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In this paper, new concepts for continuous 24/7 real-time monitoring of plants and insects with miniaturized terahertz (THz) systems are described and experimentally tested. Thus, for the first time, small-scale insights into the instantaneous plant health but also in their long-term growth can be obtained. Using such compact THz systems, e.g. water uptake, insect infestation and the behavior of pollinators (i.e. honey bees) and pests can be measured dynamically and non-invasively at virtually any position in the close biotope surrounding them. In addition to general understanding, this can be used to optimize crop yield and reduce resource consumption as well as for identifying characteristics of insect-plant interactions induced by potential environmental stressors. Given such a holistic approach, the proposed concepts may provide a significant advancement in environmental monitoring technology.

Terahertz time-domain spectroscopy (THz-TDS) is a promising tool for high-resolution 3D imaging of objects due to the high center frequency and bandwidth compared to microwave systems. In addition, terahertz waves have a higher penetration depth than visible or near-infrared radiation. Typically, optics are used to focus the terahertz radiation onto an object under test. This limits the imaging capability in the axial dimension to the depth of field and limits simple imaging of complex surfaces. In this work, we adapt a backpropagation algorithm from synthetic aperture radar (SAR) imaging to reconstruct high-resolution 3D images from time-domain traces acquired with a lensless THz-TDS system. For this purpose, an inverse cylindrical aperture is used and an equation that describes the maximum achievable resolution as a function of the beam pattern, the bandwidth, and the length of the synthetic aperture is derived. The calculated resolution is $960 \mu \text{m}$ for the linear dimension of the aperture and 75 $\mu \text{m}$ for the rotationally symmetric dimension of the cylindrical aperture for a bandwidth of $2\,\text {THz}$ . The resolving power is verified by measurements on metallic and dielectric objects. In the future, this method can be used for non-destructive testing of objects with complex shaped surfaces and internal structures.

Terahertz (THz) systems open up the possibility of new applications of electromagnetic waves. Enormous bandwidths up to several terahertz enable the development of powerful spectroscopy systems that can provide insights into the structure and material of objects with micrometer resolution. New high power and compact THz time-domain spectroscopy (TDS) systems will be presented. The wide bandwidth also enables high-resolution imaging under far-field conditions to analyze arbitrary objects from a distance. In addition, a near-field system-on-a-chip imaging system is presented that achieves a resolution of 10 μm. Additionally, the application of terahertz waves presents challenges due to the low transmit power of the sources, high attenuation in free space, and the high noise figures of the receivers. These challenges can be overcome by antennas with high gain. Since - depending on the application - spatial scanning or focusing in a time-varying direction is required, beam steering is an essential component of many THz systems. In terms of communications, terahertz carrier frequencies offer wide bandwidths, enabling correspondingly high data rates of 100 Gbit/s and more. As a result, the frequency bands between 250 GHz and 450 GHz have already been identified and/or allocated for communication services and are being discussed as a component of 6G mobile communications. Concepts and demonstrations for 6G terahertz mobile communications will be presented. The high bandwidth of terahertz waves can also be used for high-accuracy indoor localization.

With the increase in potential uses of terahertz technology, the need for terahertz transceivers with application-oriented adaptive radiation patterns has emerged. Reconfigurable reflectarrays consisting of actuated sub-wavelength reflectors have been successfully used for terahertz beam steering and beamforming. They do not require a complex feeding network and associated electronics, enabling a compact and power-efficient terahertz beam steering system. However, the current reflectarray-based beam steering is accomplished by forming the reflectarray as a grating structure, which is accompanied by the problems such as grating lobes, limited steering range, and discrete steering angles. Here, we configure a MEMS-based reflectarray with the genetic algorithm to eliminate the grating lobes and open up the possibility of customizing its radiation pattern. We used single- and multi-objective optimization to find the optimal height profile of the reflectarray and verified the results by full-wave electromagnetic simulations. We measured the radiation patterns of four reflectarray phantoms, i.e. reflectarrays without the MEMS actuation systems. The measurement results agree well with the calculated ones, with the main beam deviating at most 2° from the target direction. Our work demonstrates how a genetic algorithm is used to shape a reconfigurable terahertz reflectarray to eliminate the grating lobes and tailor some specific featuress in its radiation pattern.

