Recent years have witnessed successful developments of detection techniques of terahertz (THz) pulse radiation and its imaging applications such as security, medicine and environmental sensing, to name an important few. Progress of detection techniques has been made in many aspects, including detection sensitivity, real‐time detection, room‐temperature operation, detection bandwidth and dynamic range, spatial (wavefront) and temporal profiles and so on. New detection techniques utilizing cutting‐edge materials, sensors, systems and even novel detection mechanisms contribute to advances in terahertz pulse detection. While detection techniques continuously improve, terahertz pulsed imaging (TPI) also finds broad and intriguing applications. For instance, TPI has shown applications in nondestructive evaluation in pharmaceutics, biomedical characterization of tissues, medical diagnosis of cancers, identification of explosive hazards and examination of art and archeology. The chapter highlights recent progress of terahertz pulse detection techniques and imaging applications.
- pulsed terahertz detection
- terahertz pulsed imaging
Terahertz (THz) electromagnetic spectrum from 0.1 to 10 THz (3 mm–30 μm wavelength) is a scientifically rich frequency band that involves research in physics, chemistry, material science, biology and medicine. In the past five years, there has been a prosperous rise of research activity related to THz pulses, partly due to the advancement of various new technologies. There is a range of applications based on the study of the interaction between matter (solid, liquid, or gaseous) and THz pulses. To achieve further understanding and utilization of THz pulses, it is essential to advance THz pulse detection techniques. Furthermore, there is a need to develop novel and reliable THz pulse detectors that can facilitate diverse THz pulse applications. Another motivation for ameliorating detection techniques is the applications of THz pulsed imaging (TPI). TPI has exceptional potential for applications in security, nondestructive evaluation, biological sciences and medicine.
The objective of this chapter is to review the state‐of‐the‐art technology for THz pulse detection and imaging developed in recent five years. A search in the Web of Knowledge (Thomson Reuters) with “THz pulse” in “Title” has returned 589 articles during 2011–2015, while the number of articles returned is 379 during 2006–2010 (the search was conducted on June 24, 2016). Another search on Google Scholar with the same keyword “THz pulse” generates 4020 and 2540 articles for the five years 2011–2015 and 2006–2010, respectively. The two results show an increase of 55% and 58%, respectively, in the total number of publications between the two five‐year periods, which indicates the increasing interest in the area of THz pulses. As stated earlier, the research field of THz pulse detection and TPI is of unique significance. Therefore, the chapter is limited to, what the authors consider, the most interesting recent research findings in the area of THz pulse detection techniques and TPI applications including THz spectroscopy. The chapter is presented in the following arrangement: Section 2 will elaborate on the significance of pulsed THz technology, Section 3 will present recent THz pulse detection techniques, Section 4 will focus on recent TPI applications and Section 5 will provide a summary and future outlook in this field.
2. Significance of THz pulses
The THz pulse radiation is of great interest by several unique features. First, the use of a short THz pulse enables the study of THz fields and the collection of information in time domain, which is the underlying principle of THz time‐domain spectroscopy (THz‐TDS). A variety of physical phenomena and material characteristics can be studied utilizing short THz pulses . For example, by studying the absorption of THz photons in doped semiconductors, carrier dynamics can be studied . For intrinsic semiconductors, the complex permittivity or THz absorption coefficient and refractive index can be determined . Second, since a single‐cycle THz pulse can be intense and short, it is experimentally possible to tap into the regime of extreme nonlinear optics  where the usual approximation (e.g., the conditions for complete state transfer) no longer holds. THz pulse radiation with high energies has found many applications such as nonlinear spectroscopy , high harmonic generation , molecular alignment  and charged particle acceleration [8, 9], to name an important few. Third, the spectral bandwidth of pulsed THz waves can be of the order of several hundred percent of the center frequency and therefore, pulsed THz waves show promise in short‐distance data transmission at high bit rates.
