Terahertz Antenna Technology for Imaging and Sensing Applications

The terahertz (THz) regime of the electromagnetic spectrum is rich with the emerging possibilities in imaging applications with unique characteristics to screening for weapons, explosives and bio-hazards, imaging of concealed objects, water content, and skin, and these advantages can be harnessed by using the effective THz sources and detectors. In THz imaging systems, the pulsed THz sources and detectors find unique applications and thus we have emphasized on re-visiting these kinds of systems. Several novel imaging techniques which exploit the distinctive properties of the THz systems have been presented. Moreover, the THz antenna is one of the most important components of a THz imaging system as it plays a significant role in both impedance matching and power source. Therefore, the recent developments in THz antenna design for imaging applications are reviewed and the potential challenges of such THz systems are investigated. The photoconductive antennas form the basis of many THz imaging and spectroscopy systems and finds promising applications in various scientific fields. However, for the imaging applications, there is a requirement of planar and compact THz antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution. Therefore, the key modalities of improving photoconductive dipole antennas performance are identified for imaging applications. Also, the ways to improve the directivity of the photoconductive dipole antenna are discussed. The main purpose of this review is to provide an assortment of all relevant literature to bring researchers up-to-date on the current state-of-the-art and potential challenges of THz antenna technology for imaging applications.

The area of Terahertz (THz) is one of the fastest growing research fields in recent years. THz antennas based upon photoconduction techniques are the most common devices in THz systems. However, the radiated THz power from these devices and the efficiency are very low. Also, different antenna excitation and current generation process in THz antennas, as compared to microwave antennas, requires new analyses approaches. Therefore, the motivation of this thesis is to theoretically establish why the THz antenna is inefficient; from which, general methods to improve the performance of such antennas are explored and validated. These investigations are essential to gain a better understanding of THz photoconductive antenna performance. In this research a new equation for the source conductance of a THz antenna is firstly developed. This is a prerequisite for further antenna radiated power analysis. Next, a new equivalent circuit, modelling the underlying physical behaviour of the device through the use of a lumped-element network, is developed. Through this model, various factors which affect the radiated power and efficiency of the THz photoconductive antenna are examined and compared with measurement results. This model can be applied to maximize the optical-to-THz conversion efficiency. Also, temporal voltage behaviour of the antenna can be predicted more realistically. Furthermore, a computational simulation procedure, solving both optoelectronic and electromagnetic problems, is proposed and validated by measurement results. This approach facilitates prediction of THz photoconductive antenna performance before antenna fabrication. In addition, considering the requirement of high THz power and good SNR devices for various THz applications, a new top loaded THz antenna embedded on a conical horn with the trapezoidal photomixer is proposed. The generation of THz photocurrent, impedance matching and coupling of the THz wave to air (the necessary factors for power enhancement) are improved. Moreover, the new trapezoidal photomixer is examined and the measurement results show that it has better radiated THz power and SNR than the bare gap and rectangular photomixers.

The rapid development of various advanced wireless applications in diversified fields makes the microwave spectrum congested, thereby demanding higher frequency bands (terahertz) with high transmission data rates (tbps) for next-generation wireless systems. Terahertz frequency band will eliminate communication spectrum shortages and lead to various novel applications at terahertz radiation, such as advanced wireless sensing and next-generation medical applications, detection of explosive materials, etc. The stable terahertz communication link can play a crucial role to achieve higher data rate transmission within a short distance. Antenna is an integral, essential element in any wireless communications system, and choosing the right antenna is critical to the successful use of wireless communication links. However, almost all the terahertz antennas suffer from high attenuation losses, which drive the requirement of high gain and wideband antennas. In this chapter, the various applications at terahertz radiation, the assorted antennas for terahertz frequencies, and microstrip terahertz antennas for detection of explosive materials with novel methods that achieved high gain and good bandwidth and their designs and development challenges are discussed.KeywordsMicrowave spectrumHigh transmission data rateNext-generation wireless systemsTerahertz antennasAttenuation lossesApplications at terahertz radiationDetection of explosivesMedical applications

We have studied the generation of terahertz (THz) waves by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals. Using parametric oscillation of MgO-doped LiNbO 3 crystal pumped by a nano-second Q-switched Nd:YAG laser, we have realized a widely tunable coherent THz-wave sources with a simple configuration. We have also developed a novel basic technology for THz imaging, which allows detection and identification of chemicals by introducing the component spatial pattern analysis. The spatial distributions of the chemicals were obtained from terahertz multispectral trasillumination images, using absorption spectra previously measured with a widely tunable THz-wave parametric oscillator. Further we have applied this technique to the detection and identification of illicit drugs concealed in envelopes. The samples we used were methamphetamine and MDMA, two of the most widely consumed illegal drugs in Japan, and aspirin as a reference.

