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Planar metasurface-based quarter wave plates have significant advantages over conventional devices in terms of compactness, flexibility, and simplicity of manufacture; however, they offer a relatively narrow operational bandwidth. A broadband terahertz achromatic quarter wave plate is realized in the 0.68-1.48 THz spectral region in this work, which consists of several separate metasurface layers of dielectric elliptic pillars stacked together with various rotation angles. Meanwhile, an improved simulated annealing method is proposed, which introduces the evolutionary strategies (ES) to optimize the distinct fundamental microstructural unit cells, and the running speed is greatly increased. Furthermore, the proposed multi-layers metasurface may pave the way for arbitrary polarization control of incident waves and be ideally suited for application by virtue of subwavelength thickness in the other frequency bands as well.

Unique features of terahertz (THz) wave have made THz spectroscopy a potential and ideal tool for spectroscopy analysis. When a conventional THz spectroscopy system is used to measure a sample with birefringence, sometimes it is highly possible to misinterpret a polarization filter effect as normal spectral absorption, thus leads to a wrong identification and conclusion. Polarimetric terahertz spectroscopy (PTS) is one kind of newly emerged technology upgraded from conventional terahertz spectroscopy, in addition to all the traditional functions, it is able to automatically measure additional state of polarization of THz wave, as well as additional polarization information of samples. Here in this article we will comprehensively review the principle and progress of such PTS technology.

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