Conventionally, optical sampling is used to perform time‐resolved measurement of THz responses. Free‐space electro‐optic (EO) sampling is one of the most common sampling techniques. Another common technique for detecting THz pulses is photoconductive antennas [10, 11]. The single‐cycle THz pulse can cover a wide spectral range (0.1–50 THz). The two detection schemes are ideal for THz‐TDS to investigate the spectral response of materials. On the other hand, commercially available thermal detectors such as bolometers, pyroelectric detectors and Golay‐cell detectors are generally used for measuring THz pulse energy. The three devices are maturely developed and can provide stable performance. However, bolometer provides high sensitivity, but cryogenic cooling is necessary, while the other two detectors show low sensitivity at room temperature, restricting detailed measurement. The rapid development of THz pulse detection techniques such as time‐domain profile detection and energy measurement offers many advantages and will be discussed in Section 3.
One primary application of THz pulse detection is TPI, which can provide a three‐dimensional (3D) map of the object by using the time of flight of THz pulses . TPI can be regarded as an extension of the THz‐TDS. TPI can acquire not only valuable spectral information but also 3D images. In image acquisition, a THz pulse is launched to the sample and the reflected echo is measured in amplitude and/or phase. The time‐of‐flight information of the echo pulse indicates the presence of the boundaries or inner structures along the propagation direction of the THz, which extracts the one‐dimensional depth profile. By performing a two‐dimensional (2D) scan from pixel to pixel, a 3D image of the target can be visualized. Thus, TPI is possible to provide 3D views into a layered structure. Unlike THz CW imaging, TPI can attain distinctive knowledge of the target, such as the spectral and depth information, by using the acquired amplitude and phase information of THz waves in the time domain. The critical benefit renders TPI valuable for diverse applications, such as detection of breast cancer  and inspection of pharmaceutical tablets [14–17], to name but a few.
Note that in the subject of THz pulses, there are issues relating to generation and detection, devices and systems and applications. We focus our discussion on recent development, basically in five years, in THz pulse detection techniques and TPI applications.
3. Terahertz pulse detection techniques
In order to attain the potential offered by pulsed THz technology, the generation and characterization of THz pulses play an important role in THz pulse applications. There are many methods for THz pulse emission, such as photoconductive antenna , optical rectification  and laser‐induced plasma [20–24]. On the other hand, several approaches for THz pulse measurement are well established because of historically long‐term needs in detection technologies such as radio astronomy. The newly developed technology for THz pulse detection in recent years will be described in this section. The progress has been made due to the advance of novel detection mechanism and material science.
3.1. Time‐domain THz sampling
In recent years major efforts have been made to improve the performance of photoconductive and EO sampling of pulsed THz wave. Photoconductive and EO methods are able to provide large detection bandwidth for THz pulses, which is useful for spectroscopy. As for photoconductive sampling, there are many studies and promising results [25–32]. (i) The sensitivity, THz bandwidth and dynamic range of THz pulse detection are improved by fabricating photoconductive antennas on InGaAs/InAlAs multilayer heterostructures [25, 26]. (ii) A nanowire‐based detector can be well suited for near‐field THz sensing . (iii) The detection of highly confined THz fields is demonstrated by employing nanostructure of optical materials . (iv) Plasmonic contact electrodes are used to enhance THz detection sensitivity . (v) Novel optical gating technique is used to realize subpicosecond temporal resolution in pulse detection . (vi) Some work is related to adapting photoconductive THz detectors to THz‐TDS systems [31, 32]. As for EO sampling, similarly, there are various progress such as improvement of detection efficiency [33, 34] and dynamic range , polarization sensing of THz pulses  and a new detection scheme based on the amplitude variation of optical pulse .
3.2. Energy or power measurement
One important need in THz pulse applications is to measure the energy or power of THz pulses. Recently there have been many investigations on new technologies.
