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  1. 1-page abstract for submitted to the URSI Commission K as part of the 2020 IEEE International Symposium on Antennas and Propagation and North American Radio Science Meeting. Spectroscopic Terahertz (THz) imaging has demonstrated utility in several biomedical applications, including identification of tissue malignancies (M.-A. Brun, F. Formanek, A. Yasuda, M. Sekine, N. Ando, and Y. Eishii, “Terahertz imaging applied to cancer diagnosis,” Phys. Med. Biol., vol. 55, pp. 4615–4623, 2010), detection of axon demyelination in human brain due to Alzheimer’s (W.-G. Yeo, O. Gurel, N. Srinivasan, P.D. King, N.K. Nahar, S. Park, N.L. Lehman, and K. Sertel, “Terahertz Imaging and Electromagnetic Model of Axon Demyelination in Alzheimer's Disease”, IEEE Transactions on Terahertz Science and Technology, Volume: 7, Issue: 6, pp. 711 – 721, August 2017), and skin burn healing (M. H. Arbab, D. P.Winebrenner, T. C. Dickey, A. Chen, M. B. Klein, and P. D. Mourad, “Terahertz spectroscopy for the assessment of burn injuries in vivo,” J. Biomed. Opt., vol. 18, no. 7, 2013, No. 077044), to name a few. Although the state of the art THz instrumentation is effective in capturing the reflection and/or transmission spectral of biomedical samples, the assessment of these images in a diagnostic setting almost always requires concurrent microscopic assessment of sectioned and stained samples of the same tissue conducted by a trained pathologist. In an effort to break out of this conundrum and demonstrate potential of THz imaging for diagnostic utility, here we propose machine learning as a tool to automatically classify biomedical images and determine the regions of interest. In particular, we present a vector-network-analyzer-based, fully-polarimetric THz imaging setup with 3.4× improved resolution, operating at 1THz to capture images of formalin-fixed, paraffin-embedded human brain tissue samples to identify regions most impacted by the demyelination of the axons. Due to the elongated morphology of the axon bundles, the 4-fold polarimetric reflection image data provides additional information to effectively classify such tissues. 
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  2. We present a fully-polarimetric THz imaging system using full-duplex frequency extenders driven by a conventional vector network analyzer (VNA). 
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  3. We analyze the Gaussian character of multiple reflections in extended hemispherical lenses which are widely used in terahertz spectroscopy. In particular, the first, second and third order reflections from a high-refractive-index extended hemispherical lens illuminated by a plane wave are characterized using high-frequency approximation. To demonstrate the importance of the Gaussicity of the incident and reflected beams on coupled power levels, we study a quasi-optical link involving a horn antenna and an off-axis parabolic reflector. Although such multiple reflections with distinctly different Gaussian character can be time-gated in time-domain spectroscopy systems, care must be exercised in continuous wave systems. Depending on the quasi-optical link, i.e. positioning of the antennas and reflectors, second and third-order reflections may induce significant variations of the sensed signal. 
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  4. We present a reflection-mode, polarimetric imaging system that can achieve 45µm sample resolution and measure the co-polarized and cross-polarized reflected field components individually over the 750GHz1.1THz band. The system is based on a vector network analyzer (VNA) in conjunction with frequency extenders and illuminates the sample under test through a high-resistivity Silicon (HRSi) lens to achieve image resolution 3.42-times better than the free-space diffraction limit. The two ports of the VNA are used to capture the co-polarized and cross-polarized images of the same sample. A simple quasi-optical setup is used to isolate and direct the cross-polarized reflected signal without significantly degrading the copolarized signal. The utility of the proposed system is demonstrated using biomedical samples in form of formalin-fixed paraffin-embedded (FFPE) tissues. 
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  5. Multiple reflections from electrically large hemispherical lens surfaces of lens-integrated antennas are investigated using an iterative Huygens’ integral approach. In particular for mmW- and THz-band applications, double-slot antennas on extended hemispherical high-resistivity Silicon lenses have been widely used due to the high Gaussisicity of their radiation/ reception patterns. Previous studies assumed an electrically-large lens and evaluated the antenna pattern using first-order physical optics approximation. Although this approach is fairly accurate for estimating the radiation pattern of such antennas, the reception pattern and the associated performance of receiving sensors need a more careful consideration due to the relatively large level of internal reflections from the concave boundary of the high index lens. Here, we present an iterative method to compute and study the effects of multiple reflections inside electrically large lenses. The rich nature of quasi-optical wave behavior is demonstrated through several examples corresponding to individual bounces of the incident, reflected, and transmitted waves from a double slot antenna. 
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