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Many established terahertz imaging modalities are on one side restricted by the tradeoff between resolution and field of view such as in the case of focal plane arrays and on the other side suffer from a limited depth of field such as in the case of quasi-optical terahertz imaging configurations. Furthermore, typical scanning solutions require time-consuming measurement procedures and restrict significant potential industrial deployments of terahertz imaging technology. Imaging with sparse multistatic line arrays in combination with digital beam forming (DBF) techniques enables us to overcome these limitations and offer three-dimensional (3D) terahertz image reconstructions of the object. This contribution addresses the design of such terahertz imaging systems from a general point of view with the focus on the design of sparse line arrays, while considering objects scattering properties. Based on this design concept, the realization of a novel highly sparse 3D terahertz imaging system is discussed. The sparse line array of the system is operating within a frequency range from 75 to 110 GHz and is used in combination with a conveyor in order to generate a synthetic sampling aperture. The system is capable to generate 3D terahertz images with tens of megavoxels at feed motions of up to a few 10 cm/s. Also, a sparse array design in regard to an imaging system operating at 240 GHz with integrated SiGe sensor elements is discussed. In addition, three different DBF algorithms are compared in regard to their computational efficiency.

We present a simulation and data-processing framework that incorporates full 3-D configuration, simulation, and reconstruction functionalities for hybrid synthetic aperture THz systems, which combine mechanically scanned real imaging projection (1-D) and synthetic imaging reconstruction (2-D). The simulation is based on a ray-casting approach including models for reflections at rough surfaces and angular antenna sensitivity. A near real-time GPU reconstruction is introduced to process large amounts of data in parallel. Techniques for fast fusion of reconstructed data from several acquisition directions are presented. They serve as basis to visualize a fully consistent 3-D object. The modular framework allows a fast, easy, and efficient evaluation and prediction of system properties including scattering behavior for these types of scanner setups.

This paper presents the research and development of two terahertz imaging systems based on photonic and electronic principles, respectively. As part of this study, a survey of ongoing research in the field of terahertz imaging is provided focusing on security applications. Existing terahertz imaging systems are reviewed in terms of the employed architecture and data processing strategies. Active multichannel measurement method is found to be promising for real-time applications among the various terahertz imaging techniques and is chosen as a basis for the imaging instruments presented in this paper. An active system operation allows for a wide dynamic range, which is important for image quality. The described instruments employ a multichannel high-sensitivity heterodyne architecture and aperture filling techniques, with close to real-time image acquisition time. In the case of the photonic imaging system, mechanical scanning is completely obsolete. We show 2-D images of simulated 3-D image data for both systems. The reconstruction algorithms are suitable for 3-D real-time operation, only limited by mechanical scanning.

Zusammenfassung Terahertz- und Millimeterwellen sind gesundheitlich unbedenkliche elektro-magnetische Wellen im Bereich von 100–10.000 Gigahertz. Bei diesen Frequenzen sind die meisten nicht-leitfähigen Materialien transparent, sodass eine kontaktlose, zerstörungsfreie Prüfung der inneren Struktur moderner (Kunststoff-Komposite-) Werkstücke mit einer Orts- und Tiefenauflösung im Submillimeterbereich möglich ist. Die hier vorgestellte vollelektronische Technologie ermöglicht hochqualitative Bilder in einer extrem kurzen Messzeit.

Die Verwendung der THz-Strahlung bietet diverse Anwendungsmöglichkeiten für die Messtechnik, wie z.B. die hochgenaue berührungslose Bestimmung der Oberflächentopographie technischer Materialien. Interferometrische Methoden im Visuellen und nahen IR scheitern, wenn die mittleren Rautiefen der zu untersuchenden Oberflächen-Werte größer als ein Zehntel der benutzten Wellenlängen annehmen. Wir stellen zwei Methoden zur hochgenauen Formmessung technischer Oberflächen vor: THz-Dunkelfeld-Reflektometrie und THz-Heterodyn-Profilometrie.

We give an overview over several all-optoelectronic measurement systems which we have developed for transmittive and reflective imaging in the terahertz (THz) frequency range. The systems employ either pulsed or continuous-wave THz radiation. In both cases, they work on the basis of single-pixel scanning. Addressing the potential for imaging in the medical and dental field, and the application of THz radiation for industrial surface and interface characterization, we explore dark-field imaging where the imaging contrast originates from diffraction and scattering effects coming from topography or refractive-index variations.

