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Abstract
The birth of terahertz imaging approximately coincides with the birth of the journal Optics Express. The 20th anniversary of the journal is therefore an opportune moment to consider the state of progress in the field of terahertz imaging. This article discusses some of the compelling reasons that one may wish to form images in the THz range, in order to provide a perspective of how far the field has come since the early demonstrations of the mid-1990’s. It then focuses on a few of the more prominent frontiers of current research, highlighting their impacts on both fundamental science and applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the last two decades, Optics Express has matured into one of the leading journals in the field of optics. Coincidentally, the field of terahertz imaging has also been maturing for about two decades – the seminal publication from Hu and Nuss that launched a tremendous wave of activity came out in 1995 [1]. This article was published in a sister journal, Optics Letters, which is also currently celebrating an anniversary. By another coincidence, the 40th anniversary of that journal also roughly coincides with another milestone in terahertz imaging: the first terahertz image ever published (which was created using a THz gas laser) [2]. Although a causal relationship between the launch of a new journal and a milestone in terahertz imaging seems unlikely, these coincidences nevertheless suggest that it might be fitting to examine the state of this research field, which has for the most part grown up alongside Optics Express. Since the mid-1990’s, the science and technology of terahertz (THz) imaging has made tremendous progress. THz techniques have proven to be valuable for basic research and applications, in a diverse array of different contexts. Like this journal [3], the THz field is still growing. New ideas continue to emerge, and new technologies are transitioning from research to commercialization. In this article, a few of the latest research advances in THz imaging will be discussed.
It is worthwhile to first review some of the basic features which motivate the use of THz radiation for imaging, as well as a bit of the background history. Although the potential value in using long wavelength radiation for imaging has been recognized for quite some time [2, 4], the majority of research in imaging was spurred by advances in femtosecond optoelectronics which took place in the late 1980’s and early 1990’s [5–7]. This led to the development of THz time-domain spectroscopy (THz-TDS) as a versatile technique for generating and detecting single-cycle THz pulses [8], and ultimately to its use in forming transmission [1] and reflection [9] images. As more varieties of THz sources have emerged, it has become clear that no single source technology is ideally suited for all problems; different challenges will require a different set of properties, including the source power, spatial and temporal coherence, size, price, and so on. By now, many different THz technology platforms have been used to form images via active illumination of a target [10–13]. It is worth noting the role of millimeter-wave and THz passive imaging systems, which use no illumination source but instead rely on capturing ambient THz radiation, either emitted directly by the object under study or scattered from the object by a uniform blackbody source such as the sky. Such systems have also become an important topic of research [14–16] and commercialization [17].
The use of THz radiation to form images has numerous advantages. Most basically, the THz range is a different region of the spectrum, so THz images can provide complementary information to that obtained with microwaves, infrared, visible, ultraviolet, or x-ray images. Compared with images formed using lower frequencies, THz images have the advantage of superior spatial resolution due to the shorter wavelength. Compared with infrared and higher frequencies, many common materials are relatively transparent, including common packaging materials such as paper and cardboard, as well as many plastics and composites. Polarization-resolved measurements [18] can also provide image contrast, particularly in the case of samples with anisotropic conductivity due to structural properties [19]. Materials with large electrical conductivity (i.e., metals) or with large static dipoles (e.g., water) tend to be strong absorbers, which can provide a source of contrast in image formation. The high sensitivity to water content (along with the fact that THz radiation is non-ionizing and therefore poses no known health risk to cells except for heating) has motivated many studies of THz imaging for agricultural [20, 21] or biomedical [22–24] sensing. Even though the penetration depth into living tissue is small (e.g., 100 microns), nevertheless several promising diagnostic applications have been identified and explored. In addition, some materials, especially molecular crystals, possess vibrational absorption bands in the THz range, which can act as spectroscopic fingerprints for material identification [25–28].
