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Terahertz time-domain systems are known as a precise imaging tool. These systems make use of parabolic mirrors or lenses to illuminate a small spot of a sample under test. By moving the sample or the sensor head, an image can be recorded. This imaging technique guarantees a high signal-to-noise ratio and large bandwidth. However, using this method a priori knowledge of the sample shape is needed for the correct focusing of the system. This limits the performance and robustness of the imaging system as only specular reflections are considered for the image. Here, we propose a fast reflection-based broadband terahertz time-domain imaging method that overcomes these hurdles by making use of both the specular and diffuse reflections of a divergent terahertz beam. The proposed method employs no optical lenses or mirrors but uses signal processing and classical radar migration techniques. High-resolution imaging is achieved by focusing the divergent terahertz beam via post-processing. To compensate the inherent poor signal-to-noise ratio of the unfocused terahertz beam, calibration and post-processing methods are used. For the evaluation of the imaging method, geometrically complex samples are scanned by a fast terahertz time-domain spectroscopy system based on electronically controlled optical sampling. The bandwidth achieved with the divergent beam is 2.5 THz with a signal-to-noise ratio of around 30 dB. We demonstrate that this method is capable to generate high-resolution 2D terahertz images of objects with arbitrary size, shape, orientation and relative position to the emitter and detector antennas. Objects with sub-mm dimension can be clearly reconstructed for arbitrary positions and orientation achieving resolution in the µm region. Furthermore, the presented method can be applied for any reflection-based scenarios and antenna configuration.
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