Analytical and Bioanalytical Chemistry

, Volume 397, Issue 3, pp 1009–1017

Chemical sensing and imaging with pulsed terahertz radiation

Authors

    • Freiburg Materials Research CenterUniversity of Freiburg
  • Bernd M. Fischer
    • French-German Research Institute of Saint Louis
    • University of Adelaide,School of Electrical and Electronic Engineering
  • Alex Ortner
    • Freiburg Materials Research CenterUniversity of Freiburg
  • Andreas Bitzer
    • Freiburg Materials Research CenterUniversity of Freiburg
  • Andreas Thoman
    • Freiburg Materials Research CenterUniversity of Freiburg
  • Hanspeter Helm
    • Freiburg Materials Research CenterUniversity of Freiburg
Review

DOI: 10.1007/s00216-010-3672-1

Cite this article as:
Walther, M., Fischer, B.M., Ortner, A. et al. Anal Bioanal Chem (2010) 397: 1009. doi:10.1007/s00216-010-3672-1

Abstract

Over the past decade, terahertz spectroscopy has evolved into a versatile tool for chemically selective sensing and imaging applications. In particular, the potential to coherently generate and detect short, and hence, broadband terahertz pulses led to the development of efficient and compact spectrometers for this interesting part of the electromagnetic spectrum, where common packaging materials are transparent and many chemical compounds show characteristic absorptions. Although early proof-of-principle demonstrations have shown the great potential of terahertz spectroscopy for sensing and imaging, the technology still often lacks the required sensitivity and suffers from its intrinsically poor spatial resolution. In this review we discuss the current potential of terahertz pulse spectroscopy and highlight recent technological advances geared towards both enhancing spectral sensitivity and increasing spatial resolution.

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Online abstract figure

Artist's view of a terahertz pulse emitted from a photoconductive antenna probing the vibrational modes of a sugar molecule.

Keywords

Terahertz spectroscopyTerahertz imagingLab-on-chip

Introduction

In recent years the popular press, companies, and research institutes have frequently reported on the great potential of terahertz radiation for medical, biological, and security-related sensing and imaging. The terahertz frequency range of the electromagnetic spectrum indeed offers a range of exciting possibilities for chemical sensing [1, 2], characterization of biological material [3, 4], standoff detection of explosives [5] and biological warfare agents [6], the analysis of low-frequency vibrations in RNA, DNA, and proteins [710], as well as the detection of skin cancer [11], to name only a few examples. Although these reports fueled enthusiastic and sometimes unrealistic expectations for this new technology, the prudent researcher in the laboratory views its potential from a slightly more realistic angle.

With frequencies of approximately 100 GHz up to some terahertz (1 THz = 1012 Hz), the terahertz frequency region is located in the electromagnetic spectrum between microwaves and the near infrared, as illustrated in Fig. 1. The mechanism leading to terahertz absorption in molecular and biomolecular systems is dominated by the excitation of intramolecular as well as intermolecular vibrations between weakly bound molecular entities with hydrogen bonds and weak interactions such as van der Waals forces playing an important role. Owing to the highly collective character of these low-frequency vibrational modes, involving the movement of large portions of the molecules, spectroscopy with terahertz radiation enables the investigation of materials with respect to their molecular structure and composition. As shown in Fig. 1, the modes at terahertz frequencies are, for example, associated with collective intramolecular vibrations, such as torsional modes in large molecular chains, or with intermolecular vibrations between neighboring molecules, e.g., in molecular crystals. In addition intra- and intermolecular modes may mix considerably, rendering an unambiguous assignment of the individual absorption bands extremely difficult and only recently have extensive molecular dynamics calculations been able to reproduce the terahertz spectra of selected molecular systems [12, 13]. Nevertheless, their collective and therefore delocalized character makes these low-frequency modes strongly dependent on molecular structure and arrangement, with a terahertz spectrum representing a characteristic ''fingerprint'' of a molecular substance.
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Fig. 1

Electromagnetic spectrum with corresponding molecular excitations. The frequency-dependent amplitude of a typical terahertz pulse (0.1–5 THz) is indicated in the spectrum by the dark-yellow area

Since the pioneering work by Auston et al. [14] and Grischkowsky et al. [15], who introduced ultrafast laser techniques to generate and detect short and broad-bandwidth pulses of terahertz radiation, a number of experimental barriers in this spectral range have been overcome, and today compact terahertz spectrometers are commercially available. Over the past 5 years an extended atlas of molecule-specific spectra in this spectral range has been accumulated and the general applicability for chemically selective characterization and imaging has been demonstrated. However, for many realistic applications in chemical analysis and imaging of biological systems, the technology still lacks the required sensitivity and also suffers from its intrinsically poor spatial resolution.