We demonstrated a novel scheme based on photoacoustic conversion of carbon nanotube (CNT) nanocomposite to realize efficient and real‐time measurement of THz pulse energy . Conventionally used thermal detectors utilize continuous heat integration to measure the power of pulsed THz radiation. The power can be converted to energy with the pulse repetition frequency (PRF). Due to the mechanism of heat integration, most thermal detectors have slow response times, which limit the characterization of energy of each THz pulse at high values of PRF. Unlike conventional thermal detectors, we utilize photoacoustic effect to realize real‐time detection of THz pulse energy. Specifically, the transient and localized heating in an absorber by the absorption of THz pulse energy produces ultrasound, which is subsequently detected by a sensitive acoustic sensor. Moreover, our method responds only to the pulse excitation while rejecting other continuous radiations. In other words, in THz pulse detection, the ultimate sensitivity will not be restricted by the background continuous radiation, thus showing the potential for the efficient detection of THz pulse energy. In order to achieve efficient detection, it is essential to optimize the efficiency for photoacoustic conversion and subsequent acoustic sensing. We choose a CNT‐polydimethylsiloxane (PDMS) nanocomposite to achieve efficient THz‐to‐ultrasound conversion. This is because CNTs can efficiently absorb THz radiation and then convert it into heat via THz absorption capability and low specific heat of CNTs, while PDMS has a high thermal coefficient of volume expansion. On the other hand, we employ a photonic device, a polymer microring resonator, as a highly sensitive acoustic sensor. The resonator has a high optical quality (Q) factor of 1.3 × 105. The acoustic pressure impinges on the microring resonator, thus changing its effective refractive index and the resonance wavelength. To enable sensitive conversion of the ultrasound pulse to the modulated optical intensity, we identify a wavelength where the local slope in the transmission spectrum is high and then probe the microring using this wavelength. The modulated optical intensity is further recorded by a low‐noise high‐speed photodetector. A high Q factor of the resonator correlates with a high slope in the transmission spectrum and therefore high sensitivity of ultrasound detection. The setup of the PA detection of THz pulses is shown in Figure 1. A two‐color air ionization scheme is used to generate broadband THz pulses. The noise‐equivalent detectable energy of the technique is calibrated as ~220 pJ. We expect that three orders‐of‐magnitude improvement in sensitivity are possible by configuring the nanocomposite in the form of an acoustic lens as well as employing a microring resonator with a higher Q factor. The fast response time less than 0.1 μs is achieved, which is several orders faster than that of a commercial pyroelectric detector (~0.1 s). In addition, the novel method possesses other advantages such as room‐temperature operation, compact detector size in mm scale and wide spectral response for THz spectroscopy.
Sensitive room‐temperature detection of THz radiation is highly difficult. A detection mechanism based on the hot‐electron photothermoelectric effect in graphene can be a promising approach . First, photo‐excited carriers thermalize rapidly due to strong electron‐electron interactions but lose energy to the lattice slowly. Next, the electron diffusion due to the electron temperature gradient, as well as asymmetry or dissimilar contact metals, produces a net current by the thermoelectric effect. Figure 2 shows optical and atomic‐force micrographs of the graphene THz detector made by microfabrication technologies. Each of the two metal electrodes consists of partially overlapping Cr and Au regions and contacts the monolayer graphene flake. Electron temperature gradient ΔT is produced across the device, then resulting in potential gradient ΔV. The photoresponse is obtained by integrating ΔV over the length of the device. The graphene thermoelectric THz detector shows sensitivity exceeding 10 V/W (700 V/W) at room temperature and noise‐equivalent power less than 1100 pW/√Hz (20 pW/√Hz), referenced to the incident (absorbed) power. The sensitivity is comparable with that of the best room‐temperature THz detectors. Further improvements on orders‐of‐magnitude sensitivity is possible indicated by studying a model of the response including contact asymmetries. A fast intrinsic response time of 10.5 ps due to electron‐phonon relaxation is estimated for THz time‐domain measurements.
There are more examples besides the above two highlights. (i) Sensitive THz pulse energy detection is demonstrated by wavelength conversion from THz waves to near‐IR light using LiNbO3 crystals . (ii) Sensitive THz detection is possible through THz light amplification in optically pumped graphene . (iii) A THz line array detector with 20 elements is demonstrated with an average noise equivalent power of 106.6 pW/vHz and the -3‐dB bandwidth of 0.18 THz . (iv) A fast response time of 45 ps allowing the measurement of ultrashort THz pulses is achieved . (v) Electroluminescence effect is utilized to develop a low‐cost, probe‐beam‐free THz detection system .
