway into spectral interpretation, Korter, an assistant professor of chemistry at Syracuse University, in New York, combines experimental methods with computational analysis.

In one study, Korter focused on phenol solutions, noting that spectral features below 200 cm-1 arise from interactions between phenol molecules that are held together as hydrogen-bonded clusters. Using quantum mechanical methods, he determined the structures of the most stable clusters and then combined information from the low-frequency (THz) and high-frequency (mid-IR) regions to model the spectra. The results indicate that the THz spectral features are due primarily to dimers, trimers, and tetramers undergoing intermolecular vibrations. Using a computer animation, Korter explained that in these types of motions, entire phenol molecules within a single cluster vibrate relative to one another. 

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Click image to View a video in Quicktime format (584 KB)
Click image to View a video in Quicktime format (584 KB)

THZ SPECTRA of solid-phase samples are also starting to yield to theoretical analysis. Because of potential applications in security and defense, a number of research groups have probed explosives such as TNT, HMX, and PETN with THz spectroscopy. But according to Korter, the origin of the spectral features is not well understood. So Korter's group recently began analyzing HMX and PETN spectra that were provided by TeraView, a manufacturer of THz spectrometers based in Cambridge, England.

After working with a host of computational methods in the weeks just prior to the conference, Korter concluded that gas-phase theoretical methods fail completely to describe the spectra of solid explosives. In contrast, careful modeling of crystal (solid-phase) unit-cell geometries and vibrations (via a technique known as periodic boundary conditions) yields theoretical results that closely match the experimental measurements.

YOURS FOR $1,499
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Credit: PHOTO BY MITCH JACOBY
Off-the-shelf components can be used to build (relatively) inexpensive terahertz spectroscopy equipment such as the terahertz generator in Cheville's hand.
Credit: PHOTO BY MITCH JACOBY
Off-the-shelf components can be used to build (relatively) inexpensive terahertz spectroscopy equipment such as the terahertz generator in Cheville's hand.

THz spectroscopy methods for detecting explosives also are being studied at Osaka University in Japan. Kohji Yamamoto, a postdoc at the university's Institute of Laser Engineering, reported that small amounts of the powerful explosive C-4 hidden in sealed envelopes are readily detected by the compound's THz fingerprint (from 590 cm-1) (Jpn. J. Appl. Phys. 2004, 43, L414). Yamamoto pointed out that C-4, which is composed of about 90% RDX and has been fingered in numerous terrorist bombings, cannot be detected via X-ray methods or metal detectors.

In a related study, Yamamoto showed that THz spectroscopy is useful in analyzing chiral compounds. Specifically, he reported that the pure enantiomers L- and D-alanine are easily distinguished from the racemic mixture (Appl. Phys. Lett. 2005, 86, 053903). The finding underscores the sensitivity of THz spectroscopy to crystal structure, which may be attractive to pharmaceutical researchers for identifying crystal polymorphs.

David A. Newnham, a senior engineer at TeraView, elaborated on other pharmaceutical applications of THz spectroscopy. For example, he noted that the common pharmaceutical agent carbamazepine, which exists in at least four polymorphic forms, undergoes phase transitions as the compound is heated, melts, and recrystallizes. Newnham showed that the phase transitions can be monitored in situ by using a THz spectrometer equipped with a sample-heating stage. He also showed that THz imaging methods can be used to prepare three-dimensional chemical maps of the interior of drug tablets.

From fundamental science to mailroom security, THz spectroscopy is a potentially powerful tool that is just starting to make news in the chemistry community. According to Zhang and Schmuttenmaer, however, the field still suffers from several shortcomings, such as a lack of convenient access to affordable spectrometer components, publicly available spectral libraries, and a large pool of THz know-how. But with THz practitioners' willingness to help newcomers--as demonstrated, for example, by Cheville's instructional website--and a new THz community network being developed by Schmuttenmaer and others to address the shortcomings, it's only a matter of time before the technique finds its place in analytical chemistry textbooks.

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