Here, we discuss selected examples of chemical sensing applications by terahertz pulse spectroscopy and review recent technological advances geared towards both enhancing spectral sensitivity, e.g., by confining the terahertz fields to waveguides, and increasing spatial resolution well beyond the diffraction limit, by near-field microscopy techniques.

The technology

In contrast to the radio and microwave parts of the electromagnetic spectrum, as well as the infrared and visible regime, where efficient electronic or optical radiation sources have been widely available, there has been a lack of efficient coherent sources and detectors for terahertz radiation in the past. The advent of modern femtosecond laser technology in the early 1990s helped to bridge this gap in the electromagnetic spectrum. Such ultrashort pulse lasers allow the generation and detection of short pulses of terahertz radiation, either in nonlinear optical crystals [16, 17] or by photoconductive antennas [15]. Terahertz pulses produced by these methods correspond to short bursts of broadband far-infrared light, which can readily be used for spectroscopic applications in this interesting part of the spectrum. Based on this technology, the terahertz time-domain spectroscopy (TDS) approach has been developed and eventually established as a powerful spectroscopic tool.

A typical terahertz TDS system based on photoconductive antennas for generation and detection of far-infrared light is schematically sketched in Fig. 2a. The output of a mode-locked Ti:sapphire laser (80-fs pulses, 800 nm, 75-MHz repetition rate) is split into an excitation and a detection beam. As shown in Fig. 2b, the excitation pulses are focused onto the terahertz emitter antenna consisting of two coplanar electrodes (50-μm separation) on a semi-insulating GaAs substrate. The charge carriers generated by the laser excitation in the substrate are driven by an external field applied between the electrodes (40-V bias). The resulting transient polarization emits a short electromagnetic field pulse (terahertz pulse). Collimating silicon lenses and mirror optics (off-axis parabolic or elliptical) can be used to generate a terahertz focus at the position of the sample (about 1-mm diameter) and to refocus the transmitted terahertz field onto the detector antenna.
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Fig. 2

a A terahertz time-domain spectrometer based on the emission and detection of terahertz pulses from photoconductive antennas, shown in b. c Measured time trace of a terahertz reference and a sample pulse (the upper curve is shifted vertically for better representation). d Corresponding spectral amplitudes of the reference and sample pulses. SI semi-insulating, LT low-temperature-grown

The terahertz detector typically consists of an H-shaped electrode structure (5-μm gap) processed on a photoconductive material. Fast photoconductors, e.g., low-temperature-grown GaAs, are typically used to provide the required fast response of the terahertz detector. By scanning a variable delay, one can continuously delay the laser pulse that gates the detector with respect to the incoming terahertz pulse, allowing direct sampling of the temporal profile of the electric field of each pulse trace.

In a conventional terahertz TDS transmission measurement, pulses are recorded with and without the sample material placed at an intermediate focal plane in the beam path of the spectrometer. The two pulse traces are then Fourier-transformed to the frequency domain to obtain their complex-valued spectra. Figure 2d shows the spectral amplitudes obtained from Fourier transforming the terahertz pulses in Fig. 2c, measured with and without a pressed sample of crystalline sugar sucrose in the beam. From the amplitude ratio A and phase difference \( \phi \) of the complex sample and reference spectra the dielectric properties of the sample material can be extracted, taking into account Fresnel losses at the interfaces of the sample material. For a free-standing sample the frequency-dependent index of refraction and absorption coefficient are calculated as [7]
$$ n\left( \nu \right) = 1 + \frac{c}{{2\pi \nu d}}\phi \left( \nu \right) $$
(1)
and
$$ \alpha \left( \nu \right) = - \frac{2}{d}\ln \left( {A\left( \nu \right)\frac{{{{\left[ {n\left( \nu \right) + 1} \right]}^2}}}{{4n\left( \nu \right)}}} \right), $$
(2)
where ν is the frequency, c the speed of light, and d the thickness of the sample. Note that for inhomogeneous samples, scattering effects have to be considered [18]. As an example, the index of refraction and the absorption coefficient of the sucrose sample extracted from the measured terahertz pulses in Fig. 2c are plotted in Fig. 3. For this measurement, a polycrystalline sample of sucrose was prepared by milling the powder and pressing it to a pellet of an approximate thickness of 1 mm by a hydraulic press. Also, in this example the pellet was cooled to 10 K, which results in sharper and clearly separable bands in the spectrum [1].
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Fig. 3