In recent years terahertz (THz) technology has attracted great interest in biosensing applications. Due to the interaction between analyte and electromagnetic (EM) field, a THz resonator is sensitive to changes in the refractive index of the analyte and can be used as a thin-film sensor for rapid pathogen diagnosis. To achieve high sensitivity and reliability, the sensor should have a high Q factor, a high field concentration at the site of the analyte, and the ability to compensate for temperature effects. However, conventional metamaterial methods have a low Q factor, which may lead to ambiguous detection and no attention has been paid to address the temperature effects. Here, we present a photonic crystal (PhC) based chip consisting of a reference channel and a sensing channel with two identical PhC slot resonators. The resonance difference between the resonators is temperature invariant and can be used for analyte detection. The dual-channel PhC chip has a Q factor of 5063 and a figure of merit of 3.1 /RIU/ $\mu \text{m}$ (RIU is refractive index unit), which are higher than that of metamaterial sensors. The chip is designed and simulated for the W band by 3D field simulations and is verified by measurements. Our results suggest that THz PhC resonators can provide high sensitivity and high resistance to environmental effects. We anticipate our work to be a starting point for future biosensing applications.

Terahertz time-domain spectroscopy enables the extraction of electrical properties from materials. An extraction of the complex permittivity can be carried out with measurements in transmission or reflection geometry enabling the identification of materials. To perform an exact identification, the sample thickness, and the angle of incidence of the terahertz radiation must be known. However, when those parameters are unknown and additionally the materials show strong absorbances, a precise differentiation between materials is challenging. A promising approach is the use of a neural network for automated material classification of terahertz images from different materials. Here, we show that a trained neural network can differentiate between 16 3D printed dielectric materials with a high accuracy of 98 % from measurements taken in transmission mode. For unknown thicknesses, the accuracy is reduced to 35 %. As the constitution of the dataset has a big impact on the accuracy, various data preparations were investigated as well as the number of traces needed for achieving a well-trained network was determined. Finally, the trained neural network was evaluated with different sample thicknesses, revealing the huge impact of the materials absorbance on the extrapolation ability. This approach can be used in security application to classify harmful substances as well as for the automated generation of material maps.

The potential of pulsed THz radiation for time-of-flight imaging applications is well recognized. However, advances in this field are currently severely limited by the low average power of ultrafast THz sources. Typically, this results in impractically long acquisition times and a loss in resolution and contrast. These limitations make imaging of the objects in real-life scenarios impossible. Here, conclusively, the potential of state-of-the-art high-average power THz time-domain spectrometer (TDS), driven by a 100-W class, one-box ultrafast oscillator for imaging applications is shown by demonstrating lensless THz imaging in reflection mode of a dielectric sample with low reflectivity. Images obtained with our home-built 20-mW average power THz-TDS system show a significant contrast enhancement compared to a state-ofthe-art commercial THz-TDS with less than 200 μW of average power. Our unique setup even allows us to obtain images of such an object with high-contrast hidden inside a medium-density fiberboard (MDF) box. This opens the door to THz time-of-flight imaging of concealed objects of unknown shape and orientation in various real-life scenarios which were so far impossible to realize.

In this work we demonstrate that 3D printed Al2O3 is a promising material for prototyping and precise fabrication of quasi-optical devices in the terahertz frequency range. The 3D printed Al2O3 exhibits a low absorption coefficient (α -1at 1 THz) and a high refractive index (n > 3). The printing resolution in the sub 50 μm range allows for the implementation of structures in the 0.3-3.0 THz range on the subwavelength scale. Furthermore, the printing process enables the realization of crystalline solids, which allows the use of the Al2O3 birefringence effect. Here, a Δn ≈ 0.05 was achieved and used for the implementation of λ/2-wave plates working at ~1 THz. The material properties and wave plates were characterized using a terahertz time-domain spectrometer.