The millimeter and sub-millimeter wavelength scale also provides a potentially valuable contrast mechanism. One prominent example is the location of buried defects (e.g., air bubbles) embedded in a low-density foam, which itself consists mostly of air. This sort of defect presents relatively little contrast for most imaging modalities; however, in the THz range, the dielectric foam is highly transparent, while the internal surface of a buried defect is typically rough on the scale of the wavelength, leading to enhanced scattering at the interface. Figure 1 shows an example, in which a low-contrast defect in foam is clearly identified. This idea motivates numerous possible applications, for example in the aerospace industry [29]. When a low-coherence broadband source is used (e.g., a THz-TDS system), one can perform time-of-flight measurements to enable high depth resolution [9, 30] or, with an interferometer, even sub-coherence resolution in a configuration akin to optical coherence tomography [31]. Tomographic image reconstruction is also possible, either in transmission [32–34] or reflection [35] mode. In one interesting implementation, a continuous-wave THz system was used for tomographic imaging, with a photoconductive antenna used as both the source and receiver in a double-pass transmission geometry [36]. Tuning the source frequency permits spectroscopic analysis of the reconstructed two-dimensional image, as illustrated in Fig. 2.
Fig. 1 A photograph and a THz transmission image of a portion of an automobile dashboard, consisting of a ~1 cm layer of foam padding sandwiched between two black polyethylene sheets. The THz image clearly reveals an air bubble defect embedded in the foam. Adapted from [10].
Fig. 2 Reconstructed 2D images of a hollow-core Teflon cylinder filled with α-Lactose at a terahertz frequency of (a) 0.19THz and (b) 0.54THz. A homodyne self-mixing technique is applied to measure the 1D projections of the object, from which the 2D image is reconstructed using the simultaneous algebraic reconstruction technique. The 2D images reveal the dimensions of the hollow core Teflon cylinder, and also allow for an identification of the α-Lactose filling by its specific absorption line. Reproduced from [36] with permission of the authors.
In this article, the intent is to highlight a few of the hottest topics in THz imaging research. The selection of these topics is admittedly a highly subjective judgment, and obviously it is not possible to exhaustively cover all of the exciting research being reported worldwide. The topics discussed here should be understood as a representative cross-section of the current trends in a vibrant global effort.
2. Faster image acquisition
One of the key limitations in many THz imaging applications has been the amount of time it takes to form an image. In most of the well-known imaging examples, image formation has relied upon a pixel-by-pixel acquisition in which the data were acquired serially, most often with mechanical scanning of either the object under study [1] or of the THz illumination beam [37]. This process is usually slow, and limits the image acquisition speed. As a result, it has been recognized for some time that an array of detectors, operating in parallel rather than serially, would be extremely valuable. At higher frequencies, a variety of technologies exist for focal plane array detection, mostly relying on the fact that the detected photon’s energy exceeds the band gap of a semiconductor, so that absorption of light leads to a measurable photocurrent. This strategy fails in the THz range, for several reasons. For example, the frequency of thermal (i.e., blackbody) radiation from, e.g., a room-temperature object, is considerably larger than 1 THz, so narrow-gap semiconductors are not useful for direct detection of THz photons at room temperature. At frequencies below about 100 GHz, array detectors have employed conventional antenna structures and integrated electronic amplifiers at each pixel [38, 39]. Despite recent progress [40], the fabrication of such multi-pixel devices in integrated form remains challenging in the terahertz range.
The development of a fast and sensitive focal plane array detector for the THz range has been something of a ‘holy grail’ for the field, for many years. One of the earliest innovative attempts to implement focal-plane detection was described by the group of X.-C. Zhang in the late 1990s. This method used a large-area electro-optic crystal, onto which a THz image beam (generated using a femtosecond optical pulse) was projected. As with conventional free-space electro-optic sampling, this THz beam induced a transient birefringence in the crystal, which could be detected using a femtosecond probe pulse and polarization-resolving optics [41]. In this case, the optical probe beam was expanded to fill the large nonlinear crystal, in order to spatially overlap the large THz beam containing the image. Thus, a THz image could be encoded onto the spatially varying polarization state of the femtosecond pulse’s wave front. This spatial modulation of the femtosecond probe beam could be detected using a conventional CCD camera. With a large enough signal so that data averaging was unnecessary, this image data could be acquired at the read-out rate of the camera (or the repetition rate of the laser), i.e., video rate. This technique was the basis for the first THz ‘movies’ ever created [42]. Unfortunately, the large area of the optical read-out beam means that this idea cannot easily be implemented using a low-power femtosecond laser; an amplified laser system is necessary. As a result, this imaging system is necessarily quite large and complex (and expensive), and so the technique has not been very widely used. Even so, this early work demonstrated the possibility and the significance of video-rate THz imaging, for the first time.