3.3. Other sensing works
There are some intriguing research works for THz pulse sensing. We incorporate some examples here. (i) THz detection capable of acquiring the entire spatiotemporal profile of THz radiation in a single laser shot is demonstrated, which is based on space‐to‐time grading by using a converging probe intensity front . The approach does not require any specially designed optics or precise alignment. The scheme has several merits such as a simple setup, high temporal resolution and fast acquisition. Compared to the conventional EO sampling, the technique reduces the time taken for data acquisition and thus may offer a decent option for real‐time detection of THz radiation. (ii) The wavefront characterization of THz pulses is presented , which is realized by using a Hartmann sensor associated with a 2D EO imaging system composed of a ZnTe crystal and a CMOS camera. The wavefront sensing is crucial to applications such as optimization of far‐field intensity distribution of time‐domain imaging or enhancement of the peak power of intense THz sources. (iii) Measurement of spectra of THz pulses is realized by using a system consisting of channels for measuring amplitudes of pulses and an algorithm based on the iteration method or the amplitude‐frequency method . The spectrum measurement is essential to THz‐TDS applications.
4. Terahertz pulsed imaging applications
THz imaging systems have been improved in recent years. The systems are intrinsically safe, nondestructive and noninvasive and can answer many of the questions left unresolved by complementary techniques, such as optical, Raman and infrared imaging . Two types of imaging, TPI and THz CW imaging, are used with their respective strengths and weaknesses . TPI renders data richer in information. Specifically, depth information can be retrieved in TPI, while THz CW imaging acquires intensity image data without depth information. In this section, recent progress on the development of TPI systems will be introduced. Besides, we will give an overview over a broad range of TPI applications such as medical imaging and diagnosis, evaluation of tablets in pharmaceutics, painting investigation for art and archeology, material characterization and detection of concealed weapons.
4.1. THz pulsed imaging system
TPI system has shown progress in different aspects such as development of a compact system design suited to specific applications and enhancement of system performance including resolution, imaging time and dynamic range. For example, the performance of THz computed tomography (CT) is improved [50, 51]. THz CT can acquire and render 3D images in the THz frequency range, as in the optical, infrared, or X‐ray regions of the electromagnetic spectrum . A THz CT system using an injection‐seeded parametric source for frequency‐tunable, Fourier transform‐limited and high‐power THz emission and a heterodyne detector for sensitive THz detection is demonstrated, as shown in Figure 3 . This system covers a frequency range of 0.95–2.7 THz and achieves a dynamic range greater than 90 dB, enabling high‐resolution 3D THz CT images of samples with strong THz absorption. For illustration, 3D imaging of a pencil and a plastic product is obtained. The hidden lead as the internal structure of the pencil and the internal defect of the plastic product can be successfully revealed, demonstrating the system's capability and potential in nondestructive testing and evaluation of a variety of industrial products, such as semiconductors, pharmaceuticals, plastics and ceramics.
Besides the above highlight, there are more examples. We discuss some here. (i) A THz InGaAs Schottky barrier diode array detector with 20 elements is built for real‐time, compact and portable scanners in a TPI system . (ii) Another real‐time TPI system is built using a palm‐size THz camera that contains a microbolometer array and a compact quantum cascade laser (QCL) . (iii) A real‐time transmission‐type THz microscope is proposed by employing a THz penetration‐enhancing agent, glycerol, to improve the THz penetration depth in tissues because the glycerol has low absorption of THz waves compared with water .
The use of novel THz sources, waveguides and detectors is valuable for THz imaging systems. The QCL in THz frequency is a compact source of THz radiation with high power and high spectral purity. As such, the source is useful for many TPI applications such as long‐range imaging and materials analysis. The QCL‐based THz imaging approaches and their key advancements are reviewed . Another example is that by utilizing a split tapered waveguide with a subwavelength aperture, near‐field THz imaging is accomplished .
4.2. Biomedical applications
Interest in biomedical THz research is growing rapidly [56, 57]. THz radiation has very low photon energy and thus does not cause any ionization hazard for biological tissues. Unique absorption spectra over the THz band have been found in different biological tissues. The feature makes THz attractive for biomedical applications because THz can provide complementary information to existing techniques. TPI or THz‐TDS is one essential approach in biomedical THz research. In the following, we will describe TPI in various biomedical imaging applications.