Summary form only given. The terahertz (THz) band of the electromagnetic spectrum has long held interest in the science and engineering community for its unique applications in imaging and spectroscopy. However, due to the difficulty in generating and receiving THz radiation, only very recently practical and efficient THz systems become available. Due to recent advances in the development and manufacturing of commercially available THz spectroscopy and imaging system many of these applications are coming to fruition. As such, the practical implementation of THz in applications needs to be investigated in order to determine the advantages of THz technology. In this work, THz spectroscopy of photovoltaic semiconductor devices will be performed throughout various critical stages in the fabrication process. Due to the few millimeter penetrating and none-destructive properties of THz radiation it will be possible to monitor semiconductor wafers using THz spectroscopy. This technology has a potential to provide insight into the development of wafer defects earlier in the fabrication process. Using a pulsed Terahertz imaging and spectroscopy system at the University of Arkansas, our group will investigate the electrical properties of semiconductor photovoltaics and will conduct THz imaging of defected ones. Testing the wafers throughout various key steps in fabrication is a process known to produce a functional device. This will then be repeated for the same process, only now with a critical fatal defect such as a buried crack, surface crack or a defect in the doping level. These defects will be intentionally inserted at an early stage to determine if THz spectroscopy can be used to detect them early on. THz characterization of the semiconductors for the impurity doping levels and crystal orientation to be studied will be performed to determine the electrical properties in this frequency range. This will allow for accurate numerical simulation to compare with the experimental results.

Ion mobility spectrometry (IMS) is an analytical technique that has primarily been used as a trace chemical monitor in the field. The benefits of IMS as a field detector are its simplicity, sensitivity, fast analysis times, portability, ruggedness, and its operation at ambient pressures. IMS has successfully been used for many years to detect traces of explosives at airports and other security checkpoints; illicit drugs in prisons, cargo, and in border security. The military have long used IMS for detection of chemical warfare agents. Innovations in drift tubes, nonradioactive ion sources, and hyphenated IMS techniques in recent years have expanded the range of forensic applications. A sample introduced into the IMS undergoes atmospheric pressure chemical ionization and the resulting product ions are separated by their mobilities in the drift tube. Several factors such as temperature, humidity, concentration and presence of interferences can affect the ionization and separation process. This article describes the principles of IMS, developments in technology, and their implications for forensic science. IMS trends in detection of explosives, illicit drugs, and chemical warfare agents are discussed. The article also provides an overview of some unconventional applications and some interesting emerging forensic applications of IMS.

Terahertz (THz) is an electromagnetic spectrum with a frequency range from 0.1 to 10 THz, which is located in between the microwave and infrared regions. The unique features of THz waves make them eligible for use in various medical applications. The THz imaging is one of them, which is mainly based on the analysis and processing of the transmission and reflection spectrum information of the sample. This chapter mainly presents the research status and prospects of several THz medical imaging systems and their applications for medical imaging in biological tissues. As the demand for technology grows, high-performance THz imaging systems are becoming indispensable. The ability of the THz time-domain spectroscopy (THz-TDS) system to extract spectra from amplitude and phase information opens virtually unlimited possibilities for imaging applications. The performance achievements of THz-TDS-based imaging with potential research on its fast-imaging components for solving the existing limitations of imaging speed are also presented. Furthermore, the latest developments of several THz-TDS-based imaging methods, including tomography imaging and near-field imaging, are highlighted with their performance improvement. Additionally, the rapid development of THz-TDS as a highly versatile analytical tool for the characterisation of pharmaceutical materials has drawn considerable attention. One of the greatest biomedical potentials of THz imaging is the use of molecular spectroscopy for diagnostics, which is exponentially advanced and moving closer to progress. It is now evident that different types of biomolecules leave distinctive spectral fingerprints in the THz region, which considerably widens the coverage of its technology application including in-vitro and in-vivo measurements of small molecules of clinical importance in point of care and diagnostic systems. In-vivo molecular imaging is considered the next frontier in medical diagnostics, which would be ideally performed non-invasively. Recent achievements in the field of medical imaging have dramatically enhanced the early detection and treatment of many pathological conditions. THz imaging systems can help in detecting early cancer before it is visible or sensitive to any other identification resources. The THz images can distinguish between healthy tissue and basal cell carcinoma and therefore help in mapping the exact margins of early-stage tumours. By obtaining both frequency and time-domain information, the THz imaging can ensure enhanced detection of cancer and provide sharper imaging and molecular fingerprinting. THz biomedical imaging has become a modality of interest due to its ability to simultaneously acquire both image and spectral information. Advanced digital image processing algorithms are greatly needed to assist in screening, diagnosis and treatment. Finally, we summarise the obstacles in the way of the application of THz biomedical imaging application technology in clinical detection, which need to be investigated and overcome in the future.