We give an overview of opto-electronic terahertz systems based on femtosecond amplifier lasers, and present a comparison of the relative performance and design issues of the various suitable downconversion photomixers employed for the generation of intense terahertz radiation pulses in such systems. The survey includes large-area biased semiconductor antennas, large-area nonlinear optical crystals and laser-generated gas plasmas as sources for terahertz radiation. The performance of these emitters is reviewed in terms of the terahertz pulse energies achieved and their associated spectral and signal-to-noise properties.

We present an all-optoelectronic THz imaging system for ex vivo biomedical applications based on photomixing of two continuous-wave laser beams using photoconductive antennas. The application of hyperboloidal lenses is discussed. They allow for f-numbers less than 1/2 permitting better focusing and higher spatial resolution compared to off-axis paraboloidal mirrors whose f-numbers for practical reasons must be larger than 1/2. For a specific histological sample, an analysis of image noise is discussed.

We propose two terahertz reflectometry modalities that are optimized to be sensitive to the curvature of surface features. The first is a dark-field technique that allows the detection of protrusions and dents on surfaces with high sensitivity. It cannot distinguish, however, between convex and concave shapes. This becomes possible with the second technique, which combines out-of-focus imaging with suitable beam filtering. Both methods may be of interest for surface inspection in fabrication environments; for example, for online monitoring during metal processing.

We present an all-optoelectronic THz imaging system based on photomixing of two continuous-wave laser beams using photoconductive antennas. For a specific biological sample, we compare continuous-wave THz imaging and pulsed THz imaging at 1 THz with respect to data-acquisition time and signal-to-noise ratio, and discuss image formation from both amplitude and phase data. In addition, we introduce the application of hyperboloidal lenses which allow tighter focusing and a corresponding improvement in spatial resolution compared to off-axis paraboloidal mirrors.

We report realization of a THz camera based on electro-optical detection of THz radiation from an optical parametric oscillator, using a silicon-based CCD camera as the optical detector. The dynamic range of 16.5 dB for a 6000-frame average (2-minute data acquisition time) is limited by laser noise.

We investigate how all-electronic Terahertz (THz) / Millimeter wave imaging allows the evaluation of special material structures used in aerospace applications. One of the structures is a foam material intended for the isolation of cryogenic tanks, the other a glass fiber composite sample intended for several applications.

The ability of terahertz and millimeter-wave imaging to detect suspicious hidden objects underneath or in luggage has led to increased interest in these techniques. Several approaches have been demonstrated in the past few years, amongst which active, all-electronic terahertz imaging has proven to be particularly adapted for safety and security applications. It combines a large dynamic range and the ability to perform range measurements with increased spatial resolution. At the French-German Research Institute of Saint Louis (ISL), we use an all-electronic 3D imaging system for a comprehensive study on various suspicious objects and cloth types. We demonstrate an enhanced detection capability for hidden suspicious objects if the range information is extracted and visualized in appropriate ways.

We successfully realized electro-optical detection of nanosecond THz pulses based on a THz optical parametric oscillator. A maximum dynamic range of ∼30 dB/ Hz is achieved in the electro-optical measurements.

We demonstrate all-electronic 3D Terahertz tomographic imaging with an extraordinary dynamic range of higher than 40 dB combined with an extremely short acquisition time of 240 μs per pixel. Next to several application examples we discuss the potential for real-time operation for production line and security applications.

We demonstrate a hybrid system for THz raster scan imaging using frequency-stabilized DFB lasers for electro-optic detection. The system combines a 0.65-THz micro-electronic narrow-band emitter with two synchronized diode lasers, providing two-color laser radiation. The functional principle promises three-dimensional real-time imaging capabilities.

We report on the realization of an active fully electronic THz camera operating at 645 GHz and room temperature. It currently acquires images with about 9000 pixels in 10 seconds. Each pixel contains the amplitude as well as the phase information of the reflected THz signal. Frequency modulation allows to select a specific working distance and to suppress spurious reflections. The typical image size is in the order of hundreds of cm2.

Characterization of the surface figure of technical materials by interferometry in the visible or near-IR wavelength regime becomes difficult or impossible if the surface is rough on the length scale of a tenth of the wavelength used. In this case, THz radiation can provide an interesting alternative. We demonstrate two methods for the accurate determination of the surface topography: THz heterodyne profilometry and THz ESAD (extended shear angle difference) deflectometry.

We report on the simulation of a terahertz optics setup with the commercial, optic-design software package Zemax. We show that the analysis of both diffraction and abberation effects are well corroborated by the experimental results.