A next important example of THz video-rate imaging was enabled by the invention of the THz quantum cascade laser (QCL) in 2002 [43, 44]. Although not the first lasers operating in the THz range, they offer many of the significant advantages of solid-state sources: relatively high output power in a compact package, without the cumbersome size and infrastructure requirements of gas lasers. In one notable case, the QCL output power was high enough to be detectable using a microbolometer focal plane array, which was compatible with real-time read-out at video rate [45]. In this case, the camera had a noise-equivalent power of about 300 pW·Hz−½, and was located over 20 m from the QCL source. The demonstration of THz videos using these compact components had a significant impact on the THz community. If a picture is worth a thousand words, then a video is worth a few tens of thousands of words per second.
More recently, the world of THz imaging is being transformed by a number of different breakthroughs in focal-plane array technologies. At least four different technology platforms have been used as the basis for pixel arrays in commercial THz cameras: III-V high-electron-mobility transistors (HEMTs), silicon CMOS circuits, microbolometers, and pyroelectric devices. Table 1 shows a comparison of some of the typical specifications of these four cameras, all of which operate at room temperature and with video-rate read-out. Although these are all still fairly expensive, the existence of so many options has truly changed the game in THz imaging research. For example, high-intensity THz pulse systems [47] often employ a THz camera for optimizing the beam alignment and focusing in order to produce the highest possible THz peak field strength [48], or for characterizing the (sometimes quite complicated) THz wave fronts generated in these non-perturbative nonlinear interactions [49]. Figure 3 illustrates one such image, showing a conical THz wave front generated by a two-color air plasma. Sensitive THz cameras have also been used for digital holographic reconstruction, with a variety of THz sources [50, 51].
Table 1. – A summary of the typical specifications of four types of commercially available focal plane arrays that can be used to visualize THz beams. Experimental comparisons of some of these cameras has been discussed in [46].
Fig. 3 Beam profiles for a THz beam generated via the two-color air plasma mechanism, measured using a camera based on a microbolometer focal-plane array. (a) the unfocused beam exhibits a conical profile, although (b) it is close to a Lorentzian profile when focused. Reproduced from [49], with permission of the authors.
Meanwhile, another complementary approach to speed up image acquisition has also recently moved to the forefront of research activity. This topic goes by the general label of computational imaging, and has become a powerful paradigm throughout the world of imaging. The basic concept is that it is often possible to assemble an image, which consists of N pixels, from M measurements where M << N, using an algorithmic approach. This general idea, which is related to synthetic aperture imaging [52], has been considered for terahertz measurements in several different contexts [53, 54]. One powerful approach is known as compressed sensing, which enables single-pixel imaging [55]. Here, one projects the image to be measured onto a basis which is 2D white noise; this was originally demonstrated using visible light, with a spatial light modulator where each pixel of the modulator was randomly set to either “on” or “off” with equal probability, and then the cumulative signal from all of the “on” pixels was measured using a single-pixel detector (i.e., a simple photodiode). Such a measurement produces a single number. The spatial light modulator was then reconfigured to a different (but still random) configuration of “on” and “off” pixels, and the measurement was repeated. It has been shown that the number of such measurements M which are required in order to reassemble the full image is generally much less than the number of pixels N in the image. The image assembly is a nonlinear optimization problem which converges for most images [55]. Since M << N, this enables a smaller image acquisition time, compared to measuring every pixel.
Such an approach seems ideally suited to the THz range, where single-pixel detectors have been relatively much more common than focal plane arrays [56]. Unfortunately, it does require a multi-pixel spatial light modulator (SLM). The idea of computational imaging motivated the initial work on THz SLMs, which was enabled by work in active metasurface arrays [57]. Here, a split-ring-resonator array was patterned on a lightly doped substrate, with each element electrically connected to its neighbors so that the elements which provided the resonant metasurface response also served as electrodes to control the size of the depletion region in the substrate [58]. Grouping the meta-element electrodes into pixels resulted in a SLM with modulation contrast of the metasurface (in this first demonstration, about 3 dB) [57]. More recent work has demonstrated a 64-pixel array, which was sufficient for image formation via various computational imaging methods [59]. In another example, the spatial modulation of the THz wave front was accomplished using a patterned optical illumination of a semiconductor wafer, which produces a spatially varying free carrier distribution that selectively attenuates different parts of the THz beam. Since the optical pattern can contain features that are much smaller than the THz wavelength, this can enable computational THz imaging with subwavelength resolution, as shown in Fig. 4 [60].