Synthetic aperture radar (SAR) is a well-known imaging technique and most commonly used up to the microwave frequency spectrum (below 30 GHz) which provides spatial resolution in the sub-m range. To enhance the resolution, higher frequency spectra such as millimeter-wave (mmWave) and terahertz (THz) regions are being investigated. The mmWave and THz spectral ranges extend the SAR applications to non-destructive testing (NDT), material characterization, and sub-mm resolution imaging. However, the higher frequency spectrum suffers from higher path loss and potentially higher atmospheric absorption that limits the propagation distance. Nevertheless, the mmWave/THz spectrum is suitable for short-range applications such as indoor room profiling. From theoretical analysis, it can be summarized that the higher frequency spectrum provides better resolution but a comparative study on the impact on the image quality of the frequency spectrum ranging from GHz to THz has not been presented. Besides, as of the hardware complexity of the THz devices, the optimum range of the spectrum is always under investigation. The optimum range is defined where no strong improvements in the image quality are achievable with further increases in the frequency spectrum. Therefore, this paper presents an overview of electronics-based imaging using the SAR technique for the frequency spectrum ranging from GHz to THz with the focus on NDT and high-resolution imaging. Seven frequency bands: 5-10 GHz, 68-92 GHz, 75-110 GHz, 0.122-0.168 THz, 0.22-0.33 THz, 0.325-0.5 THz, and 0.85-1.1 THz are selected for a comparative analysis. The results are presented for 2D and 3D imaging using the backprojection algorithm. Additionally, state-of-the-art imaging based on SAR technique with electronics transceiver modules has only been demonstrated up to the sub-0.75 THz, whereas in this paper the spectrum up to 1.1 THz has been addressed.

Imaging in the terahertz frequency range has attracted growing interests since the first image of a leaf more than 20 years ago, due to its countless applications in basic and applied research, medical imaging, and nondestructive testing. However, most terahertz imaging approaches rely on focusing optics which require knowledge about the imaging scene before the actual imaging takes place. Further, imaging is mostly restricted to short distances and high resolution is only achieved for systems with a high bandwidth. Here, we present a method that enables high-resolution imaging of small metallic and dielectric objects at distances up to 2 m based on a synthetic aperture. We derive a simple approximation for the resolution of partial circular synthetic apertures with limited bandwidth. The bandwidth limitation is encountered by replacing the measured signals with replica signals of high bandwidth and equal round-trip time so that the resolution is only limited by the carrier frequency and signal-to-noise ratio of the measurement system.

Material characterization in the 0.1 - 10 THz range has been a major topic of research since its first demonstration 30 years ago. Advances in terahertz generation, detection, and data acquisition have contributed to improved bandwidth, signal power and signal-to-noise ratio. However, material characterization is still performed using conventional spectroscopic measurement schemes which require detailed information about the test object's shape, location, orientation relative to the measurement system, and a reference measurement. Here, we present a method for reference-free material characterization of dielectric objects using a terahertz time-domain spectroscopy system without a priori knowledge about the object and its position. The proposed method is based on ellipsometry combined with lensless imaging for the estimation of the refractive index. The method is a multistage procedure designed for small test objects in reflection mode. The diffracted terahertz radiation is separated from the specular reflected radiation in a post-processing step to enable a valid material parameter estimation. In this way, small dielectric objects can be located, imaged with a sub-mm resolution, and their material parameters extracted.

In this paper, we propose a fast terahertz time-domain imaging method using a radar migration algorithm. We demonstrate high-resolution imaging in reflection without any collimating or focusing optics in the terahertz beam. In the proposed method, the sample is illuminated with a divergent terahertz beam, and the receiver collects both specular and diffuse reflections. We further present calibration and post-processing methods that allow us to compensate for the inherently low signal-to-noise ratio of an unfocused terahertz beam. The feasibility of the novel imaging method is demonstrated with geometrically complex samples and a fast terahertz time-domain spectroscopy system based on electronically controlled optical sampling. We show that our concept is capable of generating images of the objects regardless of their size, shape, orientation and position relative to the transmitter and receiver antennas. Objects with edge lengths well below 400 μm can be clearly detected. The method presented here thus lends itself to arbitrary scenarios and antenna configurations.