A subgroup of photoexcited carriers in biased few-mum-sized LT-GaAs switches is shown to recombine on a time scale of nanoseconds. This can induce field screening amounting to tens of percent of the applied field.

We report the realization of a hybrid system for stand-off THz reflectrometry measurements. The design combines the best of two worlds: the high radiation power of sub-THz micro-electronic emitters and the high sensitivity of coherent opto-electronic detection. Our system is based on a commercially available multiplied Gunn source with a cw output power of 0.6 mW at 0.65 THz. We combine it with electro-optic mixing with femtosecond light pulses in a ZnTe crystal. This scheme can be described as heterodyne detection with a Ti:sapphire fs-laser acting as local oscillator and therefore allows for phase-sensitive measurements. Example images of test objects are obtained with mechanical scanning optics and with measurement times per pixel as short as 10 ms. The test objects are placed at a distance of 1 m from the detector and also from the source. The results indicate diffraction-limited resolution. Different contrast mechanisms, based on absorption, scattering, and difference in optical thickness are employed. Our evaluation shows that it should be possible to realize a real-time multi-pixel detector with several hundreds of pixels and a dynamic range of at least two orders of magnitude in power.

The availability of few-cycle optical pulses opens a window to physical phenomena occurring on the attosecond timescale. To take full advantage of such pulses, it is crucial to measure1,2,3,4 and stabilize1,2 their carrier-envelope (CE) phase, that is, the phase difference between the carrier wave and the envelope function. We introduce an approach to determine the CE phase by down-conversion of the laser light to the terahertz (THz) frequency range by means of plasma generation in ambient air, an isotropic medium where optical rectification (down-conversion) in the forward direction is only possible if the inversion symmetry is broken by electrical or optical means5,6,7,8,9,10. We show that few-cycle pulses directly produce a spatial charge asymmetry in the plasma. The asymmetry, associated with THz emission, depends on the CE phase, which allows determination of the phase by measurement of the amplitude and polarity of the THz pulse.

We report a new method for carrier-envelope-phase determination, based on electro-optical detection of the THz pulse emitted from a plasma generated by the few-cycle pulses being characterized.

We present a microscopic model for the process of THz emission from mixing a fundamental wave with its second-harmonic in a laser-induced plasma. As demonstrated in (I ,2), THz pulses can be generated by nonlinear mixing of an intense laser pulse with its second- harmonic pulse in air. We performed such experiments with a CLARK CPA 2001 amplifier laser system which supplies pulses with a pulse duration of 150 fs at a repetition rate of 1 kHz. The wavelength of the fundamental wave is 775 nm. In the experiment the main beam is split into pump and probe. The pump beam is focused into the air, where a plasma is created. Between plasma and lens, a BBO crystal generates second-harmonic radiation. Phase adjustment between the fundamental and second-harmonic signals is achieved with a rotatable glass plate placed between BBO crystal and focus. The emitted THz radiation is focused by two parabolic mirrors into a ZnTe crystal where the THz pulse co- propagates with the optical probe pulse, which can be shifted in time by a delay stage. The distortion of the polarization is measured by an ellipsometry set-up (2). A scheme of the set-up is shown in Fig. 1. Cook et. al. (I) described the mixing process as a x'j) effect. It remained open whether the x(3' nonlinearity is to be attributed to the air or the plasma. In our experiments, we have found evidence that x(3) is associated with plasma generation, because in our experiments the THz signal disappeared when the laser pulse energies fell short of the threshold for the ionization of the air molecules. The microscopic model presented here is based on non-linear effects introduced in conjunction with the ionization process. This can be understood as follows. The ionization of a molecule occurs at a certain electrical field, which in our case is that of the optical waves. Because ofthe temporal and spatial asymmetry of the combined electrical field of fundamental and second-harmonic waves, all charge carriers move into the same direction upon ionization. This leads to a polarization of the plasma, and a plasma wave which is strongly damped. A short polarization pulse is created at each plasma volume element at the moment when the ionization level is reached. As a consequence of these considerations, the THz signal amplitude is expected to depend only on the

A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.

Summary form only given. Dark-field imaging deals with contrast enhancement by the detection of that part of the radiation which is deflected out of the beam-propagation direction by either diffraction or scattering in the sample, and requires blocking of the ballistic part of the radiation.

We demonstrate that diffraction and scattering of coherent terahertz (THz) radiation by texture variations on surfaces allows characterization of sub-millimeter-sized surface topographical features of metals and other technical materials.