Fig. 4 (a) An illustration of the imaging setup from [60]. A digital micromirror device is used to spatially modulate an optical pump beam, which is then projected onto a silicon wafer. The photoexcited carriers in the silicon lead to a similar spatial modulation of the coincident THz pulse, which then passes through an object on the other side of the Si wafer, and is then measured using a single-element THz detector. The inset shows an optical image of a resolution test target (cartwheel), which is a gold patter on the Si wafer. The lower panels (b)-(g) illustrate the results. On the left-hand side, several images are assembled with different Si wafer thicknesses. The right-hand panels show the corresponding predictions. Reproduced from [60] with permission of the authors.
3. Near-field imaging
One of the most exciting frontiers in modern optics is the study of near fields. The ability to form images on a sub-wavelength scale underlies many advances in both science and technology. The interest in near-field techniques for sub-wavelength imaging and spectroscopy extends to the THz range as well [61]. One key motivation is spectroscopy: the THz range is rich with spectroscopic features, but many interesting samples are far too small for interrogation by conventional diffraction-limited optics. This challenge has been recognized for some time, at least since the first near-field imaging experiments [62]. Subsequently, a variety of techniques have been developed for terahertz imaging below the diffraction limit.
In some cases, the approach to terahertz near-field imaging is based on the idea of adapting techniques from other frequency ranges. Yet, one can often exploit the unique aspects of the terahertz range to further enhance the capabilities of these techniques. For instance, the classic idea for near-field imaging is to use a subwavelength aperture, and place the object to be imaged in the near field of the aperture. This approach has a long history in the THz range [62, 63]. Yet, with careful design of the aperture, one can improve the performance of such systems considerably. For example, the coupling of radiation through a small aperture in a metal screen can be improved by decorating the screen with a plasmonic structure designed to channel light through the aperture [64]. This technique is much more effective in the THz range than in the visible and near-infrared, due to the lower loss for surface plasmon propagation. One can also integrate a detector directly into the near field of the aperture, exploiting plasmonic enhancements for improved detection efficiency [65]. Tapered waveguides can also be more readily engineered in the THz range [66–68], leading to substantially improved spatial resolution in comparison to the wavelength. As an important note, these waveguides can support extremely broadband propagation, so that the subwavelength field confinement at the output can be employed for broadband spectroscopy.
In contrast, other sub-wavelength imaging techniques have been demonstrated which were not adapted from more conventional photonics or electronic approaches, but which are unique to the THz range. One fascinating example is laser terahertz emission microscopy (LTEM) [71]. Here, a sample to be imaged is illuminated with a femtosecond laser pulse, focused using conventional optics to the diffraction limit. Many materials illuminated with femtosecond pulses may emit a burst of THz radiation, due to ultrafast photogeneration of charges followed by carrier acceleration in a local potential. This THz signal can be detected in the far field, using common optoelectronic techniques. By raster-scanning the optical illumination spot, an image can be created using the emitted THz radiation. This image contains information about the THz response of the sample, but it has a spatial resolution determined by the optical spot size (a few microns), rather than by the THz wavelength (hundreds of microns). Figure 5 illustrates an example of this imaging modality, for detection of defects in integrated circuits [72]. A variety of possible applications of LTEM have been demonstrated, including quantitative evaluation of a supercurrent distribution [73], spontaneous polarization domain imaging [74, 75], molecular adsorption dynamics [76], and solar cells evaluation [77].
Fig. 5 LTEM images of LSI fabricated by 180nm process (a) without and (b) with a defect. The localization of LSI defects is one of key issues for the current and future LSI developments [69]. CMOS indicated by the red arrow in (b) includes a signal line disconnected artificially while fabricating, which makes the LTEM amplitude smaller, and the corresponding CMOS image becomes darker. (c) Note that the fs laser pulses are irradiated from the back side and the THz signals are detected from the back side because the front side is covered by metal layers. A GaP solid immersion lens is attached to the back side of the LSI, to improve the spatial resolution [70]. The whole LTEM image size is 30 × 30 mm2, and a resolution estimated by the transition width from 20% to 80% of the amplitude is 360 nm as indicated in (d). Images courtesy of M. Tonouchi, used by permission.