Studies of cancer by THz imaging or spectroscopy have gained increasing interests in recent years . First, the presence of cancer often induces increased blood supply to the affected tissues and a local increase in tissue water content, which can be utilized as contrast for THz imaging of cancer. For instance, a sample of dehydrated human colon tissues embedded in paraffin is studied . The results demonstrate the potential of THz imaging to distinguish adenocarcinoma‐affected colon areas and the ability to image dehydrated tissues. Second, the structural changes in affected tissues can also be an important sign for diagnosis and can be observed by THz imaging. As another example, a reflection THz imaging system is used to identify tumors in freshly excised whole brain tissue from normal brain tissue based on structural observation . Because the THz reflection intensity is higher in brain tumors than in normal tissue, the difference in the THz reflection intensity between the normal and tumor brain tissues can be adopted for the diagnosis of cancer. Figure 4 shows the visual, THz and MR images of fresh whole brain tissues with and without tumors. Figure 4(a)–(c) shows the brain tissue with tumors and Figure 4(d) shows the normal brain tissue. The tumor boundaries in the THz images agree well with those visible images, indicating that the THz imaging technique could be useful for diagnosing brain tumors. Potentially, THz imaging could be employed as a complementary label‐free technique allowing surgeons to determine tumor margins in real time. In addition, THz image contrast differences are also observed in the normal brain tissue image in Figure 4(d) and these correspond to the gray and white matter areas, showing the ability of THz imaging to study brain structure. Furthermore, this study shows that the THz signals correspond to the cell density when water was removed. In other words, the THz contrast between normal and cancerous brain tissues can be distinguished not only by differences in the water content but also by differences in the cell density.
One other promising application of TPI is the study of breast cancer, which has been explored extensively. (i) Breast tumor phantoms that match the refractive indices and absorption coefficients in the THz band are developed to facilitate the study of breast cancer THz imaging . Phantom properties are verified through THz‐TDS. (ii) THz‐TDS and TPI are used for characterization of paraffin‐embedded breast cancer tissue . (iii) A pulsed THz system is used to enact a quick and reasonable estimation of the breast cancer margin thickness of embedded breast cancer tissue . (iv) A similar work investigates TPI for the application of surgical margin assessment of breast cancer in 3D, where the depth information is retrieved using time‐of‐flight analysis . (v) A linear sampling algorithm can be applied to TPI data for identifying breast cancer tumor margins . (vi) THz measurement of normal and breast cancer tissue in the range of 0.1–4 THz is presented , showing the ability of THz technology for characterization of cancerous and normal breast tissue. (vii) A clinical study that fifty‐one samples from patients in Cambridge and Guy's Hospital in London is conducted , showing the ability of THz technology to classify tumor and normal breast tissue with good accuracy.
4.3. Pharmaceutical applications
Pharmaceutical applications are one of the emerging opportunities offered by TPI and THz pulsed spectroscopy . Solid dosage forms are the pharmaceutical drug delivery systems of choice for oral drug delivery . These solid dosage forms are often coated to modify the properties of the active pharmaceutical ingredients, in order to help release kinetics [69, 70]. The critical coating attributes such as coating thickness, uniformity and density have chief influence on the tablet performance; advanced quality control techniques are required. TPI is an emerging nondestructive method to quantitatively characterize coating quality. Compared with established imaging techniques, e.g., near‐infrared and Raman spectroscopy, TPI has the advantage to enable structural features of coated solid dosage forms at depth by the ability of THz radiation to penetrate many pharmaceutical materials. A typical THz time‐domain waveform used for characterization of a single‐layer coated tablet is illustrated in Figure 5 .
A number of studies have been done recently. (i) TPI is used for nondestructive evaluation of film‐coated tablets by deriving parameters such as film thickness, film surface reflectance and interface density differences between the film layer and core tablets . (ii) Spectral domain optical coherence tomography (OCT) and TPI for quantifying film coating thickness of tablets are studied . The finding shows that OCT is suitable for characterizing pharmaceutical dosage forms with thin film coatings, whereas TPI is suited for thick coatings. (iii) To ensure robust measurements, the evaluation of film coating thickness using TPI should take some factors into account, such as signal processing of the raw data and signal distortions that can occur at tablet edges or areas with defects . (iv) Enteric coatings in tablets are widely used to reduce gastrointestinal side effects and to control the release properties of oral medications. TPI is used to identify structural defects within enteric coating tablets with poor acid resistance . (v) TPI is used for evaluation of the intra tablet and inter tablet coating uniformity and identification of critical process parameters in a coating process [75, 76]. (vi) TPI can also be used in evaluating the effect of coating equipment on tablet film quality . (vii) TPI is used to quantify the hardness and surface density distribution of tablets . (viii) TPI is a feasible and rapid tool to characterize ribbon density distributions , which are important parameters in dry granulation process in pharmaceutical industries. (ix) Tablet dissolution is crucial in medication and is strongly affected by swelling and solvent penetration into its matrix. A reflection mode TPI is used to measure swelling and solvent ingress in pharmaceutical compacts . (x) Layer separation is a crucial defect in many bilayer tablets. TPI is used to provide a precise estimate of the layer separation risk .