Since its first demonstration by Nuss et al. [1] in 1995. optoelectronic THz imaging has become a rapidly- expanding field of research [2] with the potential for applications in package inspection and in the chemical and biomedical field. Typical optoelectronic THz measurement systems employ set-ups based on femtosecond laser systems which both are expensive and have problems to obtain narrow linewidth spectral data. For these reasons, there is an interest to find conceptual alternatives. We present a THz imaging system based on the coherent generation and detection of cw THz radiation by photomixing as demonstrated by Verghese et al. [3]. Our set up is given in Fig. 1.

Focusing of high-energy laser pulses overlapped with its second harmonic into air leads to THz generation. We present a detailed study on the enlargement of this effect when a plasma is formed in the focus.

Summary form only given. Far-infrared (terahertz) pulses can be generated by photoionization of electrically biased gases with amplified laser pulses. We demonstrate experimentally that the efficiency of this generation process can be significantly increased, when the absolute gas pressure is raised up to 46 bar. The stronger THz output is a result of the extremely high bias fields of up to 200 kV/cm which can be applied under these conditions. The laser pulses are supplied by a CLARK CPA 2001 laser system. They are focused into the gap between two electrodes within a pressure chamber. The THz radiation is detected by an electro-optic detection scheme using a 1-mm ZnTe crystal.

We report the realization of coherent electro-optical detection of nanosecond terahertz (THz) pulses from an optical parametric oscillator, which is pumped by a Q-switched nanosecond Nd:YVO4 laser at 1064 nm and emits at approximately 1.5 THz. The beam profile and wavefront of the THz beam at focus are electro-optically characterized toward the realization of a real-time THz camera. A peak dynamic range of approximately 37 dB/radical Hz is achieved with single-pixel detection.

We have characterized the homogeneity of large-area (10 mm x 10 mm) CdTe(110) and ZnTe(110) crystals using a raster electro-optic scanning method to assess their usability in two-dimensional electro-optic terahertz (THz) imaging with parallel read out. The spatial variation in the detected THz signal (at 0.2 and 0.645 THz, respectively) is due to nonuniform residual birefringence and scattering. For CdTe, this depends critically on the growth method, and has an important contribution from slip planes in the crystals, as is evident in the scanned images. For the highest-quality CdTe(110) crystals investigated, the rms signal variations are less than 15%, comparable to those for ZnTe(110). For electro-optic scanning, we introduce a hybrid measurement system based on a fs Nd:glass laser and a continuous-wave electronic THz source.

Characterization of the topography of materials by interferometry in the visible or near-IR wavelength regime becomes difficult or impossible if the surface is rough on the length scale of a tenth of the wavelength and more. In this case, THz radiation can provide an interesting alternative. We demonstrate heterodyne profilometry at 600 GHz as a method for the accurate determination of surface topography with an achievable expanded standard uncertainty of 0.5 mum.

We report terahertz emission experiments on low-temperature-grown GaAs photoconductive antennae. Two field-screening effects determine the device response: space-charge screening on a long time scale and radiation field screening of the local electric field. This latter effect is the principal cause for saturation of terahertz emission observed when the emitters are driven hard with high-repetition-rate femtosecond laser pulses. We present an equivalent-circuit model consisting of three elements: a resistor with time-dependent conductance (photoswitch), a time-dependent voltage source (space-charge screening), and the antenna impedance (terahertz emission and radiation field screening). The simulations with this voltage divider reproduce the measured data well.

The authors have developed a hybrid system for coherent raster-scan imaging at 0.6THz. It combines a high-power electronic source (a multiplied Gunn emitter) with a femtosecond Ti:sapphire laser in order to achieve a high dynamic range via electro-optic detection. The single-scan dynamic range of 60db at a lock-in time constant of 10ms is sufficient to permit detection of scattered terahertz radiation in addition to specularly reflected/transmitted light. Active synchronization of the electronic source and the laser is not needed because of the remarkably low jitter between the two radiation sources.

We present a detailed experimental and theoretical study of terahertz (THz) generation and beam propagation in an optoelectronic THz system consisting of a large-area (ZnTe) electro-optic emitter and a standard electro-optic detector, and provide a comparison to typical biased GaAs emitters. As predicted by theory, in the absence of saturation the generated THz pulse energy is inversely proportional to the area of the optical pump beam incident on the emitter, although the detected on-axis electric field amplitude of the subsequently focused THz beam is practically independent of this area. This latter result promotes the use of larger emitter crystals in amplifier-laser-based THz systems in order to minimize saturation effects. Moreover, the generation of an initially larger THz beam also provides improved spatial resolution at intermediate foci between emitter and detector.