Absorption coefficient and index of refraction of polycrystalline sucrose. (From [19])

Chemical sensing and substance identification

As discussed already, terahertz vibrational spectra represent characteristic fingerprints for many chemical substances; hence, this technology may be used for contactless substance identification. The terahertz spectrum of sucrose shown in Fig. 3, for example, exhibits various absorption bands, with their central frequencies, widths, and absorption strengths being characteristic for this particular compound, rendering its spectrum clearly differentiable from the spectra of other sugars [19]. This chemical specificity of terahertz spectra opens up a range of useful applications in basic research, industry, or for security applications. As an example, we have shown previously that closely related chemical substances such as acetylsalicylic acid (aspirin) and its chemical precursors (hydroxylated benzoic acids) can be clearly differentiated [1]. Moreover, by comparing relative absorption strengths, one can perform a quantitative analysis, allowing mixing ratios in mixtures to be determined, which is potentially useful to detect unwanted contamination during drug production in the pharmaceutical industry.

Free-space spectroscopy

In conventional terahertz TDS the pulsed terahertz beam is guided through free space by collimating and focusing optics (lenses or mirrors) to the object under investigation and, depending on the setup, either the transmitted or the reflected terahertz pulses are subsequently detected. Transmission spectroscopy is preferred if the sample is sufficiently transparent as is the case for many solid-state systems, such as plastics and molecular crystals. Many liquids, however—in particular polar liquids such as water and alcohols—strongly absorb terahertz radiation. In this case, terahertz TDS in reflection geometry is typically used to probe their dielectric properties [2023].

Experiments based on free-space guiding of the terahertz light facilitate standoff detection of chemical compounds, e.g., for safety and security applications. For example, substances hidden in clothing, luggage, envelopes, or suitcases can be detected, owing to these objects usually being transparent at terahertz frequencies. Figure 4 shows a demonstration experiment for detection of a bag filled with a polycrystalline test substance (lactose) hidden in a shoe. The absorption spectrum obtained in a transmission spectrometer is compared with reference spectra of well-prepared sample pellets. In this demonstration the dominant absorption band of α-lactose monohydrate at 0.53 THz can be clearly identified even through the strongly scattering shoe sole.
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Fig. 4

a α-Lactose monohydrate in a sealed plastic bag is hidden in a shoe and the spectrum is measured by transmission mode terahertz time-domain spectroscopy. b Comparison of the measured absorption coefficient with reference data

Aside from its use for chemical sensing and substance identification applications, broadband terahertz spectroscopy can also reveal details on molecular arrangement. The strong sensitivity for intermolecular vibrations clearly implies the capability of terahertz TDS to detect and probe the crystalline configuration of a sample. This can be best seen in the case of racemic crystals [24]. In a racemic conglomerate—or racemic mixture—pure crystals of the left- and right-handed enantiomers of a given molecular substance are physically mixed together. In the case of the racemic compound, the racemic crystal consists of a pure crystalline phase of both enantiomers in a well-ordered ratio. So both samples contain the same types of molecules in the same ratio, the only difference is the order of the enantiomers in the crystal, whether they are bound to molecules of the same enantiomer or to molecules of the other enantiomer. In Fig. 5 we show the spectrum of a 1:1 mixture of polycrystalline d-tartaric acid and l-tartaric acid and the racemic compound dl-tartaric acid. Although both samples consist of the same type of molecules, i.e., 50% d-tartaric acid and 50% l-tartaric acid, the terahertz spectra differ significantly from each other and each spectrum has its own characteristic set of spectral signatures. Consequently, it is not only possible to differentiate between the pure racemic compound and the pure racemic mixture, but also to determine the ratio of both crystalline forms in a given sample [24, 25].
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Fig. 5

Terahertz absorption spectrum of a mixture of enantiopure polycrystalline d-tartaric acid and l-tartaric acid (upper curve) and polycrystalline racemic dl-tartaric acid (lower curve). The clear difference in the spectra is due to the different crystalline structures and hence different intermolecular vibrations. (From [24])

