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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. 

We present a fully-polarimetric THz imaging system using full-duplex frequency extenders driven by a conventional vector network analyzer (VNA). 

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.