Another idea for near-field imaging, originally developed for microwave measurements, has found important applications in the THz range. This is based on a scattering-type microscopy, in which a tapered metal probe is held in close proximity to the sample to be imaged [78]. The radiation scattered from the tip contains information about the dielectric properties of a subwavelength volume of the sample, and can therefore be employed for both imaging and spectroscopy. The resolution is typically determined by the size of the tapered tip, rather than by the wavelength of the illumination source [79, 80]. This idea also works in the THz range, offering the possibility for extreme subwavelength imaging [81] and spectroscopy [82, 83]. Indeed, one need not even have a sample present; immersing a tapered metal tip into a propagating THz field can enable imaging of the propagating wave front with tip-size-limited resolution [84]. This can be valuable for characterizing fields that vary on sub-wavelength scale, for example at the output of tapered waveguides [67, 85, 86], as illustrated in Fig. 6. Moreover, the metal near-field probe can also act as a very effective antenna, since it can easily be comparable in length to the wavelength. As a result, antenna effects can play an important role in the measured signals, particularly in the case of broadband illumination [87, 88]. Similar ideas have subsequently become important in spectroscopic near-field imaging at higher frequencies [89, 90].
Fig. 6 The spatial distribution of the longitudinal (z) component of the THz field at the output of a tapered parallel-plate metal waveguide. Here, the plates are tapered in both transverse dimensions: the plate width (x) and the plate separation (y) both taper down to roughly 100 μm at the waveguide output facet. The field is measured using a scattering-probe technique [84], with a tapered metal probe with an apex size of ~10 μm. Adapted from [86].
Tip-based microscopies are now revolutionizing the field of THz near-field imaging, opening up new realms of nanoscience in the process. In one prominent example from the group of R. Huber, the high spatial resolution of a nano-tip was combined with the high temporal resolution of a femtosecond pump-probe configuration. This powerful combination enabled measurements of photoexcitation dynamics in a single nanostructure, with 100-femtosecond temporal resolution and 10-nanometer spatial resolution [91]. This work has inspired a number of important ideas, for example the use of these tip-based technique for the study of polaritons in 2D materials [92, 93]. In another recent example, the nano-tip was illuminated with a femtosecond optical pulse, thereby mediating the generation of THz radiation in the sample underneath the tip, as well as the out-coupling of this generated THz pulse into the far field [94]. This work offers an interesting contrast to other tip-mediated nonlinear optical measurements, which have mostly been performed in the near-infrared or visible spectrum. In these other measurements, the nonlinear interaction is also manifested by a change in the wavelength of the incident and out-going waves [95–97]. However, in the more recent THz case, this wavelength change is much larger (i.e., from 800 nm to 300 microns). This distinction is important in understanding the role of, for example, antenna effects in the measured spectrum. This recent work offers the intriguing possibility of characterizing the THz generation mechanism in a very localized area, much smaller than was previously possible [71, 72]. Nano-resolved LTEM can provide a direct measurement of the local electric field strength on a length scale relevant for individual nanostructures or circuit elements.
A final important example of tip-based THz imaging techniques is in the area of scanning tunneling microscopy (STM). In a conventional STM, a small DC voltage applied to a tip-sample junction induces a tunneling current, which (because of the nature of quantum mechanical tunneling through a potential barrier) is extremely sensitive to the tip height above the surface, and also to the electronic states directly underneath the tip. As is well known, STM can be used to map electronic states in surfaces or individual molecules, with better than single-atom resolution. Combining this idea with THz pulses opens a new possibility for STM: femtosecond temporal resolution. One can use an ultrafast THz pulse to transiently increase the tip-sample bias and drive a tunneling current. The single-cycle sub-picosecond THz pulse can be viewed as the fastest transient electrical bias that could possibly be applied between the tip and the surface [98]. The THz-induced tunnel current therefore acts as a fast sampling gate for STM measurements. Moreover, since the THz pulse is naturally synchronized with the femtosecond laser used to generate it, one can first pump the surface using an optical pulse (or another THz pulse), and then probe with time-resolved STM at a controllable time delay afterwards. This permits the measurement of the evolution of the tunneling state of a surface after an optically induced change, with femtosecond temporal resolution. THz-assisted STM offers the unique possibility of bringing time-resolved spectroscopy and imaging to the level single atoms or molecules [99–101].