Intense radiation in the terahertz (THz) frequency range can be generated by focusing of an ultrashort laser pulse composed of both a fundamental wave and its second-harmonic field into air, as reported previously by Cook et al. [Opt. Lett. 25, 1210 (2000)]. We identify a threshold for THz generation that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain our measurements. An additional THz field component generated in the type I beta-barium borate crystal used for second-harmonic generation has to be considered if one is to avoid misinterpretation of this kind of experiment. We conclude with a comparison that shows that the plasma emitter is competitive with other state-of-the-art THz emitters.

This paper investigates the nature of the signals obtained in double-pulse-excitation THz-emission experiments performed on terahertz antennas with low-temperature-grown GaAs as photoconductive material. Results of such measurements on devices containing photoconductive gaps with an area below 100 µm2 have been interpreted in the past as evidence for screening effects by space charge build-up. We argue that this interpretation leads to discrepancies with what is known about charge-carrier dynamics. In analogy to the much-better-studied response of large-area emitters (photoconductors with an active area on the order of a square centimeter), we argue that radiation-field screening must be considered when interpreting the data of small-area emitters.

Terahertz emission from laser-generated air plasmas has recently been identified as an interesting source for THz radiation. High intensities and a large bandwidth of the THz pulses can be achieved. We briefly review several mechanisms which were employed to generate the quasi-static dipole moment needed for the optical rectification process. This leads us to a discussion of a specific application of THz emission from an air plasma, namely the investigation of the carrier-envelope phase of few-cycle optical pulses. Such pulses of a duration of less than 10 fs induce a spatial charge asymmetry in the plasma directly via non-linear tunneling ionization. The asymmetry, and with it the emission of the THz radiation from the plasma, depend on the carrier-envelope phase, with the consequence that one can determine the phase by measurement of the amplitude and polarity of the THz pulse.

With state-of-the-art amplifier laser systems one easily reaches pulse energies sufficient to ionize atoms and molecules in focused beams. If this is done in a gaseous medium at ambient pressure (e.g., in air), a plasma with a length of a few hundred μm and a diameter of up to 100 μm can be produced by a 1 kHz 150 fs Ti:sapphire laser system with laser pulse energies in the 500 μJ regime. The plasma formation process is strongly influenced by the optical non-linearity of the plasma itself which leads to strong defocusing of the beam and a limitation of the maximum plasma density to a few 10% of the particle density [1]. A CCD image of a laser-induced plasma is shown in Fig. 1. Terahertz (THz) emission from laser-induced plasmas was found for the first time by Hamster et al. in 1993 [2–4]. Their experiment was based on the acceleration of the charged electrons and ions by the ponderomotive forces exerted by the intense light pulses. Recently, we demonstrated that the emission process can be enhanced and steered to the forward direction by applying a static bias field to the plasma [5]. In following reports, Cook et al. showed that even more efficient ∗corresponding author; e-mail: t.loeffler@physik.uni-frankfurt.de

THz-emission from laser-generated plasmas opens a way to measure the carrier-envelope phase of few-cycle optical pulses, a parameter of critical significance for numerous experiments with sub-10-fs light pulses.

We investigate dark-field imaging in the terahertz (THz) fre-quency regime with the intention to enhance image contrast through the analysis of scattering and diffraction signatures. A gold-on-TPX test structure and an archived biomedical tissue sample are examined in conventional and dark-field transmission geometry. In particular, the capability of the technique for tumor detection is addressed.

Summary from only given. We demonstrate cw THz imaging with complex sample and compare the results with images taken with a state-of-the-art pulsed system based on an Ti:sapphire amplifier laser operating at 1-kHz repetition rate.

We present a microscopic description for the polarization induced in a plasma by few-cycle optical pulses in isotropic gaseous media. The resulting asymmetric polarization allows for optical rectification and THz emission.

'Visualization' in imaging is the process of extracting useful information from raw data in such a way that meaningful physical contrasts are developed. 'Classification' is the subsequent process of defining parameter ranges which allow us to identify elements of images such as different tissues or different objects. In this paper, we explore techniques for visualization and classification in terahertz pulsed imaging (TPI) for biomedical applications. For archived (formalin-fixed, alcohol-dehydrated and paraffin-mounted) test samples, we investigate both time- and frequency-domain methods based on bright- and dark-field TPI. Successful tissue classification is demonstrated.