Evolving from past black‐and‐white images, through present red‐green‐blue spectral colors, future remote imaging systems promise spectroscopic functionalities extending well beyond the visible wavelengths. This allows real‐time spectral information to be gathered from multiple wavelength bands that is applicable to numerous remote sensing spectroscopy/imaging applications and aids target recognition. This paper reviews the wavelength tunable microelectromechanical systems (MEMS) optical filter technologies developed for the important infrared and the emerging terahertz wavelength bands of the electromagnetic spectrum with the fabrication effort being enabled by the Western Australian Node of the Australian National Fabrication Facility. A low size, weight and power (SWaP) platform solution is demonstrated delivering mechanically robust, field‐portable, spectroscopic, chem/bio sensing suitable for deployment in remote sensing and imaging applications.

Traditional terahertz (THz) equipment faces major obstacles in providing the system cost and compactness necessary for widespread deployment of THz applications. Because of this, the field of THz integrated circuit (THz IC) design in CMOS and SiGe HBT technologies has surged in the last decade. An interplay of advances in silicon process technology, design technique, and microelectronic packaging promises to narrow the gap between the requirements and the reality of system cost and performance of THz components. Furthermore, the scalability, reconfigurability, and signal processing features of silicon technology have initiated research in complex THz ICs that expand the functionality of THz systems; this has enabled new applications, methods, and algorithms. This paper reviews the progress in THz IC research and investigates several realizations of THz imaging and sensing applications with silicon-based components regarding their motivation, system performance, and challenges. THz computed tomography, broadband multicolor imaging, high-resolution FMCW radar imaging, subwavelength resolution near-field imaging, and compressed sensing are presented.

Chemiluminescence (CL) and bioluminescence (BL) imaging technologies, which require no external light source so as to avoid the photobleaching, background interference and autoluminescence, have become powerful tools in biochemical analysis and biomedical science with the development of advanced imaging equipment. CL imaging technology has been widely applied to high-throughput detection of a variety of analytes because of its high sensitivity, high efficiency and high signal-to-noise ratio (SNR). Using luciferase and fluorescent proteins as reporters, various BL imaging systems have been developed innovatively for real-time monitoring of diverse molecules in vivo based on the reaction between luciferin and the substrate. Meanwhile, the kinetics of protein interactions even in deep tissues has been studied by BL imaging. In this review, we summarize in vitro and in vivo applications of CL and BL imaging for biosensing and therapy. We first focus on in vitro CL imaging from the view of improving the sensitivity. Then, in vivo CL applications in cells and tissues based on different CL systems are demonstrated. Subsequently, the recent in vitro and in vivo applications of BL imaging are summarized. Finally, we provide the insight into the development trends and future perspectives of CL and BL imaging technologies.

The use of coaxial waveguides with terahertz pulses is hindered by the difficulty in exciting the radially polarized fundamental mode. We propose a radially symmetric terahertz antenna and present simulations predicting a radially polarized beam.

The use of coaxial waveguides with terahertz pulses requires the difficult excitation of the radially polarized fundamental mode of the waveguide. We propose a radially symmetric terahertz antenna and present simulations predicting the generation of a radially polarized beam.

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Advanced applications of mass spectrometry imaging technology in quality control and safety assessments of traditional Chinese medicines

The depth of field (DoF) was extended 2.8-fold to achieve rapid crystal screening by retrofitting a custom-designed micro-retarder array (µRA) in the optical beam path of a nonlinear optical microscope. The merits of the proposed strategy for DoF enhancement were assessed in applications of second-harmonic generation imaging of protein crystals. It was found that DoF extension increased the number of crystals detected while simultaneously reducing the number of `z-slices' required for screening. Experimental measurements of the wavelength-dependence of the extended DoF were in excellent agreement with theoretical predictions. These results provide a simple and broadly applicable approach to increase the throughput of existing nonlinear optical imaging methods for protein crystal screening.