4. Application: art and artifacts
As a final topic of discussion, we consider one of the most exciting application areas for THz imaging: the non-destructive investigation of artwork and historical artifacts. Art historians and conservationists have used analytic tools such as x-ray and infrared imaging for many years. The possibility for using THz radiation as a complementary tool for studying artwork was first proposed in the late 1990’s [102]. However, at that time THz imaging systems were too complex and expensive to find widespread use, especially in cases where the artwork in question was too fragile, precious, or bulky to move. As a result, most of the activity in this area waited for the subsequent development of more portable and user-friendly THz systems.
The pioneering work in the demonstration of the value of THz imaging in studying paintings was led by K. Fukunaga. Starting in about 2007, her group demonstrated that THz transmission spectroscopy could be used to distinguish the composition of various different types of paint layers, including identifying different pigments and binders, and their mixtures [103]. This work inspired a great deal of activity [104, 105] in the study of various media including mural paintings [106] and documents written with lead pencil [107]. In even more exciting work, several groups have demonstrated that THz imaging can reveal previously hidden underdrawings, by using tomographic time-of-flight methods to isolate signatures from buried layers and distinguish them from obscuring overlayers [108–112]. Interestingly, in some important examples the THz signal can be used to image through a lead-based white pigment overlayer, even when this layer effectively blocks x-ray imaging [113].
The interest in THz techniques extends beyond the study of paintings. Many other objects and materials are amenable to study using THz imaging. For example, time-of-flight reflection tomography can reveal hidden air gaps in carved stone [114]. In this instance, the gaps were caused by environmental degradation over centuries of exposure. Accurate location and characterization of these defects can inform subsequent conservation efforts. Similar methods have revealed sub-surface defects in 16th-century glazed terra cotta [115] (as shown in Fig. 7) and in 19th-century Russian icons painted on wooden panels [116]. New information can be revealed even in images of mummified remains, as bone can more easily be differentiated from surrounding materials using THz image contrast [117].
Fig. 7 (a) A tomographic cut of the air gap obtained from the terahertz time-of-flight measurements. (b) An X-ray computed tomography cut of the same air gap. The two measurements both allow determination of the air-gap width, but THz imaging is a portable technology that does not require the use of ionizing radiation. Reproduced from [115] with permission of the authors.
The development of portable THz systems has opened up an additional very exciting range of possibilities for this field. There are many situations where artwork or historical objects cannot be moved, due to size, cost, or fragility. Being able to transport the imaging system to the object, to make measurements in situ, offers many new opportunities. Numerous examples have been discussed in recent years, although this area is still in its infancy. One early example involved the study of sealed Egyptian pottery jars, dating back nearly 3500 years. These jars were imaged in the museum in which they are held, in order to study their contents without opening them [118]. THz imaging has also been used to study wall paintings in a number of churches as a means to reveal sub-surface structure or to locate hidden art beneath plaster [119]. A more challenging case involves the study of neolithic wall paintings, where both the painted surface and the overlayer are rough and uneven. In this case, the combination of THz imaging with more sophisticated signal processing techniques can provide some visualization of the buried underlayer drawing [120].
5. Conclusions
Over the last two decades, the field of terahertz imaging has grown from a promising curiosity to a thriving research discipline, with ever-increasing commercial application and scientific impact. Advances in both hardware [121, 122] and signal processing [123] continue to open new frontiers. The ability to distinguish and characterize multilayer dielectric samples based on time-of-flight measurements is key to many of the most successful commercial applications of THz technology so far [124–126], but countless possibilities remain unexplored. A quick search of the phrase “terahertz imaging” in articles published in Optics Express brings up over 300 articles. Just in the last year, these have included topics as diverse as art conservation, trace gas sensing, single-pixel image reconstruction algorithms, and biological tissue analysis. It is fair to say that the early steps in terahertz imaging from twenty years ago have inspired a fantastically diverse wave of exploration and development activity that continues unabated.
Funding
National Science Foundation; Army Research Office; W. M. Keck Foundation.
Acknowledgments
I gratefully acknowledge the following individuals for illustrations and permission to use them: Masayoshi Tonouchi (University of Osaka), Rayko Stantchev and Euen Hendry (University of Exeter), Pernille Klarskov and Peter Uhd Jepsen (Technical University of Denmark), Kirsti Krügener and Martin Koch (University of Marburg), Enrique Castro-Camus (CIO Leon), and Till Mohr and Wolfgang Elsaesser (Technical University of Darmstadt). I also acknowledge helpful conversations with Ullrich Pfeiffer (University of Wuppertal) concerning the technical specifications of the TicWave CMOS terahertz camera.
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