In another example, the terahertz spectra are rather more sensitive to molecular structure than to intermolecular arrangement. In the solid state the organic molecule retinal forms extremely weakly bound crystals. Hence, intermolecular vibrations (phonon modes) occur at subterahertz frequencies and absorption bands in the terahertz regime can be attributed to vibrational modes with mainly intramolecular character [26]. Retinal can exist in different isomeric forms with the molecule backbone being twisted at different atomic positions (see the insets in Fig. 6a). The terahertz spectra of the all-trans, 13-cis, and 9-cis isomers of retinal recorded at 10 K reveal distinct variations in absorption strengths and central frequencies of their characteristic vibrational modes. For example, mode A in the twisted forms (13-cis and 9-cis) is much weaker and significantly redshifted compared with the elongated structure (all-trans). Mode B, on the other hand, is rather insensitive to the isomeric conformation. This indicates that mode A can be essentially assigned to a vibrational mode of the molecular chain, whereas mode B is localized on the unchanged ring [27]. In fact, molecular dynamics calculations for all-trans-retinal support this view and suggest that the two modes correspond to a chain torsion and a ring deformation, respectively [26], as shown in Fig. 6b.
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Fig. 6

a Absorption spectra of all-trans-retinal (black circles), 9-cis-retinal (red circles), and 13-cis-retinal (blue circles). The spectra of all three retinal isomers show different characteristic features. A careful comparison of the spectra, in combination with theoretical modeling, reveals information about the localization of the vibrational modes shown in b. Arrows indicate atomic motion and red circles indicate the approximate localization for the individual modes within the molecular chain. (a From [27])

Protein-bound retinals play the role of biological chromophores in rhodopsin and bacteriorhodopsin, and the isomerization of the retinal chromophore represents the primary step in their photocycle. The structural selectivity of terahertz spectroscopy implies its potential for sensing structural changes in chromophores, through identifying vibrational modes, which can act as a probe of their isomeric state. Unfortunately, most isomerization experiments require the chromophores to be dissolved in liquids rather than bound in a crystal lattice, which results in extremely overdamped vibrational modes and, hence, broadened absorption bands.

Waveguide spectroscopy

Often, current terahertz TDS systems do not fulfill the high requirements for potential application in chemical sensing and bioanalysis. Fundamental obstacles are mainly related to the insufficient spectral sensitivity of this approach. For instance, conventional free-space spectroscopy methods, as described in the previous subsection, typically require at least some milligrams or microliters of a sample substance to be selectively detected. In many realistic applications, however, only few nanograms to micrograms (or picoliters to nanoliters) are available, e.g., as spots on biochips.

To overcome this fundamental limitation, terahertz spectroscopy systems based on the implementation of terahertz waveguides have been developed, which allow considerable concentration and guiding of the terahertz field. Various structures have already been demonstrated that feature broadband, low-loss, and largely dispersion-free waveguiding for terahertz waves, such as porous fibers [28, 29], parallel-plate waveguides [3033], and metal wires [34, 35]. Owing to the achievable field concentrations in the waveguide or along its surface, much more sensitive measurements are feasible than for a system based on free-space beam guiding. Figure 7a shows a terahertz spectrometer that uses a metallic wire to guide and concentrate terahertz pulses, coupled from a terahertz emitter antenna onto the wire, over several centimeters before being detected at its end. This system has been demonstrated to be able to measure the characteristic absorption of a test substance (in our case lactose), requiring less than 1 mg of the material [34]. This corresponds to an improvement in sensitivity by approximately a factor of 10.
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Fig. 7

a Terahertz spectrometer consisting of photoconductive terahertz emitter and detector, and a bare metal wire as a terahertz waveguide. Terahertz pulses are guided over several centimeters, where they interact with a test substance (lactose). b Sample and reference spectra measured with this system. The inset shows corresponding spectra measured by conventional terahertz time-domain spectroscopy and with approximately 10 times more substance. Arrows indicate absorption features characteristic for lactose. (From [34])

An even more promising approach pioneered by the Grischkowsky group uses parallel metallic plates as waveguides [3033]. Using these parallel-plate waveguides, they have shown that less than 100 µg of a crystalline sample [30] or liquid layers of only few nanometers are sufficient to determine the spectrum [33].

Finally, terahertz waveguides may also be integrated as microstrip lines together with a terahertz emitter and detector on a single chip, potentially in combination with microfluidic channels, allowing for very compact and highly miniaturized terahertz sensors [36]. It can only be speculated that with expected further advancements of waveguide-based terahertz systems the sensitivities required for many biochemical sensing applications may eventually be achievable.

Spectral imaging

Not only spectral but also spatial information can be obtained by terahertz TDS. This is particularly useful for investigating strongly inhomogeneous systems, as is the case for many biological samples, such as plants and biological cells.