4.4. Art and archeology
Examination of art and archeology is important for cultural heritage scientists to understand artistic materials and to devise better conservation procedures . THz presents a number of valuable features specifically for the investigation of art and archeology such as no radiation risk with deep penetration (Figure 6), low power and noncontact mode. Recent progress shows that THz technology for art investigation is an efficient, convenient and affordable approach. We introduce several examples. (i) TPI is used to image apsidal wall painting in 3D to provide subsurface features at depths up to 1 cm from the surface . Characterization of subsurface features is useful in conservation of art history as well as in building archeology. (ii) TPI is used as a technique to image obscured mural paintings . Image processing can be used to solve the issue due to an uneven surface, enabling the visualization of the obscured painting. (iii) THz reflective tomography is used to identify the preset defects in a plaster . (iv) A portable THz‐TDS system is used to image panel paintings from a lab and a museum, offering useful information on the internal structure of the paintings and on their conditions . (v) TPI is also used to image an oil canvas painting by Pablo Picasso and the multilayer structure is clearly revealed . (vi) An artwork attributed to the Spanish artist Goya painted in 1771 is imaged and analyzed by THz time‐domain system . The study indicates that THz images present features that cannot be seen with optical inspection.
4.5. Other applications
There are still a variety of TPI applications in the fields of architecture, chemistry, material science, environmental protection and homeland security. (i) TPI and THz‐TDS are employed in characterization of construction and building materials . Different types of thermal building insulation materials are analyzed . (ii) TPI and THz‐TDS are used to identify wheat grains at different stages of germination . Specifically, the inner chemical structure during germination can be revealed from the THz spectra. (iii) TPI and THz‐TDS are also explored to identify features such as coating, pores and cracks in polymer materials . Another study employs TPI to study glass fiber‐reinforced composite laminates in polyetherimide resin . (iv) Reflective pulsed THz tomography can be a tool to monitor oil pollution. A cup of water covered by a layer of sesame oil with different densities is devised to simulate oil spills . The results show that different densities can be determined by THz images. (v) The spectral fingerprint of high explosive material is investigated using THz‐TDS . Besides, the explosive material concealed in an opaque envelope can be identified using TPI.
5. Conclusions and future prospects
In summary, a number of detection techniques of THz pulse radiation and applications based on TPI and THz‐TDS are presented in this chapter. We first elaborate the significance of THz pulses. Pulsed THz technologies bring about useful information from the interaction between THz radiation and matters. The applications based on pulsed THz technologies in part heavily rely on the technological advancement in THz pulse detection. We review the recent rapid development of THz pulse detection techniques in various aspects such as using novel materials, adopting innovative designs of detectors and even employing new detection mechanisms. These will open up new fields of applications or carry out particular tasks that cannot be attained previously. Of particular interest are the applications using TPI as well as THz‐TDS. Development and test of TPI systems have shown steady progress in recent years. In addition, a wide range of TPI or THz‐TDS applications including the fields of medical imaging and diagnosis, pharmaceutics, art and archeology, material science, architecture and so on has been intensively investigated by researchers and scientists. The exploration of THz pulsed detection technology and imaging applications will eventually lead to commercial products and systems for specific purposes, which is expected to have a profound impact on our lives. The substantial improvements over the past few years lay foundation for extending THz technology to new and potentially groundbreaking realms.
This work was supported by the National Natural Science Foundation of China (no. 61405112), the National High Technology Research and Development Program of China (863 Program) (no. 2015AA020944) and the Shanghai Pujiang Program (no. 14PJ1404400).