Far-field mapping

In its simplest form, terahertz time-domain imaging is performed by scanning an object through the focus of the pulsed terahertz beam and recording the transmitted field with a detector at some distance, i.e., in the far-field (see Fig. 9b). As a consequence, one obtains a terahertz time trace yielding a complete spectrum at each position, as shown in Fig. 8 (right-hand side). By analysis of the observed absorption bands, e.g., by measuring the absorption strength at characteristic frequencies, the corresponding substance can be assigned to each spatial pixel and visualized in false colors. In our example, visually indistinguishable tablets can be well differentiated with chemical sensitivity [37]. The spatial resolution of the experiment is given by the finite spot size of the terahertz focus, which is limited through diffraction to orders of the wavelength used (about 1 mm). In the example in Fig. 8, one pixel corresponds to a 2 mm × 2 mm spot. Similar to the intrinsically low sensitivity, also the limited spatial resolution of conventional terahertz systems is often insufficient for useful (bio-)chemical sensing applications. As discussed next, a significant improvement in terms of spatial resolution can only be achieved through alternative imaging approaches, such as near-field techniques.
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Fig. 8

Terahertz false-color map of a sample consisting of pellets of α-lactose monohydrate, aspirin, sucrose, and tartaric acid. The colors represent a measure for the presence and strength of specific spectral parameters, which are characteristic for the respective sample, as shown on the right. It can be seen that even a simple set of specific spectral parameters enables clear discrimination of the samples. (From [37])

Terahertz near-field microscopy

The problem of the diffraction-limited spatial resolution, central to far-field imaging based on conventional optics, can be overcome by measuring the terahertz transmission directly in close proximity to the sample. Such imaging systems rely on raster-scanning a near-field probe across the sample. As a consequence, the spatial resolution is determined by the interaction of the probe with the terahertz field, and not by the dimensions of the terahertz spot illuminating the sample. Various terahertz near-field probes have been demonstrated, such as static or dynamic apertures [3, 38], metal tips [39, 40], and terahertz near-field detectors [4144]. Figure 9a shows a sketch of a terahertz near-field microscopy setup based on a photoconductive antenna detecting the transmitted terahertz field directly behind a sample, which is illuminated by a focused terahertz beam. To obtain a spatially resolved transmission image, the sample is moved relative to the stationary detector, or vice versa. With this setup we have recently demonstrated imaging of a plant leaf [41]. Figure 9c shows the result of a terahertz near-field scan of a 5 mm × 5 mm section of the leaf, plotted at a frequency of 1.36 THz and featuring a spatial resolution of 50 µm, which corresponds to λ/4.4. In this particular case, absorption is mainly due to water attenuating the terahertz signal. The image clearly resolves the submillimeter leaf veins (200–300 µm in diameter) responsible for delivery of water and dissolved nutrients (plant vascular system). In comparison, this secondary leaf structure is almost invisible in a corresponding far-field image. Conventional terahertz far-field imaging has previously been used to study plant physiological parameters, such as water contents in leaves [4, 45, 46]. Near-field imaging methods now extend the applicability of this tool to the investigation of even smaller plant structures such as small leaves or the secondary structure in leaf vein networks.
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Fig. 9

A terahertz near-field imaging setup (a) and a conventional far-field imaging system (b). c Terahertz transmission image (plotted at 1.36 THz) of a small section of a plant leaf measured with a near-field setup (left) and the same section measured with a far-field setup (right) for comparison. Black corresponds to low transmission. (From [41])

Note that each spatial pixel of the image corresponds to an individual terahertz pulse scan and thus contains spectral information over the entire bandwidth covered by our experiment (50 GHz to 3 THz). Consequently, near-field microscopy can also be applied for chemically sensing small spots of substances, similar to the result in Fig. 8, however with much better spatial resolution.

Outlook

Although current terahertz TDS systems have the potential for chemically selective sensing and imaging, they still suffer from limited sensitivity and an intrinsically poor spatial resolution. Presumably, in the near future systems based on terahertz waveguides will be able to achieve the high sensitivities required for useful sensing applications in chemical or biochemical analytics. This approach also has the capability to implement very compact terahertz spectrometers, e.g., by integrating the terahertz emitter, detector, and waveguide on a single centimeter-sized chip. Various lab-on-chip applications may be feasible.

Near-field imaging methods have the potential to push achievable spatial resolutions for terahertz imaging from millimeters into the micrometer and eventually the nanometer regime. Terahertz near-field images of semiconductor devices with nanometer resolution have been reported recently [47].

Increasing sensitivities together with better spatial resolutions will lead to a significant reduction of sample amounts required in broadband terahertz spectroscopy, eventually paving the way for useful sensing and imaging applications (chemical, biological, medical, or environmental).

Acknowledgements

M.W. acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG) through grant no. WA 2641, by the Baden-Württemberg Ministry for Science and Arts Research Seed Capital (RiSC) for young researchers, and by the University of Freiburg.

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© Springer